Impact Of Mineralogical Composition And Water Chemistry On The Shear Strength Of Clay And Its Application

Shear strength of soil is one of the key parameters controlling slope stability, andcohesive soils were extensively found to be facility-sliding strata to many natural slopes orcuttings as its poor mechanical properties, so the shear strength of cohesive soil has been oneof the hot spots in engineering geology. As mineral compostion forms the skeleton of thecohesive soil and pore water (groundwater) is one of the key causes leading to reduction of itsshear strength, the mineralogical composition and the pore water chemistry were two crucialfactors controlling the shear strength of cohesive soils. However, the control law of mineralcomposition and pore water chemistry on cohesive soil’s shear strength is not clear. Thisdissertation experimentally studied the impact of mineralogical composition in clay fractionand pore water chemistry on the shear strength of artificial and natural cohesive soils, bymeans of direct shear test, triaxial shear test, mineral compositon analysis, water chemistryanalysis, electron scanning microscope etc. The mechanism was analyzed in terms ofwater-soil physic-chemical interactions and the strength control effect of each type of mineral.And the research result of natural cohesive soil was used in analyzing the mechanism ofHuangci Landslide. The following specific conclusions could be drawn:First of all, smectite has strong control effect on the shear strength of cohesive soil due toits strong expansibility and typical flake shape of particle. For cohesive soils whose clayfraction contains no detrital mineral,25%of smectite content would greatly reduce its shearstrength as the frictional strength between smectite particles was low.Secondly, the control effect of clay mineral and detrial mineral in clay fraction on theshear strength of cohesive soils depends on the relative compostion of these two types ofmineral. The control effect of detrial mineral is stronger than clay minerals only when itscontent is higher than a critical content which was found to be10~15%,8%and6%incohesive soils containing smectite, illite and kaolinite repectively.Thirdly, the control effect of of pore water salinity on the shear strength of cohesive soilwas affected by the mineral compostion and ion concentration. Smectite content decided thecontrol degree of pore water salinity, and the control effect was observed only when the NaClconcentration in0~1mol/L. The impact of pore water alkalinity/acidity on shear strength of cohesive soils was controlled by dominant mineral composition which had the key role in theshear strength of soils.Finally, the weathered red mudstone in Lanzhou and around was rich in clay mineralsand soluble salts. The soluble salts were dramatically dissolved and leached by irrigationwater (Yellow River water), and the drained residual friction angle was reduced by45.7~61.7%, and the undrained peak shear strength reduced by28.9~33.9%. It was also foundthrough numerical simulation that the shear strength reduction inducd by irrigation lixiviationplayed a major role in Huangci Landslide activity, and continous irrigation on Heitai wouldprobably cause anther sliding of Huangci Landslide.