A methodology for the determination of bearing capacity of granular soils with plate loading tests

In granular soils, due to the inability to collect undisturbed samples of soil for geotechnical laboratory tests or the impracticality of carrying out dynamic penetration tests, highlights the necessity to find a useful approach in the engineering geological practice for the determination of bearing capacity. In these soils the determination of bearing capacity can be evaluated with several formula that necessary require the estimation of the friction angle: this leads to overestimating or underestimating its real value and, consequently, non-conservative soil bearing capacity values can be obtained. An alternative method in support of bearing capacity determination, based on plate loading tests is here proposed. The methodology here proposed can be applied in isotropic and homogeneous medium soil foundation. To validate the method, we report on the results of over fifty plate loading tests performed in support of the construction of industrial buildings, in two different locations, Venaria Reale and Borgaro Torinese (NW Piedmont), where an alluvial cover (thickness of 20-40 m) consisting of gravel and sandy texture and corresponding to A-1-a(0) AASTHO Soil Classification, linked to Lanzo Fan, occurs. By the plotting, in an arithmetic scale, load-settlement curves, different curves can be obtained: in loose to medium cohesionless soil, curve does not show a clear point of failure and they are generally linear; in cohesive or dense cohesionless soils (dense sand or gravel) curve shows a clear point of failure, identified by the sudden drop down of it. This point divides the rheological behavior of soil in two parts: an elastic zone for loads and settlements lower than the failure point and a plastic zone for loads and settlements greater than the failure point. In this case, the failure point correspond to the ultimate bearing capacity of the soil (q ult ). If the curve doesn’t show a clear failure  point, a new curve can be obtained using a log scale (x-axis) and an arithmetic scale (y-axis). In this case the failure point is given by the intersections of the two tangent straight lines to this curve. The load corresponding to this point is the q ult . Finally, safe bearing capacity may be determined by dividing q ult by a suitable factor of safety. In conclusion, load-settlement curves can be utilized to determine the yield pressure and recommended allowable pressure for foundation design.