Use Of Backfill Research Articles

A study of backfill confinement to reinforce pillars in bord-and-pillar layouts

This study explores the use of backfill in hard rock bord-and-pillar mines to increase the pillar strength and extraction ratio at depth. The use of backfill will also minimize the requirement for tailings storage on surface and the risk of environmental damage. A literature survey indicated that backfill is extensively used in coal mines, but rarely in hard rock bord-and-pillar mines. To simulate the effect of backfill confinement on pillar strength, an extension of the limit equilibrium model is proposed. Numerical modelling of an actual platinum mine layout is used to illustrate the beneficial effect of backfill on pillar stability at greater depths. The magnitude of confinement exerted by the backfill on the pillar sidewalls is unknown, however, and this needs to be quantified using experimental backfill mining sections equipped with suitable instrumentation.

Controlled backfill in oncology dose-finding trials

The use of backfill in early phase dose-finding trials is a relatively recent practice. It consists of assigning patients to dose levels below the level where the study is at. The main reason for backfilling is to collect additional pharmacokinetic, pharmacodynamic and response data, in order to assess whether a plateau may exist on the dose-efficacy curve. This is a possibility in oncology with molecularly targeted agents or immunotherapy. Recommending for further study a dose level lower than the maximum tolerated dose could be supported in such situations. How to best allocate backfill patients to dose levels is not yet established. In this paper we propose to randomise backfill patients below the dose level where the study is at. A refinement of this would be to stop backfilling to lower dose levels when these show insufficient efficacy compared to higher levels, starting at dose level 1 and repeating this process sequentially. At study completion, data from all patients (both backfill patients and dose-finding patients) is used to estimate the dose-response curve. The fit from a change point model is compared to the fit of a monotonic model to identify a potential plateau. Using simulations, we show that this approach can identify the plateau on the dose-response curve when such a plateau exists, allowing the recommendation of a dose level lower than the maximum tolerated dose for future studies. This contribution provides a methodological framework for backfilling, from the perspective of both design and analysis in early phase oncology trials.

Thermal management associated with geologic disposal of large spent nuclear fuel canisters in tunnels with thermally engineered backfill

Coupled thermal-hydro-mechanical (THM) numerical modeling is conducted to study thermal management associated with geologic disposal of spent nuclear fuel (SNF) in large dual-purpose canisters (DPCs). DPCs, are containers designed for SNF storage and transportation and if determined to be feasible for permanent geological disposal could provide a cost effective disposal solution. However, one of the challenges to direct disposal of DPCs is thermal management to avoid overheating of the Engineered Barrier System (EBS), including bentonite-backfill used as a protective buffer. The model simulations show that the use of a backfill that is thermally engineered for high heat conduction can reduce the EBS temperature to acceptable levels for disposal of large waste canisters in backfilled tunnels. On the other hand, the use of high heat conduction backfill will not reduce the far field rock peak temperature that can occur several thousand years after closure of the repository. This longer term host rock peak temperature generates thermal-poro-elastic stress and geomechanical changes that must be considered in the thermal management and design of a repository.

Influence of binder content on temperature and internal strain evolution of early age cemented tailings backfill

The philosophy of the use of backfill is to return much of tailings waste to underground voids and increase working place stability to safely extract more minerals. The key issue for the safe and efficient design of cemented tailings backfill (CTB) structure is its mechanical properties and interaction with the surrounding rock. As a cemented material, CTB’s strength is a function of hydration degree. As hydration is an exothermic reaction and results in net volume reduction, thermal expansion and chemical shrinkage occur simultaneously in the mixture. Each CTB component plays a significant role during this process and the most important factor is the binder content, because it influences the amount of cement hydration products that provide binding phases between tailing particles. In this paper, a number of fiber Bragg gratings (FBGs) were directly embedded into the 7-day cured CTB specimens to measure their temperature and internal strain evolution, with settlement development measured by Linear Variable Differential Transformer (LVDT). The experimental results showed that binder content had a significant effect on CTB temperature and internal strain evolution. Furthermore, the evolution of internal strain and settlement was an indicator of the transformation in different stages (such as fluid, skeleton formation and hardening) during the hydration process.

Basic requirements of backfilling with flotation tailings in the Bor River underground mine

The use of backfill in underground mining is increased due to a need for systematic backfilling the mine openings and workings. Backfilling is applied in order to prevent fires and explosions, improve the mine ventilation, improve the rock stability, reduce the subsidence effects on surface, as well as for economical and environmental factors. Sources of materials are usually from mining industry (e.g. fly ash, gypsum, slag, infertile overburden, tailings, filter dust, residues from mineral processing) or other industries. The ore body Bor River of the Bor Copper Mines is planned to be exploited by the sublevel stopping method. In order to ensure greater efficiency in the exploitation of the ore body, it is proposed, after the excavation of chambers, to move on to the excavation of the main pillars. To ensure the stability of the excavation area, it is necessary to fill the excavated chambers with material of good physical and mechanical properties. For this purpose, a possibility of filling the chambers with backfilling consisted of flotation tailings, small amount of cement and water, will be tested. This paper reviews the basic requirements of backfilling that will be applied in the excavated chambers.

Backfill Impacts on Moisture Measurements in Fractured Rock

As the scope of hillslope‐hydrology investigations extend deeper, there will likely be an increase in the use of backfill to facilitate sensor installations, particularly in fractured rock. Because of the disparity in hydrologic properties of backfill and the native rock being monitored, discrepancies in measured values are imminent. In this study, we assessed the impact of different types of backfill that can be used to provide hydraulic continuity between sensors and the “measured” environment. During a period of 4 yr, the hydrologic response to seasonal wetting and drying was monitored with identical time domain reflectometry (TDR) sensors embedded in native rock, fracture infill, augured native rock, and silica powder. We found that while all backfills responded to wetting and drying events, there were differences in the response to individual rainfall events and in the amount of moisture measured. Our observations show that the use of any type of backfill for monitoring fractured rock hydrology will result in distorted measurements; however, our analysis suggests that simple calibrations between native rock and backfill measurements allow moisture content changes in the native rock to be quantified using backfill measurements. Backfill that is finer textured than native rock provides the best estimate of water content during long‐term, uninterrupted drying events when the backfill measurements are calibrated to native rock conditions. Irrespective of calibration, backfill materials coarser than the native rock provide the best detection of the timing and duration of the hydrologic response to precipitation.

Evolution of repository and waste package designs for Yucca Mountain disposal system for spent nuclear fuel and high-level radioactive waste

This paper summarizes the evolution of the engineered barrier design for the proposed Yucca Mountain disposal system. Initially, the underground facility used a fairly standard panel and drift layout excavated mostly by drilling and blasting. By 1993, the layout of the underground facility was changed to accommodate construction by a tunnel boring machine. Placement of the repository in unsaturated zone permitted an extended period without backfilling; placement of the waste package in an open drift permitted use of much larger, and thus hotter packages. Hence in 1994, the underground facility design switched from floor emplacement of waste in small, single walled stainless steel or nickel alloy containers to in-drift emplacement of waste in large, double-walled containers. By 2000, the outer layer was a high nickel alloy for corrosion resistance and the inner layer was stainless steel for structural strength. Use of large packages facilitated receipt and disposal of high volumes of spent nuclear fuel. In addition, in-drift package placement saved excavation costs. Options considered for in-drift emplacement included different heat loads and use of backfill. To avoid dripping on the package during the thermal period and the possibility of localized corrosion, titanium drip shields were added for the disposal drifts by 2000. In addition, a handling canister, sealed at the reactor to eliminate further handling of bare fuel assemblies, was evaluated and eventually adopted in 2006. Finally, staged development of the underground layout was adopted to more readily adjust to changes in waste forms and Congressional funding.

Effects of Backfill Soil on Excessive Movement of MSE Wall

AbstractThe use of mechanically stabilized earth (MSE) retaining walls has gained popularity as an alternative to conventional cast-in-place concrete walls. The construction of MSE walls is cost effective, requires less site preparation, and is technically more feasible compared with conventional concrete retaining walls. However, use of backfill with high fine content and poor drainage behavior can cause excessive wall movement or even failure. The current paper presents the case study of a MSE wall located at State Highway 342 in Lancaster, Texas. The top of the MSE wall has moved as much as 300–450 mm only 5 years after construction. An extensive site and laboratory investigation testing program was conducted to determine the possible causes of the MSE wall movement. The site investigation included soil test boring and resistivity imaging (RI). Perched water zones were identified at a few locations in the backfill area using RI. The bulging of the MSE wall facings was observed where the perched water z...

An analytical solution for the nonlinear distribution of effective and total stresses in vertical backfilled stopes

The increasing use of backfill in underground mines requires a proper evaluation of the stress state in and around the filled openings. This is, however, a relatively complex issue due, in part, to the large contrast in strength and stiffness between the backfill material and surrounding rock mass. In recent years, it has been shown that arching theory, based on limit equilibrium analysis, can be used to estimate the stress distribution in backfilled stopes. Nonetheless, many simplifications are involved in such analytical solutions and this affects the precision and significance of the calculated results. In this paper, a previously developed solution is enhanced by introducing the combined effects of non-uniform vertical stress distribution and positive pore water pressure. This leads to a more representative analytical solution, as demonstrated by successful comparisons with numerical simulations. The results indicate that the proposed solution can be used to estimate the effective and total stress state in submerged or partially submerged backfilled stopes with a simple geometry.

An improved analytical solution to estimate the stress state in subvertical backfilled stopes

The increasing use of backfill in underground mines requires a better understanding of the interaction between the relatively soft fill material and the surrounding rock mass. In recent years, it has been shown that stresses in backfilled stopes can be estimated using an approach based on Marston’s arching formulation developed initially for buried conduits in trenches. However, despite its advantages, this approach has some shortcomings. For instance, it postulates that both the vertical and horizontal normal stresses are uniformly distributed across the opening width. Numerical investigations conducted by the authors have shown that this assumption is not always valid. This paper presents a modification to the Marston-based solution, which leads to a nonuniform vertical stress distribution across the opening. This modification of the analytical solution involves parameters that have been calibrated against some numerical modelling results. The same equations and parameters are then shown to provide a good representation of additional modelling cases for which the stress state is correctly predicted.

Mine Rehabilitation — A New Challenge for Mining Engineers

Since 1990, with the reunification of Germany, mine rehabilitation has gained importance. Due to the sudden and often unprepared closure of mines, the number of rehabilitation cases has increased significantly. Mining engineers with their wide ranging education in mathematics, natural and engineering science as well as economics, ecology and justice are especially qualified to find solutions for these problems. The scope of mine rehabilitation contains exploration methods for contaminated sites as well as remediation methods for contaminated soils and waste dumps as well as rehabilitation methods for tailing ponds and mine damage. Extensive and detailed investigation are part of the Exploration. Enclosing, immobilisation or soil cleaning are commonly used methods for the remediation of contaminated areas. Rehabilitation of mine waste dumps is possible after the surface is covered with stabilising soil with the ability to store water and nutrients. Tailing ponds have to be more dewatered and enclosed with a solid coverage, the higher the finest-grain-quota of the containing material is. Mine damage can be avoided by flooding or use of backfill. Open-pit slopes can be stabilised by subsequently reducing the slope angle or by improving the compaction.

Effect of seismic load on a proposed nuclear waste repository at Yucca Mountain

Abstract Long term stability of waste emplacement drifts is of interest in assessing the safety of disposal of high-level nuclear waste (HLW) in the proposed repository at Yucca Mountain (YM). The repository preclosure operation period is about 100 years, and the decision regarding the use of backfill in the emplacement drifts during the postclosure period of about 10,000 years has not been finalized. Two significant factors that can induce drift instability include thermal loads generated by the decay of emplaced waste and repeated seismic loads from earthquakes. One of the failure mechanisms for an excavation in a jointed rock mass subjected to repeated seismic loading is through accumulation of shear displacements along joints. These dynamic ground motions at YM due to earthquakes (and possible nearby weapons testing) will take place in the presence of in situ stresses, excavation induced stresses, and thermal-mechanically induced stresses. This paper presents a discrete element analysis using the code UDEC (Version 3.0) to investigate the effect of repeated seismic loading on cumulative joint slip and failure around a heated, unsupported, emplacement drift in the proposed repository at YM. A 100 MTU/acre thermal loading was used. Computer results indicate that slip along joints and closing of the drift take place with both single and repeated episodes of seismic loading at peak accelerations of 0.4g if yielding has occurred in the rock around the drift during the thermal-mechanical loading stage prior to seismic loading. This cumulative effect of repeated seismic loading confirms the findings of a field investigation (Hsiung et al. 1992) and a laboratory study (Hsiung et al. 1997). However, if the rock remains in the elastic state after the thermal loading is applied, no measurable cumulative slip on joints or drift convergence is evident with repeated episodes of seismic loading up to 0.4g, aside from that which occurs during the first sequence of seismic loading.

Predicting fracture localization in folded strata from mechanical stratigraphy and fold shape: Case study of East Kaibab Monocline, Utah

Long term stability of waste emplacement drifts is of interest in assessing the safety of disposal of high-level nuclear waste (HLW) in the proposed repository at Yucca Mountain (YM). The repository preclosure operation period is about 100 years, and the decision regarding the use of backfill in the emplacement drifts during the postclosure period of about 10,000 years has not been finalized. Two significant factors that can induce drift instability include thermal loads generated by the decay of emplaced waste and repeated seismic loads from earthquakes. One of the failure mechanisms for an excavation in a jointed rock mass subjected to repeated seismic loading is through accumulation of shear displacements along joints. These dynamic ground motions at YM due to earthquakes (and possible nearby weapons testing) will take place in the presence of in situ stresses, excavation induced stresses, and thermal-mechanically induced stresses. This paper presents a discrete element analysis using the code UDEC (Version 3.0) to investigate the effect of repeated seismic loading on cumulative joint slip and failure around a heated, unsupported, emplacement drift in the proposed repository at YM. A 100 MTU/acre thermal loading was used. Computer results indicate that slip along joints and closing of the drift take place with both single and repeated episodes of seismic loading at peak accelerations of 0.4g if yielding has occurred in the rock around the drift during the thermal-mechanical loading stage prior to seismic loading. This cumulative effect of repeated seismic loading confirms the findings of a field investigation (Hsiung et al. 1992) and a laboratory study (Hsiung et al. 1997). However, if the rock remains in the elastic state after the thermal loading is applied, no measurable cumulative slip on joints or drift convergence is evident with repeated episodes of seismic loading up to 0.4g, aside from that which occurs during the first sequence of seismic loading.

The strength and deformation behaviour of model adfreeze and grouted piles in saline frozen soils

The results of a laboratory study to determine the influence of soil salinity, pile surface treatment, pile backfill material, and temperature on the adfreeze bond strength and deformation of model piles are presented. The influence of salinity and temperature on the adfreeze strength is shown to be related through the unfrozen water, and similar strength results were obtained at similar unfrozen water contents. Salinity in the backfill material of 10 and 30 ppt caused reductions in the adfreeze bond strength of 80–99%. Similar salinities within the native soil reduced its shear strength causing the location of the failure to change from the pile–backfill interface to the backfill – native soil interface when nonsaline backfill was used. Sandblasting of the pile surface to remove any surface coating resulted in adfreeze bond strengths two to three times greater than those measured on nonsandblasted piles. The use of cementitious grout to replace the sand slurry backfill resulted in greater pile load carrying capacity as the failure surface was transferred to the grout (backfill) – native soil interface instead of the pile–backfill interface. Use of backfill made from saline native soil cutting should be discouraged because of the detrimental effects of pore-fluid salinity on the adfreeze bond strength. Key words : frozen soil, saline, model pile, adfreeze, strength, deformation, grout, temperature.