Civil Engineering Reference
In-Depth Information
Figure 9.18
Brinch Hansen's 90% method. (After Brinch Hansen, 1963.)
9.2.4.3 Settlement criteria
Settlement limits at design load or 1.5 times the design load are frequently specified,
on the basis of previous experience (e.g. a maximum of 10mm settlement at design
load). Such specified settlement limits should be realistic and take into account the
likely elastic compression of the pile shaft. It is sometimes difficult to discern how the
specified criteria for a single pile will relate to the behaviour of the pile group. A much
more satisfactory approach is to use the pile test data to assess the stiffness of the
soil mass using the solutions outlined in Chapter 4. This data may then be applied to
the pile group to estimate the overall performance of the foundation under the design
loads. This approach is particularly useful in the case of piled structures carrying high
lateral loads, as the stiffness of the soil is difficult to establish and may well be affected
by the piling operation. However, a single pile will not stress the ground to a depth
comparable to the depth stressed by a pile group and some extrapolation of the results
will generally be required.
Contract test piles are frequently loaded to 1.5 times the design load as a perfor-
mance test. Such a load test should avoid significant permanent deformations in the
surrounding soil and the alteration of the load-deformation characteristics of the pile
so tested. As these tests are not taken to failure it is sometimes difficult to assess the
acceptability of the pile. In such situations it is useful to estimate the likely elastic
settlement of the pile and then to draw up a realistic and unambiguous specification
for test pile performance. Such an assessment can quickly be made by summing the
compression of the shaft and settlement of the toe. For such calculations it is nec-
essary to estimate the shaft adhesion from the site investigation data. In practice,
