COMPARISON BETWEEN THIN SPRAY-ON LINERS AND SHOTCRETE AS SURFACE SUPPORT MECHANISMS IN TUNNELS

International Journal of Innovative Research in Advanced Engineering (IJIRAE) ISSN: 2349-2163
Issue 10, Volume 5 (October 2018) www.ijirae.com
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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, 329-337. doi://10.26562/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.

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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 Phase2 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.
Phase2 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
2135.

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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:
σv = γH [1]
where γ is unit weight of the rock mass in MN/m3 and H is the depth of overburden in meter. The vertical stress
was calculated as 11 MPa assuming unit weight of 0.02 MN/m3 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):
[2]
where β = 8 × 10-6/°C (coefficient of linear thermal expansion), G = 0.024°C/m (geothermal gradient), υ is the
Poisson’s ratio and Emass 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
Emass (GPa) 2.47
Poisson’s ratio (ν) 0.33
UCS (MPa) 50
GSI (MPa) 50
σ1 (MPa) 11
σz (MPa) 5.42
σ3 (MPa) 0.6901
Material type Plastic
mi constant 13
mb constant 2.180
s constant 0.0039
a constant 0.506
mbr constant 1
sr constant 0.001
ar constant 0.5
Dilation parameter 0o
TABLE II - CHARACTERISTICS OF THE SUPPORT ELEMENTS EMPLOYED IN THE ANALYSIS
Property Shotcrete Rock bolt TSL
Young’s modulus, E (GPa) 20 20 30
Poisson’s ratio (ν) 0.2 0.2
Peak compressive strength (MPa) 20 25
Residual compressive strength
(MPa)
2.3 18
Peak tensile strength (MPa) 1.5 6
Residual tensile strength (MPa) 0 5.2
Peak load (MN) - 0.2 -
Residual load (MN) - 0.2 -
Type - 25 mm diameter – fully bonded -
Thickness (mm) 50 4

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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.
TABLE III - RESULTS FROM PHASE2 ANALYSIS
Support Case
Yielded
Elements
(No.)
Yielded Bolt
Elements
(No.)
Yielded Liner
Elements
(No.)
Maximum Total
Displacement
(mm)
Unsupported 383 - - 7.13
Bolts only 353 1 - 6.17
Bolts and shotcrete 323 None 6 6.08
Bolts and TSL (4mm) 337 1 3 6.18
Bolts and TSL (24.2mm) 324 1 None 6.09
Fig. 2 Strength factor contours and yielded elements, after plastic analysis of unsupported excavation
Fixed boundary

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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.
Fig. 4 Strength factor contours and yielded elements, for excavation with pattern bolt support only
Yielded bolt in yellow

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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.
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.
Yielded liner elements in red

_________________________________________________________________________________
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.

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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

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IV. CONCLUSION
Numerical analysis, using Phase2 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).
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3. Ferreira, P. A perspective on underground support technologies in Southern African platinum mines to reduce
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