Tunnel Engineering and Rock Mechanics

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Questions and Answers

What is the primary purpose of rock mass classification in tunnel engineering?

To determine the tunnel's route

To evaluate rock mass quality and behavior (correct)

To assess groundwater presence

To measure intact rock strength

Which of the following factors is NOT included in the Q-system formula?

Groundwater pressure (Jw) (correct)

Joint alteration (Ja)

Rock Quality Designation (RQD)

Joint roughness (Jr)

What does a high Rock Quality Designation (RQD) indicate?

Poor rock quality

High percentage of intact rock (correct)

High percentage of fractured rock

Low joint spacing

How does groundwater presence impact tunnel design?

It increases the risk of collapse Signup and view all the answers

What can joint spacing tell engineers about a rock mass?

The density of natural fractures Signup and view all the answers

What is the primary purpose of cable bolts in tunnel construction?

To provide deeper stabilization Signup and view all the answers

Which support method is typically used for immediate stabilization in tunnels?

Shotcrete Signup and view all the answers

What type of support may be utilized if rock classification suggests poor rock quality?

Rock bolts Signup and view all the answers

Which of the following would be an example of field stresses in the context of tunnel engineering?

The weight of the overlying soil Signup and view all the answers

How is the bolt length calculated for adequate anchorage in a tunnel?

$L = 2 + 0.15 imes Span$ Signup and view all the answers

What type of shotcrete contains fibers to improve strength and reduce cracking?

Fiber-reinforced shotcrete Signup and view all the answers

What does the term 'joint orientation' refer to in rock mechanics?

The angle at which joints are positioned relative to the tunnel Signup and view all the answers

Which failure mechanism involves the shotcrete detaching from the rock surface?

Adhesion failure Signup and view all the answers

In the calculation of the Q-value, what does a higher Stress Reduction Factor (SRF) imply?

Improved rock mass stability Signup and view all the answers

What does the drill core quality (fracture density) indicate about the rock?

How broken or intact the rock is Signup and view all the answers

What does a low Excavation Support Ratio (ESR) indicate for tunnel stability requirements?

Requires high stability for safety Signup and view all the answers

In what situation would fiber-reinforced shotcrete be particularly beneficial?

High-pressure tunnels Signup and view all the answers

Which type of tunnel typically requires high stability due to its purpose?

Nuclear facility tunnels Signup and view all the answers

What is the main function of surface support plates and straps in tunnel construction?

To distribute forces and prevent movement Signup and view all the answers

In tunnel engineering, what is the critical factor that varies based on rock type and tunnel purpose?

Support systems Signup and view all the answers

Study Notes

Tunnel Engineering and Rock Mechanics

Tunnel engineering involves constructing underground passages for transport, utilities, or mining. Rock mechanics studies rock mass behavior under stress for safe tunnel design.

Key Data Types for Design

Intact Rock Strength: Lab-measured resistance to force.
Field Stresses: Pre-tunnel natural stresses in the rock.
Groundwater: Impacts stability and excavation.
Drill Core Quality (Fracture Density): Indicates rock intactness.
Joint Spacing: Distance between fractures.
Joint Persistence: Length of fractures.
Joint Orientation: Fracture direction relative to the tunnel.
Joint Contour (Shape): Smooth or rough joint surfaces for stability.
Joint Aperture and Surface Condition: Joint width and condition.

Rock Mass Classification

Purpose: Evaluate rock mass quality and behavior.
Applications:
- Tunnel project feasibility.
- Determining tunnel support systems.
- Initial estimates of rock strength and deformation.

Q-System (Rock Quality Assessment)

Formula: Q = [RQD / Jn] * [Jr / Ja] * [Jw / SRF].
RQD: Rock Quality Designation (intact rock percentage in core samples).
Jn: Number of joint sets.
Jr: Roughness of the most unfavorable joint.
Ja: Joint alteration (weakness or filling).
Jw: Joint water reduction factor.
SRF: Stress Reduction Factor (rock stress/excavation impact).
Example Calculation (Fair Rock Mass Quality): RQD = 70%, Jn = 9, Jr = 2, Ja = 1, Jw = 1, SRF = 27.5, Q = 0.57 → Poor quality, needing robust support.

Support Systems for Tunnels

Reinforcement:
- Rock Bolts: Short steel bars anchored for stability.
- Cable Bolts: Longer, flexible steel cables for greater depth.
Surface Support:
- Plates & Straps: Distribute forces, prevent rock movement.
- Mesh: Steel mesh to contain loose rock.
- Shotcrete: Sprayed concrete for immediate stabilization.

Excavation Support Ratio (ESR)

Definition: Support level based on tunnel usage and safety.
Values: Lower ESR (e.g., nuclear facilities) = High stability needed. Higher ESR (e.g., temporary mine tunnels) = Less stability needed.

Bolt Length and Design

Bolt length (L) calculation: L = 2 + 0.15 * Span
Ensures adequate anchorage for stability.

Shotcrete and Fiber-Reinforced Shotcrete

Shotcrete: Sprayed concrete for immediate tunnel wall stability.
Fiber-Reinforced Shotcrete (FRS): Strengthened with fibers, reducing cracking and increasing flexibility.
Failure Mechanisms:
- Adhesion Failure: Shotcrete separates from rock.
- Shear Failure: Layer slides under pressure.
- Compression/Tension Failure: (rare, in thin shotcrete).

Applications of Tunnel Engineering

Transportation: Rail, road tunnels (under mountains/cities).
Utilities: Water supply, sewage, power transmission tunnels.
Mining: Underground ore extraction.

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Description

This quiz covers the essential concepts of tunnel engineering and rock mechanics, focusing on key data types for design and rock mass classification. Understand the various factors that influence underground construction, including rock strength, field stresses, and joint characteristics.