ULTIMATE BEARING CAPACITY OF RING FOOTINGS ON SAND

Abstract

Ring foundations are preferred to circular rafts for tall circular structures like water towers, chimneys, silos etc. due to their proven better capability of with-standing large horizontal forces which these structures are subjected to because of their exposure to the wind. In absence of any theoretical or empirical formula on the subject, an approximate method based on Terzaghi's formula for strip footing is being used presently to determine the bearing capacity of soils under such footings which renders the design uncertain. The present model study has been undertaken to investi-gate the load deformation characteristics, of ring footings on the surface of sand. Three dimensional tests have been carried out on 15 different model footings made of aluminium,5 of them being of external diameter 5 cms, 5 of 10 cms and 5 of 15 cms. Internal diameters of the footings of these three categories were kept in such a way that the ratios of the internal to the external diameter are 0.0, 0.2, 0.4, 0.6 and 0.8. Tests were conducted on poorly graded air dried fine Ranipur sand in 5 states of packing having relative densities of 74V., 65'4 , 43'! and 317! . Loads were transferred to the footings through 4 columns which were placed underneath a loading plate where central vertical load was applied by a Screwjack and a calibrated proving ring of 250 Kg capacity. -XV- On the basis of ultimate loads obtained from load settlement curves, dimensional analysis was carried out to get non-dimensional parameters for the different variables involved. By interpreting the results, an empirical formula for the ultimate bearing capacity of ring footing on sand surface has been developed. Extents of failure surface for various internal and external diameters of the footing and for various relative densities of the sand have been studied