Study On The Liquefaction Characteristics Of Q₃ Loess Under Pre-dynamic Loading

Liquefaction of loess is a phenomenon of instantaneous instability under the action of external load after the saturation of loess.The formation of liquefaction is often based on the triggering effect of rainfall,irrigation water and external loads,and liquefaction is generally divided into static liquefaction and dynamic liquefaction.The occurrence of loess landslide is mostly related to the liquefaction of loess.The fragile geological environment and special soil properties in the loess area make geological disasters such as loess landslide and loess mudflow caused by liquefaction frequent.About 1/3 of the geological disasters occur in loess area in China,which causes huge casualties and property.The loess plateau,which accounts for about 70% of China's loess area,is mainly developed on the Ordos block.The periphery is surrounded by intermittently connected Cenozoic grabens.The neotectonic movement is intense,and historical earthquakes occur frequently.The earthquake has become an important predisposing factor for loess disasters dominated by loess landslides.In general,saturated loess is liable to liquefy under seismic loading,and unsaturated loess is difficult to liquefy,but seismic loads undoubtedly cause damage to the internal structure of the soil.If the early earthquake load is used to represent the degree of influence of the historical earthquake,the characteristics of the loess when saturated with water and the characteristics of later liquefaction should have certain differences after undergoing different early earthquake loads.This will affect the formation and distribution of loess disasters.In order to deeply understand the liquefaction characteristics of Malan loess?Q₃?under the influence of early earthquakes,this paper takes Lanzhou and Tianshui loess as research object.Firstly,a new saturation method for the original loess samples of Lanzhou and Tianshui was developed by adjusting the pressure increasing rate and the pressure stabilization time in back pressure saturation process.According to the natural water content of Heping loess,the loess sample with higher water content is prepared by titration.The dynamic triaxial apparatus is used to simulate the pre-seismic load processing.Based on the combination of consolidation undrained shear test,dynamic triaxial test,mercury intrusion test,scanning electron microscopy test,physical model test and numerical calculation,the liquefaction characteristics of loess samples with different water content under the influence of early seismic loads are analysed.The initial moisture content of the sample is designed as 5%,10%,15%.The PGA is set to 0.15 g,0.30 g,0.40 g in the pre-treatment of early seismic load.By analysing the influence of the above factors,the essence of the change of liquefaction characteristics of Q₃ loess under the influence of early earthquake load is revealed.The test results obtained are as follows:?1?The saturation process of undisturbed loess samples was studied by the combination of head saturation and back pressure saturation which changed the pressure increasing rate and the pressure stabilization time,and then a new method for the saturation of undisturbed loess samples was proposed.During back pressure saturation,increasing the confining pressure and back pressure at a rate of 1 k Pa/min ensures that the degassed water uniformly enters the internal pores of the loess.After the confining pressure and back pressure reach the target value and then the pressures are maintained for 120 mins,the back pressure inside the loess can be evenly distributed so that the saturation effect is improve.The quantitative relationship between saturation Sr and B value?the ratio of pore pressure increment to confining pressure increment?can be described by an exponential function.?2?The pre-treatment of simulating the pre-seismic load,which is conducted by dynamic triaxial apparatus,is carried out on loess samples with different initial water content.The static liquefaction characteristics of pre-treated Q₃ loess were studied by consolidation undrained test.Samples with different water contents have different liquefaction properties after pre-treatment,Samples with low water content are easy to liquefy,however the liquefaction resistance is improved when water content increases.That is,the sample with a water content of 5% in the test is easy to liquefy,and the liquefaction resistance of the 10% and 15% samples is improved.When the initial water content is different,the axial strain of the sample responds differently to the dynamic load after the pre-seismic load treatment.Cohesion and internal friction decrease 1.29-3.78 ° and 2.15-6.67 k Pa after pre-treatment while the samples were in the lower water content,and increase to 1.32-3.02 °,1.42-3.98 k Pa,respectively when in the higher water content.To be specific,when the water content is 5%,the axial deformation of the sample is 0.019-0.189%.When the water content is increased to 10% and 15%,the axial deformation of the sample can reach 0.189-7.564%,which is 8-40 times higher than the 5% samples.Using the method of regression analysis,the quantitative relationship between PGA?peak ground acceleration?in pre-treatment,water content before pre-treatment and cohesive force and internal friction angle was proposed.Descriptive statistical analysis is used to calculate the confidence interval of loess shear strength at 95% confidence level when the liquefaction index M is zero.The static liquefaction trigger factor was used to reflect the effect of initial water content and of PGA on the static liquefaction.Based on this,the liquefaction trigger point of the loess static liquefaction was determined.?3?Dynamic liquefaction characteristics of pre-treated loess samples are studied by dynamic triaxial test.After the pre-treatment,when the initial water content is 5%,the vibration required for the formation of liquefaction of loess samples decreases with the increase of PGA,that is,it is easy to liquefy.After the water content increases to 10% and 15%,the vibration required to form liquefaction increases with the increase of PGA,which means the liquefaction resistance increases.When the initial water content is different,the hysteresis of the pore pressure after pre-treatment is different.That is,when the water content is low,the early earthquake loading has a promoting effect on the joint and cracking growth in the soil,which leads to the increase of soil connectivity and the decrease of pore pressure hysteresis.Under the condition of high water content,the pre-treatment leads to the destruction of the pore structure of the soil and the blocking of pore water pressure transmission channel,so that the hysteresis of the pore pressure is increased.The liquefaction stress ratio decreased at 5% water content and increased at 10% and 15% after pre-treatment.?4?The mercury intrusion test and scanning electron microscopy?SEM?are used to study the variation of pore size,porosity mode and particle contact mode in loess after pre-treatment.It is found that the microstructure change is related to the initial water content before pre-treatment and the PGA value in the pre-treatment.After the pre-seismic load treatment,the number of macropores in the low water content sample increases,while the number of macropores in the high water content sample decreases.For the loess sample with an initial water content of 5%,the number of pores of 90.603-113.180 ?m increases,while the number of micropores changes less significantly.When the water content is increased to 10% and 15%,the number of pores of 80.521-100.645?m is reduced.It is proposed that the formation of seismic damage zone increases the number of macropores when the water content is low.On contrast,the seismic dense effect decreases the number of macropores when the water content is high.A functional relationship between the liquefaction sensitivity coefficient and the pore structure parameters is established to reflect the liquefaction sensitivity due to the variation of initial water content and PGA.?5?Preliminary study on the influence of liquefaction on the stability of loess slope under the action of pre-earthquake loading is conducted by using micro-vibration table test.It is found that the liquefaction caused by the pre-treatment has caused the deformation of the loess slope to be larger in both horizontal and vertical directions,while the untreated soil block only produces the horizontal deformation.Because the influence of earthquake damage effect cannot be considered in the numerical calculation,it is proposed to reflect the influence of the previous earthquake load on the loess slope by parameter reduction and plastic zone development,whose results are consistent with the model test results.