DISTRIBUTION OF SHEAR STRESSES IN ISOLATED SQUARE CONCRETE FOUNDATION HAVING SPOTS OF VERY WEAK SOILS

Abstract

In the usual methodology for soil investigation, boreholes locations are selected in site, machinery extract, with due care, soil samples (or specimens) and tested, and then soil strength parameters for site are calculated. The soil report may contain some of the field tests, the SPT for instance, as well. The procedure is to pass this information to the structural engineer who, in turn, may design the structure. In soil investigation principle, the small soil specimen extracted from the soil strata have complete representation of the soil body underneath the footing. In soil mechanics theory the soil is well known to be nonhomogeneous and nonisotropic. This, eventually, means that soil properties and strength parameters change not due to location only but due to change of direction as well. This situation imposes the fact that the principle of representing the whole body of soil with a small sample is practically incorrect. The principle is assumed correct only as much as the soil is more and more isotropic and homogeneous. Therefore, it is in reality not uncommon for cavities to be present in soils body due to many reasons. If those cavities are detected before footing construction, then it is assumed that there is no serious problem. On the other hand, if it is not detected, the situation imposes a serious problem to the footing and structure depending on size and location of cavity.

     This study deals with cavity presence directly underneath a single separated footing of proposed multi-storey building. Location and size of cavity are changed and a finite element (FE) analysis is run for each individual case. The subgrade soil is assumed medium-dense sand with a modulus of subgrade reaction 35000 kN/cu.m. Other than cavity location, the subgrade is assumed to be homogeneous and isotropic, i.e. have same material and strength properties. Suitable graphs are used to illustrate the stresses in footing. The study, however, take into account the shear stresses in 3D only (a shear-care study) in concrete. The foundation concrete is assumed to have low compressive strength such as 20 MPa. Other properties of concrete are assumed program’s default.