Estimating engineering characteristics of gravelly soils

The Ghazi-Barotha Hydropower Project with a generation capacity of about 1450 MW is being constructed on river lndus in the northwestern part of Pakistan. The powerhouse for the project will be founded at about 70 m depth below the existing ground level, on thick deposits of variably cemented sandy gravels. This material is generally dense and is found in conglomerate form, when well cemented. The engineering characteristics of these gravel deposits have been evaluated for the design of foundations of the power complex and as well as for the design of temporary and permanent cut slopes. At the location of the power complex, the gravel deposits exist at a depth of about 48 m below the existing ground level. A crosshole seismic survey was carried out at the proposed location of the powerhouse. Due to deeper depth of the of the sandy gravel stratum, it was not possible to carry out some of the desired in-situ tests at the location of the powerhouse. However, these deposits are exposed on the left bank of river Indus, at about 1.5 km from the powerhouse location. A number of in-situ tests were carried out on this exposure for evaluation of engineering characteristics of the sandy gravel deposit. The in-situ tests included field density tests, Becker penetration tests, plate load test, and direct shear tests. In order to carry out slope stability analyses of the cut slopes, and estimation of allowable bearing pressures for the foundations of the power complex, various engineering parameters including modulus of subgrade reaction, angle of internal friction, density, equivalent standard penetration resistance and shear wave velocity were deduced from these in-situ tests. All the in-situ tests are described in this paper along with a discussion on engineering characteristics of the tested sandy gravel material.