Pile groups under compressive loading 263unpublished data, Burland and Kalra(5.17)established the relationship for London Clay:
Ev 7.5 3.9z(MN/m^2 ) where zis the depth below ground level.
Generally, the authors prefer the more logical method of considering immediate and
consolidation settlements separately. This properly takes into account time effects and the
geological history of the site. Provided that a sufficient number of good undisturbed samples
have been obtained at the site investigation stage, the prediction of consolidation settlements
from oedometer tests made in the laboratory has been found to lead to reasonably accurate
results. The adoption of the method based on the total settlement deformation modulus
depends on the collection of adequate observational data, first regarding the relationship
between the undrained shearing strength and the deformation modulus, and secondly regard-
ing the actual settlement of structures from which the relationships can be checked. Any
attempt to obtain a deformation modulus from triaxial compression tests in the laboratory is
likely to result in serious error. The modulus is best obtained from the Eu/cuand Eu/Ev
relationships, which must be established from well-conducted plate bearing tests and field
observations of settlement.
The steps in making a settlement analysis of a pile group in, or transmitting stress to, a
fine-grained soil can be summarized as follows:
(1) For the required length of pile, and form of pile bearing (i.e. friction pile or end-bearing
pile), draw the equivalent flexible raft foundation represented by the group (see Figure 5.3)
(2) From the results of field or laboratory tests assign values to Euand mvfor each soil layer
significantly stressed by the equivalent raft
(3) Calculate the immediate settlement of iof each soil layer using equation 5.22, and
assuming a spread of load of 30 from the vertical to obtain qnat the surface of each layer
(Figure 5.21). Alternatively calculate on the assumption of a linearly increasing modulus
(4) Calculate the consolidation settlement cfor each soil layer from equations 5.23 and
5.24, using Figure 5.13 to obtain the vertical stress at the centre of each layer
(5) Apply a rigidity factor to obtain the average settlement for a rigid pile group.
The consolidation settlement calculated as described above is the final settlement after a
period of some months or years after the completion of loading. It is rarely necessary to cal-
culate the movement at intermediate times, i.e. to establish the time/settlement curve, since
in most cases the movement is virtually complete after a period of a very few years and it is
only the final settlement which is of interest to the structural engineer. If time effects are of
significance, however, the procedure for obtaining the time/settlement curve can be obtained
from standard works of reference on soil mechanics.
5.3 Pile groups in coarse-grained soils
5.3.1 Estimating settlements from standard penetration tests
Where piles are driven in groups to near-refusal into a dense sand or gravel it is unlikely that
there will be sufficient yielding of individual piles under working load to permit redistribu-
tion of superstructure loading to surrounding piles as described in Section 5.2.1. Sufficient
yielding to allow redistribution may occur where bored pile groups are terminated in sand,
or where piles are driven to a set pre-determined from loading tests to allow a specified
amount of settlement under working loads.