Development of a safe and economical roadside support body (RSB) material is the key to successful backfilling gobside entry retaining (GER). By means of laboratory tests, this paper studied the effects of the watercement ratio, aggregate content, and age on the contractibility and resistance increasing speed, compressive strength, and postpeak carrying capacity of the concrete with gangues as an aggregate. It also discussed the rationality and adaptability of gangue concrete as a RSB material for backfilling GER. The experimental results show that the compressive strength of gangue concrete increases with age, and that the strength of gangue concrete demonstrates a nonlinear decreasing trend with the increase of the cementing material’s watercement ratio. The watercement ratio in the range of 0.46–0.60 has the most significant regulation effect on the strength of gangue concrete. Mixing with a certain amount of coal gangue enhances the postpeak carrying capacity of concrete, preventing the sample from impact failure. The field experimental results report that as a RSB material, gangue concrete can meet the design and application requirements of GER with gangue backfilling mining. A RSB material featuring high safety, high waste utilization rate, fast construction speed, and low costs is provided.
The gangue backfilling GER (Figure
GER of gob roof managed by different methods. (a) GER with roof managed by caving method. (b) GER with roof managed by backfilling method.
In recent years, as a safe, environmentally friendly mining method, gobbackfilled mining has been applied in China’s eastern and central regions. Many scholars have conducted creative research on backfilling mining [
In the GER development, a variety of RSB materials have been tried by engineers and scholars to maintain roadway stability, such as timber cribs, dense pillar, gangue piling, and masonry walls [
Coal gangue is a concrete material utilizing gangues as a coarse aggregate and cement as a cementing material, mixed with a certain amount of additives. Recently, scholars have studied the performance of gangue concrete for different application fields: Wang et al. [
Through laboratory tests, this paper focuses on the study of the influence of the watercement ratio, aggregate content, and age on the early stage contractibility and the transitional stage resistance increasing speed, the latestage compressive strength, and the postpeak carrying capacity of gangue concrete. Through this process, the sensitive control ranges of the various factors affecting gangue concrete performance were obtained. The adaptability of gangue concrete as a RSB material was discussed by taking into account the behavior law of mine pressure of GER with fully mechanized gangue backfilling mining. Based on the analysis of the test results, field tests were carried out in China’s Shandong mining area, the results of which suggest that the gangue concrete material designed according to the ratio obtained in this paper can meet the RSB loaddeformation requirements in different stages during the entire backfilling GER process. A safe, environmentally friendly and economical RSB material is provided for the development and promotion of GER with fully mechanized gangue backfilling mining.
Cement clinker (p.c32.5) was used, and granular gangues of 0–25 mm in diameter were used as the aggregate in the test. Considering the economy and rationality of GER engineering, the gangues were obtained directly from the gangue piles in the experimental mining area. In order to give a reasonable explanation of gangue concrete strength evolution, the particle size distribution and mechanical properties of the aggregate were measured first.
Particle size distribution characteristics of crushed gangues
The initial crushed gangue particles obtained from the field are continuously graded granular materials. A Talbot formula was used to quantitatively describe the particle size distribution characteristics of gangues:
The analysis results show that the aggregate basically satisfies the continuously graded distribution characteristics with a Talbot coefficient (
Mechanical properties of the complete gangue samples
Gangue strength and particle size distribution.
To obtain the strength characteristics of the gangue aggregate for analyzing the coordination mechanism of the aggregate and the cementing material, uniaxial compression tests were carried out on multiple complete gangue samples (Φ50 mm × 100 mm) in this paper, and the average unconfined uniaxial compressive strength obtained is 9.83 MPa. The compressive strength results of the standard gangue samples are shown in Table
Unconfined uniaxial compression test results for the standard cylindrical gangue samples.
Gangue no.  Sample size (mm)  Compressive strength (MPa)  Elastic modulus (GPa)  Poisson’s ratio  

Diameter  Height  
G01  49.8  98.3  9.17  5.11  0.21 
G02  49.8  97.7  10.33  4.32  0.26 
G03  49.9  102.3  7.32  6.22  0.18 
G04  49.8  101.7  11.82  4.83  0.22 
G05  49.9  98.6  10.52  5.08  0.22 
Samples were prepared according to the concrete sample forming and curing standards with a design size of 150 mm × 150 mm × 150 mm. To guarantee the reliability of the data, 3 samples were prepared for the same ratio and age. The test used the MTS 816 test system (Figure
Test system and formed samples.
Walker et al. [
Watercement ratio: watercement ratio is an important factor affecting the strength of the cementing material. The watercement ratios of 0.40, 0.43, 0.46, 0.50, 0.55, and 0.6 were considered in this test.
Aggregate content: four aggregate content levels were considered, respectively, which include 45%, 50%, 55%, and 60%.
Curing age: for gangue concrete samples and pure cement samples with each watercement ratio and gangue content level, three ages were considered, that is, 7 days, 14 days, and 28 days, for studying the evolution rules of the compressive strength properties of gangue concrete in different hardening stages.
Sample gangue concrete ratios (mass ratios).
No.  Gangue content (%)  Watercement ratio  Water content (%)  Cement content (%)  Age (day) 

1  50  0.40  14.29  35.71  7, 14, 28 
2  0.43  15.03  34.97  7, 14, 28  
3  0.46  15.75  34.25  7, 14, 28  
4  0.50  16.67  33.33  7, 14, 28  
5  0.55  17.74  32.26  7, 14, 28  
6  0.60  18.75  31.25  7, 14, 28  
7  0  0.40  14.29  35.71  7, 14, 28 
8  0.43  15.03  34.97  7, 14, 28  
9  0.46  15.75  34.25  7, 14, 28  
10  0.50  16.67  33.33  7, 14, 28  
11  0.55  17.74  32.26  7, 14, 28  
12  0.60  18.75  31.25  7, 14, 28  
13  45  0.4  14.29  35.71  7, 14, 28 
14  55  14.29  35.71  7, 14, 28  
15  60  14.29  35.71  7, 14, 28 
The average concrete strength with a 50% gangue content level under different watercement ratios can be obtained through uniaxial compression tests of the gangue concrete sample groups 1–6, as noted in Figure
Average strength and error of the concrete with different watercement ratios and a gangue content of 50%.
The average strength of the cementing material under different watercement ratios can be ascertained through uniaxial compression tests of pure cement sample groups 7–12, as depicted in Figure
Average strength and error of the pure cement samples with different watercement ratios.
With a comparison between Figures
Therefore, within the watercement ratio range described in this test, to allow a full harmonization between the strengths of the aggregate and the cementing material, it can be expected that the sensitivity range of the watercement ratio lies in 0.46–0.60 for controlling the strength of gangue concrete. In the application of backfilling GER engineering, the RSB load requirements may be satisfied plus a full use of the carrying capacity of the granular gangues through adjusting the watercement ratio of the cementing material within the sensitivity range based on understanding the roof load law and by considering the strength characteristics of gangues.
The average concrete strength with dissimilar gangue contents can be obtained through uniaxial compression tests of the gangue concrete sample groups 1, 13, 14, and 15, as noted in Figure
Concrete strengths at various ages with different gangue contents.
With the watercement ratio at 0.40, the increase of the gangue content has little effect on the concrete strength. Therefore, it can be deduced that when the watercement ratio and the aggregate strength are kept unchanged, the gangue content within a certain range is not a notable factor affecting the strength of gangue concrete. However, by comparing the uniaxial compression characteristics of the sample groups 7 and 15 (Figure
Destruction characteristics of the gangue concrete and the pure cement samples with the same watercement ratio.
As shown in Figure
The strength limit of the gangue concrete was obtained for various ages with various watercement ratios and a gangue content of 50% through uniaxial compression tests of the gangue concrete groups 1 to 6, which is shown in Figure
Average compressive strength of the gangue concrete at different ages with different watercement ratios (gangue content: 50%).
The field test results show that the working faces of GER with fully mechanized gangue backfilling mining stabilize at the 28day age after backfilling, and the working load of the RSB is fundamentally stable. Hence, in studying the influence rules of the age on the gangue concrete’s strength in gangue backfilling GER engineering, it is necessary to determine whether the 28day strength of the gangue concrete can meet the working load requirements in the stable period and to analyze whether the development degree of the gangue concrete’s early stage strength can satisfy the mininginduced pressure requirements in the GER process. In this paper, the development degree of the gangue concrete’s strength is expressed as a percentage of the early age strength of the samples to the strength at the 28day age:
Table
Strength development degree of the early age gangue concrete.
Watercement ratio 




Without gangue  50% gangue content  Without gangue  50% gangue content  
0.40  76.60  75.00  85.01  90.74 
0.43  77.64  77.49  83.93  94.23 
0.46  80.89  66.95  91.96  90.48 
0.50  75.27  61.81  85.94  98.32 
0.55  56.05  61.24  78.32  85.15 
0.60  46.89  62.86  66.24  89.40 
Average  68.89  67.56  81.90  91.37 
In a general GER condition, because backfilling measures are not taken for the gob, the roof shearing needs to be done in the initial stage of the roadside support. The field test results show that the roof shearing resistance is commonly required at 3–6 MPa, so the early stage strength of the RSB material is particularly critical. During the roof shearing process, a low strength of the RSB material will lead to insufficient roadside support, excessive roof deformation, and in a serious scenario roof breaking along the solid coal side, resulting in an accident. In the condition of GER with fully mechanized gangue backfilling mining, due to the support of the backfilling area, the gobside roof subsidence is smaller than in the general GER condition. Moreover, with continuous support, the roof will only bend and subside to a certain degree on the whole without obvious weighting. Therefore, the RSB does not serve as the main carrier of the roof load at the beginning of entry retaining, and so the early stage RSB strength required is smaller than that of GER with roof shearing. It can be seen from Figure
Moreover, for solid backfilling GER, it is necessary to guarantee the structural stability of the RSB under the lateral pressure of the granular backfilling material in the gob. Therefore, to ensure that the RSB does not slip toward the inner side of the roadway and loose stability due to rotation, it is required to carry a design load within a reasonable range. The results of the existing practices show that the vertical stress is gradually increasing from the working face to the backfilling area, which essentially stabilizes 30 m–50 m back from the working face (Figure
Schematic diagram of the vertical stress distribution in the backfilling area.
From the stiffness development with the gangue concrete age as shown in Figure
Average elastic modulus and error of the gangue concrete at various ages.
The test mine is in the Shandong Province of China. The ground elevation of the test zone is +32.93 to +33.08 m; the underground elevation is −642 to −636 m; the coal seam strike approximates East–West, trending South; and true dip is at 0°–3°. The main mining coal seam is seam 3_{lower}, coefficient of hardness
Strata geological log.
The width of the RSB is 2 m, and the height is 3.6 m. Crushed continuously graded native gangues at
Roadway support design and effect. (a) Roadside support design scheme. (b) Roadside support effect.
A safe RSB material should meet the following three requirements according to different mining geological conditions.
First of all, the strength of RSB should be greater than roof load at its location. Evaluation criterion of RSB stability is whether the RSB material breaks down in the process of loading and unloading caused by roof movement.
Secondly, shrinkage should be larger than the fieldmeasured deformation of basic roof. Ability of RSB to adapt to the deformation of basic roof can make full use of the bearing capacity of surrounding rock, reducing the roof load on RSB. This is an effective way to ensure that its strength is greater than the roof load.
Thirdly, there is no mutation in the postpeak strength. This means that the material can ensure that the global impact damage will not occur to RSB even if there is local high stress at the gob side. The local slow destruction can be repaired by means of reinforcement, which ensures the safety of RSB.
To analyze the effect of the roadside support, monitoring stations are arranged in the roadway to record the vertical stress and converging deformation during advancement of the working face, as shown in Figure
Monitoring result during advancement of the working face. (a) Vertical stress in RSB. (b) Roadway deformation curve.
During backfill GER, the compressive strengths of the roadside support material of various ages satisfy the onsite construction requirement, the max vertical stress in RSB is 3.8 MPa, and the strength safety coefficient is
During advancement of the working face, the rooffloor displacement of the roadway was larger than the twoside displacement. The maximum rooffloor displacement was 112 mm, and the maximum twoside displacement was 71 mm. The roadway using gangue cement as RSB material satisfies the design and application requirements.
Based on the current experimental research and field measurements, the following can be concluded:
The most effective way to adjust the strength of gangue concrete is to change the watercement ratio of the cementing material. For a fixed gangue content, with the raising of the watercement ratio, the strength of gangue concrete demonstrates a nonlinear decreasing trend. The watercement ratio within the range of 0.46–0.60 has the most significant regulation effect on the strength of the gangue concrete.
Mixing concrete with a certain amount of coal gangue as the aggregate has an important effect on the destruction mode and the postpeak carrying characteristics of the concrete. In the postpeak stage, the stress demonstrates a decreasing trend with an increase of the strain, the process of the crack development is relatively slow, and the gangue concrete has a certain postpeak carrying capacity.
Notwithstanding the watercement ratio, the compressive strength of the gangue concrete demonstrates an increasing trend with age. With the addition of the gangue aggregate, the concrete’s hydration and hardening processes are significantly accelerated. At the age of 14 days, the stiffness and strength of the gangue concrete essentially stabilize.
Based on the laboratory test results and the law of underground pressure, industrial tests were carried out at the working face of gobbackfilled GER. The monitoring results demonstrate that with a rational proportion design, the use of gangue concrete as a RSB material can meet the design and application requirements of the GER. The feasibility and rationality of gangue concrete as a RSB material for the GER were proved. A RSB material featuring high safety, high waste utilization rate, fast construction speed, and low costs was provided, guaranteeing the RSB stability for the GER.
The authors declare that they have no conflicts of interest.
This work is supported by the National Natural Science Foundation of China (nos. 51323004, 51674250, 51574228, and 51074163), Major Program of National Natural Science Foundation of China (no. 50834005, 51734009), the Graduate Innovation Fund Project of Jiangsu Province (no. CXZZ13_0924), and Open Fund of State Key Laboratory for Geomechanics and Deep Underground Engineering (SKLGDUEK1409). The authors sincerely acknowledge the former researchers for their excellent works, which greatly assisted our academic study.