Deep foundations – residential
Deep foundations can provide a good foundation for houses on sites with poor soil conditions near the surface, by transferring loads to stronger and denser soil strata. They can also resist vertical uplift loads where required.
However, deep foundations can be vulnerable to some secondary seismic effects, such as lateral spread, associated with liquefaction.
Deep piles are not considered suitable for major or severe bulk lateral spreading situations and require careful detailing for ductility to accommodate lesser levels of lateral spreading. Brittle shear failure of the piles must also be avoided. Some types of deep piles that are not inherently ductile may be designed and detailed to add ductile characteristics.
There are several types of deep foundation that can provide resistance to seismic loads.
Driven pile foundations
Driven piles are a class of pile foundation that distributes the loads from the structure, including seismic forces, vertically through a depth of soil or to a deeper stratum layer. They are driven to a sufficient depth that the cumulative bearing strength of the pile foundation is sufficient to resist these loads.
Lateral seismic loads are resisted by side bearing of the piles on the surrounding soils. Driven piles may encounter intermediate thin hard layers that are difficult to penetrate. Potential issues should be identified during the site investigation, and the foundation design should specify a pile and driving method that is suitable for the type of ground at the site. It is unacceptable to leave piles bearing on thin intermediate layers because they are too difficult to penetrate.
For some foundations, the piles can use predrilled pilot holes to help the driving process penetrate hard layers. However, predrilling should not extend into the bearing layer, and the pile should always be driven to its target depth.
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A driven timber pile foundation consists of a series of suitably treated timber poles driven vertically into the ground, usually to a depth of several metres. Depending on the type of soil, they can be designed to rely on skin friction to provide bearing strength or driven so that its bottom end bears into a stratum, such as a stiff soil layer or rock.
The process of driving timber piles tends to compact any surrounding loose soil. If they are located close enough together, this can mitigate some secondary seismic effects, such as liquefaction and minor lateral spreading. Specialist design is required for such systems.
Timber piles generally have sufficient resilience to be used on sites that are likely to experience bulk lateral movement up to 300 mm.
A steel H-pile foundation consists of a series of specially designed steel H-columns driven vertically into the ground, often to depths of 10 m or more. H-piles are usually installed with closer spacing than other pile foundations.
The steel in an H-pile foundation gives it a highly ductile characteristic, and these foundations can typically resist greater lateral spreading than other pile types. The H-piles have a relatively small end surface area and have less end-bearing resistance than other piles, but in some soils, the relatively large surface area of the H can compensate for this by providing greater skin friction.
Steel H-pile foundations are generally more suited to sites with a very dense or thick gravel bearing layer, but they also have sufficient ductility to be used on sites that are likely to experience bulk lateral movement up to 300 mm.
Similar to H-pile foundations, steel tube pile foundations consist of a series of hollow steel tubes driven vertically into the ground.
The tube piles may be either open or closed-ended, but closed-ended piles have the design option of being filled with concrete after they are installed. This adds significant ductility to the piles, making them more resistant to ground movements. In either case, the steel tube piles should be designed with sufficient wall thickness to resist the driving loads and likely seismic loads at the site.
A driven precast concrete pile foundation consists of a series of preformed concrete piles driven vertically into the ground.
The process of driving concrete piles tends to compact any surrounding loose soil. This increases the bearing strength of the pile. If they are located close enough together, this can mitigate some secondary seismic effects, such as liquefaction and minor lateral spreading. Specialist design is required for such systems.
Concrete piles are often driven to bear on a target stratum, but unless specially designed and detailed, they have limited ductility and can fail on sites where significant bulk lateral movement (greater than 300 mm) is expected.