A storage tank with a diameter of 30m (self-weight = 2000 tons) is to be constru

Question

A storage tank with a diameter of 30m (self-weight = 2000 tons) is to be constructed on a normally consolidated soft clay with a thickness of 7.5m underlain by a dense sand layer. This soft clay has an undrained shear strength of 25 kPa, effective friction angle of 24, saturated unit weight of 17.5 kNm3, and coefficient of consolidation and permeability equal to 2.7 × 10^8 m2s and 6.0 × 10^10 ms. The groundwater table is near the ground surface. To minimize the service settlement, the tank is filledwithwater up to 6m in one day to preload the soft soil for 6months.Prefabricated vertical drains (PVDs) are proposed to accelerate the consolidation. The PVDs to be selected have the cross-sectional dimensions of 100 and 5mm and penetrate the soft soil into the sand layer. The allowable discharge capacity of the drain is 0.00081 m3/s under 50 kPa normal stress and 0.00014 m3/s under 150 kPa. Assume there is no smear effect during the installation of PVDs. Determine the factor of safety against bearing failure at the end of water filling and the required spacing of PVDs (in a square pattern) to ensure 80% consolidation by the end of the 6-month preloading.

Explanation / Answer

factor of safety against bearing failure = (6.0 × 1010 )/(2.7 × 108 ) * (3.14 * 15 * 15)/50

                                                             =   0.314

required spacing of PVDs = 25000 * 50000 * 0.00081/17500

                                          =   57.85 m

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