ABSTRACT
Research is currently being conducted at the
Colorado School of Mines in the use of high-pressure water for scaling down
loose rocks in underground mine openings. In addition to improved miner safety,
the adhesion strength of shotcrete applied to rock surfaces treated with
high-pressure water is also increased significantly resulting in improved
overall support capabilities of the shotcrete layer. Test results show an increase in adhesion strength by a factor of
four on a concrete test wall cleaned with water at 21 Mpa (3000 psi) as
compared to a surface cleaned at 0.7 Mpa (100 psi).
INTRODUCTION
Shotcrete, (sprayed concrete) is defined as
pneumatically applied mortar or concrete projected at high velocity (American
Concrete Institute). Shotcrete normally
consists of a mixture of portland cement and fine aggregates and various
admixtures including microsilica, accelerators or retarders, plasticizers and
occasionally steel fibers for reinforcing.
While the use of shotcrete for the support of underground openings was
initially developed by the civil engineer industry, the mining industry has
become a major user of shotcrete for underground support in recent years.
The main purpose of the shotcrete is to
help the rock mass to maintain its integrity.
Shotcrete works by completely coating the rock surface thereby reducing
rock movement and preventing loosening of rocks around the opening. Good adhesion between the rock and shotcrete
layer is crucial. Tests performed in
Sweden (Holmgren, 2001) have shown that the primary failure of good-quality
shotcrete lining on hard rock is adhesion failure. If the adhesion strength is
low, the weight of loose blocks of rock pushing down on the shotcrete layer may
cause sections to separate and gaps can be formed between the shotcrete layer
an the rock surface (Fig.1). The
subsequent bending of the shotcrete will cause tensile failure.
In order to obtain good adhesion the rock
surface must be properly cleaned to remove dust, oil, and small pieces of loose
rock. A common practice is to clean
the rock surface using the shotcrete application equipment by blowing
compressed air and water through the shotcrete nozzle. Water pressures greater than 100 psi are
difficult to achieve and the effect is more of a wetting down of the surface
rather than effective cleaning. While
it is possible to use high-pressure water pumps for cleaning, this is seldom done
in practice.
As part of
a project funded by the National Institute for Occupational Safety and Health
(NIOSH) administered through the Western Mining Resource Center, research is
being conducted at the Colorado School of Mines (CSM) in the use of high-pressure
water for scaling down loose 
Figure
1. Tensile failure of shotcrete
resulting from insufficient adhesion strength.
rocks
in underground mine openings, (Kuchta,
2001). While the primary purpose of the project is to reduce scaling accidents
by removing miners from high risk areas, the increase in bond strength of
shotcrete applied to rock surfaces treated with high-pressure water is also
being investigated. An experiment has been set up in the CSM experimental mine
with the purpose of quantifying the increase in adhesion strength of shotcrete
as a function of the water pressure used to clean the surface. This paper describes the experiment and
summarizes the results obtained as well as similar tests performed at LKAB’s
Kiruna mine.
KIRUNA TESTS
A project
with the aim to improve the design of shotcrete as rock support was recently
initiated at LKAB's Kiruna mine
(Malmgren and Svenson,1999). The main
support system at the mine is untensioned fully grouted dowels and
shotcrete. 90% of the shotcrete is
unreinforced. The average thickness of
the shotcrete layer is about 4 cm (1.6 in). Approximately 20,000 m3
(26,000 yd3) of shotcrete is used as rock support in the mine per
year. The purpose of the shotcrete is to help the rock mass maintain its
integrity. The adhesion strength
between the shotcrete and wall rock is therefore crucial.
A mapping
program covering over 7 km (4.3 mi) of drifts reinforced with shotcrete was
carried out. Two basic failure types
were identified, (Fig. 2), fallout of shotcrete only indicating poor adhesion
of the shotcrete to the rock and fallout of rock and shotcrete together
indicating areas of weak rock.
a) b)
Figure
2. Two basic types of shotcrete
failure, a) fallout of shotcrete only indicating poor adhesion, and b) fallout
of shotcrete and rock indicating zones of weak rock, (Malmgren 1999).
Scaling is
performed in all drifts and crosscuts and usually done with a boom mounted
hydraulic scaling hammer. Before
application of the shotcrete the rock surface is cleaned using water at a
pressure of about 0.7 Mpa (100 psi). This method of surface preparation was
called the "Normal" treatment.
Water-jet scaling was tested as an alternative to mechanical
scaling. A prototype rig was built
which used a water pressure of 20 Mpa
(2900 psi) and a flow rate of 0.21 m3 (55 gal) per minute.
These
methods of surface preparation were evaluated in seven different crosscuts
situated within the iron ore (a fine-grained magnetite). Adhesion tests were performed using test
equipment designed according to the Swedish standard SS 13 72 42. The results
of the tests are summarized in Table 1.
An
analysis of the results indicates that the mean adhesion strength increased
from 0.21 Mpa (30 psi) with the normal treatment to 0.61 Mpa (88 psi) for
surfaces treated with high pressure water jet scaling. This is an improvement by a factor of about
3. It can also be seen that most of the
failures for the normal treated surface are within the rock. The distribution of failures shifts towards
failure between the shotcrete and the rock with high pressure water jet
scaling. One possible explanation for
this is that the outer layer of rock weakened by blasting is cleaned away with
water jet scaling. The shotcrete then
bonds with a cleaner more homogeneous rock surface, which is of higher strength
than the surface cleaned by the normal treatment.
CSM SHOTCRETE ADHESION TESTS
Test area and
preparation
An
experiment was set up in the Army tunnel, (Anonymous, 1984) of the CSM
Experimental Mine, Idaho Springs Colorado, with the purpose of quantifying the
increase in adhesion strength of shotcrete as a function of the water pressure
used to clean the underlying surface.
The area chosen for the tests is within a quartz-feldspar gneiss. This unit consists primarily of quartz and
potassium feldspar with minor amounts of muscovite. The rock is hard and
competent.
A section
of rib approximately 2 m (6 ft) high and 6 m (18 ft) long was divided into 6
vertical panels. The width of each panel was about 1 m (3 ft). Each panel was spayed to thoroughly clean
the rock surface using different water pressures. The pressures used were 0.69, 6.9, 13.8, 20.7, 31.0, and 41.4 Mpa
(100, 1000, 2000, 3000, 4500, and 6000 psi). Tarps were used to cover the panels
after cleaning to protect them from debris when spraying the neighboring
panels.
In addition to the rock test panels, a
concrete test panel was also constructed along the rib of the drift directly
across from the rock test wall. The
purpose of the concrete test panel was to provide a reference to compare
results of the adhesion tests. The concrete test panel was about 2 m (6 ft)
high and 3 m (9 ft) long. The panel was
divided into 6 sub-panels, each about 1 m by 1 m (3 ft by 3 ft). The panels were treated with the same water
pressures as the rock panels. The panel was constructed such that the
sub-panels were completely covered with plywood, which could be removed when
spaying the sub-panel. The purpose of
the plywood covers was to prevent the treated surfaces from being contaminated
with debris when spraying the surrounding sub-panels. Figure 3 shows
schematically the layout of the sub-panels and the water pressures used to
treat the surfaces.
Figure
3. Layout and water pressures used to
treat the 6 sub-panels on the concrete test panel.
The compressive strength of the concrete
was tested using a total of twelve 1 foot high, 6 inch diameter standard
concrete test cylinders. Four cylinders were tested at 7 days, 14 days, and 28
days. The final 28 day compressive
strength of the concrete was found to be 26.1 Mpa (3780 psi).
High pressure pump
A
high-pressure water pump capable of producing pressures up to 69 Mpa (10000
psi) at about 0.015 m³ (4 gpm) was fitted with a pressure gauge and a pressure
relief valve that could be used to regulate the water pressure and used for
cleaning of the rock panels. It could be seen that at 21 Mpa (3000 psi) a
transition occurred in the effectiveness of the cleaning. At 21 Mpa (3000 psi )
the water jet was of sufficient power to remove loose rocks fist size and
larger rocks. At 41 Mpa (6000 psi) a
sandblasting type effect was noted. The
wand operator was bombarded by small particles of rock removed from the rock
wall. When cleaning the concrete test panel it could be seen that at 41 Mpa
(6000 psi) the water jet completely removed the outermost layer of cement
creating etched lines along the surface.
Shotcrete
application
For
application of the shotcrete layer to the rock wall and concrete test panels, a
wet mix shotcrete was used. The
quantities for a one cubic yard shotcrete mix used are given in Table 2. According to the supplied specifications,
the mix will reach at least 4000 psi in 28 days with a 4 inch maximum slump and
air content of 3% to 5%. The liquid accelerator Shot-Set 250 provided by
Shotcrete Technologies, Inc was mixed at the nozzle while applying the
shotcrete.
Table
2. Quantities for one cubic yard eight-sac shotcrete.
|
Cement
|
(Type I from Holnam)
|
611
lbs
|
|
Fly
Ash
|
Class C
|
141
lbs
|
|
Fine
Aggregate
|
C-33 Washed Sand
|
2141
lbs
|
|
Course
Agg.
|
#8 Washed Rock
|
511
lbs
|
|
Air
Agent
|
AirTite 60, WR Grace
|
as
needed
|
|
Water
Reducer
|
WRDA 86, WR Grace
|
35
oz
|
|
Water
|
|
38
gal
|
The compressive strength of the concrete was
tested using a total of nine 1 foot high, 6 inch diameter standard concrete
test cylinders. The final 28 day compressive strength of the concrete was
estimated to be 27.9 Mpa (4050 psi).
Adhesion test
equipment
Equipment
for testing the adhesive strength of shotcrete was purchased, (CGE). The equipment is intended for determining
the adhesive strength of sprayed concrete (shotcrete) against rock and between
various layers according to the Swedish Standard SS137243. The equipment works in such a way that a
circular hole is first drilled through the layer to be tested and a few
centimeters into the underlying rock.
This produces a core that is still attached to the rock. A cone shaped friction grip ring is then
placed around the core, which is coupled to a tension device. The grip around the core becomes stronger as
tension is applied. The equipment consists of four parts:
·
Drill
bit
·
Core
sleeve
·
Tension
device
·
Recording
unit
The drill
bit consists of an inner drill bit with an inner/outer diameter of 72/86 mm and
an outer drill bit, variably adjustable, having an inner/outer diameter of
104/111 mm. The standard drill bit is
160 mm long. The drilling starts with
the outer drill bit fixed in its upper position. With the inner bit, a hole is drilled through the adhesion zone
and into the lower surface, i.e. the rock.
When the lower surface has been penetrated a few centimeters, the drill
is stopped and the outer bit is released by means of a socket head cap key and
is steplessly pushed down to the surface layer and re-fixed. The rock drill is started and the drilling
is terminated when the outer grove is a few millimeters deep. By this procedure, two drill grooves that
are completely parallel have been obtained.
The drilling process is illustrated in Figure 4.
When the drilling is complete, a cone
shaped friction grip ring of the core sleeve is then placed around the core.
The tension
device consists of three legs, a hand driven worm gear unit run on ball
bearings, a tension rod with a strain gauge used to obtain the breaking load,
and a threaded sleeve which is threaded into the core sleeve. The three legs of the device are placed in
the outer groove. By the fact that the
two grooves are completely parallel, a pure tensile force is obtained at
loading. The tension device attached to
the friction clamp is shown in Figure 4 c.
Figure
4. Double core drilling bit showing the
drilling of the outer groove (a) the inner core (b), and the tensioning device
(c).
The recording unit is located in a portable
case that also carries the tension device.
The signal from the strain gauge is connected with a digital display
with peak hold function for reading the breaking load. The signal from the strain gauge is also
transmitted to a printer with a paper strip where the breaking load is
recorded. The loading capacity is 10
kN, which means a tensile stress of up to 2.5 MPa can be measured. Chargeable batteries in the recording unit
are large enough to last a normal working day.
Adhesion Test
Results
Approximately 36 cores (6 per sub-panel)
were drilled and pulled on the concrete test wall. The results obtained are shown in Figure 5. A clear increase in bond strength can be
seen from about 0.45 Mpa on the surface treated with 0.69 Mpa (100 psi) water
to about 1. 90 Mpa on the surface treated with 20.7 Mpa (3000 psi) water
jet. This result indicates an increase
in bond strength by roughly a factor of four.
No significant increase in bond strength beyond this was seen on the
panels treated with higher pressure.
About half the pull tests at higher pressure resulted in failures within
the shotcrete instead of at the bond interface.
Results from the core testing on the rock
wall were difficult to interpret. The
majority of the breaks on all panels occurred within the rock itself due to the
low tensile strength of the rock.
Breaks occurred mostly along planes of weakness within the orthoclase
feldspar. In order to obtain
meaningful results, future tests should be performed on a host rock with a tensile
strength that exceeds the bond strength found with the tests performed on the
concrete wall.
Figure
5. Shotcrete to concrete bond strength
versus water pressure used to treat concrete wall surface.
CONCLUSION
Using high-pressure water to prepare
surfaces significantly increase the adhesion strength of shotcrete applied to
the surface. Test results show an
increase in adhesion strength by a factor of four on a concrete test wall
cleaned with water at 21 Mpa (3000 psi) as compared to a surface cleaned at 0.7
Mpa (100 psi). It appears that the maximum effect is achieved when a pressure
of 3000 psi or greater is used. This
pressure greatly exceeds that commonly used in underground mining operations
today. Better adhesion should increase the overall support capability of the
shotcrete layer. Further studies are required to better quantify the increased
support characteristics resulting from better adhesion in mining
environments. By combining the use of
high-pressure for scaling with shotcrete application, it should be possible to
reduce the overall ground support costs while increasing worker safety.
ACKNOWLEDGE MENTS
This
publication was supported by Cooperative Agreement number U60/CCU816929-02 from
the Department of Health and Human Services, Center for Disease Control and
Prevention (CDC). Its contents are
solely the responsibility of the authors and do not necessarily represent the
official views of the Department of Health and Human Services, CDC. Support provided by Department of Health and
Human Services, CDC, is greatly acknowledged. The work presented is part of the
Health and Safety research activities currently carried out at Western Mining
Resource Center (WMRC) at the Colorado School of Mines.
REFERENCES
Anonymous, 1984, Colorado School of Mines,
Department of Mining Engineering, Idaho Springs Tunnel Detection Test Facility,
Informational brochure.
CGE,
Swedish Concrete and Grouting Equipment AB, Box 48 s 81425 Älvkarleby, Sweden
Holmgren, J., 2001, Shotcrete Linings in
Hard Rock, in Hustrulid and Bullock, editor, Underground Mining Methods,
Engineering Fundamentals and International Case Studies, Society for Mining,
Metallurgy, and Exploration, Inc., pp 569-577
Kuchta, M.,E., 2001, The Use of
High-Pressure Water for Scaling of Loose Rocks in Mine Openings, in Proceedings
of the 32nd Annual Institute on Mining Health, Safety & Research, Salt Lake
City, Utah, August, 2001
Malmgren, L. and Svenson, T. ,1999,
Investigation of important parameters for unreinforced shotcrete as rock
support in the Kiirunavaara Mine, Sweden, in Rock Mechanics for Industry,
Amadei, Kranz, Scott & Smeallie (eds), A.A.Balkama, Rotterdam, ISBN 90 5809
052 3
Swedish Standard SS 13 72 43, 1987, Concrete
testing –Hardened concrete, shotcrete and plaster –Adhesion strength, (in
Swedish)