- Email: [email protected]
A quantitative determination of the effect of moisture on purple mudstone decay in Southwestern China
Catena 139 (2016) 28–31
Contents lists available at ScienceDirect
Catena journal homepage: www.elsevier.com/locate/catena
A quantitative determination of the effect of moisture on purple mudstone decay in Southwestern China Dan Zhang a,b, Anqiang Chen c, Xuemei Wang a, Bangguo Yan a,d, Liangtao Shi d, Gangcai Liu a,⁎ a Key Laboratory of Mountain Hazards and Earth Surface Processes, Chinese Academy of Sciences, Institute of Mountain Hazards and Environment, Chinese Academy of Sciences and Ministry of Water Resources, Chengdu 610041, China b College of Resources and Environment, Yunnan Agricultural University, Kunming 650205, China c Institute of Agricultural Environment and Resources, Yunnan Academy of Agricultural Science, Kunming 650205, China d Institute of Tropical Eco-agricultural Sciences, Yunnan Academy of Agricultural Sciences, Yuanmou 651300, Yunnan Province, China
a r t i c l e
i n f o
Article history: Received 13 April 2015 Received in revised form 25 November 2015 Accepted 7 December 2015 Available online 17 December 2015 Keywords: Rock decay Mudstone Moisture content Wetting–drying
a b s t r a c t Aims: The moisture condition is well known to be a key factor that apparently influences rock decay, but little is known about the quantitative correlations between a rock's decay rate and its moisture content, which makes it difficult to quantitatively predict the rate of rock decay under varied moisture conditions. Thus, in this paper our aim is to observe the decay rates of various purple mudstones with different moisture contents and to develop an equation to calculate the decay rate. Methods: Three types of purple mudstones were sampled from the Tuodian group (J3t), Matoushan group (K2m), and Lufeng group (J1l), respectively, all located in the Chuxiong district of Yunnan province, southwestern China. All samples were manually cut into cubes of 50 mm × 50 mm × 50 mm, divided into nine groups, wetted on a sieve with 2 mm pores according to nine treatment levels, and then dried in an oven in the laboratory. Thirty-nine such wetting–drying cycles were carried out for each treatment. Decay rates were calculated by weighing the mass remaining on the sieve after each treatment cycle, and the average decay rates were estimated from the 39 cycles. Results: The results showed that the average decay rate of the tested rocks rose with increasing rock moisture content and that the rank order of the decay rate was J3t N K2m N J1l. Conclusions: A significant exponential relationship between the average rock decay rate and rock moisture content for all three purple rocks was found to quantitatively predict the decay rate of purple rock under varied moisture conditions. © 2015 Elsevier B.V. All rights reserved.
1. Introduction It is well known that water is important for a wide range of ecological and environmental processes, including rock decay processes, that greatly influence soil formation (Zhang et al., 2013a), geomorphic evolution (Elliott, 2008), and the global carbon cycle over long time scales (Qiu et al., 2004). Many researchers have documented that the decay of rock is mainly determined by its moisture condition (Warke, 2007; Elliott, 2008; Moores et al., 2008; Doostmohammadi et al., 2009), although it is also affected by temperature conditions (Hall et al., 2008; Eppes et al., 2010). Cantón et al. (2001) found that the decay rates of investigated mudstones were proportional to the number of rainfall events during the sampling periods and confirmed, under laboratory conditions, their findings that the number of wetting– drying cycles had the greatest influence on rock decay. Porter and ⁎ Corresponding author at: No. 9, Block 4, Renminnanlu Road, P.O. Box: 417, 610041 Chengdu, China. E-mail address: [email protected] (G. Liu).
http://dx.doi.org/10.1016/j.catena.2015.12.003 0341-8162/© 2015 Elsevier B.V. All rights reserved.
Trenhaile (2007) documented that the surface downwearing rate of rocks in tidal areas is mainly due to seawater wetting and drying and has little or no relationship with rock hardness or air temperature. The alternate wetting and drying cycle was a particularly effective agent of downwearing for mudstones and other argillaceous rocks (Kanyaya and Trenhaile, 2005; Porter et al., 2010; Stephenson et al., 2010). Mol and Viles (2010) reported that an increase in weathering resulted from an increase of moisture in the sandstone of the Golden Gate Reserve. Sass (2005) suggested that the moisture content of rock was the major factor controlling the frost shattering rate in alpine and Arctic environments. All of these reports suggested that an increase of the rock's average water content would lead to an increase of the rock decay rate, which could contribute to the following roles of water in rock decay (Bozzano et al., 2006): 1) seepage of meteoric water, resulting in swelling; 2) dissemination of highly oxidizing meteoric water; 3) triggering of oxidation and dissolution of minerals; 4) water evaporation, leading to contraction; and 5) partial migration of the elements contained in the aqueous solution and consequent deposition of minerals in the joints. Furthermore, Bozzano et al. (2006) argued that
D. Zhang et al. / Catena 139 (2016) 28–31
29
2.2. Experimental procedure
the role of water during the rock decay process occurs in cyclical steps, such as the seepage of meteoric water, which accelerates rock decay. However, how the environmental surface conditions of rock moisture affect weathering processes remains largely unknown (Sumner et al., 2009). In particular, the quantitative relationship between decay of rock and its moisture conditions is not yet clear. There are large areas of purple rock covering about 18 million ha in China. Purple rock is characterized by low permeability and high hydrophilicity (i.e. good wettability) and decays easily (He, 2003). In a previous study, we showed that water made a more important contribution to the process of rock decay than temperature; the variation of moisture content and H2O phase (solid or liquid) within rock was found to play a key role in rock decay processes rather than temperature alternation (Zhang et al., 2013a). However, the quantitative relationship between the decay rate of rock and its moisture content has not yet been determined. Thus, in this study, we measured the decay rate of purple rock treated with varied moisture contents under laboratory conditions. The objectives of this paper were: 1) to reveal the quantitative effect of changes in moisture on rock decay and 2) to develop a predictive equation representing the correlation between decay rates of purple rock as a function of the number of repeated wetting–drying cycles and the rock moisture content.
All of the samples were treated during the period of March 12 to May 15, 2012 (the dry season), at the Gully Erosion and Collapse Experimental Station in Yuanmou Dry-Hot Valley (henceforth called Yuanmou station), located at 25° 50′39.2″N, 101° 49′34.0″E (Zhang et al., 2013a). Here the climate is south subtropical, characterized by hot and dry conditions, with a low daily average air humidity of 30% during the dry season of October to May. Thus, the effect of relative air humidity on rock decay is negligible. First, to ensure that samples used for each treatment were homogeneous, they were selected from the same mudstone block and cut manually into cubes of the same size (50 mm × 50 mm × 50 mm) with an electric saw. Next, the relationship between moisture content (degree of saturation) and duration of sample immersion in pure water (minutes) was established in advance by measuring the weight of the replicated samples for every immersion time for the three kinds of rocks (Fig. 1). Then, the following moisture treatments were carried out in the laboratory. Based on the water content of a completely saturated sample, representing 100% saturation and denoted as S100%, samples were treated using pure water in 10% increments of saturation from 20 to 100%. In other words, samples were subjected to nine moisture treatments, denoted as S20% to S100%, respectively. Because our previous study found that the decay of these rocks was negligible if they contained no moisture under laboratory conditions (Zhang et al., 2012), an S0% treatment (CK) was not conducted in this study. To conduct the S100% treatment, a sample laid on an iron sieve with 2 mm pores was immersed in water for around 12 h until the sample mass was constant, after which the saturation water content was calculated. The S20% and S30% treatments were conducted by manually and uniformly watering all surfaces of the treated samples laid on the sieve to achieve 20 and 30% saturation, respectively, using a sprinkling can. This method was chosen because the duration of immersion of the sample in water was too short for these two treatments, according to Fig. 1. The remaining six treatments (S40% to S90%) were carried out by immersing the samples laid on the sieve in water for various periods, based on the previously determined relationship between saturation degree and duration of immersion (Fig. 1). To avoid the effects of artificial vibrations and unequal water distribution of the above treated samples on the rock decay rate, all of the above treated samples were first sealed in plastic bags and left to stand for 24 h on the sieve under a constant temperature of 25 °C in the laboratory. Next, the samples on the sieve were put into an oven at a constant temperature of 76 °C (this temperature is the highest ground temperature of Yuanmou station and ensures a site-consistent high-temperature effect on rock decay) for another 24 h (this duration was determined by us in advance) to dry up all the moisture in the treated samples. The mass of the rock sample remaining on the sieve was weighed with an electronic balance after this (wetting–drying) procedure had been completed for each treatment, which was referred to as a cycle. Then the remaining sample on the sieve was subjected to the next cycle with the same wetting–drying procedure. In total, 39
2. Materials and methods 2.1. Materials The experimental samples were taken during the period of February 9 to 15, 2012, from three kinds of rock series, namely the Matoushan group (K2m) (25° 38′28.7″N, 101° 54′18.5″E, at an altitude of 1370 m), the Tuodian group (J3t) (24°41′50″N, 101° 37′14.7″E, at an altitude of 1928 m), and the Lufeng group (J1l) (25° 08′40.3″N, 102° 02′ 55.3″E, at an altitude of 1563 m), all in the Chuxiong district of Yunnan Province, southwestern China. These rocks are found as a hilly vegetated landscape with occasional outcrops. Bedrock is usually distributed in alternate sandstone and mudstone layers with a thickness of 2 to 5 m and coverage of 80 to 90% of the land area of Chuxiong district (He, 2003). The decayed materials of these rocks are usually eroded and transported out of watersheds by rivers, and the soil depths are often shallow: less than 2 m. The details of the in situ information and the material constituents of the samples have been described in a previous article (Zhang et al., 2013a). The basic physical properties of the samples are shown in Table 1. The methods of measurement of these indexes were as follows: (1) the bulk density and moisture content of air-dried rocks were determined by wax-sealing and oven-drying (Zhao, 2003) and the rock compressive strength was measured by uniaxial compression test (Brown, 1981; Lin et al., 2010); (2) the rate of water absorption and saturated absorption water content were determined by immersion of rock samples in water under normal temperature and pressure (Brown, 1981; Xu, 2007). Table 1 The main physical properties of the sampled rocks. Rock type
Bulk density (g·cm−3)
Porosity (%)
Natural moisture content (%)
Absorption water content (%)
Saturated absorption water content (%)
Saturation coefficient
Compressive strength (MPa) Air-dried rock sample
Saturated rock sample
Softening coefficient
K2m J1 l J3 t
2.3 2.7 2.7
7.4 7.2 12.7
0.8 1.2 0.8
4.5 3.6 5.4
7.3 6.6 7.6
0.6 0.6 0.7
50.1 78.3 31.7
39.9 69.9 13.8
0.8 0.9 0.4
Note: the natural moisture content (%) is the ratio of rock water mass under natural conditions to the mass of dried rock sample, expressed as a percentage. The absorption water content (%) is the ratio of absorbed water mass under atmospheric conditions to the mass of dried rock sample, expressed as a percentage. The saturated water absorption content is the ratio of the largest absorbed water mass under boiling conditions to the mass of dried rock sample, expressed as a percentage. The saturation coefficient is the ratio of absorption water content to saturated water absorption content. The size of rock samples used in the uniaxial compression test was 50 mm × 50 mm × 100 mm. The softening coefficient is the ratio of compressive strengths between saturated and air-dried rock samples (Lin et al., 2010).
30
D. Zhang et al. / Catena 139 (2016) 28–31
Fig. 1. Relationship between degree of saturation and duration of immersion of sample in water.
cycles were carried out for each of the above treatments, because some samples left no decayed materials on the 2 mm sieve after this number of cycles. All treatments were done with two replicates.
three types of rocks were significant at the level of p b 0.0001, meaning that this function was able to correctly and quantitatively predict the decay rate of purple rocks under varied moisture contents in laboratory conditions. This also indicates that the decay rate of all three rocks will apparently increase with an increase of rock moisture content when they experience the drying condition, quantitatively suggesting that moisture plays a substantial role in the decay of purple rock (Zhang et al., 2013a). The results (Table 2) showed that there were apparent differences between rock groups under varied moisture conditions: the rank order of the decay rate was J3t N K2m N J1l, indicating that the rock type also apparently affected the decay rate under varied moisture contents. In addition, significant differences between moisture treatments were found for the K2m and J1l groups, except between the S20% and S30% treatments. Saturation between S90% and S100% was also excluded for the K2m group, suggesting that the K2m group could reach its maximum decay rate even under conditions of 90% saturation. Significant differences between the treatments with saturation of S30% and greater than S30% were shown for the K2m and J3t groups. Significant differences were also seen between the treatments with saturation of S40% and greater than S40% for the J1l group. These results implied that the sensitivity of the decay rate to moisture variation was different for the different rock types. 4. Discussion
2.3. Calculation of the decay rate Because the tested mudstone is a lithologic product developed from sedimentary rock, particles b2 mm can be considered to be soil particles (Zhu et al., 2008), and thus we used only the mass of clastic particles b2 mm that decayed from the samples to represent the decay rate in this study (results for the other decayed particles are not presented). The rate of rock decay was calculated as the mass of clastic material b2 mm after the i-th cycle divided by the remaining mass of the sample × 100%, with i = 1, 2, 3 … 39. The average rock decay rate was calculated from the 39 cycles. 3. Results The results (Fig. 2) showed that the average decay rate apparently increased with increasing rock moisture content for all experimental rocks, especially J3t rock. The quantitative analysis showed that there was an exponential functional relationship between the decay rate and the degree of saturation of the rock and that the correlations of all
Our outcomes are consistent with many previous reports (Warke, 2007; Elliott, 2008; Moores et al., 2008; Doostmohammadi et al., 2009; Mol and Viles, 2010), suggesting that an increase in moisture causes accelerated rock decay and vice versa. On the other hand, it is possible that immersion of blocks in deionized water may also have the effect of leaching some mobile ions (Bozzano et al., 2006), such as sodium, from the surface and subsurface layers, resulting in the opening of previously blocked pores and the facilitation of movement of moisture into the rock, which would accelerate decay (Warke, 2007). Thus, the more time the rocks spent immersed in water, the more cracks formed. Moores et al. (2008) also documented that crack growth of surface clasts by hydration and/or thermal weathering was primarily a function of local water content at the crack tip and suggested that the moisture content in clasts was the limiting factor in clast breakdown. In short, water is a physical as well as a chemical agent that influences rock decay, which has direct and indirect implications for rock disintegration. Therefore, there is a significant relationship between a rock's decay rate and its moisture content (Fig. 2). Different rock types showed significant differences in decay rates (Table 2). These could be attributed to the following fac'tors: firstly, compressive strengths are the basis of the evaluation of rock hardness. The softening coefficient is the ratio of compressive strengths of
Table 2 Comparison of various rock decay rates (%) under varied moisture conditions (LSD0.05).
Fig. 2. Quantitative correlations between the average decay rate after 39 wetting–drying cycles and saturation degree.
Treatments
J3t
K2m
J1l
S100% S90% S80% S70% S60% S50% S40% S30% S20% Rock group's average
0.307 ± 0.069a 0.279 ± 0.057b 0.223 ± 0.057c 0.180 ± 0.057d 0.147 ± 0.039e 0.115 ± 0.021f 0.088 ± 0.010g 0.056 ± 0.005h 0.049 ± 0.006h 0.160 ± 0.094a
0.183 ± 0.067a 0.167 ± 0.039a 0.141 ± 0.046b 0.107 ± 0.046c 0.085 ± 0.019d 0.067 ± 0.023e 0.052 ± 0.016ef 0.039 ± 0.008fg 0.033 ± 0.017g 0.097 ± 0.056ab
0.072 ± 0.021a 0.064 ± 0.015b 0.056 ± 0.016c 0.044 ± 0.014d 0.035 ± 0.007e 0.030 ± 0.007f 0.018 ± 0.004g 0.015 ± 0.001g 0.013 ± 0.004g 0.039 ± 0.022c
Note: different letters within a column for various treatments or the row “Rock group's average” indicate a significant difference at P b 0.05; the standard deviations of each treatment were calculated for data from the 39 cycles with the replicate samples, and that of the “Rock group's average” was calculated for data from the nine treatments.
D. Zhang et al. / Catena 139 (2016) 28–31
saturated and air-dried rock samples (Lin et al., 2010). The uniaxial compressive strength and softening coefficient gradually diminished as the clay mineral content increased (Lin et al., 2010). Zhang et al. (2012) reported that higher compressive strength was correlated with greater hardness of the rock, a higher softening coefficient, and slower rock decay. Thus, J3t had the highest decay rate because it had the lowest compressive strength and lowest softening coefficient (Table 1). Secondly, all three rocks contained many mineral materials, particularly clay minerals (Zhang et al., 2013a), which cause rocks to decay easily when water penetrates them; the expansion of clay minerals results in cracks (Warke, 2007; Doostmohammadi et al., 2009). However, there are differences in the contents and types of clay minerals in the three rocks: the K2m group contains 67% illite and 17% montmorillonite, which account for 84% of the total clay content; the J1l group contains 70% illite and 8% montmorillonite; and the J3t group contains 85% illite and 6% montmorillonite (Zhang et al., 2013a), meaning that the J3t group has the highest content of swelling clay minerals and the highest decay rate (Table 2). Our results also showed that the higher the expansible clay mineral content of the rock, the higher the decay rate, suggesting that the swelling clay minerals are an important determinant of rock decay (Bozzano et al., 2006; Doostmohammadi et al., 2009). Thirdly, the rocks have different saturation coefficients, indicating their capacity to resist decay (Li et al., 1991), and the higher the saturation coefficient, the higher the rate of rock decay (Xu, 2007). The rank order of the saturation coefficients is J3t (0.71) N K2m (0.62) N J1l (0.55) (Table 1), and thus J3t has the highest decay rate. Finally, there are differences in the number, size, distribution, and connectivity of pores and cracks of various rocks. Porosity is a major factor in rock weathering, since it controls not only the mass of fluid throughout the rock but also its processes (Molina Ballesteros et al., 2011). Elliott (2008) and Akin and Ozsan (2011) also reported that porosity is one of the most important parameters influencing rock deterioration and suggested that the greater the porosity, the faster the rock decay. This was because, prior to immersion, the pores and cracks were filled with air, and when water ingressed into these pores and cracks the internal air was compressed, resulting in both the presence of water and an increase in air pressure, making the rock break down. Thus, because the J3 t group rock had the highest porosity (14.2%, Table 1) and pore area (Zhang et al., 2013b), it also had the highest average decay rate of 0.16% (Table 2). The compressive strength and softening coefficients of J1l are greater than those of K2m and J3t, and its SiO2 content (Zhang et al., 2013a) is slightly higher, while the clay mineral contents (Zhang et al., 2013a) are less than those of K2m and J3t (Table 1), respectively. Thus, the final rank order of the decay rate was J3t N K2m N J1l.
5. Conclusion In conclusion, the decay rate of the three kinds of purple rocks studied rises with increasing rock moisture content, and the rank order of the decay rate is Tuodian group (J3t) N Matoushan group (K2m) N Lufeng group (J1l). These differences can be mainly attributed to the variations in compressive strength, mineral components, and water absorbability of the three types of rocks. There is a significant exponential relationship between the decay rate and the moisture content for all the tested purple rocks, and thus this equation can quantitatively predict the decay rate of purple rock correctly under varied moisture contents in laboratory conditions.
31
Acknowledgments This work was financially supported by the Natural Science Foundation of China (grant nos. 41471232, 40971168). References Akin, M., Ozsan, A., 2011. Evaluation of the long-term durability of yellow travertine using accelerated weathering tests. Bull. Eng. Geol. Environ. 70, 101–114. Bozzano, F., Gaeta, M., Marcoccia, S., 2006. Weathering of Valle Ricca stiff and jointed clay. Eng. Geol. 84, 161–182. Brown, E.T., 1981. Rock Characterization, Testing and Monitoring: ISRM Suggested Methods. Pergaman Press, New York. Cantón, Y., Solé-Benet, A., Queralt, I., Pini, R., 2001. Weathering of a gypsum-calcareous mudstone under semi-arid environment at Tabernas, SE Spain: laboratory and field-based experimental approaches. Catena 44, 111–132. Doostmohammadi, R., Moosavi, M., Mutschler, T., Osan, C., 2009. Influence of cyclic wetting and drying on swelling behavior of mudstone in south west of Iran. Environ. Geol. 58, 999–1009. Elliott, C., 2008. Influence of temperature and moisture availability on physical rock weathering along the Victoria Land coast, Antarctica. Antarct. Sci. 20, 61–67. Eppes, M., McFadden, L., Wegmann, K., Scuderi, L., 2010. Cracks in desert pavement rocks: further insights into mechanical weathering by directional insolation. Geomorphology 123, 97–108. Hall, K., Guglielmin, M., Strini, A., 2008. Weathering of granite in Antarctica: I. light penetration into rock and implications for rock weathering and endolithic communities. Earth Surf. Process. Landf. 33, 295–307. He, Y.R., 2003. Purple Soil in China (2). Science Press, Beijing (in Chinese). Kanyaya, J., Trenhaile, A., 2005. Tidal wetting and drying on shore platforms: an experimental assessment. Geomorphology 70, 129–146. Li, Z.M., Tang, S.J., Zhang, X.W., He, Y.R., 1991. Purple Soil in China (1). Science Press, Beijing (in Chinese). Lin, Z.H., Xiang, W., Zhang, Y.M., 2010. Experimental research on influences of physical indices and microstructure parameters on strength properties of red stone from Western Hunan. Chin. J. Rock Mech. Eng. 29 (1), 124–133 (in Chinese). Mol, L., Viles, H.A., 2010. Geoelectric investigations into sandstone moisture regimes: implications for rock weathering and the deterioration of San Rock Art in the Golden Gate Reserve, South Africa. Geomorphology 118, 280–287. Molina Ballesteros, E., Garcia Talegon, J., Inigo Inigo, A.C., Gonzalez Sanchez, M., Herrero Fernandez, H., 2011. Importance of porosity and transfer of matter in the rock weathering processes: two examples in central Spain. Environ. Earth Sci. 64, 1741–1754. Moores, J.E., Pelletier, J.D., Smith, P.H., 2008. Crack propagation by differential insolation on desert surface clasts. Geomorphology 102, 472–481. Porter, N., Trenhaile, A., 2007. Short-term rock surface expansion and contraction in the intertidal zone. Earth Surf. Process. Landf. 32, 1379–1397. Porter, N., Trenhaile, A., Prestanski, K., Kanyaya, J., 2010. Patterns of surface downwearing on shore platforms in eastern Canada. Earth Surf. Process. Landf. 35, 1793–1810. Qiu, D.S., Zhuang, D.F., Hu, Y.F., Yao, R., 2004. Estimation of carbon sink capacity caused by rock weathering in China. Earth Sci. — J. Chin. Univ. Geosci. 29 (2), 177–190 (in Chinese). Sass, O., 2005. Rock moisture measurements: techniques, results, and implications for weathering. Earth Surf. Process. Landf. 30, 359–374. Stephenson, W.J., Kirk, R.M., Hemmingsen, S.A., Hemmingsen, M.A., 2010. Decadal scale micro erosion rates on shore platforms. Geomorphology 114, 22–29. Sumner, P.D., Hall, K.J., van Rooy, J.L., Meiklejohn, K.I., 2009. Rock weathering on the eastern mountains of southern Africa: review and insights from case studies. J. Afr. Earth Sci. 55, 236–244. Warke, P.A., 2007. Complex weathering in drylands: implications of ‘stress’ history for rock debris breakdown and sediment release. Geomorphology 85, 30–48. Xu, Z.Y., 2007. Rock Mechanics. China Water Conservancy and Hydropower Press, Beijing (in Chinese). Zhang, D., Chen, A.Q., Su, R.B., Duan, H.P., Liu, G.C., 2013b. Effect of hydrothermal environment on distintegration of different purple parent rocks. Acta Pedol. Sin. 50, 7–15 (in Chinese). Zhang, D., Chen, A.Q., Xiong, D.H., Liu, G.C., 2013a. Effect of moisture and temperature conditions on the decay rate of a purple mudstone in southwestern China. Geomorphology 182, 125–132. Zhang, D., Chen, A.Q., Liu, G.C., 2012. Laboratory investigation of disintegration characteristics of purple mudstone under different hydrothermal conditions. J. Mt. Sci. 9, 127–136. Zhao, X.G., 2003. A comparison of two methods for determining densities of rocks and minerals. Geophys. Geochem. Explor. 27, 202–205 (in Chinese). Zhu, B., Wang, T., You, X., Gao, M.R., 2008. Nutrient release from weathering of purplish rocks in the Sichuan Basin, China. Pedosphere 18, 257–264.
Contents lists available at ScienceDirect
Catena journal homepage: www.elsevier.com/locate/catena
A quantitative determination of the effect of moisture on purple mudstone decay in Southwestern China Dan Zhang a,b, Anqiang Chen c, Xuemei Wang a, Bangguo Yan a,d, Liangtao Shi d, Gangcai Liu a,⁎ a Key Laboratory of Mountain Hazards and Earth Surface Processes, Chinese Academy of Sciences, Institute of Mountain Hazards and Environment, Chinese Academy of Sciences and Ministry of Water Resources, Chengdu 610041, China b College of Resources and Environment, Yunnan Agricultural University, Kunming 650205, China c Institute of Agricultural Environment and Resources, Yunnan Academy of Agricultural Science, Kunming 650205, China d Institute of Tropical Eco-agricultural Sciences, Yunnan Academy of Agricultural Sciences, Yuanmou 651300, Yunnan Province, China
a r t i c l e
i n f o
Article history: Received 13 April 2015 Received in revised form 25 November 2015 Accepted 7 December 2015 Available online 17 December 2015 Keywords: Rock decay Mudstone Moisture content Wetting–drying
a b s t r a c t Aims: The moisture condition is well known to be a key factor that apparently influences rock decay, but little is known about the quantitative correlations between a rock's decay rate and its moisture content, which makes it difficult to quantitatively predict the rate of rock decay under varied moisture conditions. Thus, in this paper our aim is to observe the decay rates of various purple mudstones with different moisture contents and to develop an equation to calculate the decay rate. Methods: Three types of purple mudstones were sampled from the Tuodian group (J3t), Matoushan group (K2m), and Lufeng group (J1l), respectively, all located in the Chuxiong district of Yunnan province, southwestern China. All samples were manually cut into cubes of 50 mm × 50 mm × 50 mm, divided into nine groups, wetted on a sieve with 2 mm pores according to nine treatment levels, and then dried in an oven in the laboratory. Thirty-nine such wetting–drying cycles were carried out for each treatment. Decay rates were calculated by weighing the mass remaining on the sieve after each treatment cycle, and the average decay rates were estimated from the 39 cycles. Results: The results showed that the average decay rate of the tested rocks rose with increasing rock moisture content and that the rank order of the decay rate was J3t N K2m N J1l. Conclusions: A significant exponential relationship between the average rock decay rate and rock moisture content for all three purple rocks was found to quantitatively predict the decay rate of purple rock under varied moisture conditions. © 2015 Elsevier B.V. All rights reserved.
1. Introduction It is well known that water is important for a wide range of ecological and environmental processes, including rock decay processes, that greatly influence soil formation (Zhang et al., 2013a), geomorphic evolution (Elliott, 2008), and the global carbon cycle over long time scales (Qiu et al., 2004). Many researchers have documented that the decay of rock is mainly determined by its moisture condition (Warke, 2007; Elliott, 2008; Moores et al., 2008; Doostmohammadi et al., 2009), although it is also affected by temperature conditions (Hall et al., 2008; Eppes et al., 2010). Cantón et al. (2001) found that the decay rates of investigated mudstones were proportional to the number of rainfall events during the sampling periods and confirmed, under laboratory conditions, their findings that the number of wetting– drying cycles had the greatest influence on rock decay. Porter and ⁎ Corresponding author at: No. 9, Block 4, Renminnanlu Road, P.O. Box: 417, 610041 Chengdu, China. E-mail address: [email protected] (G. Liu).
http://dx.doi.org/10.1016/j.catena.2015.12.003 0341-8162/© 2015 Elsevier B.V. All rights reserved.
Trenhaile (2007) documented that the surface downwearing rate of rocks in tidal areas is mainly due to seawater wetting and drying and has little or no relationship with rock hardness or air temperature. The alternate wetting and drying cycle was a particularly effective agent of downwearing for mudstones and other argillaceous rocks (Kanyaya and Trenhaile, 2005; Porter et al., 2010; Stephenson et al., 2010). Mol and Viles (2010) reported that an increase in weathering resulted from an increase of moisture in the sandstone of the Golden Gate Reserve. Sass (2005) suggested that the moisture content of rock was the major factor controlling the frost shattering rate in alpine and Arctic environments. All of these reports suggested that an increase of the rock's average water content would lead to an increase of the rock decay rate, which could contribute to the following roles of water in rock decay (Bozzano et al., 2006): 1) seepage of meteoric water, resulting in swelling; 2) dissemination of highly oxidizing meteoric water; 3) triggering of oxidation and dissolution of minerals; 4) water evaporation, leading to contraction; and 5) partial migration of the elements contained in the aqueous solution and consequent deposition of minerals in the joints. Furthermore, Bozzano et al. (2006) argued that
D. Zhang et al. / Catena 139 (2016) 28–31
29
2.2. Experimental procedure
the role of water during the rock decay process occurs in cyclical steps, such as the seepage of meteoric water, which accelerates rock decay. However, how the environmental surface conditions of rock moisture affect weathering processes remains largely unknown (Sumner et al., 2009). In particular, the quantitative relationship between decay of rock and its moisture conditions is not yet clear. There are large areas of purple rock covering about 18 million ha in China. Purple rock is characterized by low permeability and high hydrophilicity (i.e. good wettability) and decays easily (He, 2003). In a previous study, we showed that water made a more important contribution to the process of rock decay than temperature; the variation of moisture content and H2O phase (solid or liquid) within rock was found to play a key role in rock decay processes rather than temperature alternation (Zhang et al., 2013a). However, the quantitative relationship between the decay rate of rock and its moisture content has not yet been determined. Thus, in this study, we measured the decay rate of purple rock treated with varied moisture contents under laboratory conditions. The objectives of this paper were: 1) to reveal the quantitative effect of changes in moisture on rock decay and 2) to develop a predictive equation representing the correlation between decay rates of purple rock as a function of the number of repeated wetting–drying cycles and the rock moisture content.
All of the samples were treated during the period of March 12 to May 15, 2012 (the dry season), at the Gully Erosion and Collapse Experimental Station in Yuanmou Dry-Hot Valley (henceforth called Yuanmou station), located at 25° 50′39.2″N, 101° 49′34.0″E (Zhang et al., 2013a). Here the climate is south subtropical, characterized by hot and dry conditions, with a low daily average air humidity of 30% during the dry season of October to May. Thus, the effect of relative air humidity on rock decay is negligible. First, to ensure that samples used for each treatment were homogeneous, they were selected from the same mudstone block and cut manually into cubes of the same size (50 mm × 50 mm × 50 mm) with an electric saw. Next, the relationship between moisture content (degree of saturation) and duration of sample immersion in pure water (minutes) was established in advance by measuring the weight of the replicated samples for every immersion time for the three kinds of rocks (Fig. 1). Then, the following moisture treatments were carried out in the laboratory. Based on the water content of a completely saturated sample, representing 100% saturation and denoted as S100%, samples were treated using pure water in 10% increments of saturation from 20 to 100%. In other words, samples were subjected to nine moisture treatments, denoted as S20% to S100%, respectively. Because our previous study found that the decay of these rocks was negligible if they contained no moisture under laboratory conditions (Zhang et al., 2012), an S0% treatment (CK) was not conducted in this study. To conduct the S100% treatment, a sample laid on an iron sieve with 2 mm pores was immersed in water for around 12 h until the sample mass was constant, after which the saturation water content was calculated. The S20% and S30% treatments were conducted by manually and uniformly watering all surfaces of the treated samples laid on the sieve to achieve 20 and 30% saturation, respectively, using a sprinkling can. This method was chosen because the duration of immersion of the sample in water was too short for these two treatments, according to Fig. 1. The remaining six treatments (S40% to S90%) were carried out by immersing the samples laid on the sieve in water for various periods, based on the previously determined relationship between saturation degree and duration of immersion (Fig. 1). To avoid the effects of artificial vibrations and unequal water distribution of the above treated samples on the rock decay rate, all of the above treated samples were first sealed in plastic bags and left to stand for 24 h on the sieve under a constant temperature of 25 °C in the laboratory. Next, the samples on the sieve were put into an oven at a constant temperature of 76 °C (this temperature is the highest ground temperature of Yuanmou station and ensures a site-consistent high-temperature effect on rock decay) for another 24 h (this duration was determined by us in advance) to dry up all the moisture in the treated samples. The mass of the rock sample remaining on the sieve was weighed with an electronic balance after this (wetting–drying) procedure had been completed for each treatment, which was referred to as a cycle. Then the remaining sample on the sieve was subjected to the next cycle with the same wetting–drying procedure. In total, 39
2. Materials and methods 2.1. Materials The experimental samples were taken during the period of February 9 to 15, 2012, from three kinds of rock series, namely the Matoushan group (K2m) (25° 38′28.7″N, 101° 54′18.5″E, at an altitude of 1370 m), the Tuodian group (J3t) (24°41′50″N, 101° 37′14.7″E, at an altitude of 1928 m), and the Lufeng group (J1l) (25° 08′40.3″N, 102° 02′ 55.3″E, at an altitude of 1563 m), all in the Chuxiong district of Yunnan Province, southwestern China. These rocks are found as a hilly vegetated landscape with occasional outcrops. Bedrock is usually distributed in alternate sandstone and mudstone layers with a thickness of 2 to 5 m and coverage of 80 to 90% of the land area of Chuxiong district (He, 2003). The decayed materials of these rocks are usually eroded and transported out of watersheds by rivers, and the soil depths are often shallow: less than 2 m. The details of the in situ information and the material constituents of the samples have been described in a previous article (Zhang et al., 2013a). The basic physical properties of the samples are shown in Table 1. The methods of measurement of these indexes were as follows: (1) the bulk density and moisture content of air-dried rocks were determined by wax-sealing and oven-drying (Zhao, 2003) and the rock compressive strength was measured by uniaxial compression test (Brown, 1981; Lin et al., 2010); (2) the rate of water absorption and saturated absorption water content were determined by immersion of rock samples in water under normal temperature and pressure (Brown, 1981; Xu, 2007). Table 1 The main physical properties of the sampled rocks. Rock type
Bulk density (g·cm−3)
Porosity (%)
Natural moisture content (%)
Absorption water content (%)
Saturated absorption water content (%)
Saturation coefficient
Compressive strength (MPa) Air-dried rock sample
Saturated rock sample
Softening coefficient
K2m J1 l J3 t
2.3 2.7 2.7
7.4 7.2 12.7
0.8 1.2 0.8
4.5 3.6 5.4
7.3 6.6 7.6
0.6 0.6 0.7
50.1 78.3 31.7
39.9 69.9 13.8
0.8 0.9 0.4
Note: the natural moisture content (%) is the ratio of rock water mass under natural conditions to the mass of dried rock sample, expressed as a percentage. The absorption water content (%) is the ratio of absorbed water mass under atmospheric conditions to the mass of dried rock sample, expressed as a percentage. The saturated water absorption content is the ratio of the largest absorbed water mass under boiling conditions to the mass of dried rock sample, expressed as a percentage. The saturation coefficient is the ratio of absorption water content to saturated water absorption content. The size of rock samples used in the uniaxial compression test was 50 mm × 50 mm × 100 mm. The softening coefficient is the ratio of compressive strengths between saturated and air-dried rock samples (Lin et al., 2010).
30
D. Zhang et al. / Catena 139 (2016) 28–31
Fig. 1. Relationship between degree of saturation and duration of immersion of sample in water.
cycles were carried out for each of the above treatments, because some samples left no decayed materials on the 2 mm sieve after this number of cycles. All treatments were done with two replicates.
three types of rocks were significant at the level of p b 0.0001, meaning that this function was able to correctly and quantitatively predict the decay rate of purple rocks under varied moisture contents in laboratory conditions. This also indicates that the decay rate of all three rocks will apparently increase with an increase of rock moisture content when they experience the drying condition, quantitatively suggesting that moisture plays a substantial role in the decay of purple rock (Zhang et al., 2013a). The results (Table 2) showed that there were apparent differences between rock groups under varied moisture conditions: the rank order of the decay rate was J3t N K2m N J1l, indicating that the rock type also apparently affected the decay rate under varied moisture contents. In addition, significant differences between moisture treatments were found for the K2m and J1l groups, except between the S20% and S30% treatments. Saturation between S90% and S100% was also excluded for the K2m group, suggesting that the K2m group could reach its maximum decay rate even under conditions of 90% saturation. Significant differences between the treatments with saturation of S30% and greater than S30% were shown for the K2m and J3t groups. Significant differences were also seen between the treatments with saturation of S40% and greater than S40% for the J1l group. These results implied that the sensitivity of the decay rate to moisture variation was different for the different rock types. 4. Discussion
2.3. Calculation of the decay rate Because the tested mudstone is a lithologic product developed from sedimentary rock, particles b2 mm can be considered to be soil particles (Zhu et al., 2008), and thus we used only the mass of clastic particles b2 mm that decayed from the samples to represent the decay rate in this study (results for the other decayed particles are not presented). The rate of rock decay was calculated as the mass of clastic material b2 mm after the i-th cycle divided by the remaining mass of the sample × 100%, with i = 1, 2, 3 … 39. The average rock decay rate was calculated from the 39 cycles. 3. Results The results (Fig. 2) showed that the average decay rate apparently increased with increasing rock moisture content for all experimental rocks, especially J3t rock. The quantitative analysis showed that there was an exponential functional relationship between the decay rate and the degree of saturation of the rock and that the correlations of all
Our outcomes are consistent with many previous reports (Warke, 2007; Elliott, 2008; Moores et al., 2008; Doostmohammadi et al., 2009; Mol and Viles, 2010), suggesting that an increase in moisture causes accelerated rock decay and vice versa. On the other hand, it is possible that immersion of blocks in deionized water may also have the effect of leaching some mobile ions (Bozzano et al., 2006), such as sodium, from the surface and subsurface layers, resulting in the opening of previously blocked pores and the facilitation of movement of moisture into the rock, which would accelerate decay (Warke, 2007). Thus, the more time the rocks spent immersed in water, the more cracks formed. Moores et al. (2008) also documented that crack growth of surface clasts by hydration and/or thermal weathering was primarily a function of local water content at the crack tip and suggested that the moisture content in clasts was the limiting factor in clast breakdown. In short, water is a physical as well as a chemical agent that influences rock decay, which has direct and indirect implications for rock disintegration. Therefore, there is a significant relationship between a rock's decay rate and its moisture content (Fig. 2). Different rock types showed significant differences in decay rates (Table 2). These could be attributed to the following fac'tors: firstly, compressive strengths are the basis of the evaluation of rock hardness. The softening coefficient is the ratio of compressive strengths of
Table 2 Comparison of various rock decay rates (%) under varied moisture conditions (LSD0.05).
Fig. 2. Quantitative correlations between the average decay rate after 39 wetting–drying cycles and saturation degree.
Treatments
J3t
K2m
J1l
S100% S90% S80% S70% S60% S50% S40% S30% S20% Rock group's average
0.307 ± 0.069a 0.279 ± 0.057b 0.223 ± 0.057c 0.180 ± 0.057d 0.147 ± 0.039e 0.115 ± 0.021f 0.088 ± 0.010g 0.056 ± 0.005h 0.049 ± 0.006h 0.160 ± 0.094a
0.183 ± 0.067a 0.167 ± 0.039a 0.141 ± 0.046b 0.107 ± 0.046c 0.085 ± 0.019d 0.067 ± 0.023e 0.052 ± 0.016ef 0.039 ± 0.008fg 0.033 ± 0.017g 0.097 ± 0.056ab
0.072 ± 0.021a 0.064 ± 0.015b 0.056 ± 0.016c 0.044 ± 0.014d 0.035 ± 0.007e 0.030 ± 0.007f 0.018 ± 0.004g 0.015 ± 0.001g 0.013 ± 0.004g 0.039 ± 0.022c
Note: different letters within a column for various treatments or the row “Rock group's average” indicate a significant difference at P b 0.05; the standard deviations of each treatment were calculated for data from the 39 cycles with the replicate samples, and that of the “Rock group's average” was calculated for data from the nine treatments.
D. Zhang et al. / Catena 139 (2016) 28–31
saturated and air-dried rock samples (Lin et al., 2010). The uniaxial compressive strength and softening coefficient gradually diminished as the clay mineral content increased (Lin et al., 2010). Zhang et al. (2012) reported that higher compressive strength was correlated with greater hardness of the rock, a higher softening coefficient, and slower rock decay. Thus, J3t had the highest decay rate because it had the lowest compressive strength and lowest softening coefficient (Table 1). Secondly, all three rocks contained many mineral materials, particularly clay minerals (Zhang et al., 2013a), which cause rocks to decay easily when water penetrates them; the expansion of clay minerals results in cracks (Warke, 2007; Doostmohammadi et al., 2009). However, there are differences in the contents and types of clay minerals in the three rocks: the K2m group contains 67% illite and 17% montmorillonite, which account for 84% of the total clay content; the J1l group contains 70% illite and 8% montmorillonite; and the J3t group contains 85% illite and 6% montmorillonite (Zhang et al., 2013a), meaning that the J3t group has the highest content of swelling clay minerals and the highest decay rate (Table 2). Our results also showed that the higher the expansible clay mineral content of the rock, the higher the decay rate, suggesting that the swelling clay minerals are an important determinant of rock decay (Bozzano et al., 2006; Doostmohammadi et al., 2009). Thirdly, the rocks have different saturation coefficients, indicating their capacity to resist decay (Li et al., 1991), and the higher the saturation coefficient, the higher the rate of rock decay (Xu, 2007). The rank order of the saturation coefficients is J3t (0.71) N K2m (0.62) N J1l (0.55) (Table 1), and thus J3t has the highest decay rate. Finally, there are differences in the number, size, distribution, and connectivity of pores and cracks of various rocks. Porosity is a major factor in rock weathering, since it controls not only the mass of fluid throughout the rock but also its processes (Molina Ballesteros et al., 2011). Elliott (2008) and Akin and Ozsan (2011) also reported that porosity is one of the most important parameters influencing rock deterioration and suggested that the greater the porosity, the faster the rock decay. This was because, prior to immersion, the pores and cracks were filled with air, and when water ingressed into these pores and cracks the internal air was compressed, resulting in both the presence of water and an increase in air pressure, making the rock break down. Thus, because the J3 t group rock had the highest porosity (14.2%, Table 1) and pore area (Zhang et al., 2013b), it also had the highest average decay rate of 0.16% (Table 2). The compressive strength and softening coefficients of J1l are greater than those of K2m and J3t, and its SiO2 content (Zhang et al., 2013a) is slightly higher, while the clay mineral contents (Zhang et al., 2013a) are less than those of K2m and J3t (Table 1), respectively. Thus, the final rank order of the decay rate was J3t N K2m N J1l.
5. Conclusion In conclusion, the decay rate of the three kinds of purple rocks studied rises with increasing rock moisture content, and the rank order of the decay rate is Tuodian group (J3t) N Matoushan group (K2m) N Lufeng group (J1l). These differences can be mainly attributed to the variations in compressive strength, mineral components, and water absorbability of the three types of rocks. There is a significant exponential relationship between the decay rate and the moisture content for all the tested purple rocks, and thus this equation can quantitatively predict the decay rate of purple rock correctly under varied moisture contents in laboratory conditions.
31
Acknowledgments This work was financially supported by the Natural Science Foundation of China (grant nos. 41471232, 40971168). References Akin, M., Ozsan, A., 2011. Evaluation of the long-term durability of yellow travertine using accelerated weathering tests. Bull. Eng. Geol. Environ. 70, 101–114. Bozzano, F., Gaeta, M., Marcoccia, S., 2006. Weathering of Valle Ricca stiff and jointed clay. Eng. Geol. 84, 161–182. Brown, E.T., 1981. Rock Characterization, Testing and Monitoring: ISRM Suggested Methods. Pergaman Press, New York. Cantón, Y., Solé-Benet, A., Queralt, I., Pini, R., 2001. Weathering of a gypsum-calcareous mudstone under semi-arid environment at Tabernas, SE Spain: laboratory and field-based experimental approaches. Catena 44, 111–132. Doostmohammadi, R., Moosavi, M., Mutschler, T., Osan, C., 2009. Influence of cyclic wetting and drying on swelling behavior of mudstone in south west of Iran. Environ. Geol. 58, 999–1009. Elliott, C., 2008. Influence of temperature and moisture availability on physical rock weathering along the Victoria Land coast, Antarctica. Antarct. Sci. 20, 61–67. Eppes, M., McFadden, L., Wegmann, K., Scuderi, L., 2010. Cracks in desert pavement rocks: further insights into mechanical weathering by directional insolation. Geomorphology 123, 97–108. Hall, K., Guglielmin, M., Strini, A., 2008. Weathering of granite in Antarctica: I. light penetration into rock and implications for rock weathering and endolithic communities. Earth Surf. Process. Landf. 33, 295–307. He, Y.R., 2003. Purple Soil in China (2). Science Press, Beijing (in Chinese). Kanyaya, J., Trenhaile, A., 2005. Tidal wetting and drying on shore platforms: an experimental assessment. Geomorphology 70, 129–146. Li, Z.M., Tang, S.J., Zhang, X.W., He, Y.R., 1991. Purple Soil in China (1). Science Press, Beijing (in Chinese). Lin, Z.H., Xiang, W., Zhang, Y.M., 2010. Experimental research on influences of physical indices and microstructure parameters on strength properties of red stone from Western Hunan. Chin. J. Rock Mech. Eng. 29 (1), 124–133 (in Chinese). Mol, L., Viles, H.A., 2010. Geoelectric investigations into sandstone moisture regimes: implications for rock weathering and the deterioration of San Rock Art in the Golden Gate Reserve, South Africa. Geomorphology 118, 280–287. Molina Ballesteros, E., Garcia Talegon, J., Inigo Inigo, A.C., Gonzalez Sanchez, M., Herrero Fernandez, H., 2011. Importance of porosity and transfer of matter in the rock weathering processes: two examples in central Spain. Environ. Earth Sci. 64, 1741–1754. Moores, J.E., Pelletier, J.D., Smith, P.H., 2008. Crack propagation by differential insolation on desert surface clasts. Geomorphology 102, 472–481. Porter, N., Trenhaile, A., 2007. Short-term rock surface expansion and contraction in the intertidal zone. Earth Surf. Process. Landf. 32, 1379–1397. Porter, N., Trenhaile, A., Prestanski, K., Kanyaya, J., 2010. Patterns of surface downwearing on shore platforms in eastern Canada. Earth Surf. Process. Landf. 35, 1793–1810. Qiu, D.S., Zhuang, D.F., Hu, Y.F., Yao, R., 2004. Estimation of carbon sink capacity caused by rock weathering in China. Earth Sci. — J. Chin. Univ. Geosci. 29 (2), 177–190 (in Chinese). Sass, O., 2005. Rock moisture measurements: techniques, results, and implications for weathering. Earth Surf. Process. Landf. 30, 359–374. Stephenson, W.J., Kirk, R.M., Hemmingsen, S.A., Hemmingsen, M.A., 2010. Decadal scale micro erosion rates on shore platforms. Geomorphology 114, 22–29. Sumner, P.D., Hall, K.J., van Rooy, J.L., Meiklejohn, K.I., 2009. Rock weathering on the eastern mountains of southern Africa: review and insights from case studies. J. Afr. Earth Sci. 55, 236–244. Warke, P.A., 2007. Complex weathering in drylands: implications of ‘stress’ history for rock debris breakdown and sediment release. Geomorphology 85, 30–48. Xu, Z.Y., 2007. Rock Mechanics. China Water Conservancy and Hydropower Press, Beijing (in Chinese). Zhang, D., Chen, A.Q., Su, R.B., Duan, H.P., Liu, G.C., 2013b. Effect of hydrothermal environment on distintegration of different purple parent rocks. Acta Pedol. Sin. 50, 7–15 (in Chinese). Zhang, D., Chen, A.Q., Xiong, D.H., Liu, G.C., 2013a. Effect of moisture and temperature conditions on the decay rate of a purple mudstone in southwestern China. Geomorphology 182, 125–132. Zhang, D., Chen, A.Q., Liu, G.C., 2012. Laboratory investigation of disintegration characteristics of purple mudstone under different hydrothermal conditions. J. Mt. Sci. 9, 127–136. Zhao, X.G., 2003. A comparison of two methods for determining densities of rocks and minerals. Geophys. Geochem. Explor. 27, 202–205 (in Chinese). Zhu, B., Wang, T., You, X., Gao, M.R., 2008. Nutrient release from weathering of purplish rocks in the Sichuan Basin, China. Pedosphere 18, 257–264.