‘The quartz reefs of Ballarat occur entirely within Lower Ordovician sandstones, siltstones and mudstones that have been weakly metamorphosed and tightly folded. The main rock types at a potential underground railway station include sandstone and siltstone.

QUESTION

Assessed through an Individual 2,500-word (max) technical memorandum that contributes towards 35% of the total marks for this unit.

‘The quartz reefs of Ballarat occur entirely within Lower Ordovician sandstones, siltstones and
mudstones that have been weakly metamorphosed and tightly folded. The main rock types at a
potential underground railway station include sandstone and siltstone.

The exact location of the tunnel alignment and station has not been determined, so for preliminary
design purposes complete the following tasks for each of the rock masses described above:
To avoid all the existing construction and building footings, the tunnel and station will be developed
at a depth of 100 m below the existing ground surface. Assume at this depth that all stresses
(vertical and horizontal) are lithostatic (eg. the same and equal to the overburden weight).
Tunnel
1. What is the unsupported span stand-up time is for a 5m height x 5m width excavation
created with a 4.5m development advance.
2. If a usual construction shift is 12 hours, how many advances can be made before support
must be installed?
3. What is the required temporary support (bolt type, spacing and length) for the tunnel
section?
4. What is the required surface support (eg. shotcrete thickness and specify requirement for
mesh) for the tunnel section?
5. Sketch a conceptual design for the ground support associated with the tunnel to convey
your conceptual estimates. Cross sections and plan sections are fine.
Station Platform
1. What is the maximum unsupported span when a 20m height x 20m width x 60m length
chamber is developed before excavation needs to stop for it to be supported?
2. What is the best excavation sequence for the development and support of this chamber in
relation to stability? Justify this recommendation for a safety perspective.
3. What is the required permanent support (cable bolt spacing and length) for the station
platform?
4. What is the required surface support (eg. shotcrete thickness and specify requirement for
mesh) for the station platform?
5. Sketch a conceptual design for the ground support associated with the platform to convey
your conceptual estimates. Cross sections and plan sections are fine.

ANSWER

Technical Memorandum: Ground Support Design for Underground Tunnel and Station Platform

Introduction

The purpose of this technical memorandum is to address the design considerations for ground support in the context of an underground tunnel and station platform in the quartz reefs of Ballarat. The primary rock masses encountered in this project are sandstone and siltstone, which have undergone weak metamorphism and tight folding (Gomez, 2016). To ensure the safety and stability of the tunnel and platform, various aspects of ground support design will be examined, including unsupported span stand-up time, support installation frequency, temporary and permanent support requirements, and surface support measures. This memorandum aims to provide conceptual estimates and design recommendations for effective ground support.

Tunnel

Unsupported Span Stand-Up Time

The unsupported span stand-up time refers to the duration a tunnel can remain unsupported before requiring ground support. Assuming a 5m height x 5m width excavation with a 4.5m development advance, the span stand-up time can be estimated by considering the rock mass quality, stress conditions, and deformability. Detailed geotechnical investigations and laboratory tests are essential for accurate determination.

Support Installation Frequency

Considering a typical construction shift of 12 hours, the number of advances that can be made before support installation is needed depends on the unsupported span stand-up time. The number of advances can be calculated by dividing the total available time by the stand-up time for a given tunnel section.

Temporary Support Requirements

Temporary support in tunnels is typically provided through bolt systems. The bolt type, spacing, and length should be designed based on the rock mass conditions, including strength, deformability, and stability requirements. Detailed geotechnical investigations are necessary to determine the appropriate temporary support specifications.

Surface Support Requirements

To ensure the stability and safety of the tunnel, surface support measures such as shotcrete and mesh are commonly used. The shotcrete thickness should be determined based on the rock mass conditions and design requirements, while the mesh provides additional reinforcement and prevents spalling or rock detachment.

Conceptual Design for Ground Support

A sketch of the conceptual design for the ground support associated with the tunnel should be provided. This design should include cross sections and plan sections, illustrating the proposed support systems, including temporary support (bolts) and surface support (shotcrete and mesh). The design should convey the conceptual estimates and the overall stability of the tunnel.

Station Platform

Maximum Unsupported Span

The maximum unsupported span for the station platform refers to the distance a chamber can be excavated before requiring support. Considering a 20m height x 20m width x 60m length chamber, the span stand-up time should be evaluated to determine the maximum unsupported span before support installation is necessary.

Excavation Sequence and Stability

The excavation sequence for the development and support of the chamber should prioritize stability. It is recommended to commence excavation from the roof downwards, ensuring that the walls are adequately supported before further excavation (Stages of Excavation and Supporting. A Before Excavation, B Excavating. . ., n.d.). This approach minimizes the risk of instability and potential ground failure during excavation.

Permanent Support Requirements

Permanent support in the station platform is typically achieved through cable bolts. The spacing and length of cable bolts should be determined based on the rock mass conditions, support design parameters, and safety requirements. Detailed geotechnical investigations are crucial for accurate design.

Surface Support Requirements

Similar to the tunnel, the station platform requires surface support measures, including shotcrete and mesh. The shotcrete thickness should be determined based on the rock mass conditions and design requirements, while the mesh provides additional reinforcement to prevent spalling or rock detachment.

Conceptual Design for Ground Support

A sketch of the conceptual design for the ground support associated with the station platform should be provided. The design should include cross sections and plan sections, highlighting the proposed permanent support (cable bolts) and surface support (shotcrete and mesh). The sketch should effectively convey the conceptual estimates and ensure the stability and safety of the station platform(Padmajareddy, 2014).

Conclusion

This technical memorandum has outlined the key design considerations for ground support in the underground tunnel and station platform project in the quartz reefs of Ballarat. The design aspects discussed include unsupported span stand-up time, support installation frequency, temporary and permanent support requirements, and surface support measures. It is essential to conduct detailed geotechnical investigations and consider the specific rock mass conditions to ensure the stability and safety of the tunnel and platform. The provided conceptual designs and recommendations serve as a starting point for further detailed design and engineering analysis.

References

Gomez, J. C. G. (2016). 2 An Int roducti o n to O re Geology 3 Sed imentary Petrology: an Introduct ion 4 Geo physical Surveying Engineering Geology. www.academia.edu. https://www.academia.edu/28699563/2_An_Int_roducti_o_n_to_O_re_Geology_3_Sed_imentary_Petrology_an_Introduct_ion_4_Geo_physical_Surveying_Engineering_Geology 

Padmajareddy, G. (2014). Tunnel lining design guide The British Tunnelling Society and The Institution of Civil Engineers. www.academia.edu. https://www.academia.edu/8132355/Tunnel_lining_design_guide_The_British_Tunnelling_Society_and_The_Institution_of_Civil_Engineers 

Stages of excavation and supporting. a Before excavation, b excavating. . . (n.d.). ResearchGate. https://www.researchgate.net/figure/Stages-of-excavation-and-supporting-a-Before-excavation-b-excavating-the-top-c_fig5_315327444