underground workshop cavern design

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UNDERGROUND WORKSHOP/CAVERN DESIGN Rock Mass – Support Interaction This technical report investigates potential stabilisation techniques that may be used in a proposed permanent underground workshop facility at 2000m below surface. This paper summarises the approach taken, key results and recommendations. The key results will include comparison between rock mass classification and rock mass, support interaction concepts, and consideration of support recommendations for long-term stability of the excavation. Client: Mr. J. Coggan Content length: 6 pages Appendix length: 16 pages Word Count: 3651 Author: 630024723

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Page 1: Underground Workshop Cavern Design

UNDERGROUND WORKSHOP/CAVERN DESIGN

Rock Mass – Support Interaction

This technical report investigates potential stabilisation techniques that may be used in a proposed permanent underground workshop facility at 2000m below surface. This paper summarises the approach taken, key results and recommendations. The key results will include comparison between rock mass classification and rock mass, support interaction

concepts, and consideration of support recommendations for long-term stability of the excavation.

Client: Mr. J. Coggan

Content length: 6 pages

Appendix length: 16 pages

Word Count: 3651

Author: 630024723

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TABLE OF CONTENTS1.0 Introduction.....................................................................................................................................2

2.0 Design considerations Q................................................................................................................2

3.0 Design considerations RMR...........................................................................................................3

4.0 Excavation design Considerations South West..............................................................................3

4.1 Examine 2D.................................................................................................................................3

4.2 Roclab.........................................................................................................................................4

5.0 Excavation Design Considerations South East.............................................................................4

5.1 Examine 2d.................................................................................................................................4

6.0 Rocsupport SW and SE......................................................................................................................5

7.0 Unwedge & Phase 2............................................................................................................................7

8.0 Recommendations & Conclusion........................................................................................................7

Appendix A – Design Considerations Q&RMR, Geotechnical Information, Equations.............................8

Appendix B – Examine 2D......................................................................................................................12

Appendix C - Rocsupport........................................................................................................................16

Appendix D – Unwedge...........................................................................................................................21

Appendix E – Phase 2.............................................................................................................................22

Appendix F – Stereonet...........................................................................................................................24

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1.0 INTRODUCTIONThe following report highlights key recommendations from calculated results for a proposed excavation. The workings will be a 6m high, 10 m wide permeant excavation with a 50m length for an underground workshop. The depth of the project is set to 2000m in “relatively” competent quartzite. A summary of the geotechnical data can be seen in Appendix A. Information on joint sets is displayed in Appendix B with stereonets computed using Dips 6.0. The information provided was analysed using examine 2D and RocLab for visualization of the stresses and strength factor of the surrounding rock mass. Phase 2, Unwedge and RocSupport have been used to show the support recommendations for the project. The aim of this software is to provide evidence to support a decision put forward in the conclusion on whether the working should be orientated in the South West plain or the South East and the support needed.

The K ratio for each orientation has been calculated in section 4.0 and 5.0 for South West and South East workings respectively. In each of these sections a discussion of the benefits and limitations of the software can be found. Unwedge and phase 2 have been shown as alternative uses of support software however focus will be paid to RocSupport as the principal software for computing support recommendations.

This report will be laid out to discuss each orientation of the workings with all the computational software and a comparison will be made in section 6.0 the recommendation.

2.0 DESIGN CONSIDERATIONS QIn the initial stages of ground stabilization investigation calculation of rock mass classification can be beneficial as a quick way of estimating the required support. From the data provided shown in Appendix A, it is possible to identify the support required for the potential workings. It must be noted these are basic and quick calculations and may miss out important factors such as joint orientation. The information provided shows that the rock mass has a Q value of 4. The ESR for a permanent mine working is given as 1.6. The rock mass quality table shown in Appendix A is able to provide an approximation for the rock class and the reinforcement for this. Q is taken on a logarithmic scale.

Q= 4 De = 6.25 (See Equation 1)

From the table in Appendix A, it can be shown that the rock quality is fair to poor and the support category is 4. Which equates to systematic bolting with unreinforced shotcrete, 4-10cm. The calculation for bolt length is shown in Equation 2 and the spacing is worked out from Langs formula (Equation 3).

Bolt length calculated (m) = 2.9375m or 3.0m (1sf)

Bolt spacing Calculated (m) = 1.5m (interpolated from the graph spacing is 2.5m)

For economic purposes it is probable that a bolt length of 2.9375m will not be found and therefore 3.0m will be sufficient. The Q system can be compared later to the RocSupport software to see if the calculation will provide the best factor of safety. The Q system is limited by engineering judgement and cannot reliably replace the software that will be covered in the following sections. It is ideal however for predictions and quick calculations.

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3.0 DESIGN CONSIDERATIONS RMRRMR can be calculated using data collected in the field similar to Q (See Appendix A for the Table 1). However a lack of ground investigation information prohibits us from calculating RMR using the tables therefore from information provided by the client RMR is equal to 75 (see Equation 4). Table 2 in Appendix A shows for a RMR of 75 we expect good rock quality requiring support of bolt length 3.0m spaced 2.5m apart. Shotcreting the crown with a thickness of 50mm. Support should be constructed 20m from the face. Compared to Q this is similar. Length of bolt required is the same and spacing is the same. Q does not specify where from the face the support should be installed but RMR does. Again this interpretation is designated by engineering judgement and bias can be considered. When compared to Roc support this report can conclusively see if the predictions made by RMR and Q will provide a sufficient factor of safety for the excavations.

4.0 EXCAVATION DESIGN CONSIDERATIONS SOUTH WESTThe coefficient of earth pressure at rest or the K ratio can be calculated using Equation 5. The k ratio provides a number for the horizontal stress and the out of plane stress that can be put into the computational software to aid in the design of the excavation, show the stresses around the excavation and help design the support for the workings. The in-situ stresses are as follows:

σv= 50MPa (vertical) σh= 40MPa (045 degrees bearing) σH= 60MPa (135 degrees bearing)

The K ratio for the in plane horizontal stress equals; 1.2, the K ratio for the out of plane stress is equal to; 0.8.

4.1 EXAMINE 2DExamine 2D software provides a simple and quick elastic analysis for investigating the different K ratios and changes to in situ stress of the rock mass from the excavation. It also has the ability to measure the changes in stresses due to the appearance of discontinuities in the rock. Stress contour patterns highlight the induced stresses surrounding the working. Using the parameters of quartzite and those set by the client this report was able to analyze the strength factor and the in plane stress. The following figures will be analyzed for the south west tunnel design:

Figure 4.1.A – Strength factor South West (see Appendix B) Figure 4.1.B – Sigma 1 stress South West (see Appendix B)

The contours for strength factor represent a ratio of material strength to the induced stress. A value less than one represents failure of the material. It can be clearly seen in Figure 4.1.A that failure will occur under the conditions set. The contours for the sigma 1 stress represent the horizontal stresses and how the shape and size of the tunnel redistributes around the working. Figure 4.1.B shows that the stress will be redistributed to the corners of the excavation.

The effect of discontinuities on the structure can be shown in figures:

Figure 4.1.C - Strength factor South West with jointing (see Appendix B) Figure 4.1.D - Sigma 1 stress South West with jointing (see Appendix B)

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Shown in Figure 4.1.C and D are the worst case scenarios for the discontinuities represented in the data. Each strike of the joint runs parallel with the tunnel direction. The one joint set characterized is shown to spaced 3m apart from the other. As can be seen in the Figures in Appendix B the discontinuities have an effect on the strength factor and Sigma 1 stress plane. In Figure 4.1.C it is clear that there are less zones where the strength factor is less than one and therefore less zones of potential failure. Figure 4.1.D shows the horizontal stresses are not focused on the corners of the excavation anymore but show reduced stresses in the roof and floor of the tunnel. When designing support recommendations it is important to design for the worst case scenario. As the exact jointing is unknown it is seen as best practice to account for the maximum possible number of joints in the set.

- The limitations of examine 2D?

Examine 2D assumes that the material modeled is isotropic, consistent and linearly elastic. It is however the case that in reality this is very unlikely. Although the excavation is 2000m deep in quartzite it is wrong to assume that this rock mass is homogenous. For deformation, the data produced by examine 2D is only shows elastic distortion. What is missing is plastic deformation that the excavation causes. It is unclear whether in a real life scenario the plastic deformation zones would have much effect on the excavation, but at a depth of 2km they should not be ignored. The lack of 3D representation does not take into account the real stress flow around the ends of the excavation. With the knowledge that the excavation is 50m long this can be modelled using examine 3D. The main problem with examine 2D is it only models in two dimensions and assumes the length is therefore infinite with no ends.

4.2 ROCLABThis software is used in conjunction with examine 2D in order to evaluate the young’s modulus (Em) by inputting the parameters shown in Appendix A. It was possible to calculate a value of Em which works out as 30000Mpa (1sf). This is the same for both South West orientation and South East.

5.0 EXCAVATION DESIGN CONSIDERATIONS SOUTH EASTThe K ratio for the in plane horizontal stress equals 0.8 and the out of plane stress ratio is 1.2.

5.1 EXAMINE 2DIn section 4.1 the limitations and introduction to examine 2D can be seen, this section will only focus on the data collected from the software and the data when discontinuities are taken into account. Shown below are a list of the figures discussed in this section:

Figure 5.1.A - South East strength factor, Examine 2D Figure 5.1.B - South East Sigma 1 plane stress, Examine 2D Figure 5.1.C - South East strength factor with added discontinuities, Examine 2D Figure 5.1.D - South East Sigma 1 plane stress with added discontinuities, Examine 2D

The strength factor shown in Figure 5.1.A shows for the South Easterly orientation there is more chance of failure in the side wall of the excavation, this is also supported by figure 5.1.B where the horizontal stress is less in value compared with the South West orientation. With a dominant vertical stress there are zones of reduced stress in the roof and foot of the excavation, the blue contouring in figure 5.1.B shows this. There are similarities to the South Western orientation where the highest zones

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of stress are the corners of the excavation. It is probable that the corners of the tunnel will be rounded to reduce these zones of stress and areas of failure. The biggest difference shown between figures 4.1.A and 5.1.A is the strength factor shows failure in the roof and sidewalls through strength factor where by the strength factor is less than 1.00 in value.

When the discontinuities are added the worst case scenario is once again taken into account. As can be seen in figure 5.1.C and 5.1.D there are two joint sets opposing one another. This creates wedges in the roof and sidewall. The effect that these discontinuities have on the strength factor and sigma 1 stress is drastic compared to the discontinuities in the South Western alignment. In Figure 5.1.C the strength factor shows zones of failure in the left side of the excavation where a small wedge and large wedge meet. It is assumed that this area will suffer wedge failure and subsequent support will be needed to prevent this undesired effect. There is also likely to be large wedge failure in the roof. For this worst case scenario it is also assumed that the wedge runs for the length of the cavern therefore there is a chance of large wedge failure across the entire length of the working. The two joint sets increase the zone of relaxation in the roof and floor of the excavation when comparing figures 5.1.B and 5.1.D.

6.0 ROCSUPPORT SW AND SERocsupport estimates deformation in circular tunnels. This report is looking into support parameters for a rectangular 6mx10m tunnel. Therefore this report has had to convert the area of 60m2 into a circular shape which gives a tunnel with a diameter of 4.37m. The software can work to find different solution based upon the solution input. For the purpose of this study the chosen solution is Carranza-Torres based on Hoek-Brown failure criterion. The software allows this report to compare support methods for different scenarios. The support methods include: Shotcrete, Rock bolts, Steelsets and a custom option. Because the software cannot distinguish orientation and jointing the following section will show a table (Table 4) comparing support parameters in a variety of scenarios but negating the SW and SE orientation. The print screens in Appendix E show:

The ground reaction curves both long term and standard for each scenario. This is the relationship between the interior pressure and the deformation of the walls. It is shown as a blue line for standard and black for long term.

Support reaction curves for each scenario. Characterized as the relationship between support pressure and the strain of the support methods used. This is shown as purple line.

The Tunnel section views showing the cross sections of the tunnel diameter for all the scenarios. This includes the plastic zone radius and added support.

For the purpose of comparison there is a scenario with no support. Long term scenario is taken into account because the working is permanent. Rocsupport makes many assumptions, the circular tunnel is one already mentioned. It also assumes the K ratio is 1 or the in situ stress field is hydrostatic and therefore as mentioned before doesn’t account for tunnel direction. The support modelled in the software is set to a uniform internal pressure around the circumference. Rockbolts and cables are fitted in a regular design and shotcreteing is a closed ring around the entire tunnel. In a real life scenario this may not be needed. The factor of safety calculated by Rocsupport can only compute when support is added a certain distance from the face. If the support is added to far away factor of safety rises to large

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amount that are unfeasible. Therefore for the purposes of continuity the support for all scenarios shown in Table 4 will be set 1m from the face.

Scenario Support added Convergence Long term Convergence

Factor of safety (ST/LT)

1 None 0.29% 0.32% N/A2 - 34mm Rockbolts at a

spacing of 1.5m.- 100mm Shotcrete

thickness

0.27% 0.30% 1.46 (ST) / 1.18 (LT)

3 - 34mm Rockbolts at a spacing of 1.5m.

- 50mm shotcrete ().5 day curing time)

- 150mm Steelsets at 32kg/m

0.28% 0.32% 1.37 (ST) / 1.09 (LT)

4 - 34mm Rockbolts at a spacing of 1.0m.

- 1000mm shotcrete- 307mm Steelsets at

97kg/m with a 0.1m out of plane spacing

0.22% 0.23% 2.37 (ST) / 1.96 (LT)

5 (1.35m from face) - 34mm Rockbolts at a spacing of 1.0m.

- 307mm Steelsets at 97kg/m with a 1.1m out of plane spacing

- 300mm shotcrete

0.25% 0.28% 1.9 (ST) / 1.51

Table 4: Comparison table for the five scenarios (See Appendix C)

Figures used:

- Figure 5.2.A: Graph and diagram with no support (Scenario 1)- Figure 5.2.B: Graph and diagram using parameters set by Q (maximum) (Scenario 2)- Figure 5.2.C: Graph and diagram for minimum max support pressure(Scenario 3)- Figure 5.2.D: Graph and diagram for best support solution. (Scenario 4)- Figure 5.2.E: Graph and diagram for the most practical support solution (Scenario 5)

Although scenario one has a small convergence long term and short term it is still likely to fail. If it is added to the examine 2d data the report has shown that in both orientations there is chance of failure. It is to this reports conclusion that the cavern will require support. Scenario 2 uses the maximum Q values calculated in section 2.0. There is no specification on rock bolt diameter so for the purpose of worst case scenario this report has compared the maximum diameter available on the software of 34mm. The Q system scenario does show a small change in long term convergence but a long term factor of safety of 1.18 is too small. A factor of safety of 1.00 is technically deemed safe, however practically the design of the excavation needs to have a long term factor of safety above 1.5. Scenario 3 is there to compare the minimum max support pressure for all of the support. Its long term convergence is the same as unsupported and factor of safety of 1.09. This scenario is unpractical for those reasons. Scenario 4 looks at using all the support at maximum max support pressure with small spacing’s between supports. It gives the best convergence value and the change between long term and short term convergence is negligible (0.23% and 0.22% respectively). The factor of safety is also sufficient for the tunnel being 1.96 for long term. In reality this scenario is probably too expensive with

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all the support and inefficient with all the small spacing’s. Scenario 5 is most likely the best option. Factor of safety in the long run is above 1.5 and convergence is 0.28% long term, the second best option shown in table 4. It will be cheaper to install the support in this scenario compared with scenario 4.

One of the biggest limitations with the software is the in ability to separate the spacing’s of support. In a practical solution this would be different. It is important to compare the data from Rocsupport with Unwedge and phase 2 which have been explained in the next sections.

7.0 UNWEDGE & PHASE 2 UnWedge

3 dimensional representation of excavation with discontinuities, wedges and joints can be easily seen with color coding (Appendix D)

Support mechanisms can be input on the 3d diagram and easily visualised, also takes into account orientation.

Safety factors and weight (tonnes) can be calculated and shown for individual wedges. The data from the file shows the wedges in the South west orientation are smaller, require less support and tunnel has better safety factor.

Phase 2

The Phase2 consists of three program components: Model, Compute and Interpret. Support structures can be added separately. A wider variety of support mechanisms that can be modelled compared with RocSupport.

In Appendix E the phase two computation and interpretation can be seen for the South East orientation. The interpretation offers a similar view to the Examine 2D software, therefore it is easy to analyse the data. The added support reduces the sigma 1 stress around the excavation drastically. No south West file was found therefore there can be no comparison.

8.0 RECOMMENDATIONS & CONCLUSION Tunnel orientation must be in the South West direction; Examine 2D showed better strength

factor and responses to sigma 1 stress with discontinuities. Discontinuities in South East orientation have potential to form large wedges (see UnWedge

and examine 2D) South West tunnel requires less support (See Appendix D) The South West tunnel will ideally need systematic rock bolts spaced 1.0m apart with a

diameter of 34mm and a length of 3m. It will require 300mm of shotcrete and 307mm Steelsets spaced 1.1m apart. This will reduce the long term convergence from 0.32% to 0.28% and the factor of safety to 1.51.

In reality the Q system support parameters calculated will be sufficient for the cavern. The factor of cost will be the decision between the two scenarios in the conclusion.

The solutions presented in this report should not be taken as absolute and support must be dynamic with advance of the excavation. These figures and results should be used for planning and construction design.

All the software discussed has limitations the two focused in this report (Examine 2D and Rocsupport) the main limitation being not taking into account plastic deformation.

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APPENDIX A – DESIGN CONSIDERATIONS Q&RMR, GEOTECHNICAL INFORMATION, EQUATIONS

Graph 1: Q graph

List of Figures and Equations

Equation 1: De=SpanESR

= 101.6

=6.25

Equation 2: L=2+ 0.15(ExcavationWidth)ESR

=2+ 0.15(10)1.6

=2.9375m∨3.0m(Rounded ¿1 sf )

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Equation 3: Spacing=L2=32=1.5m

Equation 4:RMR=GSI+5=70+5=75

Equation 5: K=σ hσ v

Table 1: Rock mass rating system (RMR) after Bieniawski 1989

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Table 2: Guidelines for excavation and support of 10 m span rock tunnels in accordance with the RMR system, after Bieniawski 1989.

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Table3: Geotechnical data.

(Below)

Stress and joint sets

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APPENDIX B – EXAMINE 2D

Figure 4.1.A – South West strength factor, Examine 2D

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Figure 4.1.B – South West Sigma 1 plane stress, Examine 2D

Figure 5.1.A – South East strength factor, Examine 2D

Figure 5.1.B – South East Sigma 1 plane stress, Examine 2D

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Figure 4.1.C - South West strength factor with added discontinuities, Examine 2D

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Figure 4.1.D - South West Sigma 1palne stress with added discontinuities, Examine 2D

Figure 5.1.C - South East strength factor with added discontinuities, Examine 2D

Figure 5.1.D - South East Sigma 1 plane stress with added discontinuities, Examine 2D

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APPENDIX C - ROCSUPPORT

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Figure 5.2.A – Unsupported tunnel diagram and ground characteristic curve

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Figure 5.2.B – Support ground characteristic curve using Q parameters.

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Figure 5.2.C – Support parameters and ground characteristic curve for all support at minimum max support pressure.

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Figure 5.2.D – Best case scenario for support, ground characteristic curve and diagram.

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Figure 5.1.E – Selected of scenario that accounts for practical FS and convergence.

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APPENDIX D – UNWEDGE

Figure 5.3.B – South East prospective, Unwedge

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Figure 5.3.A – South West prospective, Unwedge

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APPENDIX E – PHASE 2

Figure 5.3.A – Stress field and mesh grid for no excavation, Phase 2

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Figure 5.3.B – Stress field and mesh grid for underground excavation with support

APPENDIX F – STEREONET

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