BEARING CAPACITY OF RIGID ANNULAR FOUNDATIONS UNDER VERTICAL LOADS

Abstract

Annular foundations are generally used for structures like smoke stacks, water towers etc. These foundations are preferred to circular ones in view of full utilization of soil capacity and no tension condition under the foundation. Information regarding contact stress distribution, bearing capacity and settlement is essential for a proper design of the foundation and is of funda mental importance to both Geotechnical and Structural engineers. Till date there is no systematic study available in the literature regarding the behaviour of annular foundations and particularly the contact pressure distribution. The present investigation thus aims to study the contact pressure distribution below the base of annular foundations on sand and to understand the behaviour of such foundations with respect to bearing capacity and settlement. In case of annular foundations on sand the important factors influencing the behaviour are (i) outer diameter of the foundation, (ii) ratio of internal to external diameter, (iii) rigidity of footing, (iv) foundation depth, (v) soil properties and (vi) load ing conditions. Tests were carried on rigid model footings with external diameter, 2R of 30 cm, 20 cm and 10 cm with five different ratios of internal to external radius, n of 0.0, 0.2, 0.4, 0.6 and 0.8. Footings were tested embedded at three different depth ratios of- 0, 1/3 and 2/3. Tests were conducted on locally available dry sand at three different relative densities of 75, 55 and 20 per cent, deposited in a tank using rainfall technique. In all 135 tests have been conducted out of which 108 tests were conducted on instrumented footings, fitted with specially designed monoblock IV stainless steel pressure cells for measurement of contact press ures. The rigid footings were subjected to central vertical load only through a suitable loading assembly. Under each increment of load, settlement of footing and contact pressures were noted. The tests were conducted upto failure and extent of rupture surface were also noted. The contact pressure diagrams for circular surface foot ings have been studied by Brandt (1968), Murzenko (1968) and Arora and Varadarajan (1984). The contact pressure diagram in general for circular surface footings on sand is reported to be parabolic. The present study confirms this pattern. However, it has been noted that the contact pressure diagram for circular surface foot ing (n = 0.0) is elliptical at lower load intensities and tends to be parabolic as the intensity of stress increases. Further, the contact pressures for annular footings are noted to have typical curvilinear shape with maximum at the inner edge of the annular footing. This trend is noted for all depths of footings tested. However, as the depth of footing increases, the point of maximum pressure tends to move away from the edge of the annuli. The same trend is noted for other densities. Also, it is noted that with increasing annuli diameter, the point of maximum contact pressure tends to move away from the edge of the annuli and for diameter ratios, n > 0.6 the diagrams are parabclic and symmetrical about the central section of the ring. These diagrams suggest that at n ^ 0.6 the annular footing behaves as a footing of width b = (R-r) with no interference and the contact pressure diagram of width b is analogous to that of a strip footing. A non-dimensional analysis of the data was also carried out for correlating various variables influencing the behaviour of annular foundations. Based on the non-dimensional analysis and the test data an equation has been formulated for obtaining the bearing capacity of annular footings on cohesionless soils. The bearing capacity in case of annular foundation is noted to have an optimum value at 0.2 <n <0.4. This is due to the inter ference of the footing and confinement of soil. As the value of 'n' increases this influence of interference starts vanishing and the footing behaves as an individual footing with width B = (R-r) and hence the bearing capacity decreases. The settlement pattern indicates a decrease in settlement as the value of 'n1 increases suggesting that a circular footing is likely to settle more for same loading intensity as compared to an annular footing. For annular footing clear failure surfaces were observed in the case of dense sand. At failure the shape of the rupture surface was noted to be circular in plan. The extent of rupture surface is more when the value of 'n' (r/R) is lower. Based on the study, it is concluded that (i) the contact pressure diagram for a surface circular footing on sand is para bolic with maximum occurring at the centre, (ii) the contact pressure diagram for a ring footing with small annuli is a curvi linear triangle with the maximum occurring towards the inner edge of annuli, (iii) the contact pressure diagram changes over to parabolic distribution again as the inner diameter of the ring foundation increases beyond n = 0.6 and is symmetrical about the central section of the ring, (iv) for shallow foundations, the depth of foundation has practically no effect on the shape of the contact VI pressure diagram, though with increasing depth there is a slight shift in the position of maximum pressure point away from the annuli, (v) the proposed equation based on non-dimensional analysis gives the bearing capacity of annular footings and takes into account the properties of the soil and characteristics of footing, (vi) the bearing capacity for annular footings is maximum for the ratio of internal to external radius between 0.2 to 0.4, (vii) the extent of rupture surface decreases with increase in 'n* value and decreases with decrease in relative density, (viii) the settlements under the same loading intensity is more in case of footing with smaller 'n' ratio.