Small strain stiffness assessments from in situ tests

The small strain stiffness of the ground can be measured directly using laboratory tests or indirectly using seismic wave velocity measurements however both these techniques are expensive and this generally precludes their use in normal site investigations. Analysis of the stiffness characteristics of soils shows there is a fixed soil specific relationship between small and large strain behaviour and this should allow the correlation of small strain stiffness to the data from large strain in situ tests. BRE has for many years been undertaking research into the in situ measurement of small strain stiffness and so has built up a significant database of information from test bed sites on which to make such correlations. This paper considers some of the correlations presently available to interpret ground stiffness from in situ tests but includes the BRE data on a range of soils from normally consolidated through to heavily overconsolidated deposits. Anisotropy of stiffness is shown to play an important role in the reliability of the correlations and better agreement is found between CPT and DMT parameters when they are compared to the horizontal small strain stiffness.