Mechanised longwall mining is one of the more commonly employed exploitation methods in underground mines in the north of Spain as well as in the rest of the world. It is continuously changing and evolving, with new techniques, technology, equipment, and face management practices and systems appearing for the purposes of improving aspects such as operational and financial performances and, above all, the safety of the miners. Despite its importance, there are no regulations for the mining of longwall coal seams. This work aims to contribute to an advance in the design and optimisation of the roof support in longwall mining, analysing the stability of the roof using a method based on the resistance of materials, which considers the characteristics of the properties of the roof materials. The influence of not only the individual elements of support but also the coalface, which is considered one more supporting element, is investigated. The longitudinal and transverse spacings of the support and the number of walkways constituting the exploitation panel are analysed. The proposed formulation is validated by information gathered in a mine located in the region of Castilla-Leon.
Important reserves of coal exist in Spain, which can contribute to reducing the energetic dependence on other countries [
Obtaining the existing reserves requires using levels of mechanisation of labour that allow economic competitiveness as well as safety within the context of the regulation RGNBSM (General Regulation of Basic Procedure of Mining safety, 1985) [
Mining labour supported by individual hydraulic props.
But longwall mining is not a new approach to coal mining. In fact, the basic principles of longwall mining have been traced back to the latter part of the 17th century, to Shropshire and other counties in England [
Longwall mining [
From that moment, the roof immediately above the seam is allowed to collapse into the void that is left as the face retreats. Miners working along the coalface, operating the machinery, are shielded from the collapsing strata by the canopy of the hydraulic roof supports. As the roof collapses into the goaf behind the roof supports, the fracturing and settlement of the rocks progresses through the overlying strata and results in sagging and bending of the near surface rocks and in some cases subsidence of the ground above [
Mechanised longwall mining is ever changing and evolving with new techniques, technology, equipment, face management practices, and systems appearing as a direct means to continually improve all aspects of operational and financial performances [
Nevertheless, and in spite of the importance of this method of exploitation, no legal regulation exists regarding the types of support and thickness and characteristics of the rock mass surrounding the excavation, which would guarantee the safe functioning of these developments.
Calculation of the pressure that the working roof wall is to exert on hydraulic props is essential for support design, both to ensure working global stability and to avoid prop punching on gables [
This work tries to contribute to an advance in the design and optimisation of the roof support in longwall mining, analysing the stability of the roof with a method based on the resistance of materials [
In addition, two very important aspects of the safety and the productivity of the exploitation have been considered: the dimensions and the number of walkways of the panel. The width of the rear walkway at waist level needs to be considered from an ergonomic aspect to allow for lamps and self-rescuers worn by underground personnel. The horizontal dimension across an average person’s waist is between 620 and 700 mm, so a rear walkway width of ≥750 mm at waist height is required to permit ergonomically effective passage of people along the face so their productivity is not unnecessarily impeded [
It is considered as a longwall exploitation (Figure length: Young’s modulus: thickness: load:
Scheme of longwall mining.
In addition, it is considered that the stiffness stiffness of the uniform support: stiffness of the props:
Along with these assumptions is imposed the restriction that within each span the roof rock is perfectly elastic, homogeneous, and isotropic and the neutral axis coincides with the centre line of the thickness.
Considering each roof of a span as a beam subjected to a uniformly distributed load,
Single section.
The solution of the differential equation (
Once the deflection is obtained, the angle of rotation at any point of the panel is given by the first derivative of the deflection (
For this purpose, the panel has been analysed in the phase of take-off by two configurations commonly used in the mines of Castilla-León, with two walkways (panel type 1) and with one walkway (panel type 2). Panel type 1 has two walkways, three elements of support, and six spans (Figure Panel type 2 has one walkway, two elements of support, and five spans (Figure
Panel type 1.
Panel type 2.
The application of the boundary and compatibility conditions gives rise to the system of (
Once the efforts have been calculated, it is possible to calculate the factor of safety (FS) of the panel dividing the tensile strength of each section
To calculate this FS in each section of the roof, the shear stress (
A panel in the take-off phase is analysed with two configurations commonly used in the mines of Castilla-León and more specifically in the Feixolin mine [
To calculate the stiffness of the siltstone, the pressures obtained in the penetration test are considered, and to calculate the stiffness of the props [
Properties of the materials in each section or span.
Section 1 |
Section 2 |
Section 3 |
Section 4 |
Section 5 |
Section 6 | |
---|---|---|---|---|---|---|
Length of span (m) | 1.25 | 1.25 | 1.25 | 1.25 | 1.25 | 1.25 |
Thickness (m) | 1.5 | 1.5 | 1.5 | 1.5 | 1.5 | 1.5 |
Young’s modulus (MPa) | 3550 | 3550 | 3550 | 3550 | 3550 | 3550 |
Load (kN/m) | 17200 | 17200 | 17200 | 17200 | 17200 | 17200 |
Supporting stiffness (MPa/m) | 47.8 | 47.8 | 47.8 | 47.8 | 47.8 | 47.8 |
Tensile strength (MPa) | 4.24 | 4.24 | 4.24 | 4.24 | 4.24 | 4.24 |
Prop number | 1 | 2 | 3 | |||
Prop stiffness (MPa/m) | 1500 | 1500 | 1500 | |||
Spacing of props (m) | 0.55 | 0.55 | 0.55 |
One of the most important parameters from the safety point of view as well as in terms of accessibility is the deflection of the roof. In this case, it is considered that negative values of the deflection are equivalent to an increase of the deflection and vice versa.
As it is observed in Figure
Deflection.
In any case, the deflection of the roof can be considered low and barely affects working conditions.
The fall of the roof, that is, the increase of its deflection, depends directly on the stiffness of the hydraulic props. As it is observed in Figure
Deflection with stiffness of props 100 times minors.
On the contrary, considering an increase in the stiffness of the hydraulic props, it does not suppose a change over the results obtained in the initial conditions (Figure
The bending moment (Figure
Bending moment.
In this case, a decrease of the stiffness of the props does not produce a change in the shape of the curves and only produces small variations in the maximum values obtained (panel type 1: 5.24 kNm, and panel type 2: 5.50 kNm).
A change in the materials of the roof does not alter the shape of the curve, but it modifies the maximum and minimum values, which does not happen with the deflection. This behaviour is due to how the bending moment is obtained (
The representation of the rotation angle of the spans produces symmetrical curves from the centre of the panel, as much in the case of type 1 as type 2. In Figure
Rotation angle.
The shear force (Figure
Shear force.
Although most of the parameters shown in Table
A decrease in the length of the spans (Figure
Deflection versus length of spans.
An increase in the length of the spans increases the value of all parameters and specifically the value of the deflection. So it is necessary to find a compromise between length and safety/productivity.
With the advance in the exploitation, the depth of the panels grows and therefore the load on spans increases. This increase results in a greater deflection of the roof (Figure
Deflection versus depth of the panel.
The analysed parameters, and specifically the shear force and the bending moment, let us know one of the most important points in the design of a panel of longwall: the FS. In the two examples with the properties shown in Table
From (
The influence of the distribution of the props, their stiffness, or the number of walkways in the panel over the FS has been analysed. Nevertheless, unless extreme values were used, the FS scarcely varied in its value. On the contrary, changes in the properties of the materials, and especially changes in the Young's modulus, produce great variations in the FS.
The calculation of the stability of roofs in longwall mining can be resolved by employing the classic resistance of materials. In addition, because the calculation process is very fast, it is possible to design a more appropriate roof support for a specific longwall mining workshop, to know the behaviour of the roof as a support element, and to analyse the influence and capacity of the individual support elements and the effect of their longitudinal and transverse spacing. While the most influential factor in the deflection of the roof is the stiffness of the props, the bending moment depends directly on the properties of the materials and specifically on their Young's modulus. In all cases, the disposition of two walkways in the panel against one walkway reduces the maximum values of the parameters analysed with the exception of the shear force. In any case, these maximum values are placed, for both configurations analysed, in the second and last but one sections. So, these sections are critical in the analysis of the FS. Finally, the variables that have most influence in the FS, are undoubtedly the depth of the panel and above all the changes in the properties of the rock mass of the roof.
Analyse the influence that the dimensions and the number of walkways of the panel have on the stability of the roof. Study the influence of not only the individual elements of support but also the coalface, which is considered one more supporting element. Analyse the configuration of the support (longitudinal and transverse spacing) in longwall mining. Know in a simple way the safety factor for a workshop.
The not null elements of the matrices