In Situ Soil Testing for Foundation Performance Prediction

Yueyang Zhao, Cambridge University
Geotechnical Engineering Group

Abstract

Our understanding of non-linear soil behaviour at small strains hinges on laboratory static and dynamic element testing of ideal undisturbed samples. However such samples are prohibitively expensive for day to day geotechnical engineering projects, especially in sandy soils. In addition to soilís complex nature, construction processes also have an important effect on foundationsí behaviour under working load. This is especially true in piling engineering. It is well acknowledged that the base stiffness of a displacement pile is much higher than that of an equivalent bored pile. To be able to understand and predict such effects of geotechnical processes on soil-foundation performance, it is necessary to develop practical techniques to measure both the in situ non-linear soil behaviour and any construction-induced changes. This thesis presents an attempt at fulfilling both of these objectives within the limited scope of wished-in and pressed-in model piles in a dense silica sand. Simplified methods of pile base settlement prediction are then developed using the analogy with spherical cavity expansion.

To measure monotonic soil properties in situ, a new miniature pressuremeter was developed for the centrifuge. The design successfully avoided any installation disturbance by adopting a wished-in installation process, and produced repeatable results in the small to intermediate strain rage. However, membrane penetration errors proved severe for the current design at small strains. The large-strain cross-hole method of Salgado et al. (1997a) was successfully implemented in the centrifuge. Small-strain stiffness and its dependency on mean stress level were successfully measured and compared well against data obtained from triaxial tests. The dynamic shear stiffness-strain relationship was also estimated assuming a power law constitutive model of Bolton and Whittle (1999).

The in situ shear stiffness field around a penetration pile was measured directly using the large-strain seismic method both during pile jacking and after pile unloading. An estimate of the corresponding in situ stress field was made from the stiffness field, giving direct evidence of stiffer soil behaviour and the existence of large locked-in stresses after pile jacking. These higher values of shear stiffness and large locked-in stress are shown to be the main reason behind the stiffer load-settlement behaviour of a displacement pile vs a non-displacement pile. The effect of soil densification due to the action of pile penetration does not contribute significantly to the above difference in behaviour.