Weak rocks in the foundations and adopted remedial measures at some river valley projects in North Gujarat, India

Aravalli Supergroup (lower Proterozoic) of rocks consisting mainly of quartzites, phyllites, schists, graywackes and dolomites are extensively developed in North Gujarat. These rocks are closely jointed, intricately folded, fractured, sheared and faulted due to intense tectonic activity and intruded by pegmatites and younger dolerite dykes. The rocks are often found deeply weathered, soft and weak. Kadana, Panam and Uatrak dams, respectively, across ñahi, Panam and Uatrak rivers on such weak rocks are selected to illustrate the variety of geotechnical problems encountered and the remedial measures adopted. The geotechnical problems could broadly be grouped under six heads :- (1) deep excavation to eliminate weathered rock and reach the foundation grade, (2) differential settlement in the foundation of gravity dam blocks located on alternating sequence of competent and incompetent rocks, (3) sliding and settlement of foundations traversed by clay lubricated, downstream dipping and low-angle faults, (4) excessive uplift pressures and seepage due to discontinuities in the foundation, (5) scour with prospect of retrogression in the tail channel, and (6) channel slope stability due to slumping and sliding. The details of the investigations to understand the significance of these unfavourable geological conditions are recorded to evaluate their ponderable and imponderable aspects. The remedial measures adopted separately or in combination to overcome the foregoing weak geological features are : dental treatment, provision of sizeable concrete olug in properly shaped excavations, underground shear keys tying the hanging and foot walls of the low angle faults, combining the stable gravity dam blocks with unstable ones, raft foundations for even distribution of loads, downstream cut-off walls or aprons and proper slope cuts with berms.