Bearing Capacity Analyses and
Optimization of U-shaped Steel Support
under Non-uniform Load
Fei Xue*, Nong Zhang, Xianyang Yu, Baoyu Li
School of Mines, Key Laboratory of Deep Coal Resource Mining, Ministry of
Education of China, China University of Mining and Technology, Xuzhou,
Jiangsu 221116, China;
e-mail: [email protected]
ABSTRACT
Due to the non-uniform load, the U-shaped steel support (USS) is easy to damage and its support
performance can’t be brought into full play. In order to improve the traditional USS to efficiently
support and stabilize the roadways. The finite element numerical simulation method is used to analyze
and optimize the bearing capacity of the USS under non-uniform load. The results showed that the
ability of the USS to defense pressure from top is strong, but the ability to defense pressure from sides
is weak. In some degree, non-uniform load on top is beneficial to the whole bearing capacity. Nonuniform load on shoulder is the worst to the support. Bolts (cable) lacked the support can improve the
bearing capacity for non-uniform load on sidewall, shoulder and single side. Three techniques to
optimize the bearing capacity of the USS have been promoted: bolts (cable) locked the support,
improving the bending stiffness of support structure, back filling of the support using T-shape pipes.
These techniques are successfully applied in the special return airway, long time stability has been
guaranteed.
KEYWORDS: U-shaped steel support, Non-uniform load, Roadway stability, ANSYS
INTRODUCTION
U-shaped steel supports (USS) are commonly used supports in coal mines [1-2]. It has many
advantages such as high increase resistance speed, high yielding, easy to install, wide adaptability
and reusable. However the cost of the USS is high and the transportation and installation of the
USS are more complex than the bolt [3]. It matters success or not of roadway support whether
promoting bearing capacity of the USS reasonably and reducing loss to increase its reusing times.
Due to the existence of the interspace between the support and surrounding rocks, the contact
state of supports and surrounding rocks is always irregular point-line and the support would bear
very large concentrated load and non-uniform load [4]. Therefore, the support is easy to damage
and its support performance can’t be brought into full play, as shown in Figure 1.
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Figure 1: Floor heave and support damage [5]
Much research has been conducted domestically and abroad to improve the traditional USS to
efficiently support and stabilize the roadways. Luo [6-7] has analyzed and expounded the backfill
technology for U-steel support and the performance requirements of backfill material. The filed
application shows that the support result of backfilling behind the USS with the designed ratio of
the backfill material is satisfactory and the stress distribution in the USS is uniform. You et al. [8]
has researched the mechanical structural model of the USS, the force-transferring mechanism and
yielding analysis of clamp overlapping using theoretical analysis method. Chen et al. [3] carried
out some measurements on the ultimate bearing capacity of the USS under five kinds of load
conditions. Then they have proposed an optimization scheme of support section and some
technical measures to improve the ultimate bearing capacity. Jiao et al. [5] proposed an integrated
shed system which is based on backfilling of chemical grouting material behind yieldable steel
sets, i.e., metal mesh plus backfilled chemical grouting material plus double resistant
geomembrane plus USS. These researches provide a theoretical basis and technical support to
improve the bearing performance of the USS and promote the development of the USS
technology.
In this study, the bearing capacity of the USS under non-uniform load was analyzed and
optimized by using the finite element numerical simulation method. And then some techniques to
optimize the bearing capacity of the USS have been promoted and successfully applied in the
roadway controlling.
BEARING CAPACITY ANALYSES OF USS UNDER
SINGLE SIDE NON-UNIFORM LOAD
The establishment of calculation model
The typical feature of the load borne by the USS is unilateral partial load. That is some or all
of load of the roadway left side is higher than the right side. Simple diagram for calculating as
shown in Figure 2 can be used to analyze support bearing capacity under unilateral partial load
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conditions and the improvements of bolts (cable) lacked the support on the bearing capacity of
the support in different parts and different levels of partial load.
F3
q3
F2
60
¡ã
q2
q4
q1
F1
Figure 2: Mechanical analysis diagram of the USS under unilateral partial load
Four kinds of typical partial load, such as non-uniform load on side (q1 > q2 = q3 = q4) nonuniform load on shoulder (q2 > q1 = q3 = q4) non-uniform load on top (q3 > q1 = q2 = q4) and
unilateral partial load as a whole (q1 = q2 = q3 > q4) were analyzed respectively. The level of
partial load can be expressed by the ratio of the load concentration of partial load parts to the load
concentration of right side. The function of locked bolts (cable) can be simplified as the
concentrated force, F1, F2, F3. F1 is the working resistance of the bolts, with a value of 20 kN. F2
and F3 are the working resistance of the cables, with a value of 40 kN.
The ANSYS software is used to analyze nonlinear plastic. Applying a percentage of the load
on each part of the supports when building model. The load gradually increases in proportion
along with load step until difficult to convergence. At this time the force borne by the whole
support is the bearing capacity.
The cross section of calculation model is shown in Figure 3. The ANSYS finite element
model is shown in Figure 4. Kinematic hardening model was used in FEM analysis and the
response behaviors of transient local stress strain for material was also considered. The elasticity
modulus, the Poisson's ratio and the yield strength of material is 200 GPa, 0.3 and 335 MPa
respectively.
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Figure 3: Cross section of calculation model
(a) Local of Entity model
(b) Global model
Figure 4: Finite element model
ANALYSIS OF THE CALCULATION RESULT
Bearing capacity of support was calculated with different partial load degree at different
positions, meanwhile the calculated results Q were used to compare with bearing capacity of
support under condition of uniform load Qu, as shown in Figure 5.
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1.2
1.2
Bolt
Non-bolt
Difference
1.0
1.0
0.8
Q/Qu
Q/Qu
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0
Cable
Difference
Non-cable
0.0
1.5
2.0
2.5
3.0
3.5
1.5
4.0
2.0
2.5
3.0
q1/q4
(a) Non-uniform load on side (q1 > q2 = q3 = q4)
Non-cable
Cable
Difference
1.2
1.0
0.8
Q/Qu
Q/Qu
0.6
0.4
-0.4
0.6
0.4
0.2
0.0
Bolt
Non-bolt
Difference
1.0
0.8
-0.2
4.0
(b) Non-uniform load on shoulder (q2 > q1 = q3 = q4)
1.2
1.4
3.5
q2/q4
1.5
2.0
2.5
3.0
3.5
4.0
q3/q4
(c) Non-uniform load on top (q3 > q1 = q2 = q4)
0.2
0.0
1.5
2.0
2.5
q1/q4
3.0
3.5
4.0
(d) Unilateral partial load as a whole (q1 = q2 = q3 > q4)
Figure 5: Bearing capacity curves of the support with different non-uniform load degree
at different positions
From Figure 5, we can give the law of bearing capacity of support under non-uniform load
condition, as follows:
(1) Under condition of non-uniform load at different positions of side, shoulder and
unilateral, bearing capacity of support decreases sharply with the degree of non-uniform load
increasing. When ratio of non-uniform load to uniform load exceeds 2.5, the descending speed
begins to be flat, but the support already at lower bearing status.
(2) When ratio of non-uniform load to uniform load at top of support is between 1.0 and 2.0,
the bearing capacity increases, which indicates the bearing ability of support top when imposed
roof pressure is stronger and the certain degree of non-uniform load of support top is beneficial
for the bearing status of whole support. Therefore, we should assure intimate contact between
support top and surrounding rock.
(3) Comparing with side and top of support, bearing capacity of shoulder has the biggest rate
of decrease when affected by non-uniform load. So non-uniform load at shoulder is the worst
condition for the bearing of support.
(4) The ratio of non-uniform load to uniform load in figure was used to show the effect of
bolt and cable. Under condition of non-uniform load at positions of side, shoulder and unilateral,
the working effect decreases gradually with the degree of non-uniform load increasing.
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(5) When the degree of non-uniform load is lower at position of support top, the increase of
number of bolts and cables will lower the bearing capacity on the contrary. With the degree of
non-uniform load ascending, bolts and cables comes into effect, and becomes more and more
vital.
BEARING CAPACITY OPTIMIZATION TECHNIQUES
Bolts (cable) lacked the support
From the bearing capacity analyses results of the USS above, we can know that the
improvement of bolts (cable) lacked the support on the bearing capacity of the USS is obvious.
Bolts (cable) lacked the support can provide a local stress compensation to partial load positions.
If the USS and the bolts (cable) are constructed separately, there is no an effective connection
between them. Due to the stiffness is different and the deformation is difficult to coordinate, the
USS and the bolts (cable) are easy to damaged one by one and the total bearing capacity of them
can’t be brought into full play. Thus, the USS and the bolts (cable) should be connected firmly for
the coordination of bearing.
A reliable connection clamp is shown in Figure 6. The shape of the clamp bending part is
consistent with the USS so that they can contact closely. There are two fixed “ears” on the
bilateral of the clamp. Each ear has an anchor-hole which is used to install the bolt (cable). In
order to punch easily, the anchor-hole is designed to be a long and shallow waist-shape hole. The
clamp can realize the reliable connection of bolts (cable) with the USS under the premise of not
weakening the mechanical performance of the U-shape steel. The position and the number of the
clamp can be adjusted according to actual needs. The clamp can improve the stability of the
support effectively and prevent the partial load parts from bending, reverse deformation and
dumping.
Bolt (cable)
USS
Anchor-hole
Ears
Nut or lockset
Clamps
Figure 6: Schematic diagram of clamp to fix the support
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Improving the bending stiffness of support structure
USS could work with higher resistance to roof pressure however with lower resistance to side
pressure. Even if working with uniform load, the middle part of USS legs has maximum bending
moment, which means that the leg of USS is the weak link of the whole support structure. So, it is
very important to improve legs’ bending stiffness for the capacity of whole USS.
The common method to deal with great deal of USS damage is adopting heavier USS.
Generally speaking the broken USS has similar yield position. For low side bending stiffness,
most USS has broken legs and good condition roof beams. Considering the connection between
roof beam and legs, we have to enlarge both of them, which is both costly and difficulty for
miners. By enhancing the weak part, the USS capacity could be improved without adopting
heaver USS.
The most convenient way to improve bending stiffness is change the section shape of USS,
one of which is shown in Figure 7. The section shape could be changed from groove to closed
loop by welding a piece of steel with 110 mm width and 5 mm thickness at the opening position
of U29. After welding, the module of bending section of U29 at three direction is W x1 = 117 cm3,
W x2 = 132 cm3, W y = 109 cm3, respectively, comparing with original U29 steel W x1 = 106 cm3,
W x2 = 106 cm3, W y = 102 cm3, which are improved by 11.3%、42.3% and 7.4%. The piece of
steal could be melted at USS legs from end to stop block or any needed position and its thickness
and width could be chosen by condition.
5mm
100mm
U29
Solder joints
Flat steel
110mm
(a) Flat steel welding manner
(b) Section properties after welding
Figure 7: Improving flexural behavior by welding flat steel
T-sharp pipe backfilling
Theory and practice show that backwall cavities play an important role in the stability and
bearing capacity of supports. The essence of backfilling is that filling materials (e.g. gangue,
cement, high water material) acting as the force-transmission medium of supports and
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surrounding rock, fill back cavities under a certain power(e.g. by hand, pumping, pneumatic
stowing) make the supports and surrounding rock in close contact and form a "surrounding rock –
backfilling - supports" mechanical bearing system.
T-sharp pipe backfilling is a practical technique for filling backwall cavities using grouting
equipment. As shown in Figure 8, consisting of grouting part and exposed part, T-sharp pipe is
welded by two DN15 seamless steel tubes. Internal between two tubes were connected to ensure
the slurry flow. Nozzle of grouting part is smashed into flat shape, and pipe body of grouting part
is slotted to make slurry outflow.
In order to connect with grouting pump, we processes thread in the end of exposed part.
Instead of processing thread, we use quick coupling. But the rubber sleeve or other forms of
protection must be used to protect exposed part. The specific implementation process of
backfilling: at the same time of shelving and supporting net, preset T-sharp pipe in design
position according to the support pattern(as shown in Figure 9); after the completion of shelving
and supporting net, preliminary shotcrete behind the supports a certain distance to closure the
surrounding rock and prevent leakage. When the initial shotcrete has a certain strength, begin
grouting backfill using T-sharp pipe to fill backwall cavities to form a whole back filling layer
wall.
The measures not only improve stress state and improve bearing capacity of supports, but
also form a grouting pad benefitting for improving grouting pressure of subsequent shallow hole
or deep hole.
300mm
500mm
Figure 8: Processing schematic diagram of t-shape grouting pipe
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Figure 9: Backfilling with t-shape grouting pipe
ENGINEERING APPLICATION
The problematic roadway is the ventilation roadway of the first north mining area of Gubei
Coal Mine, Huainan Mining Corporation, China. The roadway is located in sand mudstone and
shale interbed. The sliding surface and fracture of rock were well developed. The main roof of the
roadway is mainly fine sandstone, coal and mudstone, and the main floor of the roadway is sand
mudstone interbed, silt-finestone. The workface layout of north mining section of Gubei Coal
Mine is shown in Figure 10.
0
-62
The ventilation roadway of the first north mining area
Figure 10: Workface layout of north mining section of Gubei Coal Mine.
Early 350 m of the roadway is supported by the USS alone. Due to the impact of the fault and
sliding surface, the USS is loaded by non-uniform force. Thus, the roadway presents regional
non-uniform deformation and failure, as shown in Figure 11. The results show that individual
using the USS is difficult to guarantee the long-term stability of the roadway.
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a Subsidence deformation of left shoulder
(b) Subsidence deformation of right shoulder
(c) Subsidence deformation of left shoulder and top
(d) Subsidence deformation of right shoulder and
beam
top beam
Figure 11: Field photos of the non-uniform deformation and failure of the roadway
The rest of the roadway is supported by the combined support technology consisting of the
USS, bolts locked the supports, T-sharp pipe backfilling, which incorporates the support and the
surrounding coal into a flexible support system. After implementation of the comprehensive
support technology, the non-uniform deformation of roadway has been effectively controlled, as
shown in Figure 12. Long time stability of the roadway has been guaranteed.
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Figure 12: Field photo of the support effect
CONCLUSIONS
This research aims at researching the bearing capacity of the USS under non-uniform load
and providing a optimization techniques to improve the traditional USS to efficiently support and
stabilize the roadways. Firstly, the bearing capacity of the USS under non-uniform load is
investigated by ANSYS. The analyses results indicate that the ability of the USS to defense
pressure from top is strong, but the ability to defense pressure from sides is weak. In some degree,
non-uniform load on top is beneficial to the whole bearing capacity. Non-uniform load on
shoulder is the worst to the support. Bolts (cable) lacked the support can improve the bearing
capacity for non-uniform load on sidewall, shoulder and single side. Secondly, three techniques to
optimize the bearing capacity of the USS have been promoted: 1) Bolts (cable) lacked the
support, 2) Improving the bending stiffness of support structure, 3) T-sharp pipe backfilling.
These techniques can incorporate the support and the surrounding coal into a flexible support
system. It can increase the contacting area between the supports and the coal walls; consequently
the self-stability of the surrounding coal can be improved. At last, the research results are
successfully applied in the support of the ventilation roadway of the first north mining area of
Gubei Coal Mine, Huainan Mining Corporation, China. Which indicates that the implementation
of the combined support technology consisting of the USS, bolts locked the supports, T-sharp
pipe backfilling is a pretty good solution in supporting the roadways.
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ACKNOWLEDGEMENTS
This study was funded by the Program for Changjiang Scholars and Innovative Research
Team in University (PCSIRT) under Contract No. IRT1084 and by the Research Fund for the
Doctoral Program of Higher Education of China (RFDPHEC) under Contract No. 20110095110013
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