wallap and rs2

Rock Anchor Details
Total length – 4 m, Diameter – 24 mm, Reinforcement direction – 150, Bond diameter – 0.13 m, Bond skin friction – 250 kPa, Bond resistance – 25.52 kN/m, Spacing - 2 m x 2 m, Bar capacity – 180 kN, Safety factor – 1.8, Grout Strength - 40 MPa

Construction Sequence
1. Excavate 2.5 m and install the 1st anchor at RL 172.0m
2. Excavate up to 4.5 m and install the 2nd anchor at RL 170.0m
3. Excavate up to the final level (RL159.5m) with 4 more equal lifts (2.5m high)

Loading and water table assumptions
• During the construction – Short term – Live load over the surface is 10 kPa
• Short term water table – 3.0m above SS-III
• At service stage – Long term live load over the surface is 25 kPa
• Long water table – (i) 1.0m below SS-III, (ii) 1.0m below SS-V
Questions 1:
Calculate Global Factor of Safety (FoS) for following cases using SLOPE/W
1. Stage 1 excavation
2. Stage 2 excavation
3. Calculate FoS for each excavation lift
4. Long term for final excavation, with fluctuation of water table (low and high)
5. Plot excavation depth vs FoS

Question 2:
Provide three reasons for difference in FoS at final excavation in short term and long term.