Model Studies of Long Piles in Clay

Abstract

Abstract Model studies were conducted to study the fundamental behavior of long pipe piles driven into two stiff, canpacted clay soils. The purpose of this paper is to report on the phenanena observed and not to provide guidelines for design. The specific observations reported are:The effects of pile penetration and diameter on the magnitude of displacement required to mobilize maximum skin resistance:'Ihe time required for soil "setup";Skin friction development and proportioning at increasing penetrations:The influence of tension loading on the development of skin friction:Verification of a soil-to-soil failure; andEvaluation of driveability studies using model piles. Model Data This investigation was aimed at modeling the behavior of piles driven to varying penetrations in clay. A modeling factor, denoted as relative stiffness, was used to gauge the simulitude of the test pile to typical slender piles driven into clays. Relative stiffness simulitude was maintained to insure similar tendencies for deveopnent of load distribution and rapid progressive failure between model and prototype. The relative stiffness is defined as the ratio of the pile flexibility (axial foreshortening under a generic load) to the vertical elastic displacement of the soil at the pile tip produced by skin friction. Assuming uniform skin friction and zero tip load, the pile flexibility, pp is given by;(Mathematical equation available in fullpaper) Although it was possible to simulate relativestiffness, it was not possible to simulate the state of effective stress along the pile due to the restricted size of the model and lack of an apparatus to apply confining pressures. Test Soils Two test soils were used that represented reasonable values of liquid limit and plasticity index found in recent deposits in the Gulf of Mexico. Each soil was compacted in a test chamber at near standard Proctor effort at a water content several percent wet of optimum in order to effect as high a degree of saturation as possible. The soils were compacted in thin layers to achieve a uniform undrained shear strength. The first test was conducted in a kaolin soil, having a liquid limit of 53% and a plasticity index of 23%. The material classified as MH, according to the Unified Soil Classification System. The second test was conducted in a soil made of a mixture of 90% kaolin and 10% montrrorillonite, which had a liquid limit of 67% and a plasticity index of 37%. This soil is classified as CH in the Unified Soil Classification System. Approximately 90% saturation was achieved in both soils