Analysis of two layer soil system beneath rigid footings - a global approach

The bearing capacity and settlement behaviour of a rigid foundation on a weak soil can be improved by providing a reinforced soil bed or a geocell mattress. Mechanisitically this will result in a two-layer soil system below a rigid footing. The upper reinforced layer will have a relatively high value of modulus of elasticity. Often the ratio of the moduli of the upper and lower layer being more than 100. There is no simple method available in literature (excluding numerical methods) to estimate the load settlement behaviour and ultimate load of such a system. The contact pressure distribution below rigid footings placed on the surface of a two layer granular soil will depend on the elastic and plastic zones formed, the thickness of the upper layer, the load level in relation to the ultimate load of the system and the ratio of elastic moduli of the soils of the two layers. Solution for this type of problem can be obtained without using the complex constitutive modelling approach. The adopted method in this investigation has been termed as 'Global Approach'. This is more suitable for reinforced earth where modelling is complex. In this investigation, the basics of the global approach has been presented. The numerical results obtained from this approach have been presented in non-dimensional form, which can be used to estimate the ultimate bearing capacity and load displacement behaviour of a two layer system below rigid footing. The results have been validated in relation to the published experimental data.