STABILITY OF OIL TANK FOUNDATIONS

Abstract

Oil storage tanks form an important component of petroleum refineries and chemical plants. An oil tank mainly consists of a bottom plate, shell and a roof which may be of the fixed or floating type. The loading pattern in an oil tank is quite different compared to normal structure. It is essentially a case/of circularly loaded area. In the case of oil tanks, however, almost all of the tank loading is brought to bear on the soil suddenly, after the completion of the structure. One of the basic features of the flexible oil storage tank is its ability to sustain a large magnitude of uniform settlement. However, it is the differential settlement which causes problems because its occurrence may lead to a rupture of the tank base. The shear criterion has two main components, namely, 'base shear' and 'edge shear' , in the case of oil tanks, the 'edge shear' criterion is more critical then the 'base shear'. The current exercise is an attempt to compute the settlement of oil tank foundations resting on sand/and or clay soils and computation of factor of safety value for 'edge shear' failure conditions. The program developed has been used to compute settlements at the tank base and to examine the effect of replacing a certain thickness at the top, of the clay layer by a dense, granular layer. The analysis carried out shows that (i) I,t is possible to work out the pattern of settlement at the base of the oil tank foundation using an interactive- computer analysis. (i ) (ii) Replacement of a 2m thick clay layer at the top by a compacted/granular layer resulted in a reduction of settlement by 22.6% at the centre and 24.6% at the tank edge, while the reduction was 37.8% at the centre of the tank and 39.6 % at the tank edge for a 4m replacement. (iii) The factor of safety against `edge shear' failure for a certain case was found to be 3.2.