Influence of ground-water conditions on the bearing capacity of sand

Abstract

This study investigates ground-water effects on the bearing capacity of sand. Laboratory model tests were conducted to observe the influence of water table on the bearing capacity, settlement and rotation of surface footings. Rough based rigid strip, square and circular footings were used. The least dimension (width or diameter) of all the footings used, was 100 mm. Sand samples were collected from four different locations of Bang 1adesh. A mechani ca 1 sand spreader was used to prepare a un iform dense bed of air-dry sand in a transparent glass-sided model tank. The ratio of sand bed depth (H) to the footing width (B) was retained within 3.0 to 3.5. Relative density (Dr) of sand in the beds varied between 73% to 87%, and the angle of internal friction from triaxial tests (~t) was in the range of 34° to 39°. Strain controlled vertical load was applied on the footings, using an electric motor operated loading ram. Load, settlement and rotation measurements were taken using load and strain dial gauges, A syphon system was improvised to supply water into the model tank for creating desired submergence conditions. Influence of depth of water table, together with the effects of friction angle of sand and footing shape, on the bearing capacity were stud ied. The resu lts are present ed, analysed and compared wit h the predictions of existing methods. Test results reveals that on fully submerged (Dw = 0) sand bed, ultimate bearing capacity of surface footing is reduced by nearly 50% to that of a dry sand bed, and the footing settlement approximately doubles. Influence of submergence on bearing capacity and settlement gradually diminishes with the increasing depth of water table; when the water table is at about twice the footing width (Dw = 2B) below bed surface, its effect almost ceases. The outcome of the test results are in general agreement with the predictions of Terzaghi (1925), Meyerhof (1955), Taylor (1966) and D'Appolonia (1968). Experimental results on dry sand bed shows that the predicted values of general bearing capacity factor (N,), using methods of Terzaghi (1943), Meyerhof (1963), Hansen (1970) and Vesic (1973) are much smaller. Corresponding ultimate bearing capacity is found to vary in a range of 18% to 31% of the experimental results. However, the predicted values of Feda (1961), and Abdul Baki and Beik (1970), in respect of N, appears to have better conformity and lies in a range of 31% to 53% of the observed data. It is further noted that, bearing capacity factor increases with the increase of friction angle (~t) of sand and the rate of increase is notably higher for ~t > 36°. Recorded settlements at peak load are 1.6 to 1.85 times higher than the predicted Terzaghi (1943) value. Footin9 rotation is limited within 1.4°, but the sense of rotation was unpredictable. Statistical analysis of test results furnishes correlation of friction angle (~t) of sand samples with: (1) initial porosity of sand (no), and (2) bearing capacity factor (N,). vi excellent Therefore, these relations are proposed to estimate the friction angle of local sands as well as the bearing capacity factor for strip footings placed on Bangladeshi sands. Also, an attempt was made to study the effects of footing shape on bearing capacity. Experimental shape factors (s,) for both square and circular footings are seen to be larger than the values suggested by Terzaghi (1943), Meyerhof (1951) and Vesic (1970). Test data displays that, s, deceases as friction angle increases and shape factor for square footing is higher than that of circular footing. Predominantly general shear failure mode is observed in the bearing capacity tests on both dry and submerged sand beds. At dry condition, failure outcrop arises generally on all sides of the footing at a distance of approximately 2B to 3B for strip footing and B to 2B for square and circular footings. At full submergence state, failure outcrop was not visible on the bed surface, whereas for partially sUbmerged bed failure outcrop grossly resembled that of the dry sand bed.