Three-dimensional Analysis of Cone Penetration into the Ground

Abstract

In the present study, the mechanism of the penetration of a cone into the ground was investigated analytically and experimentally. The dimensions of the slip line fields around the cone and the values of the contact pressures on the cone surface were calculated by the slip-line method, assuming that the ground is a plastic-rigid body. The values of the bearing capacity factor of the cone (N_cra) were calculated using the mean values of the contact pressures and were compared with the ratios (I_c/c_u) of the cone index (I_c) to the undrained shear strength of the clay (c_u), which were obtained by experiments. As a result, it was established that this method of analysis is applicable to cone penetration into pure cohesive soils (φ=0). The results obtained by this investigation are summarized as follows:
1) The slip line field for a cone with rough surface (a rough cone) is larger than that of a cone with smooth surface (smooth cone). The value of the intermediate principal stress (σ₂) does not affect the shape of a slip line field.
2) The distribution pattern of the contact pressure changes greatly with the value of σ₂ The contact pressure increases rapidly toward the central axis of the cone when (σ₂=σ₃, and it increases slowly when σ₂=σ_m.
3) The value of Ncra varies greatly not only with the apex angle of the cone.(2α) and the roughness of the cone surface, but also with the value of σ₂. It has a peak value (minimum) at 2α =30°-40° for a smooth cone, while the value for a rough cone decreases hyperbolically with the value of 2α.In general, the value is larger for a rough cone than that for a smooth cone, and larger when σ₂=σ₃than when σ₂=σ_m.
4) The cone penetration resistance (P) is in proportion to the square of the depth of cone penetration (h), which is measured from the tip of the cone, and expressed as, P= (π·tan²α·I_c) h², within the range when the penetration depth is smaller than the height of the cone.
5) The speed of cone penetration (ν) does not affect the value of I_c/c_u within the range of the experiments (speed range is 1.0 cm/sec.(commonly used speed in situ)-1/2, 000 cm/sec.).
6) The surface of commonly used cones can be considered rough in a considerable range of shearstrength for clays having plastic indices larger than 30 (I_P≥30). The values of c_u for these clays are therefore easily obtained using the values of I_c and N_cra for a rough cone.
7) The surface of commonly used cones can not be considered rough for clays having plastic indices smaller than 30 (I_p<30). In this case, it is necessary to take the adhesive characteristics of the clay into account.