149x Filetype PDF File size 1.41 MB Source: old.amu.ac.in
Types of Tunnels & Construction Methods (a) Mountain Tunnel -- Drilling and blasting (D&B) method: NATM (New Austrian Tunneling Method) is the most common method. It originates in hard rock tunneling and utilizes rockbolts and shotcrete applied immediately after blasting. This is often followed by a cast in-situ concrete lining using formwork. -- Tunnel Boring Machine (TBM) method: TMB is used as an alternative to drilling and blasting (D&B) methods. TBMs are used to excavate tunnels with a circular cross section through a variety of subterranean matter; hard rock, sand or almost anything in between. As the TBM moves forward, the round cutter heads cut into the tunnel face and splits off large chunks of rock. The cutter head carves a smooth round hole through the rock -- the exact shape of a tunnel. Conveyor belts carry the rock shavings through the TBM and out the back of the machine to a dumpster. Tunnel lining is the wall of the tunnel. It consists of precast concrete segments that form rings, cast in-situ concrete lining using formwork or shotcrete lining. (b) Shallow-buried Tunnel or Soft Soil Tunnel Shallow tunnels are of a cut-and-cover type (if under water of the immersed-tube type). Deep tunnels are excavated, often using a tunnelling shield. For intermediate levels, both methods are possible. -- Cut-and-cover method: Cut-and-cover is a method of tunnel construction where a trench is excavated and roofed over. Strong supporting beams are necessary to avoid the danger of the tunnel collapsing. -- Shield method: The Shield method uses one or two shields (large metal cylinder) to cut out a tunnel through the soft ground. A rotating cutting wheel is located at the front end of the shield. Behind the cutting wheel is a chamber where, depending on the type of the TBM, the excavated soil is either mixed with slurry (called slurry TBM) or left as is (earth pressure balance or EPB shield). Systems for removal of the soil (or the soil mixed with slurry) are also present. Behind the chamber is a set of hydraulic jacks supported by the finished part of the tunnel which are used to push the TBM forward. Once a certain distance has been excavated (roughly 1.5-2 meters), a new tunnel ring is built using the erector. The erector is a rotating system that picks up pre-cast concrete segments and places them in the desired position. Behind the shield, inside the finished part of the tunnel, several support mechanisms can be found that are part of the TBM: dirt removal, slurry pipelines if applicable, control rooms, and rails for transport of the precast segments, etc. (c)Underwater Tunnel -- Immersed-tube method: The immersed tube tunnel technique uses hollow box sectioned tunnel elements that have been prefabricated in reinforced concrete. These are floated out into the harbor and placed into a trench that was pre-dredged in the harbor bed. When in position, the elements are joined together to form a tunnel. The trench is then refilled and the harbour bed returned to its original level. -- Shield method: As previously stated. Tunnel Construction Methods and their comparison 1. Introduction – This paper gives a general description of the tunnelling techniques such as cut and cover, drill and blast, bored tunnelling and sequential mining construction, reviewed for possible use in various projects. A summary of environmental merits and demerits associated with these methods are also given. Above tunnelling techniques are mostly used to construct small tunnels and find their applications in utility projects to a great extent. 2. Construction Methods: a. 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 underground 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. b. Drill and Blast – This tunnelling method involves the use of explosives. Drilling rigs are used to drill blast holes on the proposed tunnel surface to a designated depth for blasting. Explosives and timed detonators (Delay 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 by Tunnel 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. c. Bored Tunnelling by Tunnel Boring Machines (TBM) – 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. d. Sequential Excavation Method – This method is also known as the New Austrian Tunnelling Method (NATM). 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 road- headers 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. 3. Environmental Merits and Demerits – Selection of the techniques to be adopted for construction of a tunnel section shall take into account the nature of the substrata and the levels of the tunnel involved. A summary of the environmental advantages and disadvantages associated with the construction methods is tabulated below: Tunnel Construction Environmental advantages and disadvantages Methods (on relative terms) Disadvantages: More dust and noise impact may arise, though these can be mitigated through implementation of sufficient control measures; Temporary decks are often installed before bulk excavation to minimise the associated environment impacts; Larger quantity of C&D materials would be generated from the excavation works, requiring Cut and cover tunnelling proper handling and disposal. Advantages: 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; Compared with the cut-and-cover approach, quantity of C&D materials generated would be much reduced; Compared with the cut-and-cover approach, disturbance to local traffic and associated environmental impacts would be much reduced; Blasting would significantly reduce the duration of vibration, though the vibration level would be higher compared with bored tunnelling (with proper blast design & techniques vibration can be reduced); Drill and blast Disadvantages: 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: 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; Compared with the cut-and-cover approach, disturbance to local traffic and associated environmental impacts would be much reduced; Compared with the cut-and-cover approach, quantity of C&D materials generated would be Bored tunnelling by TBM much reduced; Advantages: Similar to the drill-and-blast and bored tunnelling methods, only localised potential environmental impacts would be generated; Disadvantages: As the method is relatively slow, duration of Sequential Excavation potential environmental impacts would be Method longer than that of the other methods. Tunnel Excavation by Tunnel Boring Machine (TBM) & Parameters Major components of this Tunnel Boring Machine include: 1. Cutter head, with cutting discs/tools and 2. Muck buckets to carry and dispose excavated muck 3. Power supply Systems 4. Cutter head rotation & thrust 5. Bracing system for the TBM during mining 6. Equipment for ground support installation 7. Shielding to protect workers 8. Steeling system
no reviews yet
Please Login to review.