Mechanism Of Landslide In Nanfen Open-pit Mine And Its Reinforcement-Control-Early Warning Integrated Measures

For a long period of time, the safety of slope landslide hazard has been plagued by human activities. Due to long-term large scale mining activities in the open pit mine, high slope was formed. Coupled with the relative concentration of personnel and equipment in mining areas, great influence of slope disaster should arouse enough attention. After Nanfen open-pit iron mine in deep open-pit mining model, there have been several different sizes landslide events in history. Therefore, in order to reduce or prevent the loss of landslide disasters and to protect the security of personnel and equipment, carrying out slope disaster research in mining area is essential.For open mine high slope landslide disaster development and governance countermeasures, lots of literatures were consulted and inspection analysis of slope disaster was made. Rock mechanics and mining sciences, engineering geology, test science, information science, comprehensive knowledge of modern communications technology and many other subject areas are full used. By combining interdisciplinary research methods, Nanfen iron mine, Benxi, Located in Liaoning Province, northeast China(Asia’s largest single open pit iron ore mine) as the research background, field geological survey, and indoor mechanics tests on the samples sampling were carried out. The main factor of slope disasters induced by coal mining area was pointed out. Use of indoor physical model experiment and Numerical Calculation of three-dimensional finite-difference software, and a detailed study of weak structural plane mechanism slope landslide disaster were made. Based on the research foundation, reinforcement-monitoring-early warning measures of open-pit mine slope disaster were raised, and good practice effects in the field application was obtained.Therefore, the related research work has been done and the main research contents and achievements are as follows:(1) In order to identify landslide geology situation in Nanfen iron open-pit mine and obtain first-hand geological data, the comprehensive geological survey was firstly carried out. After scene investigation, rock in mining area can be divided four types of structures, namely the broken plate crack, crack block, and split the four-block structure types. As the focus of study and treatment landslide priority area, landslides occurred many times in history at the upper slope of footwall.Rock mass at 334 m platform, 346 m platform and 370 m, in Nanfen open-pit iron mine, become loose crushing serious. Based on the different characteristics of the dielectric caused by different propagation characteristics of electromagnetic wave, GR-IV geological radar was used. A measuring line in each of the three steps on the footwall is set and the shallow fracture zones within ten meters are detected respectively. After analysis of the sectional view generated from the measuring line in each step, it shows that fracture zones among different measuring points on the three platforms are unevenly distributed, as rock mass is complete and fracture appears. The geological radar detection result is consistent with the field geological drilling results. By the extraction of detecting cross-sectional view of the above three steps, a slope sectional view I-I’ is drawn. It shows that the maximum depth is in the 370 m platform fracture zone, which reaches 6-9 m.(2) In order to do indoor experimental research on the mechanical properties of rock mass, site acquisition of slope epidote amphibolite rock samples on the footwall of mine is made, using TGQ-Q5 shallow sampling rig which is researched and developed by State Key Laboratory for GeoMechanics and Deep Underground Engineering(Beijing) independently. Uniaxial compressive test, triaxial compression test, tensile test and analysis of mineral composition and microstructure are done.(3) Based on the similarity principle, Nanfen iron open-pit slope geological information was analyzed and three-step test excavation and weak layer strength soften experiment were designed.The bottom of the slope excavation including three-step test, each step interval is set to 300 seconds. Infrared thermal imaging infrared energy is recorded during the entire excavation experiment process. Using real-time dial & Electronic records the toe of slope horizontal displacement and the vertical displacement situation at point 3. 1 dial indicator measurements horizontal and vertical displacement of slope. 2 dial indicator to measure the horizontal and vertical displacement of the sliding bed rock. Using the camera records the law effect of excavation on the slope.Displacement Monitoring results show that every excavation step work makes horizontal displacement increased of monitoring point 3. Maximum horizontal displacement of the excavation end is up to 0.06 mm, and the vertical displacement of point 3 is smaller, maximum displacement reached 0.01 mm. The horizontal and vertical displacement of 1 and 2 monitoring points had not changed significantly. Comparison of the three points’ displacement monitoring shows that excavation activities at the slope foot has the greatest impact, and influence on horizontal displacement of slope at the foot is greater than the vertical effects.During the process of three steps excavation, infrared radiation energy density distribution of slope has no significant changes. That is, there has not been a significant change of temperature field model of slope describes the deformation of slope excavation not apparent or slip movement.(4) In order to study the effects of weak strength of structural plane on stability of slope excavation, after the excavation test, reducing the strength of weak plane tests is carried out.Strength parameters of thermal material mechanical properties decreased with the increase of temperature. Weak structural planes simulated by heat-sensitive materials and rock mass simulated by finite element planes. The main measuring instruments including the displacement of dial indicator, cameras and infrared thermal imager.Experimental results show that, influence of excavation on slope stability is limited. Weak structural plane strength weakening, however, has great impact on slope stability. Hence, the phenomena "no rain no sliding" are well explained. Weak structural plane of slope sliding mass is general broken. Long weathering agent makes weak structural plane strength decline even in rainfall situation, which leads to a worsening the mechanical indexes of soft layer and extremely slope instability.Based on the above experimental results, summarized the following rules: 1th monitoring sites proves weak resistance to decline due to overlying layer of horizontal and vertical displacement of rock mass displacement increased significantly. 3 monitoring points in the toe area, monitoring results show that weakening caused by the landslide strength of weak interlayer slip, the sliding slope foot parts affecting the vertical and horizontal displacement of rock mass displacement effect. Results of 2nd monitoring points show that the sliding body did not affect sliding bed.(5) Firstly, comparative calculations under strength reduction of general slope with weak structural plane and slope without weak structural plane are made, using three-dimensional finite difference numerical calculation program FLAC3 D. For the slope without weak structure plane, the shear strain increment and plastic zone of slope gradually expand with the reduction factor increases, until penetrated. The maximum displacement occurs at the foot of the slope. The final failure surface of slope shows arcuate surface damage. For the slope with structural plane, the shear strain increment and plastic zone of slope gradually expand with the reduction factor increases. But the place where the shear strain increment and plastic zone first expand appears near the weak structural plane. And the shear strain increment and expansion of the plastic zone is along with the development of weak structure plane. The displacement of weak structural plane is relatively large. Comparing the two groups of weak structural plane with different length and location, it’s seen that the longer the weak structural plane length, the more significant the adverse influence on the slope it results in.(6) Concerning the characteristic of Nanfen open-pit iron mine that the footwall of slope contains a 48° weak structural plane in the upper part and a 13° weak structural plane in the lower part, research of the influence of stepwise excavation on stability of slope is done. In order to highlight the comparison of different effects on the slope whether there is structural plane or not, a three-dimensional model of the slope which contains 2 weak structural planes and a three-dimensional model of the slope which contains no weak structural plane are established.In the case that no excavation is conducted, the x-direction displacement field of the slope containing weak structural plane is different from the slope without weak structural plane. It’s embodied that the displacement to free surface is larger of the rock mass which is cut by the weak structural plane. The maximum displacement occurs at the foot of the lower part of the weak fault which shows that the maximum displacement of the slope declines under weak faulting. Influenced by the weak structural plane, the displacement value and range in z-direction all increases. For the slope containing weak structural plane, the shear strain increment cloud appears in the lower part of 13 ° weak structural plane. But for the slope without weak structural plane, the shear strain increment shows an arc, and not obvious. There is a penetration in plastic zone of free surface both in slope with weak structural plane and slope without weak structural plane. This may be the result of the existence and penetration of plastic zone on the surface of slope which is caused by the excavation unloading. But there is also a certain extent of penetrations in the plastic zone of the slope, which has weak structural plane along the upper and lower two weak structural planes. It shows no cases of penetration in plastic zone outside the slope surface without weak structural plane. Therefore, the existence of weak structure plane changes the displacement in x and z directions, shear strain increment, plastic zone distribution, and reduce the slope stability in the slope.(7) Slope monitoring and warning system combines the users and slope project site to a closed loop system of information collection, transmission, feedback, and provides a powerful real-time guarantee of the safety in slope monitoring area on site. At the same time, strengthening, monitoring and early warning integrated solution is used to avoid slope hazards owing to the use of cables with constant resistance and large deformation.That the slope sliding force is greater than the stabilizing force is a necessary and sufficient condition for the occurrence of landslides. Based on this scientific proposition, a strengthening, monitoring and warning system of slope hazard is developed, which mainly consist of four subsystems: Constant resistance and large deformation anchor cable subsystem, two-way data communication subsystem, data receiving and processing subsystem and intelligent power supply subsystem.Based on the previous studies, five kinds of monitoring and warning modes of slope hazard monitoring curve are further improved: Stable mode, secondary stable mode, fracture mode, shuffle mode and disturbance mode. Therefore, good application effect in field is achieved.