SURFACE
PRE-GROUTING AND FREEZING FOR
SHAFT SINKING IN AQUIFER FORMATIONS.
Mine Water and the
Environment (2005) 24:
209-212 IMWA Springer-Verlag 2005.
Chunlai
Yang: Beijing
Research Inst of Mine Construction, Qingniangou Rd E, Hepingli 10013
Beijing, China;
Zonggmin
Wang: Zhengzhou Coal Corp Engineering Co, 2 Hangai Rd, Zhengzhou
450006, Henan, China; corresponding author's e-mail:
chunlai_y@yahoo.com.
Abstract
Special
techniques are used for shaft sinking in aquifer formations. In
the unstable aquifer formations encountered in Chinese coal mines,
freezing is
always adopted for shaft constructions in an aquifer alluvium, while
surface
pre-grouting is generally adopted in bedrock formations. So far,
freezing has
been used to construct 500 vertical shafts, comprising a total length
of 80 km,
and surface pre-grouting has been used in more than 150 vertical shaft
sinking
projects. The maximum depth of freezing is about 700 m, while the
maximum depth
of surface pre-grouting exceeds 1000 m
Shaft Freezing
Freezing
in shaft
sinking was invented by F.H. Poetsch in 1883. Its essential feature is
the
solidification, by freezing, of water-bearing ground in which the shaft
is
sunk. The method was introduced to China
from Poland
in 1955 and first applied in the Linxi Mine ventilation shaft of the
Kailuan Coal Mining Bureau. In the 50 years since then, the method has
rapidly
developed due to professional and academic research and enhanced
equipment
technology and construction methods. Today, freezing is one of the most
important methods of underground construction in the complex and
unstable
water-bearing alluvium of Chinese coal mines due to its high
reliability.
Freezing has also recently become popular in urban construction projects
To prepare a site for
freezing, a series of equally spaced boreholes are drilled on a
concentric
circle enclosing the site of the shaft. Heat is removed from the ground
via
probes placed into the boreholes. The probe itself consists of an
external
pipe, 50-150 mm in diameter, closed at the lower end and containing an
open-ended inner tube of slightly shorter length. The inner tube can
vary
between 20-75 mm in diameter. The freezing tubes are connected with two
circulation mains, in such a manner that cold brine may be pumped down
the
inner tube and allowed to return along the annular space between the
two tubes
and then through the collection main back to the refrigeration plant.
Here it
is pumped through a chiller (normally a shell and tube heat-exchanger),
where
it is cooled down again, and then delivered via a distribution main
back to the
inner tubes of the probes. The coolant is therefore confined in a
closed,
recirculatory flow path. An industrial refrigeration plant is required
to cool
the brine. Extracted heat is dissipated into a nearby watercourse if
available,
or into the atmosphere by forced-draught cooling towers or evaporative
condensers
The effect of
circulating a coolant through the complete system is to produce long
cylinders
of frozen strata around each column, which gradually increase in
diameter until
they merge, forming a" circular barrier of frozen ground, known as the
ice
wall. During excavation, refrigeration is maintained, generally in
excess of
the level required to offset heat transfer from the warmer strata
surrounding
the shaft
Freezing has been used to construct 500 vertical
shafts in the northeastern, northern, and eastern part of China,
with a total length of 80 km. The auxiliary shaft in Dingji Mine of
the Huainan Coal Mining Bureau in Anhui
Province
has a freezing depth of 570 m, and an alluvium thickness of 524 m. The
auxiliary shaft has a designed depth of 880.5 m, a net diameter of 8.0
m, and a
throughput of 6.0 Mt/a. The Guotun Mine, which is being constructed in
the Juye
mining area in Shandong
province, is expected to have the deepest freezing depth (702 m), with
an alluvium thickness of 570 m
Simultaneous
Freezing
and Grouting Technique
The
steps to construct
vertical mine shafts through water-bearing strata are: GPG ==> Shaft
freezing ==>
Drilling frozen section ==> Drilling bedrock. Usually freezing
and grouting
holes are distributed around different concentric circles, and the
diameters of
freezing holes are larger. It is normally impossible to drill grouting
holes
and freezing holes simultaneously due to the limited working space and
interference between the two operations. The key to overcoming this
problem is
to place the grouting holes on a circumference outside that of the
freezing
holes. To assure grouting quality, it is required to drill most
grouting holes
in an S-shape, not vertically. That means the starting point of
grouting holes
is away from the excavated diameter. After drilling through the
alluvium,
boreholes head to grouting positions close to the excavated diameter.
While
drilling and grouting the bedrock sediments, passive freezing,
excavation, and
bricklaying can proceed in the frozen sections
Table 1. Final
injection pressures are selected based on hydrostatic pressure,
which is controlled by depth
Depth of
grouting
|
<300
|
300-500
|
500-700
|
>700
|
Injection
pressure (mPa)
|
7 - 9
|
8 - 14
|
10 - 18
|
18 - 25
|
Grouting holes are
usually drilled using a thousand-meter boring machine and other
specialized
equipment. JJX-3 clinometers are used in normal grouting segments while
JDT-6
turbine clinometers are employed to detect the gradient of boreholes,
and to
help correct the final boreholes. The normal distance between
observations is
20 m. Directional drilling uses JDT-6 turbine clinometers, along with
5LZ120*7.0 or 5LZ95*7.0 stages of screw drills and oriented wedges.
Group pumps
and group termination plugs are used for grouting
The simultaneous process
is outlined in Figure 1. First, crews simultaneously drill the freezing
and
grouting holes on different circumferences. Meanwhile, shaft digging
and
bricklaying crews get ready (setting up the headframe, preparing
construction
stabilization and winding basis, etc.). As soon as the freezing holes
are
completed, the freezing station is installed and freezing is started.
Meanwhile, grouting is going on in the bedrock sediments. When active
freezing
is completed, shaft trial-digging commences while grouting in the
bedrock
sediments is still under way. Grouting in the bedrock sediments is
finished by
the time that excavation and bricklaying is finished in the alluvium,
allowing
the process to continue in the bedrock sediments
This simultaneous
technique shortens construction time (by 25-33%) and assures quality of
freezing and grouting, allowing earlier coal extraction. By way of
example,
details are provided on the application of simultaneous freezing and
grouting
in constructing the Xuandong No. 2 ventilation shaft in Hebei
(now
underway). The shaft has a diameter of 6.0 m and a depth of 870.0 m.
The
alluvium thickness is 92.2 m and the depth of freezing is 110.0 - 130.0
m. The
diameter of the circumference for both the freezing and grouting holes
was 11.0
m, while the diameter of the circumference for the S-shape grouting
holes was
21.0 m.
Sequence of Construction
(Refer to Figure 1)
a. Three crews drilled three
vertical grouting
holes at a circumference of 11 m. The height of the grouting stage was
110-300
m. The purpose of drilling these holes
was to
enhance
grouting effectiveness in the
fractured rock
under
the alluvium. Furthermore, such holes can be used as freezing
holes
later during shaft construction.
b. Another three crews
drilled on a circumference of 21.0 m.
Two of them constructed30
separate freezing holes, with depths ranging from 110 to 130 m
and 6
oriented grouting holes with a depth of 880 m.
c. Meanwhile, crews prepared
for excavating and
bricklaying the shafts.
d. After the freezing holes
were drilled,
freezing stations were installed
and active
freezing commenced. When active freezing
was completed, excavation and bricklaying started.
e. Digging and
bricklaying in the alluvium part of the shaft was carried out
simultaneously with
passive freezing in the freezing station, and GPG for bedrock
sediments. The
GPG was completed when excavation and bricklaying was finished in the
alluvium,
allowing operations to continue in the bedrock sediments
The shaft is under
construction now. The expected time for freezing, grouting, and shaft
digging
is 584 days. Many shafts have now been built using this simultaneous
technique,
and it is clear that it generates significant economic benefits.
Submitted
June
14,2005;
accepted Sept.
19,2005