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More information about Resalat Tunnels project;

 

Resalat Tunnels

 

Client:  Tehran Minucipality

Engineer:  Mahab Ghods

Contractor:  Perlit Company

Instruments:  Pressure cells and extensometers

 

Resalat Tunnel is a part of Resalat Highway which runs East-West through the northern part of Tehran in Iran.  This is a dual tunnel with a width of approximately 15 meters which is being built in an unconsolidated Conglomerate (called Hezar Darreh Formation in Iran).  Due to the shallowness of the tunnel, the effect of surface waters and have resulted in a very soft and low strength rock mass.  This body acts more like a soil than a rock mass in fact.

The importance of monitoring ground movements while driving this tunnel, specially when it runs through a very dense and confined civil area, a monitoring program was designed by the contractor to record the whole deformation history of the tunnel passing by the surface structures.  This program included installation of long borehole extensometers right from the ground surface before the tunnel face reaches the monitoring station.  According to the following schematic diagram this included 3 extensometers as long as 50 meters with 10 anchor points and also three extensometers 35 meters long.  To imagine a 50 meter long extensometer you can compare it with a 17 story building which is a huge instrument to design, manufacture and install. Twelve electrical pressure cells were also put into the concrete lining to evaluate the pressure acting on the lining as well.

                                                                    

               

 

 

|Since the excavation of the tunnels can cause convergence inside the tunnel and at the same time settlement on the surface, the extensometer was designed so that it can record both extension as well as compression.  In other words, the extensometer was infact a combination of extensometer and settlement cell.  To achieve this the instrument was designed and was manufactured at the companies workshop.

Due to the huge length of the instrument and difficulties of transportation, it was decided to assemble the instrument at site (figures bellow).

             

When the drilling of the boreholes was finished the extensometers were pushed into the holes and got ready to be grouted.  Careful depth measurement for stage grouting was performed to make sure the distance between the anchors are left free from the grout to allow the instrument to contract if necessary.  In fact a compressive material was filled between the two anchors and this was carefully controlled by a digital depth meter.

 

                                    

        

 
The holes were then grouted and extra length of the extensometer end was cut to its specified length and the head assembly was installed after the cement was hardened.  Zero reading was then done and regular readings were performed to obtain the required information during the tunnel passing by the monitoring station.

  

                 

 
 

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