Study On Deformation Mechanism And Support Of Roadcut Slope In Red Beds

Based on extensive geological investigation and analysis, deformation and failure mechanism, stability analysis and support measures of road cut slope in red beds are studied systematically and thoroughly in this paper. The author uses theoretic, numerical analysis and large-scale model tests to assist his research. Methods oriented by the typical projects of the red bed road cut slopes involved in the highway from Nanchong to Chongqing and the highway from Anning to Chuxiong in Yunnan Province. The mechanic parameters of rock mass in red bed are simultaneously studied by using large numbers of laboratory test and field test. The main results of this paper can be summed up as follows:1. On the basis of geologic engineering investigation, the formation structure of red bed slope is classified as dip slope, accumulative formation slope and approximate horizontal slope, which is classified as three types in slope structure: mudstone, interbedded strata of sandstone and mudstone, heavy sandstone. According to lithological combination characters, approximately horizontal slope has eight failure modes. Considering the relationship between the dip of bedding plane and the dip of slope, the dip red bed slope is further divided into three subgroups: consequent bedding rock slope, countertendency bedding slope and oblique crossing bedding slope. First, the deformation and failure mode consists of three types among consequent bedding rock slope: bedding sliding, sliding and fracturing, bending and fracturing. Secondly, the deformation and failure modes of coutertendency bedding slope are bending, toppling and sliding. Finally the failure modes of oblique crossing bedding slope consist of three types: ladder consequent bedding sliding, ladder consequent joint sliding, and wedge sliding.2. The principle of choosing mechanic parameters of rock mass in red bed is also put forward, we get integrated understanding on mechanic, hydrolytic and weathering characters of red beds via anlysis on test results and collected data in the first place. Then, the method using Hoek-Brown empirical strength criterion to decide mechanic parameters of jointed rock mass is expounded, the relationship between BQ and RMR is studied. From which empirical formula of using BQ to estimate elasticity modulus is summarized. Then, the principle of choosingmechanic parameters ot rock mass in red beds is proposed on formation structure and mechanic parameters in red bed slope. Mechanic parameters of rockmass with less than four groups joints is mainly obtained through test results of rock and structure planes. The test results have been processed with the random-fuzzy statistical method. Hoek-Brown empirical strength criterion is suggested to confirm mechanic parameters of rockmass with more than four groups joints.3. Studies are carried on stability of approximately horizontal slope. Numerical analysis has been done on complex interbedded strata of sandstone and mudstone slope to validate failure mechanism. Taking tensile strength as ripping falling criterion is suggested. Special study are carried on the horizontal impulse landslide with dense high angle joints on back of slope, the results show that the stability factor is minimal when the high angle joints with water are located in the top line of slope surface.4. Studies are carried on stability of dip slope. The author suggests using stereographic projection of structure planes to judge the failure modes of dip slope in red beds. Stereographic projection relationship of structure planes with two-faced slope is presented. The essential factors involved in stability of dip slope are presented as following: the formula from stress field used to calculate the bedding slip range of the consequent bedding rock slope with failure mode of sliding, the formula used to calculate critical slope angle, bending failure stress and breaking length of countertendency bedding rock slope, the formula used to calculate stability factor of wedge failure with different friction angles on two structure planes using limit equilibrium method and theory of plasticity upper limit method.5. The stabilities of red bed high slope excavated by step are studied. To cover the shortage of bench design based on experience, the influence on entire stability after setting bench and the width of wide bench in the middle of slope are studied. The results show slope stability after setting bench is slightly higher than that without bench. The excavation disturbed zone of slope is independent when the width of bench is wide enough. The K56 width of wide bench in Nanchong to Chongqing and highway is decided by numerical analysis.6. The methods to calculate internal force of lattice beam is presented. According to the difference of mechanical mode between rope-bolt and bolt, themethod calculating for single beam internal force, the mechanical model of lattice rope-bolt (bolt) frame design are put forward, whole winkle elastic foundation model for rope-bolt frame and for elastic bearing in node model for bolt frame. Examples on internal force of frame tallied with this theory is studied by FEM(fmite element method).7. The large-scale geomechanical model test is carried for discovering the mechanism and support measure. The results have testified that the failure mechanism of interbedded strata of sandstone and mudstone slope which is mainly caused by waters, setting the wide bench in the middle of slope is favorable to reducing the disturbed area due to excavation. The test results show the moment of resistance in lattice rope-bolt frame is consistent with theoretical calculation in the paper.