Saturated Backfill Research Articles

LEAP centrifuge tests on retaining walls at Cambridge University

Retaining walls and other waterfront structures were seen to suffer severe damage due to soil liquefaction in previous earthquakes. As part of the LEAP project, cantilever retaining walls with loose, saturated backfill were tested at various centrifuge centres participating in this endeavour. The toe of the retaining wall penetrated about 0.5 m into the dense sand layer underlying the loose sand layer. Retaining walls with different ratios of the retained height h over the penetration depth d were tested. As part of the LEAP project, additional testing was carried out at Cambridge to consider the effect of the wall size on its deformation following liquefaction. It will be shown that a larger wall will suffer more rotation and wall top displacement than a smaller wall with the same h/d ratio. This can have implications for numerical modelling in terms of how well the constitutive models capture the suppressed soil dilatancy at higher confining pressures.

Liquefaction numerical analysis of cantilevered retaining walls with saturated backfill containing fines content

Liquefaction numerical analysis of cantilevered retaining walls with saturated backfill containing fines content

Experimental Research on Strain Transfer Behavior of Fiber-Optic Cable Embedded in Soil Using Distributed Strain Sensing

The strain transfer between fiber-optic cable and soil plays a critical role in the deformation characteristics of a cable–soil interface. Existing findings cannot provide a clear understanding of the effects of key influencing factors including horizontal confining pressures, anchorage property of cables, and saturation of soils on the strain transfer and shear characteristics at the interface. A group of pullout tests of cables in soil were conducted to examine the strain transfer efficiency using the optical frequency domain reflectometry (OFDR) technique. Two kinds of cables were pulled out from sandy soil and sand–gravel–clay mixtures under the confining pressures of 0–1.2 MPa. Typical strain-hardening behavior was observed for cables with confining pressure and anchorage, and the interface shear strength could not be evaluated within the strain measurement range. To address this problem, the so-called interface shear coefficient was adopted, and the key influential factors were discussed quantitatively. The interface shear coefficient keeps a linear relationship with the confining pressure, and that of the anchored cable in saturated backfill mixtures is 2–3 times than that of the unanchored cable. These findings will guide the methods of gaining reliable data for revealing the failure mechanism of geostructures via distributed strain sensing.

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.

Seismic Rotational Displacements of Gravity Quay Walls Considering Excess Pore Pressure in Backfill Soils

The effect of excess pore pressure developed in backfill soil during earthquake is an important consideration in rotational displacement prediction of gravity quay walls. Based on Newmark’s sliding block concept and stress-based excess pore pressure model, a new method is proposed to predict the critical rotational acceleration and angular acceleration time histories considering the development process of excess pore pressure in earthquake events. Then, the rotational displacement of gravity quay walls is predicted according to the calculated angular acceleration time histories. By using the proposed method, the effects of various parameters involved in the calculation have been studied by carrying out a parameter study. Analysis results reveal that the influence of excess pore pressure on the rotational displacement of gravity quay walls with saturated backfill soil is significant, so, can not be ignored; and rotational displacement is sensitive to the magnitude of earthquake, horizontal and vertical seismic accelerations of ground motion, wall and soil friction angle, and soil relative density. When the rotation and sliding of wall occur simultaneously, rotation and sliding will be inhibited by each other.

Experimental study on seismic response of geogrid reinforced rigid retaining walls with saturated backfill sand

To study the seismic performance of geogrid reinforced rigid retaining walls with saturated backfill sand, large-scale shaking table test models were constructed in a large laminar shear container, which included geogrid reinforced rigid retaining walls and unreinforced soil retaining walls. The geometric similitude rules of test walls were confirmed by the Froude constant in dimensional analysis theories, which could be expressed by the shear velocity of the soil, and the similitude ratios of the other variables were deduced from the geometric similitude ratio. The shear wave velocities of the model grounds were tested using a seismic dilatometer. Considering the effects of ground motions, the seismic response of the retaining walls were investigated, along with wall accelerations, lateral displacements of walls, seismic settlements of the backfill sand surfaces, and the excess pore water pressures in the backfill sand and geogrid strains. Comparing the results of the test models indicated that seismic waves with long-time high acceleration values from the far field had greater effects on the seismic behaviors of the reinforced retaining wall than those from the near field, and the geogrid could still effectively improve the seismic resistance deformation abilities of the saturated geogrid reinforced rigid retaining wall. Geogrid layers could decrease the development of excess pore water pressures and accelerate the dissipation of excess pore water pressures. The middle-layer geogrids in the reinforced soil played important roles in the seismic stability of the retaining wall, and the geogrid roots connected with the walls were key parts in the seismic design of the geogrid reinforced rigid retaining wall. Seismic waves from far or mid-far fields must be considered in the seismic designs of geogrid reinforced rigid retaining walls.

Functions of Chisan dams in large-scale granular mass-flow experiments on Sakurajima volcano pumiceous clasts

We investigated the fundamental behaviour of Chisan (check) dams in response to the presence or absence of impounded sediment with different levels of saturation. Large-scale model flume experiments were conducted by use of a model Chisan dam that had been backfilled with pumiceous clasts taken from the Sakurajima volcano in Kagoshima prefecture, Japan. In these experiments, the miniature dam was situated midway down the flume under different backfill sediment conditions and the basal pore-fluid pressure, normal flow depth, and impact load of the granular mass were carefully monitored. Saturated sediment with a volume of 0.6 m3 was released along the slope of the flume segment at 30° and excess pore-fluid pressure was generated, corresponding to a sediment velocity of approximately 4.8 m s−1. Direct collision of the sediment with the Chisan dam meant the peak impact loads of granular mass against the back surface of the dam exceeded 157 N in experiments without backfill sediment but were as low as 32.1 N in experiments with sediment. Accordingly, placing the backfill sediment substantially reduced peak impact loads. The sediment captured by the Chisan dams was also calculated. For experiments using saturated backfill sediment, the dam captured only 35 % of the material, indicating that most of the granular mass passed over the dam. However, in experiments without backfill sediment or with unsaturated backfill sediment, over 90 % was captured. Although Chisan dams with unsaturated backfill sediment lacked pocket space, almost equivalent granular masses were trapped in these experiments. On the basis of these results, we suggest that Chisan dams with unsaturated backfill sediment could effectively counter debris flow, reducing sediment loads along the streams draining Sakurajima volcano.

Effect of drainage and sequential filling on the behavior of backfill in mine stopes

Underground backfilling offers significant economic and environmental advantages to mining operations. There is however a limited knowledge and understanding of how the backfill behaves within mine stopes, which creates some concern regarding the risk of accidents with potentially serious consequences. It is thus important to investigate further the response of backfill to ensure safe working conditions and optimize the filling sequence. This paper presents key results from a numerical study aimed at analyzing the hydrogeotechnical response of backfill in a narrow vertical stope. The simulations illustrate how stresses are influenced by stope geometry, water drainage, and filling rate. Three main cases are presented here to illustrate these effects; namely, (i) simulation of dry (or drained) backfill, (ii) a rapidly filled stope with progressive drainage and consolidation, and (iii) sequential backfill placement with different filling rates. The third case includes a simulation with evolving properties due to the binder added to the backfill. The results from the numerical analyses show that arching effects develop within narrow backfilled stopes because of the stiffness contrast between the rock and the fill material. This can produce a significant reduction of the stresses (horizontal and vertical) in comparison with the overburden pressure. The simulation results also show the development of excess pore-water pressures after the placement of the saturated backfill within the stope. Drainage tends to reduce these pressures and increase the frictional stresses along the rock walls. The sequentially filled stope simulations show that a rapid filling rate produces much higher total stresses and excess pore-water pressures, compared to slower rates. The simulation of the cemented backfill, with evolving properties, indicates that the progressive changes can have a significant effect on the total and effective stresses in the stope. A discussion follows on the implications of these results.

Some aspects of the mechanics of arching in backfilled stopes

In current underground mining using “open stoping” methods, it is common to backfill mined-out voids (“stopes”) using hydraulically placed backfill, which is commonly composed of tailings, to which cement is often added. Knowledge of the stress state within a backfilled stope is required for safe design of drawpoint barricades and for other operational reasons. This stress state depends, inter alia, on the degree of “arching” that occurs, resulting from the development of shear stress between the fill and the stope walls. This paper presents a numerical modelling study of aspects of the arching phenomenon, carried using the computer code Plaxis. The backfill is characterized using the Mohr–Coulomb soil model, and both dry backfill and saturated backfill are considered to completely cover the full spectrum of backfill types that are used in practice. It is shown that even with dry backfill, the behaviour is governed by a complex interaction between the soil parameters. The behaviour is more complex with saturated backfill, with a key parameter being the permeability of the backfill relative to the rate of filling — i.e., whether the backfilling operation can be considered to be “drained” or “undrained” or somewhere between these two extremes.

Horizontal pressure on barricades for backfilled stopes. Part I: Fully drained conditions

Stope backfilling is used in underground mines to provide a safer working space for personnel and machinery. To ensure a secure application of backfill, it is critical to accurately evaluate the pressures exerted on the walls and barricades placed near the base of the stope to retain the fill materials. In this paper, existing solutions for evaluating the stresses on barricades are reviewed. The authors then propose a new three-dimensional analytical solution that takes into account the stress transfer from the backfill to the walls in both the stope and access drift. In particular, the solution considers the influence of the barricade position within the drift on the stress state. Comparisons are made between the proposed solution, physical model test results, and numerical simulations. The good agreement between various results indicates that the proposed formulation provides a good estimate of the pressure on a barricade. The solution presented here is applicable to fully drained backfill (without water-pore pressure); the solution for saturated backfill is given in a companion paper (see Part II, this issue).

Seismic Response of Gravity Quay Walls. I: Centrifuge Modeling

Failure of gravity quay walls induced by earthquakes has been widely observed in the field. This paper studies the behavior of gravity quay walls under earthquake loading using data from three centrifuge tests. Failure modes similar to those observed in the field were replicated in centrifuge tests. It is shown that excess pore pressure generated in the backfill increases both the angle of failure wedge and the horizontal thrust on a retaining wall. Lateral displacement of a gravity retaining wall with dry backfill can be estimated using Newmark's method. However, for a retaining wall with saturated backfill, the presence of excess pore pressure makes it difficult to apply this type of calculation. Cyclic shear stresses and excess pore pressures induced by base shaking can lead to deterioration of soil strength and stiffness, which reduces the fundamental frequencies of wall vibrations. Implications for design are discussed.

Liquefaction analysis of a model anchored bulkhead in centrifuge

The seismic response of anchored bulkheads with saturated backfill has been simulated in a series of centrifuge tests involving various intensities of shaking, two relative densities of the backfill and two pore fluids with different viscosities. The time history of input accelerations, excess pore pressures and displacements during each test were continuously recorded at several points in the backfill and on the bulkhead. In the following, these data are used to evaluate the accuracy of a numerical method developed for simplified computations of earthquake-induced excess pore pressures and permanent displacements in saturated soils. Two sets of predictions are evaluated: Stage I predictions, based solely upon an initial evaluation of the input parameters and no feedback from the results of the centrifuge tests, and Stage II predictions, based on revised values of input parameters estimated from back-analysis of the centrifuge test results. Stage I predictions are in good agreement with measurements of excess pore pressures and outward wall displacements in the tests with a high viscosity, glycerol solution as pore fluid. For tests with water as pore fluid, good predictions are obtained with Stage II predictions when the stiffness of the anchor is reduced by about 15%, and the permeability coefficient during shaking is reduced to one quarter of the value measured in falling head permeability tests.

Analyses of Soils from the Commercial Radioactive Waste Disposal Sites at Barnwell, Sc, and Hanford, Wa

ABSTRACTBrookhaven National Laboratory, in order to provide technical assistance to the NRC, has measured a number of physical and chemical characteristics of soils from two commercial radioactive waste disposal sites: one at Barnwell, SC, and the other near Richland, WA. Materials commonly used for the containment and the solidification of radioactive waste are subject to underground corrosion. The data reported can be used to assess the corrosivity of each burial environment on a waste form or high integrity container. Samples of trench backfill soil, believed to be representative of the soil that will contact the buried waste forms were collected and analyzed. Earth resistivities were measured at each site. Laboratory measurement of the resistivities of water saturated backfill soils from both sites show values greater than 104ohm-cm. This is indicative of soils having relatively small amounts of soluble ions and of soils likely to be very mildly corrosive to steel. The backfill soil from the Barnwell site is acidic having a pH ∼5. Measurements on Hanford backfill soil indicate a neutral environment; pH ∼7. The exchange acidity of the backfill soils from both sites was <2 meq per 100 g of soil and in most cases an alkaline reaction was observed. The pHs and exchange acidities of these soils are consistent with those of mildly corrosive soils. Aqueous extracts of saturated pastes were used to determine the concentrations of the following ions: Ca2+, Mg2+, K+, Na+, HCO3−, SO4=, and Cl−. The sulfide content of each of the soils was measured in a mixture of soil and an antioxidant buffer solution.