Terzaghi’s consolidation theory assumes strain in soil due to applied overburden pressure is small, and permeability and thus the coefficient of consolidation (Cv) remain constant during consolidation. Although valid for soft soil consolidation, the Cv for hydraulically placed slurry materials with low solids content may change significantly during the consolidation process as the material transitions from a slurry to a solid. In this study, a spatial and temporal variable Cv was used to predict the time rate of consolidation of fine coal refuse placed in an impoundment as a slurry. The results suggest the variable Cv method may best reflect the low degree of consolidation at shorter times when the material is still essentially slurry. At longer times, a similar degree of consolidation can be obtained with a constant Cv value obtained from consolidation measurements at low stress levels. The consolidation response of hydraulically placed slurry at shorter times is important for construction staging and personnel safety, but the constant Cv approach may be useful for estimating the time for greater degrees of consolidation.

Lignocellulosic biomass material sourced from plants and herbaceous sources is considered as a prospective feedstock of inexpensive, potentially carbon-neutral energy. Lignocellulosic biomass is structurally built on cellulose, hemicellulose, and lignin, which are present in varying concentrations based on the feedstock type and play distinct and not well understood mechanical functions in the flow behavior. The frictional characteristics of lignocellulosic biomass particulates influence their flow behavior in biorefineries. Thus, it is important to fundamentally investigate the relative contribution of cellulose, hemicellulose, and lignin to the frictional behavior. However, these three biopolymers are interwoven into a complex matrix in the lignocellulosic biomass, thus making it hard to quantify the contribution of each biopolymer. In this study, we selectively remove hemicellulose from switchgrass and investigate the effects of its diminishing concentration on the coefficient of friction. We observed that the angle of repose and, therefore, the coefficient of friction for a loose assembly of the control and treated switchgrass samples decrease with decreasing hemicellulose content. This indicates the frictional resistance to flow for biomass particulate assemblies is at least proportional to the hemicellulose content. We also established that the observed changes in the frictional behavior were not due to particle morphological characteristics. (C) 2020 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

The stability of impounded slurry materials in part depends on the potential for flow and undrained shear strength. The purpose of this study is to investigate the effect of an added flocculant on the flow behavior and undrained shear strength of fine coal refuse (FCR). Specimens were formed from fresh FCR slurry samples obtained from an active impoundment slurry discharge pipe, as well as solids recovered from the same slurry mixed with either distilled water or a dispersant solution. Specimens were consolidated over a range of low stresses with different initial void ratios and moisture contents. Flow table and vane shear tests were performed to investigate the influence of background solution type on flow behavior and undrained shear strength. The background solution chemistry has a significant impact on the FCR void ratio, flowability, and the undrained shear strength. The flocculant present in the FCR slurry contributes to higher flow resistance and shear strength at higher values of moisture content and void ratios. A reasonable correlation was observed between flowability and undrained shear strength. For the materials tested in this study, the results indicate an estimate of undrained shear strength may be determined indirectly by conducting a modified flow table test.

Natural slopes are often observed to have a concave, convex, or a combination concave/convex profile, yet constructed slopes are traditionally designed with planar cross-sectional geometry. In this paper, the stability of two planar slopes was compared with that of companion concave slopes, designed to have similar factor of safety (FOS) under gravity loading. The stability of these slopes was then investigated in response to a suction event followed by a precipitation event, and it was shown that both the planar and the concave slopes experienced similar changes in stability. Additional analyses were conducted with a simulated erosion mechanism to investigate how the planar and concave shapes would evolve under a sequence of three similar suction/precipitation/erosion cycles. The results suggest that for these slopes, the second and third simulated weather cycles reduced the stability of the slopes, yet had a lesser effect on the concave slopes than the planar slopes. This is in spite of the fact that the planar slopes became moreconcave-likedue to the simulated erosion and suggests that slopes designed to be concave may perform better than the planar slopes.

A field exploration study was performed on an inactive coal refuse impoundment in the Appalachian region (USA) to investigate in situ characteristics of fine coal refuse (FCR). A series of standard penetration tests (SPT) were conducted from the crest of the impoundment and relatively undisturbed Shelby tube samples were obtained from various depths within the impoundment to a depth of about 60 meters (m). The SPT blow counts (N60-values) in the FCR ranged from 7 to 15 blows per foot suggesting medium to stiff material consistency. The split-spoon samples revealed intermittent layering over short intervals, with thin layers of coarser FCR separated by layers of the finer very soft FCR. Particle size analyses identified two groupings of material, a “sandy” FCR and a “silty” FCR, with no apparent trend with depth. Index testing identified a few locations where the liquidity index was greater than one. It appears thin layers of coarser FCR material produce blow counts that obscure the presence of the soft layers of FCR. This suggests the SPT is not an effective means to investigate these slurry placed materials, if the intent is to identify flowable, under-consolidated materials.

Traditional grain size distribution measurement techniques assume that the measured particle diameters are of individual grains. However, for suspended particulate materials, particles may be dispersed or associated in flocs. Further, particle-level associations depend on the surrounding fluid chemistry, which may change over time. The purpose of this study is to compare two different methods of grain size analysis for detecting particle and floc sizes in suspensions of fine coal refuse (FCR): hydrometer analysis, the well-known traditional method, and laser diffraction, a lesser known method. The influence of background solution-flocculant, dispersant, or distilled water-on the apparent grain size of FCR was also investigated. Analogous slurry suspensions made from a well-characterized kaolin were analyzed for comparison. Results from the grain/floc size analysis indicate that there are several advantages of laser diffraction over hydrometer analysis including a short measurement period, small sample size requirement, and the ability to measure a wide range of particle sizes in the same analysis. Moreover, this study highlights the ability of the particle size analyzer (PSA) to accurately measure changes in apparent particle size over time for the same suspension and thereby indicate the presence of flocs. Finally, the PSA has the capability of capturing dynamic particle interactions-flocculation and deflocculation- in real time.

Fine coal refuse (FCR) is the waste material generated by the processing of raw coal. FCR is typically mixed in slurry form with a flocculant and placed hydraulically in on-site impoundments behind tailings dams, setting out over time to form a solid. Currently, little is known about the compressibility of these materials. In this study, FCR slurry was obtained from the inlet of the slurry discharge pipe of an active impoundment in Kentucky (U.S.). The liquid slurry was consolidated in columns over a range of stress to create samples representing the slurry in an adjacent established slurry impoundment. These results are compared with consolidation tests performed on in-situ samples obtained from the impoundment. The consolidation results indicate that both the fresh slurry samples and the in-situ FCR are highly compressible, and the in-situ FCR was found to exhibit values of the pre-consolidation stress that were much less than stresses corresponding to the equivalent overburden pressures. This implies that the impounded FCR has not yet reached the end of primary consolidation, even though the material was placed decades ago. Typical calculations suggest that the time for 99% consolidation is on the order of 90-300 years depending on the actual drainage.

While manmade slopes are traditionally constructed with planar cross-sections, natural stable slopes are usually curvilinear with significant concavity of the profile. This concavity occurs as a result of evolutionary processes in which rain-driven erosion and sediment transport are balanced through slope shape adjustments. At the point of equilibrium, a relatively steady concavity with a uniform erosion rate is observed over time. Nevertheless, a true equilibrium state is possible only if mechanical stability is satisfied. In this study, concave profiles in rainfall erosion equilibrium were found based on the principles of the well-known RUSLE2 model. Results showed the existence of a family of steady shapes satisfying the condition of uniform normalized erosion rate. Those steady concave shapes that also satisfied long-term mechanical stability were then investigated. The overall results suggest that concave slopes can be constructed to achieve both minimal steady-state erosion equilibrium and mechanical stability, leading to more natural and sustainable landforms with minimal sediment delivery during initial slope adjustments.

The growth of precision autoguidance systems on construction equipment suggests that nonplanar slopes and landforms now can be constructed readily. Slopes with concave cross sections not only appear more like natural slopes, but can also have superior stability and erosion resistance. Thus, it is desirable to have the description of concave slopes that provide mechanical stability for a given set of soil properties. In this paper, an approximate solution that defines the geometry of critical concave slopes (factor of safety approximate to 1) in a frictional medium is developed, based on the slip-line field method. The approximate solution is compared with previous numerical results and validated via limit-equilibrium method and FEM analyses. The proposed solution is simple in form, and, when implemented with precision construction equipment, will allow the construction of embankments and reclaimed mine lands that appear more like those in nature and yet are more resistant to erosion. (c) 2014 American Society of Civil Engineers.

Subgrade soil, as the critical underlying support for other pavement layers and traffic loads, should be stiff enough to maintain the integrity of pavement structures and the smoothness of pavement surface. The resilient modulus, as an indicator of subgrade stiffness, is an essential input in the AASHTO Mechanistic-Empirical Pavement Design Guide (MEPDG). At input level 1 of MEPDG, the MEPDG generalized model is required to describe resilient modulus of subgrade soil, and the coefficients of this model are used for pavement design. The change of the resilient modulus model has raised the interest of many state highway agencies and made it necessary to convert old resilient modulus test data into new ones required for the MEPDG model. In this study, the coefficients of the generalized and the universal models for soil resilient modulus were obtained through regression of the results of 13 soils in Tennessee. The coefficients of the two models were also compared. There is a potential risk that the coefficients from the universal model may be mistakenly used in the MEPDG instead of those coefficients from the generalized model. The consequence of this improper use was demonstrated in the comparison between the miscalculated and the real resilient moduli. The coefficients of the generalized model were correlated to soil physical properties, which provided an alternate time-saving and economical method to obtain soil resilient modulus as level 2 inputs. The coefficients were obtained at different post-compaction water contents, to allow the estimation of pavement response under seasonal moisture variation of subgrade. Rutting and roughness of two typical pavement sections were analyzed to investigate the influence of the seasonal variation of soil resilient modulus on pavement performance. The results showed that moisture variation had a significant effect on subgrade resilient modulus and, subsequently, on pavement performance. It is recommended that seasonal change in soil resilient modulus be considered in the analysis on pavement performance.

The piling framed retaining wall (PFRW) is an innovative earth-retention system applicable for soils underlain by rock, which is ideal for applications where only limited right-of-way is available or adjacent structures limit the use of tieback anchors. Two PFRWs were successfully built along the I-40/I-75 corridor in Knoxville, Tennessee, with significant cost savings over traditional retaining wall designs. Although the walls were designed using conventional earth pressure theories, the soil pressures and forces acting on the wall face are not fully understood, and a rational design method has not been fully developed. Traditional theories of lateral earth pressure assume rigid translations or rotations as the fundamental deformation mode, when in reality more complex mechanisms of deformation and earth pressure distributions may exist. A series of FEM analyses was used to evaluate the soil stresses on the face of the wall for various configurations of wall geometry, backfill slopes, and soil properties. From the results, simplified design equations were developed to predict the earth pressures on the wall face and the overturning moments for stability analyses. The proposed design equations were validated against traditional expressions and compared with earth pressures measured on the wall over a 3-year period. The measured stresses and the numerical results suggest that the typical earth pressure distribution of the PFRW is neither linear nor monotonically increasing, and the proposed design equations yield conservative results for practical combinations of geometry and soil properties. The proposed design methods offer a reliable way to predict wall pressures and overturning moments and eliminate the need to conduct extensive numerical analyses for each wall to be constructed. (C) 2014 American Society of Civil Engineers.

While manmade slopes are traditionally constructed with planar cross sections, natural stable slopes are usually curvilinear rather than planar. These curvilinear slope shapes with significant concave portions are obtained as a result of evolutionary processes where entropy is maximized through slope shape adjustments. Evidence suggests that this adjustment results in equilibrium between rain-driven erosion and sediment transport, yielding stable concave slopes with relatively constant shapes. This equilibrium shape is characterized by a parallel retreat with a uniform erosion rate over time. Concave slopes are more likely to maintain stable profiles than are planar slopes, where large amounts of soil are removed and delivered during the process of erosion equilibrium adjustment. Since a truly stable slope must be stable in terms of both mechanical and erosion processes, it becomes important to describe concave slopes in rainfall-erosion equilibrium and evaluate their mechanical stability. In this article concave profiles in rainfall erosion equilibrium are identified and described based on the well-known RUSLE2 model. Results indicate the existence of a family of potential slope shapes satisfying the condition of uniform erosion rate along the slope. Making use of the critical slope contour concept for a given mechanical soil strength, those steady concave shapes that also satisfy long-term stability are investigated. A true steady-state equilibrium slope is obtained when both erosion equilibrium and long-term mechanical stability are achieved. This suggests that concave slopes can be constructed to achieve both minimal steady-state erosion equilibrium as well as mechanical stability, leading also to more “natural” landforms. Constructing slopes to reflect these stable shapes will provide more aesthetically pleasing results and minimize sediment delivery during initial slope adjustment.

While traditional mine reclamation methods emphasize compaction to increase the strength of the materials and ensure stability of the restored slope, high compaction restricts the successful reforestation of reclaimed mine sites. The Forest Reclamation Approach (FRA), which uses low compaction in the uppermost 1.2–1.5 m of the surface has been shown to facilitate the establishment of healthy native forests. Stability analyses of three steep FRA slopes from the southern Appalachian region have shown that the long-term static stability is not compromised, and that the infinite slope method provides a rational method to evaluate the stability of steep FRA slopes. In this article, modifications of the infinite slope equation are utilized to (a) include the effects of matric suction due to unsaturated soil conditions and (b) evaluate the seismic performance of FRA slopes based on spectral accelerations. Monthly variation of the water content at three research sites demonstrated the seasonal stability variation of FRA slopes due to matric suction, while seismic analyses illustrated the conditions under which instability may occur.

Constructed slopes are traditionally given a planar form. However, natural slopes are more likely to be concave in cross section. In addition, laboratory and computational studies have demonstrated that concave slopes yield less sediment than planar slopes. With current autoguided construction equipment, it is now possible to construct slopes with concave profiles and a more natural appearance, yet a simple method to describe such concave slopes for a given level of mechanical stability does not exist. This article begins with an examination of concave shapes satisfying a desired degree of stability and compares results with those from the FEM and limit-equilibrium method of analysis. An erosion model is used to demonstrate that the concave slopes proposed here yield 15-40% less sediment than planar slopes with the same factor of safety. Finally, a sensitivity analysis suggests that reasonable construction deviations do not compromise the stability of typical concave slopes. This work is made available under the terms of the Creative Commons Attribution 4.0 International license, http://creativecommons.org/licenses/by/4.0/.

The use of loose spoils on steep slopes for surface coal mining reclamation sites has been promoted by the US Department of Interior, Office of Surface Mining for the establishment of native forest, as prescribed by the Forest Reclamation Approach (FRA). Although low‐compaction spoils improve tree survival and growth, erodibility on steep slopes was suspected to increase. This study quantified a combined KC factor (combining the effects of the soil erodibility K factor and cover management C) for low compaction, steep‐sloped (>20°) reclaimed mine lands in the Appalachian region, USA. The combined KC factor was used because standard Unit Plot conditions required to separate these factors, per Revised Universal Soil Loss Equation (RUSLE) experimental protocols, were not followed explicitly. Three active coal mining sites in the Appalachian region of East Tennessee, each containing four replicate field plots, were monitored for rainfall and sediment yields during a 14‐month period beginning June 2009. Average cumulative erosivity for the study sites during the monitoring period was measured as 5248.9 MJ·mm·ha−1·h−1. The KC ranged between 0.001 and 0.05 t·ha·h·ha−1·MJ−1·mm−1, with the highest values occurring immediately following reclamation site construction as rills developed (June – August 2009). The KC for two study sites with about an 18–20 mm spoil D84 were above 0.01 t·ha·h·ha−1·MJ−1·mm−1 during rill development, and below 0.003 t·ha·h·ha−1·MJ−1·mm−1 after August 2009 for the post‐rill development period. The KC values for one site with a 40 mm spoil D84 were never above 0.008 t·ha·h·ha−1·MJ−1·mm−1 and also on average were lower, being more similar to the other two sites after the rill development period. Based on an initial KC factor (Ke) measured during the first few storm events, the average C factor (Ce) was estimated as 0.58 for the rill development period and 0.13 for the post‐rill development period. It appears that larger size fractions of spoils influence KC and Ce factors on low‐compaction steep slopes reclamation sites. Copyright © 2013 John Wiley & Sons, Ltd.

The soil subgrade, which supports the above pavement layers and traffic, should be stiff enough to maintain the integrity of the pavement structures and the smoothness of the pavement surfaces. The resilient modulus, as a property of subgrade stiffness, is an important input in the American Association of State Highway and Transportation Officials (AASHTO) Mechanistic-Empirical Pavement Design Guide (MEPDG). In the MEPEG input level 1, the generalized universal model is used to represent the resilient modulus, and coefficients for this model are required. The change to this model has raised the interest of states converting old resilient modulus test data, which may have been presented in terms of other models, into the general universal model as implemented in MEPDG. Based on cyclic triaxial load test data of clayey soils in Tennessee (TN), coefficients of the generalized model were regressed. Also the coefficients were regressed from soil physical properties, which can be utilized as an alternate time-saving and economical method to obtain soil resilient modulus. The coefficients were obtained at different post-compaction water contents, to allow the estimation of pavement response under natural seasonal variation of subgrade water content. Two typical pavement sections, I-40 Knox and SR-36 Washington, were evaluated for pavement performance utilizing a multiple layered software Wealsea 3.0 and the version 1.1 MEPDG software. The results showed that because moisture variation significantly affected the subgrade resilient modulus, without including these effects with appropriate coefficients for the generalized model the full pavement performance is not captured.

Since the Surface Mining and Control Reclamation Act of 1977, US coal mining companies have been required by law to restore the approximate ground contours that existed prior to mining. To ensure mass stability and limit erosion, the reclaimed materials have traditionally been placed with significant compaction energy. The Forest Reclamation Approach (FRA) is a relatively new approach that has been successfully used to facilitate the fast establishment of native healthy forests. The FRA method specifies the use of low compaction energy in the top 1.2–1.5 m of the contour, which may be in conflict with general considerations for mechanical slope stability. Although successful for reforestation, the stability of FRA slopes has not been fully investigated and a rational stability method has not been identified. Further, a mechanics-based analysis is limited due to the significant amount of oversize particles which makes the sampling and measurement of soil strength properties difficult. To investigate the stability of steep FRA slopes (steeper than 20°), three reclaimed coal mining sites in the Appalachian region of East Tennessee were investigated. The stability was evaluated by several methods to identify the predominant failure modes. The infinite slope method, coupled with the estimation of the shear strength from field observations, was shown to provide a rational means to evaluate the stability of FRA slopes. The analysis results suggest that the low compaction of the surface materials may not compromise the long-term stability for the sites and material properties investigated.

Controlling erosion and sediment delivery on constructed slopes is crucial to limit the environmental impact. Laboratory and computational studies have demonstrated that slopes with concave profiles have greater erosion resistance than planar slopes. Also, in nature slopes are seldom planar in cross section and more complex contour geometries are common, with the cross sections often tending to be concave. Nevertheless, not all concave profiles provide the desired mechanical stability. In this article, an approximate solution for a concave slope profile based on plasticity theory is presented, with results validated by Finite Element analysis. With the growing use of precision GPS-controlled construction equipment, more natural appearing concave slopes with superior erosion resistance may become an attractive alternative to planar slopes.

In the coal-mining region of the Appalachians, the Forestry Reclamation Approach (FRA) is currently promoted by the U.S. Dept. of Interior, Office of Surface Mining (OSM). FRA's goal in establishing native forest cover on reclamation sites requires the use of loose spoils rather than traditionally compacted spoils during hillslope reconstruction. Loose spoils improve tree planting survival and enhance overall forest growth. Although FRA has been shown to be successful on low-gradient slopes without excessive runoff leading to erosion, rainfall-runoff relationships have not been studied on steep slopes (>20 degrees) where most surface-mine reclamation typically occurs. A curve number (CN) range representing low-compaction steep-sloped conditions was needed for Appalachian FRA sites. Three active surface-mine sites in East Tennessee were monitored during a 1-year period (June 2009-July 2010) for rainfall (5-min intervals) and runoff hydrology (discharge volume, peak discharge) using a unique Pinson-type collection system. CNs were estimated by the traditional NEH-4 (Part 630) method and two asymptotic, frequency-matching techniques. It was found that an initial abstraction () of 0.2 best fits the data. CNs generated by the NEH-4 method among all three mining sites were not significantly different suggesting common shale-based loose spoils on steep slopes have similar runoff behavior. The CN determined by the asymptotic method provided an estimate with a practical range between 58.5 and 60. This CN range is lower than one would expect from surface-mine sites, suggesting infiltration influenced the measured rainfall-runoff relationships. Further research is needed to investigate the influence of slope and infiltration on runoff behavior at surface-mine sites applying FRA.