| The red beds of Badong formation are widely distributed in east Sichuan, west Hubei, northwest Hunan and north Guizhou. The purplish red soft rock of this easy sliding strata is also widely distributed in Badong area. Because of a certain amount of illite and montmorillonite, which make the interaction between water and rock happen easily, the mudstone has the certain features of swelling and disintegration, that further accelerates its physical weathering process. The weak intercalation, which determines the strength and deformation characteristics of rock mass, is widely distributed in the Badong strata. Many sliding failures of the rock mass are along the weak intercalation. With the advancement of the western development strategy, the constructions of highway and railway have vigorous development. Because the railway, highway constructions in Badong region would inevitably encounter the above strata, the harms of weak intercalation to the slope engineering frequently happen. There are30kilometers-length highway through the Badong formation stratum, in where lots of cutting slopes formed with weak intercalation in. There are a risk of instability and failures for cutting slopes that could be generated along weak intercalations, which there are no mature theory or experiences can be drawn lessons from now.The weak intercalation from a typical cutting slope in Yichang-Badong high way is enumerated as the research object in this paper. In order to study the effect of water on weak intercalation and analyze the influence of weak intercalation on the stability of cutting slope comprehensively, some unsaturated characteristic tests were conducted. Those researches would provide some important engineering experiences to the local engineering practices.Based on the sedimentary information of the soft rock and weak intercalation, the disaster characteristics of weak intercalation on the cutting slopes, the X-ray diffraction mineral appraisal test, chemical composition analysis test, the basic physical and mechanical properties test, the soil-water characteristic test, unsaturated strength test, unsaturated creep test and numerical simulation test were conducted in order to investigate the mineral composition, chemical composition and physical properties indexes, soil-water characteristics, unsaturated strength characteristics, unsaturated creep characteristics of the weak intercalation and the effects of weak intercalation on the stability of slope. Based on the above test results, the engineering characteristics of weak intercalation were studied through qualitative analysis, the influences of moisture contents on the strength and deformation characteristics of weak intercalation through quantitative analysis, the effect of different water contents of weak intercalation on the stability of slope engineering are summarized. Based on the above researches, the engineering properties of weak intercalation have been achieved as following.(1)The engineering geological properties, physical and mechanical properties of weak intercalation. The sedimentary environment and distribution characteristics, the formation process of weak intercalations in the study area were mastered by the geology survey and researched on the engineering geological properties of weak intercalation development in soft rock slope in Yichang-Badong highway. In view of the main engineering problems of cutting slopes in the study area, the research mainly focused on investigating of the distribution characteristics of weak intercalation and its potential hazards to soft rock cutting slope. This kind of soft rock cutting slopes should be taken more attention to in engineering practice. The mineral and chemical composition, the basic physical property indexes and the influences of water on the strength characteristics of weak intercalation in the study area were preliminary mastered by X-ray diffraction experiment test, chemical analysis test and basic physical and mechanical properties test, which provided some references for later quantitative research of water effects on the strength and deformation properties of weak intercalations. The study has shown that the clay mineral contents of weak intercalation are high, the water sensitivity of it is strong, the influence of water on its engineering property is great. The direct shear test shows that the cohesive force and internal friction angle of weak intercalation have a non-linear decrease with the increase of moisture content. The greater the moisture content of weak intercalation is, the worse the intensity is. The cohesive force and the overall strength increase with the dense of weak intercalation at the same time.(2) The soil-water characteristics of weak intercalation. The soil-water characteristic curve test of the weak intercalation with different dry density, different initial moisture content, different grain size distribution was conducted in order to investigate the soil-water characteristics of weak intercalation. The VG model was adopted to fit the soil-water curves and the fitting parameters, by which a complete soil-water characteristic curve was draw, were calculated. The study on soil-water characteristics of weak intercalation shows that the matrix suction of weak intercalation decreases with the increase of the moisture content. With the same dry density, the soil-water characteristic curve of soil sample with higher initial moisture content is above the curve of sample with lower initial moisture content. The higher the initial moisture content of sample is, the more sensitive the matrix suction change of sample with the moisture content is. The inlet value and residual moisture content of sample increase with the increase of initial moisture content. The range of the inlet value increasing with the initial moisture content decreases with increase of dry density of sample. With the same initial moisture content, the higher the dry density of sample is, the higher the inlet value is. The growth range of this change decreases with the increase of the initial moisture content of the soil sample. The location and shape of soil-water characteristic curves of soil samples with the same initial moisture content and different dry density are different. Due to the lower dry density soil samples with higher saturated moisture content, the soil-water characteristic curve of low dry density sample is over the curve of the high dry density sample in low matrix suction range. In high matrix suction range, the soil-water characteristic curve of small dry density sample is under the curve of high dry density sample, which shows that the matrix suction of small dry density sample changes with the moisture content more sensitively. With the Dry density higher, the changing susceptibility of matrix suction with the moisture content of samples reduces, also with the residual moisture content getting higher and the ability of water-holding getting intensive. Compared the soil-water characteristic curves of different particle size distribution samples, it can be found that the soil-water characteristic curve of the sample with high content of coarse particles is visibly below. With the same moisture content, the matrix suction value of lower content coarse particles samples is higher, but the saturated moisture content is lower. These above studies show that the soil-water characteristic of weak intercalations in Badong formation is significant and the pore structure (dry density, initial moisture content), particle size of particles have obvious influences on the properties, which have important significance for analyzing the engineering properties of weak intercalation with different physical state and the influence on the stability of cutting slopes.(3)The unsaturated strength and deformation properties of weak intercalation. The saturated triaxial compression and unsaturated triaxial compression tests of weak intercalation were carried out by GDS triaxial test system to realize the unsaturated strength and deformation properties of weak intercalation. The results show that the strength and deformation characteristics of weak intercalation are affected by matrix suction obviously. It shows that the stress-strain curves of weak intercalation under different confining pressure all present the hardening behavior, from which it can be seen that the deviator stress of sample increases with the strain increase to the peak strength finally. Under the same strain, the deviator stress and the peak intensity of samples increase with the increasing of confining pressure. By the strain-volume deformation curves, it can be seen that the volume shrinkage of soil samples occurs in shear progress and the volume change of sample tends to be stable in the end. The volume change of sample increases with the increase of confining pressure, which shows that the deformation properties of weak intercalation are obviously affected by the confining pressure. The saturation intensity index are got from the test, cohesion is2.25kPa, effective internal friction angle is31°The unsaturated strength test shows that the stress-strain curves of weak intercalation under different confining pressure all present the hardening behavior, from which it can be seen that the deviator stress of sample increases with the strain increase to the peak strength. It can be found that the deviator stress and peak strength of samples under the same strain increase with the increase of the matrix suction from the stress-strain curves of samples under different matrix suction, which also shows that the shear strength of weak intercalation increases obviously with the increase of matrix suction. By the strain-volume deformation curves, it could be seen that the volume shrinkage of soil samples occurs in the shear progress and the volume change of samples tends to be stable in the end. The volume change of sample increases with the increase of matrix suction, which shows that the deformation properties of weak intercalation are obviously affected by matrix suction. According to experimental results, the strength indexes of the unsaturated weak intercalation are as follows, cohesion is2.25kPa,φ’is31°, φb of samples in the matrix suction of100kPa,200kPa,300kPa,400kPa are respectively30°,24°,20°,23°. The index φb, which decreases continuously with the increase of matrix suction, is not constant. In engineering practice, the index φb can be selected for strength calculation according to the actual moisture content of weak intercalation. It is shown that the total cohesive force and shear strength that increase continuously with the increase of matrix suction of unsaturated weak intercalation are much higher than that of saturated weak intercalation from the test, which tells us the strength and deformation properties of weak intercalation are significantly affected by the matrix suction.(4) The unsaturated creep properties of weak intercalation. In order to study the unsaturated creep behaviors of the weak intercalation, a GDS unsaturated triaxial test system was used. Then the triaxial creep tests on intercalated soils under varying matric suction were conducted to obtain the unsaturated creep curves, according to which some unsaturated creep models were built. At the same time the long-term strength were calculated, which provided a reference for the stability analysis of cutting slope. The results show that the weak intercalated soils have apparent creep behaviors, which are significantly affected by the moisture contents of the samples. It should be paid more attention to that the strength of weak intercalation decreases after the creep deformation, which would have significant impact on the stability of cutting slopes. The strain of weak intercalation is in nonlinear relationships with the stress and time. The higher the deviator stress level is, the more obvious the growth of deformation with time is. Under a constant deviator stress load, the creep deformation increases with the reduction of matric suction and this trend is more obvious when the deviator stress load is larger. According to the characteristics of creep curves, the unsaturated creep is divided into three stages, creep occurrence stage, creep attenuation stage and steady creep stage. Based on the creep test data, three experiential creep models were built to fit the creep behaviors of weak intercalation. Two of them were improved from the Singh-Mitchell creep model and Mesri creep model. In order to predict the creep behavior of weak intercalation better, the third kind of unsaturated creep model was tried. In comparison of the calculated values with the experimental values, the improved Mesri model and the third kind creep model can well predict the creep behaviors in every stage of creep of weak intercalated soils. Though at high stress levels, the creep models deviate from the test results in some extent, combined with the stress status of practical engineering in real weak intercalation their forecast precisions satisfies requirements for engineering application. By comparing those kinds of unsaturated creep models, it can be found that the effect of improved Mesri model to predict creep behavior of weak intercalation is the most accurate, the third kind unsaturated creep model predicts the creep behavior of weak intercalation preferably, the effect of improved Singh-Mitchell model is not good, the worst models for weak intercalation are Mesri and Singh-Mitchell models.According to the creep curves, the long-term strength is calculated by the isochronous curve method. The long-term strength of the samples under different suction is reduced greatly than the instantaneous intensity. The loss rate of strength is between51%and73%, which shows that the strength of weak intercalation under the action of load for a long time decreases obviously. With the increase of matrix suction, the long-term strength of weak intercalation shows a trend of linear increasing, which also shows that the long-term strength of weak intercalation with lower the moisture content will be higher when under the load. When with higher moisture content, the long-term strength of weak intercalation under load is lower. The higher the moisture content is, the greater the strength loss is.(5) The influences of weak intercalation and its matrix suction on the stability of soft rock cutting slope are analyzed by using the numerical simulation technology. The results show that the influence of weak intercalation on the stability of soft rock cutting slope is great. The soft rock cutting slope excluding weak intercalation under natural conditions is very stable with a high safety coefficient. Although the soft rock cutting slope including weak intercalation after excavation under natural conditions is also stable, the plastic yield zone in weak intercalation, the shear strain and displacement increment zone in the leading edge of slope appear and the weak intercalation will become the sliding surface of the slope under the condition of the external force (earthquake) or rainfall. To the soft rock cutting slope including weak intercalation, the different matrix suction states of weak intercalation have a great influence on the stability of the slope. When the matrix suction of weak intercalation is continuously reduced, the plastic yield zone in weak intercalation and the rear of slope increase, the range of shear strain zone in the front of weak intercalation and the rear of slope increase obviously at the same time, the displacement in every slope area is also increasing, which result in the sliding failure of the slope eventually along the weak intercalation as the sliding surface.The stability calculation and analysis of soft rock cutting slope show that the stability of cutting slope including weak intercalation under natural conditions is far lower than that excluding weak intercalation. For the cutting slopes including weak intercalation, the matrix suction of weak intercalation has a great influence on the stability of the slopes. The lower the matrix suction is, the worse the stability of the slope is. There is a boundary value for matrix suction of the weak intercalation, under which the slope is in the limit equilibrium state. When the matrix suction of the weak intercalation is higher than this value, the slope is stable. When it is lower than this value, the flow of plastic yield and shear failure of weak intercalation in the slope under the action of stress are more likely to occur, which results in the failure of slopes along the weak intercalation. For practical engineering, according to local climate conditions and the distribution of weak intercalation, the stability of this kind slope and moisture content scope of weak intercalation that could cause the failure of slope would be predicted in the slope excavation process, which would provide references for the protective engineering of the slope.(6) The soft rock cutting slopes including weak intercalation in Badong formation, which formed inevitably due to the excavation in the highway construction process, will result in many engineering diseases. Based on the analysis of the engineering properties, unsaturated properties of weak intercalation and its impacts on the slope stability, some protective suggestions and advices are put forward to prevent such slope diseases from the view of the survey, protection design, construction and later operation of the cutting slopes. |
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