Reinforced Earth Structures Research Articles

Characterization and Modeling of Reinforced Earth Structures

Characterization and Modeling of Reinforced Earth Structures

Multi-objective optimization of geosynthetic reinforced soil structures

Optimization models for reinforced earth structures such as foundation pads, bridge abutments, and embankments based on the Eurocode standard are presented. The developed optimization models, which take into account construction costs and environmental footprint, are used to determine an optimal design for each earth structure. The optimization model uses discrete variables, making the results more suitable for actual construction practice and fully exploiting the geotechnical and structural capacity of earth structures with geosynthetic reinforcement. The multi-objective optimization was performed to find a set of solutions that represent the best trade-off between construction cost and environmental footprint. The results show that the correct selection of geosynthetics leads to a significant reduction in costs and environmental impact. The general observation that emerges from the multi-objective optimization is that when designing the earth structures using geosynthetic reinforcements, due to the discrete set of variables, there are not so many optimal solutions that the designer can choose from. The entire optimization process is illustrated with the help of a numerical example. This study can help engineers to select earth structure and geosynthetic reinforcements that are economical and sustainable.

Effect of gas-oil contamination on the mechanical behavior of sand-woven geotextile interface: Experimental investigation and constitutive modeling

The accurate estimation of the frictional resistance of interfaces between soils and geosynthetics plays a central role in stability and serviceability of geosynthetic reinforced earth structures. Contamination with hydrocarbons generally impairs soil geotechnical properties; however, its effect on the behavior of soil-geosynthetic interfaces has seldom been examined precisely. For this reason, an extensive series of direct shear tests was performed to investigate the consequence of gas-oil contamination on the mobilization of shear strength and volume change response of gas-oil contaminated angular sand in contact with woven geotextile (WGTX). Complementary tests on the interfaces between glass beads as a replicate for sands with high degree of sphericity and roundness in contact with WGTX were also performed to explore the effect of particle shape. Gas-oil contamination is observed to causes decrease of the peak and critical state friction and dilation angles in both the sand-WGTX and glass bead-WGTX interfaces. However, gas-oil contamination-induced decrease in the frictional efficiency in the glass bead-WGTX interfaces was greater than that in the angular sand-WGTX interfaces. Calibration of a state-dependent sand-structure interface model against the laboratory data of gas-oil contaminated soils-WGTX interfaces results in a reasonable agreement between the model simulations and the laboratory data.

Assessment of geotechnical and geo-environmental behaviour of recycled concrete aggregates, recycled brick aggregates and their blends

The infrastructure development involves the demolition of non-functional establishments and the construction of the desired infrastructure needs, which generates construction and demolition (C&D) waste in enormous volumes. The demolished concrete and brick wastes are found mixed together, if not separated before crushing. Therefore, the geotechnical behavior of blends of recycled concrete aggregate (RCA) and recycled brick aggregates (RBA) with different proportions should be identified in order to find their suitability in geotechnical applications. This paper presents the geotechnical and geo-environmental characterization of RCA, RBA, and their blends (75RCA-25RBA, 50RCA-50RBA, 25RCA-75RBA). The recycled waste aggregates and their blends under this study have excellent shear strength, hydraulic conductivity, and compaction characteristics. The values of cohesion intercept (c) and angle of internal friction (ф) are found in the range of 50 to 63 kPa and 55˚ to 71˚, respectively. It is observed that the optimum values of shear strength parameters (c = 63 kPa, ф = 64˚), hydraulic conductivity (3.12 × 10-5 m/s), and compaction characteristics (OMC = 11.50 %, MDD = 1.96 gm/cm3) are found for 50 % RBA content in RCA-RBA blends. The water absorption is found on the verge of upper bounds considering pavement applications. The impact and crushing resistance of blends are found comparably low and decrease with increased RBA content in RCA. Therefore, their applicability to the surface course, even to the base course in pavements, may be restricted. Based on experimental results current study suggested that up to 50 % RBA content in RCA-RBA blends may provide satisfactory performance in various geotechnical applications like: pavement base and sub-base, fill material in embankments and reinforced earth structure, etc.

Evaluation of Performance of Pavement Founded on the Reinforced Earth Sections along Outer Ring Road, Nairobi

There are no existing local design manuals and construction guidelines for reinforced earth structures in Kenya. The objective of this study was to evaluate the performance of the pavement found on reinforced earth sections along Outer Ring Road. The structural evaluation was determined from deflection measurements using Falling Weight Deflectometer (FWD), while the functional evaluation was by visual condition survey, roughness, and rut depth measurements. Classified traffic counts and axle load surveys were undertaken. It was established that the design traffic would be exceeded in year 10 after opening the road. The surface condition survey indicated that the pavement surface for the entire road was generally in good condition. The roughness rating for the entire road was rated "Good", while ten sections of reinforced earth embankment were rated "Fair". The characteristic rut depths on the reinforced earth sections were higher than the lane characteristic rut depth for the entire road. Three sections of the reinforced earth showed low severity, while the rest had no rutting. The PCI for the pavement on reinforced earth sections were lower than pavement on other sections of the road due to a higher International Roughness Index (IRI). The FWD deflections were higher on reinforced earth sections. It was found that the mean pavement moduli for surfacing and base, crack relief and subbase layers at reinforced earth sections were lower than for the pavement in other sections hence requiring thicker overlays. The overall residual life of the road was 17 years, while for the reinforced earth sections was 15 years. Based on the findings, monitoring of the traffic and performance of pavement on the reinforced earth embankments is vital for interventions to prevent premature pavement failure. There is a necessity to develop a design manual and construction guidelines for reinforced earth structures based on the local conditions in Kenya.

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%.

SUGGESTED GUIDELINES FOR DESIGN AND CONSTRUCTION OF SHORT—SPAN BRIDGE ABUTMENTS WITH REINFORCED EARTH SYSTEM

Construction of small bridges is one of the real challenges in road construction, because it has so many problems. The reinforced earth system (mechanically stabilized earth wall) using gabion can be suggested as better alternatives for foundation of short—span bridges, especially in the remote areas. The latest research was about to find the design of reinforced earth abutment on various heights of abutments and various lengths of bridge span on soft to very soft consistency cohesive soil. However, the results of this research were less representative because the field conditions can vary from very soft to stiff cohesive soil and very loose to dense non-cohesive soil. Therefore, further research for wide range of soil conditions was conducted. Based on internal and external stability analysis, known that the number of geotextile needed for MSE wall (reinforced earth structure) ranging from 2 to 5 layer per meter depth, depending on the grade and the depth placed of the reinforcement, while the length of geotextile needed ranging from 3.2 to 22.5 meter, depending on the bridge span, embankment height, and parameters of the soil. MSE Wall cannot be built on soft to very soft soil (Cu < 2.79 Ton/m 2 ) without soil improvement to be done in the first place. Based on circular failure analysis (overall stability), known that in cohesive soils with stiff consistency (Cu = 6 Ton/m 2 ) to very stiff (Cu = 12 Ton/m 2 ) and non-cohesive soils with dense consistency (ф = 38 0 ) to very dense (ф = 42 0 ) does not require additional reinforcement. While on other soil consistency, some need additional reinforcement ranging from 0 to 22 layer of geotextile and from 0 to 35 pieces of micropiles, depending on the bridge span, embankment height, and grade of the reinforcement. Number of gabion needed as a facing of MSE wall ranging from 5 to 8 pieces per 2-meter width of abutments, depending on the embankment height.

Seismic internal stability of bilinear geosynthetic-reinforced slopes with cohesive backfills

The bilinear geosynthetic-reinforced slope (BGRS) is a new two-tier reinforced earth structure without an offset. Analyzing its seismic internal stability is aimed to determine the resultant reinforcement force. In this paper, using a top-down log spiral mechanism, the moment limit equilibrium equations are established to formulate the resultant reinforcement forces for upper and lower tiers of the BGRS, considering influence of angle of lower tier, depth of tension crack, cohesion of backfill and pseudo-static seismic loads. Results show that ignoring cohesion of backfill and/or vertical upward seismic load is beneficial to conservative stability analysis, and the existence of tension cracks changes the distribution of reinforcement forces in upper and lower tiers. Additionally, the critical slip surface is also studied, which is found that the volume of the failure body decreases under the global stability with increase of angle of lower tier, cohesion of backfill or depth of tension crack, or with the decrease of seismic coefficients.

Seismic Behavior of Flexible Geogrid Wrap‐Reinforced Soil Slope

In this study, the failure mode of flexible reinforced soil slopes under earthquake action was investigated by shaking table tests. The distribution law of a potential failure surface of a flexible no‐faceplate reinforced soil slope under earthquake action was obtained based on the analysis results. A simplified trilinear failure surface suitable for flexible reinforced soil slopes without faceplate was proposed. Subsequently, based on the upper‐bound theorem of limit analysis, we derived the formula for calculating the yield seismic acceleration coefficient of a flexible no‐faceplate reinforced soil slope under a seismic load. The main parameters that affect its seismic performance were determined. The flexible geogrid reverse‐packed reinforced earth structure can effectively limit the fracture of a slope body and improve the stability of the slope. This provides a theoretical basis for facilitating the engineering of flexible reinforced soil slopes.

Large-Scale Prototype Testing of a Composite Geosynthetics Reinforced Earth Structure for Riverbank Slope Protection Using Fine Grained Sandy Soil

This paper presents the field evaluation of large scale prototypes of an innovative composite geosynthetic reinforced earth (CGRE) structure as an alternative for the conventional earth retaining structures, hence a green engineering design for riverbank slope protection. This reinforced earth structure comprises of a geogrid that serves as the main reinforcement and the cover of the facing member of the structure; and a geotextile as separator medium between the infill soil and intermediate horizontal gravel drainage. Three large scale prototypes at full, half, and no gravel drainage were built, monitored, and tested. Clayey sand soil was the fill material used and a 3/2” to 2” sub-rounded gravel was used for the drainage and facing members. CGRE prototypes were evaluated in saturated conditions, and tested under incremental surcharge loadings with a total applied stress of 40 kPa. Lateral displacements for every layer of the structure were determined using horizontal rods where readings were taken from line gauges. Results showed that prototypes with gravel drainage displaced lower than the permissible limit of 4.0%. The CGRE prototype with full gravel drainage had the smallest average total lateral displacement of 1.28%. Hence, it had a higher interface friction resistance, and pull-out capacity than the one with less or no gravel drainage. Overall, the incorporation of gravel drainage improved the performance of the reinforced earth system by reducing the lateral movements of the structure. Therefore, the CGRE structure is a viable slope protection alternative even under saturated conditions.

Distributed fibre-optic sensing applications at the Semmering Base Tunnel, Austria

The Semmering Base Tunnel in Austria is currently one of the largest tunnel construction sites in Central Europe. Two tunnel tubes, each 27.3 km long, are being built from the portal and three intermediate construction sites. Each access point poses specific challenges, and structural monitoring is a crucial element to ensure both economic and safe construction. In this paper, the authors describe one of the most comprehensive distributed fibre-optic sensing installations on a tunnel construction site worldwide. At the Semmering Base Tunnel, fibre-optic sensing is used at every construction location to monitor tunnel linings, shafts, reinforced earth structures and pipelines. The authors also discuss the challenges posed by the installations and the results of long-term monitoring programmes. The installed monitoring systems will, furthermore, be used throughout the expected 150-year operational lifetime of the tunnel. Unlike conventional sensors, fibre-optic sensors do not comprise electric or moveable components at the measurement location. A longer lifetime can, therefore, be expected for fibre-optic sensors, which makes them promising candidates for condition-based maintenance of civil infrastructure.

Experimental study of the effect of different backfilled soils on the stability of mechanically stabilized earth walls

ABSTRACTIn recent years, a variety of geosynthetic reinforced structures have been designed to be durable, flexible, and deformable. The mechanically stabilized earth wall (MSEW) is one such kind of protective structure which has unique characteristics. The flexible walls of the MSEW have the ability to absorb deformation and have higher resistance to dynamics than reinforced concrete structures. As a consequence, the construction of reinforced soil structures has increased around the world. Although according to the guidelines, cohesionless soil is required as backfill for the construction of MSEW, in some circumstances, this type of soil is not available. To understand the behaviors of MSEW constructed using clayey soil, when high-quality backfill is unavailable, it is necessary to examine other scenarios. In this study, two arrays of geosynthetic reinforced earth walls were constructed using clayey and sandy backfills for geotechnical centrifugal testing with varying offset distances and reinforcement lengths. The behaviors of these superimposed geosynthetic reinforced earth walls (SGREW) were observed to determine the reasons for failure of the reinforced walls. A modified lateral earth pressure design method and reinforced earth structure stability assessment have been proposed to evaluate the internal stability of the SGREW.

Stability analysis of 3D geosynthetic–reinforced earth structures composed of nonhomogeneous cohesive backfills

Soils are assumed to be homogeneous in most designs of geosynthetic-reinforced earth structures (GRESs), thus ignoring the influences of the possible cohesion and cohesion nonhomogeneity of backfills. However, the actual stability of GRESs is directly influenced by the presence of cohesion and cohesion nonhomogeneity of backfills along with the three-dimensional (3D) character of GRESs. In the present study, a chart-based stability analysis of a 3D GRES composed of nonhomogeneous cohesive backfills subjected to the seismic excitation/pore water pressure is conducted by means of the limit analysis method. Work rates by seismic forces and pore water pressure are calculated based on the pseudo-static method and the pore water pressure coefficient, respectively; thereafter the energy balance equation is derived by equating the external work rates of the soil weight and seismic forces/pore water pressure to the sum of the work rate of the geosynthetics and internal energy dissipation caused by soil cohesion. The analytical expression of the required unfactored reinforcement strength is derived and the optimized solutions are consequently captured. A comparison is drawn to verify the present study and a parametric analysis is conducted thereafter to investigate the effects of the 3D character, cohesion nonhomogeneity, seismic excitation and pore water pressure on the stability of GRESs. Finally, a set of stability charts considering the 3D effects, cohesion nonhomogeneity, seismic force and pore water pressure on long-term stability is proposed for preliminary design purposes.

Compacted expansive elastic silt and tyre powder waste

Building on/with expansive soils with no treatment brings complications. Compacted expansive soils specifically fall short in satisfying the minimum requirements for transport embankment infrastructures, requiring the adoption of hauled virgin mineral aggregates or a sustainable alternative. Use of hauled aggregates comes at a high carbon and economical cost. On average, every 9m high embankment built with quarried/hauled soils cost 12600 MJ.m-2 Embodied Energy (EE). A prospect of using mixed cutting-arising expansive soils with industrial/domestic wastes can reduce the carbon cost and ease the pressure on landfills. The widespread use of recycled materials has been extensively limited due to concerns over their long-term performance, generally low shear strength and stiffness. In this contribution, hydromechanical properties of a waste tyre sand-sized rubber (a mixture of polybutadiene, polyisoprene, elastomers, and styrene-butadiene) and expansive silt is studied, allowing the short- and long-term behaviour of optimum compacted composites to be better established. The inclusion of tyre shred substantially decreased the swelling potential/pressure and modestly lowered the compression index. Silt-Tyre powder replacement lowered the bulk density, allowing construction of lighter reinforced earth structures. The shear strength and stiffness decreased on addition of tyre powder, yet the contribution of matric suction to the shear strength remained constant for tyre shred contents up to 20%. Reinforced soils adopted a ductile post-peak plastic behaviour with enhanced failure strain, offering the opportunity to build more flexible subgrades as recommended for expansive soils. Residual water content and tyre shred content are directly correlated; tyre-reinforced silt showed a greater capacity of water storage (than natural silts) and hence a sustainable solution to waterlogging and surficial flooding particularly in urban settings. Crushed fine tyre shred mixed with expansive silts/sands at 15 to 20 wt% appear to offer the maximum reduction in swelling-shrinking properties at minimum cracking, strength loss and enhanced compressibility expenses.

English

English

Transverse Pullout Response of Smooth-Metal-Strip Reinforcements Embedded in Sand

The pullout resistance of reinforcement is an important parameter in the design of reinforced earth structures. Existing design procedures consider the pullout resistance of reinforcement from axial pull. However, the kinematics of failure clearly establish that the reinforcement is pulled obliquely along the slip surface. The response of reinforcement to oblique pull is equivalent to an application of axial and transverse components of oblique pull. In this study, details are provided of a novel experimental test setup used to examine the response of smooth-metal-strip reinforcement subjected to transverse pull at one end. A large-size test chamber of dimensions (length, width, and depth) with an arrangement to conduct transverse pullout of reinforcement is developed. The pullout response of smooth-metal-strip reinforcements to transverse pull is obtained for three different normal stresses of 17, 52, and 87 kPa. Transverse pullout resistance factors of smooth-metal-strip reinforcements corresponding to different transverse displacements of the reinforcement are proposed and found to range from 0.6 to 2.9, corresponding to transverse displacements of 20–30 mm.

Large-scale pullout testing of a new ‘rooted’ geogrid

A true understanding of soil–geogrid interaction is one of the major parameters in designing reinforced earth structures. Alongside the introduction of a new and simple system consisting of transverse geogrids connected to a base geogrid with a 45° angle, this paper is concerned with the performance of this system, called ‘rooted geogrid’ (R-G), which is tested experimentally on a large-scale in granular soil for the increase in pullout resistance. The pullout test results have shown an 88% increase in soil–reinforcement interaction in the R-G system compared with ordinary geogrids. In other words, the length required by the base geogrid in this system with the same pullout force is almost half that of an ordinary geogrid, which can be considered beneficial to mechanically stabilised earth wall designs facing a space limit in the backfill.

Visualization of pullout behaviour of geogrid in sand with emphasis on size effect of protrusive junctions

Geogrid has been extensively used in geotechnical engineering practice due to its effectiveness and economy. Deep insight into the interaction between the backfill soil and the geogrid is of great importance for proper design and construction of geogrid reinforced earth structures. Based on the calibrated model of sand and geogrid, a series of numerical pullout tests are conducted using PFC3D under special considerations of particle angularity and aperture geometry of the geogrid. In this work, interface characteristics regarding the displacement and contact force developed among particles and the deformation and force distribution along the geogrid are all visualized with PFC3D simulations so that new understanding on how geogrid-soil interaction develops under pullout loads can be obtained. Meanwhile, a new variable named fabric anisotropy coefficient is introduced to evaluate the inherent relationship between macroscopic strength and microscopic fabric anisotropy. A correlation analysis is adopted to compare the accuracy between the newly-proposed coefficient and the most commonly used one. Furthermore, additional pullout tests on geogrid with four different joint protrusion heights have been conducted to investigate what extent and how vertical reinforcement elements may result in reinforcement effects from perspectives of bearing resistance contribution, energy dissipation, as well as volumetric response. Numerical results show that both the magnitude and the directional variation of normal contact forces govern the development of macroscopic strength and the reinforcing effects of joint protrusion height can be attributed to the accelerated energy dissipation across the particle assembly and the intensive mobilization of the geogrid.

Influence of Water Content on Pullout Behaviour of Geogrid

The interaction between geogrid and soil is fundamental and crucial factor on safety and stability of geogrid-reinforced earth structure. Therefore, the interface index between geogrid and soil is of vital importance in the design of reinforced earth structures. The pullout behaviour of geogrid in soil is studied, an experimental investigation is conducted using geogrid in four groups of soil with 20%, 24%, 28%, 32% water contents, which correspond to normal stresses of 50, 100, 200 and 300 kPa respectively. The results indicate that the geogrid embedded in soil mainly represents pullout failure, and the ultimate pullout force is sensitive to water content. It decreases with the increase of the water content firstly. Besides, the water content influences the process of the pullout behaviour. The increase of water content leads to the ultimate pullout force soon.

Pullout Characteristics of Geosynthetics Reinforced Earth Using Multilayer Spreading Pullout Test

Pullout test equipment for examining pullout characteristics in a reinforced earth structure is relatively larger than direct shear testing machines and requires much time and expenses in preparing samples. Moreover, because of irregular stress distributions with respect to the length of reinforcements, it is difficult to analyze pullout test results. In this study, we developed a multilayer spreading pullout apparatus enabling the pullout test in the order of a top stage, middle stage, and bottom stage with different loads once a ground model is prepared, suggesting an efficient method of a multilayer spreading pullout test. The pullout test is carried out at least three times with the prepared ground model while changing confining loads. Jumunjin sand was used to verify the developed pullout test apparatus and to analyze pullout characteristics of each reinforcement. The analysis reveals that the difference between angles of pullout friction is approximately 0.86 to 1.3° which is distributed within the error range of a pullout test. As a result, the multilayer spreading pullout apparatus is applicable as a new pullout apparatus, and the suggested method of multilayer spreading pullout test is identified as a pullout test technique efficiently to obtain pullout parameters.