Field behaviour of stiffened deep cement mixing piles

Full-scale pile load tests were performed on soft Bangkok clay improved by stiffened deep cement mixing (SDCM) piles and deep cement mixing (DCM) piles installed by jet-mixing to compare their performance. The SDCM pile is a DCM pile with a precast reinforced concrete core pile inserted in the middle. A series of full-scale tests consisting of axial compression, lateral and pullout interface between the concrete core pile and surrounding DCM material were performed. The length of the concrete core pile influenced both the ultimate axial bearing capacity and the settlement of the SDCM piles more than its section area. Furthermore, the section area of the concrete core pile affected both the lateral ultimate bearing capacity and the lateral displacements of SDCM piles significantly. Moreover, the SDCM piles with area ratio (Acore/ADCM) of 0.17 and length ratio (Lcore/LDCM) of 0.85 increased the axial and lateral ultimate bearing capacities so that they were as much as 2.2 and 15 times higher than the corresponding values of DCM piles, respectively. The flexural strength of the DCM pile obtained from the laboratory was 16% of its unconfined compressive strength whereas that obtained from full-scale lateral load tests was much lower at 4-7% of its unconfined compressive strength. The strength reduction factor, Rinter, at the interface between the concrete core pile and DCM pile in the field averaged 0.40, which agreed with the data from laboratory tests of 0.38-0.46.