DYNAMIC BEHAVIOUR OF EMBEDDED ^LT BLOCK FOUNDATION

Abstract

Safe and economic design of machine foundations is the back bone of any industry. Design and analysis of machine foundat ions thus has become an important topic of studies. Machine foundations are usually partly or wholly embedded into the ground. In this investigation, arational analytical analysis using mass-spring dashpot mathematical model has been developed for predicting the response of embedded block foundation. The vibrations in vertical, horizontal, rocking and coupled horizontal and rocking modes have been considered. The solutions have been obtained in the form of expressions giving the amplitude at operating frequency and the resonant frequency of the foundation-soil system The analysis brings out clearly the effect of embedment on amplitude, resonant frequency, elastic constants and damping factor. Experiments have been carried out in the field on concrete block of size 1.5m x0.75m x0.7m high. The block was instru mented with specially designed shear resistance cells and contact pressure cells, in addition to frequency and amplitude measurement devices. The tests were performed at two different sites one having uniform deposit of silty sand and the other clayey deposit. In case of tests on silty sand strata, 11 tests on embedded block with air gap were also conducted- The block was excited in vertical and coupled horizontal and rocking modes of vibration. Using the proposed analytical method of both vertical and coupled mode of vibration, the experimental data have been analyzed to study the influence of embedment depth on (i) ampli tude, (ii; resonant frequency, (ill] damping factor and (iv, stiffness coefficient. It has been observed that for a constant value of embedment ratio, D/B as the dynamic forc^ to weight ratio, F/VJ increases the damping factor and the stiff ness coefficient of the' soil decrease. For the same value of F/W, as the embedment ratio D/B increases the damping factor and stiffness coefficient increase. Further, in case of vertical vibration, studies regarding in-- phase soil mass,contact pressures and side soil resistance have al.^o been made. It has been noted that for the constant valee, of embedment ratio, D/B as the dynamic force to weight ratio, F/U increases, the in-phase soil mass increases. For the seme value of F/W, as the embedment ratio increases, the inphase soil mass increases. The static contact pressures at the edges are more than that ox in the centre. Also as the depth of embedment increases, the static pressures tend to become uniform. The dynamic pressure is observed to be maximum at the centre. The dynamic pressure increases with increase in frequency. The variation of elastic side shear resistance with embedment depth has also been observed. Non-dimensional correlations have been obtained to relate D /D , K /K ?nd mo /m with embedment Ill ratio D/B,where D , K and m are damping factor, stiffness coefficient and in-phase soil mass respectively for embedded foundation and D ,. K and m „ are corresponding values for zo} zo so to foundation resting on surface. Tests performed with air gap have indicated a similar trend in results compared to embedded block without air gap. An attempt has been made to compare the results of proposed theory with experimental data. A reasonable good tally has been observed.