| Semi-formed sandstone widely exists in human engineering activities,and the weakening of its physical and mechanical properties when it meets water brings great security risks to geotechnical engineering activities.Therefore,it is of great engineering significance to study the rock mechanical properties of semi-formed sandstone under the action of waterforce.Based on the foundation pit project of Citizen’s House in Yuhua District,Changsha,this paper studies the strength weakening mechanism of semi-formed sandstone under hydraulic action through laboratory test,numerical simulation and theoretical analysis.The main research contents and conclusions are as follows:(1)XRD and SEM microscopic tests show that the main components of semi-formed sandstone are quartz and kaolinite,the content of kaolinite is 26.4% ～ 49.7%,the content of quartz is 41.4% ～ 73.6%,and there is a small amount of illite.Therefore,the cementation force of semi-formed sandstone is mainly provided by kaolinite.(2)The direct shear tests of undisturbed and remolded semi-formed sandstone samples show that the cohesion of semi-formed sandstone increases first and then decreases with the increase of water content,and the internal friction Angle decreases with the increase of water content,among which the change of cohesion is greater than that of internal friction Angle.The cement structure inside the original sample makes the limit water content of the original sample lower than that of the reconstructed sample.Under the condition of low water content,the cohesion of undisturbed sample is much greater than that of remolded sample.However,the rock structure of semi-diagenetic rocks is unstable.With the increase of water content,the internal structure is gradually destroyed,while the remolded samples are soil samples without rock structure,so the decline of cohesion of the original samples is greater than that of the remolded samples.(3)Through PFC simulation of shear test of semi-formed sandstone under different water-force action in actual engineering,it is found that the optimum confining pressure of semi-formed sandstone is between 100 and200 k Pa,which is called limit confining pressure in this paper.When the confining pressure exceeds this value,semi-formed sandstone will be reshaped,part of the cemented structure will fracture,rock porosity will change,and uneven ground settlement will appear on the macro level.(4)Through the analysis of the monitoring data of the foundation pit of citizen’s House,it is found that the decrease of lateral pressure in semiformed sandstone will lead to a significant decrease in compressive strength.When the compressive strength is less than the compressive stress,semi-formed sandstone structure will be destroyed and compressed,and then the surrounding buildings will have uneven settlement and cracks in the foundation.The semi-formed sandstone is easy to soften when it meets water,and the creep of rock mass increases significantly,which leads to a great loss of prestress of anchor cable.In this kind of foundation pit construction,pile-anchor support system is recommended to control the change of confining pressure of semi-formed sandstone. |
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