INVESTIGATION OF SETTLEMENT AND LOAD SHARING ON PILED RAFTS BY MONITORING FULL-SCALE STRUCTURES

This paper offers five recent case histories of piled raft foundations in Japan. The buildings are 19 to 162 m in height above the ground surface and were completed in 2005-2009. The piled rafts were designed based on a numerical analysis using the simplified method developed by Yamashita et al. (1998) with soil deformation parameters evaluated from the soil's shear modulus at very small strains. To confirm the validity of the foundation design, field measurements were performed on the foundation settlements and the load sharing between the rafts and the piles by monitoring the five structures from the beginning of their construction to 17 to 60 months after the end of their construction. The measured settlements were 19 to 24 mm, and the ratios of the load carried by the piles to the effective load of each structure in the tributary area, alpha(p)', were estimated to be 0.61 to 0.93 at the end of the observation period. The predicted maximum settlements and angular rotations of the rafts in the design were generally consistent with the measured values, and the ratios of the load carried by the piles to the total load assumed in the design were also consistent with those estimated from the measurements. Based on the measurement results from ten case histories, namely, five previously published case histories and five from the present study, it has been found that the value of alpha(p)' generally decreases as the pile spacing ratio is increased. The value of alpha(p)' seems to decrease gradually with a pile spacing ratio larger than about six, whereas the value of alpha(p)' seems to decrease significantly as the pile spacing ratio is increased from about four to six. Based on the measurement results, it is suggested that piled rafts work more effectively at a pile spacing ratio of larger than about six, where at least 30% of the effective load of the structure can be carried by the rafts.