Mechanical Properties And Microscopic Mechanism Of Composite Modified Expansive Soil In Short Age

Expansive soil has the characteristics of water absorbing expansion and water loss contraction,which causes great damage in engineering.Therefore,reinforcement treatment is needed in expansive soil area.As a common chemical curing agent,cement has been widely used in engineering construction at home and abroad.Nanomaterials(nano-MgO and nano-clay),as new materials,have wide application value for the improvement of cement-modified expansive soil.In addition,fiber and fly ash,as incorporation agent,also have research value for them.Based on the fact that the expansive soil used in the test is from the Xuma Mountain Flood Diversion Road in Anhui Province,the water content of the expansive soil is relatively high under the engineering environment,so the water content of the samples in the test is 65%.Due to on-site construction,the curing time of cement stabilized soil base is at least 7d at normal temperature.Therefore,the study of mechanical properties of composite modified expansive soil and the analysis of consolidation mechanism are of great significance and engineering value.The main work and achievements of the research are as follows:(1)The unconfined compressive strength test of cement-modified expansive soil with different cement,nano MgO and nano clay contents and with proper amount of fiber and fly ash as admixture was carried out for 7d.The test results were fitted by BP neural network algorithm.(2)After analyzing the stress-strain curve characteristics fitted by BP neural network,it can be concluded that when the cement content increases from 6% to 20%,the corresponding compressive strength increases by 14.9 times.Considering factors such as free expansion rate and crack resistance,10% is the best cement content.Nano MgO increased from 0‰ to 30‰ and its compressive strength increased by 2.32 times.Considering factors such as free expansion rate and crack resistance,the best content of MgO is 10‰.After adding 10‰ fiber to the cement-modified expansive soil,its compressive strength is slightly increased by 7% ~ 8%.Nano-clay and fly ash can not effectively improve their compressive strength under the condition of curing age of 7d.(3)According to the consolidated undrained shear triaxial shear test,after adding nano MgO,the effective internal friction angle and effective cohesion of cementmodified expansive soil increase by 39 % and 167 %,respectively,and the improvement effect is most obvious.After adding fiber,the effective internal friction angle and effective cohesion increase by 9.0 % and 85.0 %,respectively.After adding nano clay,its effective internal friction angle decreased by 15.5 %.However,the effective cohesion increased by 28.4 %.(4)The SEM and EDX micro-tests were carried out on the above-mentioned composite modified expansive soil to analyze the consolidation mechanism.The microscopic test results show that cement,nano-MgO and fiber can make the whole structure of expansive soil more compact when the sample curing age is 7d;nano-clay and fly ash can not improve the internal structure of expansive soil.The microscopic test results are in line with the results of macroscopic mechanical tests.