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 Watrak dams, respectively, across Mahi, Panam and Watrak 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:-
deep excavation to eliminate weathered rock and reach the foundation grade,
differential settlement in the foundation of gravity dam blocks located on alternating sequence of competent and incompetent rocks,
sliding and settlement of foundations traversed by clay lubricated, downstream dipping and 10w-angt8 faults,
excessive uplift pressure. and seepage due to discontinuities in the foundation,
scour with prospect of retrogression in the tail channel, and
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 plug 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.
Kadana, Panam and Watrak dams across Mahi, Panam and Watrak rivers, respectively, are located in north Gujarat. This semiarid region covered with wind blown sands is underlain by Aravalli Supergroup (lower Proterozoic), comprising quartzites, phyllites, schists, graywackes, limestone, dolomites etc. The rocks are deformed due to tectonic activity and intruded by granites, pegmatites and younger basalts and dolerites. Routine geotechnical investigation - geological mapping, core drilling and permeability testing - revealed the ponderable features such as thickness of overburden, rock types, joints, shears, fractures, steep faults, depth of weathering etc.; but failed to detect, the size, shape and nature of low-angle clay lubricated faults of unique disposition and concealed locally decomposed beds or seams of rock. Besides, the stability of slopes in cuts and scour pattern in bed rocks were unpredictable. These imponderable weak features encountered in the foundation or tail channel excavations, their significance to engineering structures, special investigations to identify them, elaborate and expensive remedial measures and rescheduling the construction programmes ere briefly outlined to dissiminate the knowledge and experience.
A 66 m high and 2.5 km long masonry cum earth dam, completed in 1979, across Mahi river, is founded on intricately folded, alternating sequence of steeply dipping quartzites and schistose or quartzose phyllites travesed by several low and high-angle faulte, bedding sheers, closely spaced joints and fractures. The deeply weathered rocks are soft and weak.
