Experimental Study on Mudstone’s Strength Characteristics in Deep-Buried Coal-Measure Formation: A Case Study of Permian Longtan Formation

Abstract

To investigate the strength characteristics of mudstone in deep-buried coal-measure formation, four types of experiments have been conducted: (i) the X-ray diffraction (XRD) test; (ii) the scanning electron microscope (SEM) scanning test; (iii) the point load strength index test; and (iv) the uniaxial compressive strength test. It was concluded that the mudstone of the deep-buried coal measures in the Longtan Formation is dominated by chlorite, quartz, and albite using the XRD test, of which chlorite is primary, accounting for 74.3%. It was found that the three minerals in the mudstone are unevenly distributed using the SEM scanning test, albite is irregularly distributed in chlorite, and quartz is present in the albite and chlorite. Sixty-five specimens were tested for the point load strength index. After processing the data using the method suggested by the International Society for Rock Mechanics and Rock Engineering(ISRM), it was found that the maximum value of I_s(50) was 6.10 MPa, the minimum is 0.14 MPa, and 53% of the specimens’ I_s(50) values are below 2.0 MPa. The RMT-150C rock mechanics testing machine was used to conduct uniaxial compression tests on six specimens. The maximum uniaxial compressive strength (UCS) value is 59.26 MPa, the minimum value is 31.77 MPa, and the average is 45.64 MPa. Linear fitting and logarithmic fitting are carried out for the correlation between UCS and I_s(50). The goodness of fit R² of the linear fitting is 0.863, and that of the logarithmic fitting is 0.919, indicating a strong correlation between them. When it is challenging to make standard specimens, I_{s (50)} can be used to estimate UCS.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

UCS of deep-buried coal-measure formation is an important index to evaluate its stability and an essential reference basis for arranging roadway system and selecting mining technology [1–3]. Generally, the rock depositional environment of deep-buried coal-measure formation is complex and cooccurs with coal seams, resulting in a complex composition, high heterogeneity, and a high degree of joint and fissure development, which significantly reduces the integrity and strength of coal-measure formation [4–6]. According to the suggested method of ISRM, there are strict requirements for the specimen when testing UCS. The most basic requirement is that the length/diameter ratio of the specimen is greater than 2 [7, 8]. However, in the field sampling with a high fracture development program or high degree of rock fragmentation, it is challenging to obtain a complete core for routine UCS tests in many cases.

Andrea and Fisher [9] first used the point load test to estimate the uniaxial compressive strength of rock and considered a linear relationship between them. Broch and Franklin [10] considered that the index could be obtained whatever the shape of the specimen is, and the point load strength results correlated closely with those from uniaxial (unconfined) compressive strength testing. ISRM [7] published “Suggested method for determining point load strength” in 1972 and revised it in 1985. The American Society for Testing and Materials (ASTM) [11] released the standard test method for testing the point load strength index in 1995 and revised it in 2016. In the past 50 years, there has been many research works on the point load strength test. Ulusay and Türeli [12] considered that the point load test is most importantly employed in estimating the compressive strength of rock materials. Bieniawski [13] discussed the practical applications of the point load test in geotechnical practice and proposed that the diametral point load test is most convenient and reliable in use. Broch [14] considered that the most reliable point load strength index would be obtained when cores are drilled normal or near-normal to weakness planes. Brook [15] considered that the general usefulness of the point load strength test was applying compressive strength estimation, rock mass classification, estimation of triaxial behavior, and small-scale physical model testing. Şahin et al. [16] studied the point load strength index of half-cut core specimens and its correlation with uniaxial compressive strength. Fan et al. [17] considered that the distance between two loading points and the width of the actual fracture section played an essential and nonnegligible role for the failure of rock specimens. Lei et al. [18] proposed that there is no significant difference in the shape distribution with the block size. Sha [19] considered that the fitting equations could align with reality for relatively hard and homogeneous rocks.

The relationship between the point load strength index and UCS is the focus of the point load test. In recent years, there are many reports on the relationship between them, which are aimed at different types of rocks. ISRM [7], Forster [20], Ghosh and Srivastava [21], Chau and Wong [22], Smith [23], Tsiambaos and Sabatakakis [24], Palchik and Hatzor [25], Singh et al. [26], Kohno and Maeda [27], Li and Wong [28], Şahin et al. [16], Kaya and Karaman [29], Liu et al. [30], Rabat et al. [31], Xue et al. [32], and Xie et al. [33] considered that the two were linear with zero intercept. Andrea et al. [9], Ulusay et al. [12], Kahrama [34], Diamantis et al. [35], Yilmaz [36], Kaya and Karaman [29], Heidari et al. [37], and Kong and Shang [38] et al. believed that the two are linear relations of non-zero intercept. Kahraman [34], Tsiambaos and Sabatakakis [24], Santi [39], Selçuk and Süleyman Gökçe [40], and Kallu and Roghanchi [41] considered the two to be a power function relationship. Kılıç and Teymen [42] considered the two to be a logarithmic function relationship. Quane and Russell [43] considered the two to be a quadratic function relationship. Their research is aimed at different rocks, and the expressions are fitted according to the laboratory test results. They have reference value for specific rocks in a certain range. That is, targeted research is needed for specific rocks to obtain applicable expressions.

This study conducted XRD test, SEM scanning test, point load test, and uniaxial compression test in the laboratory to study the strength characteristics of mudstone in deep-buried coal-measure strata in detail. Their strength characteristics were analyzed, and the possibility of replacing the uniaxial compression test with point load test was studied, to provide references for scientific decisions such as arranging roadways, selecting support methods, and selecting mining engineering.

2. Main Composition and Structure of the Rock Specimens

2.1. Sample Source

The specimens in this research are from the Huopu coal mine in Panzhou City, Guizhou Province, a province in southwest China. The specimens are taken from the rock formation between the 23# and 24# coal seams of this coal mine. The distance between the two coal layers is 14.3 m. The buried depth of the specimens is 1349 m-1355 m, the specimen diameter is 50 mm, and the primary lithology is thin dark gray mudstone. The rock cores drilled on-site are shown in Figure 1, and the statistics show that its RQD value is only 28.9%.

[figure omitted; refer to PDF]

Table 1

The quantitative test results of mineral composition.

No.	Mineral name	Chemical formula	Mineral content accounted (%)
1	Chlorite	Mg1.3Fe3.4Al2.6Si2.7O10(OH)8	74.3
2	Quartz	SiO2	16.2
3	Albite	NaAlSi3O8	9.5

2.3. Observation of Rock Fracture Morphology

To observe the detailed structure of the rock, a Nova Nano SEM 450 hot field emission scanning electron microscope was used to scan the surface of the rock after fracture, and SEM images with different magnification were obtained, as shown in Figure 3. From these SEM images, it can be seen that the microcracks extending from the rock surface to the content are extremely developed. PCAS software is used to analyze the pore data in 500 times SEM images, as shown in Figure 4. The calculated surface porosity is 3.01%. The number of pores directly affects the mechanical properties of rock. As the proportion of rock’s pores increases, the strength of the rock becomes lower.

[figures omitted; refer to PDF]

[figure omitted; refer to PDF]

It can be seen from Figure 3 that the distribution of various mineral components in the mudstone is uneven, albite is distributed irregularly in chlorite in blocks, and quartz occurs in spots in albite and chlorite, showing substantial heterogeneity.

The wide distribution of pores and fractures in mudstone and its heterogeneity reduce its strength to a great extent, which is why it is difficult to obtain a complete core during field sampling.

3. Point Load Strength Index Test and Results Analysis

3.1. Point Load Strength Index Test

After processing the specimens in Figures 1, 6 specimens for uniaxial compression and 65 specimens for point load test are obtained. The specimens before the test are shown in Figure 5(a). The experimental instrument is the STDZ-3 point load tester. There were 46 valid tests and 19 invalid ones. There were three reasons for the invalid tests: firstly, some samples only damaged one corner, which belongs to the invalid test specified by ISRM [7]. Secondly, some specimens are broken into massive rocks after testing, and their failure surface cannot be measured. Thirdly, some specimens have internal cracks, which are not observed on the surface, which significantly reduce their strength and destroy the test so fast that the instrument cannot effectively monitor the failure load. That is, there are no valid test data in these tests. The specimens after the test are shown in Figure 5(b). The test results are shown in Table 2.

[figures omitted; refer to PDF]

Table 2

Point load test results.

Group no.	Specimen no.	W (mm)	D (mm)	D′ (mm)	P (N)	A (mm2)	De (mm)	Is (MPa)	F	Is(50) (MPa)	Mean value (MPa)
1	1a1	45.18	16.88	14.02	260	633.35	28.40	0.32	0.78	0.25	1.82
	1a2	52.64	16.96	11.81	3210	621.62	28.13	4.06	0.77	3.13
	1a3	49.79	5.72		1410	284.80	19.04	3.89	0.65	2.52
	1a4	51.55	13.20	9.98	1530	514.47	25.59	2.34	0.74	1.73
	1a5	54.46	18.54	15.04	860	819.08	32.29	0.82	0.82	0.68
	1a6	51.50	18.62		370	958.93	34.94	0.30	0.85	0.26
	1a7	48.69	15.65	11.12	5230	541.43	26.26	7.59	0.75	5.68
	1a8	49.96	15.51	11.84	4120	591.53	27.44	5.47	0.76	4.18
	1a9	47.36	28.01		258	1326.55	41.10	0.15	0.92	0.14
2	2a1	49.32	13.74	9.09	3830	448.32	23.89	6.71	0.72	4.81	1.60
	2a2	49.44	16.33		750	807.36	32.06	0.73	0.82	0.60
	2a3	50.54	10.14	5.43	310	274.43	18.69	0.89	0.64	0.57
	2a4	47.84	23.46	19.19	390	918.05	34.19	0.33	0.84	0.28
	2a5	47.03	26.61		2910	1251.47	39.92	1.83	0.90	1.65
	2a6	48.87	20.18	13.84	2340	676.36	29.35	2.72	0.79	2.14
	2a7	51.21	15.78		1310	808.09	32.08	1.27	0.82	1.04
	2a8	53.64	16.61	11.9	3740	638.32	28.51	4.60	0.78	3.57
3	3a1	52.55	20.73	16.73	3710	879.16	33.46	3.31	0.83	2.77	1.48
	3a2	53.50	16.37	10.3	1340	551.05	26.49	1.91	0.75	1.43
	3a3	48.76	18.60	12.69	4670	618.76	28.07	5.93	0.77	4.57
	3a4	53.27	25.08		780	1336.01	41.24	0.46	0.92	0.42
	3a5	49.83	21.31		1040	1061.88	36.77	0.77	0.87	0.67
	3a6	50.15	29.54		470	1481.43	43.43	0.25	0.94	0.23
	3a7	50.62	26.61		3910	1347.00	41.41	2.28	0.92	2.09
4	4a1	48.71	28.14	22.19	4340	1080.87	37.10	3.15	0.87	2.76	2.16
	4a2	49.92	24.46	20.14	3610	1005.39	35.78	2.82	0.86	2.43
	4a3	45.52	18.72	12.98	6010	590.85	27.43	7.99	0.76	6.10
	4a4	47.12	22.68		510	1068.68	36.89	0.37	0.87	0.33
	4a5	49.28	25.53	20.65	4230	1017.63	36.00	3.26	0.86	2.82
	4a6	45.08	34.14		1530	1539.03	44.27	0.78	0.95	0.74
	4a7	47.23	25.01		5200	1181.22	38.78	3.46	0.89	3.08
	4a8	48.18	32.83	27.45	2110	1322.54	41.04	1.25	0.91	1.15
5	5a1	46.83	19.82	16.77	5170	785.34	31.62	5.17	0.81	4.21	2.58
	5a2	52.11	21.01		450	1094.73	37.33	0.32	0.88	0.28
	5a3	42.25	26.67		5230	1126.81	37.88	3.65	0.88	3.22
	5a4	52.37	29.73	25.1	3640	1314.49	40.91	2.17	0.91	1.99
	5a5	49.50	21.27	16.77	5370	830.12	32.51	5.08	0.82	4.19
	5a6	48.62	17.46	13.87	1030	674.36	29.30	1.20	0.79	0.94
6	6a1	50.32	44.45		630	2236.50	53.36	0.22	1.03	0.23	2.05
	6a2	46.54	23.13	18.49	3420	860.52	33.10	3.12	0.83	2.59
	6a3	49.40	36.38	30.86	1570	1524.48	44.06	0.81	0.94	0.76
	6a4	48.31	28.68	21.61	7510	1043.98	36.46	5.65	0.87	4.90
	6a5	48.68	29.58	24.45	4560	1190.23	38.93	3.01	0.89	2.69
	6a6	51.51	42.85		9630	2206.99	53.01	3.43	1.03	3.52
	6a7	45.75	32.92		4010	1505.93	43.79	2.09	0.94	1.97
	6a8	54.37	18.22		1120	990.62	35.51	0.89	0.86	0.76

3.2. Experimental Data Processing

According to the suggested method of ISRM [7], the point load test data can be processed with the following equation:$$\begin{array}{}\text{(1)}& \begin{array}{c}{I}_{S50}=F\times I_S,\\ I_S=\frac{P}{D_e^2},\\ D_e={\frac{4A}{\pi }}^{0.5},\\ A=W\times D,\\ F={\frac{D_e}{50}}^{0.45},\end{array}\end{array}$$where I_S(50) is the modified point load strength, MPa. I_S is the unmodified point load strength, MPa. F is the size modification factor. P is the point load strength, kN. D_e is the equivalent core diameter, mm. A is the damaged area, mm². W is the width of the damaged surface, mm. and D is the height of the damaged surface, mm.

In addition, when the rock is damaged, the spacing between loading points is generally not equal to D, but it is damaged after being pressed into the rock for a short distance. Therefore, ASTM D5731-16 [11] and other technical standards proposed that if significant plate penetration occurs in the test, such as when testing weak sandstones, the value of D should be the final value of the separation of the loading points, D′. during the experiment in this study, it is found that significant platen penetration often occurs, as shown in Figure 6. Therefore, when processing the test data in this study, the specimens of significant platen penetration are calculated by D′, and others are calculated by D.

[figure omitted; refer to PDF]

Sorting out equation (1), the point load strength calculation equation of the specimen without significant platen penetration is$$\begin{array}{}\text{(2)}& I_{S50}=\frac{P}{{50}^{0.45}}\cdot {\frac{\pi }{4WD}}^{0.775}.\end{array}$$

The calculation equation of the point load strength of the specimen with significant platen penetration is$$\begin{array}{}\text{(3)}& I_{S50}=\frac{P}{{50}^{0.45}}\cdot {\frac{\pi }{4W{D}^{\prime }}}^{0.775}.\end{array}$$

Using equations (2) and (3) to process the experimental data, I_s(50) is obtained, as shown in Table 2. In addition, according to the suggested method of ISRM, each group of data should delete the two maximum values and two minimum values and then take the average of the other values as the I_s(50) of the group of experiments. The number of successes in each group of experiments in this study is less than 10, so this study only deletes a maximum value and a minimum value of each group, and the average value of I_s(50) for each group is shown in Table 2.

According to the above test results, before removing the extreme value, among the 46 tests, the maximum value of I_s(50) is 6.10 MPa, and the minimum value is 0.14 MPa. The statistical distribution range is shown in Figure 7(a). It can be seen that, among the 46 data, 24 have I_s(50) below 2.0 MPa, accounting for 53% of the total, and 17 have I_s(50) below 1.0 MPa, accounting for 37% of the total. Remove a maximum value and a minimum value for each group. That is, after removing the data of 12 specimens, among the 34 tests, the maximum value of I_s(50) is 4.19 MPa, and the minimum value is 0.25 MPa. The statistical distribution range is shown in Figure 7(b).

[figures omitted; refer to PDF]

4. Uniaxial Compression Test

4.1. Test Methods and Equipment

The test specimen is from the core in Figure 1. Because the rock is very broken, only six specimens are obtained, and the length of these specimens is less than 100 mm. After the specimens were cut by the cutting machine, the TX-SHM200 C program-controlled double-ends planishing machine was used for polish. After the specimen is ground flat, the ends of the specimens were flat to 0.02 mm and were moved from permanency to the axis of the specimens by less than 0.05 mm.

The uniaxial compression test was carried out in the Mechanical Laboratory of Hunan University of Science and Technology. The loading equipment is a RMT-150C rock mechanics testing machine. DH3816 N static strain tester and wire-wound resistor were used for deformation monitoring. The test system is shown in Figure 8.

[figure omitted; refer to PDF]

Linear incremental force loading control is adopted, and the loading rate is 1 kN/s, that is, 0.5 MPa/s. When the test piece is damaged, the pressure head of the loading system will return automatically. Each specimen is pasted with four strain gauges, two to monitor axial deformation and two to monitor radial deformation. The average value is taken as the axial and radial deformation value, respectively. DH3816 N static strain tester is used to monitor the deformation data of the specimen in real-time during loading.

4.2. Test Results

The test data of 6 specimens are shown in Table 3, and the uniaxial compressive stress-strain data of 6 specimens are shown in Figure 9. Among the six specimens, the largest UCS is 59.26 MPa, the smallest is 31.77 MPa, and the average value is 45.64 MPa. The results of each specimen are quite different. It is speculated that the reason is that the distribution of chlorite, quartz, and albite minerals in the specimen is not uniform, resulting in a high degree of heterogeneity in the specimen, resulting in a significant difference in UCS.

Table 3

Specimens parameters and UCS test results.

Specimen no.	Length (mm)	Diameter (mm)	Weight (g)	Density (kg/m3)	Load at failure (kN)	UCS (MPa)	UCS2 (MPa)
617	53.08	48.55	253.8	2583	58.82	31.77	28.90
621	80.90	48.89	413.6	2724	95.84	51.05	49.81
622	99.42	48.63	480.4	2602	99.74	53.70	53.84
623	80.43	48.49	417.0	2808	61.54	33.32	32.52
624	86.30	48.47	430.0	2700	82.58	44.75	44.10
625	68.36	48.73	370.4	2905	110.52	59.26	56.38
Mean value				2720	84.84	45.64	44.26

[figures omitted; refer to PDF]

It can be seen from Figure 9 that the strain of each specimen is slight in the early stage of loading. That is, it is not apparent in the compaction stage, and the rock shows obvious brittleness.

4.3. Data Correction

ASTM [45] suggested an equation to convert UCS values of test specimens having an L/D ratio less than 2 : 1 to that of a specimen with a ratio of 2 : 1 (equation (4)):$$\begin{array}{}\text{(4)}& {UCS}_2=\frac{UCS}{0.88+0.24D/L},\end{array}$$where UCS₂ is the corrected value for a L/D ratio of 2 : 1, L is the length of the specimen, and D is the diameter of the specimen, while the UCS is the measured value on cores with a L/D ratio less than 2 : 1.

Equation (4) can be used to convert the UCS values of specimens with L/D ratios <2 to a standard ratio, which is accepted as 2. The revised data using equation (4) is shown in Table 3.

5. Correlation between UCS and I_s(50)

The average I_s(50) of the 46 specimens tested in this study is 2.11 MPa, the revised average USC₂ is 44.26 MPa, and the ratio of the two is 21.0, which is very close to the ratio of 24 given by ISRM [7], and many reported that the relationship between the point load strength index and UCS is a linear relationship with zero intercepts. To further qualitatively and quantitatively characterize the relationship between UCS and I_s(50) of deep-buried coal-measure formation mudstone, based on previous research results, this study performed linear fitting and logarithmic fitting on UCS and I_s(50) obtained from the experiment. The results are shown in Figure 10.

[figures omitted; refer to PDF]

It can be seen from Figure 10, within the range of six groups’ average I_s(50) obtained in this study, there is little difference between ISRM and the three USC₂ prediction equations obtained in this study. However, beyond the scope of the results of this study, the result obtained by logarithmic fitting is quite different from those obtained by the other three prediction methods. According to the results of logarithmic fitting, when I_s(50) is less than 1.5 MPa, USC₂ increases sharply with the increase of I_s(50), and their slope is much higher than the linear results. When I_s(50) is greater than 3.0 MPa, USC₂ increases slowly with the increase of I_s(50), and their slope is much lower than the linear results.

It can be seen from the fitting results that the goodness of fit R² of UCS and I_s(50) is 0.863 for linear fitting, and R² for logarithmic fitting is 0.919, indicating that there is a strong correlation between them. The logarithmic expression is slightly better than the linear expression. The fitting expression is shown in$$\begin{array}{c}\text{(5)}& UC{S}_2=\text{a}+b\times I_{S50},UC{S}_2=\text{c}+d\times \ln I_{S50},\end{array}$$where a is the linear intercept, a = −7.50 ± 9.26, b is the linear slope, b = 26.57 ± 4.67, and c and d are the logarithmic fitting constants,c = 9.44 ± 4.77, d = 53.62 ± 7.06.

The fitting parameters are shown in Table 4.

Table 4

UCS and I_s(50) fitting parameters.

Item	Fitting type	Weight	Residual sum of squares	Pearson’s r	COD(R2)	Adjusted R2
Value	Linear	No weighting	70.728	0.943	0.890	0.863
	Logarithmic	—	10.416	—	0.935	0.919

6. Conclusion

(1) It was concluded that the mudstone of deep-buried coal measures of Longtan Formation is mainly chlorite, quartz, and albite using the XRD test, of which chlorite is the main, accounting for 74.3%. It was found that the three minerals in the mudstone are unevenly distributed using the SEM scanning test, albite is irregularly distributed in chlorite, and quartz is dotted in albite and chlorite, resulting in significant heterogeneity of the mudstone.

(2) Sixty-five specimens were tested for the point load strength index. After processing the data using the method suggested by ISRM, it was found that the maximum value of I_s(50) was 6.10 MPa, the minimum is 0.14 MPa, and 53% of the specimens’ I_s(50) values are below 2.0 MPa.

(3) The uniaxial compression tests of six specimens were carried out in the laboratory using a RMT-150C rock mechanics testing machine. The maximum UCS was 59.26 MPa, the minimum was 31.77 MPa, and the average value was 45.64 MPa. The results of each specimen were quite different. It is speculated that the reason is due to the uneven distribution of chlorite, quartz, and albite minerals in the specimen.

(4) Linear fitting and logarithmic fitting are carried out for the relationship between UCS and I_s(50). The goodness of fit R² of the linear fitting is 0.863, and that of the logarithmic fitting is 0.919, indicating that there is a strong correlation between them. When it is challenging to make standard specimens, Is(50) can be used to estimate UCS.

Acknowledgments

This research was financially supported by National Natural Science Foundation of China (Nos. 51974117 and 52174076), Hunan Provincial Natural Science Foundation of China (No. 2020JJ4027), New Talent Training Project of Guizhou Institute of Technology (No. GZLGXM-02), and Science and Technology Project for Outing and Young Talents of Guizhou (No. QKHPTRC[2019] 5674).

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