for spread foundations in various classes of soils and rocks such as Table 1 of BS8004.
Similar tables are not generally available for piles except those giving allowable base
pressures for piles bearing on rock.
The observational methodis not usually relevant to piled foundation design. The method
involves the observation during construction of the behaviour of the whole or part of the
structure and its foundation. Typically, the method involves measuring the total and differential
settlements as the loading increases and making any necessary modifications to the design
if the movements are judged to become excessive. At this stage the piling would have been
long completed and too late to make any changes to the design without demolishing the
superstructure or introducing underpinning piles.
EC7 requires that geotechnical design by calculationshould be in accordance with BSEN
1990:2002, Basis of structural design. It is emphasized that the quality of the information
on the ground conditions is more significant than precision in calculation models and the
partial factors employed. Also the interaction between the structure and the ground should
be considered to ensure that the strains in the structure are compatible with the ground
movements resulting from the applied loading.
EC7 defines actionson the foundations. They include earth and groundwater pressures
as well as the dead and imposed loading from structure and ground movements such as soil
swelling and shrinkage, frost action and drag-down. Duration of actions such as repetitive
loading and time effects on soil drainage and compressibility are also required to be
considered.
Ground propertiesare required to be obtained from field or laboratory tests, either
directly or by correlation, theory or empiricism. The effects of time, stress level and defor-
mation on the properties are to be taken into account.
Geometrical datato be considered include the slope of the ground surface, groundwater
levels and structural dimensions.
Characteristic values of geotechnical parameters are selected from the available
information, usually in the form of a site investigation report. A cautious appraisal of the
data is made within the zone influenced by stresses transmitted to the ground (e.g. the zones
beneath a pile group as shown in Figure 5.21). The selected values may be lower ones which
are less than the most probable ones, or an upper range of values higher than the most probable
ones. The latter selection would apply where high values have an unfavourable effect on
foundation behaviour, for example, when considering drag-down on piles or differential
settlement. Statistical evaluation of geotechnical data is permitted by EC7 provided that it
conforms to local knowledge of comparable materials. A very cautious approach is necessary
when selecting values from published tables of typical properties of soils and rocks.
Design values of actionsare required to be determined in accordance with BSEN 1990.
In the case of piled foundations the design value (Fd) can be assessed directly, or derived
from representative values (Frep) by the equation:
Fd FFrep (4.2)whereFis the partial factor for an action.
Actions can be either structural, i.e. the loads transmitted from a structure directly to the pile
head or through a raft, or they may be geotechnical. The latter are caused by ground movements,
for example axial compression loads on the embedded surface of a pile caused by negative skin
friction (drag-down); or tension loads caused by swelling of the surrounding soil. Geotechnical
Resistance of piles to compressive loads 145