STUDY OF ELASTIC CO-EFFICIENTS OF SOILS FOR DESIGNING MACHINE FOUNDATIONS

Abstract

Machine foundations are subjected to vibrations due to the unbalanced forces produced by machines. The foundations in turn transmit these vibrations to the soil underlying it. For rationally designing the machine foundations, it is important to know the natural frequency of the machine-foundation-soil system. Out of the various methods of designing the foundations for slow speed machines, B a.rkants method has the advantage of being simple in application . The method requires the determination of the coefficient of elastic uniform compression, Cu, of soil. Theoretical evaluation of the coefficient of elas-tic uniform compression needs the evaluation of elastic properties of soil viz E and - 11. However, the E and i., values of soils are not well defined and unique properties of soils, as the soils do not obey Hookers law in the true sense. It therefore appears more rational to use field investigations for the determination of the coefficient of elastic uniform compression of soils. The two methods for determining the value of Cu In the field are dynamic model tests and cyclic plate load. tests. The dynamic model tests represent the field loading conditions to a great extent but require special instruments. The data interpretation technique is based on a number of _ 2 simplifying assumptions and empiricpl dorrelations whiLh are not strictly valid. On. the other hand, cyclic plate load tests need only conventional equipment and do not require great technical skill. In the present investigation, cyclic plate load test technique on rigid plates has been adopted to eva- luate the value of Cu. Th-} soil exploration of the site indieat~3d silty sand upto a depth of 3.64 metres below ground level. Tests have been conducted on square, rectangular and circular plates. Four areas viz. 1000 cm2 , 2500 cm2 4000 cm2 and 5500 cm2 of each shape of the plate have been tested to determine the effect of test area of the plate and its shape. on the value of Cu. The sides of the rectangular test plates were in the ratio of 2 % 1 (Length : Width) . Tests have been carried out at 75 ems below th- ground level. The load was applied by mechanical jack and measured on a pre—calibrated Aimil proving ring. The reaction in each case was taken by reaction truss. Value of Cu in each case is determined. Shape effect has been studied. The validity of the empirical relationship between Cu and bearing capacity of soils as given by Barkan has been checked for this soil. 3 _, Limitations of the testing programme and sugges-tio_ns for further research have also been out-lined.