A multi-storey building was planned for a site where unusual subsoil conditions were encountered. The architect indicated that only very small differential settlements could be tolerated for the proposed structure. This requirement could have been satisfied by founding the structure on bedrock but the only expedient and economical scheme consisted of placing strip footings directly on the surface and ensuring that excessive differential settlements would not occur.
It is the purpose of this paper to describe the analyses that were made to predict the magnitude of both the total settlement and the differential settlement for strip footings placed on the surface; these analyses considered soil-structure interaction.
The building site is located in the Middle East, and as illustrated in Figure 1, a layer of jointed rock overlies a layer of stiff to very stiff marl of varying thickness. The presence of the surficial rock layer is attributed to a slump mechanism wherein the rock has slipped into its present position thereby confining the marl layer (Harrison and Falcon, 1936).
The surface rock layer, after: site levelling, consists of limestone which is about 1.5 m thick and is jointed along essentially vertical planes at horizontal spacings of about 0.10 to 0.50 m with an average spacing of 0.25 m. The joints have an average width of 0.01 m and are filled with stiff marl. The upper rock stratum is underlain by stiff to very stiff marl having a thickness varying from about 0.5 to 10.5 m within the foundation area. Bedrock is found below the marl and dips about 10 degrees.
(Figure in full paper)
For purposes of the analysis, only a few soil properties were required; these were the shear modulus, G, of the marl within the joints of the upper rock layer which was estimated to be 8000 kPa, and the coefficient of volume compressibility, mv' of the marl layer which was estimated to be 6×10−5 m2/kN.
Layered elastic systems have been studied by many investigators, and the findings have been summarized by Poulos and Davis (1974). In general, it is shown that the change in stress, 6q, throughout a layered system is a function of the relative values of the modulus of elasticity of each layer.
The vertical displacement or settlement may be estimated from the expression:
(Equation in full paper)
where mv is the coefficient of volume compressihility (=l/F),and ∆q is the change in vertical stress with denth.
Experimental studies by Sowers and Vesic (1962) have indicated that there are limitations to the applicability of the theoretical solutions. A layered system consisting of an asphalt pavement, a base course and a sandy silt subgrade was studied. The prediction of the vertical stress in the base and subgrade was found to agree better when Boussinesq's theory was used rather than a theory for layered systems. Similar conclusions were given by McMahon and Yoder (1960).
