The main Beris Dam is founded on a sequence of thick bedded conglomerates and pebbly to fine grained sandstones with minor mudstone mapped as the Semanggol Formation of Triassic age. Ultrasonic pulse measurements show velocities of compressional and shear waves through the sandstones to increase with decreasing grain size; the pebbly sandstone with velocities of 2.210, and 5.171, km/s, and the coarse grained sandstone with velocities of 2.477, and 5.612, km/s, respectively. The medium grained

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... ndstones have compressional and shear wave velocities of 2.457, and 5.793, km/s and the fine grained sandstones, velocities of 2.572, and 5.867 km/s, respectively. Dynamic elastic constants computed from the ultrasonic velocities also increase in values with decreasing grain size; Poisson's ratio varying from 0.36 to 0.39, the modulus of elasticity from 35.076 to 48.210 GPa, the bulk modulus from 52.260 to 67.362 GPa and the modulus of rigidity from 12.637 to 17.468 GPa. Increasing velocities and elastic constants with decreasing grain size are considered to result from a denser arrangement of constituent grains as shown by increasing dry unit weights. Comparison with the results of an unconfined compression test on a fine grained sandstone indicate that the ultrasonic elastic constants are good approximations of static elastic constants.