Rapid growth in Bangkok’s population and industries and a corresponding rise in electricity consumption pushed the Metropolitan Electricity Authority of Thailand’s capital city to develop new infrastructure for distributing electric power.

Part of the authority’s plans involve a new electricity substation at Vibhavadi which would be connected with the existing Ladprao substation. The connection is to be done via 8km of underground transmission line, to avoid adding to the city’s tangle of overhead electricity cable.

A design and build contract to construct and install the underground line was awarded to Skanska Lundby together with a local partner and a Japanese electrical contractor. The work covers installing two 8km long circuits of 230kV cable and one 5km long 115kV circuit, together with all the ancillary installations necessary for proper operation of the line. The civil works involve constructing an 8km long pipejacked drive of 2.6m finished diameter

The tunnel is mainly located under the canal beside the road, about 13m-15m below ground level and is divided into 20 spans by 21 shafts. There is also a cooling plant building for the cooling system in the tunnel. The contract started in March 1999 for a total construction time of 44 months.

The shafts were constructed on temporary working platforms in the canal, which was diverted during construction. After the pipejacking of the tunnel is completed the upper 5m of the shafts will be removed. A shaft roof slab will be cast, and only a smaller access pass from ground surface will be left. The canal will be reinstated above the shafts. The tunnel is designed for permanent access for maintenance and a rail track for a service car is installed on an invert concrete slab along the tunnel. The cable is installed on cable racks on the tunnel walls.

Careful site planning, including the methods for shaft sinking, and an advanced jacking system produced a high production rate from the beginning. The construction and sinking of the shafts took place in 60 to 70 shifts, which included the construction and sinking of the shafts and construction of the bottom slabs. Pipejacking takes place 24 hours a day. With each machine jacking about 18-20 pipes (45m-50m) per day, average advance rate is about 30m/day.

The project includes 19 rectangular shafts measuring 8.8m x 4.5m inside, and two round shafts of 9m id. The shafts are founded on very stiff clay at about 20m depth. All except one were constructed by caisson sinking method. They were mainly placed in the canals and, at some locations, very near buildings. The use of sheet-piles and construction inside the pits were rejected because of the large size and depth of the shafts, the risk of settlement, and the high costs.

Skanska Lundby had used hydraulic controlled shaft sinking before, but for the Vibhavadi project it had to develop a new sinking method to suit the shaft sizes and ground conditions.

Varying ground properties were met during the sinking of the shafts. The first metres of the sinking were very unpredictable, with fill material and utilities, but the following 8m-10m was through soft clay, where the shafts sank relatively easily. After 10m-15m there was medium to hard clay and the shafts would not sink through their own weight. The tolerance for inclination was 1:100, which seems moderate, but sinking these large, heavy shafts, without being able to steer and still keeping the tolerance, would have been very difficult and risky.

Hanging shaft

These problems were solved by hanging the shaft liings during the construction from two overhead ‘sinking beams’. The shaft was held in four locations near the corners by high-tension stress bars connected to hydraulic cylinders mounted on the sinking beams. The beams were placed on supports made of piled H-beams, which carried the load by ground friction. The hydraulic cylinders were designed to work in two directions, making it possible to hold the shaft while sinking in soft clay and push the shaft when sinking in hard clay. The total capacity of the hydraulics was nearly 800 tonnes force.

The shafts were cast in situ in 2.5m sections. After each section had sunk the construction of the next section took place. The shafts were guided into position while sinking by holding or pushing the four corners of the shaft, as required. During sinking, the clay was excavated by hydraulic clamshells. When the shafts reached the hard clay the lining had to be jacked while excavation was carried out under the shaft walls. The excavation was done without water filled shafts because of the very hard clay.

The designed tunnel centreline was 13m-15m under the surface and the water pressure corresponded to the depth. The ‘soft eyes’ in the shaft lining for TBM breakout and entry were constructed with concrete of normal strength but with thinner walls compared with the shaft walls. They were reinforced with glass fibre bars with the same tension strength as ordinary steel bars. These bars were used to enable the EPB machine to pass the reinforcement without problems.

Jet grouting was also used outside the soft eyes so that the concrete could be broken out before the jacking machine exited or entered the manholes.

Pipe jacking

The shafts were spaced at 400m-500m intervals along the route. The centre of the tunnel was levelled at around 12m-14m under the surface and located in medium hard clay. The 8,000m long tunnel was divided into 20 spans for pipejacking of 2.6m id (3.04m od) spun concrete pipes. Each span was designed with a vertical curve with the highest level in the middle. The design included horizontal and, in some locations, S-curves to avoid foundations from bridges or pump stations and to enable the right of way (wayleave) to be followed. The minimum horizontal radius was 400m.

These relatively sharp curves and the jacking lengths of up to 400m put high oblique forces on the concrete pipes especially in the curves. The limited construction time forced the use of two pipejacking machines working 24 hours a day.

Two Herrenknecht EPB 2600 remote control tunnelling systems with main jacking stations were used. The system was also equipped with clay pumps, automatic pipeline lubrication system and VMT guiding system (type SLS-RV).

Lubrication

Skanska started off using bentonite as lubrication together with the automatic lubrication system from Herrenknecht. In the beginning there were relatively high friction loads of around 0.2t-0.3t/m². Investigation revealed that the high friction was probably due to the bentonite in the voids in combination with the compact clay. The bentonite continued to swell after it had been pumped into the void, thereby creating a pressure between the concrete pipe and the clay.

Changing to the use of a polymer lubricant lowered the jacking forces remarkably. Using the automatic lubrication system together with the polymer reduced the skin friction to under 0.05t/m². It made possible the jacking of spans up to 500m without using intermediate jacking stations and still keeping the maximum jacking load below 400t.

The automatic lubrication system is controlled, via computer, from a cabin at the surface. The injection stations are located every 15m along the pipeline in a jacking pipe that has three injection ports at five, seven and 12 o’clock.

The operator can automatically lubricate the entire pipeline from the shield and all the way back to the jacking shaft. The lubricant is injected station by station in a pre-programmed sequence.

Muck system

To excavate the clay without interrupting the jacking progress the material is pumped through a pipeline up to the surface into specially made containers. The containers are emptied by an excavator onto road trucks.

Excavated clay is conveyed from the face by a screw conveyor in a closed circuit to the muck pump. The 160kW piston pump transports the excavated material over 500m along the length of the pipeline and lifts it about 20m from the shaft to the muck containers. A 180mm dia steel pressure pipeline is used with a ring nozzle that allows injection of water to reduce the friction in the pipeline.

Guidance system

The long drives and the design with both vertical and horizontal curves means the surveying is very complex. Therefore an advanced guidance system was procured together with the EPB machines. The VMT guidance system type SLS-RV is incorporated in the Herrenknecht operation system and the EPB 2600 shield machine is guided by a laser, which strikes the ELS laser target in the shield. The precise centre of the beam in relation to the centre of the target is then determined.

The designed alignment for the tunnel is given to the computer and the guidance system automatically follows that alignment and gives the operator direct indication on a screen in the control cabin about his location compared to the theoretical design.

Surveying

The surveying for these long curved drives is in three phases. In the first phase the automatic total station with laser is placed in the shaft and the jacking can be steered as long as the laser reaches the ELS target. It is possible to use this method for about 100m-200m, depending on the tunnel alignment.

After the first phase the theodolite is moved into the tunnel and a back target prism is placed in the shaft. Two reference prisms are placed in the pipeline just in front of the theodolite. The theodolite will automatically check its position by sighting the back target prism.

Phase three starts when the reference back target in the jacking shaft is out of sight and the prism is moved from the shaft into the pipeline. The program will calculate the position of the theodolite and the prisms when they are moving during the jacking and thereafter point the laser on the ELS target.

Manual control surveys are necessary at certain intervals, normally of about 100m, to take account of external effects such as irregularities in the concrete pipes or over-cutting on the curves.