Advances in Coal Mine Ground Control

Syd S. Peng

• Publisher: Woodhead Publishing
• Year: 2017
• ISBN:
  9780081012802
• Topic: Technologie et ingénierie
• Subtopic: Combustibles fossiles

Advances in Coal Mine Ground Control is a comprehensive text covering all recent advances in coal mine ground control, the most advanced subsystem of the rapidly advancing coal mining systems.

This complete resource is written by Professor Syd Peng who, alongside leading experts from the world's major coal producing countries, has contributed extensively to the understanding of subsidence from underground coal mining, longwall operations and ground control in underground mines.

Syd and the team of contributors bring together key advances from the past decade into one comprehensive resource that is accessible to all those studying, researching and working in the mining industry.

This book is an essential text for undergraduate and graduate students of mining engineering and related programs, and a must-have reference for mining, civil and geotechnical engineers.

• Written and edited by the world's leading experts on ground control in coal mining
• Covers all aspects of ground control practices in coal mines
• Focuses on advances over the past decade, equipping readers with the most up-to-date knowledge regarding current research and practices in the field

1

Research developments that contributed to the landscape of longwall roof support design over the past 25 years

Thomas M. Barczak

Chapter Outline

1.1 Introduction 1

1.2 The Mine Roof Simulator 2

1.3 Longwall Shield Supports 3

1.3.1 Development of the two-leg shield 3

1.3.2 Control system technology 5

1.3.3 Support performance testing 6

1.3.4 Stiffness characteristics 7

1.3.5 Shield load monitoring 8

1.3.6 Design capacity considerations 12

1.3.7 Assessing and understanding shield loading behavior 15

1.3.8 Remaining research focus 19

1.4 Standing Roof Supports 21

1.4.1 Conventional wood cribs and timber posts 23

1.4.2 Engineered timber supports 23

1.4.3 Yieldable cementitious supports 24

1.4.4 Steel props 27

1.5 The Ground Reaction Curve 30

1.6 Closing Remarks 32

1.7 Disclaimer 33

References 33

1.1 Introduction

Longwall mining has had a major impact on both productivity and safety in the US coal mining industry. Many changes have occurred as longwall mining matured from its infancy in the 1970s. Much of it was experienced based, but there were also focused research efforts that addressed all aspects of the longwall method. Included in this was significant research dedicated to the ground control aspects. This paper summarizes a body of research conducted over a 25-year period that was concentrated on support technologies and support design, specifically investigations and research studies devoted to shield and gate road design.

1.2 The Mine Roof Simulator

The largest single investment ever made in ground control research was the Mine Roof Simulator (see Fig. 1.1). This unique load frame was designed and built by MTS Corporation to US Bureau of Mines specifications at a cost of approximately $10 million and was commissioned into service by the Department of Energy in 1981. The machine was designed with the primary goal of being able to test longwall shields under active loading that fully simulated the ground response associated with longwall mining. Constructing a load frame sufficiently large enough to accommodate a full-size shield and incorporating several “first-ever” operational capabilities provided this requirement.

Size: The load frame was fabricated with a 20×20 ft upper and lower platen and a maximum roof-to-floor height of 16 ft.

Loading capability: The longwall ground response required a multi-axis loading capability to simulate both the vertical convergence of the longwall face area and lateral movement of the mine roof from face toward the gob as the panel was extracted. The MRS satisfies this biaxial loading requirement with the capability of providing up to 3,000,000 lbs of vertical force through a 24 in. stroke of the lower platen and up to 1,600,000 lbs of horizontal force through a 16 in. stroke of the lower platen. Rates of load application of up to 6 million lbs/min have been used. The maximum platen displacement is 5 in./min.

Load control: Precise load application is achieved by closed-loop servo-controlled actuators with six degrees of freedom control of the lower platen. The six degrees of freedom are three displacements (vertical, horizontal, and lateral) and three rotations (pitch, yaw, and roll). The pitch, yaw, and roll movements are commanded to keep the upper and lower platen parallel at all times during specimen testing. The lateral displacement is limited to 0.5 in. and exists primarily to maintain yaw control of the lower platen.

Control modes: The machine can be operated under either force or displacement control. Initially, an analog control system provided four vertical operating load ranges (200, 1000, 2000, and 3000 kips) and three horizontal load ranges (400, 800, and 1600 kips) and four vertical displacement ranges (5, 10, 20, and 24 in.) and four horizontal displacement ranges (2, 5, 10, and 16 in.). A fully digital control system replaced the analog system in 2009, which now provides load control to any specification within the full operational ranges.

Stiffness: The load frame stiffness when loading over small-area (2-ft-diameter) specimens with a load of 1000 kips in the middle of the platen is 25,000,000 lbs/in., which is similar in magnitude to a triaxial rock testing frame that has a stiffness of 60,000,000 lbs/in. Considering the size of the platens, this is a remarkable achievement. Furthermore, the platen deflection is measured and is incorporated into the feedback response allowing it to be subtracted from displacement control so that the intended displacement is maintained by the control system. The system also incorporates shock absorbers to absorb energy released during brittle specimen failure. The shock absorbers limit lower platen movement to less 0.1 in when sudden specimen failure occurs. These capabilities allow for testing of both stiff and soft specimens.

Figure 1.1 Mine Roof Simulator.

Since its commission in 1981, 7838 tests have been conducted in the MRS. The ability to test full-scale roof support structures under load conditions that replicate in-mine service loads has greatly enhanced the ability to fully develop roof support products prior to utilizing them underground, thereby eliminating the need for lengthy in-mine trials and accelerating the maturing of new roof support developments.

1.3 Longwall Shield Supports

Much of the success in the safety and productivity enhancements in longwall mining during the past 25 years can be attributed to the development of the shield support. The shield support provided unprecedented capacity and stability in a hydraulic support structure, and the basic structural design has been unchanged in the past 25 years.

1.3.1 Development of the two-leg shield

The development and maturity of the two-leg shield design changed the landscape of powered roof support design in longwall mining. The two-leg shield design provides superior strata interaction than a four-leg shield design. The primary difference is the ability of the two-leg shield to provide an active horizontal force. Since the leg cylinder in a two-leg shield is inclined toward the face, horizontal components of the leg force push the canopy toward the coal face as shown in Fig. 1.2. This works to induce a force into the immediate strata attends to maintain the strata in a state of compression. In comparison, the legs of a four-leg shield are inclined in opposite directions to one another and the horizontal components of the leg force can cancel one another out resulting in no active horizontal force acting toward the coal face.

Figure 1.2 Active horizontal force with two-leg shield.

The unbalanced distribution of loading between the front and rear legs also make the four-leg shield less effective in cavity prone strata. As the example in Fig. 1.3 illustrates, the force in the rear legs causes the canopy to rotate up into the cavity, which causes a loss of roof contact at the canopy tip. This condition ultimately results in further cavity formation and requires the front legs to do most of the supporting work. Since the front legs of a four-leg shield are considerably smaller than they would be in a two-leg shield of equivalent support capacity, the four-leg shield provides much less supporting force than would a comparable two-leg design.

Figure 1.3 Reduced stability of four-leg shield in cavity prone roof conditions.

The primary disadvantage of a two-leg shield is generally higher contact pressure on the canopy and base. High toe loading, caused by the moment created by the line of action of the resultant vertical forces acting on the canopy and base (see Fig. 1.4), can be a problem in high capacity two-leg shields and should be a primary consideration in the support design.

Figure 1.4 Resultant force actions on two-leg shield induce tendency for high toe loading on shield base.

1.3.2 Control system technology

Enhancements in shield control have been made with the incorporation of electro-hydraulic control systems that automate the support function. Included in this technology development was the ability to incorporate a shearer-initiated shield advance capability, whereby the shearer location is sensed by a receiver on the shield and activates the advance cycle automatically. Another feature of the advancements in electro-hydraulic control technology is the ability to provide a programmable and consistent setting force across the face. Early in the application of this technology, full setting pressure was not consistently achieved, as the demand placed upon the system was greater than the capability of the hydraulic distribution system to provide sufficient hydraulic fluid at the required pressure. Tapping off the hydraulic pressure following the shield setting operation subsequently solved this problem.

1.3.3 Support performance testing

The MRS was commissioned into service shortly after the introduction of longwall mining into the United States. This also happened to be the period in time when the some of the early shi...