| The gully development is widely distributed in the Ming-Ningqiang county area of southern Shaanxi province, where there are the strata of Cambrian system, Ordovician system and Silurian system, the lithology mainly is mudstone and silty mudstone which belong to the soft rock. The engineering problem of the soft rock in the mountainous area during the highway duilding is very conspicuous. With the implementation of the western development of China, the engineering problem of the soft rock in the mountainous area during the highway duilding is needed urgently to be studied. In this paper, based on the geological conditions and the nature of rock mechanics of soft rock of Mianxian - Ningqiang in the southern mountain area of Shaanxi province, the engineering geological conditions of the region's soft rock slope and the geomechanical model of slope deformation damage are studied.Through the rheological test of the soft rock slope , the rock material parameters in the rheological mechanical model is determined, and the actual rheological mechanical model of the slope is established.From the engineering geological foundation, rheological characteristics of soft rock, the stability analysis and evaluation of soft rock slope, the rerheological characteristics and stability problems of the soft rock slop in the typical regional are systematically studied. Major research results are as follow:(1) Based on the full study in the engineering geological environment of the soft rock slope, using the electron microscopy and water immersion test , the microcosmic and macrocosmic fracture development process of the soft rock are studied ,and the main factors impacting the rock fracture development and the microcosmic reasons of mudstone failure surface are analyzed.Based on the research of the indoor experiment of rock samples, the physical mechanic property of soft rock is described in detail. The rock in the field is tested with resiliometer.The correlation equations of the resilience value and compression strength of the rock are presented by the least square method. And by this, the rock intensity along the Mianxian-ningqiang freeway was classified.(2) Based on the characteristics of soft rock slope along the Mianxian-ningqiang freeway, the different grade quality of slope rock are estimated by RMR and fractal method. The fractal dimension D of rock joint distribution as classify index is provided on the base of above research. According to the indoor and outdoor wave velocity test results , the fractal damage parameters for the different grade quality of slope rock is provided.(3) According to analyzing the lithologic features of the soft rock slope along the freeway, the rock weathering degree and the structural characteristics, the deformation and destruction modes of soft rock slope are divided into five kinds. Through the analysis of statistics of the soft rock slopes of 47, 1 model of the sliding-compression cracking deformation, accounting for 2.1% for the total number of slopes; 32 models of the bending-fracturing deformation, accounting for 68.1% for the total number of slopes; 4 models of the sliding-fracturing deformation, accounting for 8.5% for the total number of slopes; 6 models of the creep sliping -fracturing deformation, accounting for 12.8% for the total number of slopes; 4 models of the sliding-bending deformation, accounting for 8.5% for the total number of slopes. The deformation mode of the soft rock slopes mainly is the bending-fracturing deformation, of which the main failure mode are toppling, buckling failure, collapse and sliding failure.(4) According to the 3-axis compression test, it is showed that the constitutive equation of viscoelastic Burgers can better explain the mechanical properties for the soft rock of Mianxian-ningqiang freeway after undergoing pressure, particularly for its long-term creep characteristics.Test samples need 23 kN of the axial pressure to be destroyed . Before the destruction, the maximum strain in axial direction around 0.02. The long-term strength for soft rock is 40.0 MPa. Based on test results, the parameters values in the constitutive equation of Burgers are made. According to the present situation of the specimen being made with difficulty , a squareness and a cylinder test sample are designed, and the simulation was calculated.The characteristics of different forms for the test samples of rock creep deformation is discussed,namely:The overall numerical variable of cube samples should be less than that of cylinder test specimen. In low confinement pressure state(1~5 MPa), the difference between the two value is smaller, about 5 to 10%. In high confinement pressure (10 MPa), the difference between the two value is larger, about 10 to 30 %.(5)Based on the test results of the 3-axis compression tes, 3-D Numerical Simulation of the the creep characteristics and the fractal damnification characteristics for the typical soft rock slope are calculated by considering the groundwater respectively. The rules of the stress, strain, plastic area, main influence factors and failure mode etc.in above two states during the excavation process and the creep process are analyzed and studied. The safety factors of the soft rock slope in different states are calculated.(6) According to the different damage forms of the soft rock slope,the coefficient of stability of some slope are calculated by the method of limit equilibrium, the quantitative evaluation are made for the stability of soft rock slope.According to the results of field investigation and lab analysis of the affecting factors for the stability of the slope, 20 BP neurons evaluation index is eventually identified by BP neurons evaluation method. 27 slope samples are the raining sample back-testing. The comprehensive evaluation of 21 slopes is made, and the corresponding slope stability types are got. For the bedding soft rock slope, the rock stratum model was adopted. The method of apply catastrophe theory has been studied the lose steady mechanical mechanism when the rock stratum is under the horizontal and vertical stress working together.According to the established mechanical model of four sides simply back rock stratum, a layered rock system's total potential function expression is provied, and the cusp catastrophe model of the system is established, the tee-offs anthology at the level crustal stress and vertical control of space in the instable system is given, the gradual change of acting force led to the gradual process of mutation status.
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