energies Article
Mechanical Characteristics of Superhigh-Water Content Material Concretion and Its Application in Longwall Backfilling Xufeng Wang 1,2,3 , Dongdong Qin 1, *, Dongsheng Zhang 1,4 , Chundong Sun 5 , Chengguo Zhang 6 , Mengtang Xu 7 and Bo Li 1 1 2 3 4 5 6 7
*
School of Mines, China University of Mining & Technology, Xuzhou 221116, China;
[email protected] (X.W.);
[email protected] (D.Z.);
[email protected] (B.L.) The Jiangsu Laboratory of Mining-Induced Seismicity Monitoring, China University of Mining & Technology, Xuzhou 221116, China Key Laboratory of Deep Coal Resource Mining, China University of Mining & Technology, Xuzhou 221116, China State Key Laboratory of Coal Resources and Safe Mining, China University of Mining & Technology, Xuzhou 221116, China Jizhong Energy Handan Mining Industry Group, Handan 056002, China;
[email protected] School of Mining Engineering, University of New South Wales, Sydney, NSW 2052, Australia;
[email protected] School of Mines, Guizhou Institute of Technology, Guiyang 550003, China;
[email protected] Correspondence:
[email protected]; Tel.: +86-152-1087-6910
Academic Editor: Vijay Kumar Thakur Received: 19 September 2017; Accepted: 9 October 2017; Published: 13 October 2017
Abstract: Superhigh-water content material (SCM) has been widely utilized for goaf backfilling, grouting, and fire prevention and extinguishing. In this paper, the engineering mechanical characteristics of superhigh-water content material concretion (SCMC) were studied for two types of backfilling technologies in longwall mining—open-type and pocket-type backfilling. The mechanical properties and responses of the SCMC were assessed under different cementation states, varying loading conditions and at different scales. The results indicate that: (1) the compressive and tensile strengths of SCMC specimens in different cementation states increase as the curing time increases—the SCMC formed by a mixture of SCM and gangues has higher strength than that of pure SCM; (2) the SCMC is under different loading and confinement conditions when different backfilling technologies is applied; however the strength of SCMC increases with curing time and decreases with water volume percentage; and (3) large-size specimens of pure SCMC enter into an accelerated creep state at a leveled load of 1.4 MPa. The effects of SCM backfilling on subsidence control has been verified by field applications. The results presented in this paper can provide data support for the optimization of backfill mining technology using SCM, as well as for the design of hydraulic supports parameters at longwall faces. Keywords: superhigh-water content material; engineering mechanical properties; cementation state; confined compression; scales effect
1. Introduction A critical factor in controlling the mining-induced subsidence of overlying strata in mined-out areas is the performance of backfilling material [1,2]. The current backfilling technologies can be classified into three major groups depending on the material used—dry backfill, water-sand backfill, and cemented paste backfill (CPB). Cemented paste backfill is most widely used due to its high strength, Energies 2017, 10, 1592; doi:10.3390/en10101592
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fast backfill speed, and simple application technique. A number of studies have been conducted on CPB that focus on three main areas: (1)
(2)
(3)
Various backfilling materials have been developed based on CPB. In the early 1980s, a mixed cementation material was developed which was called Aquapak that can be solidified with a volume ratio of water up to 85%. This work laid the foundation for subsequent research and development of high-water backfilling materials [3,4]. Sun et al. [5] developed a high-water rapid-setting material with a water volume ratio up to 90%. By improving the sand-soil solidifying backfilling material, they developed a sialite paste-like backfilling material along with another material called Ningtaike-A. Feng et al. [6] developed a type of superhigh-water backfilling material that features a high water content, controllable strength and setting time, as well as sound fluidity of single slurry. Systematic studies have been carried out on the mechanical properties and influencing factors of CPB. Arioglu [7] proposed a formula for calculating the strength of CPB based on the field conditions and the characteristics of CPB. Thomas model is designed to predict the vertical stress at the bottom of CPB [8]. Cui and Fall [9] proposed an evolution elasto-plastic model for CPB. Fall et al. [10] established a water conductivity function model for CPB. Yi et al. [11] improved the ductility of CPB by adding in polypropylene fiber. Klein, Simon, and Hughes et al. used shear wave velocity tests [12], in-situ tests [13], and electromagnetic wave detection [14], to analyse the compressive strength [15], hydration process, and curing time of CPB that contained different cementing agents [16]. Sheng Huang, Fall M., and Nasir O. studied the effects of strain rate, curing time, and mixture ratio on the compressive strength of CPB [17–19]. Experimental studies have been conducted on the fundamental physical properties of CPB. Ghirian and Fall [20] systematically studied the changes in the physical characteristics of CPB including thermal conductivity, pore-water pressure, shear strength, and microstructure under multi-field coupling effect at early curing times. Celestin and Fall [21] and Ouellet et al. [22] analysed the influential factors of the thermal conductivity of CPB and the evolution process of its microstructure, respectively; Ercikdi et al. [23] and Yilmaz et al. [24] proposed a method of quickly evaluating compressive strength using ultrasonic waves that used statistical analysis of the velocity of ultrasonic waves through the CPB under various water-cement ratios and sizes.
Extensive studies on the effect of superhigh-water content material (SCM) in controlling overlying strata in goaf area during backfilling have been conducted, resulting in the development of a technique for assuring the filling rate of SCM, as well as a roof control technology based on the relationship between supports and rocks during longwall goaf filling using SCM [25]. During backfilling of SCM, the cementation states, load conditions and size of superhigh-water content material concretion (SCMC) all change with the roof caving, backfilling face advancing and the filling range increasing and the mechanical properties of SCMC change accordingly. Recent studies have focused on the fundamental properties of SCMC, such as the compressive strength and volume strain of SCMC under different water volume and weathering time. But few involve comprehensive studies on the effects of backfilling technologies, cementation states, confinement conditions, and size on the engineering mechanical properties of SCMC. In this paper, the strength changes and the scale effect of SCMC were explored under various cementation states and load conditions, taking into consideration the backfilling process of both open-type and pocket-type technologies. The experimental results and stability of SCMC are verified through the field application. This study aims to provide guidance on optimizing backfilling techniques and to offer design parameters for using SCM.
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2. Mechanical Behaviour of the Backfilled SCMC 2.1.Energies The Micro-Characteristics of SCMC 2017, 10, 1592
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SCMC was scanned using a scanning electron microscope (SEM) to better understand the SCMC was scanned using a scanning electron microscope (SEM) to better understand the ◦ C. The results show characteristics at at thethe micro scale. The SCMC samples were cured forfor 14 days at 19 characteristics micro scale. The SCMC samples were cured 14 days at 19 °C. The results Energies 2017, 10, 1592 3 of 15 that the concretion contains contains meshy and acicular which arewhich the main ingredients—aluminagel show that the concretion meshy and ettringite, acicular ettringite, are the main ingredients— and other substances that the characteristics is dispersed,isnonfixiform and flocculent. The SEM photos aluminagel andwas other substances the characteristics dispersed, nonfixiform flocculent. The SCMC scanned usingthat a scanning electron microscope (SEM) to betterand understand the (amplification 3000 times) of the concretion with different water volume percentage are showed in SEM photos (amplification 3000 times) of the concretion with different water volume percentage characteristics at the micro scale. The SCMC samples were cured for 14 days at 19 °C. The resultsare Figure 1a,b.in showed Figure 1a,b. show that the concretion contains meshy and acicular ettringite, which are the main ingredients— aluminagel and other substances that the characteristics is dispersed, nonfixiform and flocculent. The SEM photos (amplification 3000 times) of the concretion with different water volume percentage are showed in Figure 1a,b.
(a)
(b)
Figure 1. The scanning electron microscope (SEM) photos of the superhigh-water content material Figure 1. The scanning electron microscope (SEM) photos of the superhigh-water content material concretion (SCMC) micro-characteristics with different water volume(b) percentage. (a) Water volume (a) concretion (SCMC) micro-characteristics with different water volume percentage. (a) Water volume percentage of 95%; (b) water volume percentage of 97%. percentage (b) water volume percentage of 97%. Figure of 1. 95%; The scanning electron microscope (SEM) photos of the superhigh-water content material concretion (SCMC) micro-characteristics with different water volume percentage. (a) Water volume
Figure 1 shows that the form of ettringite will change with the water volume. The acicular and percentage of 95%; (b) water volume percentage of 97%. Figure 1 structure shows that the form of ettringite change with increases. the waterItvolume. The meshacicular rod-shaped of ettrigite becomes slenderwill as water volume is a very thin and rod-shaped structure of ettrigite becomes slender as water volume increases. It is a very thin like structure the water volume is at 97%. The research thatvolume. the main why Figure 1when shows that the form of ettringite will change withshows the water Thereason acicular andthe mesh-like structure whenofthe waterbecomes volumeslender is(SCMC) at 97%. Thehigh research shows that main reason superhigh-water content material concretion has water holding is because of rod-shaped structure ettrigite as water volume increases. It capacity is the a very thin mesh- why thethe superhigh-water content material concretion (SCMC) has high water holding capacity is because presence of the porous structure as the ettringite mutual crisscrosses with ettringite and the gel like structure when the water volume is at 97%. The research shows that the main reason why the of material. thesuperhigh-water presence of thecontent porous structure as the (SCMC) ettringite crisscrosses with ettringite material concretion hasmutual high water holding capacity is becauseand of the the presence of the porous structure as the ettringite mutual crisscrosses with ettringite and the gel gel material. 2.2.material. Engineering State of SCMC Using Open-Type Backfilling 2.2. Engineering State of SCMC Using Open-Type Backfilling Open-type backfilling using superhigh-water content (SCM) is applied for mining upwards the 2.2. Engineering State of SCMC Using Open-Type Backfilling inclined seams, and whenusing the working face has advanced for anisappropriate goaf isthe Open-type backfilling superhigh-water content (SCM) applied fordistance mining the upwards Open-type backfilling using superhigh-water content (SCM) is applied for mining upwards backfilled. Of note that the caving of face the overlying strata is during the backfilling inclined seams, and is when working has advanced fornot aninterfered appropriate distance thethe goaf is inclined seams, and when the working face has advanced for an appropriate distance the goaf is process. This method ensures that there is overlying no interference backfillingduring and mining, which backfilled. Of note is that the caving of the stratabetween is not interfered the backfilling backfilled. Of note is that the caving of the overlying strata is not interfered during the backfilling simplifies technological A schematic diagram between of an open-type backfilling of SCM is process. Thisthe method ensuresprocess. that there is no interference backfilling and mining, which process. This method ensures that there is no interference between backfilling and mining, which shown in Figure 2. simplifies the technological process. A schematic diagram of an open-type backfilling of SCM is shown simplifies the technological process. A schematic diagram of an open-type backfilling of SCM is
in Figure 2. in Figure 2. shown
Figure 2. Schematic Diagram of an Open-type Backfilling of SCM. 1-main roof; 2-immediate roof; 3Figure 2.body; Schematic Diagram of an Open-type Backfilling of SCM. roof; 2-immediate roof; 3backfilled 4-hydraulic support; 5-coal body; α-coal bed pitch.1-main Figure 2. Schematic Diagram of an Open-type Backfilling of SCM. 1—main roof; 2—immediate roof; backfilled body; 4-hydraulic support; 5-coal body; α-coal bed pitch.
3—backfilled body; 4—hydraulic support; 5—coal body; α-coal bed pitch.
In practice, SCM has varying cementation states due to the difference in backfilling timing, strata In practice, SCM has varying cementation due to the difference in backfilling strata characteristic and caving behaviours. When thestates roof is relatively intact, the SCMC are timing, primarily pure characteristic and caving behaviours. When the roof is relatively intact, the SCMC are primarily pure superhigh-water cemented paste; if the roof is fractured, the SCMC will be a mixture of SCM and superhigh-water cemented paste; if the roof is fractured, the SCMC will be a mixture of SCM and caved rocks. Based on relative position between the caved strata in the goaf and backfilled material, caved rocks. Based on relative position between the caved strata in the goaf and backfilled material,
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In practice, SCM has varying cementation states due to the difference in backfilling timing, strata characteristic and caving behaviours. When the roof is relatively intact, the SCMC are primarily pure superhigh-water cemented paste; if the roof is fractured, the SCMC will be a mixture of SCM and caved Energies 2017, 10, 1592 4 of 15 EnergiesBased 2017, 10, 4 of 15 rocks. on1592 relative position between the caved strata in the goaf and backfilled material, the SCMC can be preliminarily classified into the following four types (as shown in Figure 3): Type I—pure SCMC; the SCMC can be preliminarily classified into the following four types (as shown in Figure 3): Type the SCMC can be preliminarily classified intoof the following four types shown in Figure 3): Type Type II—substructure of a mixture SCM and andof(as superstructure consisting of I—pure SCMC; Typeconsisting II—substructure consisting of agangues mixture SCM and gangues and I—pure SCMC; Type II—substructure consisting of a mixture of SCM and gangues and a pure SCMC; Type III—substructure consisting of a pure SCMC and superstructure consisting of superstructure consisting of a pure SCMC; Type III—substructure consisting of a pure SCMC and superstructure consisting of a pure Type III—substructure consisting of pure SCMC and a mixture of SCM and gangues; andSCMC; Type whole structure of aa mixture of SCM superstructure consisting of a mixture of IV—the SCM and gangues; and consisting Type IV—the whole structure superstructure consisting of a mixture of SCM and gangues; and Type IV—the whole structure and gangues. consisting of a mixture of SCM and gangues. consisting of a mixture of SCM and gangues.
(a) Type I (a) Type I
(b) Type II (b) Type II
(c) Type III (c) Type III
(d) Type IV (d) Type IV
Figure 3. Classification of the backfilled SCMC. 1-main roof; 2-immediate roof; 3-hydraulic support; Figure Classificationofofthe thebackfilled backfilled SCMC. 1—main 2—immediate roof; 3—hydraulic Figure 3. 3. Classification SCMC. 1-main roof;roof; 2-immediate roof; 3-hydraulic support; 4-coal body. support; 4—coal body. 4-coal body.
2.3. Stress State of SCMC Using Pocket-Type Backfilling 2.3. 2.3. Stress Stress State State of of SCMC SCMC Using Using Pocket-Type Pocket-TypeBackfilling Backfilling Pocket-type backfilling using SCM is a very simple process. The prepared paste fluid is delivered Pocket-type Pocket-typebackfilling backfillingusing usingSCM SCMisisaa very very simple simple process. process. The The prepared prepared paste paste fluid fluid is delivered delivered by pipelines to a mixer that is close to the filling working face; after being mixed, the paste is delivered by by pipelines pipelines to to aa mixer mixer that that is close to the filling working face; after being mixed, mixed, the paste paste is delivered delivered to the pockets that are pre-arranged inside the goaf. After solidification inside the pockets, the paste to to the the pockets pockets that that are are pre-arranged pre-arranged inside inside the the goaf. goaf. After After solidification solidification inside inside the the pockets, pockets, the the paste paste fluid acts as a support to the overlying strata. This method has a large scope of applicability as it is fluid acts as a support to the overlying strata. This method has a large scope of applicability fluid the overlying strata. This method has a large scope of applicability asas it it is convenient to install the pockets and the pockets can impose restrictions on the cemented paste. A is convenient install pockets and pockets can impose restrictions cemented paste. convenient to to install thethe pockets and thethe pockets can impose restrictions onon thethe cemented paste. A schematic diagram of the pocket-type backfilling of SCM is shown in Figure 4. A schematic diagram the pocket-type backfillingofofSCM SCMisisshown shownininFigure Figure4.4. schematic diagram ofof the pocket-type backfilling
Figure 4. Compression State of the backfilled SCMC. 1-main roof; 2-immediate roof; 3-floor; 4Figure 4. Compression State of the backfilled SCMC. 1-main roof; 2-immediate roof; 3-floor; 4Figure 4. support; Compression the backfilled 1—main roof; 2—immediate roof; 3—floor; hydraulic 5-coalState body;of6-SCMC within SCMC. 24 h; 7-SCMC after 24 h; L-width of pocket. hydraulic support; 5-coal body; 6-SCMC within 24 h; 7-SCMC after 24 h; L-width of pocket. 4—hydraulic support; 5—coal body; 6—SCMC within 24 h; 7—SCMC after 24 h; L-width of pocket.
The pocket-type backfilling is commonly used for the working scheme with 3 shifts per day. A The pocket-type backfilling is commonly used for the working scheme with 3 shifts per day. A workThe flowpocket-type is adopted in which each backfilling should bethe conducted two cutting cycles.per Within backfilling is commonly used for workingafter scheme with 3 shifts day. work flow is adopted in which each backfilling should be conducted after two cutting cycles. Within the first 24 h (in the early stage of backfilling), the SCMC is subjected to relatively small loading from A work flow is adopted in which each backfilling should be conducted after two cutting cycles. the first 24 h (in the early stage of backfilling), the SCMC is subjected to relatively small loading from the roofthe and its24displacement or right isthe restricted. is no in front of Within first h (in the earlytowards stage ofleft backfilling), SCMC is There subjected to confinement relatively small the roof and its displacement towards left or right is restricted. There is no confinement in loading front of the SCMC in the presence of the working face, which gives an unconfined space for the SCMC. Within from the roof andpresence its displacement towards leftwhich or right is restricted. Therespace is nofor confinement front the SCMC in the of the working face, gives an unconfined the SCMC.in Within 24 h, the SCMC is in a partly confined state, specifically subjected to compression from all directions 24 h, the SCMC is in a partly confined state, specifically subjected to compression from all directions except for the direction of the working face. After 24 h (the later stage of backfilling), with the new except for the direction of the working face. After 24 h (the later stage of backfilling), with the new SCMC filling the goaf and hardening quickly, the previous SCMC is also confined in the forward SCMC filling the goaf and hardening quickly, the previous SCMC is also confined in the forward direction along with the sagging of the roof, the SCMC starts to bear a load gradually, which subjects direction along with the sagging of the roof, the SCMC starts to bear a load gradually, which subjects the SCMC to completely confined compression. the SCMC to completely confined compression.
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of the SCMC in the presence of the working face, which gives an unconfined space for the SCMC. Within 24 h, the SCMC is in a partly confined state, specifically subjected to compression from all directions except for the direction of the working face. After 24 h (the later stage of backfilling), with the new SCMC filling the goaf and hardening quickly, the previous SCMC is also confined in the forward direction along with the sagging of the roof, the SCMC starts to bear a load gradually, which subjects Energies 2017, 10, 1592 5 of 15 the SCMC to completely confined compression. 3. Laboratory Testing 3. Laboratory Testing
3.1. Experiments Facilities and System Setups 3.1. Experiments Facilities and System Setups (1) Mold and Curing Box (1) Mold and Curing Box In this study, a cylindrical and cuboid molds are available for making SCMC specimens. The In this study, a cylindrical and cuboid molds are available for making SCMC specimens. cylindrical mold has an outer diameter of 70 mm, an inner diameter of 50 mm, and a height of 100 The cylindrical mold has an outer diameter of 70 mm, an inner diameter of 50 mm, and a height of mm; while the cuboid mold has a size of 4 × 100 mm, with both height and width of 100 mm, which 100 mm; while the cuboid mold has a size of 4 × 100 mm, with both height and width of 100 mm, can make four cube specimens at one time. which can make four cube specimens at one time. In order to simulate the field moist environment, the curing box is used to adjust the temperature In order to simulate the field moist environment, the curing box is used to adjust the temperature and humidity and cure the specimens. and humidity and cure the specimens. (2) Large-size Mechanical Properties Testing System for SCMC (2) Large-size Mechanical Properties Testing System for SCMC To study the scale effect of SCMC, a 10,000 kN large-size testing system (1500 mm × 600 mm × To study the scale effect of SCMC, a 10,000 kN large-size testing system 900 mm) was developed and used for this research. The system mainly consists of limiting steel bars, (1500 mm × 600 mm × 900 mm) was developed and used for this research. The system mainly bearing plates, hydraulic pillows, stress stabilizing systems, displacement sensors, and static consists of limiting steel bars, bearing plates, hydraulic pillows, stress stabilizing systems, displacement resistance strain gauge, as shown in Figure 5. sensors, and static resistance strain gauge, as shown in Figure 5. Bearing plate Hydraulic pillow Limiting steel bar Large-size specimen
P
Electric bridge box
YHD-type displacement sensor
Static resistance strain gauge Pressure stabilizing system
Computer
High pressure oil pipe
Figure 5. System component schematics. Figure 5. System component schematics.
3.2. Samples Preparation 3.2. Samples Preparation (1) Experimental Materials (1) Experimental Materials The SCM is primarily made from materials A and B. Material A is obtained by refining bauxite The SCM is primarily made from materials A and B. Material A is obtained by refining bauxite and gypsum, while material B by grinding gypsum and lime. During the process, a small amount and gypsum, while material B by grinding gypsum and lime. During the process, a small amount of composite super-retarding dispersing agent (material AA) is added into material A; and a small amount of composite accelerator (material BB) is added into material B. The volume ratio of materials A and AA to materials B and BB is 1:1. The materials for the SCMC used in the experiments came from the backfilling station of Taoyi
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of composite super-retarding dispersing agent (material AA) is added into material A; and a small amount of composite accelerator (material BB) is added into material B. The volume ratio of materials A and AA to materials B and BB is 1:1. The materials for the SCMC used in the experiments came from the backfilling station of Taoyi Coal Mine of Handan Mining Group; the water came from tap water in the laboratory (water temperature being 18 ◦ C); and the gangue was obtained from the longwall panel and was crushed into required particle sizes. (2)
Standard Samples
Four types of SCMC specimens were prepared in this study. After curing, mechanical property tests were conducted to analyse the strength characteristics of the four types of SCMC at different curing times. Cylinder molds were used to prepare specimens for four types of SCMC. In the mixture the mass ratio of gangue to superhigh–water materials was 1:5 and water in the SCMC had a volume percentage of up to 95%. Standard specifications were used with the diameter to 50 mm for the compression and tension test. The specimens were placed inside the curing box at 20 ◦ C and humidity of 99%, which were cured for 2 h, 1 day, 2 days, 3 days, 4 days, and 5 days respectively. A set of four specimens were prepared for each type of SCMC at the different curing durations, for the three types of tests. (3)
Specimens for Confined Compression Tests
In order to understand the mechanical property changes that the SCMC goes through after being filled in the goaf, it is necessary to conduct both partly confined and completely confined compression tests on the SCMC based on the loading characteristics in the goaf. For this purpose, two types of specimens were prepared for two different stress states: a.
b.
(4)
Specimens subject to partly confined compression stress (early stage of backfilling, within 24 h): 324 cube specimens (100 × 100 × 100 mm), one-half with a water volume percentage of 95% and one half with a water volume percentage of 94%, were prepared and cured for 2 h, 8 h, and 24 h. Specimens subject to completely confined compression stress (late stage of backfilling, after 24 h): 160 cylinder specimens (Φ = 50 mm, h = 100 mm) in 20 groups, one-half with a water volume percentage of 95% and one-half with a water volume percentage of 94%, were prepared and cured for 1 day, 3 days, 5 days, 7 days, 9 days, 15 days, 19 days, and 20 days.
Large-size Specimen
Large-size specimen with dimensions of 1500 mm × 600 mm × 900 mm, and with a water volume ratio of 95%, was prepared using designed steel formwork. A handheld vibrating rod was used for vibration and tamping. The specimen was cured for 7 days in a sealed environment. 3.3. Testing Method and Process (1)
Mechanical Property Test of Standard Specimens
MTS815.02 (SHT5305, Mechanical Testing and Simulation Systems Corporation, Minneapolis, MN, USA) testing system is used to apply a load onto the four types of specimens until the specimens are yielded. The data collected automatically by the system is used to analyse the strength of the specimens. (2)
Mechanical Property Test of Confined Samples
After curing, the specimens are placed into the compression molds and the loading rate of the MTS815.02 testing system is set at 70 N/s to conduct confined compression tests. Compressive force is applied onto the specimens until they are fully damaged, and the test data is exported to analyse the strength of the specimens. This test process is shown in Figure 6.
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(a) (a)
(b) (b)
Figure 6. 6. Confined Confined compression compression test. test. (a) Partly confined confined compression compression test; test; (b) (b) Completely Completely confined confined Figure (a) Partly Figure 6. Confined compression test. (a) Partly confined compression test; (b) Completely confined compression test. compression test. compression test.
(3) Creep Property Test of Large-Size Specimen (3) (3) Creep Creep Property Property Test Test of ofLarge-Size Large-SizeSpecimen Specimen The large-size mechanical property testing system is used to conduct a compression test on the The mechanical property testing system is to conduct The large-size large-size testing system and is used used toeffect. conduct aa compression compression test test on on the the large-size specimen,mechanical in order toproperty study it creep property scale large-size specimen, in order to study it creep property and scale effect. large-size specimen, in order to study it creep property and scale effect. The stress stabilizing system applies a load at a rate of 0.1 MPa per level for a time duration of The stress stabilizing system applies aa load load at at aa rate of MPa level aa time of Thefor stress stabilizing system applies rate of 0.1 0.1 MPa per per DaHao level for forElectronic time duration duration of 120 min each level. A YHD100-type displacement sensor (YHD-100, Co., Ltd., 120 min for each level. A YHD100-type displacement sensor (YHD-100, DaHao Electronic Co., Ltd., 120 min forGuangdong, each level. A YHD100-type sensor (YHD-100, DaHao Electronic Ltd., Shenzhen, China) is used todisplacement monitor lateral deformation, and fixed scale stripsCo., are used Shenzhen, China) is used to monitor lateral deformation, and fixed scale strips are used to are monitor Shenzhen, Guangdong, China) is used to monitor lateral deformation, and fixed scale strips used to monitor axial deformation. The displacement sensor is connected to a TS3890A static resistance axial deformation. The displacement sensor is connected a TS3890Atostatic resistance strain gauge to monitor deformation. The displacement sensor istoconnected a TS3890A static resistance strain gaugeaxial in the form of a half-bridge connection, which collects data at intervals of 6 s and stores in the form of a half-bridge connection, which collects data at intervals of 6 s and stores data in the strain gauge in the form of a half-bridge connection, which collects data at intervals of 6 s and stores data in the database. With regards to the monitoring setup, monitoring lines #1, #2, #3, #4, #5, and #6 database. With regards to regards the monitoring setup, monitoring lines #1, #2, #3,#1, #4,#2, #5,#3, and fall in#6 a datainina the database. With to the monitoring setup, monitoring #4,#6 #5, and fall horizontal direction; and monitoring lines #7, #8, #9, #10, #11, andlines #12 are in an axial direction. horizontal direction; and monitoring lines #7, #8, #9, #10, #11, and #12 are in an axial direction. fall inAalarge-size horizontalmechanical direction; and monitoring linessystem #7, #8, is #9,used #10, #11, and #12 are in direction. properties testing to apply a load onantoaxial the large-size A large-size mechanical properties testing system is is used used to to apply aa load on to the large-size A large-size mechanical properties testing system apply load on to the large-size specimen in accordance with the loading plan. The loading and destruction process of large-size specimen accordance with the specimen in in accordance with 7. the loading loading plan. plan. The loading loading and and destruction destruction process process of of large-size large-size specimens is shown in Figure specimens is shown in Figure 7. in Figure 7.
(a) (a)
(b) (b)
Figure 7. Loading and destruction process of large-size specimen. (a) Specimen without damage; (b) Figure 7. 7. Loading specimen. (a) (a) Specimen without damage; (b) Totally destroyed. Figure Loadingand anddestruction destructionprocess processofoflarge-size large-size specimen. Specimen without damage; Totally destroyed. (b) Totally destroyed.
4. Analysis of Experiments Results 4. Analysis of Experiments Results 4. Analysis of Experiments Results 4.1. Mechanical Properties of SCMC with Different Cementation States 4.1. Mechanical Properties of of SCMC SCMC with with Different Different Cementation Cementation States States 4.1. Mechanical Properties The compressive strength characteristics of four types of specimens at the same curing time, as The compressive strength characteristics four types of specimens at same the same curing as The compressive strength characteristics ofof four types of specimens atwere the curing as well well as relationships between strength and curing time of the specimens analysed intime, this time, section. well as relationships between strength and curing ofspecimens the specimens analysed in this section. as relationships between strength and curing timetime of the werewere analysed in this section. (1) Compressive Strength Characteristics of Specimens at the Same Setting Time (1) Compressive Compressive Strength Strength Characteristics Characteristics of of Specimens Specimens at at the the Same Same Setting Setting Time Time (1) Figure 8 shows compressive strength change characteristics of the four types of specimens after Figure 8 shows strength change characteristics of the four types of specimens after the same curing timecompressive of 24 h. the same curing time of 24 h.
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Figure 8 shows compressive strength change characteristics of the four types of specimens after Energies 2017, 10, 1592 8 of 15 the same curing time of 24 h. Energies 2017, 10, 1592 8 of 15 11
σ/MPa σ/MPa
0.8 0.8
Ⅰ Ⅰ
Ⅱ Ⅱ
Ⅲ Ⅲ
Ⅳ Ⅳ
0.6 0.6 0.4 0.4 0.2 0.2 00
00
0.02 0.02
0.04 0.04
0.06 0.06
εε
0.08 0.08
0.1 0.1
four types of specimens after a curing time of 24 h. Figure Figure 8. 8. Stress-strain Stress-strain curves curves of of the the four types of specimens after a curing time of 24 h.
Figure 88 indicates indicates the the type type IV IV specimens specimens have the the highest ultimate strength, and and type-II type-II and and Figure specimens have the highest highest ultimate ultimate strength, type-III specimens specimens have about 82.76% 82.76% and IV type-III have similar similar ultimate ultimate strength, and 80.46% 80.46% of of type type IV strength, respectively, respectively, about specimens. The ultimate strength of type I specimens is the lowest, about 58.62% of type IV specimens. specimens. specimens. The ultimate ultimate strength strength of of type type I specimens specimens is is the lowest, lowest, about about 58.62% 58.62% of type IV specimens. specimens. The stress stress within within the four types specimens at five stages during the loading process: original pore The the four types specimens at five stages during the loading process: original pore within the four types specimens at five stages during the loading process: compaction stage, linear elastic stage, elastoplastic transition stage, plastic stage, and post-failure compaction stage, elastoplastic transition stage, plastic stage,stage, and post-failure stage. compaction stage, stage,linear linearelastic elastic stage, elastoplastic transition stage, plastic and post-failure stage. The pore type-I is longest, and type The original pore compaction stage of stage type-Iof is the longest, the type specimens stage. The original original pore compaction compaction stage ofspecimens type-I specimens specimens is the theand longest, andIVthe the type IV IV specimens quickly into the linear elastic stage and reach the ultimate strength. quickly intoquickly the linear stageelastic and reach strength. specimens intoelastic the linear stagethe andultimate reach the ultimate strength. (2) Strength Strength Characteristics Characteristics at Different Curing Times (2) Characteristics at at Different Different Curing Curing Times Times Figure shows the time history curves of the the four of The Figure time history curves of the of theof four specimens. The strength Figure 999shows showsthe the time history curves of strength the strength strength of thetypes fouroftypes types of specimens. specimens. The strength of four types of increases as goes and meanwhile, the increment of the four types specimen increases as curing timetime goes on,on, and strength of the the fourof types of specimen specimen increases as curing curing time goes on, andmeanwhile, meanwhile,the the increment increment decreases gradually. specimens have the highest compressive strength, followed by decreases gradually. The Thetype-IV type-IV specimens have highest compressive strength, followed by gradually. The type-IV specimens have thethe highest compressive strength, followed by typetypeII and type-III specimens, which have similar compressive strength; compressive strength of type type-II and type-III specimens, which have similar compressive strength; compressive strength II and type-III specimens, which have similar compressive strength; compressive strength of typeofII specimens is about lower that of Type have type I specimens is the lowest, about 20%than lower than that IV of specimens. type IV specimens. Type II specimens specimens is the the lowest, lowest, about 20% 20% lower than that of type type IV specimens. Type II II specimens specimens have the the highest tensile strength-about 40% higher than type-I specimens. None of the four types of specimens have thetensile highest tensile strength-about 40% higher than type-I specimens. None the four types of highest strength-about 40% higher than type-I specimens. None of the fourof types of specimens has high in the strength is 1/10 of specimens has a particularly highstrength; tensile strength; in all samples, the tensile strength is only of has aa particularly particularly high tensile tensile strength; in all all samples, samples, the tensile tensile strength is only only 1/101/10 of the the compressive strength. the compressive strength. compressive strength.
1.5 1.5
0.15 0.15
1 1
0.5 0.5 0 0
Ⅰ Ⅰ Ⅲ Ⅲ 0 0
20 20
40 40
60 60 t/h t/h
(a) (a)
80 80
Ⅱ Ⅱ Ⅳ Ⅳ 100 120 100 120
RR t /MPa t /MPa
0.2 0.2
/MPa RR c c/MPa
2 2
0.1 0.1
0.05 0.05 0 0
0 0
20 20
40 40
60 60 t/h t/h
80 80
Ⅰ Ⅰ Ⅲ Ⅲ 100 100
Ⅱ Ⅱ Ⅳ Ⅳ 120 120
(b) (b)
Figure change characteristics of specimens at curing time. (a) Compressive strength; (b) Figure 9. 9. Strength Strength at at curing time. (a) (a) Compressive strength; (b) Figure 9. Strengthchange changecharacteristics characteristicsofofspecimens specimens curing time. Compressive strength; Tensile strength. Tensile strength. (b) Tensile strength.
4.2. 4.2. Deformation Deformation Behaviour Behaviour of of SCMC SCMC under under Confined Confined Compression Compression 4.2. Deformation Behaviour of SCMC under Confined Compression Analysis Analysis was was carried carried out out on on the the strain strain (ε) (ε) characteristics characteristics of of specimens specimens under under different different confined confined Analysisconditions. was carriedThe outresults on theare strain (ε) characteristics of specimens under different confined compression as follows: compression conditions. The results are as follows: compression conditions. The results are as follows: (1) (1) Specimens Specimens Subject Subject to to Partly Partly Confined Confined Compression Compression The The stress-stain stress-stain curves curves of of specimens specimens subjected subjected to to partly partly confined confined compression compression (within (within 24 24 h h after after backfilling) are depicted in Figure 10, which shows that the strength of the specimens increases backfilling) are depicted in Figure 10, which shows that the strength of the specimens increases with with curing time; after the same curing time, the strength of the specimens is inversely related to the water curing time; after the same curing time, the strength of the specimens is inversely related to the water
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Specimens Subject to Partly Confined Compression
The stress-stain curves of specimens subjected to partly confined compression (within 24 h9after of 15 of 15 backfilling) are depicted in Figure 10, which shows that the strength of the specimens increases9with curing after theWithin same curing thetime, strength of the specimens is inversely related to the volumetime; percentage. 24 hh of oftime, curing the maximum maximum strength of the the specimens specimens is 0.65 0.65water MPa volume percentage. Within 24 curing time, the strength of is MPa volume percentage. Within 24 h of curing time, the maximum strength of the specimens is 0.65 MPa at at aa water water volume volume percentage percentage of of 94%, 94%, and and 0.24 0.24 MPa MPa at at aa water water volume volume percentage percentage of of 95%. 95%. at a water volume percentage of 94%, and 0.24 MPa at a water volume percentage of 95%. Energies 2017, 10, 1592 Energies 2017, 10, 1592
0.7 0.7 0.6 0.6 σ/MPa σ/MPa
0.5 0.5 0.4 0.4
94% 02h 02h 94%
94% 08h 08h 94%
94% 24h 24h 94%
95% 02h 02h 95%
95% 08h 08h 95%
95% 24h 24h 95%
0.3 0.3 0.2 0.2 0.1 0.1 00
00
0.05 0.05
0.1 0.1
0.15 0.15 εε
0.2 0.2
0.25 0.25
0.3 0.3
Figure 10. Stress-strain curves curves of compression. 10. Stress-strain of specimens specimens subjected subjected to to partly partly confined confined compression. compression. Figure
(2) Specimens Specimens Subject Subject to to Completely Completely Confined Confined Compression Compression (2) a. Deformation Deformation behaviour behaviour of of specimens specimens under under different differentload loadconditions conditions a. different load conditions Different loads loads were were applied applied to to the the specimens specimens to to simulate simulate changes changes at at different different burial burial depths depths and and Different to study studythe thedeformation deformation behaviour of specimens specimens at different different curing durations. As time curing time to study the deformation behaviour of at durations. As curing time behaviour of specimens at different curingcuring durations. As curing elapses, elapses, the specimens became less deformed, and there is a smaller increment in deformation—after elapses, the specimens less deformed, and is a smaller increment in deformation—after the specimens became became less deformed, and there is there a smaller increment in deformation—after 7 days, 7 days, days, the deformation of the specimens stabilized and was maintained at 5.2 mm, with 7the the deformation of the specimens stabilized and was maintained at 5.2 mm, with aa deformation of the specimens stabilized and was maintained at 5.2 mm, with a compression ratio compression ratio of of 5.2%. 5.2%. compression ratio of 5.2%. Different loads were applied to to the the specimens specimens to to simulate simulate changes changes at at different different burial burial depths depths and and Different loads were applied to study the deformation behaviour of specimens at different curing durations. Figure 11 depicts the to study the deformation behaviour of specimens at different curing durations. Figure 11 depicts the deformation curves curves of specimens under under these conditions and indicates that as curing time elapses, deformation deformation curves of of specimens specimens under these these conditions conditions and and indicates indicates that that as as curing curing time time elapses, elapses, the specimens became less deformed and there is a smaller increment in deformation—after days, the specimens became became less less deformed deformed and and there there is is a smaller smaller increment increment in deformation—after deformation—after 777 days, days, the deformation of the specimens stabilized and maintained at 5.2 mm, with a compression ratio of the deformation mm, with a compression ratio of deformation of of the thespecimens specimensstabilized stabilizedand andmaintained maintainedatat5.25.2 mm, with a compression ratio 5.2%. 5.2%. of 5.2%.
∆L/mm ∆L/mm
12 12 10 10 88 66 44 22 00
01d 01d 00
55
10 10
03d 03d
15 15 σ/MPa σ/MPa
05d 05d 20 20
07d 07d 25 25
09d 09d 30 30
Figure 11. 11. Deformation Deformation behaviour behaviour of of specimens specimens under under different different loads loads at at different different ages. ages. different Figure ages.
b. Critical Critical Deformation Deformation Load Load Characteristics Characteristics of of Specimens Specimens b. After the the specimens specimens were were subjected subjected to to completely completely confined confined compression—along compression—along with with increases increases After of load—water oozed from the specimens. The load under which water begins to ooze from the the of load—water oozed from the specimens. The load under which water begins to ooze from specimens is considered as the critical deformation load. Statistical analyses were performed on the specimens is considered as the critical deformation load. Statistical analyses were performed on the test data data in in order order to to determine determine the the critical critical deformation deformation load. load. The The curing curing time time curves curves of of the the specimens specimens test are shown in Figure 12. are shown in Figure 12.
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After the specimens were subjected to completely confined compression—along with increases of load—water oozed from the specimens. The load under which water begins to ooze from the specimens is considered as the critical deformation load. Statistical analyses were performed on the test data in order to determine the critical deformation load. The curing time curves of the specimens are shown 12. Energies 2017,in 10,Figure 1592 10 of 15
σ/MPa
16 14 12 10 8 6 4 2 0
95% 0
2
4
6
8
10 12 T/day
94% 14
16
18
20
Figure 12. 12. Critical Critical deformation deformation load load characteristics characteristics of Figure of the the specimens. specimens.
shown that, that, at athigher higherwater watervolume volumepercentage, percentage,the thecritical critical deformation load is lower. Prior It is shown deformation load is lower. Prior to 7 days, critical deformation of specimens increases rapidly; 7 days, it increases at a 7todays, thethe critical deformation loadload of specimens increases rapidly; after after 7 days, it increases at a lower lower amplitude, and remains at 14.3 about 14.3(water MPa (water percentage 94%) MPa amplitude, and remains at about MPa volumevolume percentage of 94%)ofand 10 and MPa10 (water (water volume percentage volume percentage of 95%).of 95%). 4.3. Creep Property and Scale Effect of SCMC To To analyse creep property and the the scale effect of the experimental large-size analyse creep property and scale effect of SCMC, SCMC, the experimental results of large-size specimens and small-size specimens specimens were were compared. compared. (1) Creep Property Property For the purpose of analysis, monitoring data were taken from the axial deformation monitoring lines #7 #7 and and #9, #9,from fromwhich whichdisplacement-time displacement-timecurves curvesofof the two monitoring lines were derived, the two monitoring lines were derived, as as shown in Figure shown in Figure 13.13. 0.3
monitoring line #7
0.25
monitoring line #9
ε
0.2 0.15 0.1 0.05 0 0
20,000
40,000
t/s
60,000
80,000
100,000
Figure 13. Displacement–time curves curves of Figure 13. Displacement–time of monitoring monitoring lines lines #7 #7 and and #9. #9.
Creep rupture rupture of of large-size large-size specimens specimens occurs occurs when when the the axial axial deformation deformation is mm and and axial axial Creep is 14 14 mm strain is is 0.154. 0.154. When the levelled levelled load load is is less less than than strain When the the loading loading time time is is shorter shorter than than 79,200 79,200 ss (22 (22 h) h) and and the 1.1 MPa, the average strain difference between each level is relatively small. After the levelled load 1.1 MPa, the average strain difference between each level is relatively small. After the levelled load exceeding 1.1 MPa, the average strain difference between each level begins to increase gradually and exceeding 1.1 MPa, the average strain difference between each level begins to increase gradually and the specimens state. Meanwhile, as cracks develop, the specimens still the specimens enter enterinto intoan anaccelerated acceleratedcreep creep state. Meanwhile, as cracks develop, the specimens have bearing capacity, and demonstrate strong plastic deformation features; cracks develop and propagate through the specimens in the axial direction. (2) Scale effect The experimental results show that the stress-strain change rules of specimens with different scales are basically the same, except for the compressive strengths and maximum deformations,
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still have bearing capacity, and demonstrate strong plastic deformation features; cracks develop and propagate through the specimens in the axial direction. (2)
Scale effect
The experimental results show that the stress-strain change rules of specimens with different scales are basically the same, except for the compressive strengths and maximum deformations, which are slightly different. The compressive strength of large-size specimens is approximately 1.41 MPa, while the standard specimens have a compressive strength of approximately 1.48 MPa (a 4.96% increase); the maximum strain of the large-size specimens prior to rupture is about 0.15, for standard specimens it is about 0.16 (a 6.67% increase). The above data indicates that the scale effect of the tested SCMC is not significant. 5. Application of SCMC Mechanical Characteristics in Longwall Backfilling According to the laboratory test results of the SCMC engineering mechanical characteristics, in the Taoyi Coal Mine of Handan Mining Group, backfilling using SCM is optimized. Through the monitoring data of surface subsidence and SCMC bearing stress, the stability of SCMC is verified. 5.1. Back Filling Rate and Backfilled Body Strength Design Higher filling rates can improve overburden strata control performance during longwall retreat. However, excessive filling rates complicate and increase the costs of the operations. Consequently, the filling rate of the working face should be optimized based on the seam conditions and the control standards of the surface subsidence. The designed filling rate of the working face (η f ) can be obtained using the mining height (M0 ), maximum permitted mining height (MZ ), and roof sagging rate prior to filling (η 0 ), and is given by [25]: (1 + η0 ) M0 − MZ ηf = (1) M0 In the Equation (1), mining height (M0 ) and roof sagging rate prior to filling (η 0 ) can be obtained by field measurement; maximum permitted mining height MZ = min(rimax /qcosα, r2 Kmax /1.52qcosα, rεmax /1.52bqcosα), and q is the surface subsidence coefficient, α is the dip of the coal seam, b is the horizontal movement coefficient, r is the major influence radius, imax is the maximum permitted surface dip of the building, Kmax is the maximum permitted surface curvature, εmax is the maximum amount of surface horizontal deformation. Different SCM backfilling technologies were implemented in Taoyi Coal Mine of Handan Mining Group. The average mining height of the coal seam at Taoyi Coal Mine is approximately 4 m; its cover depth, average seam dip, surface subsidence coefficient, horizontal movement coefficient, major influence radius, and roof sagging rate before filling are approximately 400 m, 10◦ , 0.78, 0.35, 200 m, and 4%, respectively. For the surface building protection level of II, the allowed amounts of surface deformations are: imax = 5.0, Kmax = 0.2, and εmax = 4.0. According to these indicators, the maximum permitted mining height of the coal seam in Taoyi Coal Mine is approximately 1.3 m and the filling rate in the working face should be not less than 72%. The details of SCM backfilling in Taoyi Coal Mine are shown in Table 1. Table 1. The details of SCM backfilling in the Taoyi Coal Mine. Working Face Backfill technology Filling rate/(%)
No. 1 BF open-type 82
No. 2 BF
No. 3 BF
No. 4 BF
pocket-type open-type 83.7
92.1
No. 5 BF
open & pocket-type pocket-type 82 98.7
BF: Backfilling face.
No. 6 BF
No. 12702 BF
No. 12706 BF
open-type
pocket-type
pocket-type
85
89
82
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The main key layer that plays a critical role in controlling the subsidence of overlying strata will not break during the backfilling process. The strength of backfilled body should be sufficient to support the weight of the caved strata under the main key layer. The required backfilled body strength can be obtained using the average density and the thickness of layers under the main key layer, and is given by: Energies 2017, 10, 1592 12 of 15 σn = ρ a gH (2)
In Equation Equation (2), (2),σσn is is the the strength strengthof ofbackfilled backfilledbody, body,ρ ρis a is the average density of overburden, H In the average density of overburden, H is n a is the thickness of layers under the main key layer, g is the acceleration of gravity. the thickness of layers under the main key layer, g is the acceleration of gravity. In the Taoyi Coal Mine, the maximum thickness of layers under the main layer is 291.7 m In the Taoyi Coal Mine, the maximum thickness of layers under the main keykey layer is 291.7 m (at −3 (at No. 12706 backfilling face), average density of overburden the overburden is 2500 kg·m . According −3 . According No. 12706 backfilling face), andand the the average density of the is 2500 kg·m to to Equation (2), the required strength of backfilled body is 7.15 MPa. Equation (2), the required strength of backfilled body is 7.15 MPa. 5.2. Impacts Impacts of of SCMC SCMC Backfilling Backfilling on on Ground Ground Control Control 5.2. (1) Surface Surface Subsidence Subsidence Characteristics Characteristics in in the the Filling Filling Region Region (1) Taking the the surface surface subsidence subsidence situations situations of of No. No. 66 backfilling backfilling face face and and No. No. 12706 12706 backfilling backfilling face face Taking in the the Taoyi Taoyi Coal Coal Mine Mine as as an an example, example, the the surface in surface subsidence subsidence control control effect effect after after mining mining is is explained. explained. The surface subsidence situations after mining are shown in Figure 14 and the corresponding values The surface subsidence situations after mining are shown in Figure 14 and the corresponding values are shown in Table 2. are shown in Table 2. Table 2. Table 2. The value of surface subsidence subsidence situations situations in in the the filling filling region. region.
Working Face Working Face No. 6 backfilling face No. 6 backfilling face No. 12706 backfilling face No. 12706 backfilling face
Maximum Surface Subsidence Subsidence Maximum Surface Filling Rate/(%) Filling Rate/(%) Subsidence Coefficients Values/mm Coefficients Subsidence Values/mm 405 85 0.101 405 85 0.101 415 82 0.103 415 82 0.103
Subsidence coefficients of No. 6 backfilling face and No. 12706 backfilling face are 0.101 and Subsidence coefficients No. 6 backfilling facesurface and No. 12706 backfilling face(0.78) are 0.101 and 0.103 0.103 respectively, which is of less than the average subsidence coefficient at Taoyi Coal respectively, which is less that thanthe theSCM average surfacetechnology subsidencehas coefficient (0.78) atsubsidence Taoyi Coalcontrol Mine. Mine. The results indicate backfilling a good surface The results indicate that the SCM backfilling technology has a good surface subsidence control effect. effect.
(a)
(b)
Figure 14. 14. The The surface surface subsidence subsidence situations situations after after mining. mining. (a) (a) No. No. 66 backfilling backfilling face; face; (b) (b) No. No. 12706 12706 Figure backfilling face. face. backfilling
(2) The Backfilled SCMC Bearing Stability (2) The Backfilled SCMC Bearing Stability In order to monitor the backfilled SCMC bearing stress in the goaf, stress sensors were buried in In order to monitor the backfilled SCMC bearing stress in the goaf, stress sensors were buried in the SCMC at rear of No. 12706 backfilling face. As shown in the Figure 15, the SCMC bearing stress the SCMC at rear of No. 12706 backfilling face. As shown in the Figure 15, the SCMC bearing stress has undergone a process of “Slowing-Increase-Stability” with working face retreated and is stable at 7 MPa finally; less than the measured critical deformation load (10 MPa).
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has undergone a process of “Slowing-Increase-Stability” with working face retreated and is stable at 13 of 15 than the measured critical deformation load (10 MPa).
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The SCMC bearing value/MPa
8
The monitoring point No.1
7
The monitoring point No.2
6 5 4 3 2 1 0 0
10
20 30 40 The distance to working face/m
50
60
The bearing bearing stress of SCMC in the goaf. Figure 15. The
6. Discussion Discussion For test results that show that the strengths of SCMC specimens in different cementation states and under underdifferent differentcompression compression conditions increase curing timedecrease and decrease withvolume water conditions increase withwith curing time and with water volume percentage, both increasing curing time and decreasing water percentage, we thinkwe thatthink boththat increasing curing time and decreasing water volume canvolume benefit can the benefit the coarsening of scanning. crystal by Hydration scanning. Hydration place the preparation coarsening of crystal by takes placetakes during theduring preparation process ofprocess SCMC, of SCMC, and the SCMCmeshy contains andettringite acicular which ettringite areingredients. its main ingredients. and the SCMC contains andmeshy acicular are which its main When the When water the water volume percentage is less than 95% or the curing duration is long, the ettringite needle-like volume percentage is less than 95% or the curing duration is long, the ettringite needle-like structure is structure is thicker, which can strength of the SCMC. thicker, which can increase the increase strength the of the SCMC. The test results indicate that SCMC formed from a mixture of SCM and gangues is stronger than pure SCM alone. The strength of the caving roof is obviously higher than that of pure SCM, and plays a skeleton role in SCMC. At the same time, the shrinkage of SCM during solidification is reduced by gangues. gangues. Therefore, Therefore, type type IV IV specimens specimens have have the the highest highest ultimate ultimate strength, strength, and type-II and type-III specimens have similar ultimate strength. The ultimate strength of type I specimens is the lowest. Specimens follow thethe same stress-strain change rulesrules and the Specimens with withdifferent differentscales scalesgenerally generally follow same stress-strain change andscale the effect of the tested SCMC is not significant. First of all, SCM paste fluid should be fully stirred during scale effect of the tested SCMC is not significant. First of all, SCM paste fluid should be fully the SCMC preparation process, ensuring the SCMC is denser and that there and are fewer cracks. stirred during the SCMC preparation process, ensuring the SCMC is denser that there are When fewer SCMC reach their ultimate strength, the deformation the plastic is pronounced. cracks. specimens When SCMC specimens reach their ultimate strength, theof deformation of more the plastic is more The SCMC specimens have high residual and a strength certain plasticity, so theplasticity, size effectsoofthe SCMC pronounced. The SCMC specimens havestrength high residual and a certain size is not of obvious. effect SCMC is not obvious. Conclusions 7. Conclusions properties of SCMC, which included strength This paper studied the engineering mechanical properties changes and scale effects of SCMC in different cementation and stress states. changes states. The following major conclusions have been drawn:
(1) (1) The Themechanical mechanical properties properties and and scale scale effect effect of of SCMC SCMC was was studied studied in in the the experimental experimental test. test. It It was was found that the compressive and tensile strengths of SCMC at different cementation states found that the compressive and tensile strengths of SCMC at different cementation states increase increase withtime; curing the strengths of that SCMC waswith mixed with gangue are than higher than with curing thetime; strengths of SCMC wasthat mixed gangue are higher that of that of pure SCMC. Under the conditions of partly and completely confined compression, the pure SCMC. Under the conditions of partly and completely confined compression, the specimens specimens show anofincrease bearingand strength and a decrease of deformation withtime; curing time; show an increase bearingofstrength a decrease of deformation with curing and the and the specimens’ and deformation critical deformation are in relation inverse relation to thevolume water specimens’ strengthstrength and critical load areload in inverse to the water volume percentage. Specimens with different scales generally follow the same stress-strain change rules, except in relation to compressive strengths and maximum deformation. (2) The engineering practice shows that, considering the engineering mechanical properties of cementation states and bearing characteristics of SCMC comprehensively, it can achieve better control effect of surface subsidence through optimizing the backfilling techniques parameters
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percentage. Specimens with different scales generally follow the same stress-strain change rules, except in relation to compressive strengths and maximum deformation. The engineering practice shows that, considering the engineering mechanical properties of cementation states and bearing characteristics of SCMC comprehensively, it can achieve better control effect of surface subsidence through optimizing the backfilling techniques parameters such as final strength and filling rate of backfilled body, according to specific geological and mining conditions.
The paper systematically investigated the characteristics of the SCMC for backfilling. The results advance the understanding and implementation of the backfilling technologies field, which further enhances subsidence control and other aspects of sustainable mining development. Acknowledgments: We acknowledge the financial support for this work, provided by the National Natural Science Foundation of China (Grant No. 51474206, No. 51404254), the National Basic Research Program of China (973 Program-No. 2015CB162500), the Fundamental Research Funds for the Central Universities (2014QNA49). Author Contributions: All authors contributed to this paper. Xufeng Wang prepared and edited the manuscript. Dongdong Qin and Bo Li participated in the data processing during the research process. Dongsheng Zhang revised and reviewed the manuscript. Chundong Sun and Chengguo Zhang partially participated in the literature research. Mengtang Xu was responsible for creating the figures. Conflicts of Interest: The authors declare no conflict of interest.
Abbreviations CPB SCMC SCM BF
cemented paste backfill Superhigh-water Content Material Concretion Superhigh-water Content Material Backfilling face
References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
Lv, W.Y.; Zhang, Z.H. Current situation and prospect of coal backfill mining. Technol. Adv. Mater. Res. 2012, 524, 421–425. [CrossRef] Zhang, J.X.; Zhang, Q.; Sun, Q.; Gao, R.; Germain, D.; Abro, S. Surface subsidence control theory and application to backfill coal mining technology. Environ. Earth Sci. 2015, 74, 1439–1448. [CrossRef] Johnson, G.; Kellet, W.H.; Mills, P.S. Aquapak: A cementitious pack material with high water content. In Proceedings of the International Strata Control Conference, Liège, Belgium, 20–24 September 1982. Buddery, P.S.; Hassani, F.P.; Singh, R.N. Study of Aquapak behaviour with reference to strata control requirements. Can. Geotech. J. 1984, 21, 621–633. Yao, Y.; Sun, H.H. A novel silica alumina-based backfill material composed of coal refuse and fly ash. J. Hazard. Mater. 2012, 213, 71–82. [CrossRef] [PubMed] Feng, G.M.; Ding, Y.; Zhu, H.J.; Bai, J.B. Experimental research on a superhigh-water packing material for mining and its micromorphology. J. China Univ. Min. Technol. 2010, 39, 813–819. Bharatkumar, B.H.; Narayanan, R.; Raghuprasad, B.K.; Ramachandramurthy, D.S. Mix proportioning of high performance concrete. Cem. Concr. Compos. 2001, 23, 71–80. [CrossRef] Thomas, E.G.; Nantel, J.H.; Notley, K.R. Fill Technology in Underground Metalliferous Mines; International Academic Services Ltd.: Kingston, ON, Canada, 1979. Cui, L.; Fall, M. An evolutive elasto-plastic model for cemented paste backfill. Comput. Geotech. 2016, 71, 19–29. [CrossRef] Fall, M.; Adrien, D.; Célestin, J.C.; Pokharel, M.; Touré, M. Saturated hydraulic conductivity of cemented paste backfill. Miner. Eng. 2009, 22, 1307–1317. [CrossRef] Yi, X.W.; Ma, G.W.; Fourie, A. Compressive behaviour of fibre-reinforced cemented paste backfill. Geotext. Geomembr. 2015, 43, 207–215. [CrossRef] Klein, K.; Simon, D. Effect of specimen composition on the strength development in cemented paste backfill. Can. Geotech. J. 2006, 43, 310–324. [CrossRef]
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14. 15. 16.
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