Soil foundations - Geotechnical Investigation and Design Tables - page 272

Table 21.9 Factors of safety for shallow foundations (Vesic, 1975).

Loading and consequences of failure Factor of safety based on extent of S I Typical structure

Thorough SI Limited SI

Maximum design loading likely to 3.0 4.0 Hydraulic structures •

occur often. Silos

•

Consequences of failure high. Railway bridges

Warehouses
Retaining walls

Maximum design loading likely 2.5 3.5 Highway bridges

•

to occur occasionally. Light industrial buildings

Consequences of failure serious. Public buildings •

Maximum design loading 2.0 3.0 Apartments •

unlikely to occur. Office buildings

•

Limit state design uses a partial load factor on the loading and a partial per-
formance factor on the Resistance. Design Resistance Effect ≥ Design Action

effect.

•

Ultimate limit states are related to the strength. Characteristic values are used.

•

Serviceability limit states are related to the deformation and durability.

•

Shear failure usually governs for narrow footing widths, while settlement governs
for large footings (typically 2.0m or larger).

21.10 Pile characteristics
•

The ground and load conditions, as well as the operating environment determine
a pile type.

•

The table provides a summary of some of the considerations in selecting a
particular pile type.

•

Prestressing concrete piles reduces cracking due to tensile stresses during driv-
ing. Prestressing is useful when driving through weak and soft strata. The pile is
less likely to be damaged during handling as compared to the precast concrete
piles.

•

Piles with a high penetration capability would have high driving stresses capability.

•

There are many specialist variations to those summarised in the table.

Table 21.10 Pile selection considerations.

Pile type Typical working Cost/ Penetration Lateral/Tension Vibration

load (kN) metre capacity level

Driven Precast 250-2000 kN Low Low Low High

Prestressed 500-2500 kN Medium Medium Low High

Steel H - pile 500-2500 kN High High High High

Timber 100-500 kN Low Low Medium Medium

Cast Bored auger Up to 6 MPa on shaft High Medium/High High Low

In situ Steel tube Up to 8 MPa on shaft Medium High Medium High