OPTIMIZATION OF SUPPORT STRUCTURE DESIGN FOR BACKFILL CHAMBERS AT THE KAULDY MINE

ABSTRACT

At the Kauldy Mine (Almalyk MMC JSC), approximately 110 wooden support structures are constructed annually to form backfill chambers with a volume of ~240 m³, using a horizontal slicing system with cemented backfill. The high cost of pine timber and its limited durability in high-humidity conditions prompted the investigation of alternative barrier structures made from D500 foam blocks and polystyrene blocks. The study includes calculations of the hydrostatic pressure exerted by the backfill mix on the support structure, strength assessments of the three materials with a twofold safety factor, and a techno-economic comparison. It was determined that foam blocks can reduce annual costs by nearly 1 billion UZS, while polystyrene blocks offer maximum durability with moderate cost savings.

АННОТАЦИЯ

На руднике «Каульды» (АО «Алмалыкский ГМК») при системе разработки горизонтальными слоями с твердеющей закладкой ежегодно возводят около 110 деревянных перемычек для формирования камер объёмом ~ 240 м³. Высокая стоимость сосны, а также её ограниченная долговечность в условиях повышенной влажности побудили исследовать альтернативные ограждающие конструкции из пеноблоков D500 и полистиролблоков. В работе приведён расчёт гидростатического давления смеси на перемычку, проверка прочности трёх материалов с двукратным запасом, а также технико‑экономическое сравнение. Установлено, что пеноблоки позволяют снизить годовые затраты почти на 1 млрд сум, а полистиролблоки обеспечивают максимальную долговечность при умеренной экономии.

Keywords: Backfill chambers, cemented backfill, Wooden support structures, D500 foam blocks, Polystyrene blocks, Hydrostatic pressure, Safety factor, Economic efficiency, Kauldy Mine, Water absorption

Ключевые слова: Закладочные камеры, Твердеющая закладка, Деревянные перемычки, Пеноблоки D500, Полистиролблоки, Гидростатическое давление, Прочностной запас, Экономическая эффективность, Рудник «Каульды», Водопоглощение

Introduction

Due to increasingly complex mining conditions and stringent requirements for resource utilization, the application of backfill systems is steadily increasing, a trend likely to continue. Backfill methods are classified into cemented, hydraulic, and dry types based on the construction of backfill masses. Cemented and hydraulic backfills are the most prevalent due to their effective void filling, minimal surface displacement, maximum ore recovery, and high levels of mechanization, technological efficiency, and automation potential. Dry backfill is less common but can be competitive in specific cases [1].

The Kauldy Mine exploits a gold ore body with a dip angle of 10–30°, a thickness exceeding 1.5 m, and a Protodyakonov strength coefficient of 13–14, justifying the use of horizontal slicing with cemented backfill. This technology requires temporary barrier structures on the exposed chamber side before each backfill pour. Traditionally, pine roundwood and sawn timber are used (~1.9 m³ of wood per structure). At a current market price of 4.8 million UZS per cubic meter, annual timber costs exceed 1.3 billion UZS. This study aims to justify replacing wooden support structures with D500 foam blocks and polystyrene blocks, demonstrating their sufficient strength and economic efficiency.

From a physical perspective, dynamic manifestations of rock pressure involve avalanche-like brittle failure processes (fracturing) within the rock mass. A key approach to addressing this issue involves studying rock mass properties and deliberately modifying their mechanical characteristics. For cemented backfill, managing geomechanical processes hinges on optimizing the parameters of artificial pillars or backfill masses, including their geometry, strength, and composition [2]. Support structures in underground mines prevent mine water ingress into active workings and facilitate the drainage of partially clarified water through structure drainage systems into mine dewatering sumps, followed by pumping to surface settling ponds. A review of studies [3-4] indicates that underground support structures include waterproof metallic and wedge-type barriers, filtration barriers made of unsorted loose rock or blasted rock fill, and isolating wooden, concrete, reinforced concrete, or metallic barriers, with or without drainage systems.

Materials and Methods

Mining and Technical Conditions

The Kauldy Mine operates under specific geotechnical conditions that influence the design of backfill chambers. Post-excavation chambers measure 3 × 4 × 20 m, yielding a volume of 240 m³, a base area of 80 m², and a support structure front of 12 m². The chamber is filled with 50 m³ of cemented backfill mix per cycle, administered in four full cycles and one partial cycle to achieve complete filling. The backfill mix, composed of cement, sand, and water, has a density of 2,000 kg/m³ and a design compressive strength of 11.5 MPa, ensuring stability and support for the surrounding rock mass. These parameters were selected based on the mine’s geological and operational requirements, including the need to minimize surface subsidence and maximize ore recovery.

Hydrostatics and Pressure Calculations

The backfill process increases the column height by 0.625 m per cycle, calculated as (50 m² / 80 m²) Hydrostatic pressure (P) was determined using the formula:

where:

 = 2,000  (density of the backfill mix),

= 9.81 ,   (gravitational acceleration),

 = column height (m).

The force acting on the support structure was calculated as:

where (S = 12 m²) (area of the structure front). To ensure a conservative estimate, a rectangular pressure distribution (uniform load) was assumed, doubling the actual triangular distribution to incorporate an additional safety margin. This approach accounts for potential variations in backfill behavior and ensures structural integrity under worst-case scenarios. The calculated values for each cycle are:

Cycle 1: (P = 12.3 kPa, F = 147 kN)

Cycle 2: (P = 24.5 kPa, F = 294 kN)

Cycle 3: (P = 36.8 kPa, F = 442 kN)

Cycle 4: (P = 49.0 kPa, F = 589 kN)

Cycle 5 (full chamber): (P = 58.9 kPa, F = 706 kN)

Support Structure Materials

Three materials were evaluated for support structure construction:

Pine: Roundwood (diameter 0.25 m, length 3 m) and sawn timber, with a compressive strength along fibers of approximately 10 MPa. Pine is traditionally used but is susceptible to moisture-induced degradation.

D500 Foam Blocks: 64 blocks (625 × 300 × 250 mm), assembled with masonry adhesive and reinforced with mesh. These blocks have a compressive strength of ≥3 MPa and are lightweight, facilitating rapid installation.

Polystyrene Blocks: 67 blocks (600 × 300 × 200 mm), bonded with MINE BLOCK MORTAR, offering a compressive strength of ≥5 MPa. These blocks are highly resistant to water absorption, enhancing durability in humid conditions.

Allowable stress for each material was calculated with a safety factor of
 (k =2), ensuring that the structures can withstand unexpected loads or material variability.

Economic Methodology

The cost of each support structure was assessed based on direct expenses, including materials (blocks, timber, adhesive/mortar, nails, canvas), and labor costs, quantified in standard hours. The analysis considered local market prices and mine-specific labor rates. Annual savings were calculated as the difference in annual expenses when transitioning from the baseline wooden structures to block-based structures, multiplied by the 110 structures constructed annually. This approach accounts for both capital and operational cost reductions, providing a comprehensive economic evaluation.

Results

Strength Assessment

The maximum operational stress on the support structure at full chamber filling was calculated as 0.059 MPa, derived from the hydrostatic pressure of 58.9 kPa applied over the 12 m² structure front. This stress was compared to the allowable stresses for each material, adjusted by a safety factor of 2 to ensure reliability under dynamic mining conditions. The allowable stresses are:

Pine:  , MPa;

Foam blocks:  , MPa;

Polystyrene blocks:  , MPa;

The actual stress represents only 1.2% of the pine’s capacity, 4% of the foam blocks’, and 2.4% of the polystyrene blocks’. This significant safety margin, even under the conservative rectangular pressure distribution, confirms that all materials are robust enough for the application. The low stress utilization highlights the potential for further optimization, such as reducing material thickness or exploring alternative block compositions, while maintaining structural integrity. The strength assessment also considered the materials’ behavior under sustained loads, with foam and polystyrene blocks showing superior resistance to deformation compared to pine, which is prone to creep in humid environments.

Economic Performance

The economic analysis revealed substantial cost savings with alternative materials:

Wooden structure: Each unit costs 11.09 million UZS, resulting in annual expenses of approximately 1.30 billion UZS for 110 structures. The high cost is driven by the price of pine (4.8 million UZS/m³) and labor-intensive installation.

Foam block structure: Each unit costs 2.00 million UZS, yielding annual savings of ~1.0 billion UZS (40% reduction). The lower cost reflects cheaper materials and faster installation.

Polystyrene block structure: Each unit costs 6.23 million UZS, achieving annual savings of ~0.54 billion UZS. While less economical than foam blocks, polystyrene offers enhanced durability.

These savings were calculated by aggregating material and labor costs, with foam blocks benefiting from lower material prices and polystyrene blocks from reduced maintenance due to their longevity. The economic evaluation also considered indirect benefits, such as reduced downtime and lower replacement frequency, which further enhance the viability of block-based structures.

Technological Benefits

The adoption of block-based structures offers multiple operational advantages. Installation of foam or polystyrene block structures takes approximately 2 hours, compared to 6 hours for wooden structures, due to the larger format and modular nature of the blocks. This reduction in construction time accelerates the stoping cycle, increasing overall mine productivity. Water absorption is significantly lower for blocks (≤8% for foam blocks, ≤4% for polystyrene blocks) compared to pine (>60%), mitigating risks of rot and warping in the mine’s high-humidity environment. Additionally, the larger volume occupied by blocks in the chamber cross-section reduces the void space, decreasing annual sand consumption for backfill by 35,000 tons. This reduction lowers material handling costs and environmental impact, aligning with sustainable mining practices. The block structures also incorporate drainage systems, ensuring effective water management without compromising structural integrity.

Figure 1. Schematic of chamber filling with cemented backfill

Discussion

The results demonstrate that alternative materials provide a substantial safety margin even under overestimated load conditions. Foam blocks offer the best economic performance, with a 40% cost reduction, but their strength limit (3 MPa) advises against use under pressures >150 kPa, which may occur in deeper horizons or with denser backfill mixes. Polystyrene blocks, with a higher strength (5 MPa) and lower water absorption (≤4%), are more suitable for aggressive hydrogeological conditions, though their higher cost limits savings to 0.54 billion UZS annually. In the Kauldy Mine’s high-humidity workings, wooden structures degrade within 3–4 months, leading to shield deformation and frequent replacements. Block structures mitigate these risks: MINE BLOCK MORTAR creates a monolithic joint, and reinforcing mesh evenly distributes forces, enhancing stability.

The study’s limitations include reliance on laboratory-based strength assessments, which may not fully capture in-situ conditions. Cyclic hydrostatic pressure loads under vibration, common in active mining environments, were not modeled, potentially affecting long-term performance. To address these gaps, an industrial trial is proposed: construct one foam block and one polystyrene block structure at the 980 m horizon and monitor deformation for 12 months. This trial will validate the materials’ performance under real-world conditions, including exposure to mine water and dynamic loads.

The maximum hydrostatic pressure (58.9 kPa) is 17–42 times below the allowable stresses of alternative materials, confirming their suitability. Replacing pine structures with foam blocks saves ~1 billion UZS annually, while polystyrene blocks save ~0.54 billion UZS. Block structures reduce stoping cycle times, lower barrier water absorption, and decrease backfill material consumption, contributing to operational efficiency and sustainability. It is recommended to adopt foam blocks as the baseline solution for standard conditions and use polystyrene blocks in areas with aggressive hydrogeological conditions following the trial.

Conclusion

This study confirms that replacing wooden support structures with D500 foam blocks or polystyrene blocks at the Kauldy Mine significantly enhances economic and operational efficiency. The maximum hydrostatic pressure on the support structure (58.9 kPa) is 17–42 times below the allowable stresses of the alternative materials, ensuring robust performance with a substantial safety margin. Foam blocks offer the highest cost savings, reducing annual expenses by approximately 1 billion UZS, while polystyrene blocks provide superior durability with savings of 0.54 billion UZS per year, making them ideal for hydrogeologically challenging areas. Block structures reduce stoping cycle times by 4 hours per installation, lower water absorption to ≤8%, and decrease sand consumption by 35,000 tons annually, contributing to sustainability and productivity. It is recommended to implement foam blocks as the standard solution and deploy polystyrene blocks in high-humidity zones following a 12-month industrial trial at the 980 m horizon to validate long-term performance. These findings offer a scalable approach to optimizing backfill operations, with potential applications in similar underground mining environments.

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