CC BY
22
- © CAO Hui, QIN Xiu-shan, YUAN Ye,
YU Shi-bo, ZHANG Jun, WANG He, 2015
УДК 622. 834: 550.348.422
CAO Hui, QIN Xiu-shan, YUAN Ye, YU Shi-bo, ZHANG Jun, WANG He
EARLY WARNING TECHNOLOGY FOR LANDSLIDE OF HIGH AND STEEP SLOPE BASED ON JOINT MONITORING OF MICROSEISM AND DEEP SLIDING FORCE
The study summarizes and analyzes the status quo of monitoring technology of open slope and points out the major existing problems of current monitoring means: hysteresis quality for monitoring results, short time for early warning of critical sliding, insufficient criteria for critical sliding, etc., thus accurate advanced warning for landside of open slope cannot be realized. It proposes to adopt joint monitoring technology of microseism and energy absorption anchor cable, combine further research on slope instability mechanism of early warning norms of mines, establish integrated monitoring system with forewarning information of slope monitoring and reserve factor of rock mass strength as instability criterion, and realize medium and long-term monitoring and accurate early warning for critical sliding during the whole process of slope instability destruction. The successful application of the technology will possess very significant guiding significance for the stability control of open high and steep slope, monitoring of open slope, as well as safe production of mines. Key words: microseism monitoring; energy absorption anchor cable; high and steep open-pitslope; early warning of landslide
1
Foreword
Along with the increasing demands of mineral resources, people are gradually seeking for necessary mineral resources deep in the earth, and deep exploration has become the trend for future mining development. At present, domestic and foreign exploration of strip mines occupies a quite large proportion of ore output. In China, iron ore through opencast working accounts for about 80 %-90 %, nonferrous metal mine accounts for about 40 %-50 %, chemical materials mine accounts for 70 % and building materials mine accounts for 100 %, indicating large development potential for future opencast working [1—3]. Since the 1990 s, along with the constant development and utilization of resources, deep and hollow strip mine becomes the development trend of strip mines in the world, and currently, the largest strip mine - America Utah Bingham Copper Mine is 4,000m wide and 1,200m deep, with an area of 7.7 km2; large-scale Chuquicamata Copper Mine in Chile is 850m deep, and will reach 1,100m in 2020; Russia Udachnaya Diamond Mine has an exploration depth of over 600 m [4].
During the exploration of deep and hollow strip mine, along with the increase of slide and exploration depth, the slope stability and safety be-
come worse and worse, and to increase the slope angle is an important manner to fully recycle resources, to lower production cost and to enhance exploration efficiency. Therefore, deep research on the instability mechanism and forecasting technique of open high and steep slope in mine can be of great theoretical and practical significance for promoting prediction and prevention of geological disaster of high and steep slope and guaranteeing safe production in mines. The paper compares and analyzes the advantages and disadvantages of existing slide monitoring technologies, combines typical cases of open slope monitoring, and emphasizes on elaborating the unique advantages of joint monitoring technology of microseism and deep stress in the field of open slope monitoring field, thus providing important technical means for effective monitoring of strip mine slope.
2. Status quo, problem and development trend of slope monitoring technology
2.1. Status quo of slope monitoring technology
The complexity of slope geological conditions and influence factors determines that the analysis of slope stability must depend on the slope monitoring means for monitoring, control and verification. Based on the different monitoring objects, the slope monitoring technology, after many years' development, mainly includes various monitoring technologies with major monitoring objects such as surface displacement, groundwater pressure change, anchoring stress change and deep displacement, and monitoring instrument and method in combination with surface and deep stress-displacement [5], which can be divided into: (1) geodetic surveying of slope surface (theodolite, water level, diastimeter, electronic total station, etc.); (2) GPS monitoring; (3) displacement meter; (4) infrared remote sensing monitoring method; (5) laser monitoring of micro displacement; (6) SAR interfer-ometry, INSAR; (7) Time Domain Reflectometry (TDR) technology; (8) deep sliding force monitoring; (9) acoustic emission monitoring technology; (10) microseism monitoring technology, etc. Among these slope monitoring systems, (1)-(7) are mainly for surface monitoring of slope, and (8)-(10) are for deep monitoring under landslide mass surface.
2.2. Major problems for slope monitoring technology
From the analysis on landslide occurring mechanism, it is found
that the slippage instability is caused by a whole process of rock slope of high and steep strip mine, which often suffers rock microfracture within slope, fracture enlargement and glide plane cut-through before final occurrence of landslide due to functions of external rainfall, temperature and artificial disturbance. If displacement can be monitored on the slope surface, then maybe within the slope, microfracture has already occurred and even the glide plane has been cut through. Thus, the slope monitoring with major monitoring object of sur-
face displacement cannot avoid the hysteresis quality. If landslide is monitored during surface displacement of slope, the engineering technicians of the mine often can only try to evacuate all personnel and equipment as soon as possible so as to reduce loss, and they find it difficult to discover the landslide at an early stage and adopt treating measures to effectively delay the development process of landslide and timely control the development of landslide.
The monitoring technology with the major object of groundwater pressure is to drill a hole at the position to be monitored and install a water manometer to explore the groundwater distribution law inside side slope; earthquake and blasting vibration monitoring refers to the test on seismic wave resulting from blasting for obtaining the parameters such as maximum displacement, velocity, accelerated velocity, master frequency, fluctuation velocity, vibration duration, etc., so as to evaluate the impact of blasting on slope; anchoring stress monitoring is to monitor the high-sensitivity sensor on anchor cable and anchor rod in mining slope strengthening engineering, convert sliding force monitoring and calculating into sensor data monitoring and calculating, calculate the sliding force of landslide mass, and realize the stability monitoring and early warning for partial high-risk slope of anchor engineering. These monitoring technologies have unique advantages under specified conditions or in one aspect of slope stability evaluation, but are lack of the condition to comprehensively monitor all the mining slopes.
The monitoring technology for deep displacement of slope has been widely applied after years of development from early-stage multi-point displacement meter method for measuring the axial displacement of borehole and borehole clinometer method for measuring horizontal displacement to time-domain reflection technology, microseism/ acoustic emission monitoring technology, etc. Deep displacement monitoring technology of slope has obtained wide application [6]. Time-domain reflection technology obtains the surface or deep deformation information of slope by monitoring the deformation status of coaxial cable in borehole laid on the earth's surface or buried in slumped mass, which can be applied to monitor both the surface displacement as well as deep displacement, but the method has a disadvantage of «a peep hole view» inescapably, so does borehole clinometer, etc.
Microseism slope monitoring technology is to obtain rock failure information from the detector pre-buried in slope and conduct positioning analysis to failure point. Since being introduced into slope monitoring field in 1967, it has become an important development direction of strip mine slope monitoring technology field and been regarded as one of the important technologies to evaluate the stability of
rock mass. Although it is able to monitor the formation process of sliding surface of slope accurately, it cannot give early warning for critical sliding of the slope landslide.
2.3. Development trend of slope monitoring technology
As landslide means rock mass failure and instability, it changes from continuous deformation to discontinuous deformation. If the whole process of slope instability is monitored from different perspectives in combination with traditional slope surface displacement monitoring method, deep sliding force monitoring method, acoustic emission method and monitoring method for potential damage and sliding zone, it will certainly play an important role in guiding the slope stability evaluation and instability predication and research. Meanwhile, a method can be established to identify the rock mass strength parameters of strip mine slope based on multi-source monitoring data coupling analysis, thus enabling strength reduction method to have an objective basis, and monitoring and analysis to have solid mechanics basis. Besides, dynamic analysis and prediction for strip mine slope stability can be realized by taking precursory information and reserve factor of slope rock mass strength as instability criteria.
3. Joint monitoring method for microseism and energy absorption anchor cable
Beijing General Research Institute of Mining & Metallurgy has independently researched and developed an integrative analysis technology for slope microseism monitoring and deep sliding force monitoring after practice and research for many years. By combining the characteristics of the two monitoring early warning technologies, the whole-process monitoring for strip mine slope landslide and early warning for critical sliding of slope landslide can be realized, which has better promotion and application prospect.
3.1. Microseism monitoring principle and slope monitoring application
Under the action of external stress, concentration phenomena of elastic-plastic performance will appear inside rock partially and when energy accumulation reaches one critical value, rock microfissure will be generated and extended and the generation and extension of microfissure spread quickly in the surrounding rock with elastic wave or stress wave. The elastic wave is called microseisms (MS) geologically. Slope microseism monitoring technology refers to a method that utilizes the elastic wave resulting from rock excavation or construction to monitor the stability of engineering rock. These micro-fractures generate elastic wave by releasing elastic energy, and each microseism signal contains rich information about the status change inside rock mass, and can be received by the sensor within effective range.
After treatment and analysis for microseism signal received by multiple sensors (as shown in Fig. 1), the time, position and nature for rock micro-fracture, namely, three elements of «time, position and intensity» in geophysics, can be obtained. And it is possible to deduce the development trend of rock macrofracture based on the size, concentration degree and fracture density of micro-fracture, which refers to the core thought of mi-croseism monitoring technology. Only this monitoring technology is able to effectively «capture» the information of rock micro-fracture with rock slope instability premonition, and warn and forecast the «gradual deformation» of slope, thus enabling microseism monitoring technology to be applied in slope monitoring system successfully.
One strip mine analyzes the distribution rule of rock micro-fracture during large-scale rock excavation and construction process (from field entry of microseism monitoring system to completion of large-scale slope excavation), and collects 1,506 microseisms incidents. After noise identification, screening and rejection, there are 1,337 microseisms incidents and 1,217 blasting incidents in the effective research area [7]. The microseism incidents during the slope excavation process are strip-shaped on the hanging wall of controlling fault and in the lower area of fissure zone. In combination with geological information, the geological condition of the place where microseism incidents gather gets worse and there is a risk of landslide (Fig. 2, see Application, p. 437).
According to the above information, microseism monitoring information can timely reflect the generation, development, gathering, migration, evolution and extension laws of rock micro-fracture resulting from slope construction from the perspectives of space and time, and effectively identify and outline the activation degree of unknown fault or fissure zone, thus marking the rock deformation area inside slope rapidly and accurately, analyzing and evaluating the overall stability of slope qualitatively, and providing basis for the focused monitoring and qualitative analysis for potential landslide mass.
3.2. Principle and application of energy absorption anchor cable monitoring
Greater sliding force than sliding resistance force is the necessary and sufficient condition of landslide. Taking it as the main criterion of
Fig.1. The principle diagram of mi-croseismic monitoring
sliding of landslide mass, the equilibrium relationship between sliding resistance force and sliding force is raised. The relationship is the results of the combined actions of various factors such as lithology, structure, displacement, inclination of strata, variation in underground water level, rainfall, etc. in landslide mass, and determines the steady state of slope. Therefore, sliding force monitoring can reflect the motion characteristics and evolvement rules of landslide mass in a best manner.
However, the sliding force inside the landslide mass as «natural mechanical system» cannot be measured, but artificial mechanical system can be measured. Therefore, the measurable artificial mechanical system can be inserted into the immeasurable natural mechanical system to form a new complex mechanical system that some mechanical quantity is measurable, and the measurable artificial mechanical quantity can be adopted to calculate the immeasurable sliding force, so as to realize the advanced and punctual forecast of landslide disaster in essence.
The unmeasured sliding force can be expressed as [8]:
Fs = k p+k2
In which, k1 = cos(a + 9) + sin(a + 9) tan ^ , k2 = G cos a tan ^ - (U + V sin a) tan ^ + cl.
In the formula, P refers to disturbing force (perturbative force) (kN); k1 and k2 are constants, controlled by three influence factors of lithology, geometry and water; ty refers to the weighted average of internal friction angle of the various soil layers of slope landslide mass (°); c refers to the cohesion force of various soil layers of sliding surface (kPa); l refers to the length of sliding surface (m); a refers to the included angle between sliding surface and horizontal plane (°); Q refers to angle of incidence of anchor cable monitoring (°); U refers to pore water pressure borne by single-width sliding surface (kPa); V refers to the total pore water pressure borne by the trailing edge single-width tension crack surface (kPa). The values of U and V will depend on the slope structure and underground water under specific circumstances (Fig. 3, see Application, p. 437).
The scientific researchers have overcome the technological bottleneck in sliding force monitoring technology of open slope, and successfully developed the energy absorption anchor cable monitoring equipment that adapts to large deformation of open slope, which has been successfully applied in various high and steep strip mines in China. The following is the hazardous area of a certain high and steep open mine determined in combination with the data analysis of micro-seismic moni-
toring. Four monitoring points are set in the potential fault and landslide area by applying energy absorption anchor cable monitoring system, and one landslide is successfully forecast through monitoring for nearly one year [9], with an early warning time of 42 days (Fig. 4).
Fig. 4. The monitoring curves of sliding force and displacement in an open pit mine
Landslide remote real-time monitoring and forecasting system can punctually forecast «time of occurrence» and «place of occurrence» before the occurrence of landslide [10], and provides scientific basis for landslide disaster prevention and control. The characteristics of the full process of landslide evolution — development - slump are shown in Fig. 5 (see Application, p. 437), D refers to the average width of fracture, and H refers to the drop height on the two sides of fracture.
3.3. Characteristics of joint monitoring technology
It can also be seen from the mine examples that microseism monitoring can determine the main potential landslide area, and with the further combination of partial point monitoring of sliding force, the critical sliding early warning in the potential landslide area can be realized. Software and hardware system based on «microseism and energy absorption anchor cable joint monitoring» developed by BGRIMM adopts the modular design and cloud service technology, and integrates the parameters of microseism monitoring, sliding force monitoring, video monitoring, displacement monitoring, etc., and can flexibly choose monitoring parameters based on the actual on-site situation, ensure the safe storage, flexible
transfer and convenient downloading of monitoring data through cloud service terminal, really realize the integration on the hardware and coupling on the software of multisource monitoring technology, conduct temporal and long-term critical sliding early warning for the whole process of slope instability failure, and then ensure the safe and sustainable opening of mines. Microseism and sliding force joint monitoring technology has the following advantages:
1. Through the study on formation mechanism and intensity parameter characterization methods in the damage and failure area within slope rock mass, it builds the mechanical model of the rock mass damage evolution based on microseism monitoring and deep sliding force monitoring information inversion, establishes the correlation of microseism, sliding force characteristics of glide plane and dynamic instability of slope of internal »spatial» damage and deterioration of rock mass in slope instability process. With microseism precursory information and safety reserve factor of sliding force of glide plane in deep slope rock mass as instability criterion, the dynamic analysis and prediction on open slope stability can be successfully realized.
2. It conducts safe monitoring for the whole process of fracture generation, expansion, cut-through, landslide mass formation, slump and destruction of the strip mine high and steep slope rock mass by use of the comprehensive monitoring and early warning analysis technology combining microseism and deep sliding force, builds standard evaluation system through fusion and analysis of multi-dimensional monitoring data, determines the quantitative criterion for slope stability safety factor, and realizes the continuous and whole-process comprehensive analysis and stage-based prediction & early warning for slope rock mass in potential landslide area.
4. Conclusion
1. Strip mine slope is in a state of dynamic development, so its stability should be tracked and monitored all the way. Meanwhile, its stability evaluation is also dynamic along with the mining development, so open slope stability evaluation is a continuous feedback analysis process with the «interaction» of theoretical calculation and on-site monitoring.
2. The combination of micro-seismic monitoring and sliding force monitoring technology can not only analyze the position of surrounding rock failure and realize the advanced early warning, but also provide scientific theoretical method for instability prediction and forecast of surrounding rock failure through the inverse analysis of rock mass strength in further combination with sliding force monitoring data.
3. Along with constant perfection and improvement of comprehensive monitoring system in combination of microseism monitoring and sliding force monitoring technology, the method can provide a basis for rock mass failure of mine slope, and lay a foundation for guiding the safe and efficient production of mine. Microseism and sliding force comprehensive monitoring technology represents the development direction of the mine slope monitoring engineering in the future.
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ABOUT AUTHORS -
CAO Hui,QIN Xiu-shan,YUAN Ye,YU Shi-bo,ZHANG Jun,WANG He - Beijing General Research Institute of Mining and Metallurgy, Beijing, China.
