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Frontiers in Earth Science ·

Volume 14 - 2026 | https://doi.org/10.3389/feart.2026.1917882

Disintegration mechanism and characteristics of water-sensitive red-bed mudstone affected by multi-factor via a self-developed test apparatus

  • Qian Dai1
  • Xin Liu2
  • Xiaosen Kang2*corresponding author
  • Ding Zhang2
  1. 1Department of Civil Engineering, Xi’an Jiaotong University City College, Xi’an, China
  2. 2Department of Geological Engineering, Chang’an University, Xi’an, China
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Abstract

The disintegration of red-bed mudstone has caused lists of geohazards, which threatens the geological environment in the upper reaches of the Yellow River, China. However, the disintegration mechanism and characteristics of red-bed mudstone influenced by multi - factor remain unclear. To solve the problem, a series of laboratory tests was conducted using a self-developed disintegration apparatus and testing method. The disintegration characteristics of high - clay - mineral - content mudstone (HCM) and low - clay - mineral - content mudstone (LCM) were investigated by analyzing the instantaneous mass-loss ratio (m i), instantaneous mass - loss rate (m ˙), and final mass - loss ratio (m f). The results show that the clay mineral content and initial moisture content markedly affect both the disintegration process and its ultimate state of unsaturated red-bed mudstone. Specifically, for unsaturated HCM, the values of m ˙ and m f initially increase to a peak value and then decrease with increasing initial moisture content. In contrast, for unsaturated LCM, both m ˙ and m f decrease monotonically with increasing initial moisture content. Comparing with clay mineral content and initial moisture content, water pressure and temperature influence the disintegration process, but have a weaker effect on ultimate state. Furthermore, the disintegration process of the red-bed mudstone can be categorized into three types, namely progressive erosion, partial fragmentation, and surficial spalling. These three types are driven by different hydro - mechanical process among mineral particles. These results provide insights into the interaction between reservoir construction and geological environment in upper reaches of the Yellow River.

Keywords

1 Introduction

Red-bed mudstone is widely exposed in the watershed gullies, landslide areas, and reservoir drawdown zones in upper reaches of the Yellow River (Zhao et al., 2025). Its disintegration has caused lists of geohazards, like landslide and collapse. In the Gaojiawan landslides in the Xining Basin, the red - bed mudstone disintegrates under the action of groundwater and surface water, which threatens the slope stability. In the Lijiaxia hydropower station and Gongboxia hydropower station, it disintegrates under reservoir water level fluctuations, which threatens the hydropower engineering (Figure 1). These demonstrate that water plays a significant role in the disintegration process of red-bed mudstone (Liu et al., 2021; Su et al., 2025), which in turn poses a critical threat to hydropower station safety (Zhang et al., 2018; Zhang et al., 2020; Dai et al., 2026).

Red-bed mudstone in upper reaches of the Yellow River **(a)** watershed gullies in the guide basin, **(b)** watershed gullies in the Jianzha Basin, **(c)** gaojiawan landslide in Xining Basin, **(d)** slopes in Lijiaxia hydropower station, **(e)** slopes in gongboxia hydropower station (modified from Zhao et al., 2025).
Figure 1. Red-bed mudstone in upper reaches of the Yellow River (a) watershed gullies in the guide basin, (b) watershed gullies in the Jianzha Basin, (c) gaojiawan landslide in Xining Basin, (d) slopes in Lijiaxia hydropower station, (e) slopes in gongboxia hydropower station (modified from Zhao et al., 2025).

The disintegration of mudstone is generally influenced by internal and extrinsic factors (Guo et al., 2012). The internal factors include mineral composition, initial moisture content, pore structure, and particle size (Yan et al., 2024; Zeng et al., 2024; Zhang et al., 2024; Zhang et al., 2019). The mineral composition affects the mechanical behavior of mudstone (Li et al., 2024; Kang et al., 2022), and higher clay mineral content promotes the disintegration (Gautam and Shakoor, 2013). The initial moisture content influences the disintegration of rock-soil masses (Liu et al., 2021; Zhang D. et al., 2016; Zhang S. et al., 2016), and higher initial moisture content impedes disintegration (Yu et al., 2023). The larger pore size induced by mineral dissolution intensifies the disintegration process (Huang K. et al., 2022; Jiang et al., 2014). Moreover, smaller grain sizes lead to more notable water - rock interactions, which in turn accelerate the evolutionary process (Fu et al., 2022). Besides, the extrinsic factors include stress and pH levels (Zhang and Gao, 2020; Zeng and Kong, 2023; Zhao et al., 2018). The stress - induced nonuniform deformation during hygrothermal cycles leads to microfractures that accelerates disintegration (Geng et al., 2023), and the concentration of H+ accelerates degradation (Xiao et al., 2021; He et al., 2023).

The coupling of the internal and extrinsic factors induces interactions among physical, chemical, and mechanical processes. The hydration and swelling of clay minerals generate shear stresses and tensile stresses, and temperature variations further alter the tensile stress (Wang et al., 2022; Fu et al., 2022; Wang Q. et al., 2024). When the tensile stress exceeds the tensile strength, internal fractures propagation occurs (Hu et al., 2019). As water permeates the internal fractures, residual gas compression increases both hydraulic and gas pressure. This further promotes fracture propagation and creates preferential pathways for water infiltration, and ultimately leading to disintegration (Su et al., 2020; Fu et al., 2023). During this process, continuous energy transition occurs, involving heat energy, surface energy, and failure energy (Huang K. et al., 2022; Zeng et al., 2021). The process can be explained by the hydro-mechanical coupling principle (Lu and Likos, 2004), as it is influenced by cementation and matric suction (Wang HM. et al., 2024; Su et al., 2022; Su et al., 2024). The cementation weakens as mineral dissolution rates rise (Zha et al., 2022), in turn inducing the matrix disintegration into fines (Zhang et al., 2023). Although the disintegration properties of mudstone were investigated, few studies have focused on the disintegration mechanism and characteristics of red - bed mudstone under the influence of the multi-factor, including clay mineral content, initial moisture content, water pressure, and temperature.

To address the problem, a series of laboratory tests was conducted by considering clay mineral content, initial moisture content, water pressure, and temperature. Firstly, a self - developed disintegration apparatus and testing method were designed and fabricated, and a series of disintegration tests were conducted. The disintegration characteristics were investigated by analyzing indices such as the instantaneous mass - loss ratio, instantaneous mass-loss rate, and final mass-loss ratio. The disintegration mechanism of the red-bed mudstone was analyzed. These results provide insights into the interaction between reservoir construction and geological environment in upper reaches of the Yellow River.

2 Specimens preparation

The Xining Basin, situated between Laji Mountain and Daban Mountain, has a special geological environment characterized by intense neotectonic movements, seismic activity, and numerous fault fracture zones (see Figure 1). As a result of long-term geological uplift, extensive deposits of mudstone formed during the Tertiary period (Xie et al., 2025). The red - bed mudstone used for testing was collected from the left bank of Honggou at the leading edge of the Gaojiawan landslide, located in Ledu District, Haidong City, Qinghai Province (see Figure 1c). The mudstone was characterized as Neogene red-bed mudstone and then cut into cylindrical standard samples (ϕ50 mm × h25 mm) from massive blocks, according to ISRM standards. All standard samples are intact and free of visible cracks. The mineral composition, as determined via X-ray diffraction (XRD), includes illite, calcite, quartz, chlorite, potassium feldspar, plagioclase, analcime, and hematite. Based on the relative mass contents of clay minerals (illite and chlorite), the red-bed mudstone was categorized into two types: high - clay - mineral - content mudstone (HCM), referring to mudstone with a total illite and chlorite content greater than 40.0%; and low-clay-mineral-content mudstone (LCM), representing mudstone with a total illite and chlorite content less than 35.0%. This categorization was not adopted from some standard classification schemes or references. Instead, it was defined in this work to distinguish the two types of red-bed mudstone exhibiting rapid and slow disintegration process. The surface of the HCM samples is smooth (Figure 2a), whereas that of the LCM samples is rough with visible sand grains (Figure 2c). This distinction is evident by visual inspection and is further confirmed under a polarizing microscope (Figures 2b,d). The natural density is 2.10 g/cm3 ∼ 2.28 g/cm3, the dry density is 1.92 g/cm3–2.16 g/cm3. The water absorption cannot be reliably measured as sample disintegration upon water immersion. The porosity is about 19.1%–23.6%, which is calculated via pixel analysis of the binary image obtained by computed tomography recognition technology (Xie et al., 2026).

Mineral compositions of the red-bed mudstone. **(a)** Surface of HCM, **(b)** polarizing microscopy image of HCM, **(c)** surface of LCM, **(d)** polarizing microscopy image of LCM.
Figure 2. Mineral compositions of the red-bed mudstone. (a) Surface of HCM, (b) polarizing microscopy image of HCM, (c) surface of LCM, (d) polarizing microscopy image of LCM.

These standard HCM and LCM samples with varying initial moisture contents w (0.0%, 4.5%, 9.0%, 15.0%) were prepared for testing (see Figure 3). To avoid their disintegration during sample preparation, different methods were adopted for preparing dry, unsaturated and saturated samples. Dry samples (w = 0.0%) were prepared through oven-drying and surface polishing until a dry weight of approximately 95 ± 0.5 g was obtained. Unsaturated samples at low initial moisture content (w ≈ 4.5%) were prepared by the water-vapor humidification method (Xie et al., 2024). Specifically, the dry samples were placed in a sealed container to absorb water vapor. The target initial moisture content of 4.5% was reached by periodically monitoring sample mass using an electronic balance. Then, the samples were wrapped in plastic film and kept in a humidity cell to maintain a uniform moisture distribution within the samples (over 48 h). Unsaturated samples at medium initial moisture content (w ≈ 9.0%) were prepared by the water-absorption method (Xie et al., 2024). Specifically, the dried samples were wrapped in gauze and placed in the saturator. Their mass during water absorption were monitored in real time. Once the target mass was attained, the samples were sealed in a bag to ensure homogeneous moisture distribution. To exactly control the initial moisture content, their mass was measured again after 48 h. If the measured mass is lower than the target mass, a predetermined amount of water was added using a dropper; if measured weight exceeded the target mass, the samples were air-dried until the target mass was reached. Saturated samples were produced through vacuum saturation in a saturation apparatus (Xie et al., 2026). That is, the samples immersed in water was set in a vacuum environment of −0.1 MPa. Their degree of saturation was not exactly calculated as water on the sample surface obviously influences the measured mass.

Samples preparation.
Figure 3. Samples preparation.

Thus, this process lasted more than 30 days to achieve a high degree of saturation, as shown in Figure 3. This time was adequate to achieve full saturation, and no mass variation was observed with extended time. Finally, the weight was measured, and the saturated water content was calculated.

3 Methodology

Based on previous apparatuses (Wang Q. et al., 2024; Huo et al., 2023; Zhang et al., 2020), a disintegration apparatus that can control the water pressure and temperature was designed, fabricated, and tested [a Chinese patent (CN202211635270.0)], as shown in Figure 4. It can be applied to study the disintegration characteristic of red-bed mudstone in complex temperature and hydrostatic-pressure conditions.

Test apparatus and test procedure: **(a)** disintegration testing apparatus, **(b)** schematic illustration of the apparatus, **(c)** M d f measurement (M d is the mass-sensor reading before the basket is immersed in water, and M d 1 is the mass-sensor reading after the metal mesh basket is fully immersed in water), M c is the mass sensor reading after the sample is fully immersed in water, and M t is the mass sensor reading at time *t*.
Figure 4. Test apparatus and test procedure: (a) disintegration testing apparatus, (b) schematic illustration of the apparatus, (c) M d f measurement (M d is the mass-sensor reading before the basket is immersed in water, and M d 1 is the mass-sensor reading after the metal mesh basket is fully immersed in water), M c is the mass sensor reading after the sample is fully immersed in water, and M t is the mass sensor reading at time t.

3.1 Disintegration apparatus

The disintegration apparatus comprises four systems, as shown in Figure 4a.

  • The sample submergence system includes a support frame ⑨, a lifting shaft, and a metal mesh basket ⑥ that is designed for holding samples.
  • The pressure-controlled system includes a pressure cell ⑧, an air compressor, an air pressure regulator ⑬, and a pressure sensor ⑭. The water pressure can be controlled through the air compressor with the air pressure regulator ⑬, which was used to adjust and stabilize the target pressure. The pressure can be measured via the pressure sensor ⑭.
  • The temperature-controlled system includes a temperature-controlled bath with a control accuracy of 0.1 °C ①, copper tubes ④, and inlet ② and outlet pipes ③. Water in the temperature - controlled bath is pumped through the inlet pipes into the copper tubes within the pressure cell to ensure precise temperature.
  • The data measuring logging system includes a temperature sensor with resolution of 0.1 °C ⑤, mass sensor with resolution of 0.01 g ⑦, and data logger with sampling frequency of 1 Hz.

The quantitative validation of the apparatus was analyzed in previous studies on the disintegration of different kinds of red-bed mudstone (Xie, 2025; Liu, 2026; Kang et al., 2026). The water pressure can be controlled to within ±5.0 kPa by means of the air compressor, since the water pressure inside the cell is imposed via air pressure, this pressure may be disturbed by the disintegration of unsaturated samples. Once the water pressure falls outside this range, the air-pressure regulator should be adjusted back to the target value. The water temperature in the pressure cell can be controlled to within ±3.0 °C via water-bath conditioning. It was controlled by the temperature-controlled bath via copper tubes. The control accuracy of the temperature is influenced by the steel pressure cell and the laboratory temperature. Generally, the water temperature was adjusted to near the target value and maintained for 1 h before the following disintegration tests.

3.2 Test method

Before testing, the apparatus should be calibrated. The mass sensor was calibrated using standard weights as samples. The temperature sensor was calibrated against a standard thermometer. The built-in water pressure sensor was calibrated through air pressure loading supplied by an air compressor. The readings from the built-in sensors were compared with those of the reference calibration sensors to verify proper sensor function. Especially, the airtight performance of the apparatus should be confirmed.

The details of the test method are illustrated in Figures 4b,c.

  • The mass sensor ⑦ is calibrated to zero before testing.
  • A target temperature is established using the temperature-controlled system ①∼④.
  • Once the water temperature in the pressure cell reaches the target value (about 30 min), a standard sample is placed in the metal mesh basket ⑥, and the top bolts are securely tightened.
  • A target pressure is established in 5 min via the pressure-controlled system. The water pressure was generated by compressed air from the air compressor. The water pressure was controlled and measured by the air pressure regulator ⑬ and the pressure sensor ⑭, respectively. They were used to adjust and stabilize the target water pressure. The pressure was stabilized until the ultimate state during disintegration.
  • The sample is immersed in water in 10 s via the sample submergence system ⑥⑨. In initial condition, the water lever above the top surface of the sample is about 2 cm, to minimize the hydrostatic depth effect. This was achieved by the lifting shaft with applies the target vertical displacement.
  • Before disintegration, the initial value M c is recorded once the sample was fully immersed in water (see Figure 4d).
  • During disintegration, the mass sensor M t is instantaneously recorded through the mass sensor ⑦ of the metal mesh basket ⑥ holding the samples (see Figure 4d).
  • Final disintegration: test is terminated upon mass sensor M t for a completely disintegrated sample, and terminated after 2 hours for an incompletely disintegrated sample.
  • Photographs are obtained to document the features of both the residue and the sediment.

3.3 Analysis method

The disintegration behavior was quantitatively evaluated via three indices: the instantaneous mass-loss ratio (m i), final mass-loss ratio (m f), and instantaneous mass - loss rate (m ˙). Following the calculation methods provided in the Standard for Geotechnical Testing Method GB/T 50123–2019, the instantaneous mass-loss ratio m i is calculated by m i = 1 − M t − M d + M d f / M c − M d + M d f. The final mass-loss ratio m f is the value of m i after disintegration. The instantaneous mass-loss rate m ˙ is the tangent slope of the mass-loss curve (m i ∼ t) at the point where when m i equals 0.5 m f, which is an intermediate disintegration state during the entire disintegration process. Moreover, M d f equals M d − M d 1, where M d is the mass-sensor reading before the mental mesh basket is immersed in water, and M d 1 is the mass-sensor reading after the metal mesh basket is fully immersed in water (Figure 4c). M c is the mass-sensor reading after the sample is fully immersed in water, and M t is the mass-sensor reading at time t.

To investigate the disintegration behaviors influenced by the multi-factor, such as clay mineral content, initial moisture content, water pressure, and temperature of red-bed mudstone in the upper reaches of the Yellow River region, three temperature gradients were selected in the disintegration tests (15 °C, 25 °C, and 35 °C) to reflect the monthly temperature distribution in the Ledu District, Haidong City, Qinghai Province, from 2021 to 2023 (see Figure 5a). The water pressure was designed to reflect the pressures at different depths, and three levels of water pressures are established (see Figure 5b): 100 kPa (10 m), 200 kPa (20 m) and 300 kPa (30 m). The three levels of water pressure were generated by compressed air from the air compressor. The initial moisture contents were set at 0.0% (dry), 4.5%, 9.0%, and 15% (saturation). Under unsaturated conditions, 4.5% represents the mean values of natural moisture content (3.71%–5.47%), and 9.0% is closing to the maximum values of moisture content attained by spontaneous imbibition. Under saturated conditions, each of the measured initial moisture content are different (13.98%–16.08%). Their mean value is about 15.0%, which was consider as a design value and selected as the represent value for saturated condition in the following analysis (Table 1). For each test control condition, three tests were conducted. Seventy-two sets of intermediate datasets were selected as representative results, as shown in Table 1. This selection does not alter the following qualitative conclusions, since the disintegration trend is evident in the following figures. More repeated-test data can be found in the literatures (Xie, 2025; Liu, 2026; Kang et al., 2026). Quantitative conclusions need further improving of accuracy of the disintegration apparatuses, as the natural variations in fabric and mineral composition among intact samples.

Determination of *w*-T-P conditions **(a)** temperature T **(b)** initial moisture content *w* and water pressure P.
Figure 5. Determination of w-T-P conditions (a) temperature T (b) initial moisture content w and water pressure P.

TABLE 1. Test control conditions and results of red-bed mudstone.

Samples IDInitial moisture content wPressure P/kPaTemperature T/°Cm f/%m ˙/% · min−1
HCMLCMHCMLCMHCMLCM
HCM-1–100–15LCM-1–100–150.0% (dry)1001579.7720.341.540.74
HCM-1–100–25LCM-1–100–250.0% (dry)1002593.2314.251.550.98
HCM-1–100–35LCM-1–100–350.0% (dry)1003596.004.143.440.17
HCM-1–200–15LCM-1–200–150.0% (dry)20015100.0053.352.930.75
HCM-1–200–25LCM-1–200–250.0% (dry)20025100.0046.001.940.87
HCM-1–200–35LCM-1–200–350.0% (dry)20035100.0028.814.060.55
HCM-1–300–15LCM-1–300–150.0% (dry)3001599.0720.342.581.13
HCM-1–300–25LCM-1–300–250.0% (dry)3002595.1017.132.230.85
HCM-1–300–35LCM-1–300–350.0% (dry)3003598.6715.253.080.46
HCM-2–100–15LCM-2–100–154.5% (unsaturated)1001598.609.447.200.25
HCM-2–100–25LCM-2–100–254.5% (unsaturated)1002589.452.794.050.07
HCM-2–100–35LCM-2–100–354.5% (unsaturated)1003592.4311.184.490.60
HCM-2–200–15LCM-2–200–154.5% (unsaturated)20015100.0027.442.931.04
HCM-2–200–25LCM-2–200–254.5% (unsaturated)2002592.9212.416.660.18
HCM-2–200–35LCM-2–200–354.5% (unsaturated)2003593.0719.066.16−0.06
HCM-2–300–15LCM-2–300–154.5% (unsaturated)3001591.5328.675.620.71
HCM-2–300–25LCM-2–300–254.5% (unsaturated)3002599.782.874.754.95
HCM-2–300–35LCM-2–300–354.5% (unsaturated)30035100.000.349.40−0.21
HCM-3–100–15LCM-3–100–159.0% (unsaturated)100152.349.890.050.06
HCM-3–100–25LCM-3–100–259.0% (unsaturated)1002521.5917.471.390.83
HCM-3–100–35LCM-3–100–359.0% (unsaturated)1003519.7422.990.210.85
HCM-3–200–15LCM-3–200–159.0% (unsaturated)2001526.9418.961.290.86
HCM-3–200–25LCM-3–200–259.0% (unsaturated)2002551.8823.578.650.66
HCM-3–200–35LCM-3–200–359.0% (unsaturated)2003540.513.462.80−0.08
HCM-3–300–15LCM-3–300–159.0% (unsaturated)3001533.3010.501.040.30
HCM-3–300–25LCM-3–300–259.0% (unsaturated)3002540.4810.073.264.30
HCM-3–300–35LCM-3–300–359.0% (unsaturated)3003526.283.300.48−0.20
HCM-4–100–15LCM-4–100–1515.0% (saturation)100156.025.880.090.03
HCM-4–100–25LCM-4–100–2515.0% (saturation)100254.565.820.170.06
HCM-4–100–35LCM-4–100–3515.0% (saturation)100350.480.82−0.100.11
HCM-4–200–15LCM-4–200–1515.0% (saturation)200158.138.370.110.11
HCM-4–200–25LCM-4–200–2515.0% (saturation)2002510.902.130.210.18
HCM-4–200–35LCM-4–200–3515.0% (saturation)200355.861.800.18−0.01
HCM-4–300–15LCM-4–300–1515.0% (saturation)300156.992.300.13012
HCM-4–300–25LCM-4–300–2515.0% (saturation)300253.901.790.540.10
HCM-4–300–35LCM-4–300–3515.0% (saturation)300357.812.280.250.45

The sample number is defined as a-b-c-d, where a denotes HCM, or LCM, and b denotes 1, 2, 3, or 4 corresponding to initial moisture contents of 0.0%, 4.5%, 9.0%, or 15.0%, respectively. Moreover, c denotes the water pressure in the pressure cell, with a value of 100, 200, or 300 kPa, and d denotes the water temperature in the pressure cell, with a value of 15 °C, 25 °C, or 35 °C. the negative values are attributed to the that water absorption exceeds the mass loss of solid particles, indicating a weak disintegration process. Besides, the representative tests were listed in Table 1, and the results of the other test groups was not listed because they exhibit no obvious regularities. So, the mean values and standard deviations of all tests were not calculated.

4 Results

4.1 Disintegration features

Figure 6 shows the disintegration features of the red-bed mudstone affected by the clay mineral content, initial moisture content, water pressure and temperature. As shown in Figure 6a, the unsaturated HCM samples with initial moisture contents of 0.0% and 4.5% strongly disintegrated with a little residue material. The HCM samples with an initial moisture content of 9.0% partially disintegrated with fragmented residue material. The HCM samples under saturated conditions yielded approximately intact residue.

The disintegration features of **(a)** HCM samples and **(b)** LCM samples.
Figure 6. The disintegration features of (a) HCM samples and (b) LCM samples.

As shown in Figure 6b, the LCM samples with an initial moisture content of 9.0% and 15.0% exhibits relatively intact residue. While, at an initial moisture content of 0.0% and 4.5%, the unsaturated LCM samples exhibits notably fragmented residue that were small and dispersed. This indicates that the disintegration process of LCM samples was more intense at lower initial moisture contents, which is not as obvious as HCM samples. The LCM samples yielded more residue under a constant water pressure and temperature and yielded sediment with a reduced radial distribution range and an increased fragment size. This finding indicates that unsaturated HCM samples and LCM samples exhibits strong and weak disintegration, respectively. Saturated HCM and LCM all exhibits weak disintegration. Their Disintegration features are influenced by the clay mineral content, initial moisture content, water pressure and temperature.

4.2 Instantaneous mass-loss ratio m i

Figures 7–9 shows the instantaneous mass-loss ratio m i of red-bed mudstone. As shown in Figure 7, the initial moisture content w affects the instantaneous mass-loss ratio m i of the red-bed mudstone. As shown in Figures 7a–c, for the HCM samples with lower initial moisture contents about 0.0% and 4.5%, m i rapidly increases to a steady value near 1.0. For the HCM samples with higher initial moisture contents about 9.0% and 15.0%, m i slowly increase to a steady value below 0.5 within 2 hours. As shown in Figures 7d–f, for the LCM samples with different initial moisture contents, m i slowly increases to a value below 0.5. This indicates that a low initial moisture content w and a high clay mineral content accelerate the disintegration process of red-bed mudstone.

Effect of initial moisture content on the instantaneous mass-loss ratio m i of red-bed mudstone. HCM samples under water pressure of **(a)** 100 kPa, **(b)** 200 kPa, **(c)** 300 kPa; LCM samples under water pressure of **(d)** 100 kPa, **(e)** 200 kPa, **(f)** 300 kPa.
Figure 7. Effect of initial moisture content on the instantaneous mass-loss ratio m i of red-bed mudstone. HCM samples under water pressure of (a) 100 kPa, (b) 200 kPa, (c) 300 kPa; LCM samples under water pressure of (d) 100 kPa, (e) 200 kPa, (f) 300 kPa.
Effect of water pressure on the instantaneous mass-loss ratio m i of red-bed mudstone: HCM samples with initial moisture content of **(a)** 0.0%, **(b)** 4.5%, **(c)** 9.0%, **(d)** 15.0%; LCM samples with initial moisture content of **(e)** 0.0%, **(f)** 4.5%, **(g)** 9.0%, **(h)** 15.0%.
Figure 8. Effect of water pressure on the instantaneous mass-loss ratio m i of red-bed mudstone: HCM samples with initial moisture content of (a) 0.0%, (b) 4.5%, (c) 9.0%, (d) 15.0%; LCM samples with initial moisture content of (e) 0.0%, (f) 4.5%, (g) 9.0%, (h) 15.0%.
Effect of water temperature on the instantaneous mass-loss ratio m i of red-bed mudstone. HCM samples with initial moisture content of **(a)** 0.0%, **(b)** 4.5%, **(c)** 9.0%, **(d)** 15.0%; LCM samples with initial moisture content of **(e)** 0.0%, **(f)** 4.5%, **(g)** 9.0%, **(h)** 15.0%.
Figure 9. Effect of water temperature on the instantaneous mass-loss ratio m i of red-bed mudstone. HCM samples with initial moisture content of (a) 0.0%, (b) 4.5%, (c) 9.0%, (d) 15.0%; LCM samples with initial moisture content of (e) 0.0%, (f) 4.5%, (g) 9.0%, (h) 15.0%.

As shown in Figure 8, water pressure P affects the instantaneous mass-loss ratio m i of red-bed mudstone. The values of m i of unsaturated HCM and LCM samples increases faster at pressures of 200 and 300 kPa than those at a pressure of 100 kPa. Notably, the values of m i does not linearly increase with increasing water pressure, that is, it initially increases but then decreases with increasing water pressure. For example, the value of m i at a pressure of 200 kPa was greater than pressure of 300 kPa, as shown in Figure 8. This indicates that water pressure plays a dual role in mudstone disintegration process, capable of both intensifying and inhibiting this process.

As shown in Figure 9, the water temperature influences the instantaneous mass-loss ratio m i of red-bed mudstone. It can be found that higher water temperatures led to shorter disintegration process. Especially, the effects of water temperature on the disintegration process is limited if initial moisture content is about 9.0% or 15.0%, according to the evidence of slight differences between the instantaneous mass-loss ratios shown in Figure 9. The results indicate that the influence of temperature on disintegration process is related to the initial moisture content.

4.3 Final mass-loss ratio m f

The final mass-loss ratio m f of red-bed mudstone was shown in Figure 10. The values of m f of the HCM and LCM samples were primarily influenced by the initial moisture content and clay mineral content, whereas water pressure and temperature exerted relatively weaker influence (see Figure 10). The HCM samples at lower initial moisture contents (about 0.0% and 4.5%) almost disintegrate completely in 1 hour, and the final mass-loss ratio m f reached nearly 1.0. The HCM samples with larger initial moisture contents (about 9.0% and 15.0%) does not disintegrate completely after 2 hours, and the final mass-loss ratio m f was generally below 0.5. While, the LCM samples in various initial moisture contents experienced incomplete disintegration. The differences between the HCM and LCM samples was greater at an initial moisture content lower than 9.0%, over which the differences gradually disappeared. According to the values of m f, the HCM and LCM samples can be classified into two types: complete disintegration (m f ≈ 1.0) and incomplete disintegration (m f <1.0). This indicates that the disintegration ultimate state of the red-bed mudstone is primarily affected by the initial moisture content and clay mineral content, whereas water pressure and temperature exert relatively minor effects.

Final mass-loss ratio m f of red-bed mudstone **(a)** *w-*P **(b)** *w*-T.
Figure 10. Final mass-loss ratio m f of red-bed mudstone (a) w-P (b) w-T.

4.4 Instantaneous mass-loss rate m ˙

The instantaneous mass-loss rate m ˙ of red-bed mudstone is shown in Figure 11. The values of m ˙ is influenced by the clay mineral content, initial moisture content, water pressure, and temperature. For the saturated HCM and LCM samples, the clay mineral content, water pressure, and temperature exerts little effect on the values of m ˙. For the unsaturated HCM and LCM samples, the values of m ˙ increases with increasing water pressure and temperature. That is, the values of m ˙ of the HCM samples initially peaks at the moisture content 4.5%, which is the moisture level producing the maximum disintegration response. It corresponds to the residual suction, according to the characteristics of soil-water retention curve (Su et al., 2026). In contrast, the values of m ˙ of the LCM samples consistently decreases with increasing initial moisture content. The results indicate that the initial moisture content and clay mineral content markedly affects the disintegration process, which is accelerated by higher water pressure and temperature.

Instantaneous mass-loss rate m ˙ of red-bed mudstone **(a)** *w*-P **(b)** *w*-T.
Figure 11. Instantaneous mass-loss rate m ˙ of red-bed mudstone (a) w-P (b) w-T.

5 Discussion

5.1 Effect of clay mineral content

As shown in Figure 12, the clay mineral content is an intrinsic factor in the disintegration process of red-bed mudstone. In the HCM samples, clay minerals, including illite and chlorite, account for over 40.0% of the overall mineral composition. In the LCM samples, clay minerals account for less than 35.0% of the total mineral composition (Figure 12). Notably, illite has obvious hydration–dispersion properties (Huang SY. et al., 2022), which leads to a volume increase according to the chemical reaction of clay minerals (Zhu, 1996). Because of this, the instantaneous mass - loss rate of the HCM samples is greater than that of the LCM samples, and the clay mineral content is the intrinsic factor of red-bed mudstone disintegration.

Mineral compositions of red-bed mudstone: **(a)** spectra and **(b)** percentages of minerals.
Figure 12. Mineral compositions of red-bed mudstone: (a) spectra and (b) percentages of minerals.

5.2 Effect of initial moisture content

For unsaturated red-bed mudstone, the instantaneous mass-loss rate is high. On one hand, the presence of microcracks and micropores provides new pathways for water molecules to penetrate with increasing initial moisture contents. In turn, the mineral particles expand and gradually produce microcracks and micropores, and the mineral particles of unsaturated mudstone exhibit point ‒ point contacts (Figure 13) (Jiang et al., 2014). On the other hand, the effect of initial moisture content is also related to clay minerals, such as illite and chlorite. Water molecules penetrate the interlayer pores and react with interlayer cations to form layers of polarized water molecules, which leads to a gradual increase in interlayer spacing (Gong et al., 2024). Water molecules also enter the interlayers of clay mineral crystals and form internal interlayer water films (Ding et al., 2023). The water films induce the expansion of clay mineral, which generates uneven stresses that act on interparticle water films. The stress facilitates fracture development in mudstone (Zhang et al., 2010). While, uneven stresses in saturated red-bed mudstone were released after saturation (Ren et al., 2023), and the instantaneous mass-loss rate of saturated mudstone is lower than that of unsaturated mudstone.

Micro-structure of mudstone under different initial moisture contents.
Figure 13. Micro-structure of mudstone under different initial moisture contents.

5.3 Effect of water pressure and temperature

In addition to the influence of clay mineral content and initial moisture content, water temperature also affects the mass-loss of mudstone. Higher temperatures increase the activity of water molecules within clay minerals and increase the diffusion coefficients of water molecules and cations in the interlayers of illite (Ren et al., 2023; Lu et al., 2017). Water molecules more easily penetrate micropores and microfractures, thereby increasing the mass-loss of unsaturated mudstone. Water pressure plays a dual role in mudstone disintegration, capable of both intensifying and inhibiting this process. That is, the pore-water pressure of mudstone increases from the surface to the interior of the sample as water pressure rises, which intensifies disintegration (Ding and Tang, 2024); Mineral particles at the surface are restricted by the hydrostatic pressure, which inhibits the disintegration process. Water pressure and temperature have a weaker effect on ultimate state comparing with those of clay-mineral content and initial moisture content, but alter the disintegration process.

5.4 Disintegration types

According to the above analysis on the instantaneous mass-loss ratio, final mass-loss ratio, instantaneous mass-loss rate, and disintegration features of red-bed mudstone, the disintegration behaviors of red-bed mudstone can be categorized three types, as shown in Figure 14.

Disintegration types of red-bed mudstone.
Figure 14. Disintegration types of red-bed mudstone.

Type I: Progressive erosion. The unsaturated red-bed mudstone experiences a considerable progressive erosion with mineral-particle detachment, which is accompanied by strong water‒air exchange. For this type, the water–air exchange gradually proceeds until complete disintegration, and the final mass-loss ratio m f rapidly increases to a steady value near 1.0 within 1 hour. This can be evidenced by the HCM samples at lower initial moisture contents (about 0.0% and 4.5%), according to Figures 6–10.

Type II: Partial fragmentation. The unsaturated red-bed mudstone slowly disintegrated along the microcracks, generating blocky fragments. This process is controlled by the initiation and propagation of microcracks (see Figure 6). The value of m i slowly increases at the disintegration stage, and the final mass-loss ratio m f approaches a value below 0.5 after 2 hours (see Figure 8). This can be evidenced by the HCM samples at larger initial moisture content (about 9.0%) and the LCM samples at initial moisture contents of 0.0%, 4.5% and 9.0%, according to Figures 6–10.

Type III: Surficial spalling. The saturated red-bed mudstone exhibits weak disintegration merely on the rock surface, generating flaky fragments. The instant mass-loss ratio m i slowly increases, and the final mass-loss ratio m f reaches approximately 0.2, which is caused by the slight spalling on the surface of the mudstone samples. This can be evidenced by the HCM and the LCM samples both at an initial moisture content of 15%.

The above three types also exhibit mutual transitioning influenced mainly by the clay mineral content and initial moisture content. As the initial moisture content increases and the clay mineral content decreases, the disintegration type progressively transitions from progressive erosion (Type I) to partial fragmentation (Type II) and surficial spalling (Type III). That is, these types can mutually transform as affected by clay mineral content and initial moisture content.

5.5 Disintegration mechanism

According to the previous results, the clay mineral content, initial moisture content, water pressure and temperature influence the disintegration characteristics of unsaturated red-bed mudstone, which is a kind of solid-water-air three phase medium. The disintegration mechanism was further explored based on the hydro-mechanical coupling process (Fredlund and Rahardjo, 1993; Lu and Likos, 2004; Chen, 2023). The simplified equilibrium equations for solid particle along the x- and y-directions can be derived, as shown in Figure 15. Based on the equations, the disintegration mechanism of the three types of disintegration was analyzed, i.e., progressive erosion (Type I), partial fragmentation (Type II), and surficial spalling (Type III) (Figure 14). For Type I and II, the forces correspond to trajectory a-b-c. For Type III, the forces correspond to trajectory d-e-f, according to Figure 15. During the disintegration process, the contact forces F A and F B gradually increase due to the swelling of clay minerals, and the matric suction u a − u w decreases with increasing r 1 and r 2. Additionally, the weight of the particles (G) increases, and the force F p 0 x decreases. Moreover, the cementation forces f x c and f y c decrease during disintegration because water immersion accelerates the hydration and diffusion of clay minerals on mudstone surfaces (Cui et al., 2024) and induces dissolution of non-expansive clay and soluble materials at the edges of pores (Guan et al., 2024). The variation in these forces changes the values of F x (red solid line a-b-c) and F y (red dashed line a'-b'-c') during the disintegration process.

Schematic illustration of the force acting on water-immersed particles: **(a)** Ideal geometric model of the gas-water interface among particles and **(b)** Forces acting on a single particle. **(c)** Force and state evolution of red-bed mudstone particles during water immersing. F A denotes the contact force between particles C and A; F B denotes the contact force between particles C and B. f x c and f y c denote the cementation forces along the *x*- and *y*-directions, respectively; F p 0 x and F p 0 y denote the forces induced by water pressure along the *x*- and *y*-directions, respectively; *G* is the weight of particle C; θ is the angle between F A and the *x*-direction; and γ is the angle between F B and the *x*-direction. u a and u w are the pore air pressure and pore water pressure, respectively. r 1 and r 2 are the principal radius of curvature.
Figure 15. Schematic illustration of the force acting on water-immersed particles: (a) Ideal geometric model of the gas-water interface among particles and (b) Forces acting on a single particle. (c) Force and state evolution of red-bed mudstone particles during water immersing. F A denotes the contact force between particles C and A; F B denotes the contact force between particles C and B. f x c and f y c denote the cementation forces along the x- and y-directions, respectively; F p 0 x and F p 0 y denote the forces induced by water pressure along the x- and y-directions, respectively; G is the weight of particle C; θ is the angle between F A and the x-direction; and γ is the angle between F B and the x-direction. u a and u w are the pore air pressure and pore water pressure, respectively. r 1 and r 2 are the principal radius of curvature.

Higher clay mineral content promotes the disintegration of red-bed mudstone owing to the hydrophilicity of clay minerals. Water immersion induces the swelling of clay minerals, as well as softening and dissolution of inter-particle cementation (Fu et al., 2022; Tang et al., 2024). The swelling of clay minerals influenced by multi-factor generate shear and tensile stresses (Wang et al., 2022; Wang Q. et al., 2024). Once the shear stress or tensile stress exceed the inter-particle shear strength or tensile strength, respectively, internal fractures propagation occurs (Hu et al., 2019). Thus, the contact forces F A and F B in HCM decrease more than those in LCM during disintegration (Figure 13), and the inter-particle cementation forces f x c and f y c decline accordingly. This process is more intense for red-bed mudstone with higher clay mineral content (Figures 6–9), due to the pronounced swelling of clay minerals in HCM.

Lower initial moisture content promotes the disintegration of red-bed mudstone owing to intense water-pore air exchange and hydro-mechanical coupling processes. At a lower initial moisture content, the red - bed mudstone is a solid-water-air three phase medium. Once immersed in water, intense water-pore air exchange occurs and induce a complex hydro-mechanical coupling processes (Fu et al., 2023). Red-bed mudstone with a lower initial moisture content exhibits greater matric suction u a − u w, and the attractive force of clay surfaces to water molecules is greater (Wei et al., 2024). Meanwhile, microcracks and micropores propagate and provide more pathways for water penetration (Fu et al., 2023), a large number of air bubbles are released from samples, as observed during the disintegration process. This accelerates the increase in particle weight G and the decrease in the forces F u a − u w induced by matric suction, and the contact forces F A and F B progressively decrease to an equilibrium in the unsaturated state (Ren et al., 2023). Thus, the particle forces in unsaturated samples change continuously during the hydro - mechanical coupling processes (red lines a - b - c and a' - b' - c' in Figure 15), whereas particle forces in saturated samples vary slightly due to the weaker hydro-mechanical coupling effect (blue lines d - e - f and d' - e' - f' in Figure 15).

Water pressure and temperature influence the disintegration process of red-bed mudstone. Temperature accelerates the expansion of clay minerals, which induces microcracks (Liu et al., 2020), which promote water infiltration that induces a decrease in forces F p 0 x and an increase in G. Moreover, the swelling force and water absorption rate of mudstone increase with increasing temperature (Meng et al., 2021), which induces increases in F A and F B and decreases in f x c and f y c. Furthermore, the pore water pressure increases with the increasing of boundary water pressure, which induces increases in G, F A, and F B, and decreases in f x c and f y c. Thus, water pressure and temperature influence the process of disintegration, but have a weaker effect on the ultimate state of disintegration.

The disintegration mechanism explored above was obtained under constant boundary conditions. However, the actual boundary condition in the field varied. Such as, for the red - bed mudstone in reservoir drawdown zones, its initial moisture content varies with water level fluctuations. That is, it experiences numerous cyclic drying - wetting, which changes the mineral composition and microstructure (Fu et al., 2023). Thus, its soil-water retention curve, contact angle, and pore size distribution will be altered, as evidenced by Su et al. (2026). Accordingly, cyclic drying - wetting may further trigger complex hydro-mechanical coupling processes that influence the disintegration behaviors of red-bed mudstone.

6 Conclusion

A self-developed disintegration testing apparatus was designed and fabricated. It comprises four systems including a sample submergence system, a pressure-controlled system, a temperature-controlled system, and a data measuring–logging system. A series of laboratory tests was conducted, thereby controlling the clay mineral content, initial moisture content, water pressure, and temperature.

Clay mineral content and initial moisture content markedly affects the disintegration process and its ultimate state. With higher clay mineral content, the unsaturated HCM samples disintegrate faster than the unsaturated LCM samples. This is primarily due to the swelling of clay minerals and the dissolution of inter-particle cementation.

With increasing initial moisture content, m ˙ and m f of unsaturated HCM initially increase to a peak value and then decrease with increasing initial moisture content. In contrast, those of unsaturated LCM decrease monotonically with increasing initial moisture content. This is mainly due to the intense water-pore air exchange and hydro-mechanical coupling processes in HCM.

Water pressure and temperature influences the disintegration process, but have a weaker effect on ultimate state comparing with clay - mineral content and initial moisture content. In unsaturated conditions, the water pressure and temperature impose a limited effect on final mass-loss ratio m f but markedly affect instantaneous mass - loss rate m ˙. In saturated conditions, this influence may disappear. Furthermore, the low clay mineral content and higher initial moisture content weaken the effects of water pressure and temperature on the disintegration of red-bed mudstone.

According the above disintegration characteristics, the disintegration of red-bed mudstone can be categorized into three types: progressive erosion, partial fragmentation, and surficial spalling. These three types are driven by different hydro-mechanical process among mineral particles. The three types can mutually transform as affected by clay mineral content and initial moisture content. The results provide insights into the interaction between reservoir construction and geological environment in upper reaches of the Yellow River.

It should be noted that the results were obtained by single-immersion tests, and did not consider cyclic drying-wetting, repeated pressurization and depressurization, or drainage. Future research will focus on the disintegration characteristics of red-bed mudstone under cyclic drying-wetting and cyclic pressurization-depressurization conditions, to further explore the long-term behavior of red-bed mudstone influenced by repeated reservoir water-level fluctuations.

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Footnotes

Edited by: Hui Lu, Ningbo University, China

Reviewed by: Yu Su, Nanchang University, China

Anrun Li, Chengdu University of Technology, China

Statements

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Funding

The author(s) declared that financial support was received for this work and/or its publication. The National Natural Science Foundation of China (Grant No. 42102317), Key Project of Natural Science, City College of Xi’an Jiaotong University (No. 2023PY02), and the Fundamental Research Funds for the Central Universities, CHD (No. 300102264201). This study was also supported by The Youth Innovation Team of Shaanxi Universities—Green Ecology-Empowered Urban Disaster Resilience Innovation Team.

Data availability

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding authors.

Author contributions

QD: Data curation, Formal Analysis, Investigation, Methodology, Writing – original draft, Writing – review and editing. XL: Investigation, Methodology, Writing – original draft. XK: Funding acquisition, Project administration, Supervision, Writing – review and editing. DZ: Formal Analysis, Writing – review and editing.

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Citation

Dai, Q., et al. (2026). Disintegration mechanism and characteristics of water-sensitive red-bed mudstone affected by multi-factor via a self-developed test apparatus. Frontiers in Earth Science, Volume 14. https://doi.org/
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14 - 2026

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Copyright © 2026 Dai, Liu, Kang and Zhang.

License: https://creativecommons.org/licenses/by/4.0/

*Correspondence: Xiaosen Kang

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