Side Friction along Drilled Shafts in Weak Carbonate Rocks

Weak carbonate rocks are characterized by shear strength and deformation properties that are inferior to those of conventional rocks but superior to those of stiff or dense soils. Although numerous research programs have been carried out on almost all types of piles in various soil profiles, the behavior of drilled shafts in weak carbonate rocks has not been thoroughly investigated. In this paper, an attempt is made to enhance the design practice for drilled shafts in weak carbonate rocks by integrating the results of full-scale static instrumented pile load tests on large-diameter drilled shafts with good quality rock properties derived from reliable geotechnical investigations. The objective of the test program is to investigate the effect of several parameters which are known to influence rock-pile interaction and skin friction mobilization. Observations of the t-z behavior of the tested piles indicate a strong correlation between the normalized skin friction t/t_max and the normalized vertical displacement along the shaft z/z_max. More importantly, results indicated that the maximum mobilized side friction t_max was found to increase with increasing rock modulus and/or decreasing pile diameter thus confirming the possible effect of the constant normal stiffness on load transfer. The predictive performance of six commonly used empirical models which are based on the unconfined compressive strength was investigated. Results indicated that all models except those proposed by Horvath and Kenney (1979) and O'Neill and Reese (1999) tend to significantly overestimate side shear. A simple proposed empirical model that combines the effect of the unconfined compressive strength and the constant normal stiffness showed a better performance when compared to all other models tested in this study.