SOME GEOTECHNICAL ASPECTS OF DESIGN OF THE INITIAL SYSTEM PROJECT IN BANGKOK

The Blue Line underground transit system in Bangkok comprises 22 kilometres of twin bore single track tunnel and 18 cut and cover stations founded in soft marine clays, stiff clays and dense fine sands. The design of the basic station box structures and tunnel works is largely complete and construction of these works has commenced, whilst the detailed design phase of the internal structures and facilities continues. The paper briefly describes the Initial System Project and some of the unique geotechnical features. The design and construction has to consider the special requirements of construction in recent marine and deltaic deposits and the effects of ongoing subsidence arising from deep well pumping. The piezometric draw down in the upper soil horizons along the tunnel alignment is discussed with respect to the changes in geological profile and the properties of the soils. The under drainage of the upper soft clay and the consequent increase in effective stresses has initiated large and continuing consolidation of the upper soil layers. This phenomenon has a significant effect on the design of the shallow tunnels and the transition zones between tunnels, entrances and the cut and cover stations. The methods of dealing with differential settlements are described. The proximity of the works to adjacent buildings requires that settlements arising from tunnelling and cut and cover construction must be monitored and compared with subsidence arising from ground water extraction. Typical instrumentation schemes comprising settlement points, inclinometers and extensometers are currently being installed to monitor the Works. For the covering abstract see IRRD E102185.