BEHAVIOUR OF RIGID RING FOOTINGS ON SAND UNDER VERTICAL AND LATERAL LOADS

Abstract

Ring foundations are commonly used for structures like cooling towers, water towers, smoke stacks etc. Besides being economical, ring foundation is often the only solution when dual condition of full utilization of soil capacity and no tension condition under the foundation is to be satisfied. Tall circular structures, for which this type of foundation is generally preferred, are subjected to large horizontal forces because of their exposure to wind or due to earthquake. Therefore, in addition to vertical loads, these foundations are subjected to large moments also The present investigation has been carried out to study the behaviour of rigid ring footings under constant vertical and progressively increasing lateral loads on dry dense sand deposit. Three dimensional tests have been carried out on rigid model footings with external diameter of 20 cm and ratio of internal diameter to external diameter of 0.0, 0.2, 0.4, 0.6 and 0.8. The values of constant vertical load have been kept as 5 percent, 20 percent, 40 percent, 80 percent and 100 percent of the ultimate vertical load. Ratio of height of lateral load application to external diameter of footing in each case has been kept as 0.0, 0.3- and 0.6. Tests were conducted on poorly graded dry Ranipur sand at a dense state of packing (DR = 78 percent). In all sixty five tests have been conducted, out of which five tests were conducted under vertical load only and sixty tests were conducted under constant vertical and progressively increasing lateral load. The test data has been presented in the form of various plots. From the analysis of the test data it has been conclu . ded that (i) under central vertical load, the bearing capacity of ring footings on sand is maximum for the annuli ration, of about 0.4 and the settlements under the same loading intensity are more in case of footings with smaller annuli ratio, (ii) under constant vertical and progressively increasing lateral load, the lateral load capacity of ring footings on sand decreases with increase in annuli ratio 'n' and h/B ratio (ratio of height of lateral load application to external diameter of footing), (iii) for all.values of twand h/B ratio, the lateral load capacity of ring footings increases with increase in constant vertical load and is maximum at about 80 percent of the ultimate vertical load, (iv) for same m4gnitude of lateral load, the lateral displacement and tilt increase with increase in h/B ratio and decrease with increase in constant vertical load,