- Open Access
Effect of bolting on roadway support in extremely weak rock
© The Author(s) 2016
- Received: 15 April 2016
- Accepted: 9 August 2016
- Published: 17 August 2016
In mine roadway support operations, floor bolting not only played a role in floor heave control, but also in reinforcing roof and its two sides. Correspondingly, bolting of roof and two sides also played a part in floor heave control. To quantify the effect of such bolting, based on roadway support in extremely weak rock, three physical models were produced and tested in laboratory. Through comparison of their displacements in three physical simulation experiments, the reinforcing effect of bolting in extremely weak rock roadways was quantified. Reinforcing coefficients was defined as displacement ratio between original support and new support regime. Results indicated that the reinforcing coefficients, for bolting of roof and its two sides, on floor, two sides, and roof reached 2.18, 3.56, and 1.81 respectively. The reinforcing coefficients for floor bolting on floor, two sides, and roof reached 3.06, 2.34, and 1.39 respectively. So in this extremely weak rock, the surrounding rock should be considered as an integral structure in any support operation: this allows for better local strength improvement, and provided future design guidance.
- Physical simulation
- Roadway supporting
- Extremely weak rock
- Reinforcing coefficient
Roadway control in soft rock is a problem in many mines (Bilir 2011; Ghiasi et al. 2012; Serafeimidis and Anagnostou 2013; Thomas et al. 2013). When roadways are excavated, pressure is redistributed. Under this mining induced stress, roadways in soft rock undergo large, continuous deformation as they are excavated.
Three rock mass classification systems, the rock mass rating (RMR), the rock mass strength (RMS), and the slope mass rating (SMR) have been widely applied to areas of hard rock and weak rock in civil engineering. Brook and Hutchinson (2008) indicated that the relative weightings of the different parameters within the RMR, RMS, and SMR classification schemes would need modifying for weak rock masses, but the precise details of this were difficult to determine. In mine, based on RMS, content of mudstone, features of joint planes, and the mechanism of plastic deformation, weak rock was classified into four types (He et al. 2002): extremely weak rock (swelling soft rock), high-stress soft rock, jointed soft rock and mixed soft rock.
Extremely weak rock, with a lower compressive strength, a lower tensile strength, a reduced apparent cohesion and poor cementation, always contains swelling (smectite) clay minerals. This rock was vulnerable to weathering, which became more serious when in contact with water. When roadways are excavated in this rock, even in lower stress conditions (<25 MPa), the cross-sections can be disrupted and the plastic failure zone in surrounding rock always extends significantly. Indeed, mudstone in Cretaceous strata always exhibits such behaviour.
While investigating the mechanisms underpinning swelling behaviour in such soft rock, Christoph et al. (2011), Luciano and Eduardo (2012) indicated that the dissolution of anhydrite and precipitation of gypsum occurred upon the uptake of water. Einstein (1996) indicated that, in argillaceous rocks, swelling is caused by one, or a combination of, intracrystalline, osmotic, and mechanical effects. In a tunnel, Butscher et al. (2011a, b) concluded that excavation-induced groundwater inflow into anhydritic layers and caused rock swelling. Pejon and Zuquette (2002) and Moosavi et al. (2006) indicated that deformation of a roadway in swelling soft rock depended on mineralogy, lithology, ground characteristics, hydrology, stress state, and weathering conditions. In addition to this, using an artificial neural network, Doostmohammadi et al. (2008) investigated the swelling potential of mudstone. Lo and Hefny (1996) developed rheological models, which could be used to predict the swelling effects on a circular tunnel.
High-stress soft rock, always with less argillaceous components, demonstrates a high compressive strength, a high tensile strength and a good cohesion. This rock can be deformed obviously under high stress conditions (>25 MPa). High stresses were the main factor causing roadway deformation. For the same strength of surrounding rock, when under different stress conditions, the roadway may demonstrate different deformation patterns. In low-stress conditions, the roadway was stable and no obvious deformation occurred; however, when under high stress conditions, the roadway may demonstrate rheological behaviour, such as soft rock deformation: the deformation can be large, and thus causes difficulties in roadway control. Strata at 1000 m below ground have always presented such characteristics.
Jointed soft rock, always with less and even no argillaceous components, contains many joints in rock body. In such soft rock, each block demonstrates high strength. But the whole body exhibits soft rock characteristic and can be deformed largely, which is induced by joints’ plane slipage and dilatation, under mining induced stress.
Mixed soft rock is the combination of above three soft rocks, including high stress-swelling soft rock, high stress-jointed soft rock and high stress-swelling-jointed soft rock.
To control soft rock deformation, numerous studies have advocated various support strategies and tested in situ.
In swelling soft rock, support measures include either the application of a strong, rigid supporting formwork to limit deformation, or allowing floor heave to release swelling pressures, or a combination of both (Christoph et al. 2011). In Canada, a tunnel situated in the Queenston Formation, South Ontario, was supported by a double shell lining system, which included an initial lining of shotcrete, steel ribs, and rock dowels, and a final lining of waterproofing membrane and cast-in-place concrete (Ansgar and Thomas 2010). In the T13 tunnel, Ankara–Istanbul High-Speed Train Project, a heavier, non-deformable support system was applied (Aksoy et al. 2012). In tertiary soft rock roadway in Liuhai coal mine, China (Yang et al. 2015), bolt-mesh-cable and double-layer-truss supports were used to control the large rheological deformation. Shen (2014) proposed a support system, which included an optimal cable/bolt arrangement, full length grouting, and high-load pre-tensioning of bolts and cables.
In high-stress soft rock, according to Chang et al. (2013), use of a double layer of 40 U-shape steel sets, cables, resin bolts in both ribs and roof, and swellex bolts in floor were used in roadway support works. Sun et al. (2014) indicated that the asymmetric coupling support of bolt net spray, with anchor cables, and floor bolts could enhance the stability of deep roadway. Li et al. (2015) proposed a combined support system including: high-toughness sealing layer, hollow grouting cables, and full-length anchor bolts. Li et al. (2014) recommended a coupling support of double yielding shells, which can control roadway deformation in high stress soft rock.
In most control measures, bolting in surrounding rock was used to better effect (Guo et al. 2012; Wang et al. 2012; Meng et al. 2013; Kang et al. 2015; Sun and Wang 2011; Wang et al. 2015; Yuan et al. 2014; Yu et al. 2015; Zhao et al. 2015). Since the surrounding rock was treated as a whole structure, bolts in the floor not only played a role in controlling floor heave, but also played a role in reinforcing roof and two sides. Correspondingly, bolts in roof and two sides not only played a part in reinforcing the roof and the two sides, but also played a role in controlling floor heave. To quantify the effect of bolting, based on extremely weak rock in the No. 1 mine of Chagannuoer, three physical simulation experiments were conducted. Through displacement comparison in three experiments, we analysed the quantitative effect of boltig in the roof and two sides, and the effect of floor bolting in reinforcing the surrounding rock, which would provide guidance for roadway support in future.
The No. 1 mine of Chagannuoer (NMC), located in Xilin Gol League, Inner Mongolia, China, with production capacity of 8.0 Mt/a, was under construction. Within the scope of mining area, all coal seams were stable with a near-horizontal dip angle. There was a slight syncline (20 km in length, 4–10 km in width) in the mining area. In general, the geological structure in this area was simple.
Mechanical parameters of the strata
Bulk density (average)/(t/m3)
Compressive strength (average)/MPa
Tensile strength (average)/MPa
Apparent cohesion (average)/MPa
Internal friction angle/(°)
Ground stress measurements
σ hmax (MPa)
σ hmin (MPa)
σ v (MPa)
σ hmax/σ v
In this situation, the maximum principal horizontal stress was perpendicular to roadways along a North–South direction and parallel to roadways along an East–West direction. When the stress acted perpendicular to the roadway, it would induce much more damage than those stresses parallel to it. This was confirmed by large deformation (19.3 mm/day of two sides moving inward, and total amount 596 mm in monitoring period) in the return air cross-cut, and small deformation in the return airway in the first month after roadway excavation and support installation. About one month later, because of large swelling pressure in this extremely weak rock, the return airway also suffered large deformation (12.5 mm/day of two sides moving inward, and total amount 426 mm in monitoring period).
Under the influence of extremely weak rock, ground stress and swelling pressures, the roadway deformed to a significant extent, in which floor heave was particularly serious. Steel sets or bolts alone had little effect in controlling roadway deformation. So, a variety of supporting methods, such as closed 36U-shape steel sets and bolting, a pair of 12I-beam steel sets and bolting, were tested in situ. Although offering significant support strength, roadways deformed continuously and those supports failed to achieve an ideal effect. Because of the presence of expansive clay in this extremely weak rock, grouting was unfeasible. So, the alternative was to improve the strength of steel sets, or to improve the strength of bolts, or to improve the strength of both. This project mainly researched the effect of bolting in this extremely weak rock, included assessment of effect of roof and both sides bolting, and the effect of floor bolting.
In prototype of Models 1 and 2, the type of anchor used in roof and two sides was Φ 20 × 2500 mm, full-size grouted. Closed 36U-shape steel set was installed in every 700 mm. At each 700 mm centre, a row of bolts was installed. In every row, each two bolts were at 700 mm in spacing (i.e. a row of bolts was installed between each pair of steel sets). The closed steel set had 1000 mm invert, which was filled with concrete.
Ratio between prototype and model
Physical tests are commonly used in geological mechanics analysis. Similarity theorem indicats that model’s each element is similar to prototype’s corresponding element. Comprised by similar elements, the field and physical phenomenon between model and prototype are similar to each other. Based on similar physical phenomenon, the mechanics analysis in prototype can be deduced by model tests.
According to similarity theorems, as in prototype, material in model should be complied with Hooke’s law. Stress state of all points in the model should satisfy equilibrium equation, compatibility equation and geometric equation simultaneously. So the ratio (Yuan 1998) between prototype and model were defined as
In which, l p , γ p , E p , δ p were the size, volume-weight, elasticity modulus and displacement in prototype, respectively. l m , γ m , E m , δ m were the size, volume-weight, elasticity modulus and displacement in model, respectively.
In each model, there were four displacement meters S 1, S 2, S 3 and S 4 for deformation measurement, which were set in roof (S 3), floor (S 1), and both sides (S 2 and S 4), respectively.
Loading grades in Model 1 (MPa)
Loading grades in Model 2 (MPa)
Loading grades in Model 3 (MPa)
The reinforcing coefficient on two sides and the floor were treated in the same way. Displacements of the floor, right side, roof, and left side can be obtained by measurements at S 1, S 2, S 3, and S 4. Two sides displacements were almost the same, so left side displacement was used in this analysis only. When Q > 1, it is indicated that the new supports could control roadway deformation. The larger the value of Q, the better the new supports in deformation control. When Q < 1, it is indicated that the new supports could not control roadway deformation.
Analysis of bolting in the roof and on two sides
Before the eighth load increment was applied to Model 1, the displacements of roof, two sides and floor reached 19.0, 22.8 and 36.0 mm, respectively. And before the twelfth load increment was applied to Model 2, the displacements of roof, two sides and floor reached 17.8, 14.3 and 29.0 mm, respectively. Under these two conditions, the roadway deformed to a significant extent, but remained stable. When the eighth load increment was applied to Model 1, and the twelfth to Model 2, the roadway deformation increased and failure ensued soon after. The displacements here were so large as to be meaningless when analysing the role of any supports, so displacements before these stages were used in subsequent analysis.
Roadway displacements upon completion of the seventh load increment
Left side convergence/mm
Right side convergence/mm
Analysis of floor bolting
Before the twelfth load increment in Model 2, and the fourteenth in Model 3, the roadway remained stable. Thereafter the roadways were damaged within a few minutes: displacements before the twelfth load increment in Model 2, and the fourteenth in Model 3 were analysed.
Roadway displacements upon completion of the 12th load increment
Left side convergence/mm
Right side convergence/mm
The surrounding rock formed an integral structure: when one area was reinforced, other areas were affected. Based on extremely weak rock in NMC, three physical models were produced and tested in laboratory. Through physical simulation, this research quantified the effect of bolting as a roadway support mechanism.
The results indicated that the reinforcing coefficients of bolting in roof and sides, on floor, two sides and roof, reached 2.18, 3.56, and 1.81 respectively. The reinforcing coefficient of floor bolting, on floor, two sides and roof reached 3.06, 2.34, and 1.39 respectively. The data show that roof and sides bolting had the biggest effect in reinforcing two sides, bigger effect in floor strengthening, and big effect in roof controlling. Although no bolting was used in floor, the reinforcing coefficient on floor reached 2.18, showing a greater effect than on roof. Based on bolting in roof and two sides, floor bolting had the biggest effect in floor reinforcing, bigger effect in two sides strengthening, and big effect in roof controlling, which indicated that the effect of floor bolting decreased as distance to floor increased.
From above we could get that the overall effect on surrounding rock was remarkable. When roof and sides were reinforced, the floor heave decreased, and when floor strength was improved, convergence of two sides was better controlled. So in this extremely weak rock, surrounding rock should be considered as an entire, integral structure, when designing a support scheme: this will lead to better results than local strength improvement.
QL made substantial contributions to research conception, as well as ground stress measurement in site, physical experiments design and implementation, manuscript revised finally. WS was mainly involved in physical experiments data analysing and manuscript drafting. ZQ was mainly involved in ground stress data analysing and manuscript revising. All authors read and approved the final manuscript.
We wish to gratefully acknowledge Xizhu Li, director of No. 1 mine of Chagannuoer, who provided great help in ground stress measurement in site.
The authors declare that they have no competing interests.
The collaborative funding supported by Opening Project Fund of State Key Laboratory of Mining Disaster Prevention and Control Co-founded by Shandong Province and the Ministry of Science and Technology (No. MDPC2013ZR03), and the funding supported by Scientific Research Foundation of Shandong University of Science and Technology for Recruited Talents (No. 2014RCJJ027).
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