Sand- and Clay-Photocured-Geomembrane Interface Shear Characteristics Using Direct Shear Test

sustainability

Article Sand- and Clay-Photocured-Geomembrane Interface Shear Characteristics Using Direct Shear Test

Lihua Li, Han Yan, Henglin Xiao *, Wentao Li and Zhangshuai Geng

School of Civil Engineering, Architecture and Environment, Hubei University of Technology, Wuhan 430068, China; [email protected] (L.L.); [email protected] (H.Y.); [email protected] (W.L.); [email protected] (Z.G.) * Correspondence: [email protected]

Abstract: It is well known that geomembranes frequently and easily fail at the seams, which has been a ubiquitous problem in various applications. To avoid the failure of geomembrane at the seams, photocuring was carried out with 1~5% photoinitiator and 2% carbon black powder. This geomembrane can be sprayed and cured on the soil surface. The obtained geomembrane was then used as a barrier, separator, or reinforcement. In this study, the direct shear tests were carried out with the aim to investigate the interfacial characteristics of photocured geomembrane–clay/sand. The results show that a 2% photoinitiator has a signiﬁcant effect on the impermeable layer for the photocured geomembrane–clay interface. As for the photocured geomembrane–sand interface, it is reasonable to choose a geomembrane made from a 4% photoinitiator at the boundary of the drainage layer and the impermeable layer in the landﬁll. In the cover system, it is reasonable to choose a 5% photoinitiator geomembrane. Moreover, as for the interface between the photocurable geomembrane

and clay/sand, the friction coefﬁcient increases initially and decreases afterward with the increase of normal stress. Furthermore, the friction angle of the interface between photocurable geomembrane

Citation: Li, L.; Yan, H.; Xiao, H.; Li, and sand is larger than that of the photocurable geomembrane–clay interface. In other words, the W.; Geng, Z. Sand- and interface between photocurable geomembrane and sand has better shear and tensile crack resistance. Clay-Photocured-Geomembrane Interface Shear Characteristics Using Keywords: photocurable geomembrane; direct shear test; shear strength; friction coefficient Direct Shear Test. Sustainability 2021, 13, 8201. https://doi.org/10.3390/ su13158201 1. Introduction Academic Editor: Guang-Liang Feng Existing geomembranes are mainly made from polyethylene (PE), polyester (PET), polypropylene (PP), and polyvinyl chloride (PVC), which can be thermally and mechani- Received: 23 April 2021 cally processed to form a continuous film [1]. Geomembrane is an excellent drainage barrier Accepted: 12 July 2021 Published: 22 July 2021 and has been widely used in landfills, water channels, detention ponds, and reservoirs [2]. However, since the geomembrane is made into fixed sizes in the processing, it is necessary

Publisher’s Note: MDPI stays neutral to cut and weld the geomembrane to make it fit into a large area of soils at the construction with regard to jurisdictional claims in site [3]. Due to the difference of interface characteristics between soil and geomembrane, published maps and institutional affil- the welding and joint between geomembrane plates are prone to failure, which accounts for iations. 20% of various failure modes of geomembrane [4]. Therefore, it is essential to investigate some novel geomembranes to match the reliable requirements. Photocured geomembrane is a new impermeable material with excellent mechanical and hydraulic properties. This geomembrane can be formed through the photocuring chain reaction between the acrylic monomer and the photoinitiator (reaction initiator) in Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. which carbon black powder is added as an ultraviolet (UV) blocker to improve durabil- This article is an open access article ity. More importantly, photocured geomembranes made from proper raw materials are distributed under the terms and environmentally-friendly and are not harmful to human health [5–7]. Therefore, studying conditions of the Creative Commons photocured geomembrane seems to be a popular issue in civil engineering. Attribution (CC BY) license (https:// In the past, many researchers have made great efforts to study the geomembrane. creativecommons.org/licenses/by/ The interface characteristics between geomembrane and clay/sand have always been an 4.0/). important direction in the field of geomembrane research. The composite liner system of

Sustainability 2021, 13, 8201. https://doi.org/10.3390/su13158201 https://www.mdpi.com/journal/sustainability Sustainability 2021, 13, 8201 2 of 16

landfills usually consists of multiple soil–geomembrane interfaces. Fleming et al. [8] found through direct shear tests that the shear mechanism of the interface between sand and the geomembrane is different under different normal stress. Punetha et al. [9] performed microscopic analysis of the geomembrane–sand interface by electron microscopy to explain the reason for the shift in the shear mechanism. Feng et al. [10] analyzed the geomembrane– sand interface by Digital Elevation Model (DEM) software and found that the roughness of the geomembrane surface was an important factor affecting the shear characteristics of the interface. Gao et al. [11] improved the shear strength of the reinforcement-soil interface to a certain extent by adding ribs to the geomembrane. Cen et al. [12] conducted a shear test on high density polyethylene (HDPE) geomembrane and sand with a piece of cyclic shear equipment and found that under different normal stresses, for large shear displacements, the interface friction angle tends toward a stable value. For small shear displacement am- plitudes, the damping ratio is greater than zero and shows a downward trend. When the shear displacement amplitude is large, the damping ratio increases with the increase of the shear displacement amplitude. Rowe et al. [13] studied the changes in tensile properties of HDPE geomembrane in landfills. The results show that setting a proper protective layer on the geomembrane (GMB) can limit the local tensile strain of the geomembrane to less than 3%; choosing a suitable slope angle and geotextile protective layer can limit the GMB strain to less than 2%. Izgin and Wasti [14] studied the influence of sand shape on the interface friction performance of geomembrane and found that the interface friction performance of sand with round particles is better than that of sand with angular particles. Gao and Koerner [15,16] found that the shear strength of the interface between the soil and the geotextile or geomembrane is less than the shear strength of the soil itself. Ling et al. [17] measured the friction coefficient and shear strength of the contact interface between the geomembrane and clay with a 100 mm × 100 mm direct shear apparatus and analyzed the relationship between the shear displacement and the maximum shear stress on the contact interface. Gao et al. [18] studied the interfacial shear strength between HDPE geomembranes of different shapes and sandy soils were investigated by extensive direct shear tests under different normal stresses. The results show that the cohesion increases and the angle of internal friction decreases for strip geomembranes, while all block geomembranes increase significantly. However, they did not study the photocurable polymer. It has been an important direction in the field of geomembrane research to further explore the interface characteristics between geomembrane and clay/sand. Natalia Abramova et al. [19] used a sensor-array-based analysis system to cover the photocured polymer film on the chip, which can realize the automated analysis of serum components. Xiang et al. [20] investigated experimentally and theoretically the dependence of the material properties of photocured polymers on the intensity of the curing light. The change in viscosity with the displacement rate and light intensity is given to show the strong dependence of viscosity on deformation displacement rate and light intensity. The effect of free chains on the mechanical response is also discussed. Kim et al. [21] used photocured film to have strong antibacterial activity against E. coli and staphylococcus aureus, and its antibacterial activity only depends on the concentration of allowable daily intake (ADI) in the polymer coating. At present, there is no relevant report on the research on the interface characteristics of photocured geomembrane and clay/sand. These studies are limited to the study of the photocurable film, and the study of the interface characteristics between the photocurable geomembrane and clay/sand is still blank. To date, there is no study addressing the aforementioned key design issues, thus, there is an urgent need to substantially increase our knowledge about. In view of the above facts, this paper studies the problems of the author’s inadequate research. The interfacial shear characteristics of different UV-cured geomembranes were analyzed, and the interfacial shear force–shear displacement curve and the interfacial peak shear stress and interfacial shear strength friction coefficient of different UV-cured geomembrane were obtained. Therefore, the research results of this paper are of great significance to the practical application of geomembrane. Sustainability 2021, 13, x FOR PEER REVIEW 3 of 17

analyzed, and the interfacial shear force–shear displacement curve and the interfacial peak shear stress and interfacial shear strength friction coefficient of different UV-cured geomembrane were obtained. Therefore, the research results of this paper are of great significance to the practical application of geomembrane.

2. Principle of Photocuring Reaction Photopolymerisation reaction: Photoinitiators absorb light at certain wavelengths. Sustainability 2021, 13, 8201 The absorption of light is accompanied by the release of the radical R•, which can3 react of 16 with the functional group of the acrylic monomer M to form an RM• with a π-bond [22]. The formed RM• acts as a radical, which can continue to bind to other acrylic monomer Ms. Thus, the polymerization reaction can continue until another radical is encountered 2.and Principle the chain of reaction Photocuring is terminated, Reaction forming a long chain of molecules with closed ends. FigurePhotopolymerisation 1 shows the process reaction: and use Photoinitiators of photocured absorb chain light reactions at certain to wavelengths.form photocured The absorptiongeomembrane. of light is accompanied by the release of the radical R•, which can react with the functionalIn this group study, of in the addition acrylic monomer to the monome M to formr and an photoinitiator, RM• with a π-bond carbon [22 black]. The powder formed RMwas• alsoacts asused a radical, as additive which to can block continue ultraviolet to bind radiation to other and acrylic prevent monomer material Ms. Thus,aging. the In polymerizationthis way, the photocurable reaction can geomembrane continue until is another made. After radical the is photocurable encountered geomembrane and the chain reactionis made isin terminated,a large area, forming it is applied a long to chain the tunnel, of molecules tailings with dam, closed and reservoir. ends. Figure 1 shows the processThe complete and use photocuring of photocured reaction chain reactionsprocess is toshown form photocuredin Figure 1. geomembrane.

Figure 1. The process and use of photocured chainchain reactionreaction toto formform photocuredphotocured geomembrane.geomembrane.

3. MaterialsIn this study, and Processes in addition for toMa thenufacturing monomer Photocured and photoinitiator, Geomembrane carbon black powder was also used as additive to block ultraviolet radiation and prevent material aging. In this 3.1. Materials way, the photocurable geomembrane is made. After the photocurable geomembrane is madeIn in this a large paper, area, acrylic it is applied monomers to the and tunnel, carb tailingson black dam, powder and reservoir.are used in the experiment,The as shown complete in photocuringTable 1 and Figure reaction 2. process is shown in Figure1. Acrylic monomers: the acrylic monomers with multiple unsaturated bonds are 3.proposed Materials as andthe Processesbasic reactants for Manufacturing for the photocuring Photocured reaction. Geomembrane Acrylic monomers are ac- 3.1.tually Materials vinyl esters containing carbon–carbon double bonds, one of which is attached to a carbonylIn this carbon. paper, Acrylate acrylic monomers monomers and are carbonused extensively black powder in polymerization are used in the reactions experiment, for asthe shown applications in Table in1 industry,and Figure construction,2. water conservation, and horticulture and have Sustainability 2021, 13, x FOR PEER REVIEWbeen widely used in photocuring because of their low price, harmlessness to4 humansof 17 and

animals, and theTable processability 1. Materials used of the in the product. test. According to statistics, over 90% of the photocuring reactions employed today are based on acrylic monomers [2,22,23]. There- Type of Materials Used in thefore,mW/cm Test the2 ofacrylic UV light monomers emitted Speciﬁc throughare Namerepresentative a UV flat light of widesource applications. to ensure Percentage consistency Range of in- Acrylic monomerstensityCarbon over a large Acrylicblack area. powder: monomers Geomembranes (CH2CHCOOH) typically contain 1% 90% to 3% carbon black PhotoinitiatorpowderIn the [1], experimental which TPO is Photoinitiator used raw as materials, an a (Cdditive22 Hphotoini21P0 to2) obtaintiators inare this selected research. at the 5%percentage of Carbon black1–5%Photoinitiator [2] while High carbon pigment and black light5000 powder source: mesh was carbon In select this blacked paper, at the itpercentage is proposed of 2% to [1]. 3%use The UV added light (200 nm Others 2%

(a) Acrylic monomers (b) Photoinitiators (c) Carbon black powder

FigureFigure 2. 2. TheThe materials materials used used in the in thetest. test.

Table 1. Materials used in the test.

Type of Materials Used in Specific Name Percentage Range the Test Acrylic monomers Acrylic monomers 90% (CH2CHCOOH) Photoinitiator TPO Photoinitiator(C22H21P02) 5% High pigment 5000 mesh carbon Carbon black 3% black Others 2%

3.2. Material Production Procedure Photocured geomembrane test ratios can be found in Table 2. The test was performed at room temperature (22 °C) and the procedure was as follows. The acrylic monomers, 1–5% photoinitiator, and 2% carbon black powder were poured into the measuring cylinder and mixed to form a monomer mixture. Then, the mixer was used to stir the mixture at 400 rpm for 30 min to ensure that the mixture fully uniform. The ultraviolet lamp was placed 3 cm away from the special round vessel. In this study, the UV wavelength was used to stimulate the UV curing reaction is 360 nm. It is emitted by a 250 W UV lamp. The mixture was then poured into a special round vessel. The UV lamp was turned on to irradiate the mixed liquid stimulated luminescence curing chain reaction, and the photocured geomembrane is generated after several hours, as shown in Figure 3. The light-cured geomembrane was taken out and the cutter was used to cut it into the desired shape. Then, the direct shear test of geomembrane and filler was directly carried out.

Table 2. Clay properties.

Sample Photoinitiator Content Acrylic Monomers Carbon Black Content (%) Number (%) (%) 1 2 1 97 2 2 2 96 3 2 3 95 Sustainability 2021, 13, 8201 4 of 16

Acrylic monomers: the acrylic monomers with multiple unsaturated bonds are proposed as the basic reactants for the photocuring reaction. Acrylic monomers are actually vinyl esters containing carbon–carbon double bonds, one of which is attached to a car- bonyl carbon. Acrylate monomers are used extensively in polymerization reactions for the applications in industry, construction, water conservation, and horticulture and have been widely used in photocuring because of their low price, harmlessness to humans and animals, and the processability of the product. According to statistics, over 90% of the photocuring reactions employed today are based on acrylic monomers [2,22,23]. Therefore, the acrylic monomers are representative of wide applications. Carbon black powder: Geomembranes typically contain 1% to 3% carbon black powder [1], which is used as an additive to obtain in this research. Photoinitiator and light source: In this paper, it is proposed to use UV light (200 nm < wavelength < 400 nm). Long-wave UV light exists in nature at extremely low intensities and will not have a major impact on the curing reaction. Due to low energy, it will not adversely affect the health of personnel on site, nor will it change the properties of the monomer [24,25]. Experiments were conducted using intensities of 1 mW/cm2 to 5 mW/cm2 of UV light emitted through a UV ﬂat light source to ensure consistency of intensity over a large area. In the experimental raw materials, photoinitiators are selected at the percentage of 1–5% [2] while carbon black powder was selected at the percentage of 2% [1]. The added photoinitiator content is generally no more than 5%. In this paper, 1–5% photoinitiator is preferable in order to study the effect of photoinitiator content on light-curing reaction.

3.2. Material Production Procedure Photocured geomembrane test ratios can be found in Table2. The test was performed at room temperature (22 ◦C) and the procedure was as follows. The acrylic monomers, 1–5% photoinitiator, and 2% carbon black powder were poured into the measuring cylinder and mixed to form a monomer mixture. Then, the mixer was used to stir the mixture at 400 rpm for 30 min to ensure that the mixture fully uniform. The ultraviolet lamp was placed 3 cm away from the special round vessel. In this study, the UV wavelength was used to stimulate the UV curing reaction is 360 nm. It is emitted by a 250 W UV lamp. The mixture was then poured into a special round vessel. The UV lamp was turned on to irradiate the mixed liquid stimulated luminescence curing chain reaction, and the photocured geomembrane is generated after several hours, as shown in Figure3. The light-cured geomembrane was taken out and the cutter was used to cut it into the desired shape. Then, the direct shear test of geomembrane and ﬁller was directly carried out.

Table 2. Clay properties.

Sample Number Carbon Black Content (%) Photoinitiator Content (%) Acrylic Monomers (%) 1 2 1 97 Sustainability 2021, 13, x FOR PEER REVIEW 5 of 17 2 2 2 96 3 2 3 95 4 2 4 94 4 52 2 4 94 5 93 5 2 5 93

Figure 3. Photocured geomembrane. Figure 3. Photocured geomembrane. 4. Direct Shear Tests To investigate the effect of photocured geomembranes in practical engineering applications, a series of direct shear tests with different soil materials (sand and clay) and photocured geomembranes were carried out in this study. The photocured geomembrane has a diameter of 300 mm and a thickness of 2 mm. The sand used in this study is in near-dry conditions with a water content of 0.79%. The sand density is 2.69 g/cm3. The coefficient of uniformity (Cu) is 5.2, and the curvature coefficient (Cc) is 1.2. The soil is well-graded sand per the Unified Soil Classification System (USCS). The particle size distribution is shown in Figure 4. The properties of the clay are listed in Table 3.

Table 3. Physical parameters of the clay.

Characteristic Index Value Natural moisture content (%) 7.71 Specific gravity 2.73 Natural Density (g·cm−3) 1.39 Optimum moisture content (%) 20.80 Max dry density/(g·cm−3) 1.62 Liquid limit (%) 34.00 Plastic limit (%) 17.80

Figure 4. Curve of particle distribution. Sustainability 2021, 13, x FOR PEER REVIEW 5 of 17

4 2 4 94 5 2 5 93

Figure 3. Photocured geomembrane.

4. Direct Shear Tests To investigate the effect of photocured geomembranes in practical engineering applications, a series of direct shear tests with different soil materials (sand and clay) and photocured geomembranes were carried out in this study. The photocured geomembrane has a diameter of 300 mm and a thickness of 2 mm. The sand used in this study is in near-dry conditions with a water content of 0.79%. The sand density is 2.69 g/cm3. The coefficient of uniformity (Cu) is 5.2, and the curvature coefficient (Cc) is 1.2. The soil is Sustainability 2021, 13, 8201 well-graded sand per the Unified Soil Classification System (USCS). The particle size5 of 16 distribution is shown in Figure 4. The properties of the clay are listed in Table 3.

Table 3. Physical parameters of the clay. 4. Direct Shear Tests Characteristic Index Value To investigate the effect of photocured geomembranes in practical engineering ap- Natural moisture content (%) 7.71 plications, a series of direct shear tests with different soil materials (sand and clay) and photocured geomembranesSpecific gravity were carried out in this study. The 2.73photocured geomembrane −3 has a diameterNatural ofDensity 300 mm (g·cm and) a thickness of 2 mm. The sand1.39 used in this study is in near-dryOptimum conditions moisture with content a water (%) content of 0.79%. The sand 20.80density is 2.69 g/cm3. The coefficientMax of dry uniformity density/(g·cm (Cu)−3 is) 5.2, and the curvature coefficient1.62 (Cc) is 1.2. The soil is well-gradedLiquid sand limit per (%) the Unified Soil Classification System 34.00 (USCS). The particle size distributionPlastic is shown limit in Figure(%) 4. The properties of the clay are 17.80 listed in Table3.

FigureFigure 4. Curve 4. Curve of particle of particle distribution. distribution.

Table 3. Physical parameters of the clay.

Characteristic Index Value Natural moisture content (%) 7.71 Speciﬁc gravity 2.73 Natural Density (g·cm−3) 1.39 Optimum moisture content (%) 20.80 Max dry density/(g·cm−3) 1.62 Liquid limit (%) 34.00 Plastic limit (%) 17.80

The test utilized a ZJ-type strain-controlled direct shear apparatus, as shown in Figure5. The test was carried out per the “Standard for Geotechnical Test Methods” (GBT 50123_2019) [26] and the “Technical Specifications for the Application of Geosynthetics” [27]. The test procedure for the photocured geomembrane-clay is as follows. Before the test, the clay was dried, crushed, ground, and then passed through a set of sieves with the opening from 2 mm (top) to 0.425 mm (bottom), and the required water was added to reach the optimum moisture content and then allowed to stand for three days. During the test, first place a hardwood base with the same height as the lower shear box in the lower shear box. Then, glue the photocured geomembrane to the hardwood base with epoxy resin. The membranes should be flat and free of folds and wrinkles to ensure that the cut surface is the photocured geomembrane–clay interface. Then, weigh 102 g of the clay after standing still, and compact it into three layers. The compaction of each layer of clay is 90%. The test procedure for photocured geomembrane-sand is as follows: During the test, first place a hardwood base with the same height as the lower shear box in the lower shear box, and then glue the photocured geomembrane with epoxy resin. For hardwood bases, the bonded geomembrane should be flat and free of folds and wrinkles to ensure that the cut surface is the photocured geomembrane–sand soil interface. The upper shear box is loaded with sand Sustainability 2021, 13, 8201 6 of 16

Sustainability 2021, 13, x FOR PEER REVIEW 7 of 17 samples into three layers according to the uniﬁed standard and layered and compacted to ensure that the sand body density is 52%.

Figure 5. The production flow chart of geomembrane. Figure 5. The production flow chart of geomembrane. The production flow chart of geomembrane is shown in Figure5. A strain-controlledstrain-controlled direct direct shear shear test test was was employed employed in the in study the withstudy a ratewith of 0.8a rate mm/min. of 0.8 Themm/min. normal The stresses normal applied stresses were applied 50 kPa, were 100 50 kPa, 100 200 kPa, 200 and kPa, 300 and kPa 300 (Figure kPa6 (Figure, from 6,right from to right left). to The left). test The is stoppedtest is stopped when thewhen shear the displacementshear displacement reaches reaches 18 mm. 18 Themm. final The peakfinal peak shear shear stress stress is reported is reported using using theaverage the average value value from from three three specimens specimens tested tested per pernormal normal stress stress applied applied and and specimen specimen composition. composition. Sustainability 2021, 13, 8201 7 of 16 Sustainability 2021, 13, x FOR PEER REVIEW 8 of 17

FigureFigure 6.6. DirectDirect shearshear apparatus.apparatus.

5.5. TestTest ResultsResults andand AnalysisAnalysis 5.1.5.1. MeasuredMeasured ShearShear Stress-DisplacementStress-Displacement ResponseResponse FigureFigure7 7shows shows the the variation variation of theof the shear shear stress–shear stress–shear displacement displacement response response and theand peak shear stress with normal stress at the interface between the clay and the photocured the peak shear stress with normal stress at the interface between the clay and the photo- geomembrane with different ratios. The shear stress increases rapidly for clay in the range cured geomembrane with different ratios. The shear stress increases rapidly for clay in of 0–4 mm shear displacement. Due to the increasing normal stress, the peak shear stress the range of 0–4 mm shear displacement. Due to the increasing normal stress, the peak of the 2% photoinitiator geomembrane is the largest and slightly higher than the peak shear stress of the 2% photoinitiator geomembrane is the largest and slightly higher than shear stress of the 4% dope level, followed by the 3% and 5% doping levels, and the the peak shear stress of the 4% dope level, followed by the 3% and 5% doping levels, and 1% doping level is relatively the smallest. The shear stress increases gradually with the the 1% doping level is relatively the smallest. The shear stress increases gradually with normal stress. The shear stress–shear displacement curve at the interface between the the normal stress. The shear stress–shear displacement curve at the interface between the geomembrane and the clay for 1%, 3%, 4%, and 5% photoinitiator goes through three geomembrane and the clay for 1%, 3%, 4%, and 5% photoinitiator goes through three stages: at the beginning, the interfacial shear stress between the clay and the geomembrane stages: at the beginning, the interfacial shear stress between the clay and the geomem- increases rapidly between shear displacement (0–4 mm), then increases approximately brane increases rapidly between shear displacement (0–4 mm), then increases approxi- linearly to the peak, and a slight non-linear decrease curve showing weak strain weak characteristicsmately linearly is to observed; the peak, ﬁnally, and a the slight interface non-linear residual decrease shear stress curve is showing reached and weak remains strain basicallyweak characteristics constant. The is geomembrane–clayobserved; finally, the interface interface of residual 1%, 3%, shear 4%, and stress 5% is photoinitiator reached and isremains under thebasically action constant. of normal The stress, geomembrane–clay and a small amount interface of clayof 1%, is embedded3%, 4%, and in the5% surfacephotoinitiator of the photocuredis under the geomembrane. action of normal In thestress, shearing and a process,small amount the surface of clay layer is em- of thebedded photocured in the surface geomembrane of the photocured is continuously geomembrane. scratched In by the the shearing clay and process, penetrates the intosur- theface interior, layer of and the the photocured interfacial geomembrane shear stress reaches is continuously its peak with scratched the increase by the of theclay shear and displacement.penetrates into However, the interior, with and the the change interfacial in the shear shear stress surface, reaches the shear its peak surface with changes the in- fromcrease the of photocuredthe shear displacement. geomembrane–clay However, interface with tothe the change internal in shearthe shear of the surface, clay. The the reasonshear surface for this changes is that from the clay the embeddedphotocured in ge theomembrane–clay photocured geomembrane interface to the is internal closely combinedshear of the with clay. the The photocured reason for geomembrane. this is that the As clay a result, embedded during in the the shearing photocured process, ge- theomembrane clay embedded is closely in thecombined photocured with the geomembrane photocured andgeomembrane. the clay not As embedded a result, during in the photocuredthe shearing geomembrane process, the clay are sheared,embedded and in thisthe statephotocured will be maintainedgeomembrane for and a long the time. clay Macroscopically,not embedded in it the shows photocured that the interface geomembrane shear stress are sheared, has a non-linear and this andstate small will decrease,be main- andtained it basically for a long remains time. unchangedMacroscopically, after reaching it shows the that interface the interface residual shear shear stress stress. has a non-linearThe shear and stress–shearsmall decrease, displacement and it basically curve remains of the 2%unchanged photoinitiator after reaching geomembrane the in- andterface clay residual interface shear shown stress. in Figure 7 has gone through two stages: the curve ﬁrst increases linearly to the peak value of the interface shear stress and then remains basically unchanged. The curve shows strong characteristics of the strain. The reason for this is that the geomembrane with the 2% photoinitiator is harder, and the clay particles slide and rub on the surface of the geomembrane during the shearing process under the action of lower normal stress. As the normal stress increases, clay particles are dented the geomembrane on the surface of the geomembrane. Under the action of normal stress, the clay particles have both sliding and scratching effects on the surface of the geomembrane, which causes the friction at the interface to increase. When the normal stress continues to increase, the clay particles become more deeply embedded into the geomembrane, resulting in deeper scratches on Sustainability 2021, 13, 8201 8 of 16

the surface of the geomembrane. The deeper the scratch on the geomembrane, the rougher the surface of the geomembrane. The clay and the geomembrane are interlocked under the action of the normal stress. The geomembrane acts as a reinforced clay, and its shear stress is significantly increased. This clay and geomembrane plowing phenomenon is consistent with the phenomenon of particle plowing in smooth geomembranes observed when the micro-geomembrane interacts with granular materials, as studied by David Frost [28]. The compaction of the clay and the smooth geomembrane becomes higher and higher under the action of high normal stress. Due to the smooth contact between the clay and the geomembrane, the pore water pressure will decrease with the increase in displacement, and the formation of negative pore water pressure will increase the effective stress. This is one of the phenomena of shear resistance on the contact surface of the clay and the smooth geomembrane. Therefore, as the normal stress increases, the contact surface of the clay and the geomembrane is continuously ploughed and compacted. The resulting shear stress is significantly greater than the shear stress of pure clay. Figure7 shows the shear stress–shear displacement response of the different ratios of the photocured geomembranes–sand interface. The peak shear stress varies with the normal stress. It can be seen that Figures7 and8 have similar trends. As the normal stress continues to increase, the shear stress also increases. Under the continuous displacement, the shear stress gradually becomes a constant state after reaching the peak. Under normal stress at the sand–geomembrane interface, the sliding of sand particles on the contact surface of the geomembrane is the main shear mechanism of the smooth geomembrane [29]. The peak shear stress at the interface between the sand and the geomembrane is higher than that of pure sand at lower stresses (50 kPa and 100 kPa). This shows that the geomembrane and the sand have a stronger restraint effect, and the geomembrane and the sand particles are correspondingly compressed and compacted, which increases the friction at the interface and leads to a larger shear stress peak. With the increasing normal stress, the plowing effect occurs at the interface between sand particles and geomembrane. Under the action of higher normal stress (200 Pa and 300 kPa), the sand particles make deeper scratching and plowing on the surface of the geomembrane. As the normal stress increases, the related interface structure is rearranged, which leads to the densification of the sand on the interface. After the peak stress is reached, since there is no obvious relative movement, the shear stress of the entire shear test remains constant. The residual interfacial friction increases due to the wear mechanism that produces deep scratches. The increase in normal stress leads to larger scratches, so the sand particles will plow together with the geomembrane during the shearing process. As can be seen from Figure8, the geomembrane–sand interface with a 4% photoinitiator shows strain hardening at the curve at a normal stress of 300 kPa. The reason for this is that the geomembrane with a 4% photoinitiator has a large amount of friction with the sand, and the surface of the geomembrane is ploughed and worn due to the large normal stress. The dislocation of the sand–geomembrane interface weakens its bite force and anchoring force to the sand and weakens the frictional resistance of the interface with the sand. The shear stress of the shear surface of pure sand mainly depends on the friction between the sands. In addition to the friction between the soil and the surface of the soil and the reinforcement, the shear force on the reinforced composite’s shear surface is also affected by the bite force between the soil and the reinforcement. This bite force increases with the vertical load.

5.2. Interface Shear Stress Peak Analysis As can be seen from Figures7 and8, the shear strength of the photocured geomembranes with different photoinitiator contents in contact with clay and sand is linearly related to the normal stress, in accordance with the Coulomb shear criterion in geotechnics. Sustainability 2021, 13, x FOR PEER REVIEW 9 of 17

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(a) (b)

(e) (f)

FigureFigure 7. Shear 7. Shear stress–shear stress–shear displacement displacement curv curveses for for different different ratios ratios of of photocur photocureded geomembrane–claygeomembrane–clay interface: interface: (a) Geomembrane(a) Geomembrane with 1%with photoinitiator—clay; 1% photoinitiator—clay; (b) (Geomembraneb) Geomembrane with with 2% 2% photoinitiator—clay; photoinitiator—clay; ( c()c) Geomembrane Geomembrane with with 3% photoinitiator—clay;3% photoinitiator—clay; (d) Geomembrane (d) Geomembrane with with 4% 4%photoinitiator—clay; photoinitiator—clay; (e (e) )Geomembrane Geomembrane withwith 5% 5% photoinitiator—clay; photoinitiator—clay; (f) clay-clay.(f) clay-clay.

The shear stress–shear displacement curve of the 2% photoinitiator geomembrane and clay interface shown in Figure 7 has gone through two stages: the curve first increases linearly to the peak value of the interface shear stress and then remains basically unchanged. The curve shows strong characteristics of the strain. The reason for this is that the geomembrane with the 2% photoinitiator is harder, and the clay particles slide and rub on the surface of the geomembrane during the shearing process under the action of lower normal stress. As the normal stress increases, clay particles are dented the geomembrane on the surface of the geomembrane. Under the action of normal stress, the clay particles have both sliding and scratching effects on the surface of the geomembrane, Sustainability 2021, 13, x FOR PEER REVIEW 11 of 17 Sustainability 2021, 13, 8201 10 of 16

(a) (b)

(e) (f)

FigureFigure 8.8. Shear stress–shear stress–shear displacement displacement curves curves for for different different ratios ratios ofof photocur photocureded geomembrane–sand geomembrane–sand interface: interface: (a) Geomembrane with 1% photoinitiator—sand; (b) Geomembrane with 2% photoinitiator—sand; (c) Geomembrane with (a) Geomembrane with 1% photoinitiator—sand; (b) Geomembrane with 2% photoinitiator—sand; (c) Geomembrane 3% photoinitiator—sand; (d) Geomembrane with 4% photoinitiator—sand (e) Geomembrane with 5% photoinitia- with 3% photoinitiator—sand; (d) Geomembrane with 4% photoinitiator—sand (e) Geomembrane with 5% photoinitiator— tor—sand; (f) sand–sand. sand; (f) sand–sand. 5.2. Interface Shear Stress Peak Analysis Figure8 shows that when the normal stress gradually increases, the interfacial shear strengthAs ofcan the be clay seen and from the Figures 2% photoinitiator 7 and 8, the content shear of strength the photocured of the photocured geomembrane ge- increasesomembranes as the with normal different stress photoinitiator increases. The cont reasonents in is contact that under with theclay action and sand of normal is line- stress,arly related the photocured to the normal geomembrane stress, in withaccord 2%ance photoinitiator with the Coulomb content hasshear a strong criterion restraint in ge- andotechnics. compaction effect with clay particles or soil-like particles. In the shearing process, the surface of the geomembrane is constantly scratched, making the surface rough and Sustainability 2021, 13, x FOR PEER REVIEW 12 of 17

Figure 8 shows that when the normal stress gradually increases, the interfacial shear strength of the clay and the 2% photoinitiator content of the photocured geomembrane increases as the normal stress increases. The reason is that under the action of normal stress, the photocured geomembrane with 2% photoinitiator content has a strong restraint and compaction effect with clay particles or soil-like particles. In the shearing process, the surface of the geomembrane is constantly scratched, making the surface rough and causing the friction coefficient to increase. Under the conditions of high normal stress (200 kPa and 300 kPa), the friction between the clay and the geomembrane Sustainability 2021, 13, 8201 surface is fully utilized during the shearing process. Under greater normal11 stress, of 16 the interfacial shear strength of the geomembrane with a 2% photoinitiator is higher than that of the other photoinitiator content, and the shear strength and tensile properties of the geomembranecausing the with friction a 2% coefficient photoinitiator to increase. are Underalso improved. the conditions of high normal stress The(200 fitting kPa and line 300 of kPa), the the photocured friction between geomembrane–clay the clay and the geomembrane interface surfaceis shown is fully in Figure 9, utilized during the shearing process. Under greater normal stress, the interfacial shear in accordance with the Mohr–Coulomb failure envelope. The shear strength parameters strength of the geomembrane with a 2% photoinitiator is higher than that of the other of clayphotoinitiator and clay/photocured content, and thegeomembrane shear strength samp and tensileles indicate properties that of the the geomembrane addition of photocured withgeomembrane a 2% photoinitiator will slightly are also improved.reduce the external friction angle and increase the cohesion of theThe clay. fitting The line ofoverall the photocured effect of geomembrane–clay the photocured interfacegeomembrane is shown reinforcement in Figure9, is to increasein accordancethe shear with strength the Mohr–Coulomb of the clay. failure The envelope.increase Thein shearthe shear strength strength parameters shows of an in- clay and clay/photocured geomembrane samples indicate that the addition of photocured creasegeomembrane with the increasing will slightly normal reduce thestress. external In frictionthe shear angle test and increaseof the theclay cohesion and photocured of geomembrane,the clay. The there overall was effect no of theinterlocking photocured geomembraneof particles, reinforcement and no swelling is to increase was the observed, subsequently.shear strength When of thethe clay. photoi The increasenitiator in content the shear is strength 2%, the shows obtained an increase shear with strength the pa- rametersincreasing are significantly normal stress. greater In the shear than test the of theshear clay strength and photocured parameters geomembrane, of pure there clay. When was no interlocking of particles, and no swelling was observed, subsequently. When the the photoinitiatorphotoinitiator contentcontent is 2%,is 2% the and obtained 3%, shearthe shear strength strength parameters parameters are significantly gradually greater increase with thethan increase the shear strengthof normal parameters stress ofand pure are clay. significantly When the photoinitiator greater than content the is 2%shear and strength parameters3%, the of shear other strength photoinitiat parametersor gradually content. increase In addition, with the 1% increase and of 5% normal of the stress photoinitiator and contentare of significantly the cured greater geomembrane than the shear is strength basicall parametersy the same of other in shear photoinitiator strength content. parameters, and theIn addition,interface 1% reinforcement and 5% of the photoinitiator effect is contentlower ofthan the cured other geomembrane content of is basicallyphotocured ge- the same in shear strength parameters, and the interface reinforcement effect is lower than omembrane.other content of photocured geomembrane.

Figure Figure9. Shear 9. Shear strength–normal strength–normal stress stress of photocuredphotocured geomembrane–clay geomembrane–clay interface interface with different with different proportions.proportions. Figure9 shows that under the action of lower normal stress (50 kPa and 100 kPa), Figurethe friction 9 shows and shear that stress under of the the photocured action of geomembrane lower normal and stress the sandy (50 soil kPa interface and 100 kPa), the frictionare very and similar shear with stress the photoinitiatorof the photocured content geomembrane of 1 to 5%. Under and the the action sandy of higher soil interface are verynormal similar stress with (200 kPa the and photoinitiator 300 kPa), a 4% photoinitiatorcontent of content1 to 5%. photocured Under geomembranethe action of higher produced more scratches, in accordance with the Mohr–Coulomb failure envelope. As normalthe stress sand particles(200 kPa and and the 300 geomembrane kPa), a 4% shear photoinitiator each other to produce content scratches photocured and the geomembrane sandproduced particles more plow scratches, the surface in of accordance the geomembrane, with the the Mohr–Coulomb friction on the surface failure of the envelope. As thegeomembrane sand particles is increased, and the and geomembrane a greater shear shear force iseach required other to shearto produce the surface scratches of and the sandthe particles geomembrane plow and the cause surface the sand of the particles geom toembrane, slide and the generate friction displacement. on the surface In of the the process of increasing normal stress, the interfacial shear strength of sand and 4% geomembranephotoinitiator is increased, content photocured and a greater geomembrane shear is force signiﬁcantly is required higher thanto shear others. the surface of Figure 10 shows the shear strength parameters of the interface between the sand and the photocured geomembrane. The addition of photocured geomembrane caused Sustainability 2021, 13, x FOR PEER REVIEW 13 of 17

the geomembrane and cause the sand particles to slide and generate displacement. In the process of increasing normal stress, the interfacial shear strength of sand and 4% photoinitiator content photocured geomembrane is significantly higher than others. Figure 10 shows the shear strength parameters of the interface between the sand and the photocured geomembrane. The addition of photocured geomembrane caused a significant increase in internal friction angle and cohesion. The shear interface of the photocured geomembrane-reinforced sand obviously increases the shear strength of the sand. The increase in the shear strength of the sand shows an increase with the increase of the normal stress. The results show that adding photocured geomembrane at the interface of sand can significantly improve the shear strength of sand. With the increase of the photoinitiator content in the photocured geomembrane, this reinforcement effect is obviously enhanced. The shear stress increased significantly in the early stages of the test and the samples exhibited hardening behavior. The increase in shear stress then continues to gradually reach a maximum, and the shear displacement is between 2 and 5 mm, depending on the photoinitiator content. At the same time, when the photoinitiator content is 4%, as the normal stress further increases, the photocured geomembrane–sand soil exhibits an obvious maximum shear stress, and their maximum and ultimate shear strengths are consistent, showing plastic behavior. The shear strength of a geomembrane at the interface with clay/sand is determined by the friction angle and cohesion. In the study, the cohesion at the interface between the geomembrane and clay/sand was influenced by the size of the photoinitiator content. From the above comparison, it is clear that there is a clear relationship between the co- Sustainability 2021, 13, 8201 12 of 16 hesion at the interface between the clay or sand and the photocured geomembrane depending on the photoinitiator content. A 2% photoinitiator content of the photocured geomembrane–clay interface and a 4% photoinitiator content of the photocured ge- a significant increase in internal friction angle and cohesion. The shear interface of the omembrane–sandphotocured geomembrane-reinforced contact surface has sand a higher obviously friction increases angle, the which shear strength is the main of the reason for its highersand. Theshear increase strength in the compared shear strength to ofthe the other sand shows specimens. an increase As withthe thegeomembrane–clay increase interfaceof the is normal subjected stress. to Thehigher results normal show thatstresses adding (generated photocured by geomembrane the solid waste at the and cover system)interface in practical of sand can engineering, significantly it improve is reas theonable shear to strength choose of sand.a 2% Withphotoinitiator the increase geomembraneof as the the photoinitiator impermeable content layer. in the In photocuredthe draina geomembrane,ge and impermeable this reinforcement layers of effect landfills, the is obviously enhanced. The shear stress increased significantly in the early stages of the geomembrane–sandtest and the samples interface exhibited is hardening subject to behavior. higher The normal increase stresses in shear (generated stress then by solid wastecontinues and the to cover gradually system) reach and a maximum, it is reasonable and the shear to choose displacement a 4% isphotoinitiator between 2 and geomembrane;5 mm,in the depending capping on system, the photoinitiator the geomembrane–sand content. At the same interface time, when is the subject photoinitiator to lower normal stressescontent (generated is 4%, as the by normal vegetation) stress further and increases,it is reasonable the photocured to choose geomembrane–sand a 5% photoinitiator soil exhibits an obvious maximum shear stress, and their maximum and ultimate shear geomembrane.strengths are consistent, showing plastic behavior.

FigureFigure 10. Shear 10. Shear strength-normal strength-normal stressstress of of photocured photocured geomembrane-sand geomembrane-sand interface interface with different with different proportions.proportions. The shear strength of a geomembrane at the interface with clay/sand is determined by the friction angle and cohesion. In the study, the cohesion at the interface between the geomembrane and clay/sand was inﬂuenced by the size of the photoinitiator content. From the above comparison, it is clear that there is a clear relationship between the cohesion at the interface between the clay or sand and the photocured geomembrane depending on the photoinitiator content. A 2% photoinitiator content of the photocured geomembrane–clay interface and a 4% photoinitiator content of the photocured geomembrane–sand contact surface has a higher friction angle, which is the main reason for its higher shear strength compared to the other specimens. As the geomembrane–clay interface is subjected to higher normal stresses (generated by the solid waste and cover system) in practical engineering, it is reasonable to choose a 2% photoinitiator geomembrane as the impermeable layer. In the drainage and impermeable layers of landﬁlls, the geomembrane–sand interface is subject to higher normal stresses (generated by solid waste and the cover system) and it is reasonable to choose a 4% photoinitiator geomembrane; in the capping system, the geomembrane–sand interface is subject to lower normal stresses (generated by vegetation) and it is reasonable to choose a 5% photoinitiator geomembrane.

5.3. Interfacial Friction Coefﬁcient The interfacial shear strength coefﬁcient f of the reinforcement–soil interface is an important parameter for the engineering design of reinforced soil, and its value is generally Sustainability 2021, 13, 8201 13 of 16

the ratio of the maximum shear stress τmax under stress in different directions to the corresponding normal stress σn, as shown in Formula (1): τ f = max (1) σn The interfacial shear strength coefﬁcient α is used to quantitatively compare the interfacial shear strength of soil and geosynthetic materials. The interfacial shear strength coefﬁcient α is the ratio of the shear strength of the soil/geomembrane material to the internal shear strength of the soil as follows: τsoil/geomembrane α = (2) (τsoil)

where α denotes the interfacial friction strength coefficient, τsoil/geomembrane denotes the shear strength values of different reinforced materials in direct shear tests, τsoil denotes the shear strength values of the internal soil in direct shear test, τ = σtanϕ,σ denotes the normal positive stress, and ϕ denotes the angle of internal friction. Figures 11 and 12 depict the friction coefficients for different scales of photocured geomembranes at the interface with two different materials. The peak friction coefficient increases with increasing normal stress as can be seen in Figures 11 and 12. When the critical normal stress is reached, the contact stress between the particles reaches the yield pressure of the material. A further increase in normal stress will lead to plowing of the material and provide a higher interface shear resistance, resulting in an increase in the Sustainability 2021, 13, x FOR PEER REVIEWinterfacial friction coefficient. As shown in Figure 11, the shear strength coefficient of most 15 of 17 clay/photocured geomembranes at different content of photoinitiator geomembrane is less than one. The interfacial friction between the soil and geomembrane is smaller than the friction inside the soil. This means that the shear strength of the interface between the soil the correspondingand the photocured friction geomembrane angle ischanges. less than thatThe of magnitude the soil. It shows of thatthe whencritical the directnormal stress shear mode is concerned, the photocured geomembrane–soil contact interface is a potential dependssliding on interface.the relative properties of the particles and the continuum material.

FigureFigure 11. The 11. The friction friction coefficient coefﬁcient of of the the photocured photocured geomembrane–clay geomembrane–clay interface with interface different with pro- different proportions.portions.

Figure 12. The friction coefficient of photocured geomembrane–sand soil interface with different proportions.

Table 4. Friction characteristic parameters.

Photoinitiator Interfacial Shear Strength Coeffi- Interfacials Shear Strength Coeffi- Content (%) cient (Compared with Clay) cient (Compared with Sand) clay/sand 1 1 1 0.68 1.90 2 1.04 1.70 3 0.99 1.95 4 0.75 2.10 5 0.71 1.80

6. Conclusions In this paper, the direct shear test was implemented to explore the shear characteristics of the photocured geomembrane–clay/sand interface with different proportions. The interface shear forceshear displacement curve, the peak value of the interface shear stress, and the friction coefficient of the interface shear strength of the photocured ge- Sustainability 2021, 13, x FOR PEER REVIEW 15 of 17

the corresponding friction angle changes. The magnitude of the critical normal stress depends on the relative properties of the particles and the continuum material.

Sustainability 2021, 13, 8201 Figure 11. The friction coefficient of the photocured geomembrane–clay interface14 ofwith 16 different proportions.

Figure 12. Figure 12. TheThe friction friction coefficient coefficient ofof photocured photocured geomembrane–sand geomembrane–sand soil interface soil interface with different with different proportions. proportions. Table4 shows that as the photoinitiator content increases, the shear strength coeffi- Tablecients 4. Friction at the sand–soilcharacteristic specimen parameters. interface are all greater than one. The upward trend in the friction coefficient of the critical normal stress represents the major plowing that occurs due Photoinitiatorto the contact stressesInterfacial of each Shear particle Strength exceeding Coeffi- the yieldInterfacials pressure of the Shear geomembrane. Strength Coeffi- ContentDuring (%) the interfacialcient shearing (Compared process, with under Clay) low normal stress,cient the(Compared position of with each Sand) sand particle is prone to change, and the dilatancy is obvious. Below the critical normal clay/sandstress, the shearing mechanism is sliding.1 After the critical normal stress is exceeded, 1 the shearing1 mechanism changes to0.68 sliding and plowing. The coefficient of friction 1.90 at the interface2 between the 2% photoinitiator1.04 geomembrane and the sand decreases with 1.70 increasing normal stress. This is due to the hard surface of the 2% photoinitiator geomembrane. 3 0.99 1.95 The sand is shallowly embedded in the geomembrane at a normal stress of 300 kPa and does4 not continuously plough the0.75 geomembrane surface, and the high normal 2.10 stress limits the5 tumbling and sliding of the sand0.71 particles. This weakens the shear expansion 1.80 of the photocured geomembrane–sand interface. However, the tendency of the interface friction coefficient to decrease slows down with increasing normal stress, and it can be expected 6. Conclusionsthat at higher normal stresses (>300 kPa) the interface friction coefficient will increase with increasingIn this paper, normal the stress. direct These shear results test clearly was reveal implemented the importance to explore of the critical the normalshear character- isticsstress of the to the photocured shear mechanism geomembrane–clay/s of the sand–geomembraneand interface interface, the with boundary different between proportions. the dominant sliding and the dominant plowing, and the corresponding friction angle The interfacechanges. The shear magnitude forceshear of the criticaldisplacement normal stress curve, depends the onpeak the relativevalue of properties the interface of shear stress,the and particles the andfriction the continuum coefficient material. of the interface shear strength of the photocured ge-

Table 4. Friction characteristic parameters.

Photoinitiator Interfacial Shear Strength Interfacials Shear Strength Content (%) Coefﬁcient (Compared with Clay) Coefﬁcient (Compared with Sand) clay/sand 1 1 1 0.68 1.90 2 1.04 1.70 3 0.99 1.95 4 0.75 2.10 5 0.71 1.80

6. Conclusions In this paper, the direct shear test was implemented to explore the shear characteristics of the photocured geomembrane–clay/sand interface with different proportions. The Sustainability 2021, 13, 8201 15 of 16

interface shear force shear displacement curve, the peak value of the interface shear stress, and the friction coefficient of the interface shear strength of the photocured geomembrane- clay/sand with different proportions are analyzed. Some main conclusions can be drawn as follows: (1). For the photocured geomembrane –clay interface, the interface shear stress peak of the geomembrane with a 4% photoinitiator is the largest under the normal stress of 50 kPa. Under the normal stress of 300 kPa, the peak value of the interfacial shear stress of the geomembrane with a 2% photoinitiator is the largest. In the anti-seepage system, it is more reasonable to choose a geomembrane with a 2% photoinitiator. (2). For the photocured geomembrane–sand soil interface, the interface shear stress peak value of the geomembrane with a 5% photoinitiator is the largest under the normal stress of 50 kPa. Under the normal stress of 300 kPa, the peak value of the interfacial shear stress of the geomembrane with a 4% photoinitiator is the largest. At the boundary between the drainage layer and the anti-seepage layer of the landfill, it is more reasonable to choose a 4% photoinitiator geomembrane. In the capping system, it is preferable to choose a 5% photoinitiator geomembrane. (3). For the photocured geomembrane–clay/sand interface, the interface friction coefficient shows a trend of first increasing and then decreasing with the increase of normal stress. When the normal stress reaches the critical normal stress, the interface friction coefficient will increase slowly. For the photocured geomembrane–sand soil interface, the geomembrane interface of 2% photoinitiator shows a trend that the interface friction coefficient decreases with the increase of normal stress. The photot-cured geomembranes of other ratios all show a trend of first increasing and then decreasing with the increase of normal stress. (4). The shear strength of the interface between photocured geomembrane and clay/sand is mainly determined by the interface friction angle. The friction angle of photocured geomembrane–sand soil is larger than photocured geomembrane–clay. Therefore, the interface between the photocured geomembrane and sand has better shear resistance and tensile crack resistance.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/su13158201/s1. Author Contributions: Conceptualization, L.L.; methodology, H.Y.; software, H.Y.; validation, formal analysis, investigation, H.Y.; writing—review and editing, H.X.; writing—review and editing, W.L.; writing—review and editing, Z.G. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by National Natural Science Foundation of China (No. 51778217), the Hubei Central Special Fund for Local Science and Technology Development (No. 2019ZYYD053), the Science, Technology Planning Project Wuhan (No. 2020020601012278), and the Innovative Group Project of Hubei Province (No. 2020CFA046). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available in Supplementary Materials. Acknowledgments: The authors are thankful for the financial support from that National Natural Science Foundation of China (No. 51778217), the Hubei Central Special Fund for Local Science and Technology Development (No. 2019ZYYD053), the Science, Technology Planning Project Wuhan (No. 2020020601012278), and the Innovative Group Project of Hubei Province (No. 2020CFA046). Conflicts of Interest: The authors declare no conflict of interest. Sustainability 2021, 13, 8201 16 of 16

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