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Population growth, number of daily-trips and motor-vehicles traffic, in addition to changes in the structure of urban life all together have caused the traffic problem in addition to its side-effects, to manifest themselves in the City of Tehran.
This is the reason why under the ground is the only place suitable for displacement of more passengers in a more rapid and safer manner.

However, construction of an under-ground tunnel proved not to be an easy task. The main difficulties in tunnel construction in this city included the quality of the soil, which was clay, existence of made-soil along the route of the tunnel, running-soil and also level of the underground water tables.

In general , there are 3 different methods worldwide for excavating metro channels as follows:

  • Cut & Cover Method
  • N.A.T.M Method
  • T.B.M Method
Cut & Cover Method

In applying this method of tunnel construction, initially Geo-technical survey and tests are made to determine layers and strength of the soil along the route of tunnel excavation. After conducting earth-moving and excavation operations, measures are taken to clear and level the bottom of the excavated area first and carrying-out the formwork-operation by means of sliding-forms (dies), prior to armature working and finally, the concrete working.

N.A.T.M Method

Geo-technical features, sliding characteristics of the soil, in addition to underground-water tables existing at the central part of Tehran, convinced experts of the company to conduct extensive surveys on that zone, prior to excavation of the tunnel.

Finally, taking inspiration from the N.A.T.M Method, an excavation method was developed to meet the requirements so far as technical specifications and features of the soil were concerned.

In the N.A.T.M Method of tunnel-excavation, firstly, similar to the method of excavating underground water-channels, a shaft or ramp is dug to reach to the intended depth and then measures are taken to excavate the tunnel.

Then earth-moving operations are carried-out for the upper and lower sections of the tunnel.
Following that, the tunnel is provisionally supported and then formwork operations are carried out by means of telescopic forms (dies).
Followed by armature working and concrete working.

T.B.M Method

Construction of tunnels by the excavator  is made at such districts that the level of underground water-tables are rather high and also at those streets with a high volume of traffic in applying this method, by construction of a ramp, it would be possible for the excavator to move deep below the ground level and then starts the excavation operation form beneath the ground level to be followed by tunnel construction.

The excavator is composed of a cylindrical sheet measuring 9 M. in diameter and 6.9 M in length , equipped with a heavy-duty rotary earth-moving machine and a number of hydraulic jacks which move in a horizontal direction.

The advancing-speed of the excavating-machine is 3 meters per hour. Therefore , the machine is able to excavate a tunnel measuring 9m . in diameter and 1.5 m . in length and transfer the soil to the rear part of the machine.
Then , the pre-fabricated concrete segments will be installed by the machine. Then , by means of a conveyor-bell , the excavation soil are transferred to soil-handling wagons placed at the near part of the excavator.
The said conveyor-belt is 45 meters and 1.5 meters , in length and width , respectively. Its speed is 91 meters per minute , with a soil-handling capacity of about 1600 tons per hour.

As a result , the said excavator is able to construct a tunnel measuring 9 meters in diameter.

Tehran municipality, Public & International Relations Department CopyRight 2005 - 2011