Underground structures

By virtue of being buried, underground structures are subject to substantial vertical and horizontal static pressure. Because of their confinement by surrounding soil, which reduces their dynamic response as compared to similar structures constructed above ground, some engineers thought underground structures were safe during earthquakes. That is, until the 1995 Kobe earthquake. Severe damage occurred to underground subways and other buried facilities when they were deformed by the modest horizontal shear movements within the encompassing soft soil.6 Now that the seismic damage mechanism is better understood and future brittle fractures can be avoided by ductile design, satisfactory seismic performance of new underground structures can be expected.

Underground structures are vulnerable to flotation in the event of soil liquefaction. If built below the water table their tendency to float to the surface increases. Where liquefaction may occur, underground structures need strong anchorage to dense soil or rock unaffected by seismic shaking, or the ground around them improved by densification and drainage.

Horizontal propping action in floor slabs

Static and - seismic soil and water pressure

Basement slab

Basement wall

Static and - seismic soil and water pressure

Basement slab

▲ 7.11 A reinforced concrete basement wall propped against horizontal soil pressures by slabs, top and bottom.

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