Granular Backfill Research Articles

Quantification of landslide velocity from active waveguide–generated acoustic emission

Acoustic emission (AE) has become an established approach to monitor stability of soil slopes. However, the challenge has been to develop strategies to interpret and quantify deformation behaviour from the measured AE. AE monitoring of soil slopes commonly utilizes an active waveguide that is installed in a borehole through the slope and comprises a metal waveguide rod or tube with a granular backfill surround. When the host slope deforms, the column of granular backfill also deforms and this generates AE that can propagate along the waveguide. Results from the commissioning of dynamic shear apparatus used to subject full-scale active waveguide models to simulated slope movements are presented. The results confirm that AE rates generated are proportional to the rate of deformation, and the coefficient of proportionality that defines the relationship has been quantified (e.g., 4.4 × 105 for the angular gravel examined). It is demonstrated that slope velocities can be quantified continuously in real time through monitoring active waveguide–generated AE during a slope failure simulation. The results show that the technique can quantify landslide velocity to better than an order of magnitude (i.e., consistent with standard landslide movement classification) and can therefore be used to provide an early warning of slope instability through detecting and quantifying accelerations of slope movement.

Shear resistance of bentonite backfill materials and their interfaces under varying hydraulic conditions in a deep rock nuclear waste repository

This paper presents the shear behaviour and the shear resistance of different bentonite based clay backfill materials and their interfaces of Finnish KBS-3V type nuclear waste repository, under varying hydraulic conditions. The interfaces between proposed precompressed Friedland clay blocks (FCB) and other granular backfill materials (GM) such as bentonite pellets (QSEP) and granules of bentonite (GB) were tested in a conventional direct shear box apparatus under various hydraulic test conditions. The main objective of this study was to get insight into the shear behaviour of different backfill interfaces and to determine their interface shear parameters. Tests were done by changing the salinity of the interface water and the amount of interface water itself. Test results showed that the internal shear strength of QSEP and GB decreased with increasing water content due to the lubricating effect of the free-water between the granules. Furthermore, test results showed that the interface shear parameters of the FCB–FCB interface increased with increasing salinity of the interface water, which produced rough FCB surfaces and asperities as the water was absorbed. The interface friction and cohesion of the FCB–GM (i.e. QSEP and FCB) interfaces under varying amounts of interface water were investigated through two different scenarios. In scenario 1, natural FCB surface was tested against GM with varying water contents, and in scenario 2, the amount of water on the FCB surface was varied whilst the water content of the GM was fixed as high as possible at around 60% (by weight). The test results showed that in scenario 1, the interface shear of FCB–GM (both FCB–QSEP and FCB–GB) decreased with increasing water contents of GM. At water contents (of GM) less than 40% (by weight), the failure took place at the interface, whilst at water contents above 40%, the failure occurred within the GM itself. Greater interface shear was observed when the FCB was wetted (i.e. in FCB–QSEP and FCB–GB interfaces) in scenario 2, compared with natural FCB surface in the same interfaces. The interface shear results obtained from this study will be incorporated in the detailed modelling of the buffer–backfill interaction of the KBS-3V.

Acoustic emission monitoring of a soil slope: Comparisons with continuous deformation measurements

Acoustic emission (AE) has become an established approach to monitor the stability of soil slopes. However, the challenge has been to develop strategies to interpret and quantify deformation behaviour from the measured AE. This paper presents the first comparison of continuous AE (measured using an active waveguide) and continuous subsurface deformation measurements. The active waveguide is installed in a borehole through a slope and comprises a metal waveguide rod or tube with a granular backfill surround. When the host slope deforms, the column of granular backfill also deforms, generating AE that can propagate along the waveguide. This paper presents results from a field trial at a reactivated soil slope in North Yorkshire, UK. The measurements confirm that AE rates generated are directly proportional to the velocity of slope movement (e.g. the AE rate versus velocity relationship determined for a series of slope movement events produced an R2 value of 0·8) and demonstrate the performance of AE monitoring of active waveguides to provide continuous information on slope displacements and displacement rates with high temporal resolution.

Modelling of solid and hollow voussoir FlexiArch systems

This paper investigates the use of solid and hollow core voussoir blocks in an innovative FlexiArch bridge system. The system is formed from precast concrete voussoirs connected by a polymeric reinforcement, enabling the arch to be built and transported as a flat pack and lifted into shape after delivery to site, thus avoiding the use of centring for construction. This research is novel in that it trials and compares the use of hollow core and solid FlexiArch rings in third-scale models with granular backfill. The use of hollow core voussoirs reduces the loading on the polymeric reinforcement during construction and should allow for increased spans. This paper presents the test results of two similar third-scale 5 m × 2 m (span × rise) arch systems, using both solid and hollow construction. The use of hollow core voussoir blocks reduces the weight of the system. Non-linear finite-element analysis (NLFEA) has been used to further compare arch behaviour and the thrustline position.

Lightweight backfill materials in integral bridge construction

Analysis of integral bridge structures shows that lateral earth pressures on the end abutments have a dominant influence on the sizing of bridge components. Thermal cyclic movements induced by deck expansion cause densification of backfill material, leading to the build-up of high pressures behind the abutments. Measures to reduce these pressures by the use of alternative backfill materials can be highly beneficial to the structure as a whole, with extra expenditure on the backfill material being offset, and in some cases exceeded, by savings in material quantities in the rest of the structure and build time. This paper examines three contrasting backfill options that were considered during the design of Cottington Road overbridge in Kent: 6N granular backfill; lightweight expanded clay; and expanded polystyrene blocks. Although more expensive as a material than the other options investigated, the expanded polystyrene block option was selected as the most economical solution for this particular structure owing to large material savings in the foundations, abutment walls and wing walls as a direct result of reduced lateral pressures. Although it did not form part of the decision-making process, this paper also shows a significant saving in embodied and transport-related carbon dioxide emissions through preferring expanded polystyrene blocks to expanded clay.

Effects of Fines Content on Hydraulic Conductivity and Shear Strength of Granular Structural Backfill

For cast-in-place abutments and retaining walls, structural backfill is preferred to be free-draining, which generally implies less than 5% fines content. This fines content is expected to eliminate the need to design for hydrostatic pressures. The availability of high-quality structural backfill with naturally low fines content is declining. This situation warrants an evaluation of whether granular backfill materials with greater than 5% fines content could be successfully used in practice. Flexible wall, hydraulic conductivity tests on a granular structural backfill with 0%, 5%, 10%, 15%, 20%, and 25% nonplastic fines content were conducted at 41, 83, and 124 kPa (6, 12, and 18 psi) confining pressures followed by consolidated drained triaxial compression tests for obtaining associated drained shear strength parameters of these gradations. The 15.2-cm (6-in.) diameter specimens were prepared at optimum moisture content and 95% of maximum standard Proctor density. To enable a comparison with respect to modified Proctor maximum densities, modified Proctor tests were also performed for all base soil–fines content mixtures. The experimental results were compared with relevant studies found in the literature. This research indicates that a nonplastic fines content up to 10% may be justified in structural backfill specifications for retaining walls and abutments.

Behavior of geotextile reinforced flyash + clay-mix by laboratory evaluation

The major factors that control the performance of reinforced soil structures is the interaction between the soil and the reinforcement. Thus it is necessary to obtain the accurate bond parameters to be used in the design of these structures. To evaluate the behavior of flyash + clay soil reinforced with a woven geotextile, 36 Unconsolidated-Undrained (UU) and 12 reinforced Consolidated.Undrainrained (CU) triaxial compression tests were conducted. The moisture content of soil during remolding, confining pressures and arrangement of geotextile layers were all varied so that the behavior of the sample could be examined. The stress strain patterns, drainage, modulus of deformation, effect of confinement pressures, effects of moisture content have been evaluated. The impact of moisture content in flyash + clay backfills on critical shear parameters was also studied to recommend placement moisture for compaction to MDDM. The results indicate that geotextile reinforced flyash + clay backfill might be a viable alternative in reinforced soil structures if good-quality granular backfill material is not readily available.

Vertical Stress Isobars for Trenches and Mine Stopes Containing Granular Backfills

AbstractThe vertical stresses within granular fills contained in trenches can be determined using Marston’s theory or numerical modeling. Marston’s theory gives only the average value of the vertical stress at a depth. Numerical modeling can be used for determining the vertical stress at any point within the fill. This paper presents vertical stress isobars for trenches filled with granular materials, developed from numerical modeling, and proposed in terms of dimensionless variables. These isobars are similar to the pressure isobars for uniformly loaded circular and strip footings found in literature. These isobars can be used for determining vertical stresses at any point within the granular material contained in trenches and can become a simple and valuable tool. The validity of the isobars was verified through several trenches with randomly selected dimensions and fill properties. It was found that the estimates using the isobars were within 9% of the estimates from the numerical model.

Earthquake Load Attenuation Using EPS Geofoam Buffers in Rigid Wall Applications

The paper is a synthesis of previously published work by the authors that is focused on the use of expanded polystyrene (EPS) geofoam buffers for seismic load attenuation against rigid basement and soil retaining walls. The paper begins with a brief description of the first documented field application followed by a description of physical 1 m-high reduced-scale shaking table tests that provided the first “proof of concept”. Next, details of the development and verification of a displacement-based model and a FLAC numerical model are described and simulation results that were verified against the physical shaking table tests presented. The numerical results include simulations using simple linear elastic constitutive models for the EPS buffers and granular soil backfill and more complex non-linear hysteretic models. Finally, the verified FLAC model was used to develop a series of preliminary design charts for the selection of a suitable seismic buffer based on characteristics of the design earthquake accelerogram.

Granular anchors under vertical loading – axial pull

Granular anchors are a relatively new concept in ground engineering with relatively little known regarding their load–displacement behaviour, failure modes, ultimate pullout capacity, and also potential applications. A granular anchor consists of three main components: a base plate, tendon, and compacted granular backfill. The tendon is used to transmit the applied load to the base plate, which compresses the granular material to form the anchor. A study of the load–displacement response and ultimate pullout capacity of granular anchors constructed in intact lodgement till and made ground deposits is reported in this paper. Parallel tests were also performed on cast in situ concrete anchors, which are traditionally used for anchoring purposes. A new method of analysis for the determination of the ultimate pullout capacity of granular anchors is presented and verified experimentally, with the dominant mode of failure controlled by the column length (L) to diameter (D) ratio. Granular anchors with L/D > 7 principally failed by bulging whereas short granular anchors failed on shaft resistance, with the latter mobilizing similar pullout capacities as conventional concrete anchors.

FEM Analysis on Tensile Force of Geosynthetic Reinforcement Arranged in GRS-RW Considering Variable Loading Rate

The combined effects of the rate-dependent behavior of both the backfill soil and the geosynthetic reinforcement have been investigated, which should be attributed to the viscous property of material. A nonlinear finite element method (FEM) analysis procedure based on the Dynamic Relaxation method was developed for the geosynthetic-reinforced soil retaining wall (GRS-RW). In the numerical analysis, both the viscous properties of the backfill and the reinforcement were considered through the unified nonlinear three-component elastic-viscoplastic model. The FEM procedure was validated against a physical model test on geosynthetic-reinforced soil retaining wall with granular backfill. Extensive finite-element analyses were carried out to investigate the tensile force distributions in geosynthetic reinforcement of geosynthetic-reinforced soil retaining wall under the change of loading rate. It is found from the analyses that the presented FEM can well simulate the rate-dependent behavior of geosynthetic-reinforced soil retaining wall and the tensile force of geosynthetic reinforcement arranged in retaining wall.

Comparison of Seismic Responses of Geosynthetically Reinforced Walls with Tire-Derived Aggregates and Granular Backfills

AbstractThis paper reports the seismic responses of geosynthetically reinforced walls with two types of backfills using shake table tests. The backfills are tire-derived aggregates (TDA) and poorly graded sand, respectively. Mechanically stabilized earth (MSE) walls with reinforced TDA backfill have not been fully tested under seismic conditions. In this study, two geosynthetically reinforced walls are tested on a one-dimensional shake table. A section of reduced-scale MSE wall (1.6 m high, 1.5 m deep, and 1.5 m long) is built in a box that is anchored on a shake table that can generate earthquake excitations obtained from actual field recordings. Layers of geogrid are used as reinforcement. The geosynthetic reinforcement is based on static external and internal stability design. In each test, the segmental MSE wall is instrumented with accelerometers, linear variable differential transformers, linear potentiometers, and dynamic soil stress gauges to record the accelerations, wall vertical deformations, h...

Influence of EPS Geofoam Buffers on the Static Behavior of Cantilever Earth-Retaining Walls

In this study, the effect of expanded polystyrene (EPS) buffers on lateral stresses and deflections of model retaining walls with various flexibility values were investigated. For this purpose, 0.7 m high model walls were instrumented and 1-g model tests were performed in laboratory environment. In the first group of tests, the wall models retain only granular cohesionless backfill whereas in the second and third group of tests, EPS deformable buffers of two different thicknesses were installed between the wall and granular backfill. Tests were repeated for four different wall thicknesses and results were discussed comparatively. As wall flexibility increases, there is a decrease in the load reduction pattern of the buffer. On the other hand, utilization of geofoam buffers with flexible cantilever walls still provides substantial decrease in wall thrust and deflections thus leading to more economical retaining structure design. The lateral earth pressure coefficients determined through model tests were compared to those calculated from Coulomb's theory for active lateral earth stresses. A graph is provided for the estimation of lateral earth pressure coefficients for various combinations of wall flexibilities and buffer characteristics.

Numerical analysis of reinforced soil walls with granular and cohesive backfills under cyclic loads

Novel approaches to the dynamic analysis of the reinforced soil walls have been reported in the literature. Use of marginal soils reduces the cost of geosynthetic reinforced soil walls if proper drainage measures are taken. Therefore the affect of using cohesive marginal soils as backfill in geosynthetic reinforced retaining structures were investigated in this research. The dynamic response of reinforced soil walls was investigated in a similar focus, using finite element analysis. The results obtained from walls with cohesive backfill were compared to the results obtained from walls with granular backfill. The height of the wall was chosen as 6 m in the two-dimensional plane strain finite element model and the base acceleration was chosen to be a harmonic motion. The effects of various parameters like the backfill type, facing type, reinforcement stiffness, and peak ground acceleration on the cyclic response of reinforced soil retaining walls were investigated. After analyzing the wall response for end of construction and dynamic excitation phases, it was determined that the deformations and reinforcement tensile loads increased during the cyclic load application and that the amount of additional deformation that occurred during cyclic load application was strongly related to backfill soil type, facing type, reinforcement type and peak ground acceleration. It was determined that a cohesive backfill and geotextile reinforcement was a good combination to reduce the deformations of geosynthetic reinforced walls during cyclic loading for medium height walls.

Load Coefficient for Ditch Conduits Covered with Geosynthetic-Reinforced Granular Backfill

Conduits buried in narrow trenches and covered with granular soil backfills are commonly used for several services. Because of soil arching action, a significant fraction of self-weight of the backfill is transferred to the wall. The load on top of the conduit is expressed as CdγB² where the γ is the unit of weight of granular backfill, B is the trench width and Cd is the load coefficient, which increases with depth. Values of Cd for ditch conduits covered with granular backfills have been presented in the literature. Recent research has shown that providing a layer of geosynthetic above the conduit, anchored at both walls, can reduce the overburden load transferred to the conduit. This paper is aimed at formulating a theoretical framework for computing the load coefficient Cdr for ditch conduits covered with geosynthetic-reinforced granular backfills. The new Cdr-value takes into account the load reduction attributable to soil arching as well as geosynthetic arching but ignores the soil-geosynthetic frictional/adhesion interaction in the analysis for simplicity's sake. It is shown that the Cdr-values depend on the stiffness of the geosynthetic layer and the rut depth, among other factors that govern the unreinforced case. The developed expression should be useful for field applications of geosynthetics in buried structures and for developing standards for such applications.

Lateral soil stiffness adjacent to deep integral bridge abutments

Our understanding of the performance of integral bridge abutments subjected to cyclic loading has been improved using well-designed laboratory experiments, and by observation of abutment performance in the field. However, there remains significant uncertainty on how the increase in lateral stress that arises due to cycling of the backfill may be assessed for abutment design. Such uncertainty is addressed in this paper by drawing on results from a series of experiments performed using the University of Western Australia (UWA) beam centrifuge. The experimental results shed light on the effects, for granular backfill, of initial density, retained height, cyclic history and particle shape. These results, as well as observations made from tests involving overconsolidated clays, are used in conjunction with the existing database of lateral stress measurements to assist in the development of recommendations for assessment of design lateral stresses on deep integral bridge abutments.

Can Soil Arching Be Insensitive to ϕ?

Arching is a common phenomenon that is encountered in backfilling behind retaining walls, trenches, or underground voids in the mine. Marston’s equation has been widely used and modified for computing stresses within backfills, duly accounting for the reduction in stresses due to arching. A critical appraisal of Marston’s equation and its improved forms reveal that the average vertical stress within the soil backfill at any depth is governed by the product of earth pressure coefficient K and wall-backfill frictional coefficient μ , which does not vary significantly with variation in friction angle ϕ of the granular soil backfill. The average normal stress depends on the value assumed for δ/ϕ and whether the lateral earth pressure coefficient has been assumed as Ka , K0 , or KKrynine . Therefore, it is not necessary to direct any effort toward determining the friction angle of the backfill precisely. Rather, attention should be paid to the value of δ/ϕ and the appropriate expression for K . Thus, it can be...

Long-Term Reinforcement Load of Geosynthetic-Reinforced Soil Retaining Walls

As increasing number of geosynthetic-reinforced soil (GRS) retaining walls are built for permanent purpose, and their long-term behaviors have become one of the most critical issues in design. However, there has been very limited study on long-term reinforcement load and its relation to various parameters of GRS walls. A finite-element procedure for the long-term response of geosynthetic-reinforced soil structures with granular backfills was first validated against the long-term model test. Extensive finite-element analyses considering the viscous properties of geosynthetic reinforcements were then carried out to investigate the load distributions in geosynthetic reinforcements of GRS walls under operational condition. Construction sequence was simulated and a creep analysis of 10 years was subsequently conducted on each model wall. The effects of wall parameters, including backfill soil, reinforcement length, reinforcement spacing, reinforcement stiffness, and creep rate of reinforcement were investigate...

Laboratory Testing to Examine Deformations and Moments in Fiber-Reinforced Cement Pipe

Two 381 mm (15 in. nominal) diameter fiber reinforced cement pipes have been tested under embankment loading conditions to study pipe response in both low stiffness, fine grained backfill, and a high stiffness graded granular backfill. Pipe deformations and strains were measured and interpreted to provide insight into the effect of soil backfill on the deformations and moments that develop. Not surprisingly, the use of silty clay backfill resulted in greater pipe deflections while the stiffer granular backfill lead to greater load transfer to the surrounding ground. Calculations using elastic soil-pipe interaction theory were effective in estimating the observed changes in pipe diameter at typical service loads (overburden pressures of 100 kPa, i.e., 14.4 psi in the lower stiffness backfill and 200 kPa, i.e., 28.8 psi in the high stiffness backfill). Measured strain distributions show that the fiber reinforced pipe exhibited ovaling response similar to that seen for flexible and semiflexible pipes. As expected, tensile strains were observed on the outer surface at the springlines and the inner surface at the crown. Strains observed at the haunch were negligible, indicating that the bending moments within the pipe have conventional hourglass distribution, with negligible moments at shoulders and haunches. Differences in strain measured at the inner and outer surfaces were used with the elastic pipe modulus to calculate the experimental bending moments. Comparisons of those experimental bending moments with the bending moment calculated for a rigid pipe indicate that these FRC pipe structures are semirigid so that moments are reduced as a result of support provided by the surrounding soil. A design expression for moment arching factor (MAF or moment divided by the rigid pipe moments) developed in an earlier paper was found to provide reasonable estimates for the experimental moment values. Moment estimated using the design soil moduli of McGrath and MAF provide moment values that are reasonable and conservative relative to those that were observed.

Lateral Performance of Full-Scale Bridge Abutment Wall with Granular Backfill

Bridge abutments typically contain a backwall element that is designed to break free of its base support when struck by a bridge deck during an earthquake event and push into the abutment backfill soils. Results are presented for a full-scale cyclic lateral load test of an abutment backwall configured to represent the dimensions ( 1.7 m height), boundary conditions, and backfill materials (compacted silty sand) that are typical of California bridge design practice. An innovative loading system was utilized that operates under displacement control and that assures horizontal wall displacement with minimal vertical displacement. The applied horizontal displacement ranged from null to approximately 11% of the wall height (0.11H) . The maximum earth pressure occurred at a wall displacement of 0.03H and corresponded to a passive earth pressure coefficient of Kp =16.3 . The measured force distribution applied to the wall from hydraulic actuators allowed the soil pressure distribution to be inferred as triangula...