Abstract: This paper presents a case history of deep pebble soil site investigations for the proposed large shopping mall. The preliminary geotechnical investigation at the project site did not fully understanding the limestone bedrock conditions, the vertical and horizontal limits of sandy and pebbly soil layer were not completely revealed. In the final subsurface investigation overcame the equipment limitations in the preliminary site explorations by using the improved geotechnical drilling methods such as, single tube drilling, and double-tube-double action and doubletubesingle action for sampling. The improved drilling methods successfully increased the recovery of rock cores. Different drilling slurries were used in the advancement of welldeveloped karst rock cavities to minimize the loss of drilling fluid. These slurries are drilling mud, cement slurry, vegetative slurry, and polymer slurry. These measures successfully minimize the borehole caving and loss of drilling fluid so that the accuracy of the in-situ geotechnical testing was improved. A total of 665 heavy dynamic cone penetrometer testing were performed in pebble soil layers and the results indicated that the pebble layer can be used as the bearing layer for the proposed pile tip.

Key words: Deep Pebble Layer, Pile Tip Bearing Layer, Drilling Methods