Experimental Research On Stress Relaxation Of Unsaturated Lime-treated Expansive Soil

After the lime improvement treatment, it has significantly improved strengthcharacteristics of expansive clay. However, due to the process of constructiondisturbance on site, stress relaxation phenomenon is occured in the lime-treatedexpansive soil, and the larger relaxation of stress in soil may result in engineeringdisasters and great economic loss. Therefore, stress relaxation research of unsaturatedlime-treated expansive soil will not only enrich the rheological theory but also provide abasis for ground treatment.Using GDS triaxial apparatus, experiment was conducted to discuss hydraulicproperties and strength characteristics of unsaturated lime-treated expansive clay.Meantime, a series of stress relaxation tests under different strain level, strain rate andload pattern were carried out, and the rheological element models were introduced to fitthe measured stress relaxation results. Besides, the scanning electron microscope(SEM)and energy dispersion analysis(EDA) tests were applied to discuss soil microstructurecharacteristics. The tests results lead to the following conclusions:(1) From the measured soil water characteristics curve of lime-treated expansiveclay, the drying and wetting curves exhibit hysteresis and the air-entry value was about300kPa.(2) The specimens presented brittle fracture during the unconfined compressionstrength tests, and the peak strength decreases with the increase of water content, whilethe peak strain exhibits a reverse trend. In unsaturated triaxial strength tests, specimensshowed strain softening characteristic, and the peak strains are less than1.5%. Besides,the slower specimen was sheared, the higher peak strength was.(3) All the stress relaxation curves were of similar shape, and the final stress wouldnot decay to zero. For relaxation tests under different strain level, stress in stablerelaxation stage generally decreased with the increase of strain level. For specimensunder different strain rates, the quicker specimen was sheared, the higher initialrelaxation rate was. In the step loading stress relaxation test, former load history willdamage specimen to certain extent, and this damage accumulates with number of step.(4) The stress relaxation curve of specimen BB is decomposed into multi-timescales series with empirical mode decomposition(EMD) method. The decomposedresults show that the series has period that about1day, which confirms that temperaturedifference between day and night affected GDS stress relaxation tests. (5) Based on micro-quantitative analysis, the selected specimen pore areas aremainly in0.3-5um2range, and the pore average diameter is1.57um.(6) Nishihara model is of good fitting result to describe stress relaxation propertiesof lime-treated expansive clay under stage loading tests. While Burgers model showedbetter fitting effect than two-element generalized Maxwell model in depicting specimenviscoelastic under step loading test.