Tunnel 142jy

A tunnel is a age way that carries people or vehicle to across a destination that have obstruction or to shorten the traveling time.

TUNNELING THROUGH HARD ROCKS: FULL FACE METHOD: This method is suitable for small tunnels through stable and self-ing rocks. Since the full section is tackled in one continuous operation, additional units of tunneling equipment and man power is needed. The present day method of shield tunneling and Tunnel Boring Machine (TBM) are very suitable for full face method. The full face method is a particularly useful for small tunnels having diameter below 6 m and face area below 19 m2. The entire section is drilled, and the holes are loaded and the explosives are discharged. The debris or broken rocks are collected through mucking equipment In case of larger diameters the full face area increases above these limits and the equipment costs and problems make the operation uneconomical. In the case of poor ground conditions the problem becomes all the more difficult. This method has the following advantages:     

The operation is simple and minimum quantity of equipment is needed. Total ground disturbances and settlement are minimum. Simplicity of work. In situations where multi-face excavation cannot be adopted this system provides definite advantage. Mucking truck can be positioned once and for further mucking the position is shifted in according to the progress of the work.

DRIFT METHOD: Driving small sized heading. Blasting the sides of the drift to the required section. Then benching is carried out in stages

HEADING & BENCH METHOD: The top portion is driven in advance of the bottom portion. Is preferred when the rock strata is weak. So the top heading is driven well ahead of the bench, and the bench is used in installing to the roof.

CANTILEVER CAR DUMP METHOD: This method is primarily for providing an improved system of throwing the muck from the bench to the floor of the tunnel. The characteristics features of the system are:   

Provision of two plate girders about 23m in length fixed at I .8 m centre to centre distance. Belt conveyor lined with a number of jacks runs on the plate girders. The plate girder ends project beyond the full face of the bench.

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Front ends of the girders ed by means of struts from top of the tunnel. The blasted material (muck) is removed to sites outside the tunnel entrance. The quick disposal of the muck makes the system economical as in other system disposal of muck s for considerable expenditure making the operation costly.

TUNNELING THROUGH SOFT ROCKS: FORE-POLING METHOD:            

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The shaft is sunk at the desired location on the surface. Wooden bent is set up few centimetres from the sheeting and securely braced. Now the holes are drilled in the timber sheeting, above and below the cap The sheeting above the cap is cut along the top lines of the holes. Now the wedge end shaped “fore-poles” are driven through the cut in the sheets, into the ground at an inclination, up to their half lengths. A timber plank is placed along the roof and wedges are placed to press the hanging sides of the fore-poles from top. The face sheeting is broken along the lower line of holes below the cap and the excavation is started below the fore poles. Now horse head is placed about 60 cm from the sheeting. Spikes are then driven to full length. The excavation is done further into the shaft because the ends of the spikes are uned. A board known as breast board is provided. Now the next cap is provided 1.2 m ahead, and is held in position temporarily by a single post set on the bench The side spikes are driven to their full length A pair of needle-beams known as boom is provided to the forward cap and the excavation in the lower portion is done. These beams are placed on the sides of the tunnel with a post under the middle. While providing the booms the breasting boards are removed one by one. The breast boards are immediately reset one at a time at the front end. Immediately after excavating the portion of the tunnel to the grade, the legs and foot blocks are set and the load from the boom is transferred to the legs and foot blocks, and the booms are taken out Now the tunnelling work is repeated again.

NEEDLE BEAM METHOD:     

A monkey drift for a short distance of 900mm is driven beyond working face. The roof of the drift is ed by lagging carried on wooden segments, which are in turn ed by two trench jacks set in hitches cut in the sides of the monkey drift. The front end of the needle beam rests on planks on the floor of the drift, while the rear end is carried on stout posts resting on the lining of the tunnel floor. A trench jack is now placed on the center –line of the needle beam to the segment, thus transferring the roof load to needle beam. The other trench jacks are removed and the drift is widened sideways and ed as before, by laggings, segments and trench jacks ed on the sides of the needle beam.

LINER PLATE METHOD:

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In this method pressed steel liner plates (91cm x 41cm) are used to the soil during excavation work. First about 40cm deep hole is excavated at the crown and a liner plate is placed in position to the roof. Now the excavation is done on the side and liner plates are provided and bolted with central plate. The ex cavation is done in the lower side and the trench jacks are provided to the liner plates. After further excavation wall plates and wedges are provided at the ends of the lower liner plate and the load of the jacks is transferred to them. Now the trench jacks are removed and the excavation is started further in the above sequence.

TUNNEL BORING MACHINE: A tunnel boring machine (TBM) is a machine used to excavate tunnels with a circular cross section through a variety of soil and rock strata.

FACTORS TO SELECTION OF TBM:      

Type of soil Method of excavation Size of tunneling Amount of settlements Excavation capacity Load action during construction

DESCRIPTION OF MACHINE:        

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A tunnel boring machine (TBM) typically consists of one or two shields (large metal cylinders) and trailing mechanisms. At the front end of the shield is a rotating cutting wheel. Behind the cutting wheel is a chamber. The chamber may be under pressure (closed machine) of open to the external pressure (open machine). Behind the chamber there is a set of hydraulic jacks ed by the finished part of the tunnel which push the TBM forward. The rear section of the TBM is braced against the tunnel walls and used to push the TBM head forward. At maximum extension the TBM head is then braced against the tunnel walls and the TBM rear is dragged forward. Behind the shield, inside the finished part of the tunnel, several mechanisms which are part of the TBM are located: soil/rock removal, slurry pipelines if applicable, control rooms, and rails for transport of the precast segments. The cutting wheel will typically rotate at 1 to 10 rpm (depending on size and stratum), cutting the rock face into chips or excavating soil. Depending on the type of TBM, the muck will fall onto a conveyor belt system or into skips and be carried out of the tunnel, or be mixed with slurry and pumped back to the tunnel entrance. Depending on rock strata and tunnel requirements, the tunnel may be cased, lined, or left unlined.

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This may be done by bringing in precast concrete sections that are jacked into place as the TBM moves forward, by assembling concrete forms, or in some hard rock strata, leaving the tunnel unlined and relying on the surrounding rock to handle and distribute the load. While the use of a TBM relieves the need for large numbers of workers at increased pressure, if the pressure at the tunnel face is greater than behind the chamber a caisson system is sometimes formed at the cutting head this allows workers to go to the front of the TBM for inspection, maintenance and repair if this needs to be done under pressure the workers need to be medically cleared for work under pressure like divers underwater and to be trained in the operation of the locks. Modern TBMs typically have an integrated shield. The choice of a single or double shielded TBM depends on the type of rock strata and the excavation speed required. Double shielded TBMs are normally used in unstable rock strata, or where a high rate of advancement is required. Single shielded TBMs, which are less expensive, are more suitable to hard rock strata.

The size of the shield is limited by: 

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The cutter-head. Indeed the bigger the size the stronger structure of the head must be. As a result there is less room for openings and therefore more difficulties to let the bored materials located closed to the axis of the head. Movement from the cutting chamber. If the muck is too dry it is difficult to remove it from the cutting chamber. If it is too wet the pressure at the entrance of the screw conveyor (hydrostatic behavior but with a high density) is too high and the ability to keep a proper pressure gradient along the screw not achievable. This means there is no pressure drop through the screw conveyor therefore a very high risk of collapse of the front face.

The speed of the shield:   

This depends on the pressure to the cutter-head and is linked to available power for the torque demand of the cutter-head. The speed is also highly related to the compressive strength of the soil or rock and how it is fractured in case of rock. The wear of the cutting tools will reduce the advance rate of the shield.

COMPONENTS OF TBM:   

Rotating cutting head Metal shields ing mechanisms.

ROTATING CUTTING HEAD:     

This is located in front of the TBM. It is the part that does the actual “digging” although it is more of a cutting action. The cutting wheel will rotate at different speeds (1 to 10 rpm) according to the soil type. Rotating head diameters can range from a meter (done with micro-TBMs) to almost 16m Diameters vary from 1 to 16 m

METAL SHIELDS: 

Used to brace the machine against the wall tunnels

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This is a large metal cylinder used to brace the machine against the wall tunnels. There can be one or two shields depending on the TBM type and type of soil to be excavated. Rock type soils can be excavated with a one-shield machine while sand soil digging requires a two-shield TBM.

ING MECHANISMS: 

Behind the cutting wheel and the shields TBMs have a vast array of ing mechanisms such as dirt removal, control rooms, rails, transporting system, pipelines (if slurry is created), etc.

SHOTCRETE: 

Shotcrete is concrete conveyed through a hose and pneumatically projected at high velocity onto a surface, as a construction technique and undergoes placement and compaction at the same time.