Seismic Response Of Cuijiayoufang Rock Slope In Wenchuan

The performance of large landslides in Wuchuan Earthquake Area is complex, and it is difficult to be explained with traditional mechanisms. The failure modes of rocks in the same region are different from each other, either tensile failure or shear failure. The fault stretches acrosses Cuijiayoufang Slope. And it is necessary to study the fault influence on its seismic response, which offers reference to the study of slopes with similar engineering geological conditions.On 12 May 2008 Cuijiayoufang Rock Slope was triggered by Wenchuan Earthquake in Beichuan Chenjiaba. The type is spray collapse. Its destructive process is this:First, the cracks in tension are generated in the upper part and post-edge of the slope, then extends, while the cracks in shear are generated in the lower part. Second, the strength decreases, and both kinds of cracks intersect. Instantaneously the friction of the slope drops to the minimum, and the slope sprays down. Third, the slip mass has the initial velocity, and flies. When it lands, it is blocked by the hill on the right side. The slip mass turns left and continues moving. As the front land is near horizontal, friction with the ground increases and rock blocks crashes each other. The energy is consumed and the slide mass stops on the frontal edge. From the engineering geological conditions, there are three main causes of Cuijiayoufang Rock Slope. First, height difference of the slope is 160m, and the slope with two empty faces is isolated. This steep and lofty topography leads to significant amplification effect of seismic wave. Second, fracture is developed in rock, and the strength of rock block is low. Gray karsts holes are seen occasionally. The rock is crumbling and of poor quality. The fault crosses the slope, and the slope is located in the meizoseismal area. The peak acceleration on the ground in some areas is high of 1.5～2.0g.The spatial distribution of discontinuities is calculated in the analysis of rock structure, and one of its previous work is measuring the orientations of discontinuities in the field. The geological compasses are used to measure the orientations generally. From the study of traditional geological compasses, the disadvantages are found, that is, complex operation, low efficiency, high bias in plunge angle of linear structure, and reading difficulty in the orientations measurement of the low pour rock roof. Due to these shortcomings of traditional geological compasses, a new type of geological compasses is invented, including pedestal, collimator, azimuth dial, goniometer, hemispherical transparent cover and liquid. When measuring plunge direction and angle of line, aim at the linear structure with the two tips of the collimators, then rotate the compass on the line which passes by the two tips, till the superposition of the goniometer center line and the cover alignment. The reading of azimuth dial which the center line of green part of goniometer indicates is the magnetic azimuthα°, and the reading of goniometer which liquid level indicates is the plunge angle b°.The plunge direction is the difference betweenα°and magnetic declination. When measuring the orientation of face, the compass bottom is taken to stick to rock bottom or top, and the reading of azimuth dial which the center line of green part of goniometer indicates is the magnetic azimuthα°. The reading of goniometer which liquid level indicates is the dip angle b°.The dip direction is the difference betweenα°and magnetic declination. Compared with traditional geological compass the new kind has the following advantages:1) the plunge angle bias of line measurement can be eliminated;2) Bias can be minimized and efficiency can be improved in the face measurement without manually adjusting the horizon;3) There can be plenty of space and bright light for reading in the orientation measurement of low pour rock roof.When assuming the discontinuity is circular, diameter becoming an important parameter of the spatial distribution of rockmass discontinuities should be deduced by the observed trace length. Nowadays:(1) Window sampling and scanline sampling for complete trace are mostly used for trace field survey, however, due to outcrop condition or time limit, both the sampling methods cannot be adopted, while it is suitable to adopt scanline sampling for semi-trace. (2) The probabilistic relation between untruncated uncensored semi-trace length and diameter in the scanline sampling for semi-trace is complex implicit function, and it is difficult to acquire exact analytical solution, previous algorithms of it are rare. (3) Though the diameter estimation formula applicable for this sampling method can be derived from the integration of Priest's and Zhang's achievements, the result is inaccurate, that is, the relative error is up to 7.1%-23.32%. Considering that an accurate algorithm applicable for scanline sampling for semi-trace is badly needed, a new trial algorithm is proposed, which takes Monte-Carlo method and analytic geometry as its theory, and Excel, C++, etc. as tools, to estimate the best diameter distribution, making the trial untruncated uncensored semi-trace length approximate the observed one. Due to the dispersion of its result, experiments to reduce the dispersion are conducted, it is found that, the result confidence degree of taking mean of ten trials is respectively 1.93 or 2 times as that of doubling the trial sample size, and 2.19 times or 2.13 times as that of single trial, when the diameter follows uniform or negative exponential distribution. So it is illustrated that taking mean of many trials can reduce the dispersion effectively. Compared to Priest-Zhang Algorithm, on accuracy, the new trial algorithm can reduce the error for an order of magnitude, and is better in fault tolerance. On calculation speed, memory requirement, dominant frequency requirement, file size and memory occupancy, the new trial algorithm is respectively 432 times,1/6,1/5,11/12 and 1/6 as that of the entity algorithm, and more convenient to program, more stable to operate. It has been applied in Wenchuan, so it is awarding to popularize it.The discontinuities developed in the upper plate rock of Cambrian ((?)q) and the lower plate rock of Silurian (Sh) of Cuijiayoufang Slope are measured with semi-trace line sampling. Grouping is taken by Strike Rose Diagram and Orientation Isopycnal Diagram. The optimal probability distribution of each group's dip direction, dip angle and gap width is derived from Probability Graph and K-S Test. Diameter distribution is calculated from semi-trace, and linear density from spacing. Finally the spatial distribution of discontinuities is obtained by adjusting the probability parameters observed. Then the type of its structure is judged as cataclastic or thin-layer. And it is better to simulate its seismic response with FLAC.Where,1) Group. The discontinuities in Cambrian ((?)q) rock mass are divided into 2 groups, and that in Silurian (Sh) slate is divided into 3 groups.2) Dip direction. GroupⅠof (?)q follows normal distribution. Its mean is 311°, and standard deviation is 29°. GroupⅡof (?)q follows normal distribution. Its mean is 160°, and standard deviation is 17°. GroupⅠof Sh follows lognormal distribution. Its mean is 3.23°, and standard deviation is 0.822°. GroupⅡof Sh follows normal distribution. Its mean is 331°, and standard deviation is 16°. GroupⅢof Sh follows normal distribution. Its mean is 148°, and standard deviation is 6°.3) Dip angle. GroupⅠof (?)q follows lognormal distribution. Its mean is 3.27°, and standard deviation is 0.557°. GroupⅡof Cq follows normal distribution. Its mean is 74°, and standard deviation is 8°. GroupⅠof Sh follows lognormal distribution. Its mean is 3.75°, and standard deviation is 0.372°. GroupⅡof Sh follows normal distribution. Its mean is 66°, and standard deviation is 11°. GroupⅢof Sh follows normal distribution. Its mean is 63°, and standard deviation is 16°.4) Gap width. The five groups follows negative exponential distribution. The means are: GroupⅠof (?)q 0.06mm, GroupⅡof (?)q 0.02mm, GroupⅠof Sh 0.5191 mm, GroupⅡof Sh 0.409mm, GroupⅢof Sh 0.313mm.5) Diameter. Diameter is estimated with the new algorithm proposed in this paper. The diameter of GroupⅠof (?)q follows normal distribution. Its mean is 0.4m, and standard deviation is 0.6m. The other four groups follow negative exponential distribution. The means of them are:GroupⅡof (?)q 0.3m, GroupⅠof Sh 3.4m, GroupⅡof Sh 0.4m and GroupⅢof Sh 0.9m.6) Linear density. The linear density of five groups is:GroupⅠof (?)q 21.8m-1, GroupⅡof (?)q 8.9m-1, GroupⅠof Sh 14.7m-1, GroupⅡof Sh 8m-1, GroupⅢof Sh 1.8m-1.The fault stretches across Cuijiayoufang Slope, and it is necessary to study the fault influence on its seismic response. The two cases i.e. existence (actual) and inexistence (assumptive) of fault, are taken to the comparison of seismic response. Its general steps are like this:(1) the maximum mesh size is calculated from the censoring limit of seismic wave. Then the meshes whose size is small than calculated maximum size are generated and the model is erected. (2) the physical and mechanical parameters, horizontal and vertical boundary constraints, Mohr-Coulomb constitutive model and yield criterion, gravity acceleration are set. The initial ground stress is generated with the solution of changing the elastic-plastic parameters. (3) The seismic wave acceleration time history is filtered with censoring limit of 10Hz and baseline-corrected. As the model bottom is soft rock and viscous boundary, the acceleration time history is converted to stress time history. (4) After generation of the initial stress, the model bottom is set as viscous boundary and surrounding as free-field boundary. The local damping is set. And the dynamic stress time history is applied at the model bottom for seismic calculation. At last Zheng Yingren's seismic failure criterion is applied to judge the slope stability.The seismic response characteristics of two cases, i.e. existence and inexistence of fault in Cuijiayoufang Slope are shown as follows:1) At the end of 5s earthquake, without fault, he shear plastic zones are joined up on the slope surface. During 5s after earthquake, the displacements in the middle and toe of the slope change abruptly and do not converge, especially the horizontal displacements. And it indicates that the slope has failed at the end of 5s earthquake. Its type is shear failure; at the end of 5s earthquake, with fault, the shear plastic belt emerges in the toe, and the rock on it is not destroyed. The tension plastic zone emerges in the middle. The shear plastic belt and tension plastic zone are not joined up. In this case, the slope does not fail, and its deformation type is tension deformation in the middle and shear deformation in the toe.2) At the end of 5s earthquake, the maximum principal stress with fault is smaller than that without fault, while the minimum principal stress with fault is greater than that without fault. The stress state with fault is better for the slope stability than that without fault by Mohr Criterion, which can be verified by the comparision of deformation and failure between two cases. Therefore, the existence of fault improves the stability of Cuijiayoufang Slope, it is probably because that the fault weakens the strength of seismic wave in the ahead rock and soil.3) At the end of 10s earthquake, with fault, the shear plastic belt in the toe and tension plastic zone in the middle are joined up. During 5s after earthquake, the displacements in the middle and toe of the slope change abruptly and do not converge, especially the horizontal displacements. And it indicates that the slope has failed at the end of 10s earthquake. Its type is the perforation of shear failure in the toe and tension failure in the middle, while the type without fault is shear failure, which has occurred at the end of 5s earthquake.The conclusions are drawn from seismic responses of Cuijiayoufang Slope with fault and without fault:The fault may weaken the strength of seismic wave in the front rock. It leads to that the stress state is good for the slope stability and it delays the time of failure. The failure type is shear failure without fault, while that is shear failure in the toe and tension failure in the middle with fault. Therefore, the fault is an important factor in the time and type of failure. The attention should be paid to the fault influence on the time and type of slope failure in the future, whose engineering geological conditions is similar to Cuijiayoufang Slope.