High-density Polyethylene Geogrid Research Articles

Effect of oxidative ageing on stiffness improvement factor for HDPE and PET geogrids

The effect of thermal-oxidative ageing on in-isolation and in-soil stiffness is investigated for two high-density polyethylene (HDPE) and two polyester (PET) geogrids when incubated in an oven at 90°C. It is shown that the in-isolation and in-soil stiffness of three out of the four geogrids decreased during the 4-month ageing time with a faster degradation of the two PET geogrids. In contrast, one of the HDPE geogrids examined with thicker rib showed slower degradation compared to the PET counterparts. It is also shown that a geogrid might have a higher unaged stiffness, but less aged stiffness compared to another geogrid made of the same polymer type. It is suggested that the ageing approach used in this study could be adopted for the selection of geogrids based on their long-term stiffness for structures such as reinforced embankments or mechanically stabilized earth walls that require a long service life. Moreover, results indicate that the stiffness improvement factor for PET geogrids decreases with ageing time. On the contrary, the stiffness improvement factor of the HDPE geogrid increases with time and hypothetically might keep increasing followed by a sudden drop to zero at full degradation of the geogrid.

DURABILITY TESTS OF GEOSYNTHETIC MATERIALS USED IN DIFFICULT ENVIRONMENTAL CONDITIONS

The article describes the ageing process of high-density polyethylene (HDPE) geogrids under the influence of chemical and environmental factors. Research on accelerated ageing of a uniaxial HDPE geogrid incubated in a water solution for a period of 12 months is presented. Three temperatures (25°, 45° and 75°C) were selected for the accelerated ageing tests in aqueous solutions simulating the conditions at a municipal waste landfill. Changes were observed using differential scanning calorimetry (DSC), and correlations with the mechanical properties of the aged geogrid were checked. No significant effect of the loss of antioxidant in the material on the mechanical properties of the uniaxial geogrid was observed. The tests made it possible to determine the mechanical properties, such as tensile strength and deformability of the geogrid – which are extremely important in the analysis of slope stability.

Analysis of tensile properties of geogrids for different morphology parameters

Following the emergence of new morphology geogrids, tensile tests on some of the new morphology geogrids were conducted in this paper for practical engineering options. The study focuses on the tensile testing of 3D printed geogrids to analyze the effects of morphology differences (material, planar structure, filling rate, rib width and rib height) on the tensile properties of geogrids. The test results show that the tensile properties of high density polyethylene geogrids are stronger than those of polylactic acid geogrids. Increasing the filling rate of geogrid can increase the tensile performance while decreasing the rate of tensile performance improvement. In contrast to the elongation at the break of the geogrid, the maximum tensile strength and the maximum tensile modulus increase in the order of bidirectional, triaxial and quadaxial geogrids. When the rib width and the rib height increase by the same amount, the rib height increases the maximum tensile strength of the geogrid more than that of the rib width.

Experimental Research on the Creep Behavior of the Interface of Compacted Loess and High-Density Polyethylene Geogrid

The stability of geogrid-reinforced soil structure is closely related to the interface characteristics between geogrid and soil. However, the creep behavior of the soil–geogrid interface is still unrevealed. In this study, using a modified stress-controlled pullout device, influence of the normal pressure, dry density, and water content on creep behavior of interface of compacted loess and high-density polyethylene (HDPE) geogrid is investigated. A three-parameter empirical model and a Merchant element model were established through fitting analysis. Analysis results show that the normal pressure, dry density, and water content have significant effects on the creep shear displacement of the reinforced soil interface. Under the same pullout level, creep displacement of the interface increases with the increase of water content and decreases with the increase of dry density and normal pressure. Both the three-parameter empirical model and Merchant element model can describe the creep characteristics of the reinforced soil interface. The Merchant model is more accurate in the early stage, while the three-parameter empirical model is more suitable for predicting the long-term creep deformation of the interface of compacted loess and geogrid.

Importance of PET geogrid in the enhancement of hill slope's safety factor: a finite element approach

Slope stability analysis is crucial since the instability of the slope contributes to the failure of many buildings in hilly areas. Numerous ground improvement techniques are used to address this issue, including vertical drains, grouting, soil replacement, geosynthetic reinforcement, and piling. Geosynthetics are used more frequently to stabilize the hill slope as an alternative and cost-effective approach. The behavior of a geogrid reinforced slope was investigated in the current work utilizing 3D numerical analyses using the finite element program MIDAS GTS NX 2021 v1.1. Five types of geogrid are made up of different materials such as Polyethylene Terephthalate (PET), High-Density Polyethylene (HDPE), Low-Density Polyethylene (LDPE), Polypropylene (PP), and Polyvinyl Chloride (PVC) were considered to stabilize the slope. The percentage Increase in Factor of safety for PET, HDPE, LDPE, PVC and PP Geogrid reinforced slopes are 18%, 17.1%, 16.7%, 15.6% and 16.3% respectively as compared to Unreinforced slope. The percentage decrease in deformations for PET, HDPE, LDPE, PVC and PP Geogrid reinforced slopes are 97%, 78.4%, 64.79%, 78.17% and 49% respectively as compared to Unreinforced slope. The PET geogrid reinforced Slope provides a higher factor of safety and lower deformation among other geogrid reinforced Slopes because PET geogrid provides maximum pull-out resistance among different types of geogrid. Similarly, the strain induced in PET geogrid-reinforced slope is minimum among other geogrid-reinforced Slopes due to the lower stiffness of PET geogrid. Hence the slope reinforced with PET geogrid performed well compared to different geogrid-reinforced slopes.

Comparison between uniaxial and pull-out tests setup for polyester and HDPE geogrids

Uniaxial tensile and/or pull-out tests are performed to obtain the stress-strain curve of geogrids. The clamp lining in contact with a specimen in the uniaxial tensile test or the grit of a sandpaper clamp liner in a pull-out test affects the results. In this study, one geogrid made of polyester and another one made of high-density polyethylene (HDPE) are tested in the uniaxial tensile test using various clamps and in pull-out test using sandpapers of different grits. Based on the results obtained, it is recommended to test HDPE in uniaxial tests with serrated steel-lined clamps and in pull-out tests with sandpaper (grit 180) lined clamps. A polyester geogrid shall be tested in uniaxial tests with plastic-lined clamps and in pull-out tests with sandpaper (grit 400) lined clamps.

Appraisal of Anchor Arrangement and Size on Sand-Geogrid Interaction in Direct Shear

Soil–reinforcement interaction is a major factor in the analysis and design of reinforced earth structures. In current research the effects of attaching elements of different size and numbers as anchors on enhancement of interaction at soil–geogrid interface under direct shear conditions were studied. Poorly and well graded sands (SC & Sf), a high density polyethylene geogrid, anchors with three different size and numbers (layouts) and clamping length of 2 cm from shear surface were used. Samples were prepared dry at a relative density of 80% in a 30 × 30 cm direct shear box and subjected to normal pressures of 12.5, 25 and 50 kPa with the shear load applied at a rate of 1 mm/min. Results of the assessment show that anchored geogrids improve shear resistance at interface mainly due to mobilization of passive soil resistance that is significantly influenced by the magnitude of the normal pressure and the number and size of anchors. Interaction enhancements achieved varied between a minimum of 8% and a maximum of 42%.

Short-term tensile behaviour of three geosynthetics after exposure to Recycled Construction and Demolition materials

Designing and building structures and infrastructures with alternative and environmentally friendly materials is, nowadays, an important step towards a more sustainable society. Recycled Construction and Demolition (C&D) materials have been considered as alternative materials in different civil engineering applications, such as unbound pavement layers and structural embankments, in which geosynthetics are also frequently applied. If the durability of geosynthetics is an important issue when conventional materials are used, it becomes more relevant when utilising alternative materials. This paper presents and discusses the chemical and environmental degradation induced by a recycled C&D material on the short-term tensile behaviour of three geosynthetics used typically as reinforcement material (two geogrids and a high-strength geotextile), after 24 months of exposure. For comparison purposes, geosynthetics samples were also exposed to a natural soil. The physical and environmental characterization of the recycled C&D material are presented and the tensile behaviour of intact (as-received) samples, immediately exhumed samples and exhumed samples after 24 months of exposure are characterized and discussed. To evaluate the potential damage in more detail, Scanning Electron Microscope (SEM) analyses were carried out. Regardless of the geosynthetic type and exposure condition, the geosynthetic’s tensile strength decreased after 24-month exposure. This loss of tensile strength was insignificant for the high density polyethylene geogrid and higher for the geotextile. The effect of exposing the geosynthetics to the recycled C&D material for 24 months had some relevance only for geotextile. For both geogrids, the loss of strength for the specimens immediately exhumed and exposed through 24 months is comparable. In general, the exposure to the recycled C&D material or to the soil induced similar effects.

Experimental investigation of the behavior of soil reinforced with waste plastic bottles under cyclic loads

The low natural degradation rate of plastic products makes the recycling or disposal of increasing post-consumer plastic materials, including Plastic Polyethylene Terephthalate (PET) bottles, a global concern. The current capacity of recycling programs for absorbing annual waste PET materials in environmentally friendly ways is limited. As a novel beneficial option, the cellular form of post-consumer bottles might act to provide sufficient confinement for infill soil and hence provide a technique by which the behavior of foundation beds under applied loads could be improved. In this study, a series of full-scale tests were conducted on unreinforced and PET (Plastic Polyethylene Terephthalate) bottle reinforced beds. For each test 100 cycles of 400 kPa load followed by 400 cycles of 800 kPa load was applied to the pavement through loading plate. Experiments were performed in a 2200 × 2200 × 1000 mm test pit on a 300 mm diameter circular loading plate. Three different sizes (i.e., different external diameters and heights) of waste plastic (PET) bottles filled with soil were tested as a reinforcing element. In some tests, a single layer of HDPE (high-density polyethylene) geogrid layer over bottles reinforced zone was examined. The results showed improvement in all reinforcing arrangement, although improvement was slightly higher for smaller bottles. For the reinforced case, a lower settlement accumulation rate and a higher resilient (elastic) settlement was observed compared to the unreinforced condition. In the case of lower cyclic loading (400 kPa amplitude), an average 42% improvement occurred for the three bottle-reinforced scenarios, however this value was 179% for higher cyclic loading (800 kPa), indicating more benefits of bottle-reinforcement in the case of critically loaded conditions. An extra layer of geogrid over the bottle-reinforced zone increases load distribution area and prevents distortion of bottles, leading to as much as 82% more reduction in soil settlement compared to the case without geogrid. The combined use of bottles and single geogrid layer eliminates the need to multilayer geosynthetic reinforcement usage, helps the economy of the project besides avoiding the high cost of waste bottle disposal.

Experimental Investigations on Pullout Behavior of HDPE Geogrid under Static and Dynamic Loading

This paper describes a series of laboratory pullout tests that were performed to investigate the pullout behavior of high‐density polyethylene (HDPE) uniaxial geogrid subjected to static and dynamic loading. Pullout tests were conducted on HDPE geogrid reinforced coarse sand under normal static loading (60–300 kPa), dynamic loading with different amplitudes (20, 40, and 60 kPa), and different frequencies (2, 4, and 6 Hz) by using the newly developed pullout apparatus. The results indicated that the pullout resistance of geogrid presented different growth patterns with the increase of normal loads under static loading. The amplitude and frequency both had significant effects on the interaction between reinforcement and soil, and the increment of the pullout resistance was 0.6 kN and 0.3 kN, respectively. The effect of dynamic loading on the soil‐geogrid interface can be gradually equivalent to that of static loading corresponding to the balance position of dynamic loading with the increase of frequency compared with the static loading. The results of this study are helpful for the selection of the strength of the reinforcement in different locations and to simplify the study on the stress of reinforcement in reinforced soil structures under traffic loads.

Experimental evaluation of an expanded polystyrene (EPS) block-geogrid system to protect buried pipes

This study presents the results of experimental tests carried on buried uPVC (unplasticized polyvinyl chloride) pipes with an external diameter of 160 mm. The behavior of buried pipes in unreinforced and reinforced trenches by a single layer of HDPE (high-density polyethylene) geogrid and expanded polystyrene (EPS) geofoam block were investigated. To simulate vehicle wheel loadings, 500 cycles of repeated load respectively with an amplitude and frequency of 450 kPa and 0.33 Hz was applied to a loading plate placed over the trench surface. Pipe behavior under cyclic loadings were assessed using vertical diameter strain measurements, circumferential strains and pressures at the crown and springline. Additionally, settlement at the soil surface was measured throughout testing. The testing program is aimed at evaluating the role of different parameters influencing pipe behavior, such as embedded depth of the pipe, implementation of the EPS block and geogrid layer simultaneously and separately, and density, width and thickness of the EPS blocks. The results illustrate that the rate of changes in the pipe circumferential strain, vertical diameter strain and soil surface settlement, which increase rapidly in initial loading cycles, decreased as loading progressed. Based on the results, the density, width and thickness of implemented EPS blocks have an impact role in improving the behavior of buried pipes. The use of geogrid reinforcement with an EPS block with density of 30 kg/m3, thickness of 60 mm and width of 1.5 times the pipe diameter showed the most benefit when balancing vertical diameter strain, pipe crown strain, and soil surface settlement.

Field monitoring and numerical modeling of 4.4 m-high mechanically stabilized earth wall

A full-scale mechanically stabilized earth (MSE) wall was constructed and monitored in a residential compound in New Cairo, Egypt. The height of the wall was 4.4 m and the facing consisted of modular concrete blocks. The wall was reinforced with high-density polyethylene geogrid. This paper presents the instrumentation program and interpretation of measurements recorded at end of construction and after applying loads on top of the wall. Strains in the geogrid layers were measured using strain gauges. Deformations of the wall face were surveyed using a total station device. Backfill soil properties and geogrid stiffness properties were characterized through laboratory testing. A two-dimensional finite element model was developed to simulate the performance of the instrumented wall at end of construction. Reinforcement loads deduced from measured strains were compared to different load prediction methods: AASHTO Simplified, K-stiffness, Simplified stiffness, and finite element modeling. The paper highlights important aspects to be considered in numerical modeling of MSE walls. Predicted horizontal wall displacements and reinforcement loads from finite element modeling compared well to field measurements. The Simplified stiffness method better predicted the reinforcement loads compared to the AASHTO Simplified and K-stiffness methods.

Tensile behaviour of an HDPE geogrid under cyclic loading: experimental results and empirical modelling

Residual deformations due to sustained and cyclic loading, acting on full-scale reinforced soil-retaining walls, could lead to an unacceptable serviceability limit state. In the present study, the writers carried out an extensive experimental programme comprising several monotonic and multistage tensile tests on a high-density polyethylene (HDPE) uniaxial extruded geogrid. Based on analysis of the test results, a combined empirical model has been developed that can predict the tensile behaviour of a specific HDPE geogrid subject to cyclic tensile loading. The paper presents the results and analyses of the effect of cyclic tensile loading histories (varying pre-stress tensile load, frequency, amplitude and number of cycles) on those parameters that are characteristic of hysteresis loops (i.e. the maximum and residual strains cumulated during each cyclic loading, tensile stiffness and area of the hysteresis loops). The results show that the average strain rate and the loading time affect the material's tensile response, as well as the combination of pre-stress tensile load and loading amplitude. The writers also investigated the difference between the load-strain relations for the monotonic test and those for the multistage tests, and concluded that the effects of cyclic loading histories are not due to material degradation.

Effect of the Impact of Chemical and Environmental Factors on the Durability of the High Density Polyethylene (HDPE) Geogrid in a Sanitary Landfill

A high density polyethylene (HDPE) uniaxial geogrid was exhumed after twenty years of service in a sanitary landfill, and its properties were examined. A geogrid installed in a landfill is exposed to mechanical and chemical factors (e.g., a wide pH range and high temperatures), as well as different weather conditions. This paper presents the results of physical and mechanical analyses of virgin and aged HDPE geogrid samples. Structural changes observed by differential scanning calorimetry and Fourier transform infrared spectroscopy (FT-IR) spectroscopy correlate with the mechanical properties of the aged geogrid. The mechanical properties were found to have changed only slightly. In the FT-IR spectrum of the topmost layer of the aged geogrid samples, no significant changes were observed compared to the spectrum of the top layer of the virgin samples. This indicates the strong chemical resistance of the HDPE material, which is able to withstand environmental conditions for at least 20 years of service in a landfill.

Creep behaviors and constitutive model for high density polyethylene geogrid and its application to reinforced soil retaining wall on soft soil foundation

The reinforced soil retaining wall has been applied extensively with its numerous advantages. However, the performance of geogrids reinforced soil retaining wall was affected by creep behavior of geogrids and soft soil foundation as well as stress relaxation of geogrids. In this paper, the creep behavior and stress relaxation of high density polyethylene geogrids have been investigated in a laboratory. Four different sustained load levels of 20%, 40%, 50% and 60% of ultimate tensile strength were employed as load levels for creep tests. A constitutive model, which was capable of simulating creep behaviors and stress relaxation of geogrids, was established based on experimental data and verified. Numerical modeling using finite element method has also been used to assess the impact of creep behavior and stress relaxation of geogrids on the long-term performance of reinforced soil retaining wall on the deep soft soil foundation. The results show that the constitutive model for creep behavior and stress relaxation of geogrids was in good qualitative agreement with experimental data under different load levels. To ensure the stability of the retaining wall, the working stress of geogrids should be less than 40% of ultimate tensile strength, and the high strength geogrids should be adopted in the middle of the wall or the spacing of geogrid reinforced layers should be reduced. The deep soft soil foundation which is treated by piles can ignore the creep behavior of soft soil.

Effects of temperature on the rupture strength and elastic stiffness of geogrids

ABSTRACT: An automated tensile loading system that can accurately control histories of both loading and temperature was developed. A series of unconventional tensile loading tests were performed on polypropylene (PP), high-density polyethylene (HDPE) and polyester (PET) geogrids using this system. The following was found. The temperature significantly affects the elasto-viscoplastic stress–strain properties of the tested polymer geogrids. The inviscid stress–viscoplastic strain relation changes with temperature. The tensile rupture strength decreases by 9.2%, 26.7% and 4.5% when the temperature rises from 30°C to 50°C with PP, HDPE and PET geogrids, respectively. The elastic stiffness of the geogrid was evaluated by applying small-strain-amplitude unload–reload cycles after a certain period of sustained loading during otherwise monotonic loading at a constant load rate. The value increases with an increase in the tensile load level at a fixed temperature, and decreases with an increase in the temperature at a fixed load level. A set of mathematical expressions are proposed to describe these trends of tensile rupture strength and elastic behaviour.

Evaluation of creep behavior of high density polyethylene and polyethylene-terephthalate geogrids

The tensile creep behavior of polyethylene-terephthalate (PET) and high density polyethylene (HDPE) geogrids was evaluated using five test methods: the short- and long-term stepped isothermal method (SIM), the short- and long-term time-temperature superposition (TTS), and the conventional method. SIM and TTS are acceleration tests using elevated temperatures. SIM uses a single specimen throughout all temperature steps in contrast to TTS in which a new specimen is employed for each temperature step. The test results indicate that at the same percentage of ultimate tensile strength, PET geogrid exhibited less creep deformation than the HDPE geogrid. The HDPE geogrid exhibited primary, secondary, and tertiary creep stages before rupture, whereas only primary creep and tertiary creep were detected in the PET geogrid. Furthermore, the strain rate of the primary creep stage was found to be independent of the applied loads for the PET geogrid, while it increased exponentially for the HDPE geogrid. The activation energies deduced from different accelerated creep tests were very similar for the PET geogrid. In contrast, the activation energies were higher from the short-term acceleration tests than from the long-term tests for the HDPE geogrid. The four-parameter Weibull model was able to predict the linear and non-linear creep behavior up to 100 years based on 10-h creep testing data. The creep reduction factor of 100 years design life was evaluated and higher values were resulted from the HDPE geogrid than from the PET geogrid.

Temperature and pressure effects on the oxidation of high-density polyethylene geogrids

Two acceleration incubation methods were used to evaluate the oxidative resistance of high-density polyethylene (HDPE) geogrids. In Series #1, forced air ovens at elevated temperatures of 55, 65 and 75 °C were utilized to assess two types of HDPE geogrids, GG-1 and GG-2. After 84 months of incubation, the tensile properties and molecular weight remained unchanged, while the amount of antioxidants, which was measured by the standard oxidation induction time (OIT), decreased as incubation time increased. The depletion mechanisms of antioxidants were found to be significantly different between the two geogrids. For geogrid GG-1, depending on the incubation temperature, the OIT versus incubation time curves can be divided into two or three linear regions: Regions I, II and III. In Region I, the OIT-retained values remained constant. Region II occurred within few months of incubation; it consisted of a steep slope indicating rapid depletion rate of antioxidants. Region III, which followed immediately after Region II, exhibited a gradual decrease of OIT. In contrast, geogrid GG-2 showed an exponential decrease of OIT with aging time. Under the temperature acceleration condition, the predicted lifetimes of antioxidants in GG-1 and GG-2 are 172 and 122 years, respectively, using the Arrhenius model at a constant site temperature of 20 °C. The second incubation method, Series #2, combined elevated temperature with high oxygen pressure. The acceleration of antioxidant depletion is governed by the oxygen partial pressure instead of total pressure in the incubation pressure cell. The antioxidant reaction rate increases exponentially with pressure. The lifetime of antioxidant is predicted to be 116 years for GG-1 at conditions of 20 °C and 1 atm.

Lateral and axial deformation of PP, HDPE and PET geogrids under tensile load

The paper describes a series of short-term in-isolation tensile tests that were carried out to investigate the load–strain–time performance of typical knitted polyester (PET), biaxial polypropylene (PP) and uniaxial high-density polyethylene (HDPE) geogrid products. The specimens were subjected to constant rate of strain (CRS) loading and variable rate of strain (VRS) loading. Variables between test specimens included specimen length and specimen aspect ratio (e.g. number of ribs). A novel feature of these tests was that internal strains in the specimens were recorded using non-contact measurements taken with a video-extensometer apparatus. The data shows that at large strains the PP and HDPE geogrid specimens exhibited large lateral strains while lateral strains were negligible for the PET product. Results of CRS tests carried out with different specimen aspect ratios showed that there was no change in tensile strength, axial strain and lateral strain at rupture for HDPE and PET materials. The PP material in this investigation did show an increase in axial and lateral strain at rupture for a specimen tested at an aspect ratio approaching one. The polyolefin geogrids (PP and HDPE) were shown to exhibit tensile stiffness and ultimate tensile load capacities that increased with rate of strain. Implications of the contractive behaviour of the polyolefin geogrid products to the selection of in-soil reinforcement stiffness values and rheological modelling are identified.

CONSTRUCTION AND POST-CONSTRUCTION BEHAVIOUR OF A GEOGRID-REINFORCED STEEP SLOPE

A geogrid reinforced steep slope was built and monitored during construction and during the first ten months of service. The slope is located between Regua and Reconcos in the new Portuguese main itinerary, IP3, and is a part of reestablishment 2. The reinforced slope has an extension of about 206.2 m, is in curve and the reinforced area reaches a maximum height of about 19.6m in the outside curve slope at 150.0 m of extension (km 0 + 150). The monitored slope cross section is at km 0 + 150. The reinforcements are high density polyethylene geogrids; materials with different tensile strength values were used. The reinforcement strains were measured at three reinforcement levels using linear extensometers. The soil vertical stresses were recorded using load cells. The internal horizontal displacements of the slope were recorded using two inclinometer tubes. The face displacements were recorded topographically in points spaced approximately 1.2 m vertically along the face of the slope on the km 0 + 150 cross section. The reinforced slope behaviour was observed during a period of about 13 months, which includes three months of construction period. This way it was possible to obtain information about the slope behaviour during and after construction (the first 10 months of service). The behaviour of the observed reinforced slope is characterized by: low values of face displacements, slope internal horizontal displacements and reinforcement strains; change of the face displacements configuration at the end of construction during service; tendency to stabilization of the horizontal displacements in a relatively short period of service; change, during service, of the position of the line passing through the points of the reinforcements where maximum strains were recorded. The reinforced slope behaviour express the conservative design of Equilibrium Limit methods and encourage the research on new design methods for geosynthetic reinforced soil systems. (A)