A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering, School of Civil Engineering

Thesis (M.Eng.) - Asian Institute of Technology

This thesis describes results of field investigation into properties of soft and stiff clay in Bangkok tlu·ough in-situ testings. In co-operation with the Port and Harbour Research Institute (PHRI) and Dia Consultant from Japan, a complete set of in-situ tests including seismic cone penetration test (SCPT), flat dilatometer test (DMT), piezocone test (CPTU) and field vane shear test (FVT) has been performed at Sutthisan site in Bangkok Metropolitan area. Of them, SCPT has been performed by Dia Consultant and DMT has been carried out by PHRI. Based on the in-situ data obtained from these tests, coupled with the results of oedometer test, the undrained shear strength, Su, the overconsolidation ratio, OCR, as well as the shear modulus at small strain, Gr have been correlated amongst the results from these different tests; Estimation of the horizontal coefficients of consolidation c11 and Cv as well as the horizontal coefficient of permeability has also been included. From these tests, it is shown that there has been a unique correlation between the shear modulus, Gscpt obtained from SCPT and the dilatometer modulus, Ect and given the constant, for the unique correlation of 5.6 and 4.0 for soft clay and stiff clay, respectively. Similarly, the correlation between GscrT and (q1 - crv0) obtained from CPTU for normally or slightly overconsolidated Bangkok clay has given the constant of 60.7 and 46.0 for soft and stiff clay, respectively. Two empirical correlation, Gr = A e0 -1. 5 crvo'o.s (Shibuya and Tanaka, 1996) and Gr = A (l+e0Y2 .4crvo'o.s (Shibuya, Hwang and Mitachi, 1996) has also been used for Bangkok clay, for which A is the constant, e0 extended over a range from 1 to 5, and the OCR ranged from 1 to 2. The constant A is found as 4,190 and 15,790, respectively. When estimating Su and OCR, empirical equations, S11 = q' - CY"'' , NkT proposed by Aas (1986), and LogOCR::::; Q + c, proposed by Powell et al (1988) (a+ bw1,) have been used. As the result, the values of NkT are 9.92 and 7.67, for the corrected field vane shear strength and the uncorrected field vane shear strength, respectively. The value of constant a is 38.76, the value of constant bis 14.0 and the value of constant c is 0.038. In addition, at Nong Ngoo Hao and Outer Bangkok Ring Road, where ground improvement using Prefabricated Vertical Drain technique was applied, DMT and FVT have also been performed on both improved areas and unimproved areas to evaluate properties change of soft soils due to PVD ground improvement. The results of FVT has shown that Su of the subsoil increased considerably by a factor of 1.3 to 2 for very soft clay and 1.1 to 1.5 for soft clay. It is also shown that DMT was a useful tool for evaluating the shear strength increase associated with consolidation in the soft clay deposit.