Influence of backfill on coal pillar strength and floor bearing capacity in weak floor conditions in the Illinois Basin

doi:10.1016/j.ijrmms.2014.11.011

International Journal of Rock Mechanics and Mining Sciences

Volume 76, June 2015, Pages 55-67

https://doi.org/10.1016/j.ijrmms.2014.11.011 Get rights and content

Highlights

•
Minimizing the distance between footings has increased the bearing capacity.
•
A 10–40% increase in pillar strength was observed at 25–75% fill of the mined height.
•
Methodology for analyzing the plastic flow behavior of a coal pillar and a footing in FLAC^3D has been presented herein.

Abstract

This paper discusses the current theoretical design equations associated with foundation design in the Illinois Basin, as well as the benefits of utilizing high density backfill in a numerical model, to improve pillar and floor stability. Based upon the results, Vesic’s bearing capacity solution for a two layered soil tends to underestimate the true bearing capacity of a foundation, especially at higher friction angles and when footings are placed in close proximity. Minimizing the distance between adjacent foundations has shown an improvement in the ultimate bearing capacity of a foundation; however, placing the foundations in too close proximity has shown the foundations may behave as a single foundation and undergo appreciable settlement. A 10–40% increase in pillar strength and ultimate bearing capacity can be expected when a cohesive fill is used between 25 and 75% fill of the mined height, respectively. The non-cohesive nature of the simulated backfill showed little influence on increased pillar strength, even at higher fill ratios. It was determined, that as the shearing resistance, tensile strength and stiffness of the backfill are reduced, increases in coal pillar strength is due more to the confinement aspect of an underground mine rather than the strength properties of the material itself. A methodology for analyzing the plastic flow characteristics of a coal pillar and a footing using FLAC^3D has also been presented herein.

Introduction

This paper directly relates to the numerical investigation of an immediate weak floor, a coal pillar and the support consisting of backfill represented as a confining material. In particular, this study will focus on the Illinois Basin which consists of Illinois, Indiana and Western Kentucky. The Illinois Basin is major coal producer in the U.S. where nearly one hundred thousand short tons of underground coal were produced in 2013 [1]. This research was conducted mainly because of the ever-restrictive legislation posed on coal mines in the United States by the Environmental Protection Agency (EPA). More specifically, the EPA have proposed new legislation which is predicted to increase the conventional surface disposal cost for coal mines in the future [2]. Additionally, public opposition to surface waste disposal hinders the ability to obtain surface disposal permits in a timely fashion. Surface disposal is the primary means of coal waste disposal in the United States, therefore the most practical, but not yet cost effective option for the future, will probably be underground disposal of waste as the form of high density (paste) backfill [3]. In this case, coal waste can constitute either washing plant tailings or combustion by-products such as fly or bottom ash. Currently use of high density backfill is not a routine operation for underground coal mines in the U.S. but has been used quite regularly in longwall coal mines in Germany and Poland for several decades [3].

Further, this paper considers backfill disposal in underground coal mines located primarily in the Illinois Basin of the United States. Total mineable reserves for the Illinois Basin are estimated at nearly 14.4 billion tons [4]. More importantly, the majority of underground coal mines in the major mineable coal seams in the Illinois Basin are associated with a weak immediate floor termed underclay, fireclay or clay-stone which is very friable and variable in nature. Generally underclay has been studied and analyzed from the basis of soil mechanics because of its behavior to that of a clayey soil. Most importantly, underclay can cause serious instability issues in the short-term including pillar punching, loss of entry, pillar sloughing and roof problems and subsidence in the long-term [5]. This issue is typically mitigated via oversized pillars, which significantly results in reduced extraction in room and pillar mines.

Since no theoretical methods exist, to the authors’ knowledge, regarding the design of such a scenario concerning the pillar, backfill and immediate floor interaction, and deriving such a relationship would be impractical or rigorous at best; a heuristic approach with numerical modeling was deemed most appropriate to observe this problem. This paper will discuss the shortcomings of current and theoretical design equations associated with foundation design in the Illinois Basin and the potential benefits of utilizing backfill as a confining agent to coal pillars in a numerical model. The results are presented in a pragmatic and useable manner for future design.

Section snippets

Theoretical background

Classical foundations engineering credo is concerned with the satisfaction of two criteria: designing against shear failure in the soil and minimizing excessive displacements or settlement of the foundation. Shear failure is associated with the term of ultimate bearing capacity, which is the load per unit area of a foundation at which shear failure in the soil occurs [6].

Foundation design as a whole has been largely geared and implemented more in civil engineering practice than any of the other

Methodology

The popular numerical modeling package of FLAC^3D 5.0 was utilized throughout the following body of work. Since this problem is representative of a three part system – pillar, floor and backfill – it is obvious that trying to model, and interpret and analyze the response of the system as a whole by using an “all at once” approach is very difficult and impractical. Therefore a “one-factor-at-a-time” approach was adopted initially until each particular factors effect on the models response was

Effect of pillar spacing

The effect of footing spacing on the bearing capacity model was studied next, as it was known from past research [16] that the effect of footings and/or pillars in close proximity increases the bearing capacity. Therefore the modeled bearing capacity was compared to the theoretical bearing capacity value.

Four footing-to-room width (F_w/R_w) ratios were simulated: 1.5, 2.0, 3.0 and 6.0. These ratios were chosen as they cover the majority of F_w/R_w ratios found in Illinois Basin Coal Mines [5] and

Conclusions

Based on the following body of work the following conclusions can be made. Vesic’s approximation for bearing capacity, which is by far the most popular design equation for governing purposes for mines in the Illinois Basin, tends to underestimate the total bearing capacity of a footing especially at higher friction angles and when multiple footings are in close proximity. This conclusion was also validated by Gadde [5].

Minimizing the distance between adjacent foundations has shown an

Acknowledgments

The authors greatly appreciate the funding from the Illinois Department of Commerce and Economic Opportunity through the Illinois Clean Coal Institute.

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