The socket length of piles in soft rock: a case study

Abstract

In case where weak soil take place under mat foundations of high, heavily loaded structures, foundation system is designed with socket piles which transfer their load through skin friction to a firm stratum located at a considerable depth below the base of the structure. End bearing is generally not allowed for foundation piles of high structures for limiting vertical displacement conditions. Therefore the load of the structure is designed to be carried by friction between the firm stratum and the pile shaft. In literature there are different empirical relations for calculating unit friction resistance along the firm stratum. In important applications, it is very crucial to obtain unit friction resistance values, their change with respect to depth by measurements, utilizing special pile load tests and prove that piles carry the load with a desired factor of safety within tolerable displacements. In this thesis, a case study is presented at Maslak, Istanbul which is named as the Mashattan project where ten 34 storey high rise residential blocks are planned to be constructed. The subsoil conditions consist of completely weathered, over consolidated clays(Belgrade Formation) which are located above the extensively fractured “graywacke” claystone- siltstone- sandstone formation, locally known as Trace Formation. Thickness of the Belgrade formations vary at the site, that is why friction between the soil and the pile shaft, with respect to depth and formation, needs to be determined. For that reason two special loading tests were performed just outside the foundation area and the test piles were loaded up to 6000 kN, which is twice the design load. In those tests, unit skin friction values are measured by taking strain measurements from the strain gauges which are installed at three different locations on the test piles. As a result, change of skin friction values with respect to depth is determined. Furthermore distribution of the applied load to skin and the tip are examined. By comparing calculated and measured unit skin friction values, safety of the design is tested prior to construction. Consequently, proper socket length of the pile to be utilized is determined.