GEOTECHNICAL CHARACTERIZATION THROUGH PRESSUREMETER AND LABORATORY TESTING FOR ALLUVIAL SOILS

Abstract

Geotechnical characterization of soils for its use in any project is conducted through a programme that comprises in-situ and laboratory tests. The main in-situ tests include pressuremeters (PMT), Dilatometer (DMT), Standard Penetration Test (SPT), Cone Penetrometer (CPT). Among these, prebored pressuremeter tests are performed in pre- drilled boreholes. The drilling methods such as hand augering and rotary drilling rig are recommended by the ASTM D-4719 for the prebored pressuremeter testing. The vertical and constant diameter boreholes are the basic requirement for the prebored pressuremeter testing to obtain quality tests curves. The verticality and constant diameter for the boreholes are difficult to be achieved by these two methods as the hand auger has no control on the vertical movement and rotary rig induces vibrations to the walls of the borehole during rotation of the bit. A cost effective mechanical drilling system (MDS) has been developed locally for the drilling of vertical and constant diameter shallow boreholes to about 10 m depth. The prebored pressuremeter test curves obtained in boreholes drilled by the MDS, hand auger and rotary rig were compared and found that the quality of the test curves obtained in boreholes drilled by the MDS was better than the hand auger and rotary rig. The site selected for the detailed study comprised alluvial soils (CL-ML and ML). In addition to prebored PMT testing, field testing comprised, SPT, CPT and laboratory testing included Triaxial testing, Resonant Column along with classification tests. The sophisticated laboratory testing like resonant column tests, isotropically consolidated undrained (CIU) and isotropically consolidated drained (CID) triaxial tests with unload- reload loops were conducted for the determination of shear modulus of soils. The unload, reload and unload-reload shear moduli from triaxial unload-reload tests were compared with those determined from pressuremeter tests. The correlations of geotechnical parameters obtained from laboratory testing and in-situ testing have been established. The precise determination of the in-situ horizontal stress is difficult by the traditional prebored PMT testing technique. A new technique has been developed for the estimation of in-situ horizontal stress keeping in mind the least disturbance/relaxing of the in-situ stresses.