Kamburu Hydroelectric Scheme is situated on the Tana River some 160 km NE of Nairobi. The scheme comprises a 90 MW power station using water impounded behind a 35 m high asphaltic—concrete faced rockfill dam. The spillway structure is set in a 30 m deep and 50 m wide rock cut channel and comprises a massive concrete base block forming the overflow crest. This overflow crest supports four massive piers between which are three 13 m square radial gates. Pier 1 is on the left.
The photograph on the facing page is taken from the right bank of the spillway cut, some 150 m downstream of the spillway axis. The outer piers of the spillway were cast directly against the cut rock surface and abutment structures were provided, principally to store stoplogs. The as blasted rock surface of the left abutment against which the left hand pier was cast is shown on (Fig. 1).
In 1982, some 8 years after construction, relative movement was observed between the left stoplog store and pier 1; it appeared as if the store were
settling at its outer end, opening the joint between it and the pier. The movement was monitored for 3 years until in 1985 further signs developed, the most noticeable of which were numerous cracks in pier 1 concrete, and interference between gate 1 and its stainless steel guides. A comprehensive survey of the piers confirmed that pier 1 had moved some 35 mm into the gate opening, while the other