Backfill Pressure Research Articles

Effectiveness of displacement compensation units for integral abutment bridges

Integral abutment bridges (IAB) have become increasingly popular in the past few decades due to their design simplicity. UK design rules limit the length of IABs to 60 m, due to the issues associated with thermal strains, settlement, and pressure build-up behind the abutment. A cyclic loading test, representing seasonal thermal fluctuations, was conducted on a 1/12 scaled-down retaining wall of a conventional full-height IAB. The test was then repeated with the inclusion of displacement compensation units (DCU) in the form of conical disc springs (CDS) and hollow rubber cylinders (HRC), which operate in a pre-deformed shape. The results show some remarkable improvements in the IAB performance with DCUs. The soil backfill pressure was reduced to 30% and 47% with CDSs and HRCs, respectively. Furthermore, no settlement was observed, as compared to the conventional IAB test which recorded a 20 mm settlement after 100 cycles. Non-linearity in the force-deflection behaviour of DCUs enables expansion and contraction of the deck to be accommodated with minimal fluctuation of backfill pressure. Finally, a finite element (FE) model of an HRC applied with two temperatures has been analysed and compared with the IAB wall test, which suggests that both analyses showed some correlation.

Measurement of Aerosol Transmission Through a Stress Corrosion Crack–Like Geometry

The formation of a stress corrosion crack (SCC) in the canister wall of a dry cask storage system (DCSS) has been identified as a potential issue for the long-term storage of spent nuclear fuel. The presence of a SCC in a storage system could represent a through-wall flow path from the canister interior to the environment. Modern, vertical DCSSs are of particular interest due to the commercial practice of using relatively high helium backfill pressures (up to approximately 800 kPa) in the canister to enhance internal natural convection. This pressure differential offers a comparatively high driving potential for blowdown of any particulates that might be present in the canister in the event of a through-wall SCC. In this study, the rates of gas flow and aerosol transmission of a spent fuel surrogate through an engineered microchannel with dimensions representative of a SCC were evaluated experimentally using coupled mass flow and aerosol analyzers. The microchannel was formed by mating two gauge blocks with a linearly tapering slot orifice nominally 13 μm (0.0005 in.) tall on the upstream side and 25 μm (0.001 in.) tall on the downstream side. The orifice is 12.7 mm (0.500 in.) wide by 8.89 mm (0.350 in.) long (flow length). Surrogate aerosols of cerium oxide (CeO2) were seeded and mixed with either helium or air inside a pressurized tank. The aerosol characteristics were measured immediately upstream and downstream of the simulated SCC at elevated and ambient pressures, respectively. The next iteration of testing involves replacing the engineered microchannel with lab-grown SCCs. Preliminary clean flow testing has been conducted on SCC samples provided by the Electric Power Research Institute. These data sets demonstrate a new capability to characterize SCCs under well-controlled boundary conditions. Preliminary testing efforts are focused on understanding the evolution in both the size and quantity of a hypothetical release of aerosolized spent fuel particles from failed fuel cladding into the canister interior, and ultimately, through a SCC.

Effects of Soil–Structure Interaction on Performance of Bridges During Earthquakes. Case Study: Integral Abutment Bridge in Pennsylvania, USA

Bridges are among the most important transportation elements that may be damaged by earthquakes. An integral abutment bridge (IAB) is a bridge linking the superstructure directly to the substructure. As soil piles, abutments, and superstructures act as a combined system to resist lateral loading on the bridge, soil stiffness has a major impact on load distribution. This research attempts to determine how the structure and soil parameters affect the IABs. The parametric study consists of four variables, namely bridge span (short, medium, and large spans were 18.3, 35.4, and 64.5 m, respectively), backfill height/pressure (3.1, 4.6, and 6.1 m, respectively), stiffness of soil mixture backfills (high, intermediate, and low), and soil density around the piles (high, intermediate, and low). Because of the small width–length ratio of the bridge, a 2D model of an IAB with soil springs around the piles and abutments was developed with finite element software. Findings show that the value of the backfill pressure affects girder axial forces and girder bending moments at the IAB. Also, the stiffness of soil mixture backfills is an important factor to change lateral displacements, while less movement is related to high stiffness of soil mixture backfills with intermediate clay around the pile. It is clear that the maximum axial girder moments at the superstructure generally decrease when the stiffness of the soil mixture behind the abutments and around piles increases, similar to pile deflection and abutment displacements. In addition to maximum abutment, the head moment decreases when abutment backfill is dense and increases when piles are located in hard clay, similar to pile moments. Lastly, dense sand backfill behind abutments is recommended since it decreases pile deflections, pile lateral forces, abutment displacements, abutment head moments, and particularly pile bending moments.

Seismic performance of mechanically stabilized earth walls with Sand-Crumb rubber backfills of varying proportion

The applicability of crumb rubber mixed with sand as alternative fill material in mechanically stabilized earth (MSE) walls is investigated in the current study. Shaking table tests have been conducted to analyze the dynamic response of model geogrid reinforced earth walls subjected to three different base input motions. Eight different sand-crumb rubber mixture combinations were used as backfill. The outcomes of this study provide quantitative results on how base excitation amplitudes could influence the seismic behavior of the MSE walls in the presence of sand-rubber backfill. The results have been analyzed in terms of deformation and rotation of the wall, settlement of backfill, acceleration amplification, and earth pressure response. Wall displacement and backfill settlement was found to reduce with the addition of rubber in the sand. The maximum displacement at the top of the wall was found to reduce by 45% at a rubber content of 30% for high peak ground acceleration (PGA) input motion. Two-wedge failure was observed with the failure envelope inclined at an angle of 49° from the vertical. Apart from this, the strain in the geogrid was decreased with an increase in the rubber content. Hence, the rubber content of 30% mixed with sand can be considered an optimum percentage to control the dynamic performance of MSE walls. In the end, a cost-benefit analysis was conducted, and it was found that using rubber as a partial substitute for sand backfill reduces the overall cost of the project. This research will also promote the use of recyclable rubber as a construction material in highway construction.

Field Monitoring and Analysis of Abutment Foundation Behavior for a Curved Integral Abutment Bridge under Thermal Loading

The construction of integral abutment bridges (IABs), including semi-IABs, has increased substantially in the past two decades, partly because of reduced construction and maintenance costs. However, temperature-driven lateral movements, associated with the elimination of expansion joints, lead to a complex interaction between the bridge structure and the soils behind and under the abutment, which could result in soil densification, void formation, and settlement in the approach slab. The bridge's structure, geometry, and location all play an essential role in how the IAB responds to those movements. In addition, if the IAB is curved, it brings more complexity to its behavior. Nonetheless, there has been very little research on the behavior of curved IABs to date. This study investigates the behavior of a newly constructed curved IAB via field monitoring and numerical simulation. Field measurements included the backfill pressure, horizontal translation, and rotation imposed on the abutments during seasonal thermal fluctuations. The abutments rotate and translate toward backfill in summer and away from it in winter because of thermal expansion and contraction of the superstructure. Such thermal movements contributed to the increase in the lateral pressure to 85 kPa in the first year and to 140 kPa in the second year of monitoring. This implies that irreversible deformation may occur during seasonal temperature changes, supported by the monitored displacements. The bridge curvature, different skew angles at both ends, and the abutment geometry resulted in different movements and backfill pressures at each corner. A numerical simulation is also performed to provide more insights into the behavior of the curved IAB with more seasonal temperature cycles.

Soil-structure interaction of integral abutments

Integral abutment bridges (IABs) are pioneering structures that offer certain advantages over conventional bridges. The main difference is the ability of IABs to eliminate the necessity of utilizing bearings and expansion joints. This results in more economical construction and enhanced structural performance. However, the intricate soil-structure interactions of IABs under the influence of seasonal changes are not yet fully understood. This paper aims to improve the understanding of these innovative structures by utilizing comprehensive numerical models to investigate their geostructural performance under various conditions. The case study selected for this research was the Middlesex bridge in Vermont, USA. A full-scale monitoring program at this bridge measured and recorded the bridge acting pressures, deformations, and internal forces, making it possible to gain insight into the thermal response of such structures. For the numerical research, a comprehensive two-dimensional numerical model, developed by using the PLAXIS software, was verified against the available field measurements. Subsequently, parametric studies were conducted to investigate earth pressure and pile bending moment variations influenced by different model conditions. Key model parameters were varied, including the constitutive soil model, thermal loading, backfill stiffness, pile size and orientation, and span length. It was found that utilizing a linear constitutive soil model could lead to significant inaccuracies in the results. Increasing the backfill stiffness was seen to increase passive earth pressures and decrease pile bending moments. With smaller pile sections, oriented for weak-axis bending, pile bending moments decreased substantially and lateral earth pressures increased. Because dead load magnitudes were closely linked to span lengths, pile bending moments and backfill pressures increased when fewer and longer bridge spans were used.

Modelling of seismic actions in earth retaining walls and comparison with shaker table experiment

This paper is concerned with the response behaviour of a retaining wall in seismic conditions from a two-dimensional perspective. The backfill was levelled behind the wall for some distance. Both the wall and the backfill were found on a stiff elastic medium. The manner in which the backfill affected the response behaviour of the wall when subject to base excitations was of interest. The wall was idealised into a linear elastic element which responded mainly in flexural actions. The novelty of the paper is in the use of shaker table testing of a scaled down model of the retaining wall and backfill to validate the accuracies of results from numerical simulations. The comparison is mainly based on displacement time - histories at the top of the wall. The alignment with displacement-based approach for the seismic assessment of structures is distinguished from many other investigations which typically put the focus on the behaviour of the backfill pressure. Laboratory testings of backfill materials including triaxial tests have been conducted to determine the value of parameters for characterising the hysteresis properties of the backfill. Calibration of backfill material model has also been shown to simulate the pre yield, post yield and damping behaviour of backfill. The ultimate objective of the paper is to present a viable means of achieving realistic and accurate modelling of the seismic response behaviour of a retaining wall and to provide data for benchmarking simplified (macro) numerical models to be developed in the future for supporting seismic design and assessment.

Simplified Analytical Method for Predicting the Lateral Ground Displacements due to Shield Tunnelling

Earth pressure balance or slurry shield tunnelling will squeeze the subsoils and lead to lateral outward ground displacement. However, current methods to estimate the shield tunnelling-induced ground displacements generally use the methods based on the face unsupported tunnelling (e.g., New Austrian tunnelling and open shield excavation), which cannot predict the lateral ground movement due to shield tunnelling. In this paper, a novel simplified analytical method is proposed to predict the ground lateral displacement during the shield advancing process. The key shield tunnelling operation factors, including the additional pressure of cutter head, the friction forces around shield body, the back-fill grouting pressure, and the soil volume loss are all considered. The lateral ground displacements induced by the four former factors are calculated by using Mindlin’s solutions. The soil volume loss-induced lateral ground displacement is calculated by employing the expression introduced by Pinto and Whittle. Combining with the displacement obtained from all the factors, the analytical method for lateral ground displacement induced by shield tunnelling is obtained. The applicability of the proposed analytical approach is verified with three well-documented case histories involving slurry shield and EPB shield machines.

Experimental study of the “short-term” pressures of uncemented paste backfill with different solid contents for barricade design

The successful application of paste backfill in underground mines depends on the stability of a confinement structure, called barricades, constructed at the base of stopes to hold the backfill slurry in the stopes. The stability analysis of barricades needs the knowledge of the pressures during or shortly after the placement of the paste backfill exerted on the barricades. The recommendation of slight construction of barricades for paste backfill given in the few handbooks on paste backfill implicitly indicates that the paste backfill pressure is very small. This does not correspond to the field pressure measurements reported in the literature, which indicated that the pressures of paste backfill during and shortly after the placement can be as high as the iso-geostatic overburden pressure. To understand this dilemma, the difference between the paste backfill defined in the handbooks and that used in underground mines is for the first time highlighted. The former requires a high enough solid content and the backfill should be unsaturated. In practice, enough water has to be added in the backfill to facilitate its transportation by pipes. The solid content cannot be as high as that required in the handbooks. The dilemma between the recommendation of handbooks and field pressure measurement is for the first time solved. However, this analysis tends to indicate that the pressure of paste backfill depends on the solid content. To verify this hypothesis, a series of laboratory tests were conducted to measure the vertical total stresses of paste backfill having different solid contents at the end of filling operation. The results show that the vertical total stresses decrease as the solid content increases. The solid content should be taken into account in the pressure estimation of paste backfill for the design of barricades.

Ground subsidence due to the backfill pressure in tunnel boring machine

Abstract. In shield tunnel boring machine constructions, backfill pressure affects surface settlement and the stability of other underground structures nearby. Therefore, it is essential to pre-calculate backfill pressure in the design stage. This study examines and compares the seven internationally known backfill pressure calculation theories and calculates and analyzes the backfill pressures in six virtual ground conditions. The calculated backfill pressure increased as the ground cover increased, but the increase rate decreased. Also, this study performs a numerical analysis to identify the impact on ground settlement and tunnel crown settlement. In the end, settlement was more impacted by face pressure than backfill pressure in the unsaturated and saturated ground conditions. Also, as the ground cover increased, the impact of backfill pressure decreased, and as the applied face pressure decreased, the impact of backfill pressure increased.

A Review: Study of Integral Abutment Bridge with Consideration of Soil-Structure Interaction

Abstract Bridges are one of the most critical parts of transportation networks that may suffer damages against earthquakes. Also, seismic responses of most bridges are significantly influenced by soil-structure interaction effects. Taking out expansion joints in the bridges may cause many difficulties in design and analysis due to the complexity of soil-structure interaction and nonlinear behavior. The secondary loads on an IAB include seismic load, temperature variation, creep, shrinkage, backfill pressure on back wall and abutment, all of which cause superstructure length and stress variations in girder changes. The purpose of this study is to recognize the most effective parameters of analysis IABs. Findings show that the backfill material behind the IABs has a significant effect on the performance of IABs. Using a compressible material behind the abutments would enhance the in-service performance of IABs. Finally, behaviour of abutment may be greatly affected by thermal load and soil pressure. Thermal expansion coefficient significantly influences girder axial force, girder bending moment, and pile head/abutment displacement.

Numerical Investigation of the Three-Dimensional Performances of a Shield-Machine-Bored Tunnel in Loose Sands

This study first explores the impact on adjacent structures caused by a shield-machinebored tunnel in loose sand; observed displacements of structures have been reported that have rarely been examined in the past. Further, considering the difficulty of the reasonable prediction in advance of the ground convergence during tunnelling, a new method for the numerical investigation of the three-dimensional deformations induced by a tunnel in said ground conditions is developed, which is associated with the properties of backfill grout and pressure in the grouting zone, and a hardening soil-small strain model is selected for the simulation of soil behaviors. The influences on the ground deformations caused by changes in the operation parameters, such as the face and backfill grouting pressure as well as the elastic modulus of the backfill grout, are also explored. Finally, the strain level of the soil caused by tunnelling is examined and compared with the results from previous works, and a detailed discussion is provided.

A fully coupled fluid-structure interaction simulation of three-dimensional dynamic ductile fracture in a steel pipeline

Long running fractures in high-pressure pipelines transporting hazardous fluid are catastrophic events resulting in pipeline damage and posing safety and environmental risks. Therefore, the ductile fracture propagation control is an essential element of the pipeline design. In this study, a coupled fluid-structure interaction modelling is used to simulate the dynamic ductile fractures in steel pipelines. The proposed model couples a fluid dynamics model describing the pipeline decompression and the fracture mechanics of the deforming pipeline exposed to internal and back-fill pressures. To simulate the state of the flow in a rupturing pipeline, a compressible one-dimensional computational fluid dynamics model is applied, where the fluid properties are evaluated using a rigorous thermodynamic model. The ductile failure of the steel pipeline is described as an extension of the modified Bai-Wierzbicki model implemented in a finite element code. The proposed methodology has successfully been applied to simulate a full-scale pipeline burst test performed by British Gas Company, which involved rupture of a buried X70 steel pipeline, initially filled with rich natural gas at 11.6 MPa and −5 °C.

Experimental and numerical distribution of pressure on the abutment with backfill from tyre bales

This article describes the tests of the pressure distribution on bridge abutment generated by the backfill made of the recycled car tyres compressed in the form of bales with interfaces of medium sands and a buffer layer with tyre shreds. Backfill pressure distribution was measuring continuously over a period of three months using a pressure sensors. Static and dynamic load tests of abutment were conducted and cyclic horizontal displacements of the abutment wall, as a simulation of thermal elongation / shortening of the bridge span were also performed. The results of pressure field measurements were compared with results of numerical analysis performed using the GEO5 program basing on finite element method. Compatibility results of field measurements and numerical analysis was pretty good, and the maximum difference was about 25%. Based on field measurements and numerical analysis, a 2÷4 times reduction of pressure on bridge abutment was found, if the tyre bales had been used, compared to conventional backfilling of medium sand.

Study on Emergency Monitoring and Deformation Law of Slope

The slope of a project in Guiyang, due to heavy rain and other causes of serious deformation, the arrangement of 40 monitoring points, the slope of the emergency monitoring monitoring shows that the use of rapid and then accurate emergency method is feasible; Backfill pressure can stabilize the slope quickly. The supporting slope is characterized by strong deformation integrity and retrograde sliding. Guizhou is a mountainous area with many engineering slopes, which has reference value for emergency monitoring of similar slopes.

Critical face pressure and backfill pressure in shield TBM tunneling on soft ground

The most important issue during shield TBM tunneling in soft ground formations is to appropriately control ground surface settlement. Among various operational conditions in shield TBM tunneling, the face pressure and backfill pressure should be the most important and immediate measure to restrain surface settlement during excavation. In this paper, a 3-D hydro-mechanical coupled FE model is developed to numerically simulate the entire process of shield TBM tunneling, which is verified by comparing with real field measurements of ground surface settlement. The effect of permeability and stiffness of ground formations on tunneling-induced surface settlement was discussed in the parametric study. An increase in the face pressure and backfill pressure does not always lead to a decrease in surface settlement, but there are the critical face pressure and backfill pressure. In addition, considering the relatively low permeability of ground formations, the surface settlement consists of two parts, i.e., immediate settlement and consolidation settlement, which shows a distinct settlement behavior to each other.

Trench effects on lateral p-y relations for pipelines embedded in stiff soils and rocks

Existing relationships for lateral backfill pressures on pipelines assume that the trench is adequately wide to contain the failure surface. This condition is commonly violated in design and construction practice, putting at risk the pipeline safety. In this context, size and shape effects for trenches excavated in stiff soils and rocks, are numerically investigated, through experimentally-calibrated parametric analyses. It is shown that, for narrow trenches, ultimate pressures and yield displacements may increase up to an order of magnitude compared to “infinite-trench” values, while excavation of inclined walls reduces the above detrimental effects. Simplified relations are developed to aid pipeline design.

Hybrid fluid–structure interaction modelling of dynamic brittle fracture in steel pipelines transporting CO2 streams

Pressurised steel pipelines are considered for long-distance transportation of dense-phase CO2 captured from fossil fuel power plants for its subsequent sequestration in a Carbon Capture and Storage (CCS) chain. The present study develops a hybrid fluid–structure methodology to model the dynamic brittle fracture of buried pressurised CO2 pipeline. The proposed model couples the fluid dynamics and the fracture mechanics of the deforming pipeline exposed to internal and back-fill pressures. To simulate the state of the flow in the rupturing pipeline a compressible one-dimensional Computational Fluid Dynamics (CFD) model is applied, where the fluid properties are evaluated using rigorous thermodynamic model. In terms of the fracture model, an eXtended Finite Element Method (XFEM)-based cohesive segment technique is used to model the dynamic brittle fracture behaviour of pipeline steel.Using the proposed model, a study is performed to evaluate the rate of brittle fracture propagation in a real-scale 48in. diameter API X70 steel pipeline. The model was verified by comparing the obtained numerical results against available semi-empirical approaches from the literature. The simulated results are found to be in good correlation with the simulations using a simple semi-empirical model accounting for the fracture toughness, indicating the capability of the proposed approach to predict running brittle fracture in a CO2 pipeline.

Time-Resolved Electron Temperature in an Argon Theta Pinch by Line Ratio Methods

Pulsed inductive argon plasma in an 80-J, 15-kV, 490-kHz theta pinch is interrogated using spectroscopic methods. Time-resolved electron temperature is obtained by coupling line intensity ratios with steady-state corona and collisional-radiative (CR) models. Neutral excited state argon transitions from the 2p to 1s subshells were utilized. The backfill pressures of 50 and 100 mtorr are of primary focus. Time-resolved electron temperature estimates are presented ranging from 1 to 11.1 eV for the corona model up to an excess of 80 eV for the CR model. Near-IR spectral emission is seen to increase rapidly near the first zero crossing of the oscillating discharge current while electron temperature increase lags by roughly one full discharge cycle later near the third zero crossing. An analysis of long exposures provides an account of substantial second-order diffracted spectra. Weak spectral signal quality for the short exposures of 0.25-μs yielded time-resolved spectral intensity trend lines of a low accuracy with an average percent difference of 69% between raw data and trend lines.

A numerical evaluation of continuous backfilling in cemented paste backfilled stope through an application of wick drains

Cemented paste backfill (CPB) is gaining popularity in many underground mines worldwide. Sufficient water is added into CPB to make a flowable material for pipe transportation. Barricades are built near the drawpoints to prevent in-rush of the fill slurry. To avoid barricade failures resulting from excessive backfill pressures, backfilling is typically performed with a plug pour followed by a final pour. The interval between the two pours should be shortened or removed to increase mining productivity and avoid pipe clogging. Recently, Li proposed to apply wick drains in backfilled stopes to promote drainage and consolidation. The preliminary simulations by considering an instantaneous filling indicated that the drainage of CPB can be significantly accelerated by using wick drains. Barricade was not considered. Here, some new numerical modellings are presented with more representative filling sequences, stope geometry, and different draining configurations. The results illustrate that the stope can be backfilled continuously by using wick drains.