This study is part of Mrs. Nguyen Phu Hoa's doctoral dissertation research, supported by the Ministry of Education and Training, Vietnam. The authors wish to thank Prof. C. Kwei Lin, Dr. Amrit N. Bart and Dr. S.L.Rannamukhaarachchi of the Asian Institute of Technology,Thailand, and Dr. Le Thanh Hung of the University of Agriculture and Forestry, Vietnam for their valuable comments and suggestions.