Backfill Method Research Articles

Theory and experimental discussion of synchronous backfilling self-compacting concrete technology behind the segment wall of double-shield TBM

The conventional method of urban subway tunnel excavation using a double-shield tunnel boring machine (DS-TBM) typically involves a two-stage backfilling process behind the segment wall using pea gravel and grouting. However, this method has inherent drawbacks such as uneven backfilling of pea gravel and delayed grouting, which can lead to structural issues such as segment dislocation, damage, and groundwater leakage during tunnel excavation. To address these problems, this study proposes a technical theory of synchronous backfilling self-compacting concrete backfilling material (SCCBM), considering the limitations of the conventional DS-TBM backfill method. This theory uses a specially designed wear-resistant seal plate suitable for a DS-TBM shield tail. The optimal proportion of SCCBM for backfilling the segment wall was determined based on Okamura's empirical method using anti-dispersion powder, fine stones (1–5 mm particle size), and coarse stones (5–10 mm particle size) as the basic materials. The SCCBM was designed and evaluated using various test methods, including the mini-slump, standard slump flow, standard L-box, and improved L-box tests. The engineering characteristics of the backfill material, such as initial setting time, blocking ability ratio, stone rate, and axial compressive strength, were considered during the evaluation. The results demonstrate that all indicators meet the standards for backfilling behind subway tunnel segment walls. Furthermore, an on-site synchronous backfilling SCCBM experiment was conducted to validate the feasibility and advantages of the new synchronous backfilling technology. The experiment involved observing the complete backfill process and monitoring the vertical displacement of the segment. Through these observations, the practicality and benefits of the proposed synchronous backfilling technology were confirmed.

Study on Deformation Response Law of Surrounding Rock in Solid Backfill Mining Stope

To fully control the roof and surface and reduce the ecological damage caused by mining, it is necessary to understand the deformation response law of the surrounding rock in solid backfill mining. The similarities and differences of overburden movement characteristics between natural caving method and solid backfill method are analyzed and compared by means of on-site investigation and monitoring, theoretical derivation and analysis, numerical simulation and effect verification. The control mechanism of backfill mining on the roof is clarified, and the relationship between stope roof displacement and support pressure of backfill body is established. Furthermore, the mechanical conditions of immediate roof in equilibrium state during backfill mining are established. The FLAC3D simulation indicates that the solid backfill mining face has a concentrated stress peak located 15 m ahead of the coal wall. Within the range from 0 to 5 m, the abutment pressure is reduced gradually. However, within the range from 5 to 15 m, the abutment pressure is increased evidently. The stress concentration is located 35 m behind the working face. With continuous backfilling of the goaf, the maximum displacement occurs in the backfill body near the working face. The total deformation of the surrounding rock in the goaf is reduced by 58%. The total deformation of coal wall is reduced by ∼37.5%. The subsidence coefficient is reduced from 0.59 to 0.013 by using the backfill mining technology. The research results provide theoretical guidance and reference for roof control and ecological protection of backfill mining.

Investigation of the settlement mechanism and control measures of a super-large section tunnel in a giant karst cave using ultra-thick backfill method

A giant cave was encountered during the construction of the Yujingshan Tunnel. There were deep deposits at the bottom of the giant cave, including soft soil, crushed stone soil, and fine sand. A comprehensive construction scheme, comprising an ultra-thick backfill and a continuous rigid frame bridge, was adopted when constructing the large-section tunnel. However, there were still problems regarding the post-construction and differential settlements of the tunnel (the differential settlement is defined here as the difference between the left corner settlement and the right corner settlement of the tunnel bottom). These problems led to significant challenges for the long-term safety of the tunnel. Control measures were proposed to address these settlement problems. Centrifugal model tests were mainly conducted to analyze the leading causes of the post-construction and differential settlements. And numerical simulation was mainly used to investigate the differences of various grouting depths in the base. Finally, regression analysis and trend simulation were performed based on in-situ monitoring data. The results indicate that the ultra-thick backfill and soft clay are both the causes of the base settlement, and the soft clay accounts for a larger proportion. The differential settlement is mainly due to the uneven soft clay at the bottom. After research, the final scheme is grouting to a depth of 30 m, the post-construction and differential settlements can be controlled at 35.3 mm and 9.6 mm respectively, and the tunnel structure will have higher safety. On this basis, it is proposed that the maximum allowable settlement is 40 mm, and the maximum allowable differential settlement is 10 mm in long-term operation. The monitoring results and regression analysis show that the construction of the Yujingshan Tunnel is significantly improved, and the post-construction and differential settlements can be controlled at approximately 12.2 mm and 6.6 mm, respectively, which are both less than the maximum allowable values proposed above.

Experimental and numerical research of backfill cooling based on similarity theory

Based on the similarity theory, this paper conducts a dimensionless analysis of air flow and selects Archimedes number as the similarity criterion number in order to study the cooling effect of mine cold storage backfill under ventilation environment. The similarity experimental platform has been built through collecting actual stope parameters and determining the geometric and velocity scales. The influence of different backfill cooling methods and air supply conditions on the temperature distribution of stope has been studied both by similarity experiment and CFD numerical simulation. Through model verification, it is proved that Archimedes number is suitable as the similarity simulation criterion number in this research and the similarity method is feasible to predict the parameters in prototype. The results show that ventilation alone often fails to meet the specified cooling requirements in deep mine, and cold storage backfill is an effective method to control the temperature in stopes. Compared with the upward backfill method, the downward backfill method is superior in terms of cooling temperature and cooling duration. Because the position of the air supply outlet is close to the top of the stope, the ventilation has a great influence on the cooling effect of the downward backfill, and the excessive wind flow reduces the cooling effect. Furthermore, ventilation accelerates air convection, which makes the flow field in the stope unstable, and leads to horizontal temperature difference in logistic distribution.

Numerical Study of the Layered Blasting Effect on a Cemented Backfill Stope

The sublevel open stoping with backfill method has recently been widely used in underground metal mines. The primary CPB stope is frequently affected by blasting in the secondary ore stope, leading to stope collapse and ore dilution, which has become a common problem and has received widespread attention. Numerical simulations are carried out in the present work, and a 1/4 numeral model consisting of a primary CPB stope and a secondary ore stope is built. The secondary ore stope is divided into four layers on average in the simulation model, and the incident stress induced by each blasting at the interface of the CPB and ore is simulated. The results show that the CPB stope in the range within the height of the explosive charge induced horizontal compressive stress and tensile stress induced from the explosive charge height, while the mined section under the charge height has no obvious blasting impact. The maximum incident compressive stress is close to 1.2 MPa and occurs in the area closest to the blast hole The maximum induced tensile stress occurs in the range above the charge height, which is about 0.2 MPa. The stress ratios of the four-layered lift blasts are 3.6%, 3.8%, 4.0%, and 4.8%, respectively, showing a slight cumulative effect of layered blasting. In addition, the positive correlation between incident stress and the stress ratio is studied in the present work, and the results show that the greater the incident stress is, the greater the incident ratio is.

Study on Multi-Layer Filling Treatment of Extra-Large Goaf and Its Underground Application.

At present, the many domestic, large mined-out areas caused by single filling ability of the slurry flow state, thin layer flow and hardening after filling in multilayer structure generally need to finish filling for many times, because after a filling experience shows that filling body in the last solidification of flow, this leads to a lower one side of the roof, and far distance part of the filling body cannot pick up top. The determination of backfill strength is the key problem of the cemented backfill method, and it is affected by many factors. Therefore, through theoretical calculation, laboratory testing and numerical simulation methods, combined with the field filling process, this paper has verified the flow accumulation and stratification characteristics of stope layered filling slurry. When the slurry concentration is 60–73%, the slope increases exponentially from 2.5° to 8°. It is revealed that the delamination and meshing state are the key factors to determine the overall strength of large-scale stope filling through the testing of particle size distribution in interlayer and flow direction. The reduction effect of the number and Angle of structural weak surface formed by layering and filling on strength is revealed: cement–sand ratio 1:12, concentration 68%, standard curing R28 > 1.81 MPa. The strength reduction coefficient is 61.31% and 92.96% when the number of layers is 1–4. The higher the number of layers, the greater is the reduction coefficient, and when the stratification angle increases by 2, the strength of backfill decreases by 20–30%. The verification of stope filling coring shows that the in situ strength reaches 2.42 MPa, which is 0.61 MPa higher than the standard curing strength, with an increase of 33.7%. When the depth is from 1 m to 5 m, the strength increases from 2.26 MPa to 2.69 MPa, with an increase rate of 18.2%. Finally, through the research and application of the comprehensive technology of mining and filling coordination under the complex goaf group, the residual ore resources of Xianglushan tungsten mine are effectively recovered, the volume of goaf is significantly reduced, and the safety of goaf is improved.

Investigation of the Effectiveness of a New Backfilling Method: “Multi-Arch Pier-Column”

Owing to the shortcomings of blindness and inaccuracy when backfilling in goafs and based on the key stratum theory, we propose the “multi-arch pier-column” backfilling method. This method involves drilling holes at specific locations on the surface to inject filling and slurry materials into the goaf and separation area under the key stratum. This allows the broken gangue to be cemented to form a stone body, to improve its overall strength. This process, along with filling in the separation area under the key stratum, ensures that the key stratum does not break, forming a joint medium of “separation area filling body + backfilled pier-columns + key stratum + coal column”, which prevents new subsidence on the surface layer. Using the Gaojialiang coal mine as an example, the effects of the proposed method on controlling surface subsidence were determined using a numerical simulation based on FLAC3D simulation software. The results indicate that this method can effectively control the key stratum and ensure that the surface subsidence is within a safe range. The multi-arch pier-column backfilling method utilises the self-bearing capacity of the overburden structure and greatly reduces the backfilling workload and the cost of backfilling for controlling surface subsidence. At present, the multi-arch pier-column system of the new backfill method is an unexplored and new area of research.

Study on the nonlinear deformation characteristics and constitutive model of cemented tailings backfill considering compaction hardening and strain softening

The deformation characteristics of the cemented tailings backfill (CTB) has a significant influence on the stability of mine stope and surface subsidence after filling mining. An uniaxial compression experiment of the CTB containing different cement-tailings ratios (c/t) has been performed and it shows that the CTB has an obvious compaction hardening in the initial compaction stage and strain softening at the plastic yield stage. However, these nonlinear deformation characteristics of CTB are not fully understood in the previous studies. In this research, considering the nonlinear deformation characteristics, a complete constitutive model was proposed for the CTB based on the experimental stress–strain curves. This constitutive model has been programmed into FLAC3D to obtain the law of surface subsidence and assess the safety of the concentrator if the backfill method is adopted. In addition, a physical model experiment of filling mining is also carried out at the same condition. The results of these two simulation experiments show that the surface subsidence is negatively correlated with the distance between the concentrator and the collapse pit, and the maximum subsidence is less than 90 mm. Therefore, the filling mining method can effectively guarantee the safety of concentrator. This study provides a better understanding of the deformation characteristics of CTB and it also contributes to the underground filling technology of using CTB.

Ground Stress Analysis and Automation of Workface in Continuous Mining Continuous Backfill Operation

The cost, complexity, lack of filling space and time create challenges in the longwall backfill operation, resulting in poor subsidence control and reduced productivity. This paper proposes an automated continuous mining and continuous backfill (CMCB) method by examining its key requirements and investigates the optimum sequence of coal panel (such as drifts) excavation to ensure ground strata control at relatively high productivity. The automated CMCB adopts the highwall mining technique underground, which enables easier automation at the workface. A numerical simulation of the Changxing coal mine in China was undertaken, and five different sequences of coal excavation were investigated, using the automated CMCB excavation parameters (assuming a 4 m width cut, 5 m mining height for a 200 m long coal slice) to determine the optimum sequence of resource excavation. The plastic zones and vertical displacement across the five models were analyzed. Simulation results of the 5 m high coal seam excavation show that the odd-even slice (OES) mining sequence, which has a vertical ground displacement of 74 mm, is the most efficient excavation method, due to its effective stress redistribution and lower induced ground displacement.

A Particle Size Distribution Model for Tailings in Mine Backfill

With the increasing awareness of sustainable mining, the cement tailings backfill (CTB) method has been developed rapidly over the past decades. In the CTB technique, the two main mechanical properties engineers were concerned with are the rheological properties of CTB slurry and the resulting CTB strength after curing. Particle size distribution (PSD) of tailings material or PSD of the slurry is a significant factor that highly influences the rheological of CTB slurry and the strength performance of CTB. However, the concentrically partial size distribution curve and existing mathematical model could not represent the PSD of tailings material. In this study, a mathematical model for the particle size distribution of mine tailings was established using three model coefficients A B and K, which mainly reflect the characteristics of particles from three aspects respectively, the average size of particles, the proportion of the coarse or the fine parts of particles, and the distribution width of particles; meanwhile, an optimal coefficient solution method based on error analysis is given. Twelve tailing materials sourced from metal mines around China were used for the model establishment and validation. The determination coefficient of error analysis (R2) for all twelve modeled PSD lognormal curves was more significant than 0.99, and the modeled PSD lognormal curves are highly consistent with the determined particle size distribution curve.

Numerical Investigation on Stratum and Surface Deformation in Underground Phosphorite Mining Under Different Mining Methods

With the ending of deep-concave open-pit phosphorite extractions and gradual exhausting of shallow mineral resources, stoping of phosphorite seams has entered or will enter into underground mining. Particularly for excavating slightly inclined thin and medium-thick phosphorite orebodies, roof and surface control under different mining methods is crucial for safe and efficient exploitations. In this study, the study area is located in Kunyang Phosphorite Mine characterized by slightly inclined thin and medium-thick deposits. Based on the occurrence conditions, orebody thickness, dip angle, and more factors, the mining methods of underground phosphorite are selected, including room and pillar mining, cement backfill mining, and caving mining. Numerical analysis on roof deformation and surface subsidence under the three methods is performed. The results show that the amount of roof and surface subsidence decreases successively by the caving method, room and pillar method, and cement backfill method. The maximum roof and surface subsidence by the caving method is 45.7 and 13.3 cm, respectively. Regarding shallow orebodies, the open-pit slope is obviously disturbed by the caving method and room and pillar mining method. Hence, slope displacement monitoring should be emphasized. Compared with the other two methods, the backfill mining method can use mined wastes as backfill materials and has less influence on the roof and surface during stoping and is better at controlling slope stability.

The Recent Progress China Has Made in High-Concentration Backfill

With the development of science and technology, backfill technology has made continuous progress, and the traditional backfill method is no longer suitable for various complicated practical engineering situations. Therefore, researchers in the field of backfill mining have gradually shifted their research focus to the study of high-concentration backfill, and Chinese researchers are no exception. In order to solve the problems caused by the traditional backfill method, China began to vigorously develop high-concentration backfill in recent years, and achieved a lot of results. In this paper, some important achievements made by Chinese researchers on high-concentration backfill in recent years are reviewed; it also presents a summary report of the latest research results from several key laboratories across China. Therefore, this paper reviews the development progress of high-concentration backfill China has made, of which the main contents include: (1) research progress of the high-concentration backfill theory in China; and (2) research progress of high-concentration backfill equipment in China. Finally, we claim that this paper serves just as a guide to start a conversation, and we hope many more experts and scholars will be interested and engage in the research of this field.

Influence of silica fume on mechanical property of cemented paste backfill

The cemented paste backfill (CPB) method, which is the most preferred method for tailings storage, is one of the most used methods for the disposal of mine tailings. However, the high rate of cement usage increases the costs. In this study, whether silica fume (SF), which is a pozzolanic material, can be used as a substitute instead of cement was investigated based on mechanical parameters or not. As a result of the experiments, it was determined that SF substituted for portland cement (PC) increased the strength of paste backfill mixtures prepared in a certain cement ratio. In addition, it was determined that SF substitution in CPB increased the durability of mixtures against sulphate attacks as it did in concrete. With the use of SF, savings of up to 20% were achieved in the amount and cost of cement used in CPB. Besides, using it without the pulverizing process required for pozzolanic materials used in other studies had a positive effect in terms of time and cost. Moreover, the use of SF in CPB mixtures resulted in less energy use and less greenhouse gas emissions, and a reduction of environmental pollution caused by SF tailings.

A Large Goaf Group Treatment by means of Mine Backfill Technology

There are few studies on the management methods of large-scale goaf groups per the specific surrounding rock mass conditions of each goaf. This paper evaluates comprehensively the stability of the multistage large-scale goaf group in a Pb-Zn mine in Inner Mongolia, China, via the modified Mathews stability diagram technique. The volume of each goaf to be backfilled was quantitatively analyzed in the combination of theoretical analysis and three-dimensional laser scanning technology. The corresponding mechanical characteristics of the filling were determined by laboratory testing while formulating the treatment scheme of the large goaf group using the backfill method. The applicability of the treatment scheme using the backfill was verified by the combination of the numerical results of the distribution of the surrounding rock failure zone and the monitored data of the surface subsidence. The research results and treatment scheme using the backfill can provide a reference for similar conditions of mines worldwide.

Research on the Detection Countermeasures of Building Concrete Raw Materials

The quality of concrete directly determines the quality, service life and safety of the building, so it is necessary to use concrete materials of excellent quality and performance in the building construction process. In addition to the production process of concrete, which directly affects its performance, the influence of raw materials is very significant. Nowadays, the production process has become more standardised and mechanised, so special attention needs to be paid to the influence of raw materials in the process of making concrete.Based on this, the article will focus on the analysis of countermeasures for the testing of raw materials for construction concrete. By the testing of raw materials, the selection of more suitable raw materials for making concrete, helps to improve the strength and quality of the concrete.The article begins with an analysis of the key points of concrete raw material testing, analysing the considerations in the selection and testing of fly ash, water, stone, cement, sand and admixtures; then analyses several methods of testing raw materials for construction concrete, such as backfill method, ultra-deep rebound synthesis method, post-loading pull-out method and core drilling method, which have their own advantages and disadvantages and need to be selected reasonably according to the testing requirements; finally proposes a few measures to increase the quality of construction concrete.

Loading rate effect on the uniaxial compressive strength (UCS) behavior of cemented paste backfill (CPB)

Underground mined-out areas and tailings are the two major harmful consequences from metal mining industry. Cemented Paste Backfill (CPB) method, refilling the tailings back to the underground mined-out areas, has been widely used worldwide. Clear understanding on the uniaxial compressive strength (UCS) behavior of the CPB is very important for the stability analysis of the backfill structure and the parameters optimization of backfill plan. However, the UCS tests can be markedly affected by the testing loading rate. And few papers reported the effect of different loading rates on the UCS of CPB. In this study, the UCS behavior of 14-day cured CPB samples are investigated using five different loading rates: 0.1, 0.25, 0.5, 1, 2 mm/min. The digital image correlation (DIC) method and scanning electron microscope (SEM) analysis were also carried out to help clearly understand the different loading rates effect on the deformation behavior and failure surfaces. The findings show that, the increase of loading rate has a strengthening effect on the UCS. The correlation between the loading rates and the UCS is more in line with an exponential function when the loading rates are between 0.1 and 2 mm/min. During the loading process with different loading rates, the axial deformation characteristics of all the tested CPB samples can be divided into uniform and non-uniform phases. And the loading rates of 0.5 and 1 mm/min resulted the smallest standard deviation for both the axial displacement and the axial strain observed from the three preset monitoring points. As the loading rate increased, the Ca/Si parameter on the fracture surface of the samples also increased slightly. The findings of this research clearly demonstrated the behavior of UCS under different loading rates, and also provided a key research evidence in selecting appropriate loading rate in assessing USC of CPB sample for laboratory tests.

Experimental study on the mechanical properties of microbial mixed backfill

Cemented backfill mining is an important technology utilized in sustainable mining practices and is fundamental in a wide variety of underground settings. However, the cement-based binder commonly used in cemented backfill materials comes at high costs, is highly alkaline, has high energy consumption, and has high carbon dioxide emissions. As an environmentally friendly technology, microbial induced carbonate precipitation (MICP) technology can produce a variety of cementitious mineral crystals to consolidate otherwise loose materials. In this paper, the MICP technology is used to replace the traditional cement-based binder frequently used in backfill mining, and the microbial mixed backfill (MMB) method is presented. The influences of various factors on the mechanical properties of MMB are also studied. It is found that the strength of MMB samples increases at first, and then decreases, when the cementation solution concentration and the volume ratio of bacterial solution to cementation solution are increased. Solids content is also an important factor that directly affects the strength of MMB samples. However, there was no obvious relationship discovered between strength variation and the total enzyme activity. In addition, different crystal morphologies of calcium carbonate, including calcite and vaterite, are observed in MMB samples. The greater the number and the denser the distribution of calcium carbonate crystals produced in the MICP reaction, the higher the strength of the MMB samples. The strength of MMB is attributed to both the amount of precipitated calcium carbonate by MICP, and the dosage of salt materials contained in the cementation solution. This research can provide basic scientific support for the application of microbial backfilling in the mining field.

Paste backfill materials for underground mining: Some experiences in Serbia: Part II

This review paper presents the backfill method and materials for paste backfill method which are used in the world practice of underground mining. The paper consists of two parts. The first part (Part I) provides a brief literature review concerning the presentation of the backfill method, mostly the paste backfill method, its application, advantages and disadvantages, and materials that can be used to form a paste. The second part (Part II) of the paper presents an overview of possible materials that could be used for the eventual application of this method in Serbia. Flotation tailings from lead-zinc mines and copper mines, fly ash from thermal power plants and metallurgical slag can be potentially used materials for this method. This paper presents the characterization of these materials from some localities in Serbia and results of some investigations in this area.

Field Evaluation of New Backfill Method for Mini Trenching Section

Field Evaluation of New Backfill Method for Mini Trenching Section

Characteristics of Roof Ground Subsidence While Applying a Continuous Excavation Continuous Backfill Method in Longwall Mining

Activities of traditional longwall mining will result in ground subsidence and therefore cause issues such as damages to buildings and farmlands, water pollution and loss, and potential ecological and environmental problems in the mining region. With advantages of the longwall backfill mining method, as well as the room and pillar mining method, a continuous excavation and continuous backfill (CECB) method in longwall mining is recommended to effectively control the ground subsidence. In this method, mining roadways (MRs) are initially planned in a panel, and then they are excavated and backfilled in several stages until the whole panel is mined out and backfilled. According to the geologic conditions of an underground coal mine, and the elastic foundation beam theory, a mechanical model was built to study the subsidence of the roof while using this new mining method. In addition, methods to calculate roof subsidence in various stages in CECB were also provided. The mechanical parameters of backfilling materials, which were used in the theoretical calculation and the numerical analysis for mutual check, were defined through analyzing the stability conditions of the coal pillars and the filling bodies. The control effect for the ground subsidence of using the newly proposed mining method was analyzed based on both simulation results and site monitoring results, including the ground subsidence, horizontal displacement, tilt, curvature and horizontal strain. This research could provide suggestions to effectively control ground subsidence for a mine site with similar geologic conditions.