Untitled Quiz

Orientation, Intensity (number of joint sets), Spacing, Persistence (length), Separation (aperture), Roughness characteristics, Infilling conditions, Infilling material. These are important because the overall quality of the rock mass is controlled by the joint characteristics and has direct influence on the overall rock mass strength, frictional properties, deformability properties, and long term stability of the structure.

Rock mass classification methods are used for a tunnel project to: Classify quantitatively the quality of rock mass, Estimate rock support based on quality description, and Use the quality rating for stability assessment using failure criterion. Three widely used rock mass classification methods include: Q-system of rock mass classification, Rock Mass Rating (RMR), and Geological Strength Index (GSI). The six different parameters of the Q-system mapped by an engineering geologist are: Rock Quality Designation (RQD), Number of Joint Sets (Jn), Roughness of the most Unfavorable Joint (Jr), Degree of Alteration or Filling in the Joint (Ja), Water Inflow (Jw), and Stress Reduction Factor (SRF).

<p>Ex-foliation joints will be formed at the outer part of the valley side slope. The main reasons for such joints are that the valley side slope is steep (over 35 degrees) and the major principal stress will follow almost parallel to the valley side slope where stress normal (minimum principal stress) to the valley side slope inclination will be too small causing stress an-isotropy. Most likely slope stability problem: In general, the surfaces of ex-foliation joints are smooth and planar (low roughness). Upon excavation on the slope for road, these joints will daylight to the road cut slope. This will increase the chance of plane slope failure.</p> Signup and view all the answers

<p>Given: Intact rock strength (σci): 150 MPa, Poisson ratio (θ): 0.2, Specific weight (γ): 0.0265 MN/m3, Rock cover (h): 600 m, Tectonic horizontal stress (otec): 25 MPa, Tunnel length (L): 6 km, Tunnel diameter (d): 10 m. Solution: Rock mass strength (cm): 0,35 x 150 = 52.5 MPa, Vertical minimum principal stress (σ3): 0.0265 x 600 = 15,9 MPa. Horizontal maximum principal stress (σ1) is: σ₁ = 1 (1-θ) × σ3 + σtec = (1-0,2) × 15,9 + 25 = 28,98 MPa. Maximum tangential stress (σθ-max) will be: σθ-max = 3σ1 – σ3 = 3 × 28,98 – 15,9 = 71,03 MPa. Minimum tangential stress (σθ-min) will be: σθ-min = 3σ3 − σ1 = 3 × 15,9 – 28,98 = 18,73 MPa.</p> Signup and view all the answers

<p>The tunnel will face a stability problem associated with rock burst due to high tangential stress. The area impacted by rock burst will be the tunnel roof because the maximum principal stress (σ1) is horizontal, and the maximum tangential stress (σθ-max) will be concentrated to the roof of the tunnel. Hence, the rock burst activity will be in the tunnel roof where stability problem will be met. The best support solution in a rock burst condition is the use of a combination support consisting of end-anchored rock bolts with large triangular plates (dynamic bolts) and the steel reinforced shotcrete.</p> Signup and view all the answers

<p>Factors influencing the choice of tunnelling excavation method include: Geology of the rock mass including its strength, hardness, rock type, and presence of faults or fractures. The presence of ground water and its impact on stability, and other limitations during excavation, including the ability to safely manage groundwater. The size and geometry of the tunnel. Environmental considerations such as noise and vibration impacts. Accessibility and availability of skilled labour and equipment, and logistics. Time constraints for the project, and cost considerations for different excavation methods. Other considerations may include: Regulations and safety standards, and the potential for environmental impacts during excavation.</p> Signup and view all the answers

<p>Factors influencing blast design that can impact excavation time and cost include: The type of explosives used. The amount of explosives used. The timing and sequence of blasts. The placement and configuration of blast holes. The timing of blasts influences drilling time and cost. The amount and type of explosive materials influences safety and efficiency. The sequence and timing of blasts significantly impact excavation time and cost, requiring highly skilled blasters. Placement and configuration of blast holes dictate the impact of vibrations, rock fragmentation, and cost of drilling. The amount of explosive required and the method of placement can influence the level of overbreak and the associated costs.</p> Signup and view all the answers

<p>Large overbreak in a drill and blast tunnel can occur due to factors related to geology, blast design, and excavation system factors (equipment, crew, and organization), Geology: The presence of weak rock, such as highly weathered or fractured rock, can lead to excessive overbreak. The presence of joints and faults can also contribute to overbreak as they can act as planes of weakness that allow the rock mass to break apart more easily. Blast design: Improper placement of blast holes. Use of excessive amounts of explosives can lead to overbreak. Inaccurate timing of blasts can also result in overbreak. Excavation system factors: Inefficient drilling techniques. Poor quality explosives, inadequate blasting equipment, or improper detonation can lead to overbreak. Crew and organization: Inadequate training among the blasting crew. Poor communication and coordination can also lead to overbreak. Lack of adequate supervision during excavation and blasting operations.</p> Signup and view all the answers