Foundation investigations

Foundation investigations are generally undertaken in several stages. After resolving the building size or massing, a preliminary investigation takes a broad and general approach similar to that undertaken during site selection. Its focus extends beyond the site to identify potential hazards due to landslides above or below, liquefaction and lateral spreading. Also at this stage the site development history is reviewed, the seismicity of the site assessed, and geological and hazard maps consulted to understand the general geology and check for any active faults crossing through or near the site. Some codes require flexible buildings close to a designated 'active fault' to be designed stronger in anticipation of intense localized earthquake shaking known as ' near-fault fling' . The level of site seismicity influences the requirements for specific seismic soil investigations. In a region of low seismicity, earthquake induced liquefaction or landslides may not be a consideration.

Column Slab Footing

Pilecap

Driven pile

(a) Shallow footing

Column Pilecap

Column of moment frame

Spread footing

Raked pile

Belled pile (especially to resist tension)

Pilecap

Steel rods or tendons grouted into soil

Raked pile

Pilecap

(c) Raft foundation

(d) Ground anchor

(e) Raked piles (not recommended as explained in text)

Column

Concrete raft

▲ 7.8 Common foundation types for seismic resistance.

▼ 7.1 Foundation types and their suitability to resist seismic forces

Foundation type Comments

Shallow footing Spread or strip footings are the foundations of choice for low-rise buildings due to their ease of construction but they are unable to resist tension forces greater than their self-weight. Individual footings should be interconnected with tie-beams or a structural slab to prevent any relative horizontal movement occurring during earthquake shaking.

Piles Most piles are designed to resist compression and tension forces.

The amount of tension-resisting friction between their shafts and the ground depends upon the method of installation. Belled piles or piles socketed into bedrock can also provide tension capacity. Piles penetrate soft layers to provide adequate bearing at depth. Tie-beams between piles as for shallow footings are recommended.

Raft Suitable for medium to high-rise construction. A raft, whose depth can exceed 2 m, integrates gravity-only and seismic resisting vertical members. It mobilizes the entire weight of the building to resist inertia-induced overturning moments. It spreads concentrated loads onto a larger area and makes the structure tolerant of minor ground subsidence.

Ground anchors Sometimes used in conjunction with shallow footings, ground anchors are an efficient method for resisting tension forces resulting from overturning. They are also suited to anchoring foundations to steep slopes. Particular care against corrosion by using double-corrosion protection is required.

Raked piles While this system is very stiff against horizontal forces due to its triangulation, it has performed poorly in past earthquakes. Its rigidity and inherent lack of ductility mean raked piles should usually be excluded from seismic-resistant foundations.

Retaining structures

▲ 7.9 A drilling crew undertaking foundation investigations. Wellington, New Zealand.

Having completed the first and preliminary phase of investigations, reported back, sought and received client approval, more thorough investigations begin. Commensurate with the scale and importance of the project and the geological complexity of the site a range of tests are undertaken. For a small building simple manual penetrometer tests and boreholes may suffice to determine the soil conditions and identify any matters for concern. For a larger project, trenches or machine-drilled in situ tests enable engineers to quantify the engineering properties of the soil (Fig. 7.9). Recovered soil samples are subject to a battery of tests in a civil engineering laboratory. For large buildings additional foundation investigations are undertaken at the design stage, and even during construction. Boreholes might be necessary to prove founding conditions are adequate under major piles.

Retaining structures

▲ 7.9 A drilling crew undertaking foundation investigations. Wellington, New Zealand.

Retaining structures located in seismic regions are subject to static soil pressures that are increased by dynamic effects. Where a structure also retains water, or any other fluid for that matter, the structural design process includes hydrodynamic pressures.

Mass concrete

Mass concrete

Steel strip Line of cut face Compacte

Precast blocks

Precast blocks

Reinforced-soil wall

Length over which the rod is grouted to the soil

Reinforced-soil wall

Length over which the rod is grouted to the soil

Cantilevered walls

▲ 7.10 Common types of retaining structures.

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