Can explain the following tunnel construction methods: 1. Mechanical drilling/cu
ID: 1845368 • Letter: C
Question
Can explain the following tunnel construction methods:
1. Mechanical drilling/cutting
a. Type of ground (Soft soil, Hard soil, Soft rock, Hard rock)
b. Types of Excavations
c. Advantages
d. Disadvatages
2. Cut-and-cover
a. Type of ground (Soft soil, Hard soil, Soft rock, Hard rock)
b. Types of Excavations
c. Advantages
d. Disadvatages
3. Drill and Blast
a. Type of ground (Soft soil, Hard soil, Soft rock, Hard rock)
b. Types of Excavations
c. Advantages
d. Disadvatages
4. Tunnel Boring Machines (TBMs)
a. Type of ground (Soft soil, Hard soil, Soft rock, Hard rock)
b. Types of Excavations
c. Advantages
d. Disadvatages
5. New Austrian Tunnelling Method (NATM)
a. Type of ground (Soft soil, Hard soil, Soft rock, Hard rock)
b. Types of Excavations
c. Advantages
d. Disadvatages
6. Immersed tunnels
a. Type of ground (Soft soil, Hard soil, Soft rock, Hard rock)
b. Types of Excavations
c. Advantages
d. Disadvatages
Explanation / Answer
Cut and cover tunnelling :-
Cut and cover tunnelling is a common and well-proven technique for constructing shallow tunnels. The method can accommodate changes in tunnel width and non-uniform shapes and is often adopted in construction of stations. Several overlapping works are required to be carried out in using this tunnelling method. Trench excavation, tunnel construction and soil covering of excavated tunnels are three major integral parts of the tunnelling method. Most of these works are similar to other road construction except that the excavation levels involved are deeper. Bulk excavation is often undertaken under a road deck to minimise traffic disruption as well as environmental impacts in terms of dust and noise emissions and visual impact.
Disadvantages:-
1) More dust and noise impact may arise, though these can be mitigated through implementation of sufficient control measures;
2) Temporary decks are often installed before bulk excavation to minimise the associated environment impacts;
3)Larger quantity of C&D materials would be generated from the excavation works, requiring proper handling and disposal.
Advantages :-
1)In locations with no important constraints on the surface cut and cover tunnels are inexpensive compared to other tunnelling techniques. Approaches to immersed or bored tunnel sections will often be constructed as cut and cover tunnels.
2)The advantages of this method consist in the reduced time of extended worksite occupations and the rate of reinstatement and release of the area for use (vehicular circulation, squares etc) for use and finally the mitigation of disturbance as to the functions of the city.
Drill and Blast:-
This tunnelling method involves the use of explosives. Drilling rigs are used to bore blast holes on the proposed tunnel surface to a designated depth for blasting. Explosives and timed detonators are then placed in the blast holes. Once blasting is carried out, waste rocks and soils are transported out of the tunnel before further blasting. Most tunnelling construction in rock involves ground that is somewhere between two extreme conditions of hard rock and soft ground. Hence adequate structural support measures are required when adopting this method for tunnelling. Compared with bored tunnelling byTunnel Boring Machine (see below), blasting generally results in higher but lesser duration of vibration levels. A temporary magazine site is often needed for overnight storage of explosives.
Disadvantages:-
1)Potential hazard associated with establishment of a temporary magazine site for overnight storage of explosives shall be addressed through avoiding populated areas in the site selection process.
Advantages:-
1)Potential environmental impacts in terms of noise, dust and visual on sensitive receives are significantly reduced and are restricted to those located near the tunnel portal;
2) Compared with the cut-and-cover approach, quantity of C&D materials generated would be much reduced;
3) Compared with the cut-and-cover approach, disturbance to local traffic and associated environmental impacts would be much reduced;
4)Blasting would significantly reduce the duration of vibration, though the vibration level would be higher compared with bored tunnelling.
Tunnel Boring Machines:-
Bored tunnelling by using a Tunnel Boring Machine (TBM) is often used for excavating long tunnels. An effective TMB method requires the selection of appropriate equipment for different rock mass and
geological conditions. The TBM may be suitable for excavating tunnels which contain competent rocks that can provide adequate geological stability for boring a long section tunnel without structural support.However, extremely hard rock can cause significant wear of the TBM rock cutter and may slow down the progress of the tunnelling works to the point where TBM becomes inefficient and uneconomical and may take longer time than the drill-and-blast tunnelling method.
Advantages:-
1)Potential environmental impacts in terms of noise, dust and visual on sensitive receives are significantly reduced and are restricted to those located near the launching and retrieval shafts;
2)Compared with the cut-and-cover approach, disturbance to local traffic and associated environmental impacts would be much reduced;
3) Compared with the cut-and-cover approach, quantity of C&D materials generated would be much reduced.
Disadvantages:-
1)The main disadvantage is cost of this construction is so expensive.
New AustrianTunnelling Method :-
This method is also known as the sequential excavation method. The excavation location of a
proposed tunnel is divided into segments first. The segments are then mined sequentially with supports.Some mining equipments such as roadheaders and backhoes are commonly used for the tunnel excavation. The ground for excavation must be fully dry for applying the NATM and ground dewatering is also an essential process before the excavation. Another process relates to the ground modifications such as grouting, and ground freezing is also common with this method in order to stabilize the soil for tunnelling. This method is relatively slow but is found useful in areas where existing structures such as sewer or subway could not be relocated.
Advantages:
1)Similar to the drill-and-blast and bored tunnelling methods, only localised potential environmental impacts would be generated;
Dis-advantages:
1)As the method is relatively slow, duration of potential environmental impacts would be longer than that of the other methods
Immerse tunnels:-
This type of tunnels partly or wholly are underwater.They don’t block the root for ships so there problem of congession traffic as in case of bridges over rivers or seas.
Advantages:-
1)Their speed of construction
2)Minimal disruption to the river/channel, if crossing a shipping route
3)Resistance to seismic activity
4)Safety of construction (for example, work in a dry dock as opposed to boring beneath a river)
5)Flexibility of profile (although this is often partly dictated by what is possible for the connecting tunnel types)
Disadvantages:-
1) immersed tunnels are often partly exposed (usually with some rock armour and natural siltation) on the river/sea bed, risking a sunken ship/anchor strike
2) Direct contact with water necessitates careful waterproofing design around the joints
3)The segmental approach requires careful design of the connections, where longitudinal effects and forces must be transferred across
4) Environmental impact of tube and underwater embankment on existing channel/sea bed.