Piles in sand and in sand overlying clay

• Main Author: Robinson, R. B.
• Other Authors: Delpak, R.
• Published: University of South Wales 1989
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.749613

This thesis examines the behaviour of single 60mm and 114mm segmented tubular steel piles driven and placed into loose sand and loose sand overlying clay. The soil was placed and instrumented under controlled conditions in a 3.0m diameter by 3.0m deep concrete tank. The 60mm pile was dynamically driven using a pneumatically controlled driving rig, whilst the 114mm pile was driven at a constant rate of penetration via a hydraulic jack. The static and dynamic axial load distributions were monitored for the 60mm pile. The variation in local shaft friction and radial effective stress were monitored along the pile shaft of the 114mm pile, together with the distribution of axial load within the pile. The pore water pressure was continuously monitored at selected points in the clay from the placement of the overburden to the final stages of the experiment. The density of the sand was carefully controlled during placement and was subsequently measured at the relevant point in the experiment. Vertical and radial displacements were monitored within the sand. For the two soil profiles radial shear and vertical effective stresses were recorded at a defined level within the strata. Data from both the pile and soil instrumentation was recorded throughout the pile installation and load testing programme by an Orion Data Logger which was interfaced with a Commodore PET micro computer. The results show: (i) During pile installation the major principal stress acting at depth within a soil profile, appears to emanate from the face of the active wedge driven ahead of the pile. (ii) The boundary of the sand/clay interface has a considerable effect on the development of soil displacements and the effective vertical stress developed within the overlying sand. (iii) The radial displacement during pile installation is directly related to the pile diameter. Within a sand profile the peak radial displacement can be predicted using an empirical compaction factor adjustment to a theoretical representation of radial soil movement. (iv) In sand, the local unit shaft friction and the radial effective stress are practically constant along the pile shaft for a given pile embedment and increases at a diminishing rate with pile embedment. (v) At full pile embedment and ultimate applied load, the local coefficient of earth pressure KZ, for a driven pile may approach or exceed the value of Kp near the top of the pile and tend to a lower limit of 0.6 near the pile base. (vi) For a placed insitu pile at ultimate applied load, the local coefficient of earth pressure Kz may be less than Kp near the top of the pile and tend to Ka near the pile base. (vii) Adjacent to the pile shaft the radial effective stress is the major stress. (viii) The development of shaft friction is directly related to displacements within the surrounding sand and on the sand/clay interface. (ix) The influence of the underlying clay layer affects the development of shaft friction to varying limits above and below the sand/clay interface. (x) For shallow pile penetrations into the clay layer the drawdown of sand and sand plug driven ahead of the pile significantly reduces the pore water pressure generated at the soil/pile interface. (xi) The development and radial distribution of pore water pressure within the clay can be represented be a logarithmic expression. (xii) The maximum compressive strain due to pile installation in a sand profile radiates from below and around the pile base. These results are compatible with and extend previous research work at the Polytechnic of Wales. They illustrate how soil behaviour and soil/pile interaction are influenced by the method of pile installation and the boundary effects of an incompatible underlying layer.