A MICROMECHANICAL STUDY OF PENETRATION TESTS IN GRANULAR MATERIAL

Abstract

Penetration tests. such as the cone (Campanella and Robertson (1988), Proc. of the 1st Int. Symp. on Penetration Testing, Vol. 1, p. 93) or flat dilatometer (Marchetti (1980), J. Geotech. Eng. Div. 106, 299) tests are important field testing methods in characterizing soil properties. For granular materials, field testing is essentially the only alternative because of the difficulty in obtaining undisturbed samples. However, lacking an understanding of the penetration mechanisms, the interpretation of these field tests are empirical. A study was carried out to improve our understanding of penetration mechanisms taking advantage of the recent developments in micromechanical behavior of granular soils. A numerical technique which couples the distinct element method and boundary element method was developed to simulate two-dimensional penetration tests in a granular material. A series of simulated penetration tests of a 60-degrees apex angle penetrometer were performed in a normally consolidated granular deposit. The results showed that as the penetrometer passed the soil element it experiences a stress loading and unloading. The distribution of contact forces concentrates in the vertical direction below the penetrometer. The direction of particle contacts and contact forces rotates towards the penetrometer tip. The average coordination number does not vary significantly around the penetrometer. It appears that the finer particles experience stresses resulted from contact forces up to three times those of larger particles.