INTERFERENCE EFFECT ON THE BEHAVIOUR OF FOOTINGS

Abstract

In practice footings are rarely Isolated and they do interfere with each other depending on their relative positions. Such types of problems are encountered at many places in CIVILENGINEERING constructions e.g. foundations of buildings in built-up areas, runway strips, railway crossties covered colonnade adjacent to a multistorey building, a covered loading area adjacent to an industrial building and grillage foundations. Information regarding bearing capacity, settlement, tilt and contact stress distribution of interfering footings are required for their adequate design. The investigation deals with the behaviour of interfering footings covering almost all practical cases namely: (i) Two strip footings of same widths loaded simultaneously (case-I). (li) Two strip footings of same widths, one of the footings representing the existing foundation loaded with one-third of the failure load of isolated footing and the adjacent footing loaded upto failure (case-II). (iil) Two strip footings of same widths, one of the footings representing the existing foundation loaded with half of the failure load of isolated footing and the adjacent footing loaded upto failure (case-Ill), (iv) Two strip footings of different widths loaded simultaneously (case-IV). (v) Two strip footings of different widths, the wider footing representing the existing foundation loaded with half of the failure load of isolated footing and the adjacent smaller footing loaded upto failure (case-V). (vi) Three strip footings of same widths loaded simultaneously (case-VI). (vii) Three strip footings of same widths, central footing representing the existing foundation loaded with half of the failure load of isolated footing and the outer footings loaded upto failure (case-VII). (viii) Three strip footings of same widths, outer footings representing the existing foundations loaded with half of the failure load of isolated footing and the central footing loaded upto failure (case-VIII). Review of literature indicated that many theories are available to find ultimate bearing capacity of two and three interfering strip footings loaded simultaneously with equal loads (Stuart, 1962; Mandel, 1963; West and Stuart, 1965; Amir, 1967; Siva Reddy and Mogaliah,1976; Khadilkar and Varma, 1977;Patankar and Khadilkar, 1981; Graham, et al. 1984; Pathak and Dewaiker,1985). Dash (1981,1982) investigated the problem of determining ultimate bearing capacity of two strip footings when one of the footings carrying certain load located nearby. No method has been reported to find settlement and tilt of interfering footings except the finite element method and the finite difference technique (Khadilkar and Varma,1977; Patankar and Khadilkar, 1981; Pathak and Dewaiker, 1985). Many investigators have experimentally studied the problem of two and three interfering footings loaded simultaneously (Stuart,1962; West and Stuart,1965; Myslivec and Kysela,1973; Singh et al.,1973; Saran and Agarwal, 1974; Deshmukh, 1979; Dembicki et al., 1981 Das and Labri cherif,1983; Selvadurai and Rabbaa,1983; Graham et al.,1984)- The effect of footing carrying certain load on the behaviour ii of an adjacent footing has also been studied experimentally by a few investigators (Murthy S.S.N,1970; and Dash, 1990). So need was felt to evolve a method to find ultimate bearing capacity, settlement and tilt of interfering footings covering many of the possible situations. Pressure-settlement and pressure-tilt characteristics are essentially the 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 relationships. The investigation has been carried out for the two types of soils, (1) clay [<f> = 0) and (li) sand (c = 0). The procedure of analysing all the eight cases mentioned above is same. Uniform contact pressure distribution and appropriate tangential stress have been considered below each footing. The soil mass below footings have been divided into large number of thin strips. Vertical, horizontal and shear stresses have been obtained along various vertical sections by superimposing the stresses of interfering footings. Principal stresses and their directions were obtained using constitutive relations and principal strains were computed. Sum of the components of these strains in vertical direction is then obtained. To evaluate the settlement at a particular load, the strains have been multiplied by the thickness of the strip and then numerically integrated from the surface to a significant depth. The determination of settlement of various points of interfering footings enable the computation of tilt. In this way the pressure-settlement and pressure-tl1t characteristics of interfering footings were obtained. The analysis was carried out for footings resting on Dhanori clay, Buckshot clay and Amanatgarh sand. Non-linear constitutive laws of these soils in the form of Kondner's hyperbola were adopted for the computations. iii The results of all the eight cases have been analysed carefully. It has been found that ultimate bearing capacity, settlement and tilt were functions of width of footings, spacing between them and the way in which these were loaded. Non-dimensional correlations to compute maximum and minimum settlement of the Interfering footings have been developed for it's use in the design of interfering footings. Further it has been concluded that tilt is sensitive to these factors and should be considered in the design. To verify the analytical solutions, model tests were conducted on strip footings of different sizes (i) 10cm x 86cm and (ii) 15cm x 86cm in a tank of size 220cm x 87cm x 120cm. The tests were conducted on Amanatgarh sand at relative density of 70% . Footings were tested at six clear spacing/width ratios (S/B = 0.25, 0.5, 0.75, 1.0, 2.0 and 3.0). The pressure-settlement and pressure-tilt characteristics of interfering footings have been obtained for each case with a particular spacing. In addition to this, contact stress distribution using pressure cells and extent of failure surface were also determined. Comparison of analytically and experimentally obtained pressure-settlement and pressure-tilt curves show reasonably good agreement. Using analytical results, non-dimensional correlations have been developed to predict the values of bearing capacity, settlement, and tilt. Suitable examples have been incorporated demonstrating the use of the methodology and the non-dimensional relationships in design. An approach for prediction of pressure-settlement and pressure-tilt characteristics of interfering footings using non-linear constitutive laws of soils, facilitates in providing complete parameters required for the design of the foundation. It is expected that the findings of this study will lead to more economical and safe design of geotechnical structures in built-up areas. iv