INVESTIGATION OF IMPACT STRESSES INDUCED IN LABORATORY DYNAMIC COMPACTION OF SOFT SOILS

The majority of currently available analytical tools to predict ground stresses due to impact are based on linear spring-dashpot dynamic mod els. Although these simple models adequately represent stiff ground po ssessing linear visco-elastic behaviour, they suffer from two striking limitations when applied to relatively softer ground; (1) the inabili ty to account for the permanent deformation resulting from impact, (2) failure to incorporate stiffness changes of softer soil within the im pact duration. In this paper, the authors present an improved analytic al approach formulated on the basis of a series of laboratory impact t ests, to address the shortcomings of the current dynamic models in rel ation to soft soils. In this procedure, the impact zone is modelled as three distinct zones; (1) a zone beneath the falling weight undergoin g non-linear axial deformation while being in vertical motion, (2) an inner zone immediately surrounding zone 1 with non-linear shear deform ation, and (3) an outer zone undergoing a relatively lower degree of(l inear) shear deformation. The soil constitutive parameters pertinent t o the model are obtained from a modified dynamic compression test that simulates the impact conditions. It is shown that analytical predicti ons of the impact stress history and penetration are in agreement with test results. The findings are useful in the exploration of dynamic c ompaction techniques that will be effective in soft soil improvement.