Backfill Conditions Research Articles

Influence of Soil Reinforcement on Horizontal Displacement of MSE Wall

Mechanically stabilized earth (MSE) walls offer simple construction techniques, pleasing aesthetics, and cost-effective solutions as an alternative to conventional gravity walls. However, design and construction should be carefully evaluated to achieve satisfactory performance of the wall. A case study is presented on a MSE wall located on State Highway 342 in Lancaster, Texas. The horizontal movement of the MSE wall was between 300 and 450 mm within 5 years of construction. A forensic investigation was performed to determine the causes of the excessive movement. It was identified that inadequate reinforcement length was one of the contributing factors that caused horizontal displacement of the MSE wall. The objective of this study was to determine the effects of soil reinforcement on excessive movement of the MSE wall. As a part of the forensic investigation, two inclinometers were installed at the site to monitor any additional movement of the MSE wall. The inclinometer results suggested that the wall continued to move at an average rate of 4.5 mm/month during the investigation period. A finite-element (FE) program was used to simulate horizontal displacement and stability of the MSE wall. It was observed that the numerical modeling results were in good agreement with inclinometer results. A parametric study was conducted to identify the effects of soil reinforcement on horizontal movement at varied wall heights and backfill conditions. Numerical analyses results indicated that the effect of reinforcement stiffness was not significant at a wall height of 4 m compared with 8 and 12 m. The wall movement varied from 74 to 29 mm for an increase in reinforcement stiffness from 250 to 42,000 kN/m at 1.0H reinforcement length. The variations in displacement with reinforcement lengths suggested that substantial reduction in displacement occurred for an increase in length-height (L/H) ratio from 0.5 to 0.7. FE modeling results were used for sensitivity analysis employing a statistical analysis program. Based on the analyses, reinforcement length and stiffness were identified as influential factors for the horizontal displacement of MSE walls at a specific height.

습윤-건조 반복작용에 노출되는 옹벽의 거동에 관한 수치해석 연구

본 논문에서는 계절적 환경변화로 인한 습윤-건조 반복과정에 노출되는 옹벽의 시간 의존적 거동에 대한 수치 해석 연구 내용을 다루었다. 이를 위해 먼저 습윤-건조 과정에 노출되는 옹벽에 대한 응력-간극수압 연계해석 기반의 모델링 기법을 정립하였으며 이러한 모델링 기법을 토대로 다양한 옹벽조건 및 강우조건에 대한 매개변수 연구를 수행하였다. 그 결과 다양한 강우 특성 중 옹벽의 거동에 미치는 주된 영향인자는 강우강도로 나타났다. 한편, 강우가 발생하기 이전 선행하는 선행강우는 지반내 포화도, 즉 모관흡수력, 분포를 좌우하는 것으로 나타나 지반구조물의 안정성은 선행강우량에 의해 큰 영향을 받는 것으로 검토 되어 강우로 인한 습윤-건조 반복 사이클에 노출되는 옹벽구조물의 거동 예측시 선행강우량에 대한 현실적인 평가가 매우 중요한 것으로 검토되었다. This paper presents the results of a numerical investigation into the behavior of retaining wall subject to cycles of wetting and drying due to rainfall. The stress-pore pressure coupled finite element modeling strategy was first established for stimulating the wall behavior. A series of finite element analyses were then performed on a range of conditions including different rainfall and backfill conditions. The results indicated that the rainfall intensity was the primary influencing factor for the wall behavior. Also revealed was that the pre-rainfall condition determines the magnitudes and the distribution of matric suction which in fact has a significant impact on the behavior of wall during a major rainfall. This result demonstrates the importance of incorporating the pre-rainfall condition for numerical modeling of walls during heavy rainfall. Practical implications of the findings from this study are discussed in great detail.

Use of geosynthetics to improve seismic performance of earth structures

After reviewing seismic performance of earth structures based on case histories in Japan and relevant model test results, advantages of using geosynthetics in improving their seismic performance are demonstrated. As one of the successful applications, geosynthetic-reinforced soil retaining walls are highlighted, focusing on several influential factors such as facing rigidity, arrangement and properties of reinforcements, and backfill and subsoil conditions. In addition, further applications of the reinforcement method using geosynthetics are introduced, which include combination with other reinforcement methods, application to bridge abutments and piers, and application to ballasted railway tracks.

Observations of dynamic and non-dynamic interactions between a quay wall and partially stabilised backfill

The dynamic and non-dynamic interactions between a gravity-type quay wall and a backfill ground were investigated by centrifuge model testing, considering cases in which a rigidly cement-stabilised ground existed at varying distances from the quay wall. In conducting the centrifuge tests, the performance of the instrumentation applied to measure the pressure from granular soils was critically assessed. At non-dynamic states, the backfill confined between the quay wall and the rigidly stabilised soil block exerted smaller earth pressure at deeper locations, at both apparently active and transient states. The calculation, based on perfect plasticity and considering friction arching, was useful in explaining these results. A similar feature was also observed during the steady-state oscillations, in the case of dry sand backfill, and was associated with the system’s increased seismic stability. In underwater cases, the pore water fluctuations in the backfill dominated the total earth pressure behaviour, with the active pressure being smaller again from a confined fill than from a fully extending one. Despite the reduced active pressure, placing the stabilised soil in the proximity of the quay wall increased the wall’s permanent seaward movement, unless the two bodies were in direct contact. This ostensible association of smaller active pressure with greater instability in the underwater cases cannot be explained by the conventional, simplified conception of the active earth pressure as a unilateral cause of instability. The evaluation of such unconventional backfill conditions seems to require rigorous consideration of the simultaneous soil–water-structure interactions.

USE OF GEOSYNTHETICS TO IMPROVE SEISMIC PERFORMANCE OF EARTH STRUCTURES

Abstract After reviewing seismic performance of earth structures based on case histories in Japan and relevant model test results, advantages of using geosynthetics in improving their seismic performance are demonstrated. As one of the successful applications, geosynthetic-reinforced soil retaining walls are highlighted, focusing on several influential factors such as facing rigidity, arrangement and properties of reinforcements, and backfill and subsoil conditions. In addition, further applications of the reinforcement method using geosynthetics are introduced, which include combination with other reinforcement methods, application to bridge abutments and piers, and application to ballasted railway tracks.

Earth pressure measurements on buried HDPE pipe

An understanding of the stress distributions that act on buried pipes plays a key role in the development and use of design methods for these structures. The radial stresses that develop on buried flexible pipes have been investigated by a number of researchers through experimental, analytical and numerical methods. However, direct measurements of soil contact stresses acting on small-diameter pipes have not been possible, since commercial earth pressure cells are large, and the stiffness of the cell relative to the soil material is known to influence the measured stresses. A new technique is described for measuring the radial contact stresses acting on buried pipes, and experimental studies are reported that demonstrate the effectiveness of this new technique for measuring contact pressures for both loose and compacted sand backfills. Comparison of the measurements with computations from elastic soil–pipe interaction theory reveals significant discrepancies in the magnitude and distribution of radial pressure around the circumference of the pipe for both loose and compacted backfill conditions. The new measurement technology provides direct support for pressure distributions inferred in earlier studies from measurements of circumferential strains around the pipe circumference.

Experimental Investigations of Backfill Conditions on the Performance of High-Density Polyethelene Pipes

In this study, a series of laboratory tests on 100-mm-diameter high-density polyethylene (HDPE) 100 PE flexible pipe buried in poorly graded Sile quartz sand with different relative densities are described. The laboratory tests were performed in a 40-mm-thick plexiglass fronted test tank that replicated a classical trench section in field conditions. The HDPE flexible pipe was positioned against the glass with its longitudinal axis perpendicular to the glass. This allowed direct observation of the backfill–pipe interactions. Three high-definition photogrammetric cameras were used to capture the photogrammetric images through the glass allowing the discrete measurement and image processing of the deformation patterns of the pipe conduit during the pipe installation and incremental surcharge loading. Vertical loads were applied in increments of 10–150 kPa using air pressure membranes. Electric resistant strain gauges measured the bending moments of the pipe walls under vertical surcharge loadings. For each loading step, the vertical deformation of the pipe crown was also measured using linear position transducers. According to the test results, it is understood that the installation technique and backfill relative density have an important effect on circumferential strains, performance, and deformation characteristics of HDPE pipes. It was also observed that close-range image processing is a very simple and appropriate method for measuring three-dimensional pipe deformations under various conditions.

Modification of moment equations in the CHBDC (2000) for soil-metal box structures

This paper evaluates the moment equations in the 2000 Canadian Highway Bridge Design Code (CHBDC) for soil-metal box structures, which are applicable to spans less than 8 m. Finite element analyses are carried out for soil-metal box structures having spans of 4-15 m using the deep corrugated metal plates under three construction stages: backfill up to the crown, backfill up to the cover depth, and live loading. The coefficients of moment equations are newly proposed based on the results of numerous finite element analyses considering various design variables, such as span length, soil depth, and backfill conditions. The validity of the proposed coefficients in the moment equations of the CHBDC (2000) is investigated and compared with the existing coefficients and numerical results of finite element analyses. The comparisons show that the moments of the CHBDC (2000) give good predictions for spans less than 8 m, but underestimate them for spans greater than 8m. In contrast, the proposed moments give good estimates of numerical results for the spans of 4–15 m.

Characterization of gold contacts in GaAs-based molecular devices: Relating structure to electrical properties

Au/octadecanethiol/GaAs devices were prepared using different metallization conditions: direct deposition at 77 K and 300 K samples, and indirect deposition (Ar backfilling). Time-of-flight secondary ion mass spectrometry data indicate that ∼4x and ∼2x more gold penetrates through the SAM after direct deposition at 300 K and 77 K, respectively, than under Ar backfill conditions. However, these devices have significantly different conductances: Ar-backfill and 77 K samples have ∼200x and ∼70x, respectively, larger conductances than 300 K devices. An electrostatic model has been developed to explain the very large conductance changes due to small differences in device structure.

Integral Abutment Bridge Behavior: Parametric Analysis of a Massachusetts Bridge

Integral abutment bridges are often a preferred bridge type for moderate spans throughout the United States. However, design methods and construction details vary from state to state. Variations between states are noted in the methods employed to accommodate deformations in the piles. The significance of these differences was evaluated through a finite-element study. The effects of backfill properties and soil restraint on piles were evaluated with regard to bridge distortions and maximum moment realized in the piles. Results show that bridge expansion is predominantly affected by backfill conditions, whereas contraction is influenced by pile restraint conditions. Pile moments are minimized when denser backfill and lower pile restraint are provided. The influence of abutment soil-structure spring modeling assumptions is addressed. Models were calibrated to the reference bridge at Orange-Wendell, Mass, which has been instrumented and data collected for 4 years.

Dynamic active earth pressure on retaining structures

Earth-retaining structures constitute an important topic of research in civil engineering, more so under earthquake conditions. For the analysis and design of retaining walls in earthquake-prone zones, accurate estimation of dynamic earth pressures is very important. Conventional methods either use pseudo-static approaches of analysis even for dynamic cases or a simple single-degree of freedom model for the retaining wall-soil system. In this paper, a simplified two-degree of freedom mass-spring-dashpot (2-DOF) dynamic model has been proposed to estimate the active earth pressure at the back of the retaining walls for translation modes of wall movement under seismic conditions. The horizontal zone of influence on dynamic earth force on the wall is estimated. Results in terms of displacement, velocity and acceleration-time history are presented for some typical cases, which show the final movement of the wall in terms of wall height, which is required for the design. The non-dimensional design chart proposed in the present study can be used to compute the total dynamic earth force on the wall under different input ground motion and backfill conditions. Finally, the results obtained have been compared with those of the available Scott model and the merits of the present results have been discussed.

Evaluation of moment equation in the 2000 Canadian highway bridge design code for soil–metal arch structures

This paper evaluates the moment equation in the 2000 Canadian highway bridge design code (CHBDC) for soil–metal arch structures. This equation is adopted from Duncan's moment equation (1978), which is based on his finding from finite element analyses that the maximum moment occurs at the quarter point of soil-metal structures. However, finite element analyses carried out for this study demonstrate that the maximum moment in soil–metal arch structures with spans greater than approximately 11 m occurs at the crown point. In this study, the location and magnitude of the maximum moment was examined for soil–metal arch structures having spans of 6–20 m under three construction stages; backfill up to the crown, backfill up to the cover depth, and live loading. Based on the location of the maximum moment, two sets of moment equations dependant on span length were found necessary. Moment coefficients and moment reduction factors in moment equations are proposed from the results of numerous finite element analyses for semi-circular arch and part-arch types of soil–metal structures considering the various design variables, such as span length, structural shapes, section properties, and backfill conditions. The validity of the coefficients and reduction factors in the moment equation of the 2000 CHBDC is investigated by comparison with those proposed in this study. The comparison demonstrates that the moment equation of the 2000 CHBDC is still valid and a little conservative. The effects of design variables on the variations of moments of soil–metal arch structures during construction stages are also examined.Key words: soil–metal arch structures, moment equations, CHBDC, soil-structure interaction.

The performance of a laboratory facility for evaluating the structural response of small-diameter buried pipes

The performance of a new laboratory facility for testing small-diameter buried pipes (less than 300 mm diameter) subject to the biaxially compressive earth pressures expected to prevail under deep and extensive overburden is examined. The new facility consists of a prism of soil 2.0 m wide × 2.0 m long × 1.6 m high contained within a stiff steel structure. Laboratory tests were performed in the new test facility to examine the appropriateness of the boundary conditions imposed during testing. Overburden pressures are successfully simulated with a pressurized air bladder. Boundary friction was limited to only minimal effects with lubricated polyethylene sheets. The stiffness of the lateral boundary is sufficiently large to induce lateral stresses close to those for zero lateral strain conditions. Overall, the effects on the pipe arising from the idealizations involved in the laboratory model were found to be small. The application of the new test cell is illustrated by using it to assess the response of a small-diameter landfill leachate collection pipe under two different backfill conditions. This comparison showed that the structural response of the pipe is significantly impacted by the coarse gravel backfill used in landfill drainage layers. Maximum pipe deflections and strains were nearly twice as large when tested in the coarse gravel compared with the sand backfill. Much greater variations of deflection and strain were also measured with the coarse gravel when compared with the sand backfill due to local bending effects from the coarse gravel.Key words: buried pipes, soil-structure interaction, laboratory testing, leachate collection pipes.

Effective Stress Analyses of Port Structures

ABSTRACT Effective stress analyses are performed on the seismic response of port structures during the 1995 Hyogoken-Nambu earthquake. The structures analyzed include: caisson type quay walls which moved up to 5 m maximum toward the sea; pneumatic caisson type bridge foundations which moved toward the sea, reducing the distance between the bridge foundations across a sea channel by a total of 0.8 m; and composite breakwaters which settled 2 m. Although these structures were all a rigid concrete type and shaken by the same earthquake, the mode and extent of deformation/failure were different depending on the backfill and embedment conditions of the structures. The constitutive model of soil used for the seismic analyses is a multiple mechanism model defined in strain space. The model can take the effect of principal stress axis rotation into account, which is known to play an important role in the cyclic behavior of sand under anisotropic stress conditions. The model parameters are calibrated based on in-situ and laboratory investigations, including in-situ freeze sampling. The results of the effective stress analyses are consistent with those measured, including the mode and extent of deformation/failure, suggesting that the cyclic behavior of soil, idealized through the effective stress model, can explain the variety of seismic response of the port structures in a unified manner.

Behavior of High-Density Polyethylene Pipe with Shallow Cover

In this investigation, a testing program was initiated to gain some understanding of the nature of high-density polyethylene (HDPE) as a structural material and as a buried structure. The testing program consisted of a series of parallel plate tests, a sequence of flexural tests, and field tests of buried pipes under varying backfill conditions. Parallel plate tests were conducted in accordance with ASTM D2412. The flexural testing consisted of applying two point loads to simply supported beam specimens. The field tests completed in this investigation were developed to study the response of large-diameter HDPE to concentrated loads under shallow cover. From the testing, it seems that in cases where high longitudinal stresses may be present (concentrated loads with shallow cover, uneven bedding, uplift, etc.) the pipeline designer should consider the longitudinal strength of HDPE pipes in addition to the circumferential and backfill properties. In addition, the designer must realize that when stresses exist in both directions, the Poisson’s ratio effect must be considered. This finding is supported by the longitudinal failure strains measured during the flexural tests and the field tests. In both types of tests, the pipes failed at approximately the same longitudinal strain level, approximately 1,300 microstrain. On the other hand, in the field tests, the pipes never reached the magnitude of strain associated with failure in the laboratory parallel plate tests.

Thermodynamic Modelling of the Sorption of Radioelements onto Cementitious Materials

AbstractA model has been developed for the sorption of radioelements onto cementitious materials based on the diffuse-layer modelling approach. The model assumes that silicon sites (>SiOH) and calcium sites (>CaOH) dominate the surface chemistry and the sorption of radioelements onto the cementitious materials. Both types of site may undergo surface protonation and deprotonation reactions. Cement-based systems vary greatly in their chemistry depending on their calcium-tosilicon molar ratio, and the corresponding variation in the surface chemistry has been incorporated by allowing sorption of calcium ions onto silicon sites. This process results in a change from a silica-type surface, at very low calcium-silicon ratios, to a calcium hydroxide-type surface for high-calcium cement-based materials. The predicted variation in the surface chemistry is consistent with literature data on measured zeta potentials of cements. The model has been applied successfully to describe the sorption of simple caesium and iodide ions at varying calciumsilicon ratios. In a Nirex repository for low and intermediate level wastes, a high-calcium cementitious backfill would be specified. This model has allowed a consistent interpretation of experimental data for sorption of key radioelements, including uranium and plutonium, onto the backfill, under saline and non-saline conditions.

Pullout force/displacement relationship of extensible grid reinforcements

A model for predicting the pullout resistance of polymer-grid reinforcement has been proposed. The influence of bearing member rigidity and spacing ratio ( S/D) are explicitly expressed in the hyperbolic model. A new bearing capacity equation is incorporated for calculating the maximum pullout force. The displacement along the reinforcement is calculated by using the proposed pullout bearing resistance model together with the elongation of the grid longitudinal member. The validity of the method is confirmed by good agreement between calculated values and actual test data. The analytically determined effective reinforcement embedment lengths (i.e. the length of the reinforcement in tension) and pullout displacement to mobilize the desired pullout resistance of polymeric grids under different backfill conditions and under different applied normal pressures, provide useful information for the design of reinforced earth structures against pullout failure.

Structural Evaluation of Box Culverts

Corrugated‐metal box culverts are employed as replacements for short‐span bridges. As a result of increasing use, there is a great demand for establishing a rational design procedure for these culverts. The available design procedures are based on experience or mathematical modeling. Only a limited number of attempts have been made to verify these design techniques with full‐scale field tests. Three corrugated‐metal box culverts were fully instrumented and field‐tested under similar backfill conditions and live loads. The first culvert was bulb‐angle‐rib‐reinforced corrugated aluminum; the second was rib‐reinforced corrugated steel; and the third was corrugated‐rib‐reinforced corrugated steel. The collected data were evaluated using a finite element analysis, and the applicability of standard design parameters was determined. Maximum bending moments obtained from field tests were compared to plastic moments calculated according to American Association of State Highway and Transportation Officials (AASHTO) specifications. It was concluded that the design for these types of structures can be improved if (1) Composite action is assumed; (2) thrust forces are included in design; and (3) construction procedures are controlled. The AASHTO specifications were conservative for rib‐reinforced, corrugated‐metal culverts.

An Investigation into the Performance Characteristics of Boundary Boxes Subjected to Vertical Loading

ABSTRACTThis paper presents the results of a series of load tests carried out on a variety of designs of boundary boxes which are intended for use as stopcock/water meter chanbers at domestic properties. A description is given of a test which has been developed at Loughborough University to measure the performance, under load, of the complete boundary‐box units in simulated site conditions. Three broad classifications of different design types are identified, namely rigid, flexible and sliding; and the load displacement characteristics of each type are presented and discussed. The effects of rate of loading and different backfill conditions are considered, and conclusions are drawn on suitable performance specifications under load.

Leach Rates of High Level Waste and Spent Fuel. - Limiting Rates as Determined By Backfill and Bedrock Conditions.

The release rate of radionuclides from a degraded canister containing radioactive waste is controlled by diffusion through the backfill and by diffusion into the water flowing past the canister. In low porosity low permeability bedrock like the Swedish granites the amount of water which can be contaminated may be very small. Two sample cases are calculated. One uses the conditions for a repository for vitrified high level waste, the other applies to a repository for spent fuel. The small amount of water which can carry the waste in combination with the low solubilities for the major longlived actinides limit the release rate from the repositories to very small values.