In the field of mining engineering, engineering rock mass classification is a crucial foundational task that plays an indispensable role in ensuring construction safety, reducing costs, and improving efficiency. In mining operations, the characteristics of rock masses are complex and variable, with significant differences in hardness, integrity, and structure across different areas. These differences directly affect multiple key aspects of the mining process.

From the perspective of construction safety, accurate engineering rock mass classification can provide reliable safety assurance for mining operations. Misjudging the properties of the rock mass may lead to serious safety accidents. For example, in areas where rock mass stability is poor, if mining is conducted according to the standards for stable rock masses, it is highly likely to cause collapse accidents, posing a great threat to the lives of workers. Through scientific engineering rock mass classification, the stability status of rock masses in different areas can be clearly understood, allowing for targeted support measures to be taken in advance to effectively prevent accidents. In terms of cost control, engineering rock mass classification also plays a key role. During mining, support costs are a significant expense. If the classification is inaccurate, over-support or insufficient support may occur. Over-support leads to unnecessary financial investment and increased mining costs; insufficient support may cause safety accidents, resulting in greater economic losses. Reasonable rock mass classification helps precisely determine the required type and intensity of support, achieving optimized resource allocation and reducing unnecessary costs. Regarding improving mining efficiency, engineering rock mass classification is also indispensable. Different grades of rock masses can adopt corresponding mining processes and equipment. For hard and intact rock masses, large and efficient mining equipment can be used to speed up mining progress; for fractured and weak rock masses, more cautious mining methods are needed to avoid instability caused by improper mining. Thus, selecting appropriate mining schemes based on rock mass classification can effectively improve mining efficiency and accelerate project progress.

BQ Engineering Rock Mass Classification Introduction

(1) Overview of the Two-Step Classification Method The national standard "Engineering Rock Mass Classification Standard" GB 50218—1994 proposes a comprehensive and scientific engineering rock mass classification method — the Two-Step Classification Method.

This method is like a blueprint for building a house, providing clear ideas and steps for accurately assessing rock mass quality and stability. In actual engineering, faced with complex and diverse rock mass conditions, the Two-Step Classification Method can analyze and judge in an orderly manner, ensuring the accuracy and reliability of classification results. The first step is preliminary classification based on the basic quality index BQ of the rock mass. This step mainly focuses on the inherent properties of the rock mass, determined by the hardness of the rock and the integrity of the rock mass. The hardness and integrity are intrinsic characteristics of the rock mass, independent of engineering factors, and represent common features of the rock mass. By considering these two key factors, the rock mass quality can be initially classified, laying the foundation for subsequent analysis. The second step involves considering other influencing factors such as natural stress, groundwater, and structural plane orientation to modify BQ according to the characteristics of various engineering rock masses, then performing detailed classification based on the modified BQ. In actual engineering, different types of projects have different requirements for rock masses, and environmental conditions also significantly affect stability. In underground engineering, the presence of groundwater may soften the rock mass and reduce its strength; natural stress may cause deformation or even damage; different structural plane orientations also affect stability. Therefore, this step requires comprehensive consideration of these factors to modify the preliminary classification results, obtaining a detailed classification that better fits actual engineering conditions.

(2) Detailed Explanation of Basic Rock Mass Quality Classification

Hard rock has a saturated uniaxial compressive strength greater than 60MPa and is hard in texture, such as granite and quartzite. This type of rock exhibits strong bearing capacity and stability in engineering, able to withstand large loads without easily failing. In mining engineering, mining hard rock usually requires large mechanical equipment and efficient mining processes to improve mining efficiency. Relatively hard rock has a saturated uniaxial compressive strength between 60 and 30MPa, with hardness and bearing capacity slightly weaker than hard rock but still good engineering performance. In some projects where rock strength requirements are not very high, relatively hard rock can serve as good foundation material. Relatively soft rock has a saturated uniaxial compressive strength range of 30 to 15MPa; this type of rock has lower strength and requires special attention to stability in engineering. In underground engineering, relatively soft rock is easily affected by groundwater and geostress, causing deformation and damage, so corresponding support measures are needed to ensure safety. Soft rock has a saturated uniaxial compressive strength of 15 to 5MPa, with even weaker strength and poorer stability in engineering. Soft rock tends to undergo plastic deformation under external forces, leading to structural damage. In mining, encountering soft rock requires cautious mining methods and strengthened support and reinforcement measures. Extremely soft rock has a saturated uniaxial compressive strength less than 5MPa and almost no bearing capacity, requiring special treatment in engineering. Extremely soft rock usually needs reinforcement or replacement measures to meet engineering requirements.

A complete rock mass has an integrity coefficient Kv greater than 0.75, with few structural planes, good overall rock mass integrity, and high stability. Engineering construction in such rock masses is relatively easier, and the safety of the project is higher. When constructing underground tunnels in complete rock masses, support measures can be relatively simple. Relatively complete rock masses have an integrity coefficient between 0.75 and 0.55, with fewer structural planes and minimal impact on rock mass stability. These rock masses also perform well in engineering, but in projects with higher stability requirements, certain reinforcement and support are still necessary. More fractured rock masses have an integrity coefficient between 0.55 and 0.35, with more structural planes, affecting rock mass stability to some extent. Engineering construction in more fractured rock masses requires enhanced support and monitoring to prevent rock mass instability. In mining engineering, when extracting more fractured rock masses, collapse accidents are prone to occur, so effective support measures such as bolt support and cable support are needed. Fractured rock masses have an integrity coefficient between 0.35 and 0.15, with developed structural planes and poor rock mass stability. These rock masses pose significant safety hazards in engineering and require strict treatment and reinforcement. In underground engineering, for fractured rock masses, support methods such as shotcrete and steel supports are usually adopted to improve rock mass stability. Extremely fractured rock masses have an integrity coefficient less than 0.15, with very developed structural planes, the rock mass almost losing integrity, and extremely poor stability. Extremely fractured rock masses are the most unfavorable condition in engineering and require special treatment, such as grouting reinforcement and strong support measures. When using this formula, the following conditions must be strictly observed: when σ > 90K+30, take σ = 90K+30 to substitute into the formula to calculate the BQ value; when K > 0.04σ+0.4, take K = 0.04σ+0.4 to substitute into the formula to calculate the BQ value. These conditions are set to ensure the calculation of the BQ value is more accurate and reasonable, avoiding errors caused by improper parameter values.

Grade I rock mass, with a BQ value greater than 550, is hard rock with a complete rock mass, representing the best quality and highest stability. Engineering construction in such rock masses almost does not need to worry about rock mass instability and can use relatively simple construction methods and support measures. Grade II rock mass, with a BQ value between 550 and 451, includes hard rock with relatively complete rock mass and relatively hard rock with complete rock mass. The quality and stability of Grade II rock masses are also good, but in some large projects or projects with high stability requirements, appropriate monitoring and support are still needed. Grade III rock mass has a BQ value range of 450 to 351, with average rock mass quality and stability. Engineering requires corresponding support and reinforcement measures based on specific conditions. In underground engineering, for Grade III rock masses, support methods such as bolts and cables may be needed to improve rock mass stability. Grade IV rock mass, with a BQ value between 350 and 251, has poor rock mass quality and stability, requiring enhanced support and monitoring to ensure project safety. In mining engineering, when extracting Grade IV rock masses, close attention to rock mass deformation is necessary, and timely support measures should be taken to prevent collapse accidents. Grade V rock mass, with a BQ value less than 250, is the poorest quality and lowest stability rock mass, requiring special treatment and strict support in engineering. For Grade V rock masses, high-strength support structures such as steel supports and concrete linings are usually needed to ensure project safety.

Key Points of Rock Mass Stability Classification

(1) Analysis of Factors Affecting the Stability of Engineering Rock Mass (Surrounding Rock)

In addition to being closely related to the basic quality of the rock mass, it is also significantly affected by various factors, among which groundwater, main weak structural planes, and natural stress are the most critical. These factors interact and influence each other, jointly determining the stability condition of the rock mass. Groundwater has multiple impacts on rock mass stability. The presence of groundwater increases the water content of the rock mass, leading to rock softening and thus reducing rock strength. In some soft rock areas, long-term soaking by groundwater can significantly reduce the compressive strength of the rock, increasing the risk of rock mass instability. Groundwater also generates hydrostatic and hydrodynamic pressures. When fractures exist in the rock mass, groundwater accumulates in these fractures, producing hydrostatic pressure, which can damage the rock mass structure. Hydrodynamic pressure causes scouring and transport of particles within the rock mass, further damaging its integrity. During tunnel construction, encountering areas rich in groundwater may cause water inflow that leads to surrounding rock collapse, posing great difficulties and safety hazards to construction. The main weak structural planes are weak links within the rock mass and play an important controlling role in rock mass stability. The presence of weak structural planes reduces the overall integrity and strength of the rock mass, making it prone to sliding, shearing, and other failure modes along these planes under stress. In blocky or layered rock masses, when the relationship between weak structural planes and the tunnel axis is unfavorable, or when two or more sets of weak structural planes appear, easily detachable separated rock blocks may form, triggering rock mass instability. In slope engineering, the presence of weak structural planes may cause slope landslides, posing serious hazards to the project and surrounding environment. Natural stress is formed during the long geological history of the rock mass and its influence on rock mass stability cannot be ignored. The magnitude and direction of natural stress affect the deformation and failure modes of the rock mass. In high ground stress areas, strong stress release and rebound occur during excavation, causing rock fracturing, spalling, and other phenomena, seriously affecting project safety and construction progress. When natural stress in the rock mass exceeds its strength, failure occurs, impacting project stability. In deep-buried tunnel construction, high ground stress may cause rock bursts and other disasters, posing serious threats to construction personnel and equipment. In summary, when grading rock mass stability, it is essential to fully consider the effects of groundwater, main weak structural planes, and natural stress to ensure the accuracy and reliability of grading results, providing a scientific basis for project design and construction.

(2) Determination of Correction Coefficient

When the angle α between the structural plane strike and tunnel axis is ≤ 30°, and the dip β is 30°~75°, this combination has a significant impact on rock mass stability, with K₂ values ranging from 0.4 to 0.6. Because in this case, the angle between the structural plane and tunnel axis is small, sliding along the structural plane more easily adversely affects the stability of the tunnel chamber. In some underground projects, when encountering this structural plane attitude, the rock mass stability significantly decreases, requiring enhanced support measures. When the angle α between the structural plane strike and tunnel axis is > 60°, and the dip β is > 75°, the structural plane attitude is relatively favorable, with less impact on rock mass stability, and K₂ values range from 0 to 0.2. At this time, the sliding direction of the structural plane has little relation to the stability of the tunnel chamber, so the correction coefficient is small. In some projects, when encountering this structural plane attitude, the rock mass stability is relatively good, and support measures can be relatively simplified. For other combinations, K₂ values range from 0.2 to 0.4, with an impact on rock mass stability between the above two cases. In practical projects, it is necessary to accurately determine the value of K₂ based on the specific structural plane attitude and its combination with the tunnel axis to reasonably assess rock mass stability.

When the BQ value is greater than 450 and groundwater is damp or dripping water outflow, the impact of groundwater on rock mass stability is small, and K₁ is taken as 0. In some areas with better rock mass quality, this groundwater condition has almost no effect on rock mass stability, so the correction coefficient is 0. When the BQ value is between 450 and 350, and groundwater is rain-like or gushing water outflow with water pressure ≤ 0.1MPa or unit water volume 10L/min, K₁ ranges from 0.2 to 0.3. At this time, the water outflow volume and pressure have some impact on rock mass stability, requiring appropriate consideration of the correction coefficient. In some tunnel projects, when encountering this situation, rock mass stability is affected to some extent, requiring corresponding drainage and support measures. For other combinations of BQ values and groundwater conditions, the value of K₁ should also be accurately determined according to the specific situation to reflect the impact of groundwater on rock mass stability.

In extremely high stress zones, regardless of the BQ value, the K₃ value is relatively large, at 1.0 or above. This is because extremely high stress has a very significant impact on rock mass stability, requiring a larger correction factor to reflect its influence. In underground engineering with high ground stress, the rock mass is subjected to great stress during excavation, making it prone to failure, so the correction factor is larger. In high stress zones, the K₃ value is relatively smaller, around 0.5. High stress has some impact on rock mass stability, but it is smaller compared to extremely high stress zones, so the correction factor is also smaller. In some medium-depth projects, although the stability of the rock mass under high stress is somewhat affected, appropriate support measures can still ensure the safety of the project. (3) Correction value [BQ] calculation and classification. The correction value [BQ] is calculated by the formula: [BQ] = BQ - 100 (K₃ + K₁ + K₂). In this formula, BQ is the basic rock mass quality index, and K₁, K₂, K₃ are the correction factors for groundwater influence, main weak structural plane attitude influence, and natural stress influence, respectively. This formula comprehensively considers various factors affecting rock mass stability to obtain the corrected rock mass quality index [BQ]. In a certain underground project, the known basic rock mass quality index BQ is 400, groundwater influence correction factor K₁ is 0.3, main weak structural plane attitude influence correction factor K₂ is 0.4, and natural stress influence correction factor K₃ is 0.5. According to the formula, the correction value [BQ] is calculated as: [BQ] = 400 - 100 × (0.3 + 0.4 + 0.5) = 400 - 120 = 280. The engineering rock mass classification is then performed based on the corrected value [BQ], following the previously mentioned BQ rock mass quality classification table.

The physical and mechanical parameters of rock masses at each level and the self-supporting ability of surrounding rock are shown in the following table:

Class I rock mass, with a corrected value [BQ] greater than 550, density greater than 2.65g/cm³, high shear strength, and a relatively large deformation modulus, with a Poisson's ratio of 0.2. In such rock masses, spans ≤20m can remain stable for a long time, with only occasional block falls and no collapse. In the construction of some large underground factories, encountering Class I rock mass means relatively low construction difficulty and higher project safety. Class II rock mass, with a corrected value [BQ] between 550 and 451, has physical and mechanical parameters and self-supporting ability slightly weaker than Class I. Spans of 10–20m can be basically stable, but local block falls or small collapses may occur; spans <10m can remain stable for a long time, with occasional block falls. In the construction of some urban subway tunnels, when encountering Class II rock mass, certain support measures need to be taken to ensure construction safety and tunnel stability. Class III rock mass, with a corrected value [BQ] between 450 and 351, has general stability. Spans of 10–20m can be stable for several days to one month, with possible small to medium collapses; spans of 5–10m can be stable for several months, with possible local block movement and small to medium collapses; spans <5m can be basically stable. In the excavation of some small mines, encountering Class III rock mass requires enhanced support and monitoring to promptly address potential rock mass instability. Class IV rock mass, with a corrected value [BQ] between 350 and 251, has poor stability. Spans >5m generally have no self-supporting ability and may experience loosening and small collapses within days to months, which can develop into medium to large collapses. At shallow depths, loosening mainly occurs at the arch; at greater depths, obvious plastic flow and extrusion damage occur; spans ≤5m can be stable for several days to one month. In the construction of some deep-buried tunnels, encountering Class IV rock mass presents greater construction difficulty, requiring advanced construction techniques and support methods to ensure safety. Class V rock mass, with a corrected value [BQ] less than 250, is the least stable rock mass with almost no self-supporting ability. Engineering in such rock masses requires special treatment measures, such as grouting reinforcement and high-strength support structures, to ensure safety. In areas with complex geological conditions, encountering Class V rock mass necessitates detailed geological surveys and analyses to develop reasonable engineering plans. By accurately calculating the corrected value [BQ] and classifying based on the physical and mechanical parameters and self-supporting ability of each rock mass class, important references for engineering design and construction can be provided to ensure safety and smooth progress.

Introduction to RMR Rock Mass Geomechanical Classification

(1) Composition of Classification Indicators RMR rock mass geomechanical classification is a widely used rock mass classification method in engineering, proposed by the Council for Scientific and Industrial Research (CSIR) of South Africa. This classification method comprehensively considers multiple factors, with indicators including rock block strength, RQD value, joint spacing, joint conditions, and groundwater—five key indicators. These indicators reflect the characteristics of the rock mass from different perspectives, collectively forming a comprehensive rock mass classification system. Rock block strength is an important indicator measuring the load-bearing capacity of the rock mass, directly affecting the stability of the rock mass under engineering loads. Rock block strength can be measured by point load strength index or uniaxial compressive strength. The point load strength index is a quick and simple test method that applies concentrated load to the rock block to measure its failure strength, thereby assessing the strength characteristics. Uniaxial compressive strength is a more precise laboratory test where axial pressure is applied to the rock block to measure the maximum pressure it withstands at failure. RQD value, or Rock Quality Designation, refers to the proportion of core length greater than or equal to 10cm obtained by drilling with a diamond drill bit of diameter not less than or equal to 75mm and double-tube core barrels, relative to the total drilling length. The RQD value intuitively reflects the integrity of the rock mass and is an important parameter for evaluating rock mass quality. A higher RQD value indicates a larger proportion of long cores in the rock mass, better integrity, and relatively higher stability; conversely, a lower RQD value indicates more broken cores, poorer integrity, and correspondingly reduced stability. Joint spacing reflects the degree of joint development in the rock mass and significantly affects the mechanical properties and stability. Larger joint spacing indicates sparser joint distribution, better overall integrity, and relatively higher strength and stability; smaller joint spacing indicates dense joint development, damaged integrity, and reduced strength and stability. In practice, joint spacing is usually measured through field geological surveys and mapping. Joint conditions include factors such as joint surface roughness, continuity, width, and hardness of the joint surface rock. Rock masses with very rough joint surfaces, discontinuous joints, zero joint width, and hard joint surface rock have relatively better stability; rock masses with smooth joint surfaces or containing weak interlayers, large openings, and continuous joints have poorer stability. Evaluation of joint conditions requires detailed analysis of various joint features through field observation and laboratory testing. The presence of groundwater affects the physical and mechanical properties of the rock mass in multiple ways, thereby influencing stability. Groundwater can soften the rock, reducing its strength; it can generate hydrostatic and hydrodynamic pressures that damage the rock structure. In the RMR rock mass geomechanical classification, groundwater conditions are reflected by indicators such as water inflow per 10m of tunnel length or groundwater pressure. Greater water inflow or higher pressure indicates a larger impact of groundwater on rock mass stability.

(2) Scoring and Correction Method: When conducting RMR rock mass geomechanical classification, scoring is first performed according to specific standards based on the values of various indicators. For rock block strength, if the point load strength index is used, a score of 15 is given when the index is greater than 10; between 4 and 10, the score is 12, etc. If uniaxial compressive strength is used, a score of 15 is given when it is greater than 250MPa; between 100 and 250MPa, the score is 12, etc. For RQD values, a score of 20 is given when it is between 90% and 100%; 17 when between 75% and 90%, etc. When joint spacing is greater than 200cm, the score is 20; between 60 and 200cm, the score is 15, etc. Regarding joint conditions, when joint surfaces are very rough, joints are discontinuous, joint width is zero, and joint surface rock is hard, the score is 30; when joint surfaces are slightly rough, width is less than 1mm, and joint surface rock is hard, the score is 25, etc. For groundwater conditions, when the water inflow in a 10m long tunnel is zero, the score is 15; less than 10L/min, the score is 10, etc. The total RMR value is obtained by summing the scores of these five indicators. After obtaining the RMR value, it needs to be corrected based on joint orientation. The influence of joint strike or dip varies for different projects (tunnels, foundations, slopes), and the scoring correction values differ accordingly. In tunnel projects, when the joint strike is perpendicular to the tunnel axis and excavation is along the dip, the joint orientation is very favorable, and the correction value is 0; when the joint strike is parallel to the tunnel axis and excavation is against the dip, the joint orientation is very unfavorable, and the correction value is -12. In foundation projects, the correction value is 0 when joint orientation is very favorable; -25 when very unfavorable. In slope projects, the correction value is 0 when joint orientation is very favorable; -60 when very unfavorable. This correction more accurately reflects the impact of joint orientation on rock mass stability, making the RMR value more consistent with actual engineering conditions.

(3) Application of Classification Results: Based on the corrected RMR value, the category of the studied rock mass and the corresponding self-supporting time and rock strength parameters (c, φ) for unsupported underground works can be determined by referring to the table. When the RMR value is between 100 and 81, the rock mass is classified as Class I, described as very good rock. In such rock, unsupported underground works with a 15m span can remain stable for 20 years, with rock cohesion greater than 400kPa and internal friction angle greater than 45°. This type of rock has good stability in engineering construction and relatively low construction difficulty. When the RMR value is between 80 and 61, the rock mass is classified as Class II, considered good rock. Unsupported underground works with a 10m span can remain stable for 1 year, with rock cohesion between 300 and 400kPa and internal friction angle between 35° and 45°. For this type of rock, certain support measures need to be taken during design and construction to ensure safety and stability. When the RMR value is between 60 and 41, the rock mass is classified as Class III, general rock. Unsupported underground works with a 5m span can remain stable for 7 days, with rock cohesion between 200 and 300kPa and internal friction angle between 25° and 35°. In practice, for Class III rock, appropriate support and reinforcement schemes should be developed according to specific conditions to meet engineering requirements. When the RMR value is between 40 and 21, the rock mass is classified as Class IV, poor rock. Unsupported underground works with a 2.5m span can remain stable for 10 hours, with rock cohesion between 100 and 200kPa and internal friction angle between 15° and 25°. For Class IV rock, engineering construction faces greater challenges and requires enhanced support and monitoring to ensure safety. When the RMR value is less than 20, the rock mass is classified as Class V, very poor rock. Unsupported underground works with a 1m span can only remain stable for 30 minutes, with rock cohesion less than 100kPa and internal friction angle less than 15°. Construction in such rock is extremely difficult and requires special treatment measures such as grouting reinforcement and strong support to ensure safety. In a certain underground tunnel project, by measuring and analyzing various rock mass indicators, an RMR value of 70 was obtained. After correction for joint orientation, the rock mass was finally classified as Class II. Based on this classification, the engineering designers selected appropriate support methods and parameters to ensure the safety and smooth progress of tunnel construction. During tunnel construction, monitoring of rock stability showed that actual conditions matched the RMR classification results, further verifying the reliability and practicality of this classification method. The RMR rock mass geomechanical classification results have wide applications in engineering. In underground engineering, it provides important basis for the design and construction of tunnels, underground plants, etc., helping engineers determine reasonable support schemes and construction methods. In slope engineering, RMR classification results can be used to assess slope stability, predict deformation and failure modes, and provide references for slope reinforcement and protection. In foundation engineering, RMR classification helps understand rock bearing capacity and deformation characteristics, providing data support for foundation design.

BQ engineering rock mass classification, as a scientific and systematic grading method, holds a pivotal position in various rock mass engineering fields such as mining engineering. Its two-step grading method is logically rigorous: the first step calculates the BQ value by considering rock hardness and rock mass integrity to achieve preliminary classification, capturing the basic properties of the rock mass and laying a solid foundation for subsequent analysis. The second step fully considers factors such as groundwater, major weak structural planes, and natural stress to correct the BQ value, thereby achieving detailed classification and making the results more aligned with actual engineering conditions. In practical applications, BQ engineering rock mass classification provides multiple key supports for mining engineering. During the planning and design phase of mining engineering, BQ classification clearly reveals the quality and stability status of the rock mass, enabling reasonable selection of mining methods and processes. For stable Class I and II rock masses, efficient large-scale mining methods can be used; for less stable Class IV and V rock masses, more cautious mining processes and enhanced support measures are required. During construction, BQ classification results guide construction personnel in reasonably arranging construction sequences and schedules, taking safety measures in advance to effectively prevent accidents. In a certain underground mine extraction, BQ engineering rock mass classification accurately judged the stability of rock masses in different areas, and reinforcement support was carried out in advance for less stable areas, avoiding collapse accidents during extraction and ensuring the safety of construction personnel and smooth progress of the project.

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