671017.pdf

b

x

z

y

Support

Support

Elastic

support-

coalface

L 1 L 2

L 3

L 4

Figure 2: Scheme of longwall mining.

y

x

qi

ei

Ksi

Figure 3: Single section.

In addition, it is considered that the stiffness푘푒 of
the supporting elements can be different and it is possible
that there exist other supporting elements with stiffness퐾푠,
for example, keys of wood, the coalface, or the protection
rock mass between two panels. So, each span, besides the
properties previously indicated, presents the following:

(i) stiffness of the uniform support:퐾푠푖(Pa);

(ii) stiffness of the props:퐾푒푖(Pa).

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,푞푖, acting in the principal plane
of the symmetric cross-section (Figure 3), the deflection of
this roof is described through a differential equation of fourth
degree [ 20 ]:

퐾푡푖⋅푦퐼푉푖 +퐾푠푖⋅푦푖=푞푖, (1)

where푦푖is the deflection in each point and퐾푡푖is a constant
given by the expression퐾푡푖=퐸푖⋅퐼푖.with퐼푖the moment of
inertia.
The solution of the differential equation ( 1 )isgivenby

푦푖(푥)=푒퐾푖⋅푥⋅(퐴푖⋅cos퐾푖⋅푥+퐵푖⋅sin퐾푖⋅푥)

+푒−퐾푖⋅푥⋅(퐶푖⋅cos퐾푖⋅푥+퐷푖⋅sin퐾푖⋅푥)+푦푝푖,

(2)

where퐾푖and푦푝푖are given by the expressions퐾푖 =(퐾푠푖/

퐾푡푖)0.25,and푦푝푖=푞푖/퐾푠푖,respectively,and퐴푖,퐵푖,퐶푖,and퐷푖,
areconstantsthatmustbedeterminedusingtheboundary

conditions, which constitute the unknown quantities of the

problem.

Once the deflection is obtained, the angle of rotation at

any point of the panel is given by the first derivative of the

deflection (휃푖(푥) = 푦푖耠(푥)):

휃푖(푥)=퐾푖⋅푒퐾푖⋅푥

⋅[(퐴푖+퐵푖)⋅cos퐾푖⋅푥+(−퐴푖+퐵푖)⋅sin퐾푖⋅푥]

+퐾푖⋅푒−퐾푖⋅푥

⋅[(−퐶푖+퐷푖)⋅cos퐾푖⋅푥−(퐶푖+퐷푖)⋅sin퐾푖⋅푥].

(3)

The bending moment (푀푖(푥) = 퐾푡푖⋅푦耠耠푖(푥))andtheshear

force (푉푖(푥) = −퐾푡푖⋅푦耠耠耠푖 (푥))ateverypointofthepanelare

given by ( 4 )and( 5 ), respectively:

푀푖(푥)

=퐾푡푖⋅[2⋅퐾^2 푖⋅푒퐾푖⋅푥⋅(퐵푖⋅cos퐾푖⋅푥−퐴푖⋅sin퐾푖⋅푥)

+2⋅퐾^2 푖⋅푒

−퐾푖⋅푥

⋅(−퐷푖⋅cos퐾푖⋅푥+퐶푖⋅sin퐾푖⋅푥)],

(4)

푉푖(푥)

=−퐾푡푖

⋅{2⋅퐾^3 푖⋅푒퐾푖⋅푥

⋅[(−퐴푖+퐵푖)⋅cos퐾푖⋅푥−(퐴푖+퐵푖)⋅sin퐾푖⋅푥]

+2⋅퐾^3 푖⋅푒

−퐾푖⋅푥

⋅[(퐶푖+퐷푖)⋅cos퐾푖⋅푥+(−퐶푖+퐷푖)⋅sin퐾푖⋅푥]}.

(5)

As mentioned previously, once the equations that define the

problem have been established, it is necessary to determine

the boundary conditions (null deflections and null rotations

at the ends of the panel) and the conditions of compatibility

(equal deflections, rotations, and bending moments in the

points between spans) in order to know the constants퐴푖,퐵푖,

퐶푖,and퐷푖.

For this purpose, the panel has been analysed in the

phase of take-off by two configurations commonly used in the

mines of Castilla-Le ́on, with two walkways (panel type 1) and

with one walkway (panel type 2).

(i) Panel type 1 has two walkways, three elements of

support, and six spans (Figure 4), the first and last

spans being supported elastically by the coalface;

(ii) Panel type 2 has one walkway, two elements of

support, and five spans (Figure 5), and as in the

previous case, the first and last spans are supported

elastically by the coalface.