Interpretation of a footing load test on a saprolitic soil from granite

Abstract

In the north-western region of Portugal residual soils from granite are dominant. Their porous cemented structure leads to a complex geotechnical behaviour, quite distinct from that of transported soils with a similar grain size distribution. The paper describes some of the studies carried out at an experimental site on which a fairly homogeneous saprolitic soil 6 m thick was encountered. These studies included: (i) a full-scale load test of a circular concrete footing (1.20 m diameter), under carefully controlled conditions and with extensive monitoring; (ii) in situ testing techniques (SPT, CPT, SBPT, CH) and (iii) laboratory testing on high-quality block samples, including triaxial tests with local strain measurement. The discussion is concentrated on the interpretation of the pressure-settlement curve from the footing load test, for a range of pressures typical of working conditions, by means of linear and non-linear elastic analyses based on stiffnesses from the laboratory and in situ tests. Some practical conclusions are extracted regarding the use of in situ and laboratory test results in analytical and empirical or semi-empirical methods for settlement estimation on these residual soils.