Determining the rock mechanics characteristics and strength is crucial in the effective design of tunnels, particularly in challenging conditions such as squeezing stratum. The focus of this study is on metamorphic basalt, especially its behavior in the Haba Snow Mountain Tunnel Project, which has experienced severe extrusion deformation and collapse of 189m long secondary lining structure. This project is currently the most severely squeezing tunnel in China. Understanding the rock strength in such conditions is of utmost importance. To analyze the rock's properties, thin section identification, XRD diffraction, and scanning electron microscope tests were conducted to determine the mineral composition and microscopic characteristics. Subsequently, the rock's strength was assessed through uniaxial compression, point load, and rebound strength tests. Alongside, the study investigates the mechanical properties of metamorphic basalt and explores the relationship between different rock strength test methods. The findings demonstrate a significant linear correlation between the uniaxial compressive strength, rebound strength, and point load strength of the rocks. The relationship between the point load index and uniaxial compressive strength is represented as y=10.97x, exhibiting a strong correlation coefficient of 0.93. Conversely, the relationship between rebound strength and uniaxial compressive strength is described by y=1.26x-7.39, with a correlation coefficient of 0.76, slightly lower than the point load. Furthermore, as the rock's point load index increases, the confidence region of the relationship between the point load index and uniaxial compressive strength expands. Similarly, the confidence region of the relationship between rebound strength and uniaxial compressive strength initially decreases and then increases with increasing rebound strength. These research findings have significant implications for tunnel design, specifically in understanding the mechanical properties of metamorphic basalt and how different rock strength test methods relate to each other. By considering these relationships, engineers can design tunnels effectively and mitigate potential risks in squeezing stratum conditions.
Research Article
Investigation of Rock Mechanics Characteristics and Strength in a Squeezing Tunnel: A Case Study of the Haba Snow Mountain Tunnel
https://doi.org/10.21203/rs.3.rs-3223599/v1
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metamorphic basalt
rock strength
point load test
uniaxial compressive test
rebound strength test
- The rock minerals that cause severe squeeze in tunnels have been proposed.
- The strength of the metamorphic basalt has been comprehensively determined.
- A new relationship between different strength testing methods has been proposed.
Rock strength is one of the most essential parameters in tunnel design. Currently, there are various methods to determine rock strength (Wang et al., 2014), such as obtaining the uniaxial compressive strength (UCS) of the rock through coring and conducting uniaxial compressive tests, applying point loads on rock blocks to obtain the point load strength (PLI) of the rock, and determining the rock rebound strength based on the size of the rebound value of the impact rock surface by the Schmidt hammer. In uniaxial compressive testing, there is a high requirement for the quality of core extraction. It is difficult to drill in highly weathered, severely metamorphosed, or low-strength rock masses (Broch and Franklin, 1972), and rock cores are often prone to damage during the drilling process. Therefore, the uniaxial compressive strength of rocks may only sometimes be obtained through traditional core extraction methods (Brook, 1985). Point load and rebound tests are relatively low-cost and accurate testing methods compared to uniaxial compression tests (Basu and Kamran, 2010; Kahraman and Gunaydin, 2009), which have been reported as indirect testing methods for rock strength (Çobanoğlu and Çelik, 2008; Fener et al., 2005). Among them, point load test has a simple testing process and sample preparation, and the sample can be various regular cylindrical rock samples or any irregular blocks (International Society for Rock Mechanics, 1985; American Society for Testing and Materials, 2008), as well as hard or fragile rocks (Heidari et al., 2012; Tsiambaos and Sabatakakis, 2004).
Many scholars have researched the relationship between uniaxial compressive strength, point load strength, and rebound strength of rocks. Hak Joon Kim (2018) studied the correlation between the point load index and uniaxial compressive strength (UCS) of different rocks. Xue et al. (2020) conducted uniaxial compressive and point load tests on 84 granite rock samples. The regression analysis results indicate a significant and strong linear relationship between rock UCS and Is(50) (UCS = 16.017Is(50)). The Schmidt hammer method is also simple and fast for predicting rock UCS (Çobanoğlu and Çelik, 2017; Daoud et al., 2017; Sengun et al., 2011). Mohammed et al. (2020) tested 112 sets of limestone cores using a Schmidt hammer to verify the reliability of existing empirical formulas for predicting rock UCS. Although there have been many studies on the strength testing results of various rock types, there is relatively little research on metamorphic basalt, and there are also few studies on the correlation between uniaxial compressive strength, point load strength, and rebound strength. Therefore, this study first analyzes rocks' mineral composition and microscopic characteristics through thin section identification, XRD diffraction, and scanning electron microscopy testing methods. Then, the rock strength is determined through uniaxial compression, point load, and rebound strength tests. Finally, the mechanical properties of metamorphic basalt and the relationship between different rock strength testing methods are discussed.
The rock samples are mainly from the Haba Snow Mountain Tunnel, a maximum burial depth of 1155m, and the primary crossing strata are metamorphic basalt and carbonaceous slate. The Haba Snow Mountain Tunnel has produced extremely serious squeezing deformation, and the secondary lining structure collapsed 189m. This project is currently the most severely squeezing tunnel in China. It is very important to study the rock strength in this kind of squeezing stratum. This article takes metamorphic basalt as the research object. Firstly, the mineral composition and microscopic characteristics of the rock were analyzed through thin section identification, XRD diffraction, and scanning electron microscopy testing methods. Then, the rock strength was determined through uniaxial compression, point load, and rebound strength tests. Finally, the mechanical properties of the metamorphic basalt and the relationship between different rock strength testing methods were discussed. The methodology is shown in Fig. 1. The uniaxial compression, point load, and rebound tests were conducted in 60 groups, 100 groups, and 82 groups, respectively.
3.1 Mineral composition and microstructure of rocks
The thin section identification method identifies minerals and rocks under a polarizing microscope (Liu et al., 2022; Wu et al., 2021). It observes and identifies rocks to distinguish their mineral types and studies the main mineral composition, mineral generation sequence, structure, structure, and rock (ore) type of rocks and ores. The thin section identification results are shown in Fig. 2. The mineral composition is shown in Table 1.
|
Group |
Actinolite |
Albite |
Chlorite |
Epidote |
Hornblende |
Calcite |
Metal minerals |
Others |
|---|---|---|---|---|---|---|---|---|
|
A |
40 ~ 50% |
25 ~ 30% |
10 ~ 15% |
5 ~ 6% |
3 ~ 5% |
1 ~ 2% |
2 ~ 3% |
1 ~ 2% |
|
B |
65 ~ 70% |
— |
15 ~ 20% |
8 ~ 10% |
3% |
— |
2% |
1% |
|
C |
60 ~ 65% |
— |
25 ~ 30% |
3 ~ 5% |
— |
— |
5% |
1% |
The rock specimen is green or grayish-green in color, with a porphyritic columnar metamorphic structure and a sheet-like structure. The main minerals are fine granular feldspar and columnar Actinolite, containing a small Chlorite group. The rock contains a minimal amount of carbonate, and a small amount of bubbles slowly form with the addition of dilute hydrochloric acid. The rock has a smaller hardness and can be easily slid with a small knife. The rock was tested by magnetite and found to have weak magnetism. Phenocryst Epidote, columnar Actinolite, and granular Albite can be seen microscopically. In Group A, Albite accounts for a large proportion.
XRD powder crystal diffractometer can be used to study the process and phase transition, surface phases, defects, and crystal structure of mineral crystallization (Banko et al., 2021; Kochetov et al., 2020). The XRD diffraction results are shown in Fig. 3.
The XRD diffraction results of the three groups of rocks are similar, and the mineral composition mainly includes amphibole, ferruginous amphibole, plagioclase chlorite group, phlogopite, glauconite, Albite, potassium microcline, magnesium-bearing calcite, rutile, and amorphous materials. The proportion of mineral components varies slightly. The content of epibole and ankerite in Group B is relatively low, but the content of Phlogopite, Glauconite, and amorphous phase materials is slightly higher. Edenite generally occurs in sodium-rich Igneous rock, the contact zone between dolomitic limestone and Igneous rock, and jadeite-bearing albite (Fu-feng et al., 2010). Sodium-rich amphibole mainly occurs in Metamorphic rock formed by sodium rocks (Parnell et al., 2022), consistent with Albite in thin section identification. Actinolite is a silicate mineral, which is a mineral formed by replacing more than 2% of magnesium ions in Tremolite with divalent iron ions. It belongs to amphibole (Akolkar and Limaye, 2021; Pan and Fleet, 1992). Albite is a sodium mineral of the plagioclase solid solution series (Zhou et al., 2022). The Chlorite group is a layered Silicate mineral, commonly called the Chlorite group, mainly composed of magnesium and iron minerals, namely the plagioclase Chlorite group and oolitic Chlorite group (Kurnosov et al., 2019). Therefore, the XRD diffraction test results are consistent with the thin section identification.
Electronic images can observe the surface morphology characteristics, morphology, and distribution of micropores of minerals at nano or micrometer scales (De Castro et al., 2022; Guanira et al., 2020), providing a basis for rock evaluation.
The microstructure of the three sets of rock samples is similar. The microstructure of the rock block sample's front surface shows many bar-like and rod-shaped structures with staggered morphology. The rock minerals are arranged directionally and regularly. The side microstructure has some lamellar and layered structures, which are relatively disordered. A large number of rock fragments and fallen Clay minerals can be seen. In addition, there are relatively more pores on the side of the rock. Actinolite is slightly altered into the Chlorite group, and the Chlorite group appears in the form of curved sheets with angular edges. It is randomly distributed along the edge of the Actinolite crystal.
3.2 Uniaxial compressive test
Drilling rock cores inside and outside the tunnel for uniaxial compressive testing, the results of the uniaxial compressive testing are shown in Fig. 5.
It can be seen from Fig. 5 that the minimum uniaxial compressive strength of rock is 18.26MPa, and the maximum uniaxial compressive strength is 79.59MPa, which follows the Normal distribution. The average uniaxial compressive strength is about 33.33MPa. Compared with the strength of surrounding rock in deep buried soft rock tunnels in the past, the average uniaxial compressive strength is slightly higher, mainly because the more complex rock during the coring process is easier to core successfully. The drilling rig used in this experiment is a water drill, which uses water during the coring process to reduce the friction between the drill bit and the rock, reduce the temperature of the drill bit, and improve the quality of the drill core. However, during the core drilling process, it was found that harder rocks had a slower drilling speed (0.3-0.6cm/min), while softer rocks had a faster drilling speed (above 1.0cm/min), making it challenging to core highly fractured rocks, as shown in Fig. 6. When the uniaxial compressive strength of the rock is less than 18MPa, it is effortless to break the rock core during the drilling process, mainly due to the development of internal cracks in the rock. During the drilling process, the friction force of the drill bit, combined with the action of water, ultimately leads to the fragmentation of the rock core. However, through statistical analysis of the uniaxial compressive strength of rocks, it was found that the rock in the tunnel area is relatively soft. In addition, the surrounding rock of the tunnel is not all soft rock, but mainly soft rock, with both soft and hard rocks coexisting. Most of the rock cores taken come from harder rocks.
3.3 Point loading test
The point load strength index (PLI) is an index test (Fan et al., 2021; Şahin et al., 2020) commonly used to estimate the uniaxial compressive strength of rocks. The peak load is used to calculate the corrected point load strength of a sample with a diameter of 50mm (), as shown in equations (1) to (4).
Through statistical analysis of 100 sets of rock point load data, the relationship between the height, width, and area of the rock failure section and the peak load is shown in Fig. 7, and the point load index after size correction is shown in Fig. 8.
From Figs. 7 and 8, it can be seen that the failure height range of the rock sample is 0.9 ~ 6.8cm, which has a linear correlation with the peak load: y = 1.4x, and the correlation coefficient R2 = 0.74; The failure width range is 2.4 ~ 16.5cm, which has a linear correlation with the peak load of loading: y = 0.81x, and the correlation coefficient R2 = 0.72; The failure area ranges from 3.77 to 89.10 cm2 and has a linear correlation with the peak load: y = 0.18x, with a correlation coefficient of R2 = 0.71; The equivalent diameter range of failure is 2.19 ~ 10.65cm, and the failure load range is 0.6-16.9kN; The rock point load index (Is (50)) ranges from 0.16 to 9.32 MPa, with an average value of 2.16 MPa, which shows a typical Normal distribution. In the point load test, most rock failure occurs from the connected joint surface or crack surface, as shown in Fig. 9.
3.4 Rebound test
The rebound strength point and bar charts are shown in Fig. 10 by conducting rock rebound tests inside and outside the tunnel.
As shown in Fig. 10, the rock rebound strength ranges from 12.12 ~ 58.99 MPa, with an average value of 28.42 MPa. Generally speaking, the rebound strength of C30 concrete is above 34.2 MPa, indicating a lower rebound strength of rocks. However, the minimum rebound strength is 12.12 MPa, which may also be determined by the strength testing method. Rocks below this value cannot accurately determine the rock's rebound strength because the instrument has specific impact energy, which may break the rock. Of course, the above results include the test results inside and outside the tunnel. During the tunnel test, the rebound strength of the unexcavated rock can be directly measured. Due to the strong constraints of the unexcavated rock mass, it is easier to conduct rebound strength tests, while tests outside the tunnel focus on relatively complete blocks of rock.
Through experiments, this study obtained 100 sets of point load, 82 sets of rebound, and 60 sets of uniaxial compressive test data for metamorphic basalt. It fitted them, as shown in Fig. 11. The correlation between the results of different rock strength test methods was explored.
As shown in Fig. 11, the uniaxial compressive strength of rocks has a linear correlation with rebound strength and point load strength. The relationship between point load index and uniaxial compressive strength of rocks is y = 10.97x, with a correlation coefficient of 0.93, indicating a high correlation; It can be seen that the uniaxial compressive strength of the rock measured in the point load test is between 1.76 and 102.24 MPa, with an average value of 23.7 MPa; The relationship between rebound strength and uniaxial compressive strength of rocks is y = 1.26x-7.39, with a correlation coefficient of 0.76, which is lower than point load; The blue area represents a 95% confidence zone. As the rock point load index increases, the confidence zone of the relationship between the point load index and uniaxial compressive strength increases; As the rebound strength increases, the confidence region of the relationship between rebound and uniaxial compressive strength first decreases and then increases. Therefore, for the determination of the strength of weak surrounding rock, considering the characteristics of rock rebound strength, it is recommended to use a comprehensive method of point load and uniaxial compressive strength to determine.
The relationship between the uniaxial compressive strength of rocks and the point load index has been extensively studied in the literature (Tandon and Gupta, 2015), including cases with intercepts of 0 and non-0. However, the loading methods of point load and uniaxial compressive test are similar, with gradually increasing loads at the upper and lower ends, but the loading forms are different. Based on this, using a linear formula with an intercept of 0 is recommended to fit the relationship between the uniaxial compressive strength of rocks and the point load index. The rebound instrument loading method differs from the above two methods. It mainly obtains the surface strength of the rock through immediate energy impact. When the rock is relatively soft, this loading method is not applicable. The above research shows that the rebound instrument is only suitable for testing the rebound strength of concrete or rock with pressure not less than 10MPa.
This article tests the strength of rocks through three test methods and many test groups, including 60 sets of uniaxial compressive tests, 100 sets of point load tests, and 82 sets of rebound tests. It obtains the correlation between the results of different test methods. The results show that for metamorphic basalt, there is a linear correlation between the uniaxial compressive strength of the rock and the rebound strength, and point load strength. The relationship between the point load index and the uniaxial compressive strength of the rock is y=10.97x, with a correlation coefficient of 0.93, indicating a high correlation; The relationship between rebound strength and uniaxial compressive strength of rocks is y=1.26x-7.39, with a correlation coefficient of 0.76, which is lower than point load. To determine the strength of weak surrounding rock, it is recommended to use a comprehensive method of point load and uniaxial compressive strength. Considering that the rock rebound test is suitable for testing the rebound strength of concrete or rocks not less than 10MPa, this loading method is not applicable when the rock is relatively soft.
Ethics Approval: The authors confirm their Ethics Approval.
Conflict of interest: The authors declare no competing interests.
Data availability statement: The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.
Author contributions: J.P. Zhao: Investigation, Data curation, Writing - original draft. Z.S. Tan: Conceptualization, Methodology, Funding acquisition, Supervision. Z.L. Li: Data curation, Writing - review & editing. B.J. Zhang: Investigation, Writing - review & editing. F.X. Wang: Investigation, Project administration, Supervision.
Funding: The study supported by the National Natural Science Foundation of China (Grant No. 51978041).
Consent to publish: The authors confirm their consent to publish.
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No competing interests reported.