Disintegration Characteristics Investigation of Carbonaceous Shale in High-Latitude Cold Regions

Abstract

To investigate the disintegration and microstructure characteristics of carbonaceous shale in high-latitude cold regions, indoor, outdoor, initial particle size, and freeze–thaw disintegration experiments were conducted concerning dry–wet cycles. Two tests were performed: sieving and microscopic scanning electron analysis. The carbonaceous shale disintegration, particle size variation rule, disintegration resistance, and fractal characteristics were studied. The microstructure variations of carbonaceous rock before and after water immersion were compared, and the microscopic mechanism of calving in water was discussed. The results show that: (1) the disintegration of carbonaceous shale is basically achieved after eight dry–wet cycles, and the particle size is concentrated at 2 mm~10 mm; (2) the temperature and freeze–thaw cycle conditions accelerated the disintegration rate and shortened the disintegration stability time of carbonaceous shale; (3) there is a negative correlation between the disintegration resistance index and the disintegrating fractal dimension of carbonaceous shale, and the disintegration resistance of disintegrating carbonaceous shale can be indirectly characterized by the fractional dimension; (4) carbonaceous shale contains two hydrophilic clay particles, kaolinite and montmorillonite, and the difference between the lattice expansion of clay particles, the expansion of adsorbed water film, and mineral dissolution are the main factors leading to the disintegration of carbonaceous rock.

1. Introduction

Shale is a rock formed by the dehydration and cementation of clay. It has complex composition, thin lamellar or lamellar joints, and is dominated by clay minerals with an apparently thin, layered joint structure [1,2]. Shale is a form of sedimentary rock. They were widely distributed in the Paleozoic and Mesozoic periods in China. These sedimentary rocks, which are black because of their carbon content, are commonly referred to as carbonaceous shale. Their initial strength during excavation is relatively high. However, softening and disintegrating under an external rainwater environment occurs, resulting in strength attenuation and poor engineering properties [3].

China’s shale mainly includes marine, marine–land transition shale, lacustrine shale, and composite shale (a mix of marine, marine–land transition, and lacustrine shale) [4]. The marine shale development area is distributed in Yunnan, Chongqing, Hunan, Hubei, northern Jiangxi, northern Guangxi, northern Zhejiang, and southern Anhui in the middle and lower reaches of the Yangtze River, and in the central basin of the Qinghai–Tibet Plateau in two large and small bands. The marine–land transition shale development areas are distributed in Shanxi, Shaanxi, Hebei, Beijing, Tianjin, the west of Shandong, and the north of Henan on the North China Plain. The lacustrine shale development areas are distributed in the central part of the northeast plain, namely the western part of the Heilongjiang, Jilin, and Liaoning provinces. The central part of the Inner Mongolia Plateau has strip distribution, and the western Junggar Basin, Turpan Basin, and Qaidar Basin are widely distributed. In addition, the Tarim Basin has a massive distribution of marine facies + marine–land transition facies + lacustrine shale development areas. As a result, shale exists in basins, valleys, mountain streams, plains, and plateau. It is characterized by “uneven distribution, composition and characteristics” due to the differences in geographical location and environmental factors [5].

In recent years, the research on carbonaceous shale has attracted extensive attention from scholars in China and abroad. Carbonaceous shale is a soft rock. The engineering properties exhibited under different conditions are different [6], which can be summarized mainly as the following aspects: (1) Expansibility: expansibility is a typical characteristic of soft rocks, and the volume increases after contact with water. (2) Strong weathering property: carbonaceous shale is not strong on the whole, poorly resistant to weathering, quickly disintegrates, and the rock is affected by a wide range of weather. (3) Disintegration: The structural linkage and strength among soil grains are easily weakened or lost by the influence of water infiltration, and the soil disintegrates. (4) Structural complexity of the rock mass: carbonaceous shales have experienced various geological movements, fracture development, and external natural environments during the formation process, and the disintegration mechanism varies [7,8]. With the rapid development of national highway network construction in recent years, roads and railways will inevitably pass through bad geological sections such as carbonaceous shale. It will bring great difficulties to the design and construction of the highway. It may be directly used as the subgrade filler in the actual project. In that case, carbonaceous shale easily collapses under rain, which will cause subgrade collapses, uneven settlement, and other issues, and even cause embankment instability, endangering the drivers’ safety. Moreover, it will cause substantial economic loss for the country and the local people. With such an engineering background, how to effectively utilize carbonaceous shale is one of the urgent problems in highway construction.

Existing studies have shown that carbonaceous shale is similar to other soft rock materials, and the excavated carbonaceous shale cannot be directly applied to the subgrade filling. It must be made stable before embankment filling. However, the disintegration characteristics vary significantly due to different factors, such as the region and the natural environment. Thus, studying its disintegration characteristics has vital theoretical and practical significance [9,10,11]. Currently, the study of the disintegration characteristics of carbonaceous shale has become a focus of attention [12,13]. Mariappa et al. [14] analyzed the shale mineral composition and disintegration properties in disintegration tests. Deo and Chapman et al. [15,16] tried to apply Indiana shale for the filling of the road subgrade, conducted a series of related experiments on the process of shale softening in water, determined its engineering performance, and classified shale according to experimental indexes such as its disintegration resistance. Yamaguchi et al. [17] studied the correlation between water absorption and the temperature of rock mass and rock disintegration. They found that the combined action of the two factors causes rock disintegration. The main controlling factor is the change in water content, while a single temperature change without a change in the water content cannot cause rock disintegration. Rune M. Holt et al. [18] used the brittleness index to carry out a unified standard comparison, elaborated on the brittleness index of different carbonaceous shale rocks, and studied the impact of hydraulic conditions on the brittleness index, as well as the impact of brittleness on the borehole stability. Kamila Gawel et al. [19] acid treated shale rock to remove carbonate from cement, and then measured its mechanical properties, thus finding a way to reduce shale’s shear strength. Based on the feasibility of using carbonaceous shale as a subgrade filling material, this study explores the disintegration characteristics of carbonaceous shale. Hua et al. [20] analyzed carbonaceous shale with different immersion times using energy spectrum analysis, X-ray diffraction, scanning electron microscopy, nitrogen adsorption, and other testing methods. It was concluded that the microstructure was gradually destroyed during flood disintegration. With the increase in the flood time, the pore size became smaller, but the porosity increased, and the distribution of the rock particles changed from surface to point surface. Fu et al. [5] conducted an experimental study on the complete disintegration of carbonaceous shale as a stable subgrade filler in an indoor simulated climate. Through the embankment filling test, the results show that the strength of the pre-disintegrating carbonaceous shale meets the requirements, but the water stability is poor, so it could only be used as the subgrade filling in zone 93. Liu et al. [21] studied the carbonaceous rock disintegration resistance and microstructural characteristics in Guangxi Province. It was found that the originally compact particles within carbonaceous rocks would increase in size due to water intrusion. The dissolution of soluble clay minerals, the loss of clay minerals, and the difference in water swelling of clay minerals were found to be the core factors affecting the disintegration of carbonaceous rocks at the microscopic level. Shao Yu [22] researched the distribution and combination characteristics, mineral composition, and organic matter characteristics of carbonaceous rocks in Guangxi through established geological data collection, geological field surveys, and sampling tests. The influencing factors of carbonaceous rock collapse and the corresponding engineering measures are discussed, and the corresponding engineering measures are put forward. The relevant research results can guide the design, construction, and practice of highway engineering in the Guangxi carbonaceous rock area.

Based on the above relevant research status, China’s research on carbonaceous shale is mainly concentrated in the north, southwest, and northwest regions. The general law of rock disintegration was obtained from the complexity of carbonaceous shale disintegration. Relevant research on carbonaceous shale in cold areas is even less common. Heilongjiang is located in the alpine region (the average annual temperature range is from −5.0 to 5.0 °C) in China [23], and the lake phase shale development area is distributed. The formed carbonaceous shale is affected by dry and wet cycles and freeze–thaw factors, so it is of great engineering significance to master the disintegration characteristics of carbonaceous shale in the cold region in Heilongjiang. Meanwhile, with the frequent trade between China and Russia, the construction of cross-border interconnected infrastructure, urban infrastructure, industrial infrastructure, and public service facilities has increased, and the construction of the Heilongjiang road network has also flourished [23]. The development and construction of road networks will inevitably cross adverse geological sections, such as shale, in cold regions. Shale may be is used directly as a roadbed filler in actual projects. In that case, it will cause roadbed collapse, settlement, and other issues that endanger the safety of road transportation if it is disintegrated or muddied by rain. If shale is distributed throughout these areas, it will affect the planning of the road network and reduce the efficiency of transportation. At the same time, if the plans are abandoned, many road construction materials will have been transported from a long distance away, which will significantly increase the construction cost and even destroy the ecological environment of the place.

This paper presents an experimental study on typical carbonaceous shale fillers along the provincial highway from the cold area to Huji Tumo Highway in Heilongjiang province. Experiments on indoor, outdoor, and dry–wet cyclic disintegration were conducted to study the effect of particle size distribution. In addition, carbonaceous shale’s disintegration and fractal characteristics were studied in a cold area. Furthermore, the disintegration mechanism of carbonaceous shale from the cold region was investigated before and after water infiltration. The results of this study are of great significance for improving the quality of engineering construction, saving land resources, saving investment costs, beautifying the surrounding environment, and supporting sustainable development in the cold area of Heilongjiang province. Furthermore, we provide theoretical references for road construction in the carboniferous shale area in Heilongjiang province in the future to help China to achieve a “carbon peak” in 2030.

2. Experimental Materials and Methods

2.1. Test Materials

In this experiment, carbonaceous shale was taken from the provincial road in Heilongjiang province to the Huji Tumo Highway, and its thin bedding structure was apparent. The samples were dark gray, and the water content of it was 4.5%. The XRD-6100 X-ray diffractometer of Shimadsu, Japan, was used to analyze the material composition of the carbonaceous shale. Figure 1 shows the content of the main minerals: quartz (26.4%), albite (7.9%), muscovite (38.1%), montmorillonite (18.7%), kaolinite (7.8%), and hematite (1.1%). The Japanese scanning electron microscope JSM-7500F tester was used. The EDS energy spectrum analysis showed that the main elements that were contained were C, N, O, Fe, Al, and Si (the Au peak in the energy spectrum was due to the test gold spray), as shown in Figure 2. (The horizontal coordinate Kev was the energy required to accelerate the electrons through a voltage difference of 1000v, and the vertical coordinate Cps was the energy spectrum acquisition X-ray count.)

2.2. Methods

2.2.1. Wetting Program

The treatment of carbonaceous shale with water is mainly divided into two methods: sprinkling water on the surface of the specimen and soaking it with water. it was considering that when we were spraying or sprinkling water directly on the surface of the specimen, the impact of water on the surface of the specimen would affect the test results and produce differences. In order to reduce the impact of this minor process on the final test, the water immersion method of slowly adding water along the wall of the container and allowing the water to submerge the specimen was used naturally. We added water until the rock’s surface was covered, so the specimen fully absorbed the water, with a soaking time of 12 h. According to the Heilongjiang Provincial Meteorological Bureau Beian temperature information, the area from Heilongjiang Provincial Road to Huji Tumo highway has an average day and night temperature in winter of −15 °C. The DX-300-40 low-temperature test chamber was used to simulate adverse temperature conditions in winter, and the test temperature was set at −15 °C. The carbonaceous shale samples were immersed in water and put into a −15 °C low-temperature test chamber, then left to stand for 12 h and freeze completely.

2.2.2. Drying Program

Referring to the disintegration test of carbonaceous shale conducted by Fu et al. [5], the indoor drying temperature in this study was 50 °C. We chose to simulate the outdoor sunshine natural dry conditions in Harbin during sunny weather and strong sunshine in July–August. The carbonaceous shale treated with a 200 mesh filter sieve was slowly filtered out of the water, and then the watered rock samples were dried indoors in an oven at 50 °C for 12 h. Finally, the rock samples were dried outdoors.

Five drying specimens were prepared using the research methodology, each with a mass of 1 kg. The specimen numbers were 1#, 2#, 3#, 4#, and 5#, as shown in Figure 3. Specimen 1# had a grain diameter of 40 mm~60 mm and was used in the water immersion + indoor oven drying test (referred to as indoor dry and wet cycle, which is the same as that below), specimen 2# had a grain diameter of 40 mm~60 mm and was used in the water immersion + outdoor natural air drying test (referred to as an outdoor dry and wet cycle, which is the same as that below), specimen 3# had a grain diameter of 20 mm~40 mm and was used in the water immersion into a −15 °C low-temperature test chamber + outdoor natural air drying test (referred to as a freeze–thaw cycle, which is the same as that below), specimen 4# had a grain diameter of 20 mm~40 mm and was used in the outdoor dry and wet cycle test, and specimen 5# had a grain diameter of 10 mm~20 mm and was used in the outdoor dry and wet cycle test. Specimens 1# and 2# were in the indoor and outdoor dry–wet cycle disintegration control group. Specimens 3# and 4# were in the freeze–thaw cycle disintegration and the dry–wet cycle disintegration control group. Specimens 2#, 4#, and 5# were in different initial particle size disintegration control groups. The test conditions for specimens 1#, 2#, 3#, 4# and 5# were slightly different. If there was no control group for the analysis and comparison, they were collectively called the wet and dry cycle ones.

2.3. Test Process

(1)

Take samples 1#, 2#, 3#, 4# and 5# and place them on the tray. Add enough water to the tray to immerse the samples’ surfaces. Soak samples 1#, 2#, 4#, and 5# for 12 h, and immediately put sample 3# into the −15 °C low-temperature test chamber and freeze for 12 h after soaking with water.

(2)

After soaking for 12 h, slowly filter the water of samples 1#, 2#, 4#, and 5# using a 200 mesh sieve.

(3)

Please specimen #1 in the oven at 50 °C to dry for 12 h; please specimens 2#, 4#, and 5# outdoors under sunlight conditions to naturally air dry. Specimen 3# is then taken out and placed directly outside under sunlight conditions to melt and air dry, and during the process of melting, slowly filter the moisture of the specimens using a 200 mesh filter sieve. If the free water during the melting process is not filtered in time, the sample will soak for a long time and further disintegrate, thus, wait for it to dry naturally.

(4)

After each completion of the wet and dry cycles, sieve the specimens using nine different sizes of standard sieves from large to small ones, such as 40 mm, 20 mm, 10 mm, 5 mm, 2 mm, 1 mm, 0.5 mm, 0.25 mm, and 0.075 mm ones.

(5)

Record the mass of the different particle size specimens with an accuracy of 0.01 g using an electronic balance and compare them with the previous results.

The above test steps from (1) to (5) were repeated for each group of samples until the particle size and the quality of the carbonaceous shale remained unchanged at each stage. See Figure 4 below for the specific operation process.

2.4. Analysis Method

We observed and recorded the disintegration phenomenon and sieved the dried specimens at the end of each wet and dry cycle in the disintegration test [24]. The particle size variation, disintegration resistance, and fractal characteristics were analyzed during the disintegration process. In addition, the change mechanisms of the microstructure of the carbonaceous shale before and after water immersion were compared.

2.4.1. Disintegration Resistance Index

Rock disintegration resistance represents the resistance of softening and disintegration as the external conditions vary [25,26]. It is used to determine the ability of carbonaceous shale to resist disintegration crushing and the disintegration strength under different disintegration conditions. Wang et al. [27] investigated soft rocks’ disintegration characteristics in the red layer in southern Anhui. The disintegration resistance index is defined as the ratio of the mass of particles that are larger than 2 mm in the rock disintegrating to the original mass of the rock sample after the nth wet and dry cycle. The following equation can calculate it.

$${\mathrm{I}}_{\mathrm{dn}}={\mathrm{m}}_{\mathrm{n}}÷{\mathrm{m}}_0\times 100\%$$

(1)

where I_dn is the disintegration resistance index of the specimen after the nth wet and dry cycle (%), mn is the drying mass of the rock sample with particle size that is greater than 2 mm after the nth wet and dry cycle (g), and m₀ is the drying mass of the original specimen (g).

2.4.2. Disintegrating Fractal Dimension

Fractals can be introduced as the theory used in the geological discipline to describe the overall and local similarity of geological bodies [5]. Many studies have shown that the fracture of the rock mass under the random action of various non-human external forces (including natural and mechanical forces) is a fractal process. Therefore, a fractal theory is introduced to simulate carbonaceous shale’s disintegration and deformation [28]. There are many methods to calculate the fractal dimension, such as the self-affine fractal method, the yardstick method, the scale-free associative dimension, the power-law spectrum method, the modified yardstick method, the perimeter–area relationship method, the box dimension method and information dimension, and other calculation methods. Since the mass percentages of carbonaceous shale after disintegration were mostly tested in the experiments, the fractal dimension of carbonaceous shale was calculated according to the mass measurement method proposed in [28,29].

2.4.3. Scanning Electron Microscope

Before and after the disintegration test, typical carbonaceous rock specimens were selected for microscopic scanning electron microscopy analysis. Moreover, microstructure photographs of the specimens were obtained. The scans of the electron microscope samples must come from the same stone and have a uniform section when they are observed by the naked eye. Therefore, the section of the observation area was flat, and the surface was treated with gold spraying to record the changes in the microscopic structure of the carbonaceous shale.

3. Experimental Results and Analysis

3.1. Carbonaceous Shale Disintegration under the Action of Dry and Wet Cycles

The wet and dry cycle is an iterative process in which the specimen is continuously drenched and dried. The initial conditions of the tests on specimens 1–5# were slightly different, but the overall patterns of disintegration development and evolution were the same. For the specimen subjected to the process of water submersion, water was gradually immersed in the specimen, followed by bubbles and a subtle “sizzling” water absorption sound. Fine cracks were gradually produced in the rock samples at 15 min due to moisture infiltration (Figure 5a), and the fissures widened and deepened into penetration cracks at 1 h (Figure 5b). The 3 h bulk rock sample gradually produced fragmentation (Figure 5c) and continued to fragment during the rest of the immersion process. The break in the rock sample gradually increased, accompanied by fragmentation and stratification.

The macroscopic disintegration process of sample 1# of carbonaceous shale that was underwater is illustrated. Figure 6a shows the initial un-disintegrated state of carbonaceous shale. When the first dry–wet cycle had been performed, most of the rock samples (1#, 2#, 3#, 4#, and 5#) were initially broken. The overall appearance of a few parts did not change, and no apparent disintegration was observed. The particles of each specimen decreased to the next size (Figure 6b). With the increase in the number of cycles, most of the rock samples disintegrated into a granular form, and the particle size was concentrated in the range of 2–10 mm (Figure 6c). After the eighth dry and wet cycle, the mass proportion of samples 1–5# that passed through the standard sieves of different pore sizes remained the same. Only the quality of the charcoal shale particles slightly changed using the 5mm standard sieve. The maximum change was only 0.07%. From this, the disintegration process had basically completed, and the state of the disintegrated rock sample tended to be stable and became a loose accumulation (Figure 6d).

3.2. Particle Size Change Patterns of Disintegrated Material under the Action of Dry and Wet Cycles

The test data obtained from sieving were processed to obtain the particle gradation change curves for eight dry and wet cycles for test specimens 1#, 2#, 3#, 4#, and 5#, as shown in Figure 7.

From the curves of the grain gradation changes in samples #1, #2, #3, #4, and #5, it can be seen that the grain gradation curves of the first six cycles of carbonaceous shale disintegration change significantly, and the curves from the sixth to the eighth cycles change less, and the gradation curves overlap. The disintegration test conditions and the test specimen’s particle sizes are slightly different, resulting in particle gradation curve differences. However, the overall change trend is similar. Figure 7a–e show that when specimens 1#, 2#, 3#, 4#, and 5# were subjected the first three wet and dry cycles, the specimens with particle sizes that were larger than 10mm disintegrated quickly. The percentage of specimens that passed through the sieve increased from 0 to more than 69%, and the percentage of specimens with particle sizes that were smaller than 10 mm was less than 33.65%. This process is apparent in the changes in large-grain-sized specimens, and the test phenomenon is mainly based on the disintegration and fragmentation of large rock samples. After the sixth wet and dry cycle, the percentage of specimens with a particle size that was larger than 10 mm that passed though the sieve was greater than 96.11%, and disintegration had almost been achieved. The maximum percentage of samples that passed though the sieve with particle sizes of 5 mm~10 mm and 2 mm~5 mm is 68.03% and 9.60%, respectively. The disintegration rate of those with 5 mm~10 mm particles is higher than the rate of those with 2 mm~5 mm particles during the process from the fourth to the sixth wetting and drying cycles. The particle size is concentrated between 2 mm~10 mm. This process is a combination of a small fraction of the rock sample being broken and small particles sliding off the rock surface. During the 7th~8th wetting–drying cycles, the samples with a particle size that was greater than 10mm completely disintegrated, and the percentage of samples with a particle size that was less than 10mm that passed through the sieve is different, which shows that the percentage passing curves of all of the samples basically overlap. The dislodgement of small particles from the surface of the rock mass with a larger particle size dominates this process of disintegration. The degree of roundness of the specimens increased as the test progressed (Figure 8).

In this study, carbonaceous shale was taken from the provincial highway of Heilongjiang province to the Huji Tumo Highway. Because this area is a cold region with a high latitude and a low average temperature in winter, the field’s carbonaceous shale is affected by freeze–thaw cycles. Therefore, the disintegration variation in test #3 is used as an example to analyze the particle gradation characteristics of carbonaceous shale under freeze–thaw cycle conditions. As seen from Figure 9, during the first three cycles, disintegration occurred mainly for those with the particle size of 10 mm to 40 mm, and the large specimens of 10 mm to 40 mm disintegrated and broke faster. The percentage of the particle size mass decreased from 68.45% to 10.86%. The proportion of particle sizes within the 2–10 mm range increased significantly, and the percentage of other particle sizes of less than 2 mm increased slightly. It can be seen that the first three cycle tests were dominated by extensive rock disintegration and crushing. When the 3rd freeze–thaw cycle had been performed, the proportion of 20 mm~40 mm particles is only 0.01%, as basically all of them disintegrated into small particles. The percentage of grain size specimens that were 5 mm to 10 mm reached a peak of 54.67%, and the amount of rock sample mass in this grain size range was much more significant than the amount of other grain sizes. During the fourth to the seventh freeze–thaw cycles, less than 5% of the mass of the specimens had a particle size of 10–20 mm. The mass ratio of specimens with a particle size of 5 mm~10 mm gradually decreased and stabilized, accounting for 32.28% of the total mass. The percentage of specimens with a particle size that was less than 5 mm still increased, among which the percentage of those with a particle size of 2 mm to 5 mm grew significantly, and the percentage of these specimens increased to 53.1%, accounting for more than half of the total mass, and this tended to stabilize at the peak. During this process, the samples with a particle size of 5 mm~10 mm were mainly broken into smaller samples with a particle size of 2~5 mm. The percentage of samples with a particle size of less than 2 mm increased, but because the mass was too small, the change in particle grading is more apparent. During the eighth freeze–thaw cycle, the percentages of each particle size gradually tended to be flat and stable, and the maximum change in the mass ratio of each particle size was only 0.56%. At this time, it was judged that the carbonaceous shale sample’s disintegration was complete and stable.

3.3. Influence of Indoor and Outdoor Dry–Wet Cycles on Disintegration Characteristics

A change in disintegrated particle content between the first indoor wet and dry cycle and the second outdoor wet and dry cycle was found. After the first two wet and dry cycles, the disintegration rate of the second outdoor wet–dry cycle was significantly more extensive than that of first indoor wet and dry cycle, and the percentage of large-sized particles that disintegrated into small-sized particles was more significant than that of the first cycle. This indicates that the outdoor conditions are more favorable to the disintegration of carbonaceous shale at this time (Figure 10a,b). However, starting from the third wet and dry cycle, the disintegration rate of specimen #1 grew to be greater than that of specimen #2 (Figure 10c). After the completion of the sixth cycle, the particle mass proportion of sample 1# tended to be stable, while that of sample 2# tended to stabilize during the seventh cycle. At this time, tiny particles mainly fell off the surface of the rock’s body, which had larger particle sizes. After completing the eighth dry and wet cycle, the mass ratio of the first disintegrating particle size of 2 mm~5 mm reached 75.51%, which is significantly greater than that of the second disintegrating particle size of 50.83%. The disintegration degree of the rock sample was large, and the small particle size accounted for a relatively large proportion of it (Figure 10d). The regular outdoor temperature monitoring (Figure 11) shows that the maximum temperature of the rock sample surface outside during the first two wet and dry cycles was higher than 50 °C. Compared with indoor drying conditions, the internal thermal stress of the rock mass reached the tensile strength of carbonaceous shale more quickly, reducing the rock mass’s stability and forming internal cracks. Furthermore, when the shale sample was dried, micro-cracks in the surface of the mesh often appeared (Figure 12), which gradually spread throughout the sample, damaging the rock structure, resulting in the loss of the rock’s integrity, and further accelerating the disintegration of carbonaceous shale. Due to continuous disintegration, the fissure gradually deepened and widened (Figure 13), and the final disintegrated material had a smaller particle size. However, with the change in the external atmospheric temperature, after the third dry–wet cycle, the surface temperature of the carbonaceous shale during the process of outdoor, natural air drying was up to 34 °C, which is less than the indoor oven temperature of 50 °C. As a result, the disintegration rate of the second outdoor dry–wet cycle is significantly slower than that of the first indoor dry–wet cycle. In this case, indoor conditions are more conducive to the disintegration of carbonaceous shale. Thus, high temperatures during dry and wet cycles can reduce carbonaceous shale’s stability, accelerate the disintegration rate, and disintegrate and crush it into finer grained gravel.

3.4. Influence of Freeze–Thaw on Disintegrate Characteristics of Carbonaceous Shale

From the comparison of the changes in the disintegrated particle content between the third freeze–thaw cycle and fourth dry–wet cycle, it can be seen that the disintegration rate of specimen 3# is significantly larger than that of specimen 4# during the dry–wet cycle. The early disintegration mainly those with a large grain size of 20 mm~40 mm, and the percent of the grains with a size in the range of 20 mm~40 mm that passed through the sieve was 100% after the fourth disintegration cycle. All of them disintegrated into shale with a grain size of less than 20 mm (Figure 14a). After five freeze–thaw cycles, the disintegration tended to level off, and the 20 mm~40 mm particles disintegrated. After six wet and dry cycles, the disintegration tended to be slower than that which occurred in the first freeze–thaw cycle (Figure 14b). After completing the sixth wet and dry cycle, the particle size disintegrated from 5 mm~10 mm to 2 mm~5 mm. At this time, the disintegration phenomenon gradually stabilized, and the number of changes for each grain group was small. The particle disintegration degree of sample 3# is more severe, which manifested as a smaller particle size of the rock sample (Figure 14c,d). The reason for this is that the conditions of the third test mainly simulated outdoor freezing and thawing. The carbonaceous shale specimens were immersed in water, put into the −15 °C low-temperature test chamber, and left to freeze entirely for 12 h. When the water is not frozen, the water will enter into the rock mass through the pores of the carbonaceous shale. Part of the free water is in the pores. Under the action of water, some minerals interact with water to form crystalline compounds. A part of them is adsorbed onto the surface of clay particles to form a water film, and as the temperature decreases, the free water in the pores begins to freeze. The weakly bound water in the soil also freezes gradually from the outermost layer inward. Because the molecular space of ice is larger than the volume of water, the water increases in volume during the freezing process, generating uneven freezing and swelling forces [30,31]. They promote the generation of cracks inside the rock, accelerating the disintegration of carbonaceous shale, which makes the carbonaceous shale disintegrate into fine fragments at an accelerated rate. Therefore, the specimen’s disintegration rate is significant and intense when it is subjected to the freeze–thaw cycle. The rock sample is broken more finely, and disintegration is achieved more quickly.

3.5. Influence of Different Initial Particle Sizes on Disintegration Characteristics

From control groups 2#, 4#, and 5# with different initial particle size disintegration rates, it can be concluded that the particle size of carbonaceous shale affects the time required for stabilization under the same external conditions. During the 1st wet–dry cycle, the largest grain size of samples 2#, 4#, and 5# decreased from the initial 100% to 2.34%, 34.5%, and 83.49%, respectively (Figure 15a). The disintegration resistance rate of the large-grain-sized carbonaceous shale was significantly greater than that of the small-grain-sized shale. After completing the second dry and wet cycle, the sizes of the disintegrating particles of samples 2#, 4#, and 5# were mainly concentrated at 2 mm~20 mm (Figure 15b). With the increase in the number of wet and dry cycles, the difference in the mass ratio of the disintegrated particles with the same particle size gradually decreased, and the mass ratios became the same. After completing the fifth wet and dry cycle test, the mass of the particles in the range of 10 mm to 20 mm accounted for less than 5%, while the disintegrating particles were concentrated in the range of 2 mm to 10 mm (Figure 15c). The mass particle size of less than 2 mm is relatively tiny until disintegration stabilization had been achieved (Figure 15d). When the particle mass ratio reached about 90% within 2 mm~10 mm in tests 2#, 4#, and 5#, the number of dry and wet cycles was seven, six, and five, respectively, and the mass ratio fluctuated at ±1.5% during the dry and wet cycles. It can be determined that the disintegration time of the larger specimen is slower than the time of the adjacent smaller specimen during one dry and wet cycle. The time required to achieve complete disintegration is longer. The main reasons for the above facts are that the clay mineral particles in carbonaceous shale have good hydrophilic and water-absorbing swelling properties. Carbonaceous shale with a large initial grain size has more internal defects due to its natural environment. After water erosion, water quickly penetrates the pores and fissures of the rock, while the adsorbed water film of the rock grains then thickens. A small number of soluble minerals and salts dissolve in water [32], destroying the rock’s internal structure, and rapidly disintegrate into flake particles of a smaller grain size. However, the charcoal shale with a smaller particle size has a weaker water absorption capacity due to the reduced structural surface. The particle disintegration is due to particle surface flaking, and the disintegration stabilization time is short. The analysis and comparison show that the disintegration resistance of the specimens with particle size of between 2 mm and 10 mm is good, and the results of this study have some practical significance for engineering applications. In areas that are rich in carbonaceous shale, replacing all of the carbonaceous shale is not conducive to the project’s economics. Carbonaceous shale needs to be disintegrated and crushed into a particle size that meets the engineering needs by more economical means such as using carbonaceous shale as an engineering filler. For carbonaceous shale with a large initial particle size, it can be subjected to several wet and dry cycles or some external disturbance to accelerate its disintegration and fragmentation to the ideal particle size to improve the stability of the structure.

4. Disintegration Resistance and Fractional Dimensional Number Research and Modeling

4.1. Disintegration Resistance

Using the disintegration resistance index formula, the specimens were analyzed under different dry–wet cycles. As a result, the relationship curves between the disintegration resistance index and cycle times of specimens 1–5# were obtained, as shown in Figure 16.

The relationship curve between the disintegration resistance index and the number of cycles is shown in Figure 16. The disintegration resistance index shows a decreasing trend with an increase in the number of wet and dry cycles, and it decreases gradually. The rate of the decrease in the disintegration resistance index was almost the same for specimens 1–5# in the first two cycles. From the disintegration resistance index formula, the change index is mainly caused by particle size changes above 2 mm. It indicates that the disintegration of carbonaceous shale is mainly caused by the disintegration of large-sized rock samples during the early stage of dry–wet cycles. The particle size after disintegration is mainly concentrated at above 2 mm. During this time, the calving index of carbonaceous shale is about 97.8%. With the increase in the number of dry–wet cycle tests, the disintegration resistance of the carbonaceous rock samples under different conditions is significantly different. The disintegration index of samples 1#, 2#, 4#, and 5# is stable at about 93%. However, due to the small initial particle size of specimen #5, the charcoal shale disintegration was mainly due to particle surface flaking. The disintegration stability time was short. Then, the disintegration resistance index reached stability after the fifth wet and dry cycle. Moreover, under the influence of freeze–thaw conditions, the disintegration rate is faster, making the carbonaceous shale more easily broken into fine particles. The rate of the decrease in the disintegration resistance index of specimen #3 during the third wet and dry cycle was significantly greater than that of the other four specimens. The lower disintegration resistance of the specimens indicated that the ability of the rock samples to maintain integrity was significantly lower than that of the other specimens. The disintegration resistance index reached 89% stability. It can be seen that freezing–thawing conditions have a greater impact on the disintegration resistance of carbonaceous shale than outdoor natural wind drying and different initial particle sizes do. The research results have a certain theoretical significance to guide the use of carbonaceous shale as a subgrade filler in construction in high-latitude cold area.

4.2. Disintegrating Fractal Dimension Research and Model Establishment

The fractal theory mainly regards some irregular curves and shapes with similarity. The disintegration of carbonaceous rocks in the disintegration resistance test is a random process. The fractal theory can also describe the shape characteristics of the disintegrated debris [33,34,35,36,37]. In this paper, the fractal dimension number is used to study the fractal characteristics of the disintegration process of carbonaceous rocks. The number of fractal dimensions for eight cycles of tests for specimens 1–5# is shown in Figure 17.

From the variation in the fractal dimension of disintegrated particles throughout the cyclic cycling of specimens 1–5# in Figure 17, it can be seen that the fractal dimension of carbonaceous shale tends to increase with the number of dry and wet cycles. The fractal dimension increases most obviously in the first three dry–wet cycles. The macro phenomenon is dominated by the fragmentation of large rocks, followed by the disintegration of carbonaceous shale. Disintegration mainly involves the surface flaking of larger particles, leading to an increase in the number of fine particles. The fractal dimension is calculated by the logarithmic fitting of the mass and particle size, and its order of magnitude is small. Therefore, a slight change in the particle size will lead to specific changes in the fractal dimension, which will fluctuate up and down at 1.78. At this time, the similarity of calving carbonaceous shale reaches its limit, and the disintegration of carbonaceous shale is basically achieved. The disintegration tests’ environmental conditions and particle sizes are slightly different. It leads to particle fractional dimension number changes during disintegration. The changes in samples 1#, 2#, and 3#’s fractional dimension numbers tended to be flat during the third dry and wet cycle. The growth of the fractional dimension of carbonaceous shale samples 4# and 5# with a small initial particle size is significantly slower than that of carbonaceous shale with a large initial particle size. The fractal dimension of the disintegrating particles of rock sample 4# tended to be stable after the fourth dry–wet cycle. In comparison, the fractal dimension of rock sample 5# tended to be gentle after the fifth cycle. Although the initial particle size of sample 3# was the same as that of sample 4#, the disintegration rate of the carbonaceous shale was accelerated by its test conditions, indoor freezing and outdoor thawing and air-drying, respectively, which meant that they showed a similar variation pattern to that of 1# and 2#. The variation trend of the fractal dimension of distribution also reflects the disintegration rate of the samples. Generally, the faster the fractal dimension of distribution increases, the greater the degree of the disintegration of carbonaceous shale, otherwise, the more stable the carbonaceous shale tends to be. Therefore, it is of interest to determine the degree of carbonaceous shale disintegration based on the particle distribution’s fractal dimension.

In this paper, we studied carbonaceous shale located in a cold area affected by freeze–thaw cycle factors. Sample 3#’s disintegration fractional dimension D is fitted with the index I_dn, as shown in Figure 18. There is a good correlation between the disintegration fractional dimension number D and the resistance index I_dn. The disintegration resistance index I_dn gradually decreases. The corresponding disintegration fractional dimension number D gradually increases, with the number of wet and dry cycles increasing. After the fundamental disintegration was achieved, the relationship between the two changes tended to level off. The disintegration fractional dimension number is negatively correlated with the disintegration resistance index. An exponential function can describe the relationship between the disintegration fractal dimension and resistance index. The correlation coefficient R² is higher than 0.993. It should be noted that the variation trend of the test results of the other four kinds of carbonaceous shale samples is consistent with that of sample 3#. However, the degree of correlation between the parameters is slightly different.

5. Microstructural Characteristics and Mechanism of Carbonaceous Shale Disintegration

Carbonaceous shale is composed of complex materials and quickly disintegrates after encountering water, so it has unique hydrologic characteristics. Therefore, we are limited to studying its properties only from a macro perspective. Therefore, scanning electron microscopy was used to study the carbonaceous shale under the action of the wetting and drying cycles in terms of the microscopic structure. After experiencing three and five wet and dry cycles of disintegration, the initial rock samples and the fragmented rock samples were dried. The specimens were scanned by microscopic electron microscopy using a Japanese scanning electron microscope (SEM) JSM-7500F tester. The SEM images of the specimens are shown in Figure 19.

As seen from Figure 19a, before water immersion, the microscopic structure of the carbonaceous rocks is relatively dense, and the particles are closely bonded. As shown in Figure 19b, after multiple increases, it can be found that the carbonaceous rocks are stacked in a sheet structure, mainly in the form of face-to-surface contact, while a few of them use point-to-surface contact. The continuity of the lamellar structure is good, and the shape is a mostly irregularly arranged continuous lamellar structure. However, there are still many pores between the particles, which provides a way for water to infiltrate. Figure 19c,d shows that there are pores of different sizes between the surface particles after the first wet and dry cycle, and the dense structure between the particles is damaged. The microstructure was mainly connected in the form of face-to-surface contact. However, it can be found that the contact area between the point and the surface begins to increase, and the lamellar particles flake off. After the multiple increases, it can be found that surface cracks appear, and the surface particles flake off obviously. As shown in Figure 19e,f, after three and five wet and dry cycles of disintegration, the original large lamellar particles became dislodged, and small flocculent clay particles were attached to the surface of the carbonaceous shale rock. The rock structure transitioned from a denser lamellar structure to an irregularly shaped structure, from a face-to-face connection to a point-to-face connection, and from an initially relatively dense structure to a loose porous structure by erosion. It intensified the further penetration and erosion of water into the rock structure. Until the rock particles became transparent, the macro manifestation is that disintegration was completely achieved in this experiment.

The indoor dry and wet cycle test was divided into two stages: soaking and wetting, and dehydration drying. After water intervention, clay mineral particles in carbonaceous shale will be hydrated, adsorbed, and dissolved during the soaking and wetting stage. Other physicochemical effects will occur under the action of water. The continuous infiltration of water and the entry of water, erosion, and destruction due to the lamellar structure make the internal porosity of the rock structure increase, which enhances the effect of the water–rock action of carbonaceous shale. The water-soaked expansion causes the structure of carbonaceous shale rock to become loose and broken [27]. Based on the X-ray diffractometer analysis, the mineral ingredient includes 18.7% montmorillonite, 7.8% kaolinite, and other fine clay particles. The chemical formula of montmorillonite crystals is K_0.9Al_2.9Si_3.1O₁₀(OH)₂. The crystal structural unit of montmorillonite is composed of cells that are parallel to each other, and its structural lattice is highly mobile and hydrophilic. Moreover, in the cell structure layer between cations Fe³⁺, Fe²⁺, Ca²⁺, and Mg²⁺ will be some exchange with Al³⁺, and there is a high ion exchange capacity, and its volume can expand to as much as seven times the original one after encountering water. When water penetrates the pores and fissures of the rock, water molecules enter between the crystal layers of montmorillonite, causing the lattice expansion of the mineral particles [20]. Although water molecules cannot enter the mineral crystal layer for kaolinite-type clay minerals, the adsorbed water molecules form an adsorption water film on the surface of the clay mineral particles. With the thickening of the water film, intergranular expansion is caused. Since the volume expansion is not uniform, tensile stress is generated inside the rock block. In addition, the water molecules entering the intergranular force also weaken in the cemented minerals. They result in the decrease in the cementation strength. The dissolution and softening of the cement severely weaken the linkage between the rock particles. Thus, the rock masses tend to form cracks under the action of tensile stress, which eventually leads to fracture and disintegration. Within 15 min, the internal cracks of the rock samples spread due to water immersion to a certain extent and gradually produced small cracks (see Figure 5a). The fissures widened and deepened in the 1h rock sample (see Figure 5b), gradually producing fragmentation in the 3h bulk rock sample (see Figure 5c). In the remaining soaking process, the fractures in the rock samples increased gradually, and this was accompanied by fragmentation and stratification.

During the drying phase, the clay particles lose water and shrink. We considered the rock mass’s inhomogeneity and the inconsistent water loss rate between the surface and interior of the rock mass. This shrinkage is also inhomogeneous, thus generating tensile stress. Tensile cracking occurs when the tensile stress is greater than the cementing strength of the rock cement [27]. As a result, the cracking and spalling of the grains occur, as shown in Figure 12 and Figure 13, and the presence of cracks makes the rock mass less monolithic. At the same time, the next soaking and wetting test provides a channel for the water molecules to enter the interior of the rock mass, where the clay minerals are more likely to absorb water and swell. So repeatedly, when the fracture development is complete, the rock specimen disintegrates into a fine gravel, its disintegration resistance index drops to the lowest, and the morphology between the gravel grains is the most similar. The disintegration fractional dimension number is gradually stabilized.

6. Conclusions

Experiments were carried out on typical carbonaceous shale fillers along the provincial road from the cold area in Heilongjiang province to Huji Tumo Highway. Dry–wet cyclic (indoor, outdoor, and initial particle sizes) and freeze–thaw cyclic disintegration were considered. The disintegration process under the action of water was analyzed. Based on the analysis of the test results, it can be seen that:

(1)

In this study, the main component of carbonaceous shale is quartz, albite, muscovite, montmorillonite, kaolinite, and hematite, with content values of 26.4%, 7.9%, 38.1%, 18.7%, 7.8%, and 1.1%, respectively.

(2)

During the dry–wet cycle, the samples with particle sizes that were greater than 10 mm during the first three cycles were severely disintegrated. During the fourth to the seventh wetting–drying cycle, the particle with a size of 5 mm~10 mm mainly disintegrated into particles with a size range of 2 mm~5 mm. After eight cycles, the particle size of the carbonaceous shale particles tended to be stable, and disintegration was basically achieved. Additionally, the particle size of the disintegration particles was concentrated in the range of 2 mm~10 mm, of which the particle size of 2 mm~5 mm accounts for a large proportion.

(3)

Temperature and freeze–thaw cycle conditions will affect the disintegration resistance characteristics of carbonaceous shale and reduce its stability. Compared with temperature, freeze–thaw actions have an obvious effect on the disintegration characteristics of carbonaceous shale, as after disintegration, the particle size is smaller, and the time required for disintegration stability is shorter. After the freeze–thaw cycles, the degree of disintegration resistance of the carbonaceous shale was poorer, and the disintegration resistance index decreased faster.

(4)

The initial particle size significantly affects the particle size distribution of disintegrates after wet and dry cycling. The specimen disintegration stability time with a larger particle size is slower than that of a smaller one. The sample with a particle size between 2 mm and 10 mm has better disintegration resistance. For carbonaceous shale with a large initial particle size, certain external conditions or external disturbances can be applied to accelerate its disintegration and fragmentation to a smaller particle size to improve the stability of the carbonaceous shale.

(5)

There is a negative correlation between the disintegration resistance index and the fractional dimension number. The similarity of disintegrating carbonaceous shale can be used to characterize the disintegration resistance of carbonaceous rocks indirectly. In the preliminary dry–wet cycle (within three cycles), the fractal dimension of carbonaceous shale changed significantly, and with the increase in the dry–wet cycle, the disintegration phenomenon weakened, the increase in the fractal dimension tended to be stable, and the fractal dimension was around 1.78 when disintegration was achieved.

(6)

The presence of kaolinite and montmorillonite in carbonaceous shale is the main cause of its disintegration. Due to the highly hydrophilic characteristic of the two clay mineral particles, after immersion, uneven expansion occurred, and this formed tensile stress, resulting in the disintegration of the carbonaceous shale. After drying, the clay particles cracked due to water loss and shrinkage, and when the water molecules intruded again, it was more likely to enter the rock and cause it to disintegrate.

7. Limitations and Recommendations

In this study, the initial particle size of the test sample is controlled. However, the initial sample is not homogeneous, which has an impact on the results. During the test, the indoor dry–wet cycle and freeze–thaw cycle were performed to simulate the natural conditions of summer and winter, but natural factors such as wind speed cannot be fully simulated. In addition, in the final sieving process, some of large samples collided with the standard sieve and broke, which had a partial impact on the test results. It should be noticed that, in water condition, carbonaceous shale filler is prone to softening, swelling, wetting deformation, and creeping. Therefore, it is recommended that further research should be focused on the stability and feasibility of carbonaceous shale in a water environment.