Underground Singapore 2011 Pre-stress is applied to flat slab with compression by taking reaction from diaphragm wall since the diaphragm wall is the retaining wall to support external soil and water pressure. When open cut excavation is adopted, oil jack is used to apply pre-stress into flat slab from inside surface of the wall. This pre-stress enables to expand the span of columns to support the flat slab that it further increases the open space for usage. If top-down construction method is adopted, flat slabs are constructed as excavation progresses. Compression from soil and water let the diaphragm wall to incline inside and this deformation induces compression inside the flat slabs. Pre-stressing will be automatically done in this case and jacking system may not be necessary. Method using open cut construction is studied in the following section. Adoption of pumping water method to prevent floating is considered. Although there still are numbers of discussion if the floating force of underground structure can be reduced by water pumping, this method is worth considering that the cost of structure to transfer the weight of the building is quite large when the pumping method is not employed. Since the soil at the bottom of underground building is mudstone, the coefficient of permeability is less than 10-6cm/s. Also the welling of the water at nearby site was measured as almost 50m3/day. By considering these facts, floating force can be technically controlled to almost zero by penetrating the bottom of the diaphragm wall into the mudstone layer and pumping up the water outside. The employment of structural control system is considered to reduce earthquake response. As described in architectural features, the structural system is divided by Sunshine court into underground building and super high rise building. These two structures are connected with dampers at ground level. Also, since super high rise building is standing from GL-100m level, the bottom of the building is considered as bedrock and there is no amplification of earthquake by surface ground characteristics. By these two configurations, the earthquake response of super high rise building is reduced dramatically. Earthquake response of underground building is very small since the outside wall of the building has the contact with surrounding soil. 3 EXTERNAL FORCES External forces to consider are earth pressure, water pressure subjected to diaphragm wall of underground building, earthquake load and wind load. The wind load to this super high rise building is same as usual super high rise building that it is not discussed in this paper. 3.1 Lateral static pressure Based on Foundation Design Guideline of 2001 (Architectural Institute of Japan, Japan Association of Wall Foundation 2006) total static earth pressure and water pressure subjected to diaphragm wall is evaluated with following equation. P0 z K 0 z Pw z Pw z
(1)
Where P0 is total pressure at depth z, K0 is the coefficient of earth pressure at rest, is the density of soil, z is depth, and Pw(z) is water pressure at depth z. The coefficient of earth pressure at rest for sandy soil is evaluated with Jaki’s theory (J. A. Jaki ). The pressure is evaluated as shown in Figure 2. When referring to design and construction guideline of retaining wall (Advanced construction technology center 1994), and assuming the soil is basically composed of clay below GL-50m, the pressure is calculated by following equation. P0 z K a z q
(2)
Where, Ka is coefficient of active earth pressure and q is vertical pressure at ground surface. While pressure at GL-100m is calculated as 1160kN/m2 using equation (1), 915kN/m2 is obtained using equation (2). Pressure using equation (1) is used in following study.
155