Water Softening Mechanism and Strength Model for Saturated Carbonaceous Mudstone in Panzhihua Airport, China

To identify the water softening mechanisms that caused landslides in Panzhihua Airport, China, property and saturation tests of the mudstones extracted from a representative landslide were proposed. In this paper, water saturation tests were carried out on samples of carbonaceous mudstone collected from the east side of the No. 12 landslide at the airport. A number of diﬀerent analytical techniques and mechanical tests were used to determine changes in chemical composition, mineral assemblages, and mudstone structural characteristics, including shear strength, after the mudstone had been softened. Three kinds of changes caused by water and three mudstone softening stages are proposed. The results show that the water has a signiﬁcant inﬂuence on the properties of the mudstone, so the stability of the mudstone in the watery period is a big threat to the upper structure. A model for water immersion mudstone strength softening is developed. The model incorporates a permeability coeﬃcient, the hydraulic gradient, and time; the model can be used to determine the mudstone’s shear strength and internal friction angle. This study provides a reference for the study of rock softened by water immersion.


Introduction
Mudstone, a soft rock, is widely distributed in southwestern China. Many projects such as dams, airports, and highways are built on mudstone bedrock. e deterioration of soft rock has been and is a long-standing problem for engineering projects all over the world. In the presence of water, many mudstones soften, disintegrate, and swell or shrink and this can lead to many kinds if engineering problems including landslides [1][2][3][4][5][6].
To solve some of the problems related to water-softened mudstones, researchers have carried out programs that have mainly focused on three aspects: the mudstone's mechanical properties including its strength, mudstones disintegration, and mudstone chemistry. e first aspect concerns mechanical properties [7][8][9][10][11]. e most obvious effect of water softening on mudstone is that the rock's strength decreases. e mechanical indices compressive strength and shear strength are the main parameters that determine rock strength. After a series of experimental studies, it has been shown that an increase in microporous cracks in the mudstone after the mudstone comes into contact with water leads to a sharp decline in its strength. is indicates that water softening is related to the shape and size of the mudstone's pores [12]. e second aspect is mudstone disintegration [13,14]. Some rocks contain expansive clay minerals, and the clay cement will disintegrate when the rock becomes wet. When this occurs, the cement between the clastic particles is destroyed and many microfissures are produced. is destroys the internal structure of the rock and eventually leads to the rock's disintegration. Mudstone disintegration studies are mainly carried out by considering energy storage and transfer using statistics. For example, KINCAL [15] carried out a quantitative analysis of soft rock disintegration using the fractal dimension of particle sizes, a fractal dimension equation, and the fractal parameters to establish a model for soft rock disintegration. Miklusova [16] studied the surface area, volume, and particle sizes of debris from disintegrated soft rock, considered the changes in surface energy during disintegration, quantitatively characterized disintegration from the energy perspective, and established a model for energy transfer and dissipation for soft rock disintegration. e third aspect is chemical effects. After they become wet, the hydrophilic minerals in soft rocks exchange ions with and absorb water. Chemical reactions lead to changes in mineral compositions and the rock's structure, and these changes alter the mechanical properties of the rock [17][18][19].
ere are many factors that can affect the softening of mudstone including the rock's clay content [20][21][22][23], mineralogy [24], moisture content [25,27], fabric and texture [20,21,28,29], and porosity [30]. In previous studies, most of the research on mudstone softening by water focused on the analysis of specific factors, but these studies lacked any comprehensive analyses of multiple factors. Moreover, the mudstones from different areas were deposited in different sedimentary environments and had their own special compositions and structural characteristics. eir physical and mechanical properties, the water chemistry at their locations, and the specific water softening mechanism by which they were affected must have been different.
Mudstone is widely distributed in southwestern China where the climate is subtropical monsoonal and humid. From July to October every year, rainfall is very heavy and long periods of heavy rain saturate the ground. Over the past two decades, the global change in climate has led to frequent periods of both extreme rainfall and aridity [31,32].
In September 2009 landslides occurred at Panzhihua Airport, Sichuan Province, China. ese landslides were mainly due to the degradation of mudstone by water saturation; this caused slope instability. is airport is a typical branch airport in mountainous areas, which is the key direction of China's future airport planning and construction. e samples collected in this paper are typical soft rock structures in Southwest China. For this paper, the rock softening mechanisms were studied using a number of techniques including SEM imaging, X-ray fluorescence spectrometry (XRF), liquid ion chromatography, and mechanical property tests. ese tests determined the chemical compositions of the rock and water, the minerals present in the rock, any changes in mudstone surface morphology, and the rock's shear strength so that the saturated permeability softening of the mudstone could be investigated. is study can provide a basis for understanding the rock softening engineering problems in this region.

Basic Physical Properties.
e mudstone samples tested are from the east side of the #12 landslide at Panzhihua Airport. e samples are dark gray or gray-black colored and slippery. Strongly weathered sample is grayish-yellow. Generally, the samples are carbonaceous mudstones. Twelve sets of moisture content, density, and porosity determinations, six sets of uniaxial compression tests, and six sets of direct shear tests were carried out on the natural mudstone samples and on mudstone samples after different water saturation times. e results of the moisture content, density, and porosity tests are listed in Table 1 and Figure 1. e results show that the porosity varies from 3.7-5.8% with the change of water content from 1.5-2.62%. e average value of moisture content and porosity are 1.89% and 4.4%, respectively.

Chemical Properties and
Testing. X-ray fluorescence spectrometry was used to determine the chemical compositions of the rock samples. e results are listed in Table 2.
e results show that the major chemical components in the mudstone are SiO 2 , Al 2 O 3 , CO 2 (by difference), total Fe (as Fe 2 O 3 ), and CaO. e SiO 2 content is the largest, at about 60%. e Al 2 O 3 content is more than 16%, which indicates that aluminum cementation may play a major role in the mudstone. e CO 2 content accounts for about 11%, so the rock mudstone contains a carbonaceous component. Generally, mudstones with a CaO content of more than 10% have high strength and good weathering resistance but the CaO content in this mudstone is relatively low, at about 2%, so the CaO probably makes only an extremely limited contribution to the rock's strength.
Polarizing light microscopy was used to identify the minerals in the samples (Figures 2 and 3 ). In Figure 2, the intensity of the quartz among the non-clay mineral content is significantly more than the other components. e accurate results are listed in Table 3. e clay mineral content is between 75% and 85%, and the non-clay mineral content is between 15% and 25%. erefore, the mudstone's strength mainly is the result of cementation by clay minerals. Figure 3 shows that the grains that make up the mudstone are only a few microns in diameter. ese grains are regular in shape and essentially polygonal and flaky in structure. ey form a thin layer of clay in a continuous and nondirectional arrangement, and the grains are in contact face-to-face or edge-to-face. e mineral grains are cemented at their contacts, and this cemented connection is easily damaged by water or an external force. is means that mudstone samples collected from an outcrop are quite soft.

Mechanical Properties.
e samples of the mudstone have been processed the uniaxial compression and the direct shear tests in the laboratory, and the test details are shown in Figure 4. e experimental results from the uniaxial compression and the direct shear tests on the mudstone specimens are presented in Tables 4 and 5  e normal stress-shear strength curve for mudstone, σ-τ, is shown in Figure 5. e results show that the original strength of the mudstone is relatively high compared with the normal samples. erefore, it is essential to understand the reason of instability and the process of water softening.

Softening Tests
To study the softening and strength changes in the carbonaceous mudstones, the changes in microphysical, chemical, and macromechanical properties of the samples before and after water saturation were investigated. e testing scheme divided the samples into five groups. e groups were saturated with water for 5, 10, 20, 30, and 60 d periods, and the samples' microstructure, mineral composition, and the chemical compositions of the rocks and the immersion water were analyzed before and after saturation. Compression and direct shear tests were also carried out to analyze softening.
e chemical composition of the mudstone, the chemical composition of water, and the microstructure of the mudstone were analyzed to investigate the softening mechanisms.

Changes in Mudstone Chemical
Composition. X-ray fluorescence spectrometry was used to analyze the chemical compositions of the mudstones. For each analysis, a dried sample was ground to 200 mesh and lowpressure polyethylene powder was used to coat the bottom. A tablet press was used to prepare samples with an inner diameter of 32 mm and an outer diameter of 40 mm for XRF analysis. e test results and the changes in SiO 2 , Al 2 O 3 , the Si/Al ratio, and the active oxides (Na 2 O and K 2 O) after the 60 d immersion period are listed in Table 6.
Over the 60 d immersion period, the SiO 2 and Al 2 O 3 concentrations are very stable but, as can be seen from the analyses in Table 6, the Fe 2 O 3 decreases during the saturation period. is decrease in Fe 2 O 3 should be compared with the changes in ion abundances in the soaking solution. e content of CO 2 in the mudstone remains essentially unchanged but the potassium, sodium, calcium, and magnesium oxide contents all decrease. As shown in Table 6, the percentage by which K 2 O and Na 2 O decrease in the 60 d period is relatively large although the abundances of those two elements stabilize somewhat after about 30 d of water saturation. e silicon aluminum ratio is between 4.95 and 5.28 and does not change significantly over 60 d.

Changes in Soaking Solution Chemical Composition.
Inductively coupled plasma atomic emission spectroscopy and ion chromatography were used to determine the abundances of several ions in the mudstone soaking solutions used in the five immersion experiments. e changes in the solution's ion concentrations were analyzed. e raw soaking solution was ultrapure water, and it contained no detectable ions-the initial content of both anions and     Table 7.
Over the course of the immersion tests, the concentration of Ca 2+ ions shows a continuous rising trend; the concentration of Ca 2+ ions increases by the largest    Table 5. e concentrations of the K + and Na + ions begin to stabilize after 0-10 d of water saturation, and the growth trend for these ions slows after 10 d and becomes generally stable after 30 d. e Si 4+ , Al 3+ , and Fe 3+ ions mainly come from relatively insoluble minerals and these minerals, being generally stable, contribute very few ions to the immersion solution. e concentrations of these ions did not change significantly although the total cation concentration shows an upward trend during the 0-60 d period of water saturation.
During the water saturation tests, the concentration of NO 3 − ions first increased, then decreased, but then increased again. Overall, the NO 3 − concentration increased considerably, and it apparently increased continuously over the 20-60 d period. After 30 d of water saturation, the NO 3 − concentration tended to be stable. At 60 d, its concentration was the highest of any anion in the solution. e SO 2− 4 , Cl − , and F − anions showed continuous growth in the 0-30 d period, and their growth rate was rapid from 0 d to 10 d but significantly reduced from 20 to 30 d. After about 30 d of water saturation, the sulfur, chlorine, and fluorine ion growth rates tended to stabilize. e F − growth rate was the slowest, and the total F − ion concentration was low. e total anions showed a continuous growth trend in the 0-30 d period and tended to be stable after 30 d of water saturation. e water was weakly acidic in the 5 d and 10 d soaking solutions, but the pH changed to weak alkaline in the 20 d and 30 d solutions.

Changes in
Mineralogy. X-ray diffraction analyses were carried out on unaltered mudstone samples and on samples after 30 d and 60 d of water saturation. Figure 6 shows X-ray diffraction patterns from an unaltered mudstone along with patterns for 30 d and 60 d water saturated mudstones and a pattern from natural state for comparison.
It can be seen that the three characteristic peaks of muscovite and feldspar are obviously weakened with the increase in water saturation time, and the content is reduced to a certain extent. After 30 d of water saturation, the three characteristic kaolinite peaks increased slightly and a small amount of newly formed kaolinite was observed. After 30-60 d of water saturation, the kaolinite peaks weakened and the content decreased.

Changes in Mudstone Surface
Morphology. Changes in mudstone surface morphology after water saturation were observed and documented by SEM imaging. An image of the surface of a mudstone sample after 60 d of water saturation is shown at two different magnifications in Figure 7.
After 60 d of water saturation, almost all the coarse grains and aggregates that constituted much of the mudstone's cement have been removed. e mudstone components that remain are loosely stacked together forming stacked structures of flaky grains. e rock's structure has changed from the original compact state to a disordered state, and the number of microfractures around the pores has increased considerably.

Mudstone Softening.
According to the analysis presented in the previous sections, the water softening of mudstone is the result of the combined influence of physical, chemical, and mechanical processes. e effects of these processes can be classified as chemical erosion, hydraulic wedge splitting, and hydrolysis. ese three agents are discussed below.
Chemical erosion: most of the salts in the minerals that compose the mudstone are soluble salts. ese dissolve into the soaking solution during water saturation. e ionic concentration of the initially pure (zero ions) immersion water increases after the saturation tests, showing that mineral dissolution must have occurred.
Hydraulic wedge splitting: water molecules infiltrate into the lattice structure of mudstone minerals along microcracks. is squeezing action is called wedge splitting, and it shows that microcrack development increases as the water saturation time increases.
Hydrolysis: the chemical reactions between the minerals in the mudstone and the soaking solution produce new salt minerals. is is shown by the changes in the X-ray diffraction patterns and the changes in the soaking solution's pH. e chemical and microstructural changes in the mudstone during water saturation, the softening phase, can be divided into three stages: (1) e initial softening stage (0-10 d): in this stage, the soluble salts are gradually dissolved from the mudstone and the ionic strength of the soaking solution increases; pores and fractures begin to develop in the mudstone and the mudstone's structure begins to break down. is stage is predominantly one of chemical erosion. (2) e advanced softening stage (10-30 d): the soluble salt in mudstone gradually decrease, the ion concentration in the soaking solution continues to rise, and the softening effect is stronger than the previous stage. (3) e stable softening stage (more than 30 d): the ion exchange tends towards equilibrium, and the effect of chemical erosion is weakening. e pores increase in size, and wedge splitting is more pronounced.
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Mechanical Properties after Different Saturation Times.
Direct shear tests were conducted after different mudstone saturation times. e σ-τ curves from those tests are presented in Figure 8. e shear strength index c and the internal friction angle φ for those mudstones were then calculated; those values and the change of mudstone strength after different saturation times are listed in Table 8. e results show that the mudstone's shear strength tends to be stable after 30 d of water saturation but the cohesion and internal friction angle decrease by 26.53% and 42.19%, respectively, after 60 d of saturation.

Strength Softening Model for Saturated Mudstone.
Analysis of the test results revealed that after water immersion, mudstone softening is due to the combined effects of hydrolysis, wedge splitting, and chemical weathering. e softening results in a reduction in the mudstone's strength.
is conclusion is the basis for a soft rock strength-softening model that postulates that the rock is softened under the influence of a permeability coefficient K and a hydraulic gradient I over time t.
is model can be expressed as follows: where K is the permeability coefficient for soft rock; I is the hydraulic gradient; t is the permeability time; and Q 0 is the initial strength index value, which can be either cohesion c or internal friction angle φ. According to the water saturation time-strength test results, the longer the water saturation time, the more the strength is degraded. After analysis, we think that the reduction of strength is a reduction under the combined action of these factors, and a reduction function with a factor of Δ is proposed.
e factor Δ, a function of time, permeability coefficient, hydraulic gradient, and pattern thickness, is recognized. is factor is used as the independent variable in e factor Δ is called the intermediate characterization quantity.
Taking the results in Table 8 as an example, the permeability coefficient K for mudstone is 1.49 × 10 −14 m/s and the hydraulic gradient I � 1 for shallow immersion. e corresponding strength reduction relationship between the infiltration amount and φ at 5, 10, 20, 30, and 60 d immersion times is shown in Figure 9.
Based on the relationship between the intermediate characterization and the friction angle attenuation, a strength model for the internal friction angle was established. at model is represented by the following equation: where φ 0 is the initial internal friction angle from the direct shear test. en, the strength-saturation time model is established by expanding equation (3) to generate the following equation: where α, β, and c are undetermined coefficients. e strength indices calculated from this strengthsoftening model are compared with the test values. e results are listed in Table 9.
e two groups of values listed in Table 9 were plotted, and the graph is shown in Figure 10.   Table 8).  mudstone. e mudstone's original water content and porosity increase by factors of 2.46 and 2.48, respectively, after 60 d. e mudstone softening results from physical, chemical, and mechanical processes that can be classified as chemical undercutting, hydraulic wedge splitting, and hydrolysis. e softening can be divided into three stages: the initial softening stage (0-10 d), the advanced softening stage (10-30 d), and the stable softening stage (more than 30 d).
erefore, the saturation time has a significant influence on the properties of the stability of mudstone which also been a threaten to the upper structure stability. To quantification the water softening process, a mudstone strengthsoftening model is proposed. e model incorporates a permeability coefficient K, a hydraulic gradient I, and time t and can be used to determine both the mudstone's the shear strength c and internal friction angle φ.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest regarding the publication of this paper.