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Backfill Mining Method—Composition and Selection Principles of Backfill Materials—Heijingang Broadcast
Backfill Mining Method—Composition and Selection Principles of Backfill Materials
In fill mining, the composition of the filling material directly determines the strength of the fill body, construction efficiency, and mining costs. The core components fall into four main categories, and their selection must adhere to well-defined practical principles and be comprehensively evaluated in light of the mine’s actual operating conditions. The specific details are as follows.
I. Core Components of Filling Materials
Filling materials constitute a multi-component system, with each component playing a distinct and indispensable role. The specific classifications and core functions are as follows:
1. Filling aggregate: the skeletal core of the fill body, primarily serving to bear in-situ ground pressure, reduce the dosage of cementitious materials, and lower costs. Common types include mine waste rock and river sand, Tailings Among these, tailings sand is currently the most widely used aggregate type in mining due to its potential for the resource recovery and utilization of waste rock. Typically, the aggregate particle size is required to be ≤15 mm, with a silt content of ≤5%.
2. Cementitious materials: the key source of filling body strength, used to bind the aggregates and form a filling body with specified compressive and shear strengths, thereby preventing roof collapse in mined-out areas. The mainstream type is cement (P.O 42.5, P.C 32.5), and some mines also use fly ash in combination. Slag powder Additious materials are used to replace a portion of the cement in order to reduce costs.
3. Water: As the medium for the hydration reaction of the cementitious material, water also lubricates the aggregates and improves the flowability of the grouting slurry. The water dosage must be strictly controlled; excessive water reduces the strength of the grout, while insufficient water makes slurry delivery difficult.
4. Admixtures: Added as required by the construction process to optimize the performance of the grouting slurry; common types include Water-reducing agent , accelerators, and retarders. Water-reducing agents can decrease water demand and enhance the flowability of the grout; accelerators are used to speed up the setting and hardening of the filling material, thereby shortening the curing time; retarders are employed to slow down the hydration reaction, preventing premature setting of the grout during transportation.
II. Principles for Selecting Filling Materials
The selection of backfill materials shall be guided by the core principles of safety, cost-effectiveness, environmental protection, and feasibility, and shall be determined through a comprehensive assessment that takes into account the scale of mining operations, the characteristics of goaf areas, and local resource conditions. The specific principles and practical calculation methods are as follows:
1. Ensuring Safe Production: As a core principle, the filling material must meet the required strength of the fill body to maintain stability in the mined-out area and prevent safety accidents such as roof collapse and surrounding rock deformation. In practical operations, the strength of the fill body shall be determined based on the burial depth of the mined-out area and the magnitude of in-situ stress. Generally, when the burial depth H is ≤300 m, the compressive strength of the fill body shall be ≥3 MPa; when 300 m < H ≤500 m, the compressive strength shall be ≥5 MPa. The calculation formula is: σ ≥ K × γ × H, where σ represents the minimum compressive strength of the fill body ( megapascal ), where K is the safety factor (typically taken as 1.2). 1.5), where γ is the average unit weight of the surrounding rock (kN/m³; typically 26). 28 kN/m³), where H is the burial depth of the goaf (m). For example, if a mine has a goaf at a burial depth of 400 m, the unit weight of the surrounding rock is 27 kN/m³, and the safety factor is taken as 1.3, then the minimum compressive strength of the backfill material σ ≥ 1.3 × 27 × 400 ÷ 1000 = 14.04 MPa.
2. Minimum Cost: On the premise of meeting safety requirements, prioritize low-cost materials to reduce the overall cost of filling operations. Cost calculations shall cover the entire process, including material procurement, transportation, preparation, and delivery. The unit cost of filling material is calculated as follows: C = (C1 × m1 + C2 × m2 + C3 × m3 + C4 × m4) ÷ V, where C represents the unit cost of filling material (RMB/m³), C1 is the unit price of cementitious material (RMB/t), m1 is the unit consumption of cementitious material (t/m³), C2 is the unit price of aggregate (RMB/m³), m2 is the unit consumption of aggregate (m³/m³), C3 is the unit price of water (RMB/m³), m3 is the unit water consumption (m³/m³), C4 is the unit price of admixture (RMB/t), m4 is the unit admixture consumption (t/m³), and V is the unit volume of the fill (m³). For example, a mine uses tailings combined with P.O 42.5 cement for backfilling. The cement costs RMB 400 per ton, with a unit consumption of 0.3 tons per cubic meter; the tailings cost RMB 50 per cubic meter, with a unit consumption of 1.2 cubic meters per cubic meter; the water costs RMB 3 per cubic meter, with a unit consumption of 0.35 cubic meters per cubic meter; and the water-reducing admixture costs RMB 2,000 per ton, with a unit consumption of 0.0024 tons per cubic meter. Therefore, the unit material cost C is calculated as: C = (400 × 0.3 + 50 × 1.2 + 3 × 0.35 + 2,000 × 0.0024) ÷ 1 = 120 + 60 + 1.05 + 4.8 = RMB 185.85 per cubic meter.
3. Environmental Protection: Prioritize the use of green, environmentally friendly materials and avoid those that are toxic, harmful, or likely to pollute the environment. At the same time, give priority to the utilization of solid waste materials such as mine waste rock and tailings, thereby achieving “using waste to treat waste,” reducing land occupation caused by the stockpiling of solid waste, and lowering environmental pollution. For example, using mine tailings as aggregate can reduce the capital investment required for constructing tailings ponds and mitigate the environmental risks associated with tailings leakage.
4. Adequate Material Supply: Select locally sourced materials that are readily available and have a stable supply to prevent mining operations from being disrupted due to material shortages. Calculate the mine’s monthly backfill volume, take into account the transportation distance from the material source, and determine the material supply capacity to ensure that the supply quantity is at least equal to 1.1 times the monthly backfill volume (allowing for a 10% buffer). For example, if a mine has a monthly backfill volume of 5,000 m³ and the unit consumption of aggregate is 1.2 m³ per cubic meter, the monthly aggregate requirement will be 6,000 m³. If local tailings sand is selected as the material source and the monthly supply capacity is ≥6,600 m³, the demand can be met.
5. Consider integrated utilization: Based on actual mining operations, achieve the comprehensive utilization of backfill materials with mine waste and by-products to enhance resource efficiency and reduce overall operating costs. For example, tailings from ore dressing and waste rock can be used as backfill aggregates, while fly ash and slag powder can serve as cementitious admixtures to replace a portion of the cement, thereby reducing waste discharge and lowering the consumption of cementitious materials.
In summary, the selection of backfill materials must balance safety, cost, environmental protection, and resource utilization. Material proportions and dosages should be determined through scientific calculations, and the selection should be optimized in light of the mine’s actual operating conditions to ensure that the backfill mining method operates safely, efficiently, and economically.
