Calibration of penetrometers and interpretation of pressuremeters in sand

Abstract

Extensive Cone Penetration Tests have been performed in large scale calibration chambers over the past 20 years under two distinct [constant stress (BC1) and mean zero lateral strain (BC3) ] lateral boundary conditions. The investigation has shown that such tests on dense sand are affected by the size of the chamber. In an attempt to achieve a more natural boundary behaviour, the boundaries of a large scale calibration chamber (on loan from NGI) were modified to correspond to the stiffness indicated by full length expanding cylinder tests for both NC and OC soils. The outer boundary in these tests was controlled to simulate soil expanding to infinity by forcing it to follow the same stress-strain behaviour as given at an earlier stage of the test by the expansion of the inner cavity. This gave an opportunity to analyse the distribution of stress with radius as well as with time. The soil was assumed to yield plastically in response to the extreme principal stresses - initially in a vertical plane and then, with more expansion, in the horizontal plane. The results show that although plastic dilation was appreciable at high shear strains it was largely (and sometimes more than) nullified in the chamber as a whole by elastic compression accompanying the higher stresses.

A series of CPT's in dense Hokksund sand were performed in the modified chamber. The results showed that for a 2 bar NC test BC1 underestimated cone resistance by 12% but that a BC3 test matched the measured values under an infinite boundary test. The cone resistance of OC samples under either BC1 or BC3 needed to be increased by 7%. Similar correction factors were also deducted for the sleeve friction. The CPT results also indicated a relationship which (at least in theory) predicted the possibility of obtaining the necessary information regarding the stress state of the soil from a single CPT.

The results of the expansion tests were then used to predict the normal stresses at the penetrometer and thus the sleeve friction. The results matched the corresponding CPT results surprisingly well.

In an attempt to model the soil behaviour during penetration, a solution was developed for a spheroidal cavity expansion in a transversely isotropic elastic medium. This was then used in conjunction with the expansion test results to obtain an elastic/plastic solution for the sleeve friction and for the distribution of lateral stress as a function of radius and depth for semi-infinite penetration. The resulting predictions at the outer soil boundary are compared with the values deducted from the strain gauge measurements. Realising that the former includes the effect of theoretical stresses from beyond the chamber top and bottom boundaries, the comparison is reasonably good. 

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