Muhamma Usman Azhar, Hui Zhou,*, Fanjie Yang, Anan Younis, Xinjing Lu,Houguo Fang, Yijun Geng

a State Key Laboratory of Geomechanics and Geotechnical Engineering,Institute of Rock and Soil Mechanics,Chinese Academy of Sciences, Wuhan,430071,China

b University of Chinese Academy of Sciences, Beijing,100049, China

c Department of Geoscience, University of Calgary, Calgary, Canada

d Yellow River Engineering Consulting Co., Ltd., Zhengzhou, 450003, China

Abstract The strength of clay-rich sandstone decreases significantly when in contact with water due to softening effects. This scenario can pose a severe threat to the stability of water diversion tunnels during construction and operation periods. To address the issues related to water-induced softening in clay-rich sandstone zones in a water diversion tunnel of Lanzhou Water Supply Project, the microscopic and micromechanical variations of rocks due to increasing water content in two different zones i.e. zones A and B, were determined by various testing methods, such as X-ray diffraction (XRD), scanning electron microscopy (SEM), thin section microscopy, micro-indentation test, sonic velocity test, and slake durability test. The microscopic analysis confirms the presence of montmorillonite mineral which is the dominant problematic geomaterial in engineering application. The integrity and durability of clay-rich sandstone were determined with sonic velocity and slake durability tests to calibrate the results obtained by the micro-indentation test. It shows that the elastic modulus and hardness of clay-rich sandstone decrease with the increase of saturation time, up to 144 h, which is more significant and rapid during early stage of saturation.After 144 h of saturation,the elastic modulus decreases by 89%and 97%, and the hardness decreases by 89% and 99% for zones A and B sandstones,respectively. The results of slake durability and sonic velocity indicate that zone A sandstone remains 56.19%durability after 144 h of saturation, while zone B sandstone loses its durability merely after 72 h of saturation. The clay-rich sandstone starts to dissolve in water when the saturation time exceeds 144 h. The significant decreases in strength and durability of clay-rich sandstone due to water-induced softening are serious threats to tunnel stability. The improvements in the strength of surrounding rock mass by grouting and permeability by installation of drainage galleries can reduce the damage caused by water-induced softening.

2020 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords:Clay-rich sandstone Microscopic properties Micro-indentation test Elastic modulus Slake durability index

1. Introduction

There is a great need of trade-off in water resources in China due to the uneven distribution of water resources in the North and the South. For this, more and more long-distance and deep water tunnels are constructed or under construction at present, where weak and/or soft rock layers are frequently encountered.However,water seepage has an unfavorable effect on the physico-mechanical properties of the soft rocks, which poses a great challenge to the long-term stability of water tunnels and lining structures.

Sandstone is widely distributed in China and is frequently reported during construction of underground projects. In this case,understanding the mechanical behaviors of sandstone subjected to external factors such as moisture and temperature is of significant importance. Various experimental studies showed the significant reductions of uniaxial compressive strength (UCS) (Colback and Wild,1965; Shakoor and Barefield, 2009; Duda and Renner, 2013;Aydan et al., 2014), shear strength, and tensile strength (Wong and Jong, 2014) of saturated sandstones. For example, 80%decreases in stiffness and strength are estimated for clay-bearing rocks when subjected to saturation (Vasarhelyi and Van, 2006;Agustawijaya, 2007; Erguler and Ulusay, 2009a; Cherblanc et al.,2016; Vergara and Triantafyllidis, 2016; Wong et al., 2016). In the presence of water, the cementing material and clay minerals associated with weak minerals surrounding rigid particles will soften,and therefore intergranular cracking is expected to occur in wet samples (Hawkins and McConnell, 1992). Extensive experimental studies have been carried out on different rocks,in order to determine the influence of saturation on rock mechanical properties (McLamore and Gray,1967; Yilmaz, 2010; Zhang et al., 2014;Cherblanc et al., 2016; Liu and Liu, 2016; Liu and Zhang, 2017).For instance, Zhang et al. (2014) conducted triaxial compression tests on sandstone under different degrees of saturation, and analyzed the influences of saturation and confining pressure on the rock mechanical parameters such as elastic modulus and peak strength. Liu and Liu (2016) conducted uniaxial and triaxial compression tests on gray-colored fine-grained sandstone sampled from a deep tunnel under natural and saturated states, and found that water has a significant effect on strength, deformation and strain energy of the sandstone.

The uniaxial and triaxial compression tests are widely used in the literature to determine the mechanical parameters of soils and rocks, but they are basically expensive and time-consuming in terms of sample preparation. Due to these limitations, nondestructive micro-indentation test was developed by Pharr and Oliver (1992), and then refined (Oliver and Pharr, 2004). In this test,the load is applied by various types of indenters and the loade displacement curves are obtained during loading and unloading which are used to calculate the mechanical parameters such as elastic modulus and hardness.Constantinides et al.(2006)and Ulm et al.(2007)developed the statistical method,i.e.grid indentation,in which histograms of hardness and elastic modulus measured by indenters are used to determine the mechanical parameters of inhomogeneous material. A rich literature is available on rock indentation which provides qualitative description of the indentation process and proposes mathematical models of material behavior under the indenters(e.g.Leite and Ferland,2001;Mateus et al., 2007; Krakowiak et al., 2011; Chen et al., 2015a,b; Liu et al.,2016; Auvray et al., 2017). These researchers obtained various indices from the indentation tests and established several correlations with mechanical properties of material,such as compressive strength, Young’s modulus and friction. For example,Mateus et al.(2007)carried out 250 indentation tests on Colombian sandstones to determine indentation modulus(IM)and critical transition force(CTF),and correlated these mechanical parameters with UCS.Leite and Ferland (2001) obtained the Young’s modulus and UCS of a porous material by indentation test and conventional compression test, and their results agreed well with each other. Auvray et al.(2017) performed nano-/micro-indentation tests and mesocompression tests on claystone samples to compare modulus of deformability at different scales. The above investigations show that the micro-indentation test is capable to measure the mechanical properties of rocks.

The engineering properties of rocks are a function of their mineralogical composition, fiber content and texture. X-ray diffraction (XRD) based on constructive interference of monochromatic X-rays is a conventional technique to identify and quantify the composition of the material(Drits and Tchoubar,1990;Epp,2016). The microscopy techniques such as optical microscopy(Murphy, 2001) and scanning electron microscopy (SEM) provide information about the composition, surface morphology, texture and overall spatial distribution of the material.In recent years,SEM is widely used in engineering geology to detect intrinsic flaws and understand deformation mechanism in rocks at micro-scale,which helps to understand the failure in rocks and soils (Chen et al.,2015a,b). Sun et al. (2017) presented a comprehensive review of relationship between mechanical properties and microscopic characteristics of different rocks.The influences of mineral content and grain size on mechanical properties of rocks have been summarized by regression equations.

The durability of rocks decreases due to the mechanical action of water. Franklin and Chandra (1972) proposed the slake durability test method to assess the resistance of a rock sample to disintegration and weakening when subjected to one or several cycles of drying and wetting.Many researchers(e.g.Dick and Shakoor,1995;Koncagul and Santi,1999; Dhakal et al., 2002; Gupta and Ahmed,2007; Erguler and Ulusay, 2009b) also performed slake durability tests on different rocks to quantify the water-induced softening effect on the mechanical properties of rocks. In addition, in conjunction with the micro-indentation test, the slake durability test is used to estimate the degradation of clay-rich sandstone.

Nevertheless, most researches on the water-induced softening properties of rocks merely focused on the extreme water-bearing states, such as dry, natural and saturated. Also, there are few studies available on the mechanical properties of clay-rich sandstones under different degrees of saturation. Therefore, this study aims to investigate microscopic and micromechanical variations of clay-rich sandstone at various degrees of saturation. The clay-rich sandstone sampled from a water diversion tunnel project was used. The sandstone is varied in color, grain size, and mineral composition; and it can be roughly classified into two zones, i.e.zones A and B. The sandstone in zone A is composed of poorly sorted sub-angular to sub-rounded coarse grains,and sandstone in zone B contains unsorted fine grains.The porosities of sandstone in zones A and B are 6% and 11%, respectively. The microscopic analysis was carried out at each saturation level to record the variations in physical properties of the sandstone.Micro-indentation test was conducted to obtain the mechanical parameters of clay-rich sandstone at different water contents. Additionally, sonic velocity and slake durability tests were performed to determine the integrity and durability of all the samples at various degrees of saturation.

2. Experimental set-up

A detailed experimental scheme was made to determine the decreases in strength and durability of clay-rich sandstone sampled from a water diversion tunnel, including microscopic analysis,micro-indentation test,slake durability test and sonic velocity test.Table 1 presents the experimental set-up for this study.

2.1. Brief description of the test

The project area is located near Lanzhou, Gansu Province,northwest of China (Fig. 1). The tunnel under construction from Liujiaxia Reservoir to Lanzhou is 31.29 km long,600e1000 m deep and 5.46 m in diameter, which is the main part of the Lanzhou Water Supply Project.The geological units along the tunnel profile include quartz schist(metamorphic),granite and diorite(igneous),sandstone with interbedded clay and glutenite(sedimentary), and andesite (metamorphic). Fig. 2 shows the geological profile of the tunnel and the positions of zones A and B.

In this project, the clay-rich sandstones observed in different locations have various water-bearing conditions. To determine a reasonable scheme of saturation time, the water content test is carried out on the clay-rich sandstone sampled from zones A and B under multiple sets of saturation times before the main experimental campaign begins. The test results of water content versus saturation time are shown in Fig. 3.

Table 1Experimental set-up for this study.

The water contents of the clay-rich sandstones in zones A and B in natural state are 0.69%and 1.05%,respectively.After saturation of 48 h, 96 h,144 h and 196 h, they are 1.73%, 2.24%, 2.28%, 2.3% and 2.94%, 4.37%, 4.58%, 4.61%, respectively. The effect of saturation on zone B sandstone is stronger as compared to zone A sandstone. It can be seen that during early stage of saturation, the effect of saturation time on water content is more significant,as the growth rate of water content decreases with elapsed saturation time.After 144 h of saturation,the changes in water content of zones A and B sandstones gradually become stable, and no significant change with the increase of saturation time is observed. At this time, the water contents of zones A and B sandstones are 2.28% and 4.58%,respectively, which are close to saturated water contents (2.35%and 4.65%).Therefore,the clay-rich sandstone can be considered as saturated after 144 h of saturation for both zones. Moreover, after 144 h of saturation, the clay-rich sandstone begins to disintegrate with water, and thus it is difficult to carry out mechanical test on complete rock sample.Therefore,the water-induced softening test results of the clay-rich sandstone within 144 h of saturation can reflect the change of mechanical properties of the clay-rich sandstone from natural to saturated state. In total, seven saturation scenarios(0 h,12 h,24 h,48 h,72 h,96 h and 144 h)are considered with respect to the microscopic analysis and micromechanical experiments.

Fig.1. Location map of Lanzhou Water Supply Project.

2.2. Sample preparation

The rock picking machine was used to extract rock core samples from the project site, which were wrapped with plastic film and then carefully transported to the laboratory to avoid water loss.Unfortunately, the samples from zone A were broken from the edges during the cutting process mainly due to large grain size and poor cementation, which caused difficulty in preparation of standard cylindrical samples. However, for the micro-indentation test,the sample requirements were satisfied. The disk-shaped samples with 20e30 mm thickness were prepared and polished following ASTM E-3(1995).At last,the prepared rock samples were wrapped with plastic film, as shown in Fig. 4.

2.3. Micro-indentation test

Fig. 2. Geological profile of the tunnel.

The micro-indentation tester CB500 manufactured by NANOVEA, United States is shown in Fig. 5. It is equipped with an automated stage, which can move 100 mm and 50 mm inx- andydirections, respectively, and 50 mm inz-direction, and a video zoom microscope.Diamond cone Vickers indenter is used to obtain the loadedisplacement data by controlling the maximum load and loading speed. The results of elastic modulus and hardness obtained from the loadedisplacement data can be automatically compared with those of traditional hardness testers.Depth sensors are incorporated in the tester to provide faster, more accurate and repeatable results even at lower loads(Zhu and Bartos,2000).The maximum load of the machine is 300 N at micro-Newton rate by a servomotor loading system,and the depth sensors can measure the penetration depth up to 300 mm.In this test,to determine a suitable load for the experiment,a test sample was run under loads of 10 N,20 N,30 N,50 N and 70 N.The corresponding microscopic images showed the presence of cracks when the loads were 20 N, 30 N,50 N and 70 N.After careful calibration,the load of 10 N with a rate of 30 mm/s was selected for the experiment.As the water content of clay-rich sandstone increased, the depth of indentation increased beyond the maximum limit of the machine. Therefore, the maximum load at higher degrees of saturation was adjusted to 7.5 N for zone A sandstones and 5 N for zone B sandstones,respectively.

Fig.3. Variations of water content with saturation time for zones A and B sandstones.

Fig. 4. Samples prepared for testing.

2.4. Slake durability and sonic velocity tests

Rocks will undergo weathering in the presence of water,which mainly depends on rock type, cementing material, porosity and permeability.The sandstone containing some soluble minerals(e.g.carbonates),cementing materials(e.g.calcite and zeolite)and weak lamination planes is more susceptible to water-induced softening.The strength (e.g. elastic modulus) of saturated rocks decreases noticeably due to water-induced weathering(softening).Therefore,slake durability and sonic velocity tests were performed in this study to quantify the effect of mechanical softening on clay-rich sandstone at different degrees of saturation. In the slake durability test,rock lumps of approximately 500 g(10 pieces)were put in a drum(100 mm in length and 140 mm in diameter).The outer walls of the drum are made of sieve mesh with an opening of 2 mm.The drum was subjected to the rotation for 10 min at a speed of 20 revolutions per minute(RPM)in a water bath and the mass loss of rock lumps was determined after being dried in an oven overnight at a temperature of 100C.

In the sonic velocity test,the time of elastic wave propagation in intact rock sample was recorded to calculate sonic velocity,which is related to the strength of rock material. The P-wave velocity was used to predict rock strength, deformation and degree of weathering.Several researchers(e.g.Kahraman,2001;Yasar and Erdogan,2004; Yagiz, 2011) reported that the P-wave velocity has a relationship with rock properties, such as hardness, UCS, and slake durability index. The P-wave velocity test was performed on the fresh samples first, and then the same samples were tested again after saturation to record the difference in sonic velocity.

3. Test results and analysis

3.1. Microscopic analysis

In this study, three petrographic techniques were used to examine the samples at different scales.Cross-polarized light(XPL)and plane-polarized light(PPL)were combined for identification of framework grain mineral and observation of spatial distribution of mineralogy. Bulk XRD analysis was utilized to estimate the rock composition quantitatively.Due to the limited magnification of the petrographic microscope, it is unlikely to identify the clays, microns,and sub-micron size particles.Bulk XRD analysis is one of the most reliable methods to quantify the rock mineralogy, especially when clay minerals are presented. SEM images and observations are helpful in understanding the overall morphology and changes in clay minerals at higher magnification.

Fig. 5. Different components of micro-indentation tester CB500.

Table 2Mineralogical composition (%) of sandstone based on XRD results.

For the samples STA-01 to STA-07 from zone A,they are mainly composed of matrix-rich (15%e20%), well-cemented and wellcompacted lithicefeldspathic wacke. Quartz, feldspars and lithic fragments form the general framework of the rock. Calcite is the major cementing material presented in intergranular pores. XRD analysis results(see Table 2)reveal the presence of illite,muscovite,kaolinite and montmorillonite as matrix minerals.Matrix clays are generally observed within the fine-grained matrix (between framework grains),but could not be identified petrographically due to its extremely small size. Fig. 6 shows the cross- and planepolarized optical images of zone A sandstone in natural state and after 144 h of saturation.

Fig. 6. Optical microscope images of zone A sandstone: (a) STA-01, cross-polarized in natural state; (b) STA-01, plane-polarized in natural state; (c) STA-07, cross-polarized after 144 h of saturation; and (d) STA-07, plane-polarized after 144 h of saturation.

Fig. 7. Optical microscope images of zone B sandstone: (a) STB-01, cross-polarized in natural state; (b) STB-01, plane-polarized in natural state; (c) STB-07, cross-polarized after 144 h of saturation; and (d) STB-07, plane-polarized after 144 h of saturation.

For the samples STB-01 to STB-07 from zone B, the overall mineralogy is similar to the samples from zone A with a higher amount of clay minerals.In terms of fabric, STB is classified as feldspathic lithic wacke due to higher percentage of lithic rock fragments. The cross- and plane-polarized images of zone B sandstone in natural state and after 144 h of saturation are shown in Fig.7.

Fig. 8. SEM images of sandstone: (a) STA-01 in natural state, (b) STA-07 after 144 h of saturation, (c) STB-01 in natural state, and (d) STB-07 after 144 h of saturation.

Fig.9. Frequency distributions of elastic modulus and representative loadedisplacement curve for zone A sandstone after saturation of(a)0 h,(b)12 h,(c)24 h,(d)48 h,(e)72 h,(f)96 h, and (g) 144 h. STD denotes the standard deviation.

Both zones A and B have undergone a higher degree of mechanical compaction which may result in grain fracturing. Such grain fracturing usually indicates the calcite cementation in the late stage. Another typical example of late-stage cementation is the presence of calcite veins in the quartz,calcite and feldspar grains,as shown in the STA-01 and STB-01 cross-polarized images. The higher amount of feldspar minerals is presented in both zones(Table 2). Feldspars and sericite are generally prone to dissolution due to water flushing. During diagenesis, feldspars are generally first dissolved by meteoric water flushing and change to the delicate kaolinite or smectite minerals.Due to its softening,delicate structure, and high cation exchange capacity, the clay minerals have been categorized as problematic materials during construction. Montmorillonite clays are the most troublesome mineral presented in the samples due to its strong swelling property.In the presence of interlayer water and loosely bounded crystal structure,water can enter the crystalline structure of the montmorillonite clay easily. It can expand when absorbing water or moisture and would shrink at high temperature. Norrish (1954) explained the swelling phenomenon in detail,depending on the type of interlayer cation. Interlayer space of montmorillonite clay is normally occupied by a monovalent Sodium cation (Nat) or a divalent Calcium cation(Ca2t).Na-montmorillonite swelling capacity is significantly higher than that of Ca-montmorillonite due to the strong packing of Ca2tion in the interlayer structure. When exposing to water, Namontmorillonite can expand 10e100 times than their original size, which would exert immense pressure on the framework grains and can result in severe damage to the rock.

Fig.10. Frequency distributions of elastic modulus and representative loadedisplacement curve for zone B sandstone after saturation of(a)0 h,(b)12 h,(c)24 h,(d)48 h,(e)72 h,(f) 96 h, and (g) 144 h.

Table 3Micro-indentation test results of zone A sandstone.

Table 4Micro-indentation test results of zone B sandstone.

The SEM images show significant variations in surface texture and morphology of zones A and B sandstones in natural state and after 144 h of saturation, as shown in Fig. 8. In natural state, the sandstone has a very compact surface morphology,but after 144 h of saturation,the pores and micro-fractures in sandstone open up.This occurs mainly because water removes the loose particles and dissolves the minerals like feldspars and calcite. It this study, no effort has been made to quantify the dissolution of the feldspar minerals based on the microscopic observations. In the future,water analysis of the saturated rocks, including elemental analysis and pH analysis of water, would be performed to compare the dissolution of the feldspars and other minerals presented in sandstone.

Fig.11. Relationship between elastic modulus and saturation time.

Table 5Fitting for test results of elastic modulus and hardness with saturation time.

3.2. Modulus of elasticity

Micro-indentation test was carried out on all the samples that have a flat and polished surface according to the guidelines of ASTM E384-17 (2017). A distance of 2 mm was kept from the edges and between two adjacent points, and about 100 readings were recorded for each sample to minimize the effect of inhomogeneity. The test results were analyzed using statistical technique, i.e. grid indentation method.Figs.9 and 10 show the frequency histogram,normal distribution curve(red),cumulative frequency curve(blue)and representative loadedisplacement curve for elastic modulus of zones A and B sandstones at each saturation level,respectively,and the results are also listed in Tables 3 and 4. In statistics, frequency histogram, normal distribution curve and cumulative frequency curve are used to show the frequent occurrence of the data. The normal distribution curve forms a symmetrical bell-shaped frequency curve which represents scattering of the data around the mean. The shape of a normal distribution curve is determined by the mean and the standard deviation. The steeper the bell curve,the smaller the standard deviation. In both zones, the value of standard deviation decreases with the increase of saturation time,which means the test results are less scattered at higher saturation.At each degree of saturation,a representative load-depth curve for both zones is also presented, from which a gradual increase in indentation depth is noted with the increase of saturation time.

It can be seen from Table 3 that the mean elastic moduli of the zone A sandstone in natural state and after 12 h, 24 h, 48 h, 72 h,96 h and 144 h of saturation are 11.23 GPa,9 GPa,7.57 GPa,5.73 GPa,4.79 GPa, 3.96 GPa and 1.27 GPa, respectively. This means that the elastic modulus is reduced by about 88.69% after 144 h of saturation.The depth of indenter also increases gradually from 24.33 mm in natural state to 146.36 mm after 144 h of saturation,suggesting an apparent softening behavior.

Also, one can see from Table 4 that, for zone B sandstone, the mean elastic moduli in natural state and after 12 h,24 h,48 h,72 h,96 h and 144 h of saturation are 17.09 GPa, 9.94 GPa, 6.8 GPa,4.75 GPa,1.81 GPa,1 GPa and 0.53 GPa, respectively. This suggests that the elastic modulus is reduced by about 96.9% after 144 h of saturation. The indenter depth increases from 29.98 mm in natural state to 152.18 mm after 144 h of saturation.

The relationship between elastic modulus and saturation time shows a steady decreasing trend with the increase of saturation time for zone A sandstone(Fig.11).For zone B sandstone,however,during early stage of saturation, the elastic modulus decreases rapidly as compared to zone A sandstone. For example, for zone A samples, 20% reduction in elastic modulus occurs after 12 h of saturation and 33% reduction is noted after 24 h of saturation(Table 3); while for zone B samples, 42% and 60% reductions in elastic modulus occur after 12 h and 24 h of saturation,respectively(Table 4).Moreover,after 144 h of saturation,the elastic moduli are reduced by 89% and 97% for zones A and B samples, respectively,which shows that the zone B sandstone is more vulnerable to water-induced softening than zone A sandstone.

Fig.12. Frequency distributions of hardness for zone A sandstone after saturation of (a) 0 h, (b) 12 h, (c) 24 h, (d) 48 h, (e) 72 h, (f) 96 h and (g) 144 h.

The relationship between elastic modulus and saturation time was analyzed by the least squares method and the results are tabulated in Table 5. It can be seen from this table that the elastic modulus of both zones A and B sandstones has a negative exponential relation with the saturation time.

3.3. Hardness

Hardness is the measure of resistance offered by a material to permanent indentation or plastic flow. The micro-indentation tester CB500 can automatically measure the hardness along with the elastic modulus. In this study, a suitable force applied on the polished surface with the help of indenter for a specific time interval and depth of indention was measured to calculate the hardness. Sufficient readings of hardness for all the samples were measured and statistically analyzed.

The hardness results of zones A and B sandstones after 0 h,12 h,24 h,48 h,72 h,96 h and 144 h of saturation are shown in Figs.12 and 13.The mean hardness values of zone A sandstone are 0.65 GPa in natural state and 0.07 GPa after 144 h of saturation; while for zone B sandstone,the hardness values are 0.85 GPa and 0.01 GPa in natural state and 144 h of saturation, respectively. A noticeable decrease in hardness was observed in both zones with increasing saturation time. After 144 h of saturation, 89% and 99% reductions of hardness were observed in zones A and B, respectively.

Comparison of hardness after saturation in both zones is presented in Fig. 14. Although the zone B sandstone has a higher hardness value as compared to zone A sandstone in natural state,it decreases more rapidly with increasing saturation time. The hardness value of zone A sandstone becomes relatively stable after 12e96 h of saturation and then decreases to the lowest value at 144 h of saturation. On the other hand, the hardness of zone B sandstone shows a consistent decrease with increase in saturation time.

Again,one can see from Table 5 that the hardness of sandstones from both zones decreases, showing a negative exponential relation with saturation time.

3.4. Slake durability

Durability is the resistance to disintegration when rocks are in contact with water.In this study,the rock slake durability test was performed on rock lumps sampled from zones A and B in natural state and after each degree of saturation according to ASTM D4644-08(2008),as listed in Table 6.For zone A sandstone,they show high durability in natural state,but decrease steadily with the increase of saturation time. For zone B sandstone, they are highly durable in natural state and remain fairly durable even after 24 h of saturation;but after 72 h of saturation,it starts to dissolve in water.Since the zone A sandstone has larger grain size, it remains 56.19%durability after 144 h of saturation,while the zone B sandstone has fine grains and shows no durability after 72 h, 96 h and 144 h of saturation. A graphical comparison of slake durability index for zones A and B sandstones is shown in Fig.15 and the fitting results show a negative linear relation with saturation time in both zones(Table 7).

Fig.13. Frequency distributions of hardness for zone B sandstone after saturation of (a) 0 h, (b) 12 h, (c) 24 h, (d) 48 h, (e) 72 h, (f) 96 h and (g) 144 h.

3.5. Sonic velocity

Fig.14. Relationship between hardness and saturation time.

Table 6Slake durability index of clay-rich sandstone.

Fig.15. Relationship between slake durability index and saturation time.

Table 7Fitting for tests results of slake durability index and saturation time.

The P-wave velocity test was performed on all the samples before and after each degree of saturation, in order to determine the degradation of sandstone with increase of water content, and the results are shown in Fig.16. The average P-wave velocities of zones A and B sandstones in natural state range from 1500 m/s to 2005 m/s and from 1774 m/s to 2737 m/s,respectively.The P-wave velocity decreases with the increase in saturation time for the samples from both zones. The difference in P-wave velocity in natural state and after saturation is smaller for zone A samples compared to that for zone B samples.Approximately,a difference of 320 m/s is noticed for zone A samples after 144 h of saturation;while for zone B samples,the difference in P-wave velocity reaches to approximately 1400 m/s after 72 h of saturation and remains the same till 144 h of saturation. The P-wave velocity results suggest that the durability of zone B sandstone drops significantly after 72 h of saturation, and zone A sandstone remains fairly durable even after 144 h of saturation.The same trend was reflected in the slake durability test. A comparison of P-wave velocity (difference) with saturation time for zones A and B sandstones is presented in Fig.17,and a positive linear fitting relation exists between P-wave velocity(difference) and saturation time (Table 8).

4. Discussion

The reduction in overall strength and durability of sandstone is inevitable due to saturation. Progressive decreases in elastic modulus and hardness were observed in clay-rich sandstone of two different zones from a water diversion tunnel. The results indicate that after 144 h of saturation,the elastic modulus decreases by 89%and 97%, and the hardness decreases by 89% and 99% for zones A and B sandstones, respectively. The decrease in elastic modulus is relatively higher in both zones as compared to previous studies(Hawkins and McConnell, 1992; Vasarhelyi, 2003; Agustawijaya,2007). Furthermore, the reduction in elastic modulus is more rapid for zone B sandstone during early stages of saturation. One can see from Figs. 11 and 14 that the influences of saturation on elastic modulus and harness for zone B sandstone are more evident compared to zone A sandstone. The slake durability and sonic velocity test results(Figs.15 and 17)show that the zone B sandstone loses durability after 72 h of saturation, while zone A sandstone remains fairly durable even after 144 h of saturation. Higher reductions in strength and durability for zone B sandstone are simply due to its higher water content.Both elastic modulus and hardness have negative exponential relations with saturation time, and the high correlation coefficients verify the significance of the relations.Therefore, the obtained results can be used to develop theoretical and numerical models to describe mechanical behavior of clay-rich sandstone under different saturation times.

Various processes such as pore pressure increase, frictional reduction, fracture energy reduction, capillary tension decrease,and chemical deterioration are responsible for decrease in rock strength. There are two types of bonds existing in rocks, i.e. temporary depositional bonds and diagenetic bonds. The depositional bonds may vanish during wetting and reappear at drying, while diagenetic bonds during dissolution lead to long-term irreversible weakening of the rock (Ciantia et al., 2014, 2015). The rapid decrease during early stage of saturation can be explained by the existence of short-term depositional bonds. The absorption of water in rocks mainly depend on porosity which has a relation with grain size, shape and packing. Generally, porosity increases as the grain size decreases, because the spherical-shaped deformation of sand grains increases with the decrease in particle size.The higher reduction in strength of zone B sandstone compared to zone A sandstone is due to the differences in porosity and grain size.

Fig.16. Comparison of P-wave velocity of zones A and B samples in dry and saturated states.

Fig.17. Relationship between P-wave velocity (difference) and saturation time.

Table 8Fitting for P-wave velocity (difference) and saturation time.

The clay fraction in rocks plays a key role in the softening of clayrich sandstone due to high hygroscopicity and presence of free water at larger water content.The microscopic analysis reveals that the percentage of minerals is not identical in both zones,and higher percentage of problematic montmorillonite mineral is presented in zone B sandstone. The loosely bonded structure of individual montmorillonite minerals and presence of cations with stronger hydration ability are the main reasons for swelling of montmorillonite.The swelling of montmorillonite can generate extra pressure on the tunnel lining which is a serious threat to the stability of tunnel. In the future, swelling pressure test should be proposed to determine the swelling pressure in the clay-rich sandstone. Also,more sophisticated tests will be carried out to quantify the dissolution of feldspars which can reduce the strength of clay-rich sandstone.

The significant decreases in strength and durability of sandstone in both zones indicate that water-induced degradation should be considered in the design of tunnel lining. Due to serious deterioration of mechanical properties of clay-rich sandstones after contacting with water, the damage can be reduced by improving the cohesion and permeability of surrounding rock mass in the tunnel.The installation of drainage galleries and grouting reinforcement are proposed in clay-rich sandstone sections of the tunnel.

5. Conclusions

To better understand the water-induced softening behavior of clay-rich sandstone in a water diversion tunnel of Lanzhou Water Supply Project, the microscopic analysis, micro-indentation test,slake durability test and sonic velocity test were conducted under different saturation times.The main conclusions are summarized as follows:

(1) The effect of saturation is more significant for zone B sandstone as compared to zone A sandstone. After 144 h of saturation, the water contents in zones A and B sandstones are 2.28% and 4.58%, respectively. The microscopic analysis shows a considerable presence of montmorillonite mineral which is the most problematic mineral in engineering application. The SEM images show the variation in surface morphology of the sandstone due to saturation effect.

(2) The elastic modulus of zone B sandstone in natural state is higher than that of zone A sandstone, but after 144 h of saturation,the elastic moduli are reduced by 89%and 97%for zones A and B sandstones, respectively. Furthermore,reduction in mean elastic modulus is more rapid for zone B sandstone than for zone A sandstone during early stage of saturation.After 144 h of saturation,the hardness reductions of zones A and B sandstones are 89% and 99%, respectively.The elastic modulus and hardness decrease with the increase of saturation time, showing a negative exponential relationship.

(3) According to slake durability and sonic velocity test results,the zone B sandstone loses its durability after 72 h of saturation, while zone A sandstone remains fairly durable even after 144 h of saturation.

(4) In the presence of water, the temporary depositional bonds vanish,resulting in a rapid decrease in strength during early stage of saturation, and long-term processes such as dissolution weaken the digenetic bonds. The swelling nature of montmorillonite mineral exerts immense pressure on the framework grains and causes severe damage to the rock.

In natural state, the zone B sandstone is stronger and more durable as compared to zone A sandstone. There is a noticeable decrease in mechanical properties of sandstone in both zones with increasing saturation time.However,the zone B sandstone is more vulnerable to saturation. The results provide basic mechanical parameters under the influence of saturation and specific guidance to analyze the stability of surrounding rock mass during tunnel construction in high groundwater areas.

Declaration of competing interest

The authors wish to confirm that there are no known conflicts of interests associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.

Acknowledgments

Muhammad Usman Azhar would like to thank the Chinese Academy of Sciences and the World Academy of Sciences (CASTWAS)for the Fellowship.This work was supported by the National Key R&D Program of China (Grant Nos. 2018YFC0809601 and 2018YFC0809600), Key projects of the Yalong River Joint Fund of the National Natural Science Foundation of China (Grant No.U1865203), and Hubei Province Natural Science Foundation Innovation Group(Grant No.2018CFA013).