Mechanized Tunnelling and Shaft Sinking Design in Rock Engineering CE7205T

1. Describe the methods of mechanized tunnelling available. What are the advantages of mechanized methods of tunnelling over drill and blast methods?

Mechanized Tunneling Methods:

Methods Available:

Tunnel Boring Machines: TBM represents very mechanized systems that tunnel into rock by rotating a cutter head fitted with disc cutters. They can be shielded or open-type depending on the ground conditions.

Road headers: These machines use a rotating, rock-cutting head mounted on a boom. They are flexible and can cut both soft rock and hard rock, and they find uses mostly in mines and civil projects.

Advantages over Drill and Blast:

Continuous Operation: Mechanized methods can provide for continuous tunneling and reduce idle-time, enhancing operational efficiency.
Safety: They reduce risks of accidents connected with the use of explosives and can provide better control over ground stability.
Precision: TBMs can create tunnels of accurate dimensions with smooth finishes. This reduces extra support or finishing work.

2. What techniques are available for shaft sinking in wet and/or unstable ground? (Minimum two techniques are expected to be briefed to get full marks.)

Caisson Method: A large prefabricated hollow structure, normally called a caisson, is sunk into the ground. It performs the dual function of guiding and forming the final concrete liner; this is suitable for unstable or water-bearing grounds.
Ground Freezing: Pipes are inserted, and refrigerant is circulated through them to freeze the surrounding soil, forming a temporary but firm barrier against water ingress for its safe excavation and lining.
Grouting: This method involves the injection of grout into the ground. It may be done prior or during the process of excavation to stabilize the ground and reduce inflow. Grouting may increase general strength as well as cohesion in the ground.

3. A shaft is planned to sink to the depth of 500m below ground surface using cast-in-place concrete liner. You are required to design the thickness of the shaft liner at the depth of 100m, 300m and 500m, respectively, assuming the unit weight of rock over the depth is 27 kN/m3. (Hint: you may have to refer to the In-situ Stress Estimation in Rock Mechanics Applications and Shaft Lining Design in Ch.12.4 of the classnote.)

The in-situ vertical stress at a given depth can be calculated using the following formula:
v=?H
Where:

• ?v = Vertical stress (kN/m?2; or MPa)
• ? = Unit weight of rock (27 kN/m?3;)
• H = Depth below ground surface (in meters)

For Depth of 100m:
?v=27?kN/m3100?m=2700?kN/m2?

For Depth of 300m:
?v=27?kN/m3300?m=8100?kN/m2?or?8.1?MPa

For Depth of 500m:
?v=27?kN/m3500?m=13500?kN/m2?or?13.5?MPa

Step 2: Determine Shaft Liner Thickness
The thickness (t) of the concrete liner is calculated as follows
t=?vD/2fc

Where:

• D= Diameter of the shaft (in meters)
• fc = Allowable compressive strength of concrete (in MPa)

Assuming a shaft diameter of 6 meters and using a typical allowable compressive strength for concrete fc = 30 MPa
For Depth of 100m:
t=2.7?MPa6?m/230?MPa=16.260?m=0.27?m?
For Depth of 300m:
t=8.1?MPa6?m230?MPa=48.660?m=0.81?m?or?810?mm
For Depth of 500m:
t=13.5?MPa6?m230?MPa=8160?m=1.35?m?or?1350?mm

4.If the excavation diameter of the shaft is 6.0m, allowable time for mucking is 4.0hr per shift and the advance rate of shaft sinking is one round of 3.0m per shift, calculate the bucket size required for the mucking unit. (Hint: Swell factor and cycle time)

The volume of excavated material per round can be calculated using the formula for the volume of a cylinder:

V=?(D/2)2h

Where:

• V = Volume of excavated material (m?3;)
• D= Diameter of the shaft (6.0 m)
• h= Depth per round (3.0 m)

V=?(6.0/2)23.0=?93.0= 84.78m3
The swell factor accounts for the increase in volume of the material when it is excavated and broken up. Assuming a typical swell factor for rock of around 1.5:

Vswell=Vswellfactor=84.78?m31.5=127.17?m3

A typical Tc of 5 minutes per round trip (loading, hauling, dumping, and returning). The total number of cycles available during the mucking time:
TotalCycles=Allowabletimeformucking/Cycletime=4?hours60?minutes/hour/5?minutes=48cycles

Determine the Bucket Size

The required bucket size is computed as

Bs=Vswell/TotalCycles=127.17?m3/48=2.65?m3

The required bucket size for the mucking unit is approximately 2.65 m?3;.

5. A 6 m diameter TBM has been selected to undertake a tunneling operation. It is to operate at 4 rpm with a cutter head power of 750 kW in medium strength rock. Determine the predicted machine excavation rate using the monograph presented in the notes.

The specific energy is
SE=P/Q

• P = Cutter head power (in kW)
• Q = Volumetric excavation rate (in m?3;/hour)

Convert the power into Joules per second (if needed):

P=750?kW=750103?J/s

Assume or derive the specific energyfrom the monograph for medium strength rock (e.g., 10 MJ/m?3;):

Q=750103?J/s10106?J/m3

Simplify:

Q=0.075?m3/s

Convert to m?3;/hour

Q=0.0753600=270?m3/hour

6.(a) Calculate the direct unit cost of a 5.0 x 5.0 m drill and blast development in sandstone given the following:

Rock S.G 2.6
Labour cost $400/shift including on costs
Productivity 1.4m/shift
Rock bolts and mesh $25 per m2 supported
Depth drilled per round 3.6 m
Depth pulled per round 3.6 m
Overbreak 10%
Overall explosives cost average $2.40/kg
Materials cost excluding explosives and ground support $150/m
Jumbo drilling at $3.50/m drilled
Mucking and trimming $3.40/tonne

Detail any other required assumptions.

Volume of Rock Excavated per Shift
Volumepershift=AreaoffaceProductivity
Area of face = 5.0 m 5.0 m = 25 m?2;
Productivity = 1.4 m/shift
Volumepershift=25?m21.4?m=35?m3/shift

2. Weight of Rock Excavated per Shift

Weightofrock=VolumepershiftRockS.G1000?kg/m3
Where:
Rock S.G = 2.6
Weightofrock=35?m32.61000?kg/m3=91000?kg=91?tonnes/shift

3. Cost of Drilling
Drillingcost=Depthdrilledperround Areaofface Jumbodrillingrate
Where:
Depth drilled per round = 3.6 m
Jumbo drilling rate = $3.50/m drilled
Drillingcost=3.6?m25?m23.50?$/mdrilled=315$

4. Cost of Explosives
Explosivescost=WeightofrockExplosivescostperkg
Where:
Explosives cost per kg = $2.40/kg
Explosivescost=91?tonnes/shift1000?kg/tonne2.40?$/kg=218400?$

5. Cost of Rock Bolts and Mesh
Supportcost=AreaoffaceSupportcostperm2
Where:
Support cost per m?2; = $25/m?2;
Supportcost=25?m225?$/m2=625?$
Cost of Mucking and Trimming
Muckingcost=WeightofrockMuckingrate
Where:
Mucking rate = $3.40/tonne
Muckingcost=91?tonnes/shift3.40?$/tonne=309.40?$
Materials Cost Excluding Explosives and Ground Support
Materialscost=ProductivityMaterialscostpermeter
Where:
Materials cost per meter = $150/m
Materialscost=1.4?m/shift150?$/m=210?$
Labour Cost
Labourcostpershift=400?$

Total Direct Cost Per Shift

TotalCostperShift=Drillingcost+Explosivescost+Supportcost+Muckingcost+Materialscost+Labourcost
Total Cost per Shift} = 315 + 218400 + 625 + 309.40 + 210 + 400 =$ 220259.40

Unit Cost Per Meter of Development

Unit Cost=TotalCostperShift/productivity
UnitCost=220259.40$/1.4m/shift =157328.14$/m

(b) If the tunnel is 260 m long, what is the total cost for the development? How many days are required for the development? (Requirements: detail the annual working days, shifts per days.)

1. Total Cost for the Development

TotalCost=UnitCostLengthofTunnel

Where:

• Length of tunnel = 260 m

TotalCost=157328.14?$/m260?m=40905316.40.

2. Time Required for the Development

TotalShiftsRequired=LengthofTunnel/Productivity
TotalShiftsRequired= 260m/1.4m/shift =185.71shifts

3. Working Days Calculation

Assume:

• Shifts per day = 1 (assuming a single shift operation)
• Working days per year = 250 days/year

TotalDaysRequired=185.71?shift/day

Thus, approximately 186 days are required to complete the development.

Summary

• Unit Cost per Meter: $157,328.14 per meter
• Total Cost for 260 m Tunnel: $40,905,316.40
• Total Time Required: 186 days

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