Research On The Behaviors At Interface Between Geogrids And Expansive Soils And The Reinforcement Mechanism

Reinforcement technology by geogrids has a special effect and important significance for controlling geo-hazards occurred in expansive soils. But at present the insufficient research into the characteristics at interface between geogrids and expansive soils leads to the constraint of its development; since the research on the interaction between geogrids and soils in the pullout test and its mechanism is not sufficient, it causes the deviation existed in the test of behaviors at interface between geogrids and soils; inadequate research into reinforcement mechanism makes the calculation theory of reinforced soils imperfect.Taking the failure mechanism and treatment technical problems of channel slopes made of expansive soils in the South-North Water Transfer Middle Route Project in China as a background, the following important problems are studied, including the behaviors at interface between reinforcements and soils and their contributory factors, the analysis method of frictional resistance at interface between reinforcements and soils as well as interface parameters, the failure pattern and deformation characteristics of reinforced expansive soils, the mechanism that reinforcements block the shear zone of soils mass and lead to its strain-hardening, the contributory factors of reinforcement effect and their action mechanism etc.A pullout test on the behaviors at interface between geogrids and expansive soils, the stress and deformation of reinforced materials as well as the influence of reinforced materials' deformation on the pullout behaviors at the interface are analyzed; the progressive development and nonuniform change processes of frictional resistance at the interface are proposed, and two influencing mechanisms of average frictional resistance at interface are analyzed. Influencing factors, including water content and dry density of expansive soils, geometrical characteristics and types of geogrids, the distance from reinforced materials to side-wall of test box, pullout rate and the magnitude of overburden load etc. are investigated. An interface model of geogrids and expansive soils is proposed, and interface parameters are estimated by average frictional resistance at interface and average relative displacement.Numerical emulation on the pullout process is carried out, the nonuniform distribution characteristics of reinforced materials' normal stress, relative displacement for reinforcements and soils as well as frictional resistance at interface, pseudo-softening and pseudo-nonlinear characteristics of interface are found, and their occurrence mechanisms are put forward. Analysis method and finished standard of pullout test results based on the progressive yield process at interface between reinforcements and soils are presented. The difference between peak values of average frictional resistance at interface and strength at interface is studied, the deviation of tangential rigidity at interface is estimated by using relative displacements at pullout and free ends or average relative displacement, and feedback modification on interface parameters are conducted. In addition, the influence of friction characteristics at rigid front wall of pullout box on the frictional resistance at interface between reinforcements and soils and their action mechanisms are presented.Unconfined compression and consolidation undrained shear tests on non-saturated compacted samples are carried out, and consolidation drained shear test on saturated samples are performed as well. The influence of reinforcement by geogrids on the strength and deformation characteristics as well as failure pattern of expansive soils is studied, and the influence of number of reinforced layers, dry density, water content and confining pressure in test is tested. It has been found that reinforcement can effectively block the extension of shear zone of expansive soils, make the occurrence of soils mass's strain-hardening, and improve the bearing capacity and deformation performance of soils mass.Strain-type loading is simulated to perform numerical emulation on the deformation and failure process of reinforced soils under the condition of plane strain and axial symmetry, and explore the restraint effect of reinforcement on lateral deformation of soils mass and the averaging effect on vertical deformation. It is found that even if strength parameters at interface between reinforcements and soils are smaller than those of landfills, reinforcement effect is also quite significant, and there exists a loading rate effect in the load-displacement response process of reinforced soils, but its influence exists only when strain of soils mass is small.Through a pullout test and numerical emulation, the disturbance of frictional resistance at interface on soil's stress and deformation is analyzed and it is pointed out that there exists a reinforcement effect within a given scope of the interface between reinforcements and soils. Based on the influence of frictional resistance at interface in reinforced soils on the shear stress of soils and the resulting deflection in principal stress direction, the mechanism that reinforcements restrain the shear zone of landfill is proposed. By means of the influence of frictional resistance at interface between reinforcements and soils on the minor principal stress of soils mass, the mechanism that reinforcement lead to the strain-hardening of soils mass is set forth. In addition, the effect that reinforcement restrains the strain-softening of landfill and the action mechanism, as well as the influence of lateral pressure, tangential rigidity at interface between reinforcements and soils and strength parameters are explored.This research provide a basis for the perfection of calculation theory for reinforced soils, the development of technologies controlling geo-hazards in expansive soils by means of reinforcements for geogrids and the treatment schemes of large channel projects of expansive soils.