Estimating the allowable bearing pressure of soils from small-scale plate load and direct shear tests: laboratory-prepared mixtures and natural subgrade soils

Abstract The plate load test (PLT) is the most direct way of obtaining the allowable bearing pressure of foundation and subgrade soils, but full-scale testing is costly and slow. This study investigates whether a small-scale plate load test (SPLT) on remoulded soil, combined with direct shear testing (DST), can provide a reliable and conservative estimate of allowable bearing pressure. Six laboratory-prepared mixtures (sand–kaolin and sand–kaolin–aggregate) and five natural soils from sites in Pune, India, were tested. Soil was compacted in steel containers at maximum dry density or at field density, and plates of 0.10 m × 0.27 m and 0.20 m × 0.30 m were loaded using a large-box direct shear apparatus. The shear criterion was evaluated from DST parameters and the settlement criterion from the SPLT with Terzaghi–Peck scaling; the smaller value governed. Natural-soil results were compared with standard penetration test (SPT) values and one field PLT. The settlement criterion governed in five of six soils; Terzaghi net safe capacities from DST ranged from 451 to 1024 kN/m², whereas SPLT safe bearing pressures ranged from 200 to 850 kN/m². For the natural soils, the 0.20 m plate gave allowable pressures equal to 88–95% of SPT-based values, and the 0.10 m plate 62–82%. At a soft clayey-silt site, SPLT values of 29–36 kN/m² bracketed the field PLT value of 33 kN/m². Combining SPLT (settlement) with DST (shear) gives conservative estimates of allowable bearing pressure where standard field equipment is unavailable, with the larger plate being less conservative.