Pullout tests conducted on clay reinforced with geogrid encapsulated in thin layers of sand

The interaction between reinforcement and backfill materials is a significant factor for analysis and design of reinforced earth structures which is simplified as pullout or direct shear resistance. This paper presents the results of pullout tests aimed at studying the interaction of clays reinforced with geogrids embedded in thin layers of sand. Pullout tests were conducted after modification of the large direct shear apparatus. Samples were prepared at optimum moisture content and maximum dry densities obtained from standard Proctor compaction tests. Tests were conducted on clay-geogrid, sand-geogrid and clay-sand-geogrid samples. A unidirectional geogrid with sand layer thicknesses of 6, 10 and 14 mm were used. Results revealed that encapsulating geogrids in thin layers of sand under pullout conditions enhances pullout resistance of reinforced clay. For the clay-sand-geogrid samples an optimum sand layer thickness of 10 mm was determined, resulting in maximum pullout resistance which increased with increasing confining pressure. The optimum sand layer thickness was the same for all the normal pressures investigated. For sandy soils the passive earth pressure offered the most pullout resistance, whereas for clayey soils, it was replaced by frictional resistance. It is anticipated that provision of thin sand layers will provide horizontal drainage preventing pore pressure built up in clay backfills on saturation.     

Compressive load response of geogrid-reinforced fine,medium and coarse sands

This paper presents data obtained from a series of laboratory plate load tests performed on geogrid-reinforced sand beds. Fine, medium and coarse sands were used as test sand beds. Circular geogrids of diameter 120 mm were used as reinforcement layers. Improvement in load–settlement response was studied. The test sand beds were compacted to a relative density of 50%. A surface footing plate of diameter 60 mm was used as the shallow foundation. It was found that the horizontal geogrid reinforcement improved the load–settlement response. The applied load for a deformation of 0.5 mm in the case of geogrid-reinforced fine sand, medium sand and coarse sand was, respectively, 83 N, 44 N and 87 N, whereas it was 63 N, 38 N and 47 N when the sands were unreinforced. Load–settlement response was found to improve further with increase in number of geogrid layers and with decrease in spacing between them. Load improvement ratio (LIR), defined as the ratio of load for a given settlement in geogrid-reinforced case to that for the same settlement in unreinforced case, increased with number of geogrids (n) for all sands, but the improvement was significant in the case of coarse sand.     

Pullout testing and Particle Image Velocimetry (PIV) analysis of geogrid reinforcement embedded in granular drainage layers

The paper investigates the feasibility of using fine-grained soil as backfill material of geosynthetic-reinforced walls and slopes, through a laboratory study on pullout behavior of geogrids in granular layers. A series of pullout tests was carried out on an HDPE uniaxial geogrid in thin sand and gravel layers that were embedded in clay specimens.Aside from different soil arrangements, the influences of moisture content and overburden pressure on the geogrid pullout behavior is assessed and discussed. The tests were carried out at four different gravimetric water contents (GWC) on the dry and wet sides of the clay optimum moisture content (OMC), and overburden pressure values within the range σ_v = 25–100 kPa. Particle Image Velocimetry (PIV) was used to capture digital images during the tests, which were processed to help with the interpretation and improved understanding of the soil-geogrid interactions at different GWC values. Results show that embedding geogrid reinforcement in layers of sand or gravel can significantly increase the pullout resistance in an otherwise moist clay backfill, and this improved pullout efficiency is greater at higher overburden pressures. The improvement in pullout capacity was observed in clay specimens compacted at both the dry and wet sides of the OMC.     

Experimental and numerical analysis of large scale pull out tests conducted on clays reinforced with geogrids encapsulated with coarse material

The pullout test is one of the methods commonly used to study pullout behavior of reinforcements. In the current research, large pullout tests (i.e. 100 × 60 × 60 cm) have been conducted to investigate the possibility of pullout resistance enhancement of clays reinforced with HDPE geogrid embedded in thin layers of sand. Pullout tests on clay–geogrid, sand–geogrid and clay–sand–geogrid samples have been conducted at normal pressures of 25, 50 and 100 kPa. Numerical modeling using finite element method has also been used to assess the adequacy of the box and geogrid sizes to minimize boundary and scale effects. Experimental results show that provision of thin sand layers around the reinforcement substantially enhances pullout resistance of clay soil under monotonic loading conditions and the effectiveness increases with increase in normal pressures. The improvement is more pronounced at higher normal pressures and an optimum sand layer thickness of 8 cm has been determined for maximum enhancement. Results of numerical analysis showed the adequacy of the box and geogrid length adopted as well as a relatively good agreement with experimental results.     

Effects of Geosynthetic Reinforcement Type on the Strength and Stiffness of Reinforced Sand in Plane Strain Compression

The effects of geosynthetic reinforcement type on the strength and stiffness of reinforced sand were evaluated by performing a series of drained plane strain compression tests on large sand specimens. The reinforcement type is described in terms of the degree of unification of the constituting components (for geocomposites) as well as the tensile strength and stiffness, the covering ratio and others (for geocomposites and geogrids). Sand specimens reinforced with different geosynthetic reinforcement types exhibited significantly different reinforcing effects. A geocomposite made of a woven geotextile sheet sandwiched firmly with two sheets of non-woven geotextile, having a 100% effective covering ratio, exhibited reinforcing effects higher than typical stiff and strong geogrids. With some geocomposite types, the reinforcing effects increase substantially by better unifying longitudinally arranged stiff and strong yarns and non-woven geotextile sheets. When fixed firm to the yarns, the non-woven geotextile sheets function like the transversal members of a geogrid by locally transmitting load activated by interaction with the backfill to the yarns. These geocomposites can exhibit reinforcing effects equivalent to those with stiff and strong geogrids. Local strain fields of the specimens are presented to show that, for reinforced sand, the peak stress state reached is always associated with the development of shear band(s) in the sand and a higher peak strength is achieved when the strain localisation starts at a larger global axial strain due to better reinforcing effects.     

Large scale direct shear tests of soil/PET-yarn geogrid interfaces

The interface shear strength of soil against geosynthetic is of great interest among the researchers in geosynthetic properties. This study conducts a series of large scale direct shear tests to investigate the interface shear strength of different soils (sand, gravel, and laterite) against PET-yarn geogrids of various tensile strengths, percent open area, and aperture patterns. First, the appropriateness of different set-ups of a lower shearing box is examined in this study. It reveals that a lower box which is filled with the test soil and is of the same size as the upper box is more suitable for testing the soil/geogrid interface. The test results show that the soil/PET-yarn geotextile interface has significantly lower shear strength than soil strength. The ratio of shear strength soil/PET-yarn geotextile interface to internal shear strength of soil is about 0.7–0.8 for Ottawa sand and for laterite, and it is about 0.85–0.95 for gravel. On the other hand, the soil/geogrid interface has higher shear strength. The ratio of shear strength soil/PET-yarn geogrid interface to internal shear strength of soil is about 0.9–1.05. It is found that the shear strength ratio of soil/PET-yarn geogrid interface is positively correlated to the transverse tensile strength of the PET-yarn geogrid. However, it is negatively correlated with the aperture length and percent open area of the PET-yarn geogrid. The interface shear test results of PET-yarn geogrid against different soils are compared with the test results predicted by a classical model for analyzing the applicability of the classical model. Further, a simple model is proposed herein to estimate the bearing resistance provided by the transverse ribs of geogrid. It shows this component to be about 0–15% when PET-yarn geogrid is against Ottawa sand or laterite, while it is smaller when the PET-yarn geogrid is against gravel.     

Interaction between geogrid reinforcement and tire chip–sand lightweight backfill

This paper deals with the interaction between the geogrid and the tire chip–sand mixture including the determination of the index properties of the backfill materials, the shear strength parameters, the interaction coefficients, and the efficiency of geogrid reinforcements in tire chip–sand backfills. Numerous experiments including index tests, compaction tests, pullout tests, and large-scale direct shear tests were conducted. Saint–Gobain (geogrid A) and Polyfelt (geogrid B) were selected as reinforcing materials. Tire chip–sand mixtures with mixing ratios of 0:100, 30:70, 40:60, and 50:50 by weight were used as fill materials. The test results revealed that the dry unit weight of tire chip–sand mixtures depended more on the sand content, and less on the water content. The mixture at the mixing ratio of 30:70 by weight or 50:50 by volume was found to be the most suitable fill material compared to other mixing ratios. The pullout resistance and the pullout interaction coefficients of geogrid A were slightly higher than those of geogrid B. In contrast, in the direct shear resistance, the direct shear interaction coefficients, and the efficiency values of geogrid B were slightly higher than those of geogrid A. Since geogrid B has the needed uniaxial reinforcement properties and its sufficient interaction characteristics with tire chip–sand mixture, the geogrid B was utilized in this study. The interaction coefficients between the tire chip–sand backfill with 30:70 mixing ratio by weight were found to be 0.71 in pullout mode and 0.92 in direct shear mode for geogrid B.     

Centrifuge model study on geogrid reinforced soil walls with marginal backfills with and without chimney sand drain

The objective of this paper is to investigate the performance of geogrid reinforced soil walls with panel facing using marginal backfill with and without chimney sand drain subjected to seepage. A series of centrifuge model tests were performed at 40 gravities using a 4.5 m radius large beam centrifuge facility available at IIT Bombay. The results revealed that a geogrid reinforced soil wall with low stiffness geogrid and without any chimney drain experienced a catastrophic failure due to excess pore water pressure that developed in the reinforced and backfill zones at the onset of seepage. In comparison, a soil wall reinforced with stiff geogrid layers was found to perform effectively even at the onset of seepage. Provision of chimney sand drain effectively decreased pore water pressure not only at the wall toe but also at mid-distance from toe of the wall and thereby resulted in enhancing the wall performance under the effect of seepage forces. However, a local piping failure was observed near the toe region of the wall. The observed centrifuge test results were further analysed by performing seepage and stability analyses to evaluate the effect of thickness of sand layer in a chimney drain. An increase in thickness of sand layer in chimney drain was found to improve the discharge values and thereby enhancing the factor of safety against piping near the toe region. Based on the analysis and interpretation of centrifuge test results, it can be concluded that marginal soil can be used as a backfill in reinforced soil walls provided, it has geogrid layers of adequate stiffness and/or proper chimney drain configuration.     

柔性与硬性地工格网在砂土与风化泥岩围压下之力学特性

地工格网（以下称格网）用於加劲土壤时，除考虑无围压下的张力行为之外，围压下之力学性质更是设计考量的重点。实际工程应用而言，基於经济考虑，期以现地土壤作为回填材料。本研究分别以拉出、围压抗张与直剪三种试验来探讨格网放土壤中之力学行为；并利用凝聚性泥岩与非凝聚性细砂作为回填材料，评估两种回填材料对加劲成效之影响。结果显示，柔性格网之肋条在拉出过程中易扭曲，造成主应力面旋转的现象，以致拉出阻抗大放硬性格网；围压下格网抗张的应力-应变行为可分为三阶段，即束制阻抗期、张力发展期与破坏期。束制阻抗期大都於3％应变内即已完成；在低围压情况拉出阻抗达20％～60％之拉出强度（相同应变），在高围压下达150％。由直接剪力试验结果可以预测：（a）格网／泥岩加劲结构-低围压时，剪力破坏面应通过格网／泥岩之界面；而高围压时，剪力破坏面应通过泥岩上体。（b）格网／细砂加劲结构-低围压与高围压下剪力破坏面应通过格网／细砂之界面。     

The humidity buffer capacity of clay–sand plaster filled with phytomass from treatment wetlands

The aim of our study was to measure the amount of moisture absorption and desorption in clay–sand plaster mixed with “fibre-wool” from Typha spadixes and chips of Typha and Phragmites. Common cattail (Typha latifolia) and common reed (Phragmites australis) are the most common plants in constructed and semi-natural wetlands for wastewater treatment, as they are highly valued in ecologically oriented construction. The cattail and reed for the experiment were harvested in a wastewater treatment subsurface flow semi-natural wetland and in two free water surface constructed wetlands which showed reliable aboveground phytomass production over a 5 years period (for Typha, 0.37–1.76 kg DW m⁻² in autumn and 0.33–1.38 kg DW m⁻², and for Phragmites, 0.61–1.32 and 0.61–1.02 kg DW m⁻², respectively). The quantity of moisture absorption and desorption was measured in a climatic chamber where the humidity of ambient air was suddenly raised from 50% to 80% (absorption) and reduced from 80% to 50% (desorption). Over 12 h, all of the samples released the same amount of water as they absorbed. The clay–sand plaster samples absorbed slower than they desorbed, whereas the gypsum wallboard required significantly more time for desorption. Added phytomass gave positive effects by reducing the weight of the clay–sand plaster, accelerating and increasing moisture absorption.     

Numerical simulation and sensitivity analyses of full-scale test embankment with reinforced lightweight geomaterials on soft Bangkok clay

A full-scale test embankment was constructed on soft Bangkok clay using rubber tire chip–sand mixture as a lightweight geomaterial reinforced with geogrid under working stress conditions. The facing of the embankment was made of segmental concrete blocks with rock filled gabion boxes as the facing to the sloping sides. This paper attempts to simulate the behavior of the full-scale test embankment using PLAXIS finite element 2D program by means of undrained analysis in the construction stage and thereafter consolidation analysis was performed during the service stage. The settlement predictions of the soft clay foundation mostly depended on the assumed thickness of the weathered crust and the OCR values of the soft clay layer. The predicted excess pore water pressures were sensitive to the OCR values of the soft clay layer. The lateral wall movements were overpredicted by the analysis due to the partially drained consolidation process at the early stage of the construction. The FEM computed geogrid movements were smaller than the observed field data due to the use of lightweight tire chips-sand backfill. The maximum tension line agreed reasonably well with the coherent gravity bilinear failure plane. The sensitivity analyses of settlements, excess pore water pressures, lateral wall movements, geogrid movements and tensions in geogrid were performed by varying the weathered crust thickness, the OCR values of soft clay, the permeability values of the soft clay and the interface coefficient of the geogrid. The settlements and the excess pore water pressures changed significantly when the OCR and the permeability values of soft clay were varied. The interface coefficient of the geogrid reinforcements affected the lateral wall movements, geogrid movements and tensions in the geogrids. The higher interface coefficient yielded less wall/geogrid movement and resulted in higher tensions in geogrids as expected. The results of analyses show that the FEM analysis using 2D plane strain conditions provided satisfactory predictions for the field performance.     

Fiber reinforcement effects on sand considering a wide cementation range

This paper describes laboratory drained standard triaxial tests conducted on artificially cemented Osorio sand specimens reinforced with randomly oriented discrete extensible polypropylene fibers. Cemented specimens were prepared with cement contents varying from 0% to 10% by weight of dry sand and cured for seven days. Fiber length and diameter were 24 mm and 0.023 mm, respectively, in the contents of 0% and 0.5% by weight of dry sand–cement mixture. Test results indicated that the addition of cement to sand increases stiffness, peak strength and brittleness. Both cement and fiber insertions affect dramatically the stress–dilatancy behavior of the sand. The fiber reinforcement increases peak strength just up to a certain cement content (up to about 5% in the present study), increases ultimate strength, decreases stiffness and changes the cemented sand brittle behavior to a more ductile one. The triaxial peak strength increase due to fiber inclusion is more effective for smaller amounts of cement, while the increase in ultimate strength is more efficacious when fiber is added to sand improved with higher cement contents. Peak strength envelopes indicate that the friction angle is about 46° for fiber-reinforced specimens containing up to 7% cement content, reaching 51.5° for higher cement contents. Cohesion intercept is drastically affected due to fiber addition to all cement contents, increasing for cement contents up to 4% and reducing for higher cement contents. It is important to make clear that the trends observed herein are relevant for the soil, cement and fiber type used in the present research and that further studies are necessary to generalize such findings.     

Mitigation of seasonal temperature change-induced problems with integral bridge abutments using EPS foam and geogrid

Expansion of bridge girders in summer moves integral bridge abutments toward backfill, causing high lateral earth pressures behind the abutment. Some backfill material slumps downward and toward the abutment when the abutment moves away from the backfill due to bridge girder contraction in winter. Placement of geogrids within the backfill can increase stability of the backfill while placement of compressible inclusions (e.g., Expanded Polystyrene (EPS) foam) can reduce lateral earth pressures behind the abutment caused by bridge girder expansion. In this study, six physical model tests were conducted with 30 abutment top movement cycles due to simulated seasonal temperature changes to study the performance of integral bridge abutments with different mitigation measures. The test results showed that geogrid reinforcements caused higher maximum lateral earth pressures at the same abutment movement, but geogrids with wrap-around facing significantly reduced the backfill surface settlements. The combination of the EPS foam and geogrids could minimize lateral earth pressure increase and backfill settlement. The EPS foam reduced the abutment toe outward movement when the abutment top was pushed against the backfill; however, the mitigation effects by the EPS foam was limited due to its small thickness and relatively high elastic modulus in this study.     

Post-construction performance of a two-tiered geogrid reinforced soil wall backfilled with soil-rock mixture

There have been very few studies on the application of soil-rock mixtures as the backfills of geogrid reinforced soil retaining walls with due concern for their long-term performance and safety. In this study, a 17-m high two-tiered reinforced soil wall backfilled with soil-rock mixture was instrumented for its performance under gravity load after construction. The instrumentation continued for 15 months. It is found that soil-rock mixtures with small rock content (<30%) have the potential to be used as the backfill materials of geogrid-reinforced retaining walls, but special attentions should be given to compaction quality, backfill–geogrid interaction, and installation damage to geogrids. Reinforcement slippage is possible because of the large particles, but it was small in this case and ceased to develop nine months after the end of construction. Compressibility difference between reinforced and unreinforced backfill might led to rotation of the upper tier. Using the estimated soil strength, the predictions of reinforcement loads by the FHWA methods were 100% higher than the estimated ones from measured strains.     

土工格栅界面摩擦特性试验研究

土工格栅与土的界面作用特性直接影响着加筋土挡墙的安全与稳定性。因此,土工格栅与填料的界面技术指标在加筋土挡墙的设计中至关重要。本文在从试验方法、加载方式、试验箱侧壁边界效应和尺寸效应、填料厚度、压实度以及筋材夹持状况等几方面分析土工格栅界面摩擦特性影响因素基础上,进行了土工格栅在砂砾料和粘性土中的拉拔试验和直剪试验。试验结果表明:土工格栅与砂砾料接触面抗剪强度较高,而与粘土接触面抗剪强度很低;对于加筋土挡墙拉拔力较大的层位,应选用刚度大的土工格栅和砂砾料为填料。直剪摩擦试验不适合确定土工格栅接触面的抗剪强度。该试验结果对土工格栅加筋土挡土墙的设计具有重要的参考价值。     

Laboratory model studies on unreinforced and geogrid-reinforced sand bed over stone column-improved soft clay

Results from a series of laboratory model tests on unreinforced and geogrid-reinforced sand bed resting on stone column-improved soft clay have been presented. The diameter of stone column and footing has been taken as 50 mm and 100 mm, respectively for all the model tests carried out. Load was applied to the soil bed through the footing until the total settlement reached at least 20% of footing diameter. As compared to unimproved soft clay, the increase in load-carrying capacity under different improved ground conditions has been observed. Influences of the thickness of unreinforced as well as geogrid-reinforced sand bed and the size of geogrid reinforcement on the performance of stone column-improved soft clay bed have also been investigated. Significant improvement in load-carrying capacity of soft soil is observed due to the placement of sand bed over stone column-improved soft clay. The inclusion of geogrid layer within sand bed further increases the load-carrying capacity and decreases the settlement of the soil. Due to the placement of sand bed, the bulge diameter of stone column reduces while the depth of bulge increases. Further reduction in the bulge diameter and increase in bulge depth are observed due to application of geogrid layer. The optimum thickness of unreinforced sand bed is twice the optimum thickness of geogrid-reinforced sand bed. Under specific material properties and test conditions, it is further observed that the optimum diameter of geogrid layer is thrice the diameter of footing.     

Evaluation of permeability characteristics of a geosynthetic-reinforced soil through laboratory tests

The objective of this paper is to examine the permeability characteristics of geosynthetic layers under confinement with soils having relatively low permeability. For this purpose, a large permeameter was custom designed and a series of permeability tests were carried-out by varying soil type and number of geosynthetic layers. Further, effect of provision of sand cushion and the thickness of sand cushion on permeability characteristics was also examined. Normal stress was increased in intervals of 50 kPa up to 200 kPa. With an increase in normal stress, a decrease in the permeability characteristics of a geosynthetic-reinforced soil was observed. The permeability characteristics were found to improve significantly with the provision of sand cushion and an increase in its thickness. Based on the definition of equivalent coefficient of permeability of stratified soils for parallel flow, an equation for estimating coefficient of permeability of soil–geosynthetic system with and without sand cushion is proposed. Considering the application of geosynthetics in reinforced slopes and walls with low-permeable backfill soils, a suitable geosynthetic with a thin layer of sand cushion is recommended. This in turn can also help in enhancing the pore-water pressure dissipation.     

经编和玻纤格栅加筋粘性土的三轴试验研究

对经编格栅和玻纤格栅加筋粘性土进行不固结不排水的三轴压缩试验。试验结果表明,在粘性土体上布置格栅筋材,都能提高土体强度,但不同的筋材,其加筋效果是不一样的,经编格栅加筋土的加筋效果要优于玻纤格栅加筋土。加筋层数越多,加筋效果越好;随着加筋土应力增加,加筋土抵抗变形的作用才能得到更充分发挥,土体加筋效果更明显。不同筋材的加筋土,其粘聚力与内摩擦角的变化规律不一致;玻纤格栅和经编格栅加筋粘性土的加筋效果与砂土不同,不仅表现在粘聚力的增加上,还表现在内摩擦角的增加上。加强筋条结点连接的牢固性,能够提高加筋效果。     

Shear behaviour of a geogrid-reinforced coarse-grained soil based on large-scale triaxial tests

In China, weathered mudstone geogrid-reinforced coarse-grained soil is used extensively for road embankments. However, the microstructure and disintegration process of weathered mudstone remain unclear. Furthermore, few studies have investigated the shear behaviour of this kind of geogrid-reinforced fill through large-scale triaxial tests against grain size effects. To bridge this gap, this study reports results from large scale consolidated undrained (CU) and consolidated drained (CD) triaxial tests as well as scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), and disintegration tests on weathered mudstone geogrid-reinforced coarse-grained soil. EDX spectrograms and SEM images show that coarse grains disintegrate rapidly mainly owing to the high clay mineral content and loose microstructure. Therefore, a suitable disintegration time (∼15 days) is recommended for embankment sits. The shear behaviour of this geogrid-reinforced fill is investigated in detail through large-scale triaxial tests. The shear deformation tends toward strain hardening behaviour with an increase in the number of geogrid layers and the confining pressure. Geogrids significantly improve the apparent cohesive strength of coarse-grained soil. The pore water pressure is found to develop rapidly in the 0%–4% axial strain phase but dissipate slowly in the 4%–12% axial strain phase. During shear, the pore pressure coefficient A values of 0.2–0.4 are indicative of the partial saturation of specimens. Consequently, pore water pressure development is mainly attributed to the movement and rearrangement of coarse particles in coarse-grained soil. Experimental data show that the geogrid-reinforcement coefficients increase with the number of geogrid layers, and a 20-cm separation between geogrid layers is recommended for embankment construction sites. The number of geogrid layers influences the geogrid–soil interface’s mobilization and the slip surface type. Test results revealed three types of slip surfaces related to the failure shapes of specimens. Then, based on CU experimental data, the parameters of the Duncan–Chang constitutive model are discussed.     

Investigation of timber harvesting impacts on herbaceous cover,forest floor and surface soil properties on skid road in an oak (Quercus petrea L.) stand

In this study, we investigated the timber harvesting effects on some soil properties (sand, silt, clay, pH, electrical conductivity, fine soil <2 mm, coarse soil >2 mm, root mass, organic carbon, moisture equivalent, total porosity, bulk density, moisture and compaction) at soil depths (0–5 and 5–10 cm), herbaceous cover and forest floor (unit mass, organic matter and moisture) on skid road of an oak (Quercus petrea L.) stand in Istanbul Belgrad Forest of Turkey.