Study On Large Strain Triaxial Tests Of Municipal Solid Waste

In recent years, with the rapid increase of the population and the acceleration of urbanization in China, MSW production increase dramatically. So far, landfills provide for economical and environmentally safe disposal of municipal solid waste(MSW). Consequently, design and implementation of more higer and steeper new landfills or expansion of existing landfills is necessary to accommodate this ever increasing volume of MSW. Geotechnical engineering properties of MSW such as compressibility and shear strength are of prime importance in design and maintenance of these landfills. MSW is known to be a heterogeneous material of varying constituent types and dimensions containing elements that degrade with time, landfill prone to large deformation and stability problems. A good understanding and knowledge of MSW properties is required for reliably evaluating and predicting its mechanical properites. As such, the above two problems happen in the landfills can be solved. So far the study on the mechanical properties of MSW has focused primarily on CD tests in low strain levels(less than30%), and rare CU tests were conducted. As a result, the systematic study of the mechanical properties of municipal solid waste in larger strain level (especially more than30%) in CD and CU test can be useful for making deformation and stability analyses.Firstly intact samples of different embedment depth and fill age were used to perform drained triaxial compression tests. The test results showed that:the MSW samples exhibited strain-softening, and the lower depth of the MSW sample, the larger shear strain is needed before MSW occur strain-softening; before the peak strength point, the mobilized cohesion didn’t have significant change rule with the increasing of the depth and the fill age, the mobilized friction angle remain more or less the same with the increasing of the depth; In the peak strength point, with the increase of the depth, the mobilized cohesion first increased and then decreased, and a slight decrease in the mobilized friction angle of shearing is obsearved with the increase of the depth at the same time. The average shear strength of the Chengdu MSW is given by the c’of0kPa and φ’ of43-45°The knowledge of the MSW mechanical response is very important in cases involving MSW with high initial water contents, the MSW may become saturated. Therefore the excess pore water pressure cannont be easily dissipate. As such, the further study is conducted with the recompacted samples in Consolidated-Undrained (CU) triaxial tests. The tests demonstrated that:MSW will exhibited strain-softening characteristics after certain depth. Before30%shear strain, the total cohesion did not show significant change trend with the increasing of the depth and the extent of MSW degradation, the total friction angle increased with the increasing of the depth and the degree of degradation; In the peak strength point, the total cohesion decreased with the increasing of the depth and degradation degree, the total friction angle increases with the increasing of the degree of degradationIn the CU tests, the pore water pressure increases quickly and stabilizes at a value almost equal to the confining stress. The effective stress paths followed during shearing in CU tests show "S" shaped curve, and the pore pressure generation casue the stress path approach to s’=t line finally. The effective stress close to zero after the MSW be sheared at certain strain levels, but instability phenomenon such as liquefaction appeared in the soil is not discovered in the MSW, the samples still keep high strength.Lasty the Consolidated-Undrained (CU) triaxial tests results were modified based on the percentage of the fiber matter that MSW contains. The pore water pressure was modified with the pore water pressure reduction coefficient Am in CU test. Shear strength based on the use of the modified effective stress equation is less than that based on the the Terzaghi effective stress equation. The use of the factor Am to calculate the pore water pressure contribution in the effective stress equation showed significant improvement in the compatibility between the parameters obtained from CU and CD tests.