18
Soils and Foundations
Chapter 18 contains the provisions for the design and construction of buildings and foundation systems. Chapter 16 regulates buildings and foundations in terms of the allowable stress design load combinations specified in that chapter. The quality and design of materials used structurally in excavations and foundations are to comply with the requirements specified in Chapters 16, 19, 21, 22, and 23. Excavations and fills are to comply with Chapter 33.
Soils and Foundations
Soil and Foundation Investigations
The building official will normally require a soils investigation to determine the stability and bearing capacity of the site soils. The height of the groundwater table should also be part of the soils investigation, as any slabs or occupied spaces below grade will require either dampproofing or waterproofing, depending on the elevation of the groundwater table. The report should also classify the type of soil, recommend the type of footing, and design criteria for the footings. Where buildings are located in Seismic Design Categories C or above, then § 1803.5.11 and § 1803.5.12 require additional investigations for potential earthquake motion-related hazards such as slope instability, liquefaction, total and differential settlement, surface displacement due to faulting, or seismically induced lateral spreading or lateral flow.
Excavation, Grading, and Fill
§ 1804 governs excavations, placement of backfill, and site grading near footings and foundations.
Dampproofing and Waterproofing
§ 1805 requires walls that retain earth and enclose interior spaces below grade to be waterproofed or dampproofed. The location of the water table determines whether dampproofing or more extensive waterproofing is required.
Subsoil Drainage System
When conditions requiring damp-proofing occur at below-grade rooms, § 1805.4 requires that additional subsoil drainage provisions be made. Such conditions are presumed not to have hydrostatic pressures.
Waterproofing
Waterproofing is required when the site soils investigation indicates that there is water under hydrostatic pressure at the site. Waterproofing is to be installed unless a groundwater control system is installed that lowers the water table to 6'' (152) below the lowest floor level.
Walls that are to be waterproofed must be of concrete or masonry construction. They must be designed to resist the anticipated hydrostatic pressures along with expected lateral and vertical loads.
Load-Bearing Values of Soils
Allowable Load-Bearing Values of Soils
Table 1806.2 contains presumptive load-bearing values for foundations and lateral pressure, based on the observed capacities for various types of rock and soils. The use of these allowable pressures for vertical and lateral loads determines the size of the footing, based on the weight of the structure bearing down on them.
The allowable vertical pressures vary from 12,000 psf (575 kN/m2) for bedrock to 1,500 psf (72 kN/m2) for clay soils.
Footings and Foundations
§ 1808 and § 1809 describe the requirements for footings and for shallow foundations.
While there are provisions for alternate materials for footings, most footings and foundations are made of concrete. We will focus on concrete footings and foundation designs.
Deep Foundations
When spread footings are not adequate or appropriate for a foundation system, pier and pile foundations are often used. Both systems use columnar structural elements either cast in drilled holes or driven into the ground to support foundations. § 1810 describes the requirements for such deep foundation systems.
Chapter 2 defines a Deep Foundation as a foundation element that cannot be considered a shallow foundation, or in other words, not a strip footing, a mat foundation, a slab-on-grade foundation, or similar elements that are installed relatively close to the ground surface.
Both pier and pile foundations are to be based on the recommendations of a soil investigation. Once the capacity of the soil is determined, a system design can be selected. The code contains detailed requirements for the design and configuration of pier and pile foundations.
Driven Piles
Driven piles are piles that rely on either end-bearing or friction on the surface of the pile to provide support for the building. They are inserted into the ground and driven into place by impact of a hammer, similar to driving a nail into wood. Piles may also be driven by vibratory drives, subject to verification by load tests.
Driven piles may be of various materials:
- Timber piles may be round or sawn. They must be preservative-treated unless the entire pile will be under water for its entire life of service. Timber piles should be capped or monitored closely as they are driven to ensure their shafts or tips are not split or shattered during driving.
- Precast concrete piles are to be reinforced with longitudinal rods tied in place with lateral or spiral ties. Concrete piles are to be designed to resist seismic forces as for concrete columns. Reinforcement for precast nonprestressed piles is to have a minimum cover of 2'' (51). Reinforcement for precast prestressed piles should have a minimum cover as specified in ACI 318.
- Steel piles consist of either H-sections or sections fabricated from steel plates. They are to have a flange projection not exceeding 14 times the minimum thickness of either the flange or the web. Their nominal depth in the direction of the web is not to be less than 8'' (203) and the flanges and webs are to have a minimum thickness of ⅜'' (9.5).
- Steel-pipe piles driven open-ended are to be at least 8'' (203) in outside diameter. Wall thickness of the pipe sections depends on the driving force used.
Cast-in-Place Deep Foundations
§ 1810.3.9. Cast-in-place deep foundations are usually constructed by drilling a hole to suitable bearing strata and placing concrete in the hole.
Drilled Shafts
A drilled shaft is defined in Chapter 2 as being a cast-in-place deep-foundation element constructed by drilling a hole and filling it with liquid concrete. It is considered “socketed” if a permanent pipe casing extends the full length of the pile to bedrock.
