PRESSURE SETTLEMENT CHARACTERISTICS OF SURFACE FOOTINGS FROM CONSTITUTIVE LAWS

Abstract

Bearing capacity and settlement are two main criteria for designing a footing foundation. Several theories and experimental methods have been propounded for computation of the above parameters separately. However, the best estimation of bearing capacity and settlement is possible only if the pressure settlement characteristics of the found at ion-soil are known. Pressure-settlement relationship is essentially a function of non-linear stress-strain behaviour of soil. Constitutive laws defining such a behaviour of soil have been used in this investigation to predict the relation ships. The investigation has been carried out for two types of soils, (i) clay (0 • 0) and (ii) sand (c = 0). A rational contact pressure distribution has been assumed in each case. The soil mass below the footing has been deivided into a large number of thin strips and stresses and strains in each strip have been consi dered as uniform at any vertical section. To obtain settlement at a.particular load, the strains have been multiplied by thickness of the strip and then numerical integration from surface to a depth upto which the strains become negligible has been carried out. viii. The pressure- settlement characteristics of flexible and rigid, strip and square footingswith smooth and rough bases and loaded uniformly and eccentrically have been analysed In case of clay. In case of sand, flexible strip footing has only been analysed. The analytical solutions have been verified by comparing the predicted load settle ments with available data on sand (Saran, 1969) and Clay (Carrol, 1963 and Saran, et. al 197 6). The triaxial tests data on sand was collected by the author to obtain the constitutive relationship. It has been found that the predicted bearing capacity is within 5f on the conservative side of the experimental value. The bearing capacity computed by Terzaghi's theory gives a value 17.25^ on higher side 7 compared to the experimental value. A non-dimensional correlation has been oooained between q/c and contact pressure distribution coefficient ou; 1 being the surface pressure intensity and cu is the ultimate compressive strength in triaxial test. The curve of q/c versus cm has been found to be a unique curve for all footing sizes. The S~/S versus BJB relationship hd,S been found to be true as given by Taylor (1962), where S„ and S are the settlements of footing of widths B „ and B respectively. The settlement at failure load has been found to be approximately 6% of the footing ax. base-width in case of s^rip footing and 12.5$ in case of square footing. The bearing capacity of rough strip footing on clay has been found to be 5$ more compared to the bearing capacity of smooth strip footing. A non-dimensional correlation has been established between settlement of the centrally loaded footing(So) and settlements of the accentrically loaded footing at its edge(S ) and below the load(SQ) at equal factor of ° m e safety of the two footings. It has been found that there are unique relationships between Se/SQ vs e/B and Sm/So vs e/B at arbitrarily defined factors of safety of 1,2 and 3. The bearing capacity of eccentrically loaded footing q^ has been correlated with e/B and has been found to be equal to *ue " V'1 - 1-6(e/B)) The angular tilt has also been determined theore tically. The angular tilt vs q/cu has been found to be independent of footing base width but vary with e/B ratio. The values have been found to tally well with experimental values of Prakash(1975). In case of sands, it has been found that the predicted and the experimental curves tally well. The settlement at failure and the bearing capacity have been found to vary with the width of the footing.