This paper presents the analytical displacement of the sheet pile braced excavation system by means of the FEM technique based on case studies of deep excavation in Bangkok city with full geotechnical instrumentation. The research sites consists of 4 projects where the field measurement was recorded corresponds to the construction techniques and sequences. These four case studies had different boundary conditions with the excavation depth varied between 8–11 m. while the bracing spans and construction techniques were different. The back-figured values for soil stiffness in terms of Young's modulus are calculated as Eu/Su = 150, 250 and 1000 for soft Bangkok clay, medium clay and stiff silty clay, respectively. The results have later been confirmed with the first self-boring pressuremeter tests in Bangkok subsoils.
Deep excavations in Bangkok City of Thailand are still necessary even though the country has faced an economic crisis. There are still required the necessary civil works to serve the demands and to solve the environmental problems such as our first underground MRT subway project as well as the waste water treatment schemes. Bangkok subsoil conditions consist of a very soft dark gray clay about 12–14 m. thick, underlain by stiff clay to about 23–25 m. depth then followed by the first dense silty sand layer. Excavation in such soft dark gray clay requires efficient retaining structures such as reinforce concrete diaphragm walls for deep excavation and temporary sheet piles for shallower excavations. The sheet pile work is generally applied for excavations up to about 10–11 m. depth. Use of sheet pile systems is very popular because the construction cost is cheaper, the piles are reusable and simple for the contractor. Lateral wall movement that will not induce any damage to the adjacent areas normally controls the depth of excavations.