The foundation of the Ust-Ilim Hydro Power Plant structures includes a thick trappean intrusion underlaid by sedimentary rock. The trappean intrusion is composed of granophyre, ophite and troctolite diabases characterized by the columnar structure with three systems of original fissures. Rock masses adjacent to contact surfaces are of block or slab structure. Rock in these areas as well as in those near to the ground surface are subjected to more intensive weathering and· disintegration. It is characterized by heavy fracturing at which diabase turns into fragments and gruss. The weathering has manifested in the formation of heavily ferruginated layers (of thickness ranged from 0.5 to 5 cm) of reduced strength along joint faces.
The sedimentary rock in the dam foundation is varied. A skerry-lens in the fluvial zone and a trappean intrusion underlayer at the right bank are composed of sandstones, the intrusion at the left bank superimposes massive siltstones with sandstone interlayers.
The concrete gravity dam is located directly on diabase while the earth-fill dam - on the sedimentary rock. Downstream from the dam a tunnel is drawn in the sedimentary rock to protect a township from the dangerous rise of the ground water table.
The foundation of the Ust-Ilim Wood- Industrial Project is also characterized by complex ground conditions. On the site the loose soil layer 3 to 5 m thick composed of deluvium clay, alluvial and sandy loam is underlaid by weak sedimentary rock including sandstone, siltstone, argillite and coal. This rock is fairly sound when undisturbed but core samples drilled out from the boreholes soften and disintegrate intensively on the ground surface.
Field tests scope for the scientific substantiation of designs of the Ust-Ilim Power Plant and Wood-Industrial Project V\6.S determined by the geological engineering conditions and by structure types.
Presented herein are some results concerning the shear strength, deformability and pressure tested in weak sedimentary rock. These test results were used for estimating settlements of heavy industrial building basements, the thickness of tunnel lining as well as for predicting dam displacements.
Diabase is characterized by high values of physico-mechanical properties, e.i.the uniaxial compression resistance Rcom. =(1667 ± 67) × 10 5Pa and the volume wight under natural humdity conditions γ0 = 2.92 × 104 N/m3.
For sedimentary rock underlaying the trappean intrusion the mean values of dry volume weight and compressive strengtr are 1.9 × 104 N/m3 and 230 × 10 5 Pa, respectively.
The volume weight and compressive strength of saturated rock above the trappean intrusion are evaluated as 2.0 × 104 N/m3 and 190 × 105 Pa, respectively.
The shear strength of the main types of sedimentary rock occurring along the, tunnel was studied in laboratory on core samples of diameters ranged from 70 to 130 mmIn all 336 tests were carried out on 88 core samples' taken from depths of 50 to 80 m.
The rock tested on samples were extremely varied from dark yellow to green-grey fine-grained sandstones interbedded by argillite layers and blue- green rounds.
