According to statistics, in all accidents in open pit mining, casualties caused by slope instability accounted for about 50%. Therefore, the effective control of such accidents will greatly improve the safety of open pit mining. After fully familiarizing with the causes of slope damage in open pit mines and the characteristics of slopes, it is necessary to conduct on-the-spot investigation and actual survey to gain an in-depth understanding of the rock type, rock mass structure surface, groundwater and surface water, and the slippage of the slope in the previous mining. And the nature and characteristics of the natural slope angle, and then identify the type and law of the failure of the predicted slope, and on this basis to improve the structure of the slope, reduce or control the serious consequences of the slope damage.
(1) Rock mass structure surface and its influence
The rock mass structure surface is a geological interface of surface, seam, layer and strip with certain direction, scale, shape and characteristics in the rock formation process. The surface is the cleavage, jointing, bedding, and morphing without any filling between the rock blocks; the seam is a crack with a certain thickness and a certain thickness; the layer is a relatively weak interlayer in the rock layer, and there are obviously two upper and lower layers. The layer is a structural fracture zone with a certain thickness, a contact fracture zone, an ancient weathering crust and a trough. The rock mass structural plane is the decisive factor affecting the stability of the slope, which directly restricts the occurrence and development of the rock mass deformation and failure. Slope failure and instability often occur along the structural plane of the rock mass.
1. Genetic types and characteristics of rock mass structural planes: Rock mass structural planes can be divided into primary structural planes, structural structural planes, and secondary structural planes. The primary structural plane is the structural plane left in the formation of rock mass, such as layer bedding, contact between rock vein and rock wall, primary condensation joint, lamella, weak interlayer, etc.; structural structural plane is joint, fault, and interlayer displacement Etc.; secondary structural surface is caused by various weathering, mining, blasting and other cracks.
2. Grading of rock mass structural plane: The grading study on the scale of rock mass structural plane and its effect on rock mass stability can help to distinguish the primary and secondary, master the structural plane that plays a leading role in rock mass stability, and determine The right governance plan. The grading standard proposed by the Institute of Geology of the Chinese Academy of Sciences is to divide the rock mass structure into five levels.
3. Impact of rock mass structure on slope stability: 1 The structural planes in the rock mass are weak, relatively broken and easy to weather. The gaps in the structural plane are often filled with weathering secondary minerals, so the shear strength is low and the integrity of the rock mass is destroyed. The formation of many landslides is not due to the ultimate strength failure of the rock, but the stability of the slope is determined by the strength of the structural plane. The landslide often occurs along the structural plane. 2 The existence of structural planes provides a good channel for the infiltration of surface water and the activity of groundwater. As a result of the water activity, the shear strength of the rock and structural surfaces is further reduced. 3 The sliding surface and edge contour of the sliding body controlled by the structural plane even determine the type of slope failure. Determining the structural plane and its strength in the slope rock mass is the main content of the slope stability design research. The role played by each structural plane is mutually influential and constrained. Therefore, the engineering geological evaluation of rock mass in any section must be specifically analyzed and researched according to the specific location. When studying the geological structure of the mining area, it is necessary to pay attention to the analysis of the existence of the tectonic stress field. If there is a tectonic stress, the nature, direction and size of the stress should be determined and considered in the slope stability analysis.
4. The structure faces the destructive effect of the slope: in the first section of the slope, there are generally several sets of dominant structural planes that play a leading role in the slope failure. The degree of damage to the stability of these structures depends on the slope. Structural surface characteristics and mutual cutting relationship and relationship between structural surface occurrence and slope free surface (slope surface). There are three typical types of damage: platform landslides, wedge landslides, and dumping landslides.
Plane landslide: The dominant structural plane has the same tendency as the slope surface, and the plane landslide often occurs when the inclination angle is slower than the slope angle.
Wedge landslide: There are two sets of dominant structural planes, and the tendency of the intersection of the structural planes is almost the same as that of the slopes. The wedge slopes often occur when the inclination angle is slower than the slope angle.
Dumping landslide: The dominant structural plane tends to be opposite to the slope, and the dumping landslide often occurs when the dip angle is steep.
(2) Lithology and its impact
1. Structure and structure of rock: Due to the different genetic types of rock, the internal structure and structure are also different, and the difference in lithology is also large.
The mechanical properties of rock depend on the mineral composition of the rock and on the type of bonding between the mineral particles (either crystalline or non-crystalline, such as cemented, loose, etc.) and the size of the crystalline particles. The crystal bond is combined and the crystal particles are small, and the mechanical strength thereof is high, and the mechanical strength is relatively low when the crystal is bonded and the crystal particles are large. For sedimentary rocks, since the strength of the crystalline particles is much greater than the strength of the inter-particle cement, under the action of external forces, the rock is always destroyed along the cement, rather than the crystal particles. Sedimentary rocks cemented with siliceous cements have greater strength, carbonates and clays have low rock strength and are easily corroded. Sedimentary rocks with basal cementation have greater strength.
The structure of the rock also has a significant impact on the mechanical properties of the rock. For example, the bedding of sedimentary rocks, the directional direction of metamorphic rocks and the mechanical properties in the vertical direction are very different and are anisotropic.
2. Porosity of rock: The porosity of rock has a certain influence on the water absorption and strength of rock. With the increase of porosity, the strength of rock decreases and the modulus of elasticity decreases.
3. Rock strength: The various strengths of rock can be listed as follows: three-way compressive strength> two-way compressive strength> one-way compressive strength> shear strength> flexural strength> tensile strength.
The compressive strength of rock is generally 3 to 10 times the shear strength and 10 to 50 times the tensile strength. The landslides in open pit mines are mostly shear failure, so the shear strength of rock is a necessary condition to measure the stability of slope rock mass. Hard and compact rock has high shear strength and is not prone to landslides. On the contrary, it is easy to landslide, and rocks are often collapsed due to shear failure.
In soft rock masses or sand bodies and loose topsoil with low cohesion, arc landslides are prone to occur. If there is a weak rock stratum at the foot of the slope, when the slope height is too high, the shearing and crushing of the slope foot is prone to occur, and the deformation is too large, resulting in the total damage of the upper rock mass.
(3) Groundwater and its effects
Infiltration of surface water and groundwater activities are often important causes of landslides in open pit mines. Generally, groundwater pressure can reduce the stability of slopes by 20% to 30%. The groundwater that is present in the fractures of the rock mass produces hydrostatic pressure on both walls of the fracture.
1. Hydrostatic pressure: It can increase the sliding force and reduce the frictional resistance, which is unfavorable to the stability of the slope. Generally, when the groundwater is higher than the sliding surface, the hydrostatic pressure can reduce the shear strength of the rock mass by 1/4 to 1/2.
2. The role of hydrodynamic pressure: When groundwater flows in a fracture or fault zone of a fractured rock mass, the pressure acting on the rock particles flowing through it is called hydrodynamic pressure, also known as osmotic pressure. When the hydrodynamic pressure is large, the rock particles in the fracture or fault fracture zone and the soluble components of the rock mass will be carried away by the water flow, so that the rock mass and friction are reduced, resulting in so-called erosive action. The erosive action will destroy the stability of the rock mass, especially when the groundwater flow and the structural surface are linked together, the threat to the stability of the slope is greater.
3. Water softening: For the rock mass developed by clayey rock mass and joint fissures, the cohesive force C and the internal friction angle φ decrease significantly with the increase of water content, and the shear strength decreases to 1/4~1/ when dry. 20.
Among the above several functions, the effect of hydrostatic pressure is more prominent. The effect of hydrostatic pressure is affected by the change of groundwater level. The groundwater level is affected by surface water, rainfall and permeability of rock mass, and changes within a certain range. Reducing the groundwater level is one of the main ways to improve the stability of the slope.
(4) Blasting and earthquakes and their effects
The effects of blasting vibration and earthquake on slope stability are basically the same. Only blasting is man-made and controllable, and earthquakes are natural disasters that humans are currently unable to control.
1. Blasting operation is a rock breaking method frequently used in open pit mines, which makes the open pit slopes subject to repeated blasting vibrations for a long time. When the blasting seismic wave passes through the rock mass, the potential damage surface of the rock mass is supplemented by the additional vibration force, which increases the scale of the primary structural plane and the structural structural plane, and generates a secondary structural plane (burst rupture gap), thereby affecting the slope. stability. 1 The damage caused by blasting vibration to the rock mass depends on the vibration speed of the rock mass. 2 When the rock mass vibration speed is less than or equal to 25.4cm/5, the intact rock mass will not be destroyed. 3 When the rock mass vibration velocity is between 25.4 and 63.5 cm/s, a small amount of spalling occurs in the rock mass. 4 When the rock mass vibrating speed is between 63.5 and 25.4 cm/s, strong tensile and several radial fractures occur.
2. Impact of earthquakes: In areas with frequent seismic activities, the impact on slope stability should be considered, especially for slopes that may cause significant losses after landslides. The seismic intensity of China is divided by 12 degrees.
(5) Slope geometry and influence
1. Slope height and slope angle: When the slope angle is fixed, the vertical height of the slope is more unstable. When the slope height is constant, the larger the slope angle, the more unstable. Therefore, the slope height and slope angle of different lithologies are clearly defined in the relevant safety regulations.
2. Horizontal section shape of the slope: Where the slope is prominent to the stope, the horizontal tensile stress on the surface is concentrated, which is easy to produce open secondary joints or expand the original joints, affecting the stability of the slope. Concave the surface of the slope of the stope, the horizontal compressive stress is concentrated, which can increase the frictional force that prevents the rock mass from falling down, thus contributing to the stability of the slope. If the slope is bent more horizontally, the part protruding to the stope is easily damaged by the tensile stress, which is not conducive to the stability of the slope.
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