Thika Dam is a 70m high embankment dam with a clay core constructed from residual soil near Nairobi in Kenya. Drainage of the downstream side of the dam was required to maintain stability during the dam construction. Consequently, a chimney drain was located on the back of the core and horizontal blanket drains were located on the foundation rock and at 5m intervals in the fill. However, high pore water pressures (higher than those assumed in the design) were observed in piezometers installed near the base of the downstream and upstream shoulders when construction level reached 30m. This cast some doubt over the subsequent short-term stability of the dam.
Before construction was allowed to continue, GCG was commissioned to perform finite element analyses to establish the amount of drainage required to maintain overall stability of the embankment during construction and to restrict the deformation to acceptable levels. Finite element analyses highlighted zones of high shear stress and shear strain and the potential formation of failure surfaces. The analyses also established the necessary level of drainage required in the constructed material to allow safe construction of the dam. Construction was resumed after the installation of additional vertical drains in the constructed material designed to work in combination with the original horizontal drainage blankets in the downstream shoulder fill.
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