Centrifuge modelling of cone penetration testing in cohesionless soils

The quasi-static cone penetration test is becoming increasing popular as a site investigating tool to determine the geotechnical parameters for geotechnical design. As the result of complex changes in stress strain relationship, no comprehensive theoretical solution to this problem has yet been developed. Many of the available interpretations of cone penetration data are made with empirical correlations to obtain the required geotechnical parameters. 50 centrifuge tests at elevated g and 52 laboratory tests at Ig together with 23 triaxial tests with cell pressure ranging from 25kPato lOMPa to determine the mobilised angle of shearing resistance and 13 direct shear tests to determine the friction angle of cone-soil interface were carried out. The penetration test results show that the stress level and the density of the soil are the most important factors that govern the penetration resistance. Three different diameter cone penetrometers were employed in the investigation, i.e 6.35, 10.0 and 19.05mm which when tested on the same specimen such that they simulated a 'common prototype' of 400mm diameter in general gave an excellent "modelling of models" correlation. Experimental results show that no grain size effect on cone resistance was observed for dB greater than 12 where B is the cone diameter and dso is the nominal grain &0 size, whereas for dB = 7.5, the grain size effect begins to appear and the difference on &0 cone resistance is approximately 10% at ~ = 25 where D is the depth of penetration. The data indicate that difference in tip resistance between same sizes of cones but with quite different surface roughness, i.e knurled (rough) and unknurled (smooth) cones, is negligible for three different nominal grain sizes of sand (B.S 14/25, 25/52, 52/100 Leighton Buzzard sand). The data indicate that the rate of penetration does not significantly affect the tip resistance in dry sand where no excess pore pressure has been generated. The distance of the cone from the bottom boundary at which the bottom boundary effect becomes evident depends on the diameter of cone and the relative density of the soil and can be approximated from an empirical correlation as ~ = 0.1139(R.D%) - 1.238 where X is the distance from the bottom boundary. Correlation of the test results were carried out using :- ·the conventional approach to relate the tip resistance qc, (Tv and the relative density R.D . • the state parameter approach to relate the state parameter tP' and the normalised factor of the tip resistance by mean normal effective stress [~] 0.5. Armed with these correlations, they can be used to determine the fundamental soil properties such as mobilised angle of shearing resistance for design or alternatively, to determine the insitu density of a model test sample such as is employed in a centrifuge test. The theoretical solution to the deep penetration problem has been analysed using the method of characteristics taking into consideration the penetration up to a characteristic depth, thereafter a modified spherical cavity expansion theory is more appropriate. Classical bearing capacity theories used by other researchers are discussed. A parametric study on the effects of soil compressibility ~ has also been carried out for deep penetration.