Tunnels

Question 1

Drilling and Blasting

Answer 1

This method is mainly used when excavating rock or where the diameter of tunnels is too small for large excavating machines.
Short length of tunnels are excavated using this method.
Traditional methods use explosives in excavations, modern technology makes it possible to be fairly accurate in achieving the desired effect.
One of the factors affecting tunneling is the stability of rock, techniques to deal with situation:
*Advance the heading without support.
*Advance the heading in drifts followed by full-face support.
*Advance the heading in drifts followed by progressive support.

Question 2

Full-face heading without support

Answer 2

Used when rock is fairly stable (after blasting) and can support itself without the need for propping.
Tunnels with cross section up to 200m² are suitable.
Holes are drilled, into which the explosives are placed, then blasting takes place.
Loose rock and debris is loaded into trucks and carted away, once cleared the operation continues until the tunnel is completed.

Question 3

Advance the heading in drifts followed by full-face support

Answer 3

The heading is opened in segments or drifts.
If the rock is fairly stable and unlikely to suffer fracture/bursting as the stresses in the rock are releases, the drifts are sequentially advanced and temporarily supported until a complete ring of permanent lining can be put into place.
Ass the headings is enlarged, props are used for initial support.
Permanent lining is constructed when a full face has been excavated and the props are then removed.
The face progressively advances in this manner.

Question 4

Advance the heading in drifts followed by progressive support.

Answer 4

Method is used when soft or fractured rock is encountered.
It becomes dangerous to leave the full face of the tunnel unsupported and permanent lining is installed progressively as the drifts advance.
Different methods such as tunneling with a shield are used for excavating and propping until lining is in place.

Question 5

non-TBM methods

Answer 5

Because of high cost of timbering and specialist skills required to install the initial supports, the traditional methods of tunnelings have gradually been modified as technical improvements to support systems and permanent linings have been developed.

Modern non-TBM methods use one or more of:

• Sprayed concrete
• Rock bolting
• Stiffening ribs and liner plates
• Full lining

Question 6

Sprayed concrete

Answer 6

The use of sprayed concrete of aggregate size up to 25mm to which chemical strengthening improves adhesion and setting times.

Question 7

Rock bolting

Answer 7

The use of steel bolts in conjunction with sprayed concrete to provide support for tunneling roofs.
Number of bolts and spacing required depends on: Diameter, Material and Type of anchorage of bolt.

Question 8

Stiffening ribs and liner plates

Answer 8

Where friable, shattered rock and certain categories of soft ground exist, and where immediate heavy ground pressure is encountered, neither spraying concrete nor rock bolting will work.
In such conditions, additional propping with steel ribs is required.
Face should be excavated in segments and parts of the rib temporarily propped until a complete unit of support can be formed.

Question 9

Full lining

Answer 9

When the permanent lining is designed in either cast iron or precast concrete, it can perform similar function to liner plates.
Method is to build a ring of lining from the bottom upward.
This method may be used in conjunction with rock bolting to increase working space ( lining and excavation are do not become congested).

Question 10

Support systems and linings

Answer 10

When constructing a tunnel, it usually requires permanent lining which can also be utilised as temporary support during construction.
This lining follows shortly after excavation of face of tunnel.
1. Segmental form: Consists of cast iron units or precast concrete segments. Cast iron are 1-2 m long by 0.5-1 m wide, are bolted together to privide strong lining that can resist external loads of surrounding ground. Precast were introduced to avoid high cost of cast iron.
2. In situ reinforced concrete: Expensive cast iron lining in rock is not required and roof can remain unsupported and reinforced sprayed concrete is adequate. If more strength is required, normal reinforced concrete can be cast specially designed travelling formwork.
3. Masonry: Is one of the older tunneling methods where complete lining is constructed of masonry brickwork.
Today are mostly used to provide protective coating for other lining systems in tunnels.