Backfill Conditions Research Articles

Settlement Characteristics of Urban Underground Comprehensive Pipe Gallery under Different Backfill Conditions

In order to solve the problem of differential settlement of pavement after the backfilling of urban underground comprehensive pipe corridors based on a pipe gallery project in Xiong’an New Area, the mechanical calculation model of the pipe gallery foundation pit backfilling process was simulated by ABAQUS, and the pavement settlement and the change of the pipe gallery model under different construction conditions were simulated and verified. The results show that many external environmental factors have an impact on pavement settlement, and methods such as increasing the compaction degree of backfill soil and the slope of the foundation pit, reducing the width of the pit floor, and controlling the vehicle load after work can avoid the pavement damage caused by uneven settlement.

Seismic performance of U-shaped spillway retaining wall from shake table tests

A series of large-scale shake table tests were performed to evaluate the seismic behavior of a U-shaped spillway retaining wall. Three test model configurations were employed using different backfill conditions in terms of in-place soil density. Two earthquake records were employed as input motion and scaled in terms of time duration and acceleration amplitude to assess various aspects of seismic response. The test results showed that dynamic wall deflection and wall bending moment generally increased with Peak Ground Acceleration (PGA) amplitude. At peak wall deflection, the seismic earth pressure increment coefficient (ΔKAE) was back-calculated from the measured bending moment along each wall. In addition, the location of ΔKAE along the wall height and the resulting base moment were evaluated and discussed. The results suggested that seismic demands were generally lower up to a PGA of about 0.8 g, compared to the classic Seed and Whitman recommendation for seismic lateral earth pressure and seismic bending moment of retaining walls. Beyond PGA of 0.8 g, test results were in general agreement with the Seed and Whitman solution. Finally, two-dimensional finite element analyses were performed, and the computational results showed overall reasonable agreement with the experimental data.

Large-scale shake table testing of shallow tunnel-ground system

Soil-structure interaction (SSI) is of critical importance in evaluating the seismic response of shallow tunnels. Experimental observations are crucial in assessing the influence of the surrounding and overburden soil near the ground surface. Therefore, this paper presents a series of shake table tests to investigate the seismic response of a scaled shallow tunnel with different backfill material properties and thickness of overburden soil (burial depth). A shallow tunnel was tested incorporating ground representative of realistic backfill conditions using the large-scale 1-g outdoor shake table at the University of California, San Diego (UCSD). The tunnel structure was designed at 1/9 scale based on the configuration of an actual recently constructed prototype structure in San Francisco, California. The test results were interpreted in full-scale, which demonstrated that the maximum tunnel deformation generally occurred at the instant of peak ground acceleration (PGA). Relatively large shear strains were developed (up to 6%) and significant reduction of the soil shear modulus were evaluated as the PGA increased during the shaking. The backfill shear strains were lower as the tunnel embedment depth increased because of the increased soil stiffness and strength due to the higher overburden stress. In addition, the tunnel racking deformation became closer to that of the ground as the tunnel embedment increased. At the largest observed soil shear strains, tunnel racking was significantly lower than that of ground deformation due to the decrease in soil. The FHWA simplified solution was used to estimate tunnel racking deformation and wall bending moments. The FHWA solution produced reasonable estimates of ground deformation. However, a discrepancy was noted in estimating the tunnel deformation in the given ground deformation known as racking ratio (Rr). This discrepancy contributed to conservatism in the FHWA racking prediction that was pronounced particularly with the decreased overburden soil stress.

Experimental study of soil responses around a pipeline in a sandy seabed under wave-current load

Wave and current coexist widely in the oceans. Ever-changing wave characteristics, such as height and length, have a significant influence on the dynamic response within the seabed soil due to the presence of currents. Some aspects of wave, seabed and pipeline interactions have been widely investigated over past years, incorporating both experimental and numerical approaches. However, previous studies on seabed dynamic response around pipelines have mainly focused on cases with wave loading. In this experimental laboratory study, the current loading and diverse backfill conditions which commonly occur far offshore are discussed in depth. It is found that when subjected to the combined wave and current loads, the response of the seabed around a pipeline is more complex than that subjected to pure wave loading as current velocity and propagation direction would significantly change the wave characteristics as well as seabed response. Most notably when comparing with the pure wave loading, the pore pressure amplitude and its attenuation rate within the seabed increases when current and wave travel in the same direction. This could increase the possibility of liquefaction around the pipeline. When current travels in opposing direction to the wave, the effect of current on the decrease of pore pressure and its attenuation rate is more significant than that of the co-current actions. This means that the counter-current will reduce the probability of soil liquefaction.

The impact of the backfill direction on the backfill cooling performance using phase change materials in mine cooling

The deep mine radiation cooling system using cold load and storage (CLS) functional cemented paste backfill (CPB) has been proven to be better in maintaining a more comfortable mine environment with lower energy consumption than conventional cooling methods. Three typical backfill methods with CLS-functional CPB, upwards backfill, downwards backfill and wall backfill, which have different positional relationships between the cold radiant surface and the cooled space, resulted in different cooling performances. A numerical model constructed by CFD was verified by experimental results to obtain the heat transfer characteristics of the three backfill methods. The dimensionless temperature, dimensionless direction, and heat transfer coefficient were defined to identify the heat transfer mechanism of the three methods. The experimental and simulation results show that the downwards backfill can accelerate the heat transfer rate, make the stope's temperature more uniform, and better meet the requirements of deep mine mining temperature. The dimensionless temperature variation perpendicular to the direction of cold transfer is expressed by the QP model for upwards and downwards backfill conditions and the ExpDec1 model for wall backfill conditions. Compared with the homologous cold radiation system, the three backfill methods have satisfactory heat transfer cooling performance. The results of this study can provide a reference for the improvement of the cooling effect of CLS-functional CPB and mine cooling system design using CLS-functional CPB.

Experimental Study of the Effect of Backfill Conditions on Soil Responses around a Pipeline under Wave–Current Load

Waves and currents coexist widely in the ocean, and the interaction of waves and currents plays an important role in the instability of submarine pipelines. So far, most studies have concentrated on discussing the dynamic reaction within the seabed around a pipeline under pure wave action, monotypic sediment, and an exposed or fully buried condition. In this study, the effect of current characteristics (e.g., current velocity and propagation direction) and backfilling conditions (e.g., backfill depth and sand property) on the dynamic response around the submarine pipeline is investigated by conducting laboratory experiments. Pipeline was buried in the excavated trench using three types of sand with the median size of 0.150 mm, 0.300 mm and 0.045 mm, respectively. Five relative backfilled depths, with the ratios of backfill depth over the pipeline diameter being 0, 1/2, 1, 3/2 and 2, were tested. The excess pore pressure was measured simultaneously by using the pore pressure sensors installed around the pipeline surface and beneath the pipeline. Results show that both the pore pressure amplitude and its descent rate gradually decrease with an increasing backfill depth, which decreases the soil liquefaction potential. Under the co-current actions, the decrease rate of the pore pressure along the vertical direction increases with an increasing current velocity. However, the increased current velocity leads to a decrease of the attenuation rate under the counter-current actions compared with the pure wave actions, and the counter-current effect on the pore pressure within the seabed is greater than the co-current. The results indicate that the dynamic response around the pipeline in coarse sand is close to that without the backfill scenario, even if the backfill depth reaches up to two times that of the pipeline diameter. It is found that the larger the median particle size of backfill sand, the smaller the impact on pore pressure within the seabed beneath the pipeline.

Dynamic Response of the Mechanically Stabilized Earth Walls with Different Reinforcement and Backfill Conditions

Dynamic Response of the Mechanically Stabilized Earth Walls with Different Reinforcement and Backfill Conditions

Optimal insulation backfill depth analysis of coaxial deep borehole heat exchanger based on multi-factor orthogonal experiments

Coaxial deep borehole heat exchanger (DBHE) coupled with ground source heat pump (GSHP) system has been widely applied in building heating. Insulation backfill is essential for the heat transfer performance of DBHE in colder regions. However, few studies focus on the combined influence of multi-factors on the optimal insulation backfill depth. In this paper, an orthogonal experiment was designed to explore the influence of multi-factors on the optimal insulation backfill depth of DBHE by using a numerical heat transfer model. Sensitivity analysis concluded that heat transfer factors such as inlet flow rate (A), inner pipe thermal conductivity (B), inlet temperature (C), geothermal gradient (D), and pipe depth (E) all affected insulation backfill depth of DBHE, and the comparison order of significance are C > D > B > A > E. An optimal insulation backfill depth exists for different DBHE heat transfer conditions. Compared with no insulation backfill, the average heat extraction of buried pipe under optimal insulation backfill conditions increased by 6.09 kW, with an average growth rate of 8.78 %. In addition, multiple linear regression analysis was utilized to examine the relationship between affecting factors and the optimal insulation backfill depth, based on the findings of orthogonal tests. The outcomes revealed that inlet temperature, inner pipe thermal conductivity, and pipe depth were positively correlated, with the highest standardized positive correlation value of 0.755 for inlet temperature. In contrast, inlet flow and geothermal gradient were negatively correlated, with the highest standardized negative correlation coefficient of −0.420 for the geothermal gradient. The linear regression equation could predict the magnitude of optimal insulation backfill depth with diverse input parameters. The relative error between the predicted and simulated values of the equation was less than 5 %. The findings of this research can guide the popularization and application of the GSHP coupled with the DBHE system in colder regions.

Numerical study of the vibration suppression effect of a new vibration suppression method based on a shielding wall

A deep underground laboratory is an important research site for fields such as particle physics, astrophysics and cosmology, which require not only sophisticated technology and precision instruments but also stable and interference-free research environments. Ground motion can introduce interference with some precision instruments, causing the instruments to fail to work normally with the highest accuracy and even causing serious errors. Therefore, effective vibration suppression measures play a vital role in underground laboratory research. Based on the shielding effect underground structures have on ground motions, a shielding wall is proposed as a new vibration suppression method. By simulating the ground motion characteristics of various models (background models, excavation models, and backfill models) in the Jinping Underground Laboratory, the high-efficiency vibration suppression effect of a shielding wall was demonstrated. The influence of different backfill conditions on the vibration suppression effect of the shielding wall was systematically analysed, and a nonbackfilled shielding wall was determined to be the best vibration suppression option. Moreover, the influence of the length, position and angle of the shielding wall on its vibration suppression effect and the potential application prospects of the shielding wall were also discussed. Shielding walls enrich and develop vibration suppression measures, providing a new method for the prevention and control of seismic hazards in underground laboratories and surface buildings under dynamic load conditions.

Thermal analyses of TN-24P dry storage cask with heterogeneous fuel assembly modeling

The wet storage capacity of used nuclear fuels (UNFs) in South Korea is expected to reach a saturation point within a few years. Dry storage of UNFs is required, and thermal safety analysis of UNFs in a dry storage cask is therefore necessary. From this standpoint, the thermal evaluations of the TN-24P cask were conducted in this study. The UNF geometries were heterogeneously composed of many fuel rods. Thermal evaluations were conducted for backfill gas conditions through simulations that were coupled with heat transfer and fluid flow. In addition, another thermal evaluation was conducted under vacuum conditions through a simulation involving heat transfer only. The maximum temperatures for the helium, nitrogen, and vacuum backfill conditions were recorded at axial locations above the center, near the top, and at the center of the fuels, respectively. The highest maximum temperature was recorded in the vacuum backfill condition, followed by the second highest temperature in the nitrogen backfill and the third highest temperature in the helium backfill condition. Moreover, the simulation results agreed well with the measured data. The maximum temperatures for the three simulations did not exceed 400 °C; thus, all evaluations of the cask met the acceptable criteria for dry storage. However, the simulation results were significantly affected by the gas pressure in the helium backfill condition and the thermal conductivity of the basket in the vacuum condition.

Numerical Investigation of the Short-Term Mechanical Response of Buried Profiled Thermoplastic Pipes with Different Diameters to External Loads

High-density polyethylene (HDPE) double-wall corrugated pipe is a flexible buried pipe widely used in municipal drainage, and its deformation is affected by the compactness of the surrounding soil. This paper uses the ABAQUS to establish a three-dimensional pipe-soil model of double-wall corrugated pipes, and the mechanical response behavior of corrugated pipes with different nominal diameters subject to external loads is studied. The results show that the strain distribution characteristics for pipes with different diameters are very similar, and the circumferential strain value at critical positions of the pipe is proportional to its nominal diameter. Under poor backfill conditions, small-diameter pipelines are more prone to damage caused by strain concentration, while large-diameter pipelines may be damaged due to local bending.

Effects of gravel layer as thrust restraint for pipe bends subjected to earthquake loading

This paper reports effects of gravel layers as a thrust restraint in a pipe bend subjected to earthquake loading based on the shaking table tests in a centrifuge. The model pipe was buried under four backfill conditions differing in the existence and layout of a gravel layer. The pipe was pulled laterally with a constant load, simulating the thrust force generated at bends in pressure pipelines with diameters of 1800 mm. Shakings were conducted on the model pipe with lateral loading. The test results showed that the gravel surrounding the pipe was effective as a thrust restraint for pipes of large diameter. A gravel layer on the passive side was found to be particularly crucial. A gravel layer on the active side provided no thrust-restraint effect, although it did reduce pipe displacement amplitude. A gravel layer on the upper half of the passive side of the pipe likely contributed to mitigation not only of lateral displacement but also of pipe uplift.

Mechanical Response of Barricade to Coupled THMC Behavior of Cemented Paste Backfill

Cemented paste backfill (CPB), which is prepared by mixing tailings, binder and water, is widely used in underground mines for waste management and ground control. Since the CPB is delivered into mined-out areas in the form of fluid, a barricade needs to be constructed for retaining it during the process of its filling and hardening. Therefore, the barricade should have enough mechanical stability to ensure the safety of the backfill operation. The behavior of CPB, which is influenced by the thermal, hydraulic, mechanical and chemical (THMC) coupled processes, acts on the barricade and thus affects its mechanical response. In the present study, a numerical model is developed to predict and analyze the barricade mechanical performance in response to the coupled THMC behavior of CPB. The validity of the proposed model is then verified against two field case studies. Acceptable agreement between the model prediction results and in situ monitoring data proves the capability of the developed model in simulating the barricade pressure and displacement. Then, the validated model is used to investigate the effect of filling strategy on the barricade displacement. The obtained results can contribute to a better understanding of the barricade performance under various backfill conditions.

Influence of Backfill Compaction on Mechanical Characteristics of High-Density Polyethylene Double-Wall Corrugated Pipelines

For flexible pipelines, the influence of backfill compaction on the deformation of the pipe has always been the focus of researchers. Through the finite element software, a three-dimensional soil model matching the exterior wall corrugation of the high-density polyethylene pipe was skillfully established, and the “real” finite element model of pipe-soil interaction verified the accuracy through field test. Based on the model, the strain distribution at any position of the buried HDPE pipe can be obtained. Changing the location and extent of the loose backfill, the strain and radial displacement distributions of the interior and exterior walls of the HDPE pipe under different backfill conditions when external load applied to the foundation were analyzed, and the dangerous parts of the pipe where local buckling and fracture may occur were identified. It is pointed out that when the backfill is loose, near the interface between the backfill loose region and the well-compacted region, the maximum circumferential strain occurs frequently, the exterior wall strain is more likely to increase greatly on the region near crown or invert, the interior wall strains increase in amplitude at springline, and the location of the loose region has a greater influence on the strain of the pipe than the size of the loose area.

Influence of Backfill Soil Saturation on the Structural Response of Buried Pipes

The structural design of new buried pipes and the calculation of the remaining service life of in-service buried pipes are usually conducted assuming that the backfill soil is at the optimum moisture content using the unsaturated soil parameters published by Boscardin et al. (1990). These unsaturated soil parameters are also implemented in the CANDE software, which is a standard software developed for designing buried culverts/pipes. However, intense rainfall seasons, floods, or water table rise due to changes in local drainage conditions may change the state of the backfill soil from unsaturated to saturated conditions. The purpose of this paper is to study the robustness of using the unsaturated soil parameters in the design of new buried pipes and the calculation of the remaining strength of in-service buried pipes by comparing the response of buried pipes for unsaturated and saturated backfill conditions using three-dimensional finite element analysis. Cast iron, concrete, and unplasticized polyvinyl chloride pipes have been considered to address the aim of the study. The results have shown that the soil saturation increases the stresses of the pipe wall, displacement of the pipe, and the soil pressure applied on the pipe. Hence, there is a need to understand and consider the unsaturated/saturated soil mechanics in the design and the analysis of buried pipes/culverts.

Centrifuge model studies on the performance of soil walls reinforced with sand-cushioned geogrid layers

This paper is to investigate the effectiveness of encapsulating geogrid layers within thin sand layers, for enhancing the deformation behavior of vertical reinforced soil walls constructed with marginal backfills. Centrifuge model tests were performed on vertical soil walls, reinforced with geogrid layers, using a 4.5 m radius large beam centrifuge available at IIT Bombay at 40 gravities. The backfill conditions, height of soil wall, reinforcement length, and reinforcement spacing, were kept constant in all the tests. A wrap-around technique was used to represent flexible facing. Three different geogrid types with varying stiffness were used in the present study. The walls were instrumented with vertical linear variable differential transformers to monitor surface settlements during the tests. Marker-based digital image analysis technique was used to determine face movements and distribution of geogrid strain along the wall height. The deformation behavior of soil walls, reinforced with geogrid layers encapsulated in thin layers of sand, were compared against a base model having no sand-cushioned geogrid layers. Provision of sand-cushioned geogrid layers and increase in geogrid stiffness were found to limit normalized face movements (Sf/H), normalized crest settlements (Sc/H), and change in maximum peak reinforcement strain (dεpmax). Sand-cushioned geogrid layers were also found to limit the development of tension cracks behind and within the reinforced zone. Significant reduction in rate of maximum face movement (dSfmax/dt) and rate of maximum peak reinforcement strain (dεpmax/dt) was observed, with an increase in value of normalized reinforcement stiffness (Jg/γH2) of geogrid layers. The analysis and interpretation of centrifuge model tests on soil walls, constructed with marginal backfills and reinforced with sand-cushioned geogrid layers, indicate that their performance is superior to the walls without sand-cushioned geogrid layers.

Strength reduction for upheaval buckling of buried pipes in blocky clay backfill

Offshore pipelines are often buried to protect the pipeline from external loads and upheaval buckling. Models for pipe uplift resistance in clay soils are based predominantly on homogenous backfill conditions. In practice, however, there will be significant soil disturbance during installation. With certain trenching techniques this may produce a backfill more akin to a matrix of lumps of intact soil connected by weaker remoulded interfaces. This research uses centrifuge modeling to assess the resistance provided by a representative lumpy clay backfill that has experienced self-weight consolidation. A model pipe is then uplifted through this model backfill in order to assess the soil uplift resistance. Results show that the uplift resistance in this material is governed strongly by the size of the lumps and, to a lesser extent, by the rate at which displacement occurs. When interpreted in terms of the strength reduction η, that may be used to correct between theoretical and measured uplifts, lower values were derived than those currently used based on intact soils. The value of η is seen to be controlled by a non-dimensional drainage parameter, but may be practically estimated based on an estimate of the size of lumps relative to the pipe diameter.

Parametric study of frost-induced bending moments in buried cast iron water pipes

While the features of frost susceptible soils have been examined in various studies, the mechanisms by which volume changes due to ground freezing can influence cast iron water pipes buried below the frost line have not been explained, and the hypothesis that frost-induced ground deformations can induce ring fractures due to longitudinal bending of these pipes has not been proven. Therefore, a parametric study employing three dimensional finite element analysis is reported, where the soil–pipe interaction associated with a pipe crossing under an intersection of a major arterial road with a residential street are examined. The arterial road is modeled as having non-frost susceptible sub-base and the local street is represented as having a lower grade pavement with frost susceptible sub-base. One specific frost loading case featuring both isotropic pore water expansion and orthotropic ice lens formation is modeled.The analysis demonstrates how volume changes due to ground freezing in soil strata above the buried pipe can induce bending moments sufficient to cause ring fracture. Changes in the relative axial stiffness of the pipe were found to have only a small effect on pipe moments. Decreases in the relative flexural stiffness of the pipe resulting from reductions in pipe modulus also had only a small effect on pipe deflections and normalized moments. Changes due to soil modulus had a significant effect on deformations, but little influence on moments. Decreases in pavement stiffness decrease pipe deflections and moments. Trench backfill conditions greatly affect deflection and moment. Reduction in burial depth from 2m to 1.5m increases deflections, and increases moments beyond the failure capacity of the grey, cast iron pipe considered in the study, and this computational result is directly supported by field evidence.

Performance evaluation of geogrid reinforced soil walls with marginal backfills through centrifuge model tests

Marginal fill materials that do not follow the guidelines are used in constructional activities due to ease in its availability and economic benefits. But several cases of geogrid reinforced soil wall failures indicate the loss of interfacial shear resistance due to wetting of backfill as a possible reason. In the present study, centrifuge tests were performed on geogrid reinforced soil wall models with wrap-around facing using a 4.5 m radius large beam centrifuge facility available at IIT Bombay at 40 gravities. A marginal soil with 21% fines was chosen as backfill in the study. Two geogrid types of different stiffnesses were modelled based on scaling considerations and used in the study. The models were prepared at wet of optimum to simulate wet backfill conditions. The surface settlements of the models during centrifuge tests were monitored with the help of Linear Variable Differential Transformers (LVDTs). Digital Image Analysis (DIA) was performed on photographs of the front elevation of the model captured during flight, to obtain face movements and reinforcement strain distribution along geogrid layers during centrifuge tests. Interpretations of centrifuge model test results reveal that the soil wall reinforced with low stiffness geogrid layers was observed to deform excessively and undergo pullout failure along soil–geogrid interface. However, the provision of geogrid layers with higher stiffness limited the excessive outward deformations of geogrid reinforced soil walls with marginal backfills. Further, the effect of moulding water content and stiffness of the geogrid on the mobilization of pullout resistance was evaluated through pullout tests in the laboratory. Based on the observations made from pullout tests and centrifuge tests, provision of stiffer geogrids in geogrid reinforced soil walls was found to be one of the viable options to mitigate the problems posed by marginal backfills.

Shake Table Test of Large-Scale Bridge Columns Supported on Rocking Shallow Foundations

AbstractThis paper presents the results of a series of shake table tests of two 460-mm-diameter columns supported on 1.5-meter-square shallow rocking foundations. The tests were conducted using the Large Outdoor High-Performance Shake Table of the Network for Earthquake Engineering Simulation at the University of California at San Diego. The first specimen was aligned with the uniaxial direction of shaking, and the second was positioned in a skew configuration. The specimens were placed inside a soil-confining box 10.1 m long and 4.6 m wide with a 3.4-m height of clean sand compacted at 90% relative density. Three series of tests were performed; each had different groundwater and backfill conditions. The test protocols included up to six historical ground motions and resulted in peak drift ratios up to 13.8%. For peak drift ratios up to 6.9%, the rocking foundations performed very well, with residual drift ratios between 0.5 and 0.9% depending on the backfill conditions and with minimal settlements and no...