|SCIENCE & TECHNOLOGY|
The Delhi Metro Rail Corporation Ltd is putting to good use three tunnel boring machines (TBMs) for construction of 4-km reach of the Phase I of the Delhi MRTS (Mass Repaid Transit System) which was approved in 1996. Two TBMs are EPBM (Earth Pressure Balance Machine) type and one is dual mode machine capable of working both soil and hard rock.
With these machines twin tunnel section of 18.7 ft diameter is being constructed to handle up and down traffic.
The bored tunnels extend from Kashmere Gate to Delhi Main and then down to Chowri Bazar-New Delhi-Connaught Place and ends at Patel Chowk. Out of these, the reach between Delhi Main to New Delhi is being tackled by EPBMs and the rest by dual mode TBMs.
Working of TBMs
In case of TBMs, tunnelling does not involve drilling and blasting but a rotating cutting head to dislodge the material by scrappers or disc cutters.
The cutter head is capable of rotating at one to six rpm and exerting torque of 4300 KNm and requires power of 945 kw. In order to stabilise the dislodged strata, injection of foam under pressure is carried out.
This operation renders the dislodged soil into a plastic form which is removed by Screw Auger and delivered to a belt conveyor which delivers the excavated material to muck removal wagons stationed at back of the machine.
The forward motion of TBM is obtained by its jacks reacting against the completed lining of the tunnel. In the initial stage the TBM attain the required thrust by reacting against a steel frame specially installed in the launching shaft.
After completion of 50 rings of lining the help of the steel frame is dispensed with. When excavation cycle is completed, in the resulting gap pre-cast RCC lining segment is placed and grouting carried out. There are five segments to one ring with one fixing key.
In case of Delhi Metro two EPBMs obtained are second hand i.e. they had worked on metros of Bangkok and Taiwan. Before being put into use at Delhi these machines were completely refurbished or overhauled.
The dual mode machine is brand new specially manufactured by Herrencht of Germany and costs about Rs 30 crore. The main difference between the two type of machines is Cutter Head design.
In the case of EPBM the cutter head is fitted with scrapping picks and in the dual mode disc cutters made of very hard metal are installed.
In the case of EPBM, the excavated face is supported by pressurisation of foam. Thereafter a screw auger starts working on the stabilised soil mix.
The screw-auger is so designed to adjust and maintain the pitch of screw that pressure is maintained within the chamber, which is slightly above the outside hydraulic pressure.
As the material is moved along the screw conveyor, the pressure within the screw drops gradually to the atmospheric pressure at the discharge end. Each TBM unit weighs about 252 tonnes and the whole system, including pickup gantries (5 nos) measures 230 ft. and out of this the length of the shield is 13 ft. and that of Tail Skin is 18 ft.
The total power requirement of the TBM is around 3 MW. The earth pressure within the cutter Head is maintained to balance the rate of advance of the spoil material by adjusting the involved parameters.
The gap between the excavated surface and the precast concrete lining rings is filled by automatic injection of grout from the tail of the machine which result in minimising the settlement of ground located above the tunnel bore.
As a part of the ‘triple sealing system’ continuous release of lubricating grease around the shield is affected, which ensures not only water tightness of the annular space between the tail skin and the segments of lining placed but also ‘facilitate’ the forward movement of the shield by reduction of skin friction.
Once the excavation is done for one ring, excavation is stopped and segments of rings or lining are erected in the space created with the help of Erector.
The jacks for one segment which is under ring placement, are withdrawn making available space. After the placement of a ring, the jacks are extended once again to react against the complete lining.
The end of the jacks which react against placed segments are provided with "PVC Pads" so as to avoid any damage to concrete lining against which jack press and obtain reacting force.
Working of rock TBM
Geology of the work areas vary from soft soil or alluvial or silty soil to quartzite rock strata in the various stages of weathering and frequently thin bands of mica schists are encountered in the quartzite rock in the Delhi Metro work area.
For such geological conditions, EPBM are not suitable and TBM with open face mode are required.
At Delhi Metro, during initial stages some problems were encountered and some changes of the cutter face had to be carried out by gradually the working condition got stabilised and the progress of placement of 4 to 5 rings per day was obtained.
Placement of rings
Rings are first erected inside the tail skin and checked for any cracks or damage. Once the tail skin moves forward, the annular gap obtained between soil and the ring is filled with released grout.
Excavation and grouting activity proceeds simultaneously. The excavation and erection of ring marks one complete cycle of operation of TBM.
The tunnel rings for the Delhi Metro are casted in the facility setup at Nangloi, where specially fabricated precision moulds obtained from France are being used. M-45 Grade concrete is used for casting the rings. Width of each ring is about 4 ft and thickness of lining ring is about 11 inches.
Each segment used as permanent ring is fitted with EPDM gasket into pre-cast grooves which extend around parameter of the ring.
Under thrust from the adjacent ring, the gasket gets compressed and acts as effective water tight seal.
Another water proofing measure is the placement of hydrophilic seal which is fixed below the EPDM gasket, so as ensure the complete water tightness of the tunnel.
The hydrophilic seal is fixed to all the segments on the two faces, one longitudinal and the other circumferential joint.
The hydrophilic seal on coming in contact with water expands 2.5 times, and thus ensures water tightness.
With the triple seal so obtained, the water tightness of the completed tunnel is obtained to give one litre per day for every hundred square foot area of the placed lining surface.
For tackling the curves in the tunnel alignment, both types of TBMs are sufficiently articulated to be able to tackle curves of radius as small as 820 ft.
The machines are guided system which is fully computerised to ascertain the actual location and altitude of the machine with respect to DTA (Design Tunnel Alignment). The steering of the machine is effected by the controlling the hydraulic pressure of the reacting jacks.
There are 40 jacks, divided in five segments, which react against the placed lining ring. Each group of eight jacks has got one control on the panel in the operation cabin, from which control is affected.
Certain facilities meeting the requirement of the machines are located at the ground surface near the shafts.
Two shafts, one at the start of the tunnel and other at the end of the tunnel, are located for lowering the machine to tunnel bed for start of the tunnel operation and the other for pulling out the machine after it had completed the work.
The facilities located are: i) Diesel Generators, 4 x 1.5 MW supplying the power of the machines, ii) air compressors for meeting the demand of the machine of keep pressure at the required level, iii) Chilled water for cooling the hydraulic oil of the machine, iv) the ventilation system which supply clean air to the ventilating duct. The length of the duct is kept extending as the tunnel length goes on.
A progress of five rings (20 ft) advance in a day has been attained for Delhi Metro is the case of Rock TBM. But in the case of EPBM, the progress attained is higher i.e. 6 rings placement in a day.
The working of the tunnel construction of Delhi, except for two cases of leakage of foam to the surface and one case of escape of excavated material into a nearby well, has been free of any accident.
In the cases listed above there was no loss of life and damage to property except some inconvenience to people residing in the building for a few hours.
All along the tunnel alignment, a close watch is being kept on the ground settlement. For the purpose, precise levelling of set of points is called out regularly.
Observed data show that except for one point where the settlement of 30 mm was observed, elsewhere it is less than 10 mm.
Tunnelling with EPBM for Delhi Metro in soft soil areas has been found to be safe, economical and efficient and can be replicated elsewhere where Metro Rail construction is to be undertaken.
Airplanes require long runways for takeoff and landing. Then how do planes take off from shorter runways of aircraft carriers?
In order to become airborne the airplanes must acquire a speed when the upward thrust on the wings becomes greater than the weight of the plane. The passenger aircraft capable of carrying hundreds of people with all their baggage, some freight and lots of gasoline are rather heavy. They do need higher speeds to take off than lighter aircraft to develop the required thrust. The airplanes used on aircraft carriers are not that heavy. Sometimes they can be given extra starting speed by using a catapult on the carrier. I feel that since they start their run on the deck of the carrier that is several stories high and is slightly sloped up near the end, the effective runway available for take off is longer than the physical length of the carrier. The important need on the carrier is to ensure that the aircraft can come to a stop after landing. For this the planes use decelerating parachutes and finally are captured by steel ropes stretched across the deck whose restraint is regulated by a massive hydraulic system under the deck.
What is the secret behind the behaviour of the "TOUCH ME NOT" plant?
Plants are living things and depending on their environment and genetic history they develop ways of nourishing and/or protecting themselves. There are plants that live on insects. In their case when an insect happens to touch the inner surface of a leaf or flower cup the leaf or the petals fold up to trap the insect inside and then the plant proceeds to devour it with the help of secretions on the inner surface. In other cases the leaves of the plant fold up on touch so as to appear like a stick or a useless needle to a hungry insect on prowl - they protect themselves from being eaten up. The physical mechanism might be an ability to vary the pressure of the liquid inside the leaf stem through energising some valve mechanism at the point where the leaf takes off from the stem of the plant. I have to confess that these answers are based more on observation of what is than on detailed understanding of the way signals are transmitted and the exact mechanisms that produce the behaviour. Living things are not robots that we have designed. Often we know the reaction to stimuli and not the way this reaction is produced.
Why do tears come when we peel onion?
Onion's tissues and cells
have some compounds that are released in a sort of a mist when the knife
cuts through them. This compound is some kind of a sulfoxcide. It is
said that the mist containing this compound forms sulfuric acid when it
encounters the wet surfaces in our noses and eyes. This causes
irritation. Eyes water, presumably to wash out the irritating effect of
this incursion, and noses run. It has been suggested that cutting onions
under a layer of water could reduce the problem. It seems to me that a
table fan blowing away the air over the cutting board used for onion
chopping might reduce the amount of vapour wafted into our face. I have
not tried it but you might like to do that. Some people suggest that if
you cut onions close to a lighted kitchen burner the nuisance is
reduced. If true this might be due to the fact that near a burner upward
convection of hot air takes the vapour out of reach of our eyes!