-
Bolt Supporting of Large-Span Soft Rockway
in
Shaqu Colliery
Abstract
The instability of
trapezoidal I-steel support is analysed for
the
compound
roof
of
main
coal
seam
in
Shaqu
Colliery,
and
the
mechanism of bolt supporting is
studied. A scheme of bolt supporting has
been
given
and
put
into
practice,remarkable
technical
and
economic
benefits have been
got.
Key words :
large-
span,compound roof, bolt supporting, mechanism
uction
In shaqu colliery a
large coal mine mining rare coking coal in China,
most
roadways
are
laid
out
in
main
coal
seam
roof of coal seam
.The
soft compound
,which
is composed of mudstone and coal seam
contains aboundant beddings and joints.
The strength of the roof is so low
that
its
uniaxial-saturated
compressive
strength
is
only
10.7
value of coal seam and is
zero ,and that of mudstone is lower than 10%.
There
is clay
minerals
in mudstone, main
compositions are
interbedded
strata
of
illite
and
montmorillonite
which
will
swell
when
soaked
by
water, The span of preperation roadways
and gateways is wider than 4m,
and
that
of
some
main
roadways
is
over
5m.
In
shaqu
colliery
,
preperation roadways and gateways were
supported by trapezoidal I-steel
support, the beams of which were bent
and damaged, and the roadways
were
destroyed seriously within a short period just
after excavated. Roof
controlling
of
Large-Span
Soft
Rockways
in
the
coal
seam
became
the
key to
the production and construction of shaqu colliery.
ting
status
and
instability
analysis
of
trapezoidal
I-steel supports
trapezoidal
I-steel
supports
were
used
in
drawing
roadways,which
roof span is
4.0m, floor span is 4.9m, and hight 2.95m and
spacing 0.5m.
Initial resistance of the
supports was almost zero because it was difficult
to
the
support
beams
contact
the
roof,
even
if
with
high
quality
of
installation.
The
trapezoidal
I-steel
supports
would
not carry
load
until
the
displacement of surrounding rock excceded 80-100
mm because the
supports increased very
slowly. Therefore, right after excavation, the
roof
would
bend
and
subside severly.
Eight
hours
after
excavation,
the
roof
strata would break
completely, and then form rock cavity. The weight
of
caving
rock
would
act
on
the
beams
of
supports,
which
forms
loose
rock
pressure.
By
calculating,
the
ultimate
load-bearing
capacity
is
smaller
than
roof pressure whether it is uniformal
or concentrated, Based on the in-situ
observation,
inflection
value
of
most
roof
reached
200-300mm.
When
paired
supports
were
used, paired
beams
were
still
bent
and
damaged;
then midprops were added, they were
also destroyed. Many roof beams
were
stabilized
only
if
2-3
props
had
been
added.
The
supports
were
damaged
completely,
and
most
of
them
could
not
be
reused.
The
part
section
of
roadways
had
become
inverted
trapezoid,
and
the
available
section was far smaller than the
designed section. Part of roadways was
out of use because it was in the danger
of serious caving.
ism of bolt
supporting
Its mechanism is to make
full use of the self-load-bearing capacity
of
surrounding
rock
by
bolting,
and
then
make
the
surrounding
rock
stabilize
by
itself.
The
stability
of
surrounding
rock
depends
on
the
equilibrium
status
of
ground
pressure,
self-load-bearing
capacity
of
surrounding
rock
and
anchoring
force
of
bolts.
Ground
pressure
is
to
make
surrounding
rock
deform
and
break;
self-load-bearing
capacity
is
the main factor to
stablize surrounding rock. Anchoring force of
bolts can
not
change
the
equilibrium
status
of
the
three
because
it
is
very
small,
compared
with
ground
pressure
and
self-load-bearing
capacity.
And
its
function
is
to
change
the
decreasing
regularity
of
self-load-bearing
capacity
versus
the
deformation
of
surrounding
rock,
and
balance
self-load-bearing capacity against
ground pressure early.
Roof pressure is
the pressure acting on the roof beams when I-steel
supports are used to control the roof.
When roof is supported by bolts, the
roof
pressure
change
to
be
the
pressure
acting
on
the
rock
within
the
bolting range because this part of rock
is change into self-bearing body.
According to the characteristics of the
roof of coal seams
can be divided into
six substrata.
, bolts strata
When the
value of roof
subsidence is zero, roof pressure is in-situ
stress; then roof pressure decreases
with the increase of roof subsidence.
The
variation
of
roof
pressure
is
analyzed
by
FLAC,
The
results
are
shown as curve 1 in Fig.1. Whenroof
subsidence reaches 19 mm, the first
roof
substratum
begins
to
bearing
tensile
stress,
then
losts
self-
load-bearing capacity, and roof pressure decreases
to 0.67Mpa. When
roof
subsidence
reaches
40
mm,
the
second
substratum
loses
self-load-bearing
capacity, and roof pressure decreases to 0.16Mpa.
When
roof
subsidence
reaches
100
mm,
the
fourth
substratum
loses
self-load-bearing capacity, and roof
pressure decreases to0.08Mpa. In the
initial stage of roof subsidence, roof
pressure decreases rapidly, and in the
later
stage
of
roof
subsidence,
roof
pressure decreases
slowly
and
then
has an increasing trend.
The
self-load-bearing
capacity
of
the
roof
without
bolting
is
calculated
upon
the
theory
of
laminated
beam,
the
result
are
shown
as
curve
2
in
Fig.1.
When
roof
subsidence
is
zero,
the
self-
load-bearing
capacity
is
at
its
utmost
value
0.0625Mpa;
when
roof
subsidence
is
100mm,roof
strata
have
broken,
most
of self-load-bearing
capacity
has
lost,
and
the
residual
self-load-bearing
capacity
is
only
self-load-bearing capacity
of the roof with bolting is calculated upon the
theory of combined beam, the result are
shown as curve 3 in Fig.1. When
roof
subsidence
is
zero,
the
self-
load-bearing
capacity
is
at
its
utmost
value
0.4Mpa;
when
the
roof
subsidence
reaches
40mm
the
self-
load-bearing
capacity
decreases
to
0.225Mpa,and
when
roof
subsidence
reaches
100mm,
the
self-load-bearing
capacity
decreases
to
0.1Mpa .
As
shown
in
Fin.
1,
the
self-load-
bearing
capacity
of
roof
strata
without
bolting
is
lower
than
roof pressure
during
the
whole
course
of
roof subsiding, so roof
strata cave inevitably. When bolted, roof strata
is
changed
from
laminated
beam
into
combined
beam
,and
the
selr-load-bearing
capacity
increases
markedly.
When
roof
subsidence
reaches 44mm, the
self-load-bearing capacity exceeds roof pressure,
then
roof strata stabilized by itself.
4
Anchoring
technology
Based
on
the
above
study
of
bolting
mechanism,
large
setting
resistance, high
speed of resistance and high final resistance are
the key
technology
to
the
large-spon
soft
rock
roadway
before
roof
strata
detaching, which
includes: (1)to improve the setting resistance
increasing
and achieve high speed of
resistance increasing, to make the real working
properties of bolts coordinate self-
load-bearing properties of roof strata ,
which enables to make full use of the
self-load-bearing capacity of roof
strata; (2)to raise bolting
reliability, and solve the difficult problems that
anchoring force between bolts and soft
rock is small and easy to lose.
4.1
Bloting scheme
The drawing roadway is taken as an
example, the bolting scheme is
shown
in
Fin.
2.
The
high
quality
compound
bolts
installed
in
ribs
are
made
of
bamboo , other
bolts
are
made
of steel.
The
bolts
installed
in
both
shoulders of roadway is 2.4m long, other steel
bolts are 2.2m long,
the diameter of
steel is 20mm and the spacing of bolts is 0.7m.
Boreholes
for steel
bolts
are
27mm
in
diameter,
and
2.1m
in
depth
expect
for
the
boreholes in shoulders which are 2.3m
deep. All steel bolts are anchored
with
four resin cartridges. The tension brace is went
through by all roof
bolts,
and pressed
tightly
on
the
roof
by
bearing
plates.
Concave
steel
plates are
used for steel bolts and bamboo plates
are used for rib bolts.
C150 crete is
shot on the roof and the upper 500mm high part of
both ribs,
the shotcrete is 50mm thick.
The mesh is made of wire 10#, and laid on
both roof and ribs.
4.2
Effect of pretension bolts
Bolts
are
tension
when
they
are
installed,
which
enables
bolts
to
have anchoring force before the
relative displacement between bolts and
surrounding is really achieved. If
fully-anchored bolts are tension to add
the pretensioned stress, the effect
will not be good because only the collar
part of bolts can carry pretension
stress. In order to add pretension stress
to the whole length of bolts, two kinds
of resin cartridges are used: quick
resin
cartridge
is
used
in
the
other
part
of
boreholes,
mid-speed
resin
cartridge is used in the other part of
boreholes as shown in Fin. 3. After a
bolt is inserted into a borehole, the
quick resin will solidify within 1.5min,
and the mid speed resin will not
solidify until 3-4min later. The difference
between
solidifying
times
of
two
kinds
of
resin
make
the
bolt
pretensioned,
which
enables
pretensioned
bolting
and
fully-anchored
bolting to be
achieved at the same time.
4.3
Effect of fully-anchored bolting
The
fully-anchored
bolting
can
improve
the
shear
strength
of
rock
joints, then improve the mechanicd
properties of surrounding rock. High
increasing
speed
of
their
resistance
enables
themselves
to
offer
high
resistance
against
the
deformation
of
surrounding
rock. Campared
with
the
point
anchored
bolts,
the
fully-anchored
bolts
have
a
better
relationship of
interaction with surrounding rock.
Large setting resistance, high speed of
resistance increasing and high
resistance are achieved by the use of
the pretensioned and fully-anchored
bolts.
4.4
Effect of brace
The brace has following effects. (1)The
brace connects the roof bolts
into
a
whole,
which
improves
the
reliability
of
the
whole
supporting
system. (2)It
offers safe assurance to the roof bolting of
large-span soft
rock
roadway.
Even
if
in
the
most
difficult
situation
that
the
self-
load-bearing
capacity
of
bolting
roof
strata
has
lost,
and
the
bed
seperation
has
occurred
between
the
strata
within
and
beyond
bolting
range, the roof
strata
will
not caves
because the shoulder bolts anchors
reliably to 1.3m deep in ribs, which
can offer enough resistance to control
the roof by the effects of brace.
5
Observed
Results of Bolts
Measuring
sites
of
surface
displacement,
bolt
load
and
rock
inner
displacement
were
set
up
in
drawing
roadway(4.3m
wide),
track
roadway(3.4m wide)and
air-intake roadway(3.0m wide).
As shown
in Fin.
4, whether for roof or ribs,
the speed of surface
displacement can
decrease to zero quite rapidly. The measuring
sites was
about 15-20m away from the
driving face when the speed reached zero.
As shown in Fin. 5, pretensioned and
fully-anchored bolts show their
following
properties:
large
setting
resistance,
high
speed
of
resistance
increasing and
high resistance. Setting resistance reaches 20kN;
the speed
of resistance increasing
2kN/mm; and the utmost resistance 100kN. The
development
of
bolt
load
shows
obvious
characteristics,
i.e,
it
can
be
divided into three
stages: rapid increasing stage of load, regulating
stage
of load and stabilizing stage of
load.
As shown in Fin. 6, after the
surrounding rock stabilized, compared
with
the
3.4m
deep
site
in
rock,
the
rock
surface
displaced
14.4mm;
compared with the
1.7m deep site, the surface displaced 8.6mm, i.e,
the
deformation
of
the
rock
beyond
1.7m
range
reached
40%
of
the
total
deformation. This shows that the rock
within the bolting range is changed