Static and seismic performance of a friction piled raft combined with grid-form deep mixing walls in soft ground

Abstract

This paper offers a case history of a friction piled raft foundation supporting a seven-story building on soft ground in Tokyo. Since the building is located on loose sandy soil, underlain by very soft clayey soil, grid-form cement deep mixing walls (DMWs) were employed as a countermeasure against soil liquefaction. To confirm the validity of the foundation design, field monitoring of the foundation settlement and the load sharing between the piles and the raft was performed over eleven years from the beginning of the construction.At the time of the 2011 off the Pacific coast of Tohoku Earthquake, the seismic response of the soil–foundation–structure system was successfully recorded, namely, the peak horizontal acceleration of 1.21m/s2 was recorded on the first floor of the building. Before the earthquake, the measured settlements of the first floor were 15–29mm; after the event, the maximum incremental settlement was 6mm. The ratio of the load carried by the piles to the net load in the tributary area was estimated to be 0.72 before the event and increased to 0.81 at maximum about three years after the event. Based on the seismic observations, it is confirmed that the residual settlements of the piled raft were caused mainly by the penetration of the piles subjected to the moment load from the superstructure. On the other hand, it is suggested that the bending moment near the pile head was caused mainly by the horizontal ground displacement near the surface, rather than the inertial force of the superstructure. It was also found that the bending moment near the pile head was significantly small, as is expected in the design of piled raft systems. This may arise because the raft had a relatively small number of piles and was in contact with both the grid-form DMWs and the soil enclosed by them, where most of the horizontal load would be carried by the raft.Consequently, it was found that the friction piled raft foundation combined with the grid-form DMWs showed a good performance in a ground consisting of liquefiable sand and soft cohesive soil under both seismic and ordinary conditions.