Curve Jacking -Paper Bangkok T-1Thoren

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International Symposium on Underground Excavation and Tunnelling

2-4 February 2006, Bangkok, Thailand

Construction of Shaft and Cable Tunnel ID 2.6m Vibhavadi,

Bangkok

Olof Mikael Thoren1

1

Gammon Construction Ltd, Singapore

ABSTRACT

This paper outlines the methods developed for controlled shaft sinking in Vibhavadi Underground

Power Lines Project in Bangkok by Skanska Lundby, a subsidiary company to Gammon Skanska and

the pipe jacking of the 2.6-m diameter tunnel of concrete pipes which made use of the latest

technology to achieve a curved alignment to a high level of accuracy and using low jacking forces.

The pipejacking works in this project consisted of an 8-km pipe jacked tunnel with inner diameter of

2600mm, 19 rectangular shafts (10.4 x 6.1m) and 2 round shafts with outer diameter of 10 m. The

shafts were constructed in situ in 2.5m high sections. The shafts were sunk to a depth of about 20m in

clay by a hydraulic sinking method, developed by Skanska Lundby AB. The weight of the shaft was

more than 1000 ton. The hydraulics worked in two directions to make it possible to either hold or push

the shaft while sinking, thereby facilitating a fully controlled and steerable sinking. The tunnels were

constructed using two EPB (earth pressure balance) machines from Herrenknecht. The machines wereequipped with muck extracting pumps, an automatic lubrication system and a SLS Guide system.

The tunnels were designed with both vertical and horizontal curves and the distances between the

shafts are 400-500m. Some drives were performed with S-curves to avoid building foundations.

Intermediate jacking stations were installed but not used, as the skin friction was kept very low due to

the use of an automatic lubricant system.

1. INTRODUCTION

The Client, Metropolitan Electricity Authority (MEA) has been faced with the rapid growth of the

population in Bangkok and consequently the demand for electricity has risen. The authorities inBangkok were therefore challenged to develop new infrastructure systems, including power

distribution, for the inhabitants as well as industrial usage. Another reason to construct and install the

electricity power lines underground was to avoid the tangles of overhead electricity cables in the city

by letting them run underground.

Aware of the increasing electrical consumption in Bangkok, MEA decided to construct a new

substation at Vibhavadi and, in order to connect the substation, to start the construction of an 8 km

underground transmission line between the existing Ladprao Substation to the new Vibhavadi

Substation.

Skanska Lundby AB in conjunction with Italian-Thai and Sumitomo, a Japanese electrical work

contractor, were awarded the project to construct and install this underground transmission line on a

design and build basis.

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The objective was to install two circuits of 230 kV cable, each of 8-km length, and one circuit of

115 kV (5km) including all accessories and site installations required for proper operation of the line.

The civil works to be constructed included a 2.6 m dia., 8 km long tunnel using the pipejacking

method. The tunnel is mainly located under the canal beside the road about 13-15m below ground

level. The tunnel is divided in 20 spans by 21 shafts. There is also a cooling plant building for thecooling system in the tunnel. The contract started in March 1999 and the total construction time was

set at 44 months.

The shafts were constructed on temporary working platforms in the canals and the canals were

diverted during the construction.

After the pipejacking of the tunnel was finished the upper 5m of the shaft was removed. The roof slab

was cast, and only a smaller access shaft from ground surface was left. The canal was 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 (see figure1).

Figure 1. Tunnel installations

The careful site planning, including the methods for shaft sinking and an advanced jacking

system, made it possible to keep a high production rate from the very beginning. In situ construction of

the shafts was found to be most suitable. The construction and sinking of the shafts were made in 60-

70 shifts, which included the construction and sinking of the shafts divided in 8-9 element and

construction of the bottom slabs. The pipejacking was performed 24 hours per day and each machine

was able to keep a rate of about 18-20 pipes (45-50m) per day while jacking. The average rate of

advance was about 30m per day.

2. SHAFTS

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2.1 Shafts

The project included 19 rectangular shafts which measured 8.8 x 4.5m internally and two circular

Shafts with inside diameter of 9m, the sizes being fixed by the requirements in the contract. The shafts

were founded on very stiff clay on ~20 m depth. With one exception, all were constructed by caissonsinking method.

The shafts were mainly placed in the canals and at some locations very near existing buildings.

The sheet-pile method with insitu construction inside the pits was rejected due to the big and deep

shafts, the risk for settlement and the high costs.

Skanska Lundby has used the method of hydraulic controlled shaft sinking before, but for the

Vibhavadi project we had to develop a new sinking method to suit the shaft sizes and the ground

conditions on this particular project.

2.2 Sinking

We were facing varying ground properties during the sinking of the shafts. The first few meters of the

sinking were very unpredictable with filling material and utilities, but the following 8-10m was carried

out through soft clay where the shafts would sink relatively easy. After 10-15m we were reaching

medium to hard clay and the shafts would not sink by their own weight. The tolerance for inclination

was 1:100, which seems moderate, but sinking these quite big and heavy shafts without possibility to

steer and still keeping the tolerance would have been very difficult and risky.

Figure 2. Shaft Construction

These problems were solved by hanging the shafts during the construction (picture 2) in two

overhead beams that we called 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

sinking beams were placed on supports made of piled H-beams; these were carrying the load by

ground friction. The hydraulic cylinders were designed to work in two directions to make it possible to

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hold the shaft while sinking in soft clay and push the shaft when sinking in hard clay, the total capacity

of the hydraulic rams was nearly 800 ton.

The shafts were cast in situ in 2.5m sections and after each section had sunk the construction of

the next section took place. By holding or pushing the four corners of the shaft individually the shafts

were guided in correct position while sinking. During the sinking the clay was excavated by hydraulicclamshells. When the shaft reached the hard clay we had to push the shaft and in the same time

excavate under the shaft walls. The excavation was able to be done without water filling the shafts due

to the very hard clay.

With this method we were able to construct the shaft in situ in 60-70 days and in a very safe

manner, keeping to the required tolerance.

2.3 Soft Eyes

The designed tunnel-centerline was 12-15m under the surface and the water pressure corresponded to

the depth. Soft eyes were constructed in shaft walls with concrete of normal strength but with thinner

walls compared to the rest of the shaft. They were reinforced with fiberglass bars with the same tensilestrength as ordinary steel bars. These bars were used so the EPB machine could pass the reinforcement

without problems (figure 3).

The area outside the soft eyes was also jet grouted, so that part of the concrete could be broken

out before the jacking machine exited or entered the manholes.

Figure 3. Soft eye

3. PIPEJACKING

The shafts were spaced at 400-500m along the route and the centre of the tunnel was maintained at

around 12-15m under the surface and located in medium hard clay. The 8km tunnel bore was divided

into 20 spans for pipejacking of spun concrete pipes with an inner diameter of 2.6m. Each span wasdesigned with a vertical curve with the highest level in the middle. The design included horizontal and,

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in some locations, S-curves to avoid existing foundations from bridges or pump stations which

required minimum horizontal radius of 400m.

These relatively sharp curves and the jacking lengths of up to 500m put high oblique forces on the

concrete pipes especially in the curves.

We were also faced with a limited construction time, which forced us to plan for two pipejackingmachines working 24 hour per day. When choosing between the different machine alternatives for the

project our criteria included high production rate, possibility of reducing the jacking forces and to

manage the curve jacking within the required tolerances for the tunnel.

We acquired two Herrenknecht EPB 2600 remote control tunneling systems complete with main

jacking stations to the project (Figure 4). The system was additionally equipped with clay pumps,

automatic pipeline lubrication system and VMT guiding system (type SLS-RV).

Figure 4. Herrenknecht EPB 2600

4. LUBRICATION FOR PIPEJACKING

From previous jacking in the marine clay of Bangkok and with manual operation of valves in the

tunnel for the lubrication of the pipeline we had experienced a skin friction on 0,2-0,5 ton per m,

using only pure water as lubrication. In this project we started to use bentonite as lubrication togetherwith the automatic lubrication system from Herrenknecht. In the beginning we were facing relatively

high friction loads around 0.2-0.3 ton per m. After an investigation we found out that the high friction

was probably coming from the bentonite in the voids in combination with the compact clay. The

bentonite continued to swell after it had been pumped into the void, and it thereby created a pressure

between the concrete pipe and the clay.

We changed the lubrication material and started to use a polymer and the jacking forces were

lowered remarkably.

When we used the automatic lubrication system together with the bentonite mix we managed to

reduce the skin friction to under 0.05 ton/m. We were now able to jack spans up to 500m without

using intermediate jacking stations and still keeping the maximum jacking load below 400 ton, which

has to be considered as very low for a 2.6m (3.04m outer diameter) pipe jacked tunnel.

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The jacking was performed continuously 24 hour per day but even when we had some production

stops we did not experience any big differences in the jacking forces.

Due to the curve jacking it was very important to maintain these low jacking forces in order not to

damage the pipes.

The operator in the control cabin at the surface controlled the Automatic lubrication system, using

a computer. The injection stations (Figure 5) are located every 15m along the pipeline in a jacking pipe

that has 3 injections ports, the ports are placed at 5, 7 and 12 oclock.

Figure 5. Lubrication pipe

The system allows the operator to 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.

5. MUCK SYSTEM

5.1 General

To be able to excavate the clay without interrupting the jacking progress the clay is pumped through a

pipeline up to the surface and into specially made clay containers (Figure 6). The clay containers are

emptied by an excavator that loads the muck on trucks. The jacking is therefore performedindependently from the muck handling.

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During the traffic rush hour it was not possible to transport the muck so the containers were

designed to store the clay during that time not to stop the tunnel advance.

Figure 6. Pumping the Clay

5.2 EPB Muck System

The excavated clay was conveyed from the face by a screw conveyor in a closed circuit to the muck

pump. The 160 kW piston pump was able to transport the excavated material over 500m along the

length pipeline as well lift it up approximately 20m from the shaft to the muck containers.

A 180mm-diameter steel pressure pipeline was used with the assistance of one ring nozzle that

allowed injection of water to reduce the friction in the pipeline.

6. GUIDANCE SYSTEM

The long drives and the design with both vertical and horizontal curves made the surveying very

complex; therefore an advanced guidance system was procured together with the EPB machine. The

guidance system from VMT type SLS-RV is incorporated in the Herrenknecht operation system and

the EPB 2600 machine is guided by a laser, which strikes the ELS laser target in the shield. The

precise center of the beam in relation to the center of the target is then determined.

The designed alignment for the tunnel was fed into the computer and the guidance system

automatically followed that alignment and gave the operator direct indication on a screen in the control

cabin about his location compared to the theoretical design.

The surveying for these long curved drives was divided into three phases.

In the first phase the automatic total station with laser was placed in the shaft and the jacking could be

steered as long as the laser reaches the ELS target. It was possible to use phase 1 for about 100-200m

depending on the tunnel alignment.

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5

6

Skanska Lundby AB

SLS - RV Guidance SystemSLS - RV Guidance System

System LayoutPhase 1

ELS Target

TBM

Laser Unit

Figure 7.

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 pointing at the back target prism.

5

7

Skanska Lundby AB

ELS Target

TBM

Survey Prism

Survey Prism

(Rear Reference Object)

Laser Unit

Survey Prism

Designed Tunnel Axis (DTA)

SLS - RV Guidance SystemSLS - RV Guidance SystemSystem LayoutPhase 2

Figure 8.

Phase 3 starts when the reference back target in the jacking shaft is out of sight and the prism is

moved from the manhole in to the pipeline. The program will calculate the position of the theodolite

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and the prisms when they are moving during the jacking and there after point the laser on the ELS

target.

5

8

Skanska Lundby AB

Survey Prism

ELS Target

TBM

Survey Prism

Survey Prism

(Rear Reference Object)

Designed Tunnel Axis (DTA)

Laser Unit

SLS - RV Guidance SystemSLS - RV Guidance SystemSystem LayoutPhase 3

Figure 9.

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 in the curves.

7. GROUND SETTLEMENT & INSTRUMENTATION

Ground movements and stability is always a concern when working below ground. In this project one

of the tunnels passed under the Vibhavadi electrical substation with its sensitive electrical installations.

MEA asked that the settlement within the substation area be kept below 6mm in order not to risk

interruption in the power distribution to the greater Bangkok area.

This tunnel was done last and information from settlement points and inclinometers from the

earlier tunnels was compiled and compared in order to find the optimal solution on advance rates, face

pressure, over cut lubrication etc to keep ground settlement to a minimum. For the Vibhavadi project it

was estimated that the ground settlement above the tunnel would be in the order of 13mm at 3%

volume loss and some 23 mm assuming 5% volume loss.Our investigation showed that during the first year of the project we had a maximum of 12mm

settlement above the tunnel, and this occurred on the longest s-curved span.

By making the last tunnel short, keeping it straight, carefully monitoring the face pressure and

reducing the jacking speed, we managed to complete the 155m long bore with a maximum of 5mm

settlement, utilizing less than 200-ton jacking force, along the span.

8. CONCLUSION

Despite very difficult ground conditions, including the notorious Bangkok clays, Gammon Skanska

were able to complete the contract in 43 months, a few weeks ahead of programme, including the

installation of over 3 200 concrete pipes in spans varying between 92 to 509 meters each, construction

of 21 shafts, substation and chilling plant, and some 63 km of cable in three circuits.

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It was the first time such long s-curved pipe jacking had been performed with such low jacking

forces and after it was commissioned in October 2002, it now forms a key element in the new

electrical infrastructure of Bangkok.

ACKNOWLEDGEMENT

Figure 7-9 kindly provided by Herrenknecht

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Curve Jacking -Paper Bangkok T-1Thoren