1810.3   Design and detailing.   Sections 1810.3.1 through 1810.3.12 are related to the design and detailing of deep foundations and address general requirements as well as special seismic requirements where applicable. The subsections of Section 1810.3 are listed in Table 1810-1.

Table 1810-1. Deep Foundation Seismic Detailing Requirements

Driven piles and helical piles are required to be designed and manufactured in accordance with accepted engineering practice to consider handling, driving, and service conditions and are addressed briefly in Sections 1810.3.1.5 and 1810.3.5.3.3.

1810.3.1.3   Mislocation.   Because of subsurface obstructions or other reasons, it is sometimes necessary to offset deep foundation elements a small distance from their intended locations so that they are not driven out of position. In such cases, the load distribution in a group of deep elements may be changed from the design requirements and cause some of them to be overloaded. To prevent major problems from displacements, the foundation system and the superstructure are required to be designed to resist the effects of deep foundation mislocation of at least 3 inches. This section requires that the maximum compressive load on any pile caused by mislocation not exceed 110 percent of the allowable design load. Deep elements such as piles exceeding this limitation must be extracted and redriven in the proper location or other approved remedies applied, such as installing additional piles to balance the group.

1810.3.1.4   Driven piles.   Except for steel H-piles, driven piles covered in Chapter 18 are the displacement type. That is, as the pile is driven, a volume of soil is displaced by the pile volume, resulting in compaction of the surrounding soils. Piles must be designed to resist driving and handling stresses in addition to anticipated service loads. In long piles, tensile stresses resulting from driving may govern the design. In shorter piles, the handling loads may dominate. Driven uncased piles are displacement piles and are constructed by driving a temporary casing, removing the soils from the casing, and placing concrete in the hole as the casing is removed. The casing is driven with a closed end, thereby displacing and compacting adjacent soil during driving. The casing is kept closed either by a detachable tip, which is left in place when the casing is withdrawn, or by a mandrel that closes off the casing tip during driving.

1810.3.1.5   Helical piles.   “Helical pile” is a manufactured steel deep foundation element consisting of a central shaft and one or more helical bearing plates. These are piles that are installed by rotating into the ground very much like screwing in a rotating plate. Provisions for helical piles are found in IBC Sections 202.1 (Helical Pile definition), 1810.3.1.5 (Helical Pile-design condition), 1810.3.3.1.9 (Helical Pile-allowable axial load), 1810.3.5.3.3 (Helical Pile-dimensions), 1810.4.11 (Helical Pile-installation), and 1810.4.12 (Helical Pile-special inspection).

1810.3.1.6   Casings.   Casings are used in many deep foundation systems. The steel-cased pile is the most widely used type of cast-in-place concrete pile. This pile type is characterized by a thin steel shell and is a displacement pile. This pile type consists of a closed-end light-gauge steel shell or a thin-walled pipe driven into the soil and left permanently in place, reinforced when required for uplift, lateral bending, or seismic-induced curvatures, and filled with concrete. The shell or pipe is usually driven with a removable mandrel. The shell is either a constant section or a tapered shape. Steel-encased piles are generally friction piles. The steel shell must have sufficient strength to remain watertight and not collapse from ground pressure when the mandrel is removed.

1810.3.2   Materials.   Deep foundations of various materials such as concrete, steel, and timber are covered in this section including seismic hook requirements and other relevant issues such as protection of materials and allowable stresses.

1810.3.2.1   Concrete.   Concrete used for the bulb type of pile must have a zero slump to be stiff enough to be compacted by the drop weight. The maximum sized aggregate allowed is ¾ inch to allow proper compaction and prevent segregation. To prevent the pull-out of the hoops, spirals, and ties in higher seismic areas of SDCs C through F, seismic hoops as defined in ACI 318 should be used at the end of such hoops, spirals, and ties.

1810.3.2.2   Prestressing steel.   Prestressing steel used in deep foundations must comply with ASTM A 416 Specification for Steel Strand, Uncoated Seven-Wire for Prestressed Concrete. Table 1808.8.1 lists when higher-strength concrete is used in prestressed piles to reduce volume changes, which reduce prestress losses, and to provide a more dense concrete to reduce cover requirements.

1810.3.2.3   Steel.   Structural steel H-piles, sheet piling, steel pipe piles, and fully welded steel piles shall conform to the appropriate and applicable ASTM standard referenced in this section of the code. H-piles are typically available in ASTM A 36 and A 572 steel. Pipe piles are fabricated from ASTM A 252 or A 283 plate. ASTM A 252 is a specification specifically for welded pipe piles. Piles using ASTM A 690 and A 992 are used in common practice. The Pile Driving Contractors Association (PDCA) Installation Specification for Driven Piles (PDCA 102-07) contains both these material specifications for steel piles.

1810.3.2.4   Timber.   Timber piles, although not having the high load capacities of steel or concrete piles, are the most commonly used type of pile, mainly because of their availability and ease of handling. Timber piles are shaped from tree trunks and are tapered because of the natural taper of the trunk. Round timber piles are generally made from Southern Pine in lengths up to 80 feet or Pacific Coast Douglas Fir in lengths up to 120 feet. Other species that are used are red oak and red pine in lengths up to 60 feet.

Untreated timber piles that are embedded permanently below the ground-water level (fresh water only, not brackish or marine conditions) may last indefinitely. If embedded above the water table, the piles are subject to decay, and if above ground are also subject to insect attack. Hence, piles should be preservative treated.

Timber piles may be either end-bearing or friction piles. ASTM D 25 sets forth the minimum circumference at the butt and the tip based on pile taper, as well as quality of the wood and tolerances on straightness, knots, twist of grain, and other requirements.

Piles must be pressure treated to prevent decay and insect attack. The IBC references AWPA U1 (Commodity Specification E, Use Category 4C) for round timber piles and AWPA U1 (Commodity Specification A, Use Category 4B) for sawn timber piles. See also discussion in Section 1809.12 for timber footings.

1810.3.2.5   Protection of materials.   Unless properly protected, deep foundation elements may deteriorate because of biological, chemical, or physical actions caused by particular conditions that exist or that may later develop at the site. The durability of deep foundation elements will be long lasting if care is taken in the selection and protection of element materials.

Some of the problems associated with deep foundation element durability are:

•   Untreated timber piles may be successfully used if they are entirely embedded in earth and their butts (cutoffs) are below the lowest ground-water level or are submerged in fresh water. The risk, however, is in situations where unexpected lowering of the water table occurs and exposes the upper parts of piles to decay and insect attack. Such conditions may occur where the water table is significantly lowered by pumping or deep drainage. There is also the remote possibility that wood piles will be damaged by the percolation of ground water heavily charged with alkali or acids.

•   Wood piles extending above the water table or exposed to air or saltwater are subject to decay and attack by insects and marine borers. The piles need to be pressure treated with preservatives conforming to the requirements of AWPA U1.

•   Steel piles that are driven and embedded entirely in undisturbed soil are generally not significantly affected by corrosion caused by oxidation, regardless of soil types or soil properties. The reason for this is that undisturbed soil is so deficient in oxygen at levels only a few feet below the ground line or below the water table that progressive corrosion is inhibited. However, where upper portions of steel piles protrude above ground into the air, where piles are placed in corrosive soils, or where ground water contains deleterious substances from sources such as coal piles, alkali soils, active cinder fills, chemical wastes from manufacturing operations, or other sources of pollutants, the steel may be subject to corrosive action. Under such conditions, steel piles may be protected by a concrete encasement or a suitable coating, extending from a level slightly above ground to a depth below the layer of disturbed earth. For piles above ground level that are exposed to air and subject to rusting, the steel should be protected by being painted, as any other type of structural steel construction would be protected.

•   Steel piles installed in saltwater or exposed to a saltwater environment are subject to corrosion, and therefore should be protected with approved coatings or encased in concrete.

•   Concrete deep elements, plain or reinforced, that are entirely embedded in undisturbed earth are generally considered permanent installations. The level of the water table does not normally affect the durability of concrete deep elements. Ground water that readily flows through either granular materials or disturbed soil and contains deleterious substances can have deteriorating effects. Concrete piles embedded in impervious clay materials will not generally suffer from ground water containing harmful substances. The primary deleterious substances that attack concrete are acids and sulfates. In the case of acids, it is best to use an alternative pile material if the acid attack is potentially destructive as coating piles may be ineffective because of soil abrasion during driving. Concrete can be attacked, however, by exposure to soils with high sulfate content. In the case of high alkaline soils with sulfate salts, Type V portland cement may be used. Where the exposures are only moderate, a Type II portland cement will usually be adequate. If the piles are in a marine environment, Type II or V portland cement is also indicated to provide the necessary sulfate resistance.

The conditions of the underground environment should be ascertained so as to protect deep foundation elements against possible corrosion of either the concrete or exposed load-bearing steel. Corrosion by oxidation is generally very minor and often disregarded. Corrosion caused by electrolytic action or by destructive chemicals on load-bearing steel can be protected by suitable coating, concrete encasement, and cathodic protection. This also applies to bare steel piles previously discussed. Concrete can be protected from chemical attack by the use of special cements, dense concrete mixtures, and special coatings.

Deep foundation elements installed in saltwater, such as for buildings or other structures in waterfront construction, are subject to chemical action on concrete materials coming from polluted waters, frost action on porous concrete, spalling of concrete, and rusting of steel reinforcement. Spalling may become particularly serious under tidal conditions where alternate wetting and drying occurs coupled with cycles of freezing and thawing. Spalling can be minimized or prevented by providing additional cover over the reinforcement; by the use of rich, dense concrete; by air entrainment and suitable concrete admixtures; and, in the case of precast piles, by careful handling to minimize stresses and avoid cracking during placement. See discussion of Chapter 19 and ACI 318 for more detailed information on concrete quality and concrete materials.

1810.3.2.6   Allowable stresses.   Allowable stresses for various types of deep foundations have been summarized in Table 1810.3.2.6. The allowable stress limits are set to provide an adequate margin of safety.

For precast-prestressed concrete, the term 0.33 is the same as conventionally reinforced piles. Because prestressing places additional compressive stresses on the pile, this stress must be subtracted from the allowable compressive stress; hence, the subtractive term –0.27f_pc. The term f_pc is the effective prestress on the gross area, which is the prestressing force remaining after losses have occurred.

For H-piles, the stresses allowed consider stability. Tests have shown that for H-piles driven to refusal in rock through soils that provide full lateral support, the stresses at failure can approach the yield stress of the material. Hence, higher stresses are allowed if a soils investigation and load tests are performed.

For concrete cast in place without a permanent steel casing, pile capacity must be based on concrete strength alone without consideration of soil capacity, and the area of the internal cross section of the pile, that is, casing inside diameter. The allowable stresses for drilled uncased piles are the same as cast-in-place piles for which holes are formed by machine drilling with auger or bucket-type drills, with or without temporary casing. Concrete is placed by conventional methods including tremies or funnel hoppers. In augered piles, concrete is injected through the hollow stem auger as the auger is withdrawn. Reinforcement, if placed without lateral ties, is also placed through ducts in the hollow stem auger. Concrete drilled or augered cast-in-place uncased piles have an allowable stress limit of 33 percent of the 28-day specified compressive strength () and an allowable compressive stress in the reinforcement of 40 percent of the yield strength of the steel (or 30,000 psi). These limits are consistent with ASCE Standard Guidelines for the Design and Installation of Pile Foundations (ASCE 20-96). For steel-cased deep concrete foundation elements, the allowable stress is 0.33 as for other concrete piles because the steel shell is too thin to act as a composite pile but does act as confinement reinforcement. The allowable stress is 0.40 if the shell meets the confinement conditions in Section 1810.3.2.7.

1810.3.2.7   Increased allowable compressive stress for cased cast-in-place elements.   The conditions discussed in Section 1810.3.2.6 and Table 1810.3.2.6 where the allowable concrete compressive stress is allowed to be increased consist of six conditions. Note that the shell thickness is not considered load carrying, the shell must be seamless or spirally welded, the yield strength of steel normally used for the casings is 30 ksi, and the maximum diameter is set so that the volumetric ratio of shell to concrete is sufficient to provide confinement for the concrete.

1810.3.2.8   Justification of higher allowable stresses.   In those sections of Chapter 18 that specifically deal with the types of elements most commonly used in the construction of deep foundations, there are limitations placed on the stresses that can be used in the deep foundation element design. In most cases, the allowable stresses are stated as a percentage of some limiting strength property of the deep foundations material. For example, in the case of piles made of steel materials, the allowable stresses are prescribed as a percentage of the yield strength of the several grades of steel that can be used for pile construction. For concrete, the allowable stress is stated as a percentage of compressive strength of the material. The allowable stresses for timber piles are based on tabulations of already reduced stresses. The reduced stresses are based on the strength values of different species of wood and reductions in strength caused by preservative treatment.

The allowable design stresses designated in Chapter 18 for each of the different types of deep foundations are intended to provide a factor of safety against the dynamic forces of deep foundation element driving that may cause damage to the element, and to avoid overstresses in the element under the design loads and other loads that may be induced by subsoil conditions.

This section allows the use of higher allowable stresses when evidence supporting the values is submitted and approved by the building official. The data submitted to the building official should include analytical evaluations and findings from a foundation investigation as specified in Section 1803, and the results of load tests performed in accordance with the requirements of Section 1810.3.3.1.2. The technical data and the recommendation for the use of higher stress values must come from a registered design professional that is knowledgeable in soil mechanics and experienced in the design of deep foundations. This registered design professional must supervise the deep foundation design work and witness the installation of the deep foundation so as to certify to the building official that the construction satisfies the design criteria. In any case, the use of greater design stresses should not result in design loads that are larger than one-half of the ultimate axial load capacity (see Section 1810.3.3.1.2).

1810.3.3   Determination of allowable loads.   The IBC specifies that the determination of allowable loads shall be based on one of three methods:

1.   An approved driving formula

2.   Load tests

3.   Foundation analysis

In most cases, the allowable loads will be determined by a combination of Items 2 and 3. However, there may be circumstances where the soil conditions, such as granular soils, and the type of deep foundation selected are such that the use of an approved dynamic deep foundation driving formula can be an aid to a qualified practitioner in establishing reasonable but safe allowable loads for the foundation system. Nevertheless, some literature indicates that “the use of a complicated formula is not recommended since such formulas have no greater claim to accuracy than the more simple ones.”⁷

The dynamic pile-driving formula included in the 1970 and earlier editions of the UBC was dropped from the code because of its unreliability for cohesive soils. It is interesting to note that the earlier editions of the UBC utilized the so-called Engineering News formula, R = 12WH/ S + c, which is the most simple of the dynamic pile-driving formulas. In 1937, the Pacific Coast formula was adopted into the UBC until its deletion prior to the 1973 edition of the code. This was one of the more complex dynamic pile-driving formulas and was based on a dynamic pile-driving formula developed by Terzaghi. However, as stated previously, in the hand of a qualified practitioner, a dynamic pile-driving formula does have some utility even though the IBC no longer provides such a formula.

There are two general considerations for determining capacity as required for the design and installation of deep foundations. The first consideration involves the determination of the underlying soil or rock characteristics. The second is the application of approved driving formulas, load tests, or accepted methods of analysis to determine the pile capacities required to resist the applied axial and lateral loads, as well as to provide the basis for the proper selection of pile-driving equipment.

Allowable axial load determination is further addressed in Sections 1810.3.3.1.1 through 1810.3.3.1.9.

1810.3.3.1.1   Driving criteria.   Deep foundation elements must be of a size, strength, and stiffness capable of resisting without damage:

•   Crushing caused by impact forces during driving.

•   Bending stresses during handling.

•   Tension from uplift forces or from rebound during driving.

•   Bending stresses caused by horizontal forces during driving.

•   Bending stresses caused by deep element curvatures.

Additionally, the deep element must be capable of transmitting dynamic driving forces to mobilize the required ultimate deep element capacity within the soil without severe elastic energy losses. Driveability depends on the deep element stiffness, which is a function of deep element length, cross-sectional area, and modulus of elasticity. Yield strength does not affect stiffness. Thus, caution should be observed in the use of high-yield-strength steels for high loads on smaller cross sections requiring high dynamic driving energy. For allowable loads greater than 40 tons, a wave equation method of analysis reflects deep element stiffness or driveability. The selection of deep foundation types and dimensional requirements for driveability is a function of soil characteristics.

For many decades, it has been the practice to try to predict the capacity of a deep foundation element from its resistance to driving. The usual procedure has been to make such determinations by the application of pile-driving formulas, none of which have been completely dependable. The singular premise used in the development of these formulas is simple and is best expressed by R.B. Peck as follows: the greater the resistance of a pile to driving, the greater the pile’s capacity to support load. With complex engineering problems, however, occasionally there are special circumstances under which there will be exceptions to general statements of this kind.

There are many pile-driving formulas. The simplest and most widely used formula in the United States is the Engineering News formula. This particular expression and other formulas in common use today have all generally shown poor correlations with load test results. Such comparisons are considerably better, however, when they are applied to the determination of deep foundation element capacity in soils consisting of free-draining, coarse-grained materials such as sand and gravel. In soils such as silt, clay, and fine sand, the water cannot escape fast enough during driving operations to not have an adverse influence on the frictional resistance of the piles. As a consequence, information may be unreliable.

This section limits the allowable compression load on a deep foundation element as established by an approved driving formula to a maximum of 40 tons. Generally, the use of pile-driving formulas to determine pile capacity should be avoided except, perhaps, in cases involving small jobs where the piles are to be driven in well-drained granular soils and load testing cannot be economically justified.

1810.3.3.1.2   Load tests.   This section specifies the standards to be used to load test deep foundation elements, where higher compressive loads than allowed in other sections of the code are exceeded or where cast-in-place deep foundation elements have an enlarged base formed either by compacting concrete or by driving a precast base. See the discussion under Section 1810.3.3.1.3 for test evaluation methods.

Questions were raised at the public hearings as to whether or not ASTM D 4945, which is a dynamic test, is sufficient by itself to verify the pile capacity. Many standards, including ASTM D 4945, indicate that a dynamic test may not be sufficient without a static test (ASTM D 1143) to calibrate the results, but leave it up to the registered design professional to decide if the dynamic test is sufficient. Other standards require a static load test to calibrate the dynamic test.

The safest method for determining deep foundation element capacity is by load test. The load-bearing capacity of enlarged base piles is specifically required to be determined from load tests. A load test should be conducted wherever feasible and used where the deep foundation element capacity is intended to exceed 40 tons per element (see Section 1810.3.3.1.2). Test deep elements are to be of the same type and size as intended for use in the permanent foundation and installed with the same equipment, by the same procedure, and in the same soils intended or specified for the work.

Load tests are to be conducted in accordance with the requirements of ASTM D 1143, which covers procedures for testing vertical or batter foundation piles, individually or in groups, to determine the ultimate pile load (pile capacity) and whether the pile or pile group is capable of supporting the loads without excessive or continuous settlement. Recognition, however, must be given to the fact that load-settlement characteristics and pile capacity determinations are based on data derived at the time and under the conditions of the test. The long-term performance of a pile or group of piles supporting actual loads may produce behaviors that are different than those indicated by load test results. Judgment based on experience must be used to predict pile capacity and expected behavior.

The load-bearing capacity of all deep foundation elements, except those seated on rock, does not reach the ultimate load until after a period of rest. The results of load tests cannot be deemed accurate or reliable unless there is an allowance for a period of adjustment. For piles driven in permeable soils such as coarse-grained sand and gravel, the waiting period may be as little as two or three days. For test piles driven in silt, clay, or fine sand, the waiting period may be 30 days or longer. The waiting period may be determined by testing (i.e., by redriving piles) or from previous experience.

This section also requires that at least one deep foundation element be tested in each area of uniform subsoil conditions. The statement should not be misconstrued to mean that the area of test is to have only one uniform stratum of subsurface material, but rather that the soil profile, which may consist of several layers (strata) of different materials, must represent a substantially unchanging cross section in each area to be tested.

The allowable deep foundation element load to be used for design purposes is not to be more than one-half of the ultimate deep element capacity, as determined by the load test in which the net settlement of the test element is not to exceed 0.01 inch per ton or more than a total of ¾ inch. The rate of penetration of permanent deep foundation elements must be equal to or less than that of the test element(s).

All production deep foundation elements should be of the same type, size, and approximate length as the prototype test elements, as well as installed with the same or comparable equipment and methods. They should also be installed in soils similar to those of the test element.

1810.3.3.1.3   Load test evaluation methods.   Three specific methods are given that are acceptable for performing deep foundation load tests. Other methods are permitted at the discretion of the building official.

1810.3.3.1.4   Allowable frictional resistance.   Resistance that is due to skin friction is limited to a maximum of 500 psf unless a greater value is permitted by the building official based on recommendations of an approved geotechnical investigation or a greater value is substantiated by load test methods described in Section 1810.3.3.1.2.

1810.3.3.1.5   Uplift capacity of a single deep foundation element.   This section gives both the designer and building official needed guidance on criteria to use for design of a single deep foundation element for uplift. The IBC requires an approved method of analysis with a safety factor of 3 or a test in accordance with ASTM D 3689. The maximum allowable uplift load cannot exceed the ultimate load capacity determined by the methods described in Section 1810.3.3.1.2 divided by a factor of safety of two.

When deep foundation elements are designed to withstand uplift forces, they act in tension and are actually friction elements. The amount of tension that can be developed not only depends on the strength properties of the element, but also on the frictional or cohesive properties of the soil. The uplift or tensile resistance of a deep element is not necessarily a function of its load-bearing capacity under compressive load. For example, the tensile resistance of a friction pile in clay will usually be about the same value as its load-bearing capacity, as the skin friction developed in such soils is very large. In contrast, a friction pile in sand or in other granular materials will develop a tensile resistance considerably less than its load-bearing capacity.

Where the properties of the soil are known, the ultimate uplift resistance value of a pile can be determined by approved analytical methods. This section requires that where the ultimate tensile value is determined by analysis, a safety factor of 3 must be applied to establish the allowable uplift load of the deep foundation element.

The best way to determine the response of a vertical or batter pile to a static tensile load (uplift force) applied axially to the pile is by applying an extraction test in accordance with the requirements of ASTM D 3689. The maximum allowable uplift load is not to be more than one-half of the total test load. This section of the code gives a limitation on the upward movement of the pile in compliance with the provisions of the ASTM D 3689 test method. The measurements of pile movement in the standard test procedure, however, are time-dependent incremental measurements and should be adhered to in determining allowable pile load.

To be effective in resisting uplift forces as tension members of a foundation system, deep foundation elements must be well anchored into the cap by adequate connection devices. In turn, the cap must be designed for the uplift stresses. Deep foundation elements must also be designed to take the tensile stresses imposed by the uplift forces. For example, concrete piles must be reinforced with longitudinal steel to take the full net uplift. Special consideration needs to be given in the design of pile splices that are intended to act in tension. When design uplift is due to wind or seismic loading, the factor of safety for the analytical method requires a factor of safety of 2 while the load test method requires a factor of safety of 1.5.

1810.3.3.1.6   Uplift capacity of grouped deep foundation elements.   The allowable uplift load on a group of deep foundation elements is to be reduced from the value obtained on a single element as described in Section 1810.3.3.1.5 of the code and in compliance with comprehensive analytical methods. In the 2012 IBC, the capacity of deep foundation groups was also limited to two-thirds of the weight of the group and the soil contained in the group plus two-thirds of the ultimate shear resistance along the soil block. This is consistent with requirements in other sections in the code on uplift and overturning, where the dead load resistance is limited to two-thirds of the weight. Previous editions of the IBC have allowed two-thirds of the effective weight of a pile group and the weight of the soil contained within the block defined by the perimeter of the group, but did not include an allowance for the shear resistance of the soil block. This was unreasonably conservative because not only the weight of the soil within the pile group resists uplift, but also the shear resistance developed contributes to the resistance to uplift of the pile group. The IBC now allows the use of two-thirds of the effective weight of the pile group, two-thirds of the weight of the soil contained within a block defined by the perimeter of the group and the length of the piles, plus two-thirds of the ultimate shear resistance along the soil block. Where the center-to-center spacing of deep foundation elements is at least three times the least horizontal dimension of the largest single element, the allowable working uplift load for the group must be calculated by an approved method of analysis.

1810.3.3.1.7   Load-bearing capacity.   The load-bearing capacity of a deep foundation element is determined as a deep element–soil system. For example, the load-bearing capacity of a single pile is the function of either the structural strength of the pile or the supporting strength of the soil. The load-bearing capacity of the deep foundation element is controlled by the smaller value obtained in the two considerations. The load-bearing capacity of a deep element group may be greater than, equal to, or less than the capacity of a single element multiplied by the number of elements in the group, depending on deep element spacing and soil conditions.

Because the supporting strength of the soil generally controls the load-bearing capacity of a deep foundation element, this section requires that the ultimate load-bearing capacity of an individual element or a group of elements be at least twice the design load capacity of the supporting load-bearing strata.

Sometimes, weaker layers of soil underlie the soil load-bearing strata supporting a pile foundation and may cause damaging settlements. Under such subsurface conditions, it must be determined by an approved method of analysis that the safety factor has not been reduced to a figure less than 2. Otherwise, the piles are to be driven to deeper load-bearing soils to obtain adequate and safe support, or the design capacity is to be reduced and the number of piles increased.

1810.3.3.1.8   Bent deep foundation elements.   Deep foundation elements that are discovered to have sharp or sweeping bends because of obstructions encountered during the driving operations or for any other cause are to be analyzed by an approved method, or a representative deep element is to be load tested to determine its load-carrying capacity. Otherwise, the deep foundation elements could be used at some reduced capacity as determined by test or analysis; or, if necessary, they can be abandoned and replaced.

1810.3.3.2   Allowable lateral load.   Because of wind loads, unbalanced building loads, earth pressures, and seismic loads, it is inevitable that individual deep foundation elements or groups of vertical elements supporting buildings or other structures will be subjected to lateral forces. The distribution of these lateral forces to the deep elements largely depends on how the loads are carried down through the structural framing system and transferred through the supporting foundation to the deep foundation elements. The amount of lateral load that can be taken by the deep foundation element is a function of (1) the type of deep foundation element used; (2) the soil characteristics, particularly in the upper 10 to 30 feet of the deep foundation element; (3) the embedment of the deep foundation element head (fixity); (4) the magnitude of the axial compressive load on the deep foundation element; (5) the nature of the lateral forces; and (6) the amount of horizontal deep foundation element movement deemed acceptable.

The degree of fixity of the deep foundation element head is an important design consideration under very high lateral loading unless some other method, such as the use of batter piles, is employed to resist lateral loads. The fixing of the deep foundation element head against rotation reduces the lateral deflection. In general, pile butts are embedded 3 to 4 inches into the pile cap (see Section 1810.1.4) with no ties to the cap. These pile heads are neither fixed nor free, but somewhere in between. Such construction is satisfactory for many loading conditions, but not for high seismic loads.

The magnitude of friction developed between the surfaces of two structural elements in contact with each other is a function of the weight or load applied. The larger the weight, the greater the frictional resistance developed. In the design of deep foundation elements, frictional resistance between the soil and the bottom of the deep element caps (footings) should not be relied on to provide lateral restraint, because the vertical loads are transmitted through the deep foundation elements to the supporting soil below and to the ground immediately under the deep element caps. Only the weights of the caps can supply some frictional resistance insofar as such footings are constructed by placing fresh concrete on the soil, thus providing a positive contact. The weight of the caps in comparison to the magnitude of loads and lateral forces transmitted to the deep foundation elements is nominal and not significant from a structural design standpoint. Also, in rare occurrences, soils have been known to settle under caps, leaving open spaces and thus eliminating the development of any frictional restraint.

Where vertical deep elements are subjected to lateral forces exceeding acceptable limitations, the use of batter piles may be required. Lateral forces on many structures are also resisted by the embedded foundation walls and the sides of the deep foundation element caps.

The allowable lateral-load capacity of a single deep foundation element or group of such elements is to be determined either by approved analytical methods or by load tests. Load tests are to be conducted to produce lateral forces that are twice the proposed design load; however, in no case is the allowable deep foundation element load to exceed one-half of the test load, which produces a gross lateral element movement of 1 inch as measured at the ground surface or the top of foundation element, whichever is lower. This criterion can be exceeded if it can be shown that the predicted lateral movement will not cause any harmful distortion of or instability in the structure and that no element will be loaded beyond its capacity.

1810.3.4   Subsiding soils.   Where deep foundation elements are driven through subsiding soils and derive their support from underlying firmer materials, the subsiding soils cause an additional load to the deep foundation elements through so-called negative friction. This negative friction is actually a downward friction force on the deep foundation elements, which increases the axial load on such elements. The code permits an increase in the allowable stress on the deep foundation elements if an analysis of the geotechnical investigation indicates that the increase is justified.

1810.3.5   Dimensions of deep foundation elements.   Deep foundation elements must have minimum dimensions as described in Sections 1810.3.5.1 through 1810.3.5.3 for precast, cast-in-place cased, and cast-in-place uncased deep foundations.

1810.3.5.1   Precast.   Eight inches is the minimum practical dimension to accommodate reinforcement.

1810.3.5.2   Cast-in-place or grouted-in-place.   Eight inches is the minimum practical dimension to accommodate reinforcement for cased cast-in-place deep foundation elements. For uncased cast-in-place deep foundation elements, the minimum 12-inch diameter is for inspection purposes. The length-to-diameter ratio is based on construction and stability considerations.

1810.3.5.2.3   Micropiles.   A micropile is defined as a bored, grouted-in-place deep foundation element that develops its load-carrying capacity by means of a bond zone in soil, bedrock, or a combination of soil and bedrock. The maximum outside diameter of a micropile is 12 inches. This dimension was originally part of the definition of a micropile when first introduced in the 2006 IBC. The 12-inch dimension is no longer in the definition but is now used in the section as the technical criterion by which a micropile is identified.

Steel deep foundation elements are generally H-piles, piles fabricated from welded plates, sheet piling, steel pipes and tubes, and helical piles. New sections covering steel piles fabricated from welded plates and sheet piling were added to the 2015 IBC.

1810.3.5.3.1   Structural steel H-piles.   H-piles are usually used as deep end-bearing piles because they are essentially nondisplacement-type piles that can readily penetrate solid strata to reach rock or other suitable hard-bearing strata such as dense gravels. Ideally, steel H-piles are driven to hard or medium hard rock.

H-piles are proportioned to withstand the impact stresses from hard driving. The flange and web thicknesses are usually equal. The flange widths are proportioned such that the section modulus, S_y, in the weak axis is approximately one-third of S_x.

1810.3.5.3.2   Fully welded steel piles fabricated from plates.   Although they are in a separate section, the requirements for steel piles fabricated from welded plates are the same as for steel H-piles.

1810.3.5.3.3   Structural steel sheet piling.   A new section for structural steel sheet piling requires that the profiles conform to manufacturer’s specifications and the general requirements in ASTM A6. The new 2011 edition of ASTM A6 is referenced in Chapter 35 of the 2015 IBC.

1810.3.5.3.2   Steel pipes and tubes.   Driven pipe piles are displacement-type piles if driven closed, and are nondisplacement-type piles if driven open. Pipe piles are made of seamless or welded pipes and are frequently filled with concrete after driving. Pipe piles conforming to ASTM A 252 are used in both friction and end-bearing applications. Pipe piles may be driven open ended or closed ended. Open-ended pipe piles are generally used when the geotechnical investigation shows rock or a suitable end-bearing stratum close to the ground surface, especially if the loads to be supported are large. The pipe is driven to bearing, the soils forced into the pipe during driving are cleaned out, and the pipe is filled with concrete. Closed-end piles are generally used as friction piles when a suitable bearing stratum is not available at suitable depths. There are several proprietary closed-end pipe piles available.

When steel pipes are driven open ended, minimum thickness to diameter is related to hammer energy by requiring a minimum area per kip-foot of energy. Open-ended pipes require a minimum area of 0.34 square inch to resist each 1,000 ft-lb. This requirement equates to a wall thickness of 0.27 inch for a 10-inch pipe driven with a hammer energy of 25 kip-feet. Note that if the wall thickness is less than 0.179 inch, a driving shoe is required to prevent local buckling at the tip from hard driving, regardless of diameter or hammer energy. The 0.179-inch thickness originally entered in the 2003 IBC to be consistent with the most common minimum thickness for closed-end pipe piles.

Concrete-filled steel pipes or tubes in structures assigned to SDC C, D, E, or F shall have a wall thickness of not less than ³/₁₆ inch. The pipe or tube casing for socketed drilled shafts shall have a nominal outside diameter of not less than 18 inches and a wall thickness of not less than ⅜ inch. The pipe is a welded or seamless pipe conforming to ASTM A 252. ASCE 20-96 Standard Guidelines for the Design and Installation of Pile Foundations as well as the recommendations of the Driven Pile Committee of the Deep Foundations Institute list 0.179 inches as the minimum wall thickness.

Concrete-filled steel piles are either seamless or welded pipe, or closed-end tubular piles with either straight or tapered sections that are driven into the soil. The piles may be installed as either friction or end-bearing piles. This pile type is characterized by a steel shell that is thicker than the thin shell used in some steel-cased piles, hence both the concrete and steel shell are assumed to carry load compositely. If driven open ended, the earth core is removed from the shell prior to concreting. The shell may be driven with an internal mandrel.

1810.3.6   Splices.   This section specifies the requirements for splicing of deep foundation elements. The 50-percent requirement provides more strength where the bending moments are low, insofar as it is based on the capacity of the deep foundation element, not the design loads. The 2009 IBC addressed splices of the same type of deep foundation elements and splices of deep foundation elements of different materials or different types. Splices of deep foundation elements of different materials or types are required to develop the full compressive strength and not less than 50 percent of the tension and bending strength of the weaker section. Although it is physically and economically better to drive piles in one piece, site conditions sometimes necessitate that piles be driven in spliced sections. For example, when the soil or rock-bearing stratum is located so deep below the ground that the leads on the driving equipment will not receive full-length piles, it becomes necessary to install the piles in sections or, where possible, to take up the extra length by setting the tip in a preexcavated hole (see discussion, Section 1810.4.4). When piles are installed in areas such as existing buildings with restricted headroom, they are also required to be placed in spliced sections. There are a number of other reasons for field-splicing piles, such as restrictions on shipping lengths or the use of composite piles.

This provision requires that splices be constructed to provide and maintain true alignment and position of the deep foundation element sections during installation. Splices must be of sufficient strength to transmit the vertical and lateral loads on the deep foundation elements, as well as to resist the bending stresses that may occur at splice locations during the driving operations and under long-term service loads. Splices are to develop at least 50 percent of the value of the deep foundation element in bending. Consideration should be given to the design of splices at locations where the deep foundation elements may be subject to tension. Splices that occur in the upper 10 feet of pile embedment are to be designed to resist the bending moments and shears at the allowable stress levels of the pile material, based on an assumed pile load eccentricity of 3 inches, unless the pile is properly braced. Proper bracing of a spliced pile is deemed to exist if stability of the pile group is furnished in accordance with the provisions of Section 18.10.2.2, provided that other piles in the group do not have splices in the upper 10 feet of their embedded length.

There are different methods employed in splicing deep foundation elements depending on the materials used in the deep element construction. For example, timber piles are spliced by one of two commonly used methods. The first method uses a pipe sleeve with a length of about four to five times the diameter of the pile. The butting ends of the pile are sawn square for full contact of the two pile sections, and the spliced portions of the timber pile are trimmed smoothly around their periphery to fit tightly into the pipe sleeve. The second splicing method involves the use of steel straps and bolts. The butting ends of the pile sections are sawn square for full contact and proper alignment, and the four sides are planed flat to receive the splicing straps. This type of splicing can resist some uplift forces.

Splicing of precast concrete piles usually occurs at the head portions of the piles. After the piles are driven to their required depth, pile heads are cut off or spliced to the desired elevation for proper embedment in the concrete pile caps. Any portion of the pile that is cracked or shattered by the driving operations or cutting off of pile heads should be removed and spliced with fresh concrete. To cut off a precast concrete pile section, a deep groove is chiseled around the pile exposing the reinforcing bars, which are then cut off (by torch) to desired heights or extensions. The pile section above the groove is snapped off (by crane) and a new pile section is freshly cast to tie in with the precast pile.

Steel H-piles are spliced in the same manner as steel columns, usually by welding the sections together. Welded splices may be welded-plate or bar splices, butt-welded splices, special welded splice fittings, or a combination of these. Spliced materials should be kept on the inner faces of the H-pile sections to avoid forcing a hole in the ground larger than the pile, causing at least a temporary loss in frictional value and lateral support that might result in excessive bending stresses.

Steel pipe piles may be spliced by butt welding, sometimes using straps to guide the sections and to provide more strength to the welded joint. Another method is to use inside sleeves having a driving fit, with a flange extending between the pipe sections. By applying bituminous cement or compound on the outside of the ring before driving, a water-tight joint is obtained.

1810.3.6.1   Seismic Design Categories C through F.   Splices of deep foundation elements in SDCs C through F are addressed in this section and require that the splices develop the lesser of two elements: (1) the nominal strength of the deep foundation element and (2) the axial and shear forces and moments from the seismic load combinations including overstrength factor of ASCE 7, Section 12.4.3 or 12.14.3.2. For a discussion of overstrength factor, see Section 1810.3.11.2.

1810.3.7   Top of element detailing at cutoffs.   The requirements of this section are intended to account for conditions where a deep foundation element encounters refusal at a shallower depth than anticipated and a portion of the deep element is cut off. It is imperative that the required reinforcement be provided at the top of the deep foundation element when the excess deep element length is cut off.

1810.3.8   Precast concrete piles.   Precast concrete piles are manufactured as conventionally reinforced concrete or as prestressed concrete. Both types can be formed by bed casting, centrifugal casting, slipforming, or extrusion methods. Piles are usually square, octagonal, or round, and either solid or hollow. Precast piles, which may be either friction or end-bearing piles, are of the displacement type and are driven into place.

1810.3.8.1   Reinforcement.   The closely spaced spirals or ties at the ends are to accommodate radial tensile principal stresses from driving.

1810.3.8.2.1   Minimum reinforcement.   (Of precast-nonprestressed piles). Four bars are the practical minimum for placement.

1810.3.8.2.2   Seismic reinforcement in Seismic Design Categories C through F.   Minimum longitudinal reinforcing steel and transverse tie or spiral confinement reinforcing is required to provide some ductility. This is required for precast-nonprestressed piles in all SDCs except SDCs A and B.

1810.3.8.2.3   Additional seismic reinforcement in Seismic Design Categories D through F.   This section contains additional transverse reinforcement requirements for buildings in SDCs D, E, and F. These are in addition to the longitudinal and transverse reinforcement requirements provided in Section 1810.3.8.2.2. Spirals or ties are spaced closer to provide for the higher ductility requirements in SDC D and higher. The details of this additional transverse requirement are found in Section 1810.3.9.4.2.

1810.3.8.3   Precast-prestressed piles.   Minimum prestress is set to minimize cracking from handling and driving stresses. The purpose of prestressing piles is to place the concrete under a compressive stress so that hairline cracks caused by any subsequent tensile stress that may occur from handling, driving, superimposed loads, or seismic imposed curvatures, and which are larger than the prestressed compression stress, will close when the tensile stresses are removed. This is easily achievable for handling and driving loads, but may not be feasible for seismic-imposed curvatures.

Prestressed piles can be either pretensioned or post-tensioned. Pretensioned piles are generally cast full length in a casting bed at a manufacturing plant and often contain only prestressing steel reinforcement. Post-tensioned piles may be plant cast or site cast, and generally contain mild steel reinforcing to resist handling stresses.

1810.3.8.3.2   Seismic reinforcement in Seismic Design Category C (precast-prestressed piles).   Spiral transverse (confining) reinforcement is required to mitigate the effects of soil-induced curvatures from seismic ground displacements. The volumetric requirement is the same as that for columns in ductile frames. (See ACI 318, Section 21.6.2.)

1810.3.8.3.3   Seismic reinforcement in Seismic Design Categories D through F.   These transverse reinforcing requirements are based on testing of prestressed piles in New Zealand and the subsequent recommendations by the Prestressed Concrete Institute (PCI). The requirements result in prestressed piles with good ductility without creating construction problems from reinforcement congestion. In Item 2, the “distance from the underside of the pile cap to the point of zero curvature” is determined as in Section 1810.3.9.4.1 below. See Sheppard⁸ and Joen et al.⁹

1810.3.9   Cast-in-place deep foundations.   Cast-in-place deep foundations are covered in Sections 1810.3.9.1 through 1810.3.9.4.2.2. Cast-in-place (CIP) concrete piles are installed by placing concrete into holes preformed by drilling or by driving a temporary or permanent casing to the required bearing depth. Drilled or augered piles are also known as cast-in-drill-hole or CIDH piles. Drilled or augured uncased piles are nondisplacement piles and are installed by drilling or augering a hole and filling the uncased hole with concrete, either during or after withdrawing the auger. CIP piles may be either cased or uncased. Uncased piles are difficult to construct when below the ground-water table. Except for enlarged base piles, the concrete in CIP piles is not subjected to driving forces, only the forces imposed by the service loads and downdrag from settlement. One advantage of drilled CIP piles is that the tip elevation can easily be adjusted to have the tip on the correct bearing stratum. Reinforcement is installed during the concreting operation. CIP-drilled piles are of the nondisplacement type, that is, the soil is not displaced or compacted by the drilling operation. CIP piles constructed by first driving a closed-end shell are displacement piles, where the soil surrounding the shell is displaced and compacted during the driving operation. CIP piles constructed by driving an open-ended casing without a mandrel or temporary tip closure are nominally a nondisplacement pile, although some compaction around the shell may occur in cohesive soils, and densification may occur from driving in granular soils.

The steel-cased pile is the most widely used type of cast-in-place concrete pile. This pile type is characterized by a thin steel shell and is a displacement pile. This pile type consists of a closed-end light-gauge steel shell or a thin-walled pipe driven into the soil and left permanently in place, reinforced when required for uplift, lateral bending, or seismic-induced curvatures, and filled with concrete. The shell or pipe is usually driven with a removable mandrel. The shell is either a constant section or a tapered shape. Steel-encased piles are generally friction piles.

1810.3.9.1   Design cracking moment.   The design cracking moment that is established as nominal moment capacity multiplied by the factor is determined from the equation

The design cracking moment in SI is:

The equation and the reinforcement requirements of Section 1810.3.9.2 clarify the present requirements, making the IBC consistent with the requirements of ACI 318, and allow elimination of the definition for flexural length. For both uncased and cased cast-in-place deep foundation elements (but not concrete-filled pipes and tubes), reinforcement must be provided where moments exceed a reasonable lower bound for the capacity of the plain concrete section, known as the cracking moment.

1810.3.9.2   Required reinforcement.   See Section 1810.3.9.1.

1810.3.9.3   Placement of reinforcement.   With three exceptions, reinforcing steel must be assembled into a cage and placed in the hole or casing prior to concreting, not stabbed after concreting. One exception is for dowels less than 5 feet in length, and the other exception is for auger-injected piles, which are placed by injecting the concrete through a hollow stem auger. The third exception is for smaller residential and utility buildings (Group R-3 and U occupancies) where the method of reinforcement placement after concrete placement must be approved by the building official.

1810.3.9.4   Seismic reinforcement.   There are four specific cases where prescriptive provisions are provided for special cases to comply with the seismic reinforcement requirements in SDCs C through F. Other than these four exceptions, cast-in-place deep foundations must comply with Section 1810.3.9.4.1 for seismic reinforcement in SDC C and Section 1810.3.9.4.2 for seismic reinforcement in SDCs D, E, and F.

1810.3.9.4.1   Seismic reinforcement in Seismic Design Category C.   Minimum steel requirements are established to provide some ductility. The minimum reinforcement must be continued throughout the flexural length of the pile. The term flexural length was added to the 2003 IBC as the “length of the pile from the first point of zero lateral deflection to the underside of the pile cap or grade beam.” The point of zero lateral deflection can be determined from the P-y analysis. In the 2009 IBC, the term “flexural length” and its definition were removed and replaced with “minimum reinforced length.” Minimum reinforced length is not a defined term in the 2012 IBC; rather, in this section there are four criteria to determine the length.

1810.3.9.4.2   Seismic reinforcement in Seismic Design Categories D through F.   The requirements in SDC D, E, or F are similar to those in SDC C, except that the minimum reinforcement ratio is higher and extends over a longer length to improve ductility. In addition, closed ties or spirals are required to provide confinement in regions of plastic hinging, that is, at the pile-cap interface, at the interface of soft to stiff layers, and in liquefaction zones. Confinement reinforcing should also be used for bay muds and sensitive clays. See Sections 1803.5.11 and 1803.5.12. The term “flexural length” has been replaced with “minimum reinforced length,” which is determined in this section by four criteria.

1810.3.9.4.2.1   Site Classes A through D.   Transverse confinement reinforcement requirements for stiffer soil and rock sites are in this section, which references applicable provisions in ACI 318.

1810.3.9.4.2.2   Site Classes E and F.   Transverse confinement reinforcement requirements for soft or sensitive soil sites are in this section, which references applicable provisions in ACI 318.

1810.3.9.5   Belled drilled shafts.   Bells are designed to increase the bearing surface on which the loads will be transferred. Bells are typically designed in more cohesive soils so that there will not be any collapsing of the bell walls or roof.

1810.3.9.6   Socketed drilled shafts.   Socketed drilled shaft deep foundations are what were previously called the caisson pile or more commonly known as a drilled-in caisson. They are installed as a special type of high-load-capacity pile and are characterized by a structural steel core, an upper-cased section extending to bedrock, and a lower uncased tip that is socketed into rock.

The socketed drilled shaft deep foundation element is a cased cast-in-place concrete pile that is formed by (1) driving a heavy-wall open-ended pipe down to bedrock, (2) cleaning out the soil materials within the pipe, (3) drilling an uncased socket into the bedrock, (4) inserting a structural steel core into the pipe, and (5) filling the entire pipe and drilled socket with concrete.

The core material is usually made of hot-rolled structural steel wide-flange or I-beam sections, or steel rails. This section specifies that the steel core is to extend full length from the base of the drilled socket to the top of the steel pipe or, as an alternative and depending on design requirements, the steel core may extend halfway up the pipe or as a stub core to a distance in the pipe at least equal to the depth of the socket. The strength of the deep foundation element is developed in combined friction and end-bearing of the rock socket.

1810.3.10   Micropiles.   Micropiles are bored, grouted-in-place deep foundation elements that develop their load-carrying capacity by means of a bond zone in soil, bedrock, or a combination of soil and bedrock. Prior to their inclusion in the code, the use of micropiles had to be approved under the alternative materials, design, and methods of construction provisions in Section 104.11. The provisions are based on the recommendations of the ADSC/DFI (International Association of Foundation Drilling/Deep Foundations Institute) Committee on Micropiles, and are intended to provide a uniform standard for micropiles, and eliminate inconsistencies in their design and installation. The IBC provisions are based primarily on the Massachusetts Building Code (MBC) with additional changes and modifications.

1810.3.11   Pile caps.   Pile caps are to be of reinforced concrete and designed in accordance with the requirements of ACI 318. For footings (pile caps) on piles, computations for moments and shears may be based on the assumption that the load reaction from any pile is concentrated at the pile center. See ACI 318 for loads and reactions of footings on piles.

The soil immediately under the pile cap should not be considered to provide any support for vertical loads. For a more detailed explanation of this requirement, see the discussion on Section 1810.3.3.2, allowable lateral load.

The heads of all piles are to be embedded not less than 3 inches into pile caps, and the edges of the pile caps are to extend at least 4 inches beyond the closest sides of all piles. The degree of fixity between a pile head and the concrete cap depends on the method of connection required to satisfy design considerations.

1810.3.11.1   Seismic Design Categories C through F.   Deep foundation elements in SDCs C through F must have a positive connection to the pile cap for sliding or uplift purposes. This is achieved by connecting the deep foundation element to the pile cap by either embedding the deep element reinforcement in the pile cap or by field-placed dowels anchored into the element and extended into the pile cap for a distance equal to the dowel development length in accordance with ACI 318.

Piles in structures subject to seismic ground shaking are likely to be subjected to uplift (tension) forces, either by design or because of insufficient resistance to overturning forces by gravity loads. Hence, concrete piles and concrete-filled steel pipe piles must be able to develop the strength of the pile (in tension) in the connection to the cap. This is accomplished by the requirement that the pile be embedded in the cap by a distance equal to the development length. The development length may not be reduced by the ratio A_required/A_supplied. Alternative means, such as increasing concrete confinement, may be used to reduce the development length.

Similarly, the various types of steel piles are required to develop the strength in tension and to transmit this strength to the cap by positive means other than bond to the bare steel; for example, welded studs or welded reinforcement must be used.

Splices must develop the full strength of the pile, both tension and compression, for all pile types.

1810.3.11.2   Seismic Design Categories D through F.   Anchorage of piles or piers into pile caps must consider the combined effects of uplift and pile fixity. The anchorage must develop at least 25 percent of the strength of the pile in tension. For piles subject to uplift or required to provide rotational restraint, the anchorage must develop the lesser of the nominal tensile strength of the longitudinal reinforcement in a concrete element, the nominal tensile strength of a steel element, and 1.3 times the frictional force developed between the element and the soil. Because of the large variability in soils, it would be prudent to design these piles for the full tensile capacity rather than 1.3 times the uplift capacity (frictional force). Exceptions allow the use of ASCE 7 Section 12.4.3 or 12.14.3.2 for design of the anchorage to resist axial tension forces or, in the case of rotational restraint, the design of the anchorage to resist axial and shear forces, and moments.

Batter piles have performed poorly in past earthquakes. This is because the batter piles are laterally stiff relative to vertical piles and resist most of the seismic-induced inertial forces. The piles are not usually designed to resist the actual forces, but are designed to resist an inertial force reduced by an assumed ductility. However, the batter piles are axially stiff and generally not detailed for ductility; hence the failures. To preclude this type of failure in batter piles, the piles and their connections must be designed to resist the anticipated maximum earthquake forces from the load combinations with overstrength factor in Section 12.4.3 or 12.14.3.2 of ASCE 7.

The load combinations with overstrength only apply where specifically required by the seismic provisions. They constitute an additional requirement that must be considered in the design of specific structural elements to account for the maximum earthquake load effect, E_m, which considers “system overstrength.” This system characteristic is accounted for by multiplying the effects of the lateral earthquake load by the overstrength factor, Ω0, for the seismic-force-resisting system involved. It represents the upper bound system strength for purposes of designing nonyielding elements for the maximum expected load. Under the design earthquake ground motions, the forces generated in the seismic-force-resisting system can be much greater than the prescribed seismic design forces. If not accounted for, the system overstrength effect can cause failures of structural elements that are subjected to these forces. Because system overstrength is unavoidable, design for the maximum earthquake force that can be developed is warranted for certain elements. The intent is to provide key elements with sufficient overstrength so that inelastic (ductile) response/behavior appropriately occurs within the vertical resisting elements. It should be noted that these load combinations are only to be applied where specified in the earthquake load provisions or in other structural chapters of the code. The requirement for using load combinations with overstrength in Section 1810.3.11.2 is such a case.

1810.3.12   Grade beams.   In SDC D, E, or F, grade beams must be designed as ductile in accordance with the provisions of Section 21.12.3 of ACI 318 unless the beam is strong enough to resist the anticipated maximum earthquake force as set forth in the load combination with overstrength factor of Section 12.4.3 or 12.14.3.2 of ASCE 7. That is, grade beams must be either strong or ductile. For a detailed discussion of overstrength factor, see Section 1810.3.11.2.

1810.3.13   Seismic ties.   Interconnection of piles and caissons (2009 IBC replaced the term caissons with socketed drilled shaft deep foundations, which are covered in Section 1810.3.9.6) is necessary to prevent differential movement of the components of the foundation during an earthquake. It is well known that a building must be thoroughly tied together if it is to successfully resist earthquake ground motion. These provisions apply to SDCs C through F.

Individual piles, piers, or pile caps required for structures in SDCs C through F must be interconnected with ties capable of transmitting the lesser of a force equal to the larger pile cap or column load times the short-period response acceleration, S_DS, divided by 10 and 25 percent of the smaller pile cap or column load. The intent of this requirement is to minimize differential movement or spreading between the footings during ground shaking. If slabs on grade, or beams within slabs on grade, are used to meet the tie requirement, the load path from footing to slab or beam/slab and across joints in the slab or beam/slab should be checked for continuity. The slab or beam must be reinforced for the design tension load. In addition, the slab should be checked for buckling under the required compression load using an assumed slab of no more than six times the slab thickness.

1810.4   Installation.   Provisions of the code dealing with the details of installations for various deep foundation types were collected and relocated to Section 1810.4 of the 2009 IBC, which is the last section of Chapter 18.

Care must be used during installation to prevent damage during handling and driving. The proper cushion must be used at the driving end. Precast concrete pile recommendations for design, manufacture, and installation are given in ACI 543R. Damage to piles can be classified into four types:

1.   Spalling at the butt or head (driving end) caused by high or irregular compressive stress concentrations. The spalling may be caused by insufficient cushioning, pile butt not square with the pile longitudinal axis, hammer and pile not aligned, reinforcing steel not flush or below the top of the pile allowing the hammer force to be transmitted through the steel, or insufficient transverse reinforcement.

2.   Spalling at the tip, which is usually caused by an extremely high driving resistance such as when the tip is bearing on a rock.

3.   Breaking or transverse cracking. This is caused by the rarefaction wave reflected from the tip. When the hammer strikes the cushion or head, a compression wave is produced that travels down the pile. The wave can be reflected from the tip as a rarefaction (tension) wave or a compression wave depending on the soil stiffness. Rarefaction waves usually occur when the soil at the tip is soft with very little resistance to penetration, causing tension waves that can cause significant tensile damage. This phenomenon usually occurs only in long piles exceeding 50 feet. Prestressed piles have more resistance to rarefaction damage than do conventionally reinforced piles. The hammer energy should be reduced when driving long piles through soft soils.

4.   Spiral or transverse cracking may be caused by a combination of torsional stress and rarefaction stress. Torsion is usually caused by excessive restraint in the leads.

In the case of precast-prestressed piles, because of the precompression, less care is needed in the handling and driving than for conventionally reinforced piles, and prestressed piles are, in general, more durable than conventionally reinforced precast concrete piles.

If a deep foundation element consists of two or more sections of various materials or different types of deep foundation elements spliced together, each section is required to satisfy the applicable installation requirements of this section. Generally referred to as “composite piles,” these refer to deep foundation elements placed in series, such as a cast-in-place concrete deep foundation element placed over a submerged wood pile.

1810.4.1   Structural integrity.   Deep foundation elements can be exposed to damage or could potentially cause damage to surrounding areas, especially piles that are generally installed by either driving, vibration, jacking, jetting, direct weight, or a combination of such methods. Most types of piles are exposed to some degree of damage during placement. However, with knowledge of soil conditions and the proper selection of equipment, installation methods, and techniques, damage may be prevented or minimized.

Due care must be exercised during pile installation to avoid interference with adjacent piles or other structures so as to leave their strength and load capacity unimpaired. If any pile is damaged during installation so as to affect its structural integrity, the damage must be satisfactorily repaired or the pile rejected.

Displacement piles have their own special issues. As displacement piles are driven within a group, progressive compaction of the surrounding soil occurs, particularly where it involves closely spaced piles. This can cause piles to be deflected off-line because of the buildup of unequal soil pressures around the piles. Soil compaction during driving operations can cause extreme variations in pile lengths within a group, with some piles failing to reach specified load-bearing material. Ground heave is another effect of soil compaction (see discussion, Section 1810.4.6).

To prevent or significantly reduce the problems associated with soil compaction, the driving sequence of pile installations becomes an important consideration. For example, if the outer piles of a group are driven first so that the inner piles, because of soil compaction, fetch up to specified sets (hammer blows) at much higher elevations than the outer piles, the total load-bearing value of the group will be adversely affected. As another example, starting pile driving at the edge of a group makes the piles progressively more difficult to drive and results in a one-sided bearing group. The general driving practice is to work from the center of a group outward. For large groups consisting of rows of widely spaced piles, driving can be done progressively from one side to the other.

The provisions within Section 1810.4 are to ensure pile structural integrity by adhering to proper installation procedures. The code cannot cover all possibilities, however, thus establishing the need for the general nature of this section.

1810.4.1.1   Compressive strength of precast concrete.   Handling and driving forces will likely govern pile strength requirements. Through the use of steam cure and Type III cements, 75 percent of specified strength can be achieved relatively quickly.

1810.4.1.2   Casing.   These requirements are intended to result in a satisfactory pile. The construction of drilled piles is fraught with problems: caving, ground water, and other issues.¹⁰ Most of these problems generally relate to soil conditions, including soil or rock debris accumulating at the base of the pile or occurring in the pile shaft, reductions in the shaft cross section caused by the necking of soil walls because of soft materials or earth pressures, discontinuities in the deep foundation shaft, hollows on the surface of the shaft, and other problems related to the drilling operations.

1810.4.1.3   Driving near uncased concrete.   These requirements are intended to prevent damage to uncured concrete in adjacent deep foundation elements from the soil displacements caused by driving adjacent elements. The spacing requirements should not be construed to mean that the center-to-center spacing must not be closer than six average diameters of a cased element in granular soils nor within one-half the pile depth in cohesive soils; just that deep elements cannot be driven at that spacing within 48 hours of placement of concrete.

1810.4.1.4   Driving near cased concrete.   The restrictions on driving within four and one-half average diameters of a cased element filled with concrete less than 24 hours old are primarily to avoid damage to uncured concrete in adjacent deep foundation elements.

1810.4.1.5   Defective timber piles.   Damage to the pile, including breakage, should be suspected when there is a sudden drop in penetration resistance while driving that cannot be explained by the soil profile. The pile should be withdrawn for examination. If penetration resistance should suddenly increase, driving should be stopped to avoid possible damage. A significant problem encountered during installation of timber piles is damage from overdriving. Overdriving can cause failure by bending, brooming of the tip, crushing, brooming at the butt end, or splitting or breaking along the pile section. See Figure 1810-2.

Figure 1810-2   Effect of overdriving timber piles.

1810.4.2   Identification.   All deep foundation materials must be identified for conformity to the code requirements and construction specifications. Information such as strength (species and grade for timber piles), dimensions, and other pertinent information is required. Such identifications must be provided for all deep foundation elements, whether they are taken from manufacturers’ stock or made for a particular project. Identifications are to be maintained from the point of manufacture through the shipment, on-site handling, storage, and installation of the piles. Manufacturers, upon request, usually furnish certificates of compliance with construction specifications. In the absence of adequate data, piles must be tested to demonstrate conformity to the specified grade.

In addition to mill certificates (steel piles), identification is made through plant manufacturing or inspection reports (precast concrete and timber piles) and delivery tickets (concrete). Timber piles are stamped (labeled) with information such as producer, species, treatment, and length.

Identification is essential when high-yield-strength steel is specified. Frequently, pile cutoff lengths are reused and pile material may come from a jobber, a contractor’s yard, or a material supplier. In such cases, mill certificates are not available and the steel should be tested to see if it complies with the code requirements and the project specifications.

1810.4.3   Location plan.   A plan clearly showing the designation of all deep foundation elements on a project by an identification system is to be filed with the building official before the installation is started. The inspector (see discussion in Chapter 17 on special inspection) must keep piling logs and other records and submit written reports based on this identification system. The use of such a system becomes particularly important at sites where the variations in soil profiles are so extensive that it becomes necessary to manufacture piles of different lengths to satisfy bearing conditions.

The building official should also be furnished copies of all modifications to the original deep foundation location plan that may be necessary as the work proceeds (as-built drawings). This would show elements added, eliminated, or relocated. Such records would facilitate the use of existing deep foundations in the future (see Section 1810.1.2) if the structure is altered or another structure is built on the site.

1810.4.4   Preexcavation.   There are several important reasons for the use of preexcavation to facilitate the installation of foundation piles. Some of these purposes are:

•   To install piles through upper strata of hard soil.

•   To penetrate through subsurface obstructions, such as timbers, boulders, rip rap, thin stone strata, and the like.

•   To reduce or eliminate the possibility of ground heave that could lift adjacent structures or piles already driven.

•   To reduce ground pressures resulting from soil displacement during driving and to prevent the lateral movement of adjacent piles or structures.

•   To reduce the amount of driving required to seat the piles in their proper load-bearing strata.

•   To reduce the possibility of damaging vibrations or jarring of adjacent structures, as well as reduce the amount of noise, all of which are associated with pile-driving operations.

•   To accommodate the placement of piles that may be somewhat longer than the leads of the pile-driving equipment.

The two most common methods employed in preexcavation operations are prejetting and predrilling. Jetting is usually effective in most types of soils, except very coarse and loose gravel and highly cohesive soils. Jetting is most effective in granular materials. Generally, jetting in cohesive soils is not very practical or especially useful and should be avoided in soils containing very coarse gravel, cobbles, or small boulders. These stones cannot be removed by the jet and tend to collect at the bottom of the hole, preventing pile penetration below that depth.

Jetting operations must be carefully controlled to avoid excessive loss of soil, which could affect the load-bearing capacity of piles already installed or the stability of adjacent structures.

Piles should be driven below the depth of the jetted hole until the required resistance or penetration is obtained. Before this preexcavation method is used, consideration should be given to the possibility that jetting, unless strictly controlled, can adversely affect load transfer, particularly as it involves the placement of nontapered piles.

Predrilling or coring before driving is effective in most types of soils and is a more controllable method of preexcavation than jetting. The risk of adversely affecting the structural integrity of adjacent piles or structures or the frictional capacity of piles is considerably less than jetting.

Predrilling can be performed as a dry operation or as a wet rotary process. Dry drilling can be done by using a continuous-flight auger or a short-flight auger attached to the end of a drill stem or kelly bar. Wet drilling requires a hollow-stem continuous-flight auger or a hollow drill stem employing the use of spade bits. When the wet rotary process of predrilling is used, bentonite slurry or plain water is circulated to keep the hole open. As in the case of jetting, piles should be driven with tips below the predrilled hole. This is necessary to prevent any voids or very loose or soft soils from occurring below the pile tip.

There are other methods used for preexcavation purposes, such as the dry tube method and spudding, but such procedures are seldom used. In any case, the methods to be employed for preexcavation are subject to the approval of the building official.

1810.4.5   Vibratory driving.   The use of vibratory drivers for the installation of piles is not applicable to all types of soil conditions. They are effective in granular soils with the use of nondisplacement piles, such as steel H-piles and pipe piles driven open ended. Vibratory drivers are also used for extracting piles or temporary casings employed in the construction of cast-in-place concrete deep foundation elements.

Vibratory drivers, either low or high frequency, cause the pile to penetrate the soil by longitudinal vibrations. Although this type of pile driver can produce good results in the installation of nondisplacement piles under favorable soil conditions, the greatest difficulty is the lack of a reliable method of estimating the load-bearing capacity. After the pile has been installed with a vibratory driver, pile capacity can be determined by using an impact-type hammer to set the pile in its final position.

One method to determine pile capacity is to calibrate the power consumption in relation to the rate of penetration. Nonetheless, the use of a vibratory driver is only permitted where the pile load capacity is established by load tests in accordance with the requirements of Section 1810.3.3.1.2.

1810.4.6   Heaved elements.   Piles that are driven into saturated plastic clay materials can often displace a volume of soil equal to that of the piles themselves. When this happens, the soil displacement sometimes occurs as ground heave and may lift adjacent deep foundation elements already driven. Under such conditions, heaved deep foundation elements may no longer be properly seated and a loss of pile capacity occurs. Heaved piles must be redriven to firm bearing to again develop the required capacity and penetration. If heaved piles are not redriven, their capacity must be verified by load tests made in accordance with the requirements of Section 1810.3.3.1.2.

This section applies only to piles that can be safely redriven after installation. Heaved uncased cast-in-place concrete deep foundation elements or sectional piles with joints that cannot take tension should be abandoned and replaced. When redriving heaved piles, a comparable driving system or the same as that of the initial driving should be employed. It should be noted that in redriving concrete-filled pipe piles, the driving characteristics of the pile have been altered and the pile is substantially stiffer than when the empty pipe was initially driven. In such cases, the required driving resistance would be less than originally required.

One method used to prevent or reduce objectionable soil displacement is to remove some of the soil in the spaces to be occupied by the piles. This is done by predrilling the pile holes (see discussion, Section 1810.4.4).

1810.4.7   Enlarged base cast-in-place elements.   Enlarged base elements are intended to be end-bearing-type deep foundation elements that spread the bearing load over a larger area than a prismatic element, thereby increasing capacity. Enlarged base deep elements may be either cased or uncased. Enlarged base elements are used only in granular soils, which, because of the voids between soil particles, allow densification of the soils around the deep element tip without creating excessive pressures. One type is the compacted base type, which consists of a bulb-shaped footing formed after driving the shaft casing to its final depth. Another type is the concrete-pedestal type, in which a truncated cone- or pyramid-shaped precast concrete tip larger than the steel casing diameter is driven into the soil with the casing. See Figure 1810-3.

Figure 1810-3   Enlarged base pile cased or uncased shafts.

Installation must employ the same methods used to install the load test piles. The compacted base pile is usually installed by driving a steel casing. A zero slump concrete plug is placed at the tip of the casing and impacted with a heavy drop weight, thereby driving the casing and plug. Sometimes a gravel plug is used rather than zero slump concrete. When the casing and plug have been driven to the required depth, the plug is driven out and the bulb is formed by progressively compacting additional layers of zero slump concrete. A welded reinforcing steel cage is added where required. If the deep foundation element is to be uncased, the shaft is formed by compacting zero slump concrete in small lifts as the casing is withdrawn. If the deep foundation element is to be cased, as would be required for piles through peats or other organic soils, the shaft is formed by inserting a steel shell inside the drive casing after forming the bulb, withdrawing the drive casing, and filling the shell with conventional concrete. The precast base-type pile is installed with the precast base placed at the tip of a mandrel-driven steel shell.

A problem that occurs with the precast base type and the cased bulb-type piles is that an annular space between the casing and the soil remains. Either the pile must be designed as a slender reinforced concrete column governed by buckling, or the annular space must be filled to provide the requisite lateral support. The usual practice is to fill the annular space by pumping grout.

1810.4.10   Micropiles.   Micropiles are defined in Chapter 2. Micropile boreholes are typically advanced by either rotary drilling or rotary percussive drilling. Installation requirements differ based on whether a steel casing is permanent or temporary or not provided (IBC Section 1810.4.10 Items 6, 1, and 2, respectively).

1810.4.11   Helical piles.   Helical piles are defined in Chapter 2. See Section 1810.3.1.5 and 1810.3.5.3.5.

1810.4.12   Special inspection.   See analysis and discussions in Chapter 17 regarding special inspection requirements for deep foundations.