Effect of the Slit on the Mechanical Tearing Behavior of High-Density Polyethylene and Polyester Geocell Strips

Abstract

Geocells are widely applied in numerous infrastructure constructions, like heavy-haul railways and ports. The mechanical tearing behavior of a geocell strip is crucial to the stability of the geocell-reinforced soil structures. At present, extensive studies have been conducted on the tensile characteristics of geocell strips, while limited research has been performed to investigate the post-damage mechanical tearing behavior of geocell strips. Meanwhile, there is also a lack of research on the comparison of performance of strips before and after damage. In this paper, a series of trapezoidal tearing tests were performed on high-density polyethylene (HDPE) and polyester (PET) geocell strips. The tearing test results and failure mode of trapezoidal specimens with a slit were investigated, and the effect of the slit on the strength and deformation characteristics of the specimen were discussed by introducing the “damage ratio of tearing force (R_TF)” and “damage ratio of tearing displacement (R_TD)”. In addition, the mechanical tearing behavior of HDPE and PET trapezoidal specimens was also compared. The test results indicated that the failure mode of HDPE and PET specimens subjected to tearing force was ductile and brittle failure. The strength and deformation characteristics of post-damage HDPE and PET trapezoidal specimens decreased. The slit had a significant impact on the tearing displacement of HDPE and PET specimens, especially the post-peak tearing displacement. The post-peak tearing displacement of HDPE was 10.99 times that of PET. The peak tearing force of the HDPE specimen without the slit was about 1.61 times that of specimen with the slit. Before local tearing, the peak tearing force of the PET specimen without the slit was about 3.27 times that of the specimen with the slit. The strength damage to the HDPE and PET geocell strips caused by the slit was 38.0%, and 69.46%. The impact of the slit on the tearing force of the PET specimen was greater than that of the HDPE, and was 1.82 times for the HDPE. This study can enhance our understanding of the mechanical tearing behavior of the geocell strip after damage and develop effective mitigation measures.

1. Introduction

In recent decades, geosynthetics have been adopted more frequently, as in civil engineering [1,2], transportation engineering [3,4], hydraulic engineering [5], and other fields, because they have excellent engineering characteristics, such as low cost [6,7], simple construction [8,9], better seismic [10], seepage-stability [11], stability [12] and performance and deformation coordination [13,14]. The geocell is an important new type of geosynthetic reinforcement [15] material that is free to expand and contract. During transportation, it can be folded as shown in Figure 1a, and when unfolded it forms a three-dimensional honeycomb structure [16,17], as shown in Figure 1b. One major challenge [18] for geocell-reinforced soil in a freeze–thaw environment is the large expansion and contraction of soil with a high water content. Based on the presence of texture and perforations on the surface of the geocell strip [19], the geocell strip can be classified into three categories: strip without perforations or textures, strip with perforations, and strip with textures, as shown in Figure 1b.

Geocells are used by expanding them into a honeycomb structure, securing them, and filling them with soil, gravel, or other fill material to form a structure with strong lateral confinement [20,21] and high stiffness [22,23]. Therefore, geocells are also referred to as cellular confinement systems. The lateral confinement provided by the geocell is superior to planar reinforcement material such as geogrid and geotextile [24]. In addition, the geocell can also be applied in slopes [22,25], because it can prevent soil erosion [26] on the slope surface subjected to wind and rain. Meanwhile, the root growth of vegetation [27] can further reinforce the slope soil, greatly reducing soil loss. Perforated geocells in rainy conditions can provide drainage and also prevent the formation of runoff.

Several authors have conducted numerous numerical and laboratory studies to gain an in-depth understanding of geocells. Abdulmuttalip [16] investigated the effect of geocell reinforcement on the mechanical behavior of shell foundations and concluded that the geocell could reduce settlement by more than 70%. Li [23] studied the bearing capacity of geocell-reinforced reclaimed construction waste (RCW) and indicated that the geocell can significantly enhance the bearing capacity of RCW. Gedela [28] used a non-contact digital image to determine the load dispersion mechanism of the geocell. Zhang [29] conducted triaxial compression tests and concluded that the geocell could improve the stability of retaining walls. Arvin [30] performed direct shear tests on the geocell in the expanded polystyrene (EPS) and demonstrated that the geocell noticeably improved the shear strength. The aforementioned studies primarily focus on the detailed investigation of the engineering characteristics of geocells, while there is relatively limited research on the mechanical properties of geocells themselves.

The stress state of geocell strips within a reinforced structure is primarily subjected to circumferential tension. Therefore, the tensile mechanical properties are mainly analyzed and studied through indoor uniaxial tensile tests, as shown in Figure 2a. Liu [17] studied the failure mechanism of geocell strips and junctions under various loading conditions through a series of laboratory tests, such as the uniaxial tensile test, shear, and peeling test. Zuo [24] investigated the effect of specimen shape and specimen width on the tensile mechanical behavior of the HDPE geocell strip. Additionally, the effect of the welding junction on the tensile properties of the geocell strip was also studied. The tests indicated that the elongation of the HDPE strip was sensitive to the specimen shape, and the effect of the welding junction on the HDPE strip cannot be ignored. Bai [31] discussed the effect of low temperatures on the mechanical properties of geocell strips made from high-density polyethylene (HDPE), polypropylene (PP), and polyester (PET). The results revealed that the tensile performance of the PET geocell strip is superior to that of the PP and HDPE geocell strips. Yang [32] conducted a series of laboratory tests on HDPE geocell strips with welding junctions, studied the effect of clamping distance on the strength of geocell strips and junctions, and presented a laying method of geocells on slopes based on the findings. The aforementioned literature studied the influence of strain rate, specimen shape and size, temperature, and clamping distance on the tensile mechanical properties of geocell strips and junctions.

In addition to tensile mechanical properties, it is equally important to pay attention to the mechanical tearing behavior of geocell strips, especially post damage. Geocell strips within reinforced structures may be subjected to a combination of axial force and moment at critical locations, such as the slope shoulder, as depicted in Figure 2b. Under the combined action, geocell strips are inevitably prone to tearing at the edges, as illustrated in Figure 2c. If the geocell strip in the geocell undergoes tearing failure, it will significantly weaken the lateral confinement provided by the geocell to the soil mass within the cell, thereby reducing the overall strength of the geocell in the slope. If the geocell strips experience tearing and failure, it will significantly diminish the lateral confinement provided by the geocell to the soil, consequently reducing the overall strength of the geocell in a slope. In addition, the progressive movement of soil may lead to an unbalanced load transfer, causing the reinforced structure to experience sliding or overall failure. Therefore, the mechanical tearing performance of geocell strips plays a crucial role in the performance and overall integrity of geocells.

However, existing studies have primarily focused on the tensile mechanical properties of geocells, while there is a lack of research on the effects of edge damage on the strength and deformation characteristics of geocell strips, as well as the failure modes and tearing paths. Furthermore, there is also a lack of discussion of the comparison and reduction of the mechanical tearing properties before and after the damage to the geocell strip. The incomplete understanding of the mechanical tearing behavior of damaged geocell strips has severely impeded the promotion and application of geocells. Therefore, it is necessary to conduct experimental research on the mechanical tearing behavior of geocell strips to ensure the safety, durability, and sustainability [33] of reinforced structures, and then to expand the application range of geocell reinforcement in infrastructure projects.

Based on the above analysis, this paper conducted a series of tests on HDPE and PET geocell strips through the universal testing machine to gain an in-depth understanding of mechanical tearing behavior of geocell strips, especially post-damage. Trapezoidal tearing tests were performed to analyze the trapezoidal tearing results and failure mode of the HDPE and PET trapezoidal specimens with the slit. Meanwhile, the mechanical tearing characteristics of trapezoidal specimens with and without the slit were compared. Additionally, the effect of the slit on the mechanical tearing behavior of HDPE and PET specimens made from geocell strips was discussed. The test results of this study can enhance our understanding of geocell behavior after damage and develop effective mitigation measures.

2. Materials and Methods

2.1. Test Equipment and Materials

The trapezoidal tearing test is generally carried out by using the tensile testing machine [19]. Therefore, the laboratory universal tensile testing machine for the geosynthetics was used in this study, as shown in Figure 3, to investigate the mechanical tearing behavior of geocell strips. The clamp of the universal testing machine is important to ensure the quality of the test results. Hence, the flat compression clamp was used in the trapezoidal tearing test. The flat compression clamp must have a minimum of three bolts, and the quantity should be an odd number to ensure even pressure distribution on the test specimen. The upper and lower flat compression clamp required the clamping surfaces to be aligned on the same plane. Furthermore, the principal axis of the specimen needed to align with the center line of the flat compression clamp after specimen installation. The technical details of the universal testing machine are given in

Table 1. The technical details of the universal testing machine.

Specifications	Value
Tensile force range (kN)	0.2~50
Load cell accuracy (%)	±0.5
Maximum tensile length (mm)	1100
Deformation measurement range (%)	2~100
Deformation cell accuracy (%)	±0.5
Test speed range (mm/min)	0.005~500

.

In this paper, the geocell strips made from HDPE and PET were used to investigate the mechanical tearing behavior. It should be noted that the height of the HDPE and PET geocell strips should be consistent, to ensure that the effect of specimen size on the test results was excluded. The physical and mechanical properties of HDPE and PET geocells are shown in

Table 2. The physical and mechanical properties of HDPE and PET geocells.

Materials	Junction Connections	Geocell Height (mm)	Geocell Thickness (mm)	Tensile Strength (N/cm)
HDPE	welding	100	1.1	280
PET	riveting	100	0.6	1500

. Additionally, to minimize the influence of perforations and textures on the test results, the geocell strip without perforations or textures was selected in this study.

2.2. Experimental Program

Currently, the mechanical tearing properties are mainly analyzed and studied through laboratory trapezoidal tearing tests [19] to simulate the combined effects of tension and moment on geocell strips. In the specification [34], as shown in Figure 4a, a trapezoidal tearing specimen with a slit is used to simulate the damage caused by the tearing of the strip. By applying axial tensile force, the tearing force of the specimen can be tested, and the failure mode can be observed to characterize the mechanical tearing performance of the geocell. Due to the irregular cross-sectional area of the specimen during the testing process, the test results are for force rather than stress. Hence, the unit of trapezoidal tearing force is typically expressed in newtons (N). Additionally, to compare the mechanical tearing properties and the reduction in performance before and after damage, a series of trapezoid tests were conducted on the geocell strip without a slit, as illustrated in Figure 4b.

It is worth noting that the standard specifies only one size for the trapezoidal specimen for geomembrane and geotextile. Due to the higher stiffness of geocell strips compared to geomembrane or geotextile, using standard sample sizes specified for geomembrane or geotextile may result in difficulties during sample installation, compromising the quality of sample installation and introducing deviations into test results. In the previous research study [19], due to the presence of perforations on the surface of the geogrid strips, it was not possible to obtain trapezoidal specimens according to the specified standards. This idea from Liu highlighted the importance of considering the unique properties of the geocell strip when designing sample preparation protocols to ensure accurate and reliable testing results. Therefore, researchers often adjust the specimen for the trapezoidal tearing test based on the specific characteristics of the geocell strip.

Based on laboratory specifications and the analysis mentioned above, modifications had been made to the dimensions of the specimen for the trapezoidal tearing test to accommodate the geocell strips in the laboratory. These modifications aim to investigate the influence of damage on different types of geocell strip specimen and enhance the accuracy of test results. The trapezoidal specimen length remained consistent with the specifications. The left clamping distance, denoted as Distance 1, was set to 50 mm, while the right clamping distance, denoted as Distance 2, was set to 100 mm, in accordance with Liu’s method. According to the specification, the width and length of the trapezoidal specimen were 75 mm and 200 mm, respectively. In addition, the slit length was 15 mm. The slit was made at the center of the left-hand side of the specimen to mimic the damage induced by tearing. To ensure the reliability of the laboratory trapezoidal test, the number of each specimen, including specimen with and without the slit, should not be less than five. Lastly, to minimize the influence of the tensile rate on the test results, the tensile rate of 50 mm/min was selected in accordance with the specifications for this study.

3. Results and Discussion

This section presents a summary of the experimental results and analyses obtained from laboratory trapezoidal tearing tests conducted on HDPE and PET geocell strips using the tensile testing machine. Firstly, the trapezoidal tearing results and failure mode of the trapezoidal specimen with the slit were analyzed. Then, the mechanical tearing characteristics of trapezoidal specimens with and without slits were compared. Meanwhile, two variables, “damage ratio of tearing force (R_TF)” and “damage ratio of tearing displacement (R_TD)”, were introduced to quantitatively evaluate the effect of the slit on the tearing force and tearing displacement. Finally, the effect of the slit on the mechanical tearing behavior of HDPE and PET specimens made from geocell strips was compared.

3.1. Analysis of Mechanical Tearing Behavior of HDPE Geocell Strips

The plot of tearing force versus tearing displacement of the HDPE trapezoidal specimen with the slit is shown in Figure 5. From Figure 5, the tearing process of the HDPE trapezoidal specimen with the slit could be divided into two stages, namely the pre-peak region and the post-peak region. To analyze the results of the trapezoidal tearing tests in more depth, the progressive tearing process of the trapezoidal specimen with the slit was recorded during the test by simultaneous video recording of the test with a camera. Combining the trapezoidal tearing test results conducted on the HDPE geocell strip with the slit and the generation process of the plastic zone in the trapezoidal specimen for the duration of the test, the tearing process of the test from loading to failure could be divided into three stages, namely stage I, stage Ⅱ, and stage Ⅲ, as shown in Figure 5. The characteristic figures of the tearing process of the HDPE trapezoidal specimen with the slit were captured in the testing videos, as shown in Figure 6. It should be noted that the characteristic figures (a) to (f) in Figure 6 corresponded to the characteristic points (a) to (f) in Figure 5. In addition, the pre-peak region included stage I and stage Ⅱ, and the post-peak region was mainly stage Ⅲ.

Stage I was the process from the start of the test to before the generation of the plastic zone on the HDPE trapezoidal specimen with the slit. In this stage, the tearing force was proportional to the displacement, and the test curve was a straight line. At the beginning of the trapezoidal tearing test, the tearing force was very small. The tearing force recorded by point (a) in Figure 5 was only 0.12 kN, and the corresponding characteristic figure of the specimen is shown in Figure 6a. From Figure 6a, we see that the slit slightly opened due to the existence of tearing force. The trapezoidal tearing force gradually increased with an increase in axial load applied by the universal testing machine. When the axial load increased to a certain value, the slit on the trapezoidal specimen was fully opened by tearing force, and the characteristic figure of the specimen is shown in Figure 6b. It is worth mentioning that the HDPE trapezoidal specimen with the slit was currently in a critical state before the generation of the plastic zone. At this point, the critical state of the HDPE trapezoidal tearing specimen was defined as stage Ia, corresponding to point (b) in Figure 5. From Figure 6a,b, it was determined that the failure process started with the initial slit, and then the tearing force was transmitted along the initial slit.

Stage Ⅱ was the process from the generation of the plastic zone on the trapezoidal specimen with the slit to the extension of the plastic zone to the edge of the trapezoidal specimen with the slit. After stage Ia, the axial load slightly increased, and the HDPE trapezoidal specimen with the slit continued to be torn along the slit, while the plastic zone was generated near the slit. With the increase in axial load, the slit on the trapezoidal tearing specimen opened continuously because of the tearing force, while the plastic zone was also expanding further and becoming clearly defined; the characteristic figure of the specimen is shown in Figure 6c, corresponding to point (c) in Figure 5. Due to the fact that the expansion speed of the plastic zone was faster than the tearing speed of the trapezoidal specimen with the slit, this stage was mainly dominated by plastic deformation, so the test results showed a curve. When the load increased to a certain value, the plastic zone extended to the edge of the HDPE trapezoidal specimen, and the characteristic figure of specimen is shown in Figure 6d. The HDPE trapezoidal specimen with the slit was currently in a critical state of peak tearing force. At this point, the critical state of the HDPE trapezoidal specimen with the slit was defined as stage Ⅱa, corresponding to point (d) in Figure 5. From Figure 6c to Figure 6d, it was determined that the shape of the plastic zone presented a horizontal cone shape, and the horizontal cone was constantly updating for the duration of the trapezoidal tearing test. It is worth mentioning that at stage Ⅱa, the tearing force of the HDPE trapezoidal specimen with the slit reached a peak.

Stage Ⅲ was the process from the generation of the plastic zone on the side of the HDPE trapezoidal specimen away from the slit to the full rupture of the trapezoidal specimen. After stage Ⅱa, the axial load slightly increased, the HDPE trapezoidal specimen generated the plastic zone on the side of the specimen away from the slit. It should be noted that when the plastic zone was generated on the side of the specimen away from the slit, the tearing force decreased with the increase in the tearing displacement. With the increase of axial load, the slit on the trapezoidal specimen opened continuously because of the tearing force, while the plastic zone was also expanding further. The shape of the plastic zone changed from a horizontal cone shape to a horizontal trapezoid shape; the characteristic figure of the specimen is shown in Figure 6e, corresponding to point (e) in Figure 5. In addition to trapezoidal tearing force, the plastic zone of the HDPE trapezoidal specimen was also subjected to tensile force. Namely, the plastic zone of the HDPE trapezoidal specimen was subjected to the combination of tearing force near the slit and axial tensile force.

After point (e) in Figure 5, the rate of decrease in the tearing force increased compared to that of point (d) to point (e), as shown in Figure 5. The characteristic figure of the specimen corresponding to point (e) is shown in Figure 6e. From Figure 6e, it was observed that the tearing width on the side near the slit was about half the width of the HDPE trapezoidal specimen. With the further increase in tearing displacement, the rate of development of the slit was gradually accelerated, while the area of the plastic zone rapidly decreased. When the tearing displacement increased to a certain value, the slit on the plastic zone near the slit side extended to the edge of the HDPE trapezoidal specimen away from the slit, and the characteristic figure of the specimen is shown in Figure 6f. The HDPE trapezoidal tearing specimen was currently in a critical state of failure, corresponding to point (f) in Figure 5. At this point, the critical state of the HDPE trapezoidal tearing specimen was defined as stage Ⅲa. The trapezoidal tearing test ended when the specimen fully ruptured. From Figure 6a–e, we can see that the failure process of the HDPE trapezoidal specimen with the slit was progressive.

Based on the above analysis of the experimental process, it was observed that the HDPE trapezoidal specimen exhibited significant plastic deformation during the test process, and also exhibited obvious signs before complete failure. Hence, the failure mode of HDPE geocell strips with the slit subjected to tearing force was ductile failure. Additionally, comparing the tearing displacement of stage Ⅱ and stage Ⅲ shows that the tearing displacement of stage Ⅲ was greater than that of stage Ⅱ in the progressive failure process. It was determined that, although the slit had an effect on the strength of HDPE geocell strips, the mechanical tearing behavior of the HDPE geocell strip with the slit was still mainly controlled by the tensile properties of the strip. Hence, the ductile failure characteristics would reduce the effect of the slit on the mechanical tearing behavior of the HDPE geocell strip to some degree.

The plot of comparison of the experimental results of the HDPE trapezoidal specimen with and without the slit is shown in Figure 7. From Figure 7, it was observed that the existence of the slit had a significant effect on the experimental results of the HDPE trapezoidal specimen. In the pre-peak region, the tearing force of the HDPE trapezoidal specimen with the slit reached peak first. The peak tearing force of the HDPE trapezoidal specimen with the slit was less than that of the specimen without the slit. In the post-peak region, the tearing force of the HDPE trapezoidal specimen without the slit remained constant after yielding and then failed. The HDPE trapezoidal specimen without the slit showed more significant plastic deformation than the specimen with the slit. The reason was that the trapezoidal tearing test conducted on HDPE geocell strips would gradually switch to the tensile test due to the plastic flow effect. Unlike the specimen without the slit, the HDPE trapezoidal specimen with the slit transitioned directly to the plastic fracture process in the post-peak region.

Additionally, the process curve of plastic fracture of the HDPE trapezoidal specimen with the slit was parallel to the yield curve of the HDPE trapezoidal specimen without the slit. Combined with the above analysis of the trapezoidal tearing process of the specimen with the slit, the reason was that the trapezoidal specimen with the slit was mainly in the plastic deformation region after the peak, while the specimen without was also in the yield plastic deformation region after the peak, so the two test results were nearly parallel. From Figure 7, the peak tearing forces of the HDPE trapezoidal specimen with and without the slit were 1.24 kN and 2.0 kN, respectively. The HDPE trapezoidal specimen without the slit was approximately 1.61 times that of the specimen with the slit. The tearing displacements corresponding to the peak tearing force of the HDPE trapezoidal specimen with and without the slit were 42.03 mm and 45.12 mm, respectively. The difference in tearing displacement of the two specimens was only 3.09 mm. From test start to end, the tearing displacements of the HDPE trapezoidal specimen with and without the slit were 109.71 mm and 243.65 mm, and the specimen without the slit was approximately 2.22 times that of the specimen with the slit. The post-peak tearing displacements of the specimen with and without the slit were 67.68 mm and 198.53 mm, and the effect on the specimen without the slit was approximately 2.93 times that of the specimen with the slit.

To investigate the effect of the slit on the strength and deformation characteristics of the trapezoidal specimen, two variables, “damage ratio of tearing force (R_TF)” and “damage ratio of tearing displacement (R_TD)”, were introduced to quantitatively evaluate the reduction in performance of the trapezoidal specimen after damage caused by the slit. The “damage ratio of tearing force (R_TF)” was the ratio of the difference in the peak tearing force of the trapezoidal specimen without the slit and with the slit to the peak tearing force of the trapezoidal specimen without the slit. The R_TF represented the impact of the slit on the peak tearing force of the trapezoidal specimen. The greater the R_TF, the worse the peak tearing force of the trapezoidal specimen with the slit was maintained, showing that the slit had noticeable influence on the tearing force of the peak trapezoidal specimen and vice versa. The “damage ratio of tearing displacement” was the ratio of the difference in the tearing displacement of the trapezoidal specimen without the slit and with the slit to the tearing displacement of trapezoidal specimen without the slit. The R_TD represented the impact of the slit on the tearing displacement of the trapezoidal specimen. The greater the R_TD, the worse the tearing displacement of the trapezoidal specimen with the slit was maintained, showing that the slit had a noticeable influence on the tearing displacement of the trapezoidal specimen and vice versa. The formulas of “damage ratio of tearing force (R_TF)” and “damage ratio of tearing displacement (R_TD)” are shown in (1) and (2), respectively.

$$R_{TF}=\frac{T{F}_{\mathrm{without}-\mathrm{slit}}-T{F}_{\mathrm{with}-}{}_{\mathrm{slit}}}{T{F}_{\mathrm{without}}{}_{-\mathrm{slit}}}$$

(1)

where $T{F}_{\mathrm{without}-\mathrm{slit}}$ and $T{F}_{\mathrm{with}-}{}_{\mathrm{slit}}$ are the peak tearing force of the trapezoidal specimen without and with the slit, kN.

$$R_{TD}=\frac{T{D}_{\mathrm{without}-\mathrm{slit}}-T{D}_{\mathrm{with}-}{}_{\mathrm{slit}}}{T{D}_{\mathrm{without}}{}_{-\mathrm{slit}}}$$

(2)

where $T{D}_{\mathrm{without}-\mathrm{slit}}$ and $T{D}_{\mathrm{with}-}{}_{\mathrm{slit}}$ are the tearing displacement of the trapezoidal specimen without and with the slit, mm.

The “damage ratio of tearing force (R_TF)” and “damage ratio of tearing displacement (R_TD)” of the HDPE trapezoidal specimen used in this study were 38.0% and 55.0%, respectively. It was observed that the strength and deformation characteristics of the HDPE trapezoidal specimen after damage decreased. It should be noted that the R_TD was greater than the R_TF, meaning that the effect of the slit on the deformation characteristics was greater than the strength characteristics. Although the peak tearing force of the specimen decreased due to the existence of the slit, the strength performance of the HDPE trapezoidal specimen with the slit was still maintained at more than 60%, while the deformation performance was maintained at less than 50%.

Based on the above analysis, it was determined that the slit had a noticeable effect on the peak tearing force and the tearing displacement after the peak, while it had little effect on the tearing displacement corresponding to the peak tearing force. In this paper, the specimen with the slit was used to simulate the damage of the specimen. The HDPE trapezoidal specimen with the slit failed faster after damage than the specimen without the slit. Hence, when the geocell strip at the transition point of the slope is damaged, more attention should be paid, and corresponding measures should be taken as soon as possible. It is recommended that sensors should be installed to monitor the stress state on the geocell strip to ensure the safety and durability of the reinforced soil structure.

3.2. Analysis of Mechanical Tearing Behavior of PET Geocell Strips

The plot of tearing force versus tearing displacement of the PET trapezoidal specimen with the slit is provided in Figure 8. From Figure 8, we can see there was no obvious yield stage in the results during the trapezoidal tearing test. Meanwhile, the failure characteristic figure of the PET trapezoidal specimen with the slit is shown in Figure 9a. From Figure 9a, we can see that the PET trapezoidal specimen was directly torn into two parts along the initial slit under the action of the tearing force, and the tearing path was very clear. In addition, there was also no significant plastic deformation of the PET trapezoidal specimen with the slit during the test. The reason was that the PET molecular chain was composed of rigid phenyl and flexible fatty hydrocarbon groups, where the adjacent benzene rings were not in the same plane. This means that PET had a high stress sensitivity. Therefore, the PET trapezoidal specimen with the slit was directly torn into two parts under the action of tearing force, meaning that the tearing process was relatively simple.

Based on the above analysis, this section mainly focused on the mechanical tearing behavior of PET trapezoidal specimens with the slit and did not undertake a detailed analysis of the tearing process of PET trapezoidal specimens with the slit. The results for the PET trapezoidal specimens with the slit could also be divided into two stages, namely the pre-peak region and post-peak region. The peak tearing force of the PET trapezoidal specimen with the slit was 1.75 kN, and the corresponding tearing displacement was 47.2 mm. The tearing displacement was only 4.83 mm from peak to failure. The tearing displacement of the PET trapezoidal specimen with the slit from loading to failure was 52.03 mm, with 91% of the tearing displacement before the peak.

The experimental results of the PET trapezoidal specimen without the slit are provided in Figure 8. At the beginning of the test, the tearing force of the PET trapezoidal specimen without the slit presented rapid growth. The tearing force reached a peak when the tearing displacement reached 22.48 mm, and then the test curve showed a sudden drop. It is worth noting that the tearing force did not directly decrease to 0. The characteristic figure of local tearing of the PET trapezoidal specimen without the slit is shown in Figure 9b. From Figure 9b, we can see that the tearing did not penetrate completely along the width of the trapezoidal specimen. Combining the trapezoidal tearing test results conducted on the PET geocell strip without the slit and the characteristic figure of local tearing, the PET trapezoidal specimen on the side with the smaller clamping spacing was subject to axial force after the start of the test. Due to the absence of a significant yield point, tearing occurred when the specimen reached tensile strength.

When the slit was generated in the PET trapezoidal specimen, the tearing force continued to increase gradually with the increase in the tearing displacement. When the tearing displacement increased to a certain value, the tearing force reached a second peak. Subsequently, the specimen was completely ruptured and the test ended. The failure characteristic figure of the PET trapezoidal specimen without the slit is shown in Figure 9c. It should be noted that the trend of the tearing curve of the PET trapezoidal specimen without the slit after the generation of the slit was relatively consistent with that of the PET trapezoidal specimen with the slit, indicating that the experiment results of the PET trapezoidal specimen damage simulated by the slit had reference value. Meanwhile, the failure characteristic figure of the PET trapezoidal specimen with the initial slit was similar to that of the specimen without the slit.

From Figure 8, we can see that there are two peak tearing forces during the test of the PET trapezoidal specimen without the slit. Before local tearing, the peak force was 5.73 kN, and after local tearing the peak force was 2.01 kN. The peak tearing force before local tearing was approximately 2.85 times greater than the tearing force after local tearing. The difference between the two peak tearing forces was that the reason for the first peak was tensile, while the reason for the second peak was tearing. It should be noted that the peak tearing force of the PET trapezoidal specimen with the slit was 1.75 kN. Although the PET trapezoidal specimen without the slit would be torn during the test process, the tearing force before and after local tearing was also greater than that of the PET trapezoidal specimen with the slit, which was 3.27 times and 1.15 times that of the trapezoidal specimen with the slit, with a decrease of 69.46% and 12.94%, respectively. Therefore, the slit had a significant effect on the ability of the PET geocell strip to resist tearing failure. Compared with before tearing, the strength of trapezoidal specimens with the slit to resist tearing failure decreased by up to 69.46%.

Before the local tearing of the PET trapezoidal specimen without the slit, the tearing displacement corresponding to the peak tearing force was 22.48 mm, and after local tearing the tearing displacement corresponding to the peak tearing force was 56.1 mm. The tearing displacement of the PET trapezoidal specimen without the slit from loading to failure was 60.77 mm. The tearing displacement of the PET trapezoidal specimen without the slit from the occurrence of the slit to complete failure was only 38.29 mm. However, the tearing displacement of the PET trapezoidal specimen with the slit from the beginning of tearing to complete failure was 52.03 mm, which was approximately 1.36 times that of the specimen without the slit, with a decrease of 26.4%. Based on the above analysis, the generation of the slit in the trapezoidal specimen before and after being subjected to tearing force would have different effects on the tearing failure displacement of the trapezoidal specimen. If the PET geocell strip had been damaged before being subjected to tearing force, the tearing failure displacement was greater than that of the damage after being subjected to the force. In contrast, if the PET geocell strip was not damaged before being subjected to tearing forces, it would fail quickly once damage had occurred.

It should be noted that there are two peak tearing forces during the test of the PET trapezoidal specimen without the slit. Because the tearing force used in Equation (1) was under the condition of being without the slit, the tearing force before local tearing was used for analysis. The “damage ratio of tearing force (R_TF)” and “damage ratio of tearing displacement (R_TD)” of the PET trapezoidal specimen used in this study were 69.5% and 14.4%, respectively. It was observed that the strength and deformation characteristics of PET trapezoidal specimens after damage decreased. It should be noted that the R_TD was less than the R_TF, meaning that the effect of the slit on the strength characteristics was greater than the deformation characteristics. Although the tearing displacement of the specimen decreased due to the existence of the slit, the deformation performance of the PET trapezoidal specimen with the slit was still maintained at more than 80%, while the strength performance was maintained at less than 40%.

Comparing Figure 10a,c, it was observed that the failure characteristics of PET trapezoidal specimens with and without the slit were basically similar, both showing gradual tearing of the specimen along the slit, with a clear tearing path. In practical engineering, tearing damage to PET geocell strips should be avoided, as the ability of PET geocell strips to resist tearing damage would significantly decrease after tearing damage occurred.

3.3. Comparison of Mechanical Tearing Behavior of HDPE and PET Geocell Strips

The comparison of the tearing test results of HDPE and PET trapezoidal specimens without the slit is shown in Figure 10. From Figure 10, we can see that the HDPE trapezoidal specimen exhibited a significant yield process during the tearing test, and the tearing displacement of the HDPE trapezoidal specimen was much greater than that of the PET. The tearing displacement of the HDPE trapezoidal specimen without the slit from loading to failure was about 4.0 times that of the PET. The reason was that the trapezoidal tearing test conducted on the HDPE trapezoidal specimen without the slit would gradually switch to the tensile test due to the plastic flow effect. Hence, the HDPE trapezoidal specimen without the slit inherited the tensile properties of this material. Unlike the HDPE, the PET trapezoidal specimen without the slit was sensitive to stress concentration, and it would not alleviate stress concentration due to deformation. Hence, the local tearing occurred when the tearing displacement increased to a certain value.

The comparison of the tearing test results of HDPE and PET trapezoidal specimens with the slit is shown in Figure 11. From Figure 11, it was observed that both the HDPE and PET trapezoidal specimens with the slit could be divided into two stages, namely the pre-peak region and the post-peak region. In the pre-peak region, HDPE and PET trapezoidal specimens with the slit reached the tearing force at a tearing displacement of 42.03 mm and 45.87 mm, respectively. The difference in tearing displacement before the peak was only 3.84 mm. The tearing forces of HDPE and PET trapezoidal specimens with the slit were 1.24 kN and 1.75 kN. The tearing force of the PET trapezoidal specimen with the slit was about 1.41 times that of the HDPE. In the post-peak region, the tearing displacement of the HDPE and PET trapezoidal specimens with the slit were 67.68 mm and 6.16 mm, respectively. The tearing displacement of the HDPE trapezoidal specimen with the slit after the peak was about 10.99 times that of the PET. The tearing displacement of the HDPE and PET trapezoidal specimens with the slit from loading to failure were 109.71 mm and 52.03 mm. Based on the above analysis, in the pre-peak region the tearing displacement corresponding to the tearing force of the HDPE and PET trapezoidal specimens with the slit was relatively consistent. However, in the post-peak region the tearing displacement of the HDPE trapezoidal specimen with the slit was approximately 10.99 times greater than that of the PET.

From the perspective of predicting failure, the HDPE trapezoidal specimen with the slit was better than the PET trapezoidal specimen, because the test results for the HDPE trapezoidal specimen with the slit were consistent with the characteristic figure of the specimen, as shown in Figure 5 and Figure 6. The decrease in tearing force of the HDPE trapezoidal specimen with the slit corresponded to an increase in the degree of tearing of the specimen. Unlike the HDPE, the tearing force of the PET trapezoidal specimen with the slit was one of nearly continuous increase before failure. However, the tearing condition of the PET trapezoidal specimen with the slit could be observed according to the characteristic figure of the specimen, as shown in Figure 9c. Hence, the test results for the PET trapezoidal specimen with the slit were not consistent with the characteristic figure of the specimen. In practical engineering, in addition to remote monitoring of the tearing force, regular on-site maintenance was still required for geocell-reinforced structures to avoid potential safety hazards caused by the tearing of the PET geocell strip.

The speed of tearing failure of geocell strips may be related to the tensile rate, strip material and other factors, but the tearing failure cross-section of the specimen can be characterized as the brittle or ductile failure mode of the specimen. The comparison of the failure characteristic figure of HDPE and PET trapezoidal specimens with the slit was shown in Figure 12. From Figure 12, there was a noticeable difference in the tearing path and failure mode. These differences could be attributed to the properties of materials. The HDPE trapezoidal specimen with the slit produced an obvious plastic zone along the initial slit during the test, exhibiting ductile failure. The tearing path was the boundary of the plastic zone, and the tearing path was inconsistent with the direction of the initial slit. However, the PET trapezoidal specimen with the slit was directly torn into two parts along the initial slit under the action of the tearing force, and the tearing path was very clear, exhibiting brittle failure.

According to Equation (1), the damage ratio of tearing force (R_TF) of the HDPE trapezoidal specimen was 38% to 69.5%, which was about 1.82 times that of the HDPE. It was determined that the effect of the slit on the damage of the PET trapezoidal specimen was greater than that of the HDPE trapezoidal specimen. Based on the above analysis, the tearing resistance force of the PET geocell strip without the slit was greater than that of the HDPE geocell strip, while the tearing resistance force of the PET geocell strip after the generation of the slit was less than that of the HDPE geocell strip. Hence, compared with HDPE geocell reinforced structures, more attention should be paid to the laying of the PET geocell in the transition zone of reinforced slopes and the backfilling in structures such as reinforced-soil retaining walls and foundations.

Based on the above research, the following ideas are summarized so that the research may contribute to sustainability. Firstly, for geocell technology, the geocell is a new type of geosynthetic material used for soil improvement which can effectively reduce soil erosion and soil loss and improve soil sustainability. Improving the understanding of the mechanical tearing behavior of geocells can optimize their structure and the characteristics of materials, improve the service life and stability of geocell-reinforced structures, reduce the frequency of repair and replacement, and reduce resource consumption and environmental impact. Secondly, for the soil medium, the application of geocells can prevent problems such as soil erosion and landslides, protect land resources and maintain ecological balance. The research can determine the optimal arrangement density and structural parameters, improve erosion resistance and load-bearing capacity, reduce the cost of land repair and restoration, and achieve sustainability of the soil medium. Lastly, for the environment, geocells are generally manufactured from renewable materials, such as polyethylene and polypropylene, which have good ageing resistance and degradability, reducing pollution and damage to the environment. By studying the mechanical tearing behavior of geocells, it is possible to optimize their material combinations and manufacturing processes, improve the degradability of the materials and reduce the impact on the environment.

4. Conclusions

This paper investigated the post-damage mechanical tearing behavior of HDPE and PET geocell strips through a series of trapezoidal tearing tests. In this study, the trapezoidal tearing results and failure mode of the trapezoidal specimen were analyzed. Meanwhile, the mechanical tearing characteristics of trapezoidal specimens with and without slits were discussed. In addition, the effect of slit on the mechanical tearing behavior of HDPE and PET specimens made from geocell strips was also compared. The following conclusions were obtained.

(1) According to the generation sequence of plastic zone, the tearing process of HDPE specimen with the slit could be divided into three stages. The HDPE specimen with the slit produced an obvious plastic zone along the initial slit, exhibiting ductile failure. The shape of the plastic zone presented a horizontal cone shaped.

(2) Both the strength and deformation characteristics of HDPE trapezoidal specimen after damage decreased, the effect of slit on the deformation characteristics was greater than the strength characteristics. The R_TF and R_TD of the HDPE specimen used in this study were 38.0% and 55.0%, respectively.

(3) The PET specimen was directly torn into two parts along the initial slit under the tearing force. Due to stress sensitivity, the PET specimen without slit experienced local tearing. The tearing force before and after local tearing of PET specimen without slit was 3.27 times and 1.15 times greater than that of the PET specimen with slit.

(4) The R_TF and R_TD of the PET specimen used in this study were 69.5% and 14.4%, indicating that the impact of slit on strength characteristics was significantly greater than that on deformation characteristics. It was determined that the deformation performance of the PET trapezoidal specimen with slit was maintained at more than 80%.

(5) The slit had a significant impact on the tearing displacement of HDPE and PET specimens, especially the post-peak tearing displacement. The post-peak tearing displacement of HDPE was 10.99 times that of PET. The impact of slit on the tearing force of PET specimen was greater than that of HDPE, which was 1.82 times for HDPE.

The follow-up work will study the coupling effect of tearing force and low or high temperature on the mechanical tearing behavior of geocell strips, to improve the understanding of the mechanical characteristics of geocells.