International Journal of Innovative Research in Advanced Engineering (IJIRAE) ISSN: Issue 10, Volume 5 (October 2018)

Transcription

1 COMPARISON BETWEEN THIN SPRAY-ON LINERS AND SHOTCRETE AS SURFACE SUPPORT MECHANISMS IN TUNNELS Samuel Jjuuko * Department of Civil Engineering, University of Cape Town, South Africa sjjuuko1@gmail.com Denis Kalumba Department of Civil Engineering, University of Cape Town, South Africa denis.kalumba@uct.ac.za Manuscript History Number: IJIRAE/RS/Vol.05/Issue10/OCAE10084 Received: 13, October 2018 Final Correction: 26, October 2018 Final Accepted: 28, October 2018 Published: October 2018 Citation: Samuel, J. & Denis, K. (2018). COMPARISON BETWEEN THIN SPRAY-ON LINERS AND SHOTCRETE AS SURFACE SUPPORT MECHANISMS IN TUNNELS. IJIRAE::International Journal of Innovative Research in Advanced Engineering, Volume V, doi:// /ijirae.2018.ocae10084 Editor: Dr.A.Arul L.S, Chief Editor, IJIRAE, AM Publications, India Copyright: 2018 This is an open access article distributed under the terms of the Creative Commons Attribution License, Which Permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited Abstract Since the 1990 s, the mining industry has introduced diverse kinds of Thin Spray-on Liners (TSLs). The aim has been to replace the conventional methods of surface support like Shotcrete. Since then, TSLs have shown potential as alternative surface support systems with increased benefits. However, their application is largely based on experience, assumptions, field observations and cost considerations. This is due to the fact that mechanisms by which TSLs provide support are not yet fully understood. In this study, Phase2 v.7.0, a numerical analysis program was utilised to compare performance of these thin spray-on liners and Shotcrete in underground tunnel applications. The computer software was used to determine the induced stresses, deformations and developed plastic zone around the tunnel, with different support linings of Shotcrete and TSL, combined with rock bolts. A tunnel 10 m wide and 15 m high was considered. The tunnel was assumed to be through sandstone at a depth of 550 m. The strength of sandstone was represented by the generalised Hoek- Brown failure criterion with the uniaxial compressive strength of the intact rock equal to 50 MPa and the Geological Strength Index (GSI) equal to 50. The other Hoek-Brown parameters were determined using RocLab software. Numerical analysis showed that TSL thickness of 24.2 mm offers the same structural capacity as Shotcrete of 50 mm thickness. This is a significant reduction in material consumption with associated economic benefits. Keywords Surface Support; TSLs; Shotcrete; Numerical Analysis; Tunnelling; I. INTRODUCTION Since the 1990 s, diverse kinds of Thin Spray-on Liners have been developed by both the mining and construction industry. The intention has been to provide replacements for the conventional surface support methods, especially Shotcrete (Yilmaz, 2011). Shotcrete negatively impacts the mining operations with regards to costs, logistics and mining cycle times, due to large material volumes involved in its application (Ozturk and Tannant, 2010). Hence, there is a need for an alternative, with enhanced methods, in order to realise the business target of the mining industry of providing the planned return on investment made, without harm and within budget. Page 329

2 TSLs have the advantages of low volume, rapid application and rapid setting. They are also applied by simple equipments (Ferreira, 2012). These are all properties that ease logistics, improve on cycle times, increase mechanisation and improve safety (Adams and Baker, 2002; Tannant, 2001; Hermanus, 2007). However, their application in the mining industry is still in the initial stages. Therefore, its design as surface support systems is still based on experience, assumptions, field observations and cost considerations (Tannant, 2001; Yilmaz, 2011). This is because the mechanisms by which TSLs act to provide support are not fully understood (Saydam, Yilmaz and Stacey, 2003). This investigation used Phase 2 v.7.0, a numerical analysis program, to determine the induced stresses, deformations and developed plastic zone around a tunnel with different support linings of Shotcrete and TSL, combined with rock bolts. II. NUMERICAL MODELLING Currently there are no clearly well-defined rules for numerical modelling of tunnel support and lining design. However, there are three wide-ranging methods that have been developed over the recent years, as explained below (Rocscience, 2009): Closed form solution methods based upon the calculation of the extent of plastic failure in the rock mass surrounding an advancing tunnel and the support pressures required to control the extent of the plastic zone and the resulting tunnel deformation, Numerical analysis of the progressive failure of the rock mass surrounding an advancing tunnel and of the interaction of temporary support and final lining with failing rock mass, and Empirical methods based upon observations of tunnel deformation and the control of this deformation by the installation of various support measures. Each of these methods has its own advantages and disadvantages and are suitable for different conditions of projects and rock masses. Sometimes, the optimal solution for a given tunnel may require a combination of different methods at different stages of the design. A preliminary analysis for temporary support requirements may be carried out by the closed form solution methods and detailed final design by numerical analysis methods. Phase 2 is a 2-dimensional elasto-plastic finite element program for calculating stresses and displacements around underground openings, and can be used to solve a wide range of mining, geotechnical and civil engineering problems (Rocscience, 2009). It was employed in this examination. The main assumptions in the analysis method were: Rock mass is isotropic and homogeneous. Failure is not controlled by major structural discontinuities, Support response is elastic-perfectly plastic, Support is modelled as an equivalent uniform internal pressure around the entire circumference of the tunnel. Therefore, Shotcrete and TSL linings are closed rings, and mechanically anchored rock bolts are installed in a regular pattern, which completely surrounds the tunnel. A. Design Problem In order to compare Shotcrete and TSLs as lining systems, a tunnel of width 10 m and height 15 m was considered, chosen randomly basing on worst case scenario. The tunnel was assumed to be through sandstone at a depth of 550 m. The strength of sandstone was represented by the generalised Hoek-Brown failure criterion with the uniaxial compressive strength of the intact rock equal to 50 MPa, as determined in the laboratory, and the Geological Strength Index (GSI) equal to 50, theoretical value. The other Hoek-Brown parameters were determined by RocLab software (Rocscience, 2007). The rock mass support was to comprise of a radial array of 5 meter long pattern bolts on a 1 x 1 meter grid. The surface support was then modelled separately as either made of Shotcrete or TSL. B. Setting up the Model Boundary Conditions: The outer model boundary was set by considering the overburden of the tunnel. Hoek- Brown failure criterion was used to estimate yielded elements and plastic zone of the rock masses in the vicinity of the tunnel. A fixed (i.e. zero displacement) condition for the external boundary was considered, assuming infinite conditions. The external boundary was automatically generated within the software. Mesh: An automatic mesh, consisting of six-noded triangular finite elements, was generated. Finer zoning was used around the excavation. The mesh was of graded type with a gradation factor of 0.1. The default number of nodes on all excavations was set at 75 (Rocscience, 2009). The total number of elements generated was 1003 with nodes of Page 330

3 The poor quality elements were defined as those having the following properties: side length ratio (maximum/minimum) greater than 30 (Rocscience, 2009), minimum interior angle less than 2o (Rocscience, 2009) and maximum interior angle greater than 175o (Rocscience, 2009). The total number of bad elements was zero. Loading conditions: Field stress determines the initial in-situ stress conditions, prior to excavation. The loading conditions for vertical stress were taken as an increasing trend with depth, due to its overburden weight, and are estimated by: σ v = γh where γ is unit weight of the rock mass in MN/m 3 and H is the depth of overburden in meter. The vertical stress was calculated as 11 MPa assuming unit weight of 0.02 MN/m 3 and H of 550 m. Horizontal stress in rock mass is known to be variable at shallow depth. However, it tends towards a hydrostatic state in a deep environment (Hoek and Brown, 1978). Therefore, it is more difficult to estimate. The horizontal stress was estimated from the equation suggested by Sheorey et al. (2001): where β = / C (coefficient of linear thermal expansion), G = C/m (geothermal gradient), υ is the Poisson s ratio and E mass is deformation modulus of rock mass in MPa. The horizontal stress was calculated as 5.42 MPa. Table I shows the utilised rock mass properties, determined in the laboratory and using RocLab software, while Table II shows the properties of the support elements similarly defined in the laboratory and also as indicated by the supplier. Figure 1 shows the generated model. TABLE I- MATERIAL PROPERTIES OF SANDSTONE FOR THE NUMERICAL MODEL Property Estimated Value Elastic type Isotropic E mass (GPa) 2.47 Poisson s ratio (ν) 0.33 UCS (MPa) 50 GSI (MPa) 50 σ 1 (MPa) 11 σ z (MPa) 5.42 σ 3 (MPa) Material type Plastic m i constant 13 m b constant s constant a constant m br constant 1 s r constant a r constant 0.5 Dilation parameter 0 o TABLE II - CHARACTERISTICS OF THE SUPPORT ELEMENTS EMPLOYED IN THE ANALYSIS Property Shotcrete Rock bolt TSL Young s modulus, E (GPa) Poisson s ratio (ν) Peak compressive strength (MPa) Residual compressive strength (MPa) Peak tensile strength (MPa) Residual tensile strength (MPa) Peak load (MN) Residual load (MN) Type - 25 mm diameter fully bonded - Thickness (mm) 50 4 Page 331 [1] [2]

4 Fixed boundary Fig. 1 Detailed tunnel geometry, mesh and conditions III. RESULTS FROM NUMERICAL MODELLING The excavated tunnel was analysed for four scenarios: unsupported; supported with bolts alone, supported with bolts and Shotcrete, and with bolts and TSL. Table III summarises the results for all the cases from the analysis. Support Case TABLE III - RESULTS FROM PHASE 2 ANALYSIS Yielded Elements (No.) Yielded Bolt Elements (No.) Yielded Liner Elements (No.) Maximum Total Displacement (mm) Unsupported Bolts only Bolts and shotcrete 323 None Bolts and TSL (4mm) Bolts and TSL (24.2mm) None 6.09 Fig. 2 Strength factor contours and yielded elements, after plastic analysis of unsupported excavation Page 332

5 The unsupported excavated tunnel had 383 elements, yielding with a maximum total displacement of 7.13 mm. Figure 2 shows the yielded elements, while Figure 3 shows the displacement of this support situation. In the vicinity of the tunnel face, there is combined shear and tension failure. As you get away from the tunnel face into the rock mass, the failure is in shear. Most of the failure is at the bottom and top of the tunnel. There is significant inward displacement of the tunnel walls, as well as significant floor heave. Fig. 3 Displacement contours and vectors around excavation (plastic analysis) The introduction of bolts into the excavation, as a rock mass support mass system, reduced the yielded elements to 353 and maximum total displacement to 6.17 mm. Figure 4 shows the yielded elements, while Figure 5 shows the displacement of the excavation with pattern bolt support. The yielded zone, based on the extent and location of the yielded elements, is not evidently different from the unsupported yield zone. However, the number of yielded finite elements decreased by 30. The displacements also reduced slightly. Only one bolt element yielded. This indicates tensile failure of a bolt element. Bolt elements for fully bonded bolts are defined by intersections with finite elements. This means that the bolt size, strength or number may have to be increased. But, in this case, the peak and residual bolt capacities were made equal. Therefore, even though the bolt has reached its yield capacity, it can still provide support. Yielded bolt in yellow Fig. 4 Strength factor contours and yielded elements, for excavation with pattern bolt support only Page 333

6 Fig. 5 Displacement contours and vectors around excavation with pattern bolt support only To support the gaps between the bolt systems, Shotcrete was introduced as the liner. It reduced the yielded elements to 323 with a maximum total displacement of 6.08 mm. No bolt element yielded, though 6 liner elements yielded. The application of a Shotcrete liner, in conjunction with the pattern bolting, was effective in reducing the failure around the tunnel, especially at the top and on the sides. Figure 6 shows the yielded elements, while Figure 7 shows the displacement of the excavation with pattern bolt and Shotcrete support. Yielded liner elements in red Fig. 6 Strength factor contours and yielded elements, for excavation with pattern bolt and Shotcrete support The replacement of Shotcrete with TSL as the liner only reduced the yielded elements to 337. It had no effect on the displacement achieved with the pattern of bolt support only. However, only 3 liner elements yielded, with one bolt element also yielding. Figure 8 shows the yielded elements while Figure 9 shows the displacement of the excavation with pattern bolt and TSL support. It may be interpreted that Shotcrete is a superior material to TSLs. However, it should be noted that the thickness of TSL was only 4 mm, compared to the thickness of 50 mm for Shotcrete. Additionally, TSLs are known to have higher productivity and lower material handling efforts than Shotcrete (Tannant, 2001). Though TSLs may have higher material costs, the additional hour per shift, if utilised, makes this new support option attractive. Page 334

7 Fig. 7 Displacement contours and vectors around excavation with pattern bolt and Shotcrete support They are associated with reduced time for support installation, since their application requires simple equipments and have shorter setting times (Tannant, 2001). Shotcrete also has a very high rebound factor leading to almost 50% wastage, (Ferreira, 2012). With this smaller thickness the TSL is able to support the excavation. It should also be noted that Phase2 software does not incorporate the shear-bond strength of TSLs. It is believed that the contribution of this bond strength will greatly reduce the displacement, since the failure of the elements around the tunnel face was both in tension and shear, (Espley et al., 1996). Fig. 8 Strength factor contours and yielded elements, for excavation with pattern bolt and TSL support of 4 mm thickness Figure 10 shows the yielded elements, while Figure 11 shows the displacement of the excavation with pattern of bolt and TSL support of the recommended thickness of 24.2 mm, (Jjuuko, 2015). The total displacement and the number of yielded elements is equal to that attained with pattern of bolt and Shotcrete of 50 mm thickness. Although Shotcrete showed yielded elements, TSL of 24.2 mm does not exhibit any yielding element. Therefore, TSLs are superior to Shotcrete in terms of surface support for underground excavations. Page 335

8 Fig. 9 Displacement contours and vectors around excavation with pattern bolt and TSL support of 4 mm thickness Fig. 10 Strength factor contours and yielded elements, for excavation with pattern bolt and TSL support of 24.2 mm thickness Fig. 11 Displacement contours and vectors around excavation with pattern bolt and TSL support of 24.2 mm thickness Page 336

9 IV. CONCLUSION Numerical analysis, using Phase 2 computer software, showed that TSL thickness of 24.2 mm offers the same structural capacity as Shotcrete of thickness 50 mm. This is a significant reduction in material consumption, hence the associated economic benefits. Additionally, TSLs are known to have higher productivity and lower material handling efforts than Shotcrete (Tannant, 2001). REFERENCES 1. Adams, D. and Baker, D. An assessment of the use of Shotcrete and thin sprayed linings in South African mines from a safety point of view. In Proceedings of 2nd International Seminar on Surface Support Liners, Johannesburg, South Africa, Espley, S.J., O'Donnell, J.D.P. Thibodeau, D. and Paradis-Sokoloski, P. Investigation into the replacement of conventional support with spray-on liners. Canadian Institute of Mining and Metallurgy Bulletin, 89(1001), , Ferreira, P. A perspective on underground support technologies in Southern African platinum mines to reduce safety risks and enhance productivity. Proceedings of the Southern African Institute of Mining and Metallurgy Platinum Conference, Johannesburg, South Africa, Hermanus, M.A. Occupational health and safety in mining - status, new developments, and concerns. The Journal of the Southern African Institute of Mining and Metallurgy, 107: , Hoek, E. and Brown, E.T. Trends in relationship between measured in-situ stresses and depth. International Journal of Rock Mechanics and Mining Science, 15: , Hoek, E. and Brown, E.T. Underground excavations in rock. Institute on Mining and Metallurgy, London, England, Hoek, E., Kaiser, P.K. and Bawden, W.F. Support of underground excavations in hard rock. Rotterdam: Balkema, Hoek, E. and Brown, E.T. Practical estimates of rock mass strength. International Journal of Rock Mechanics and Mining Science, 34(8), , Hoek, E., Carranza-Torres, C.T. and Corkum, B. Hoek-Brown failure criterion-2002 edition. In proceedings of the 5th North American Rock Mechanics Symposium, Toronto, Canada: 1: , Jjuuko, S. Investigation into Shear-Bond Strength of Thin Spray-On Liners as a Surface Support Mechanism in Underground Rock Support. MSc. Thesis; Cape Town, South Africa: University of Cape Town, Ozturk, H. and Tannant, D.D. Thin spray-on liner adhesive strength test method and effect of liner thickness on adhesion. International Journal of Rock Mechanics and Mining Sciences, 47(5): , Rocscience. Phase 2 v7.0 2D finite element program for calculating stresses and estimating support around the underground excavations. Geomechanics Software and Research. Rocscience Inc. Toronto, Ontario, Canada, Rocscience. Roclab v1.0 Rock mass strength analysis using the generalised Hoek-Brown failure criterion. Rocscience Inc. Toronto, Ontario, Canada, Rocscience. RocSupport, rock support interaction and deformation analysis for tunnels in weak rock. Tutorial Manual. Rocscience Inc. Toronto, Ontario, Canada, Saydam, S., Yilmaz, H. and Stacey, T.R. A new testing approach for thin spray-on liners: double-sided shear strength (DSS) test. International Workshop and Seminar on Surface Support Liners: Thin Spray-on Liners, Shotcrete and Mesh, Quebec City, Canada, Sheorey, P., Murali, M.G. and Sinha, A. Influence of elastic constants on the horizontal in-situ stresses. International Journal of Rock Mechanics and Mining Science, 38(1): , Tannant, D.D. Thin spray-on liners for underground rock support testing and design issues. Proceedings of International Conference on Surface Support Liners: Membranes, Shotcrete and Mesh, Australian Centre for Geomechanics, Perth, Yilmaz, H. Development of testing methods for comparative assessment of thin spray-on liner shear and tensile properties. PhD thesis, University of the Witwatersrand, Johannesburg, South Africa, Page 337