Load transfer mechanisms of drilled shaft foundations in karstic limestone : behavior under working load

Drilled shaft foundations are widely used in karst areas due to their economy and reliability, but limited data on the response is available from load tests or in-service shafts. To investigate the load transfer mechanisms of drilled shaft foundations in this geologic setting, four in-service drilled shafts were instrumented. The instrumentation was designed to measure the relative contribution of skin friction and end bearing in karstic limestone, and their variations throughout the construction and service life of the structure. The measurements began immediately after concrete was poured and continued throughout the superstructural construction. The monitoring is planned to be continued throughout the service life of the building.

Significant tip stress decrease was noted in the first 28 days after the concrete was poured. It is suggested that the tip stress decrease is caused by a small volume decrease due to concrete shrinkage which results in most of the shaft dead load being carried by skin friction. The end bearing and the variation of stress distribution along shaft in soil and rock were recorded during different stages of construction. Time effects on skin friction in the soil and rock socket were observed. The measured end bearing and skin friction from the long-term instrumentation were compared with the those calculated from different design equations, and useful results were obtained. A numerical analysis of the drilled shaft foundations was used to examine the load transfer mechanism in karst regions and the effects of adjacent shafts. Based on the obtained instrumentation results and the analysis, recommendations and a modified design method were proposed.