Once the setup of the jacking frame and surface equipment is complete, the jacking operation can commence.

We here above describing the basic procedure for microtunneling.

Alignment Control – Prior to launching the machine a laser is fixed in the pit bottom. This laser is aligned to the correct horizontal alignment and vertical grade in a position that ensures it shines onto the laser target within the machine. The laser target within the machine detects the laser beam through a series of light sensors and is able to determine the position of this beam. This information is relayed to the control cabin and a graphical display indicates the horizontal and vertical position of the machine to the operator. The machine itself is articulated and the angle of this articulation is controlled by hydraulic cylinders. By opening and closing the cylinders the operator is able to control both the vertical and horizontal direction of the cutter head. It is this control that allows the operator to ensure the laser beam remains on the target throughout the length of the tunnel drive and therefore the machine and pipeline remain within design tolerances. Machine Launch and Excavation Procedure – Each machine consists of 2 main parts, or ‘cans’, the machine can and the trailing can. The machine can is the front part of the machine and contains all the mechanical components such as the cutting wheel, main drive motor and gear box. The trailing can is empty and is there to prevent any rolling of the machine being transferred to the pipe, there are injection ports in the trailing can that allow bentonite lubrication to be injected very close to the machine if required.

The machine is powered hydraulically. The source of this hydraulic power is a power pack situated in the can immediately installed after the machine. Initially the machine is lowered onto the jacking frame and the cutter head carefully pushed into the shaft seal using the jacking frames hydraulic jacks. Once in position, all the services are connected. The service connections consist of the following:

• Hydraulic connections to the machine.
• Slurry circuit pipe work connections.
• Electrical control cables.
• Power Cable to the Power Pack Can.

Once all the connections are made, the machine is ready to advance. To begin, water is circulated through the machines bypass circuit. This circuit allows the water flow to stabilise before it is directed through the cutter head chamber to the open ground. The ground conditions are assumed to be sand, and therefore slurry being directed at the ground without the machine going forward could potentially cause over excavation and subsequent settlement, this is the reason for stabilising the flow through the bypass circuit prior to advancing the machine. Once the flow is stabilised pressure will be applied by the jacking frame to the machine, this pressure will cause the cutter head to contact the ground and commence excavation. The operator is able to see that excavation has commenced once he observes an increase in torque on the cutter head from his control screen. Once the torque has reached the correct level, depending on machine diameter, the operator will then direct the slurry flow through the cutter head chamber to remove the excavated material.

The operator will continue to apply pressure with the jacking frame and monitor the slurry flow and cutter head torque as the machine moves forward. He will also monitor the position of the laser beam on the target and make adjustments to the direction of the cutter head as necessary.

Once the machine is fully advanced into the ground (approx. 3.0m), the excavation process will stop. When the excavation process stops, remote controlled valves are closed to ensure the cutter head chamber and the ground is stabilized. All services are then disconnected and the trailing can be lowered and connected to the back of the machine can.

All services are then reconnected and the excavation process continued. Once again after the trailing can is fully advanced into the ground the excavation process will stop, services disconnected and the first pipe is lowered and fixed to the back of the trailing can. Once again the services are reconnected and the excavation process resumes. This procedure will be repeated until the machine has been pushed as far as the reception shaft, where it will be removed.

Use of bentonite in the pipe annulus – The micro tunneling machine excavates a larger diameter hole that the pipe that is being installed. During the micro tunneling operation it is necessary to fill this gap, known as the annulus, with a bentonite slurry. The bentonite pressure within the annulus is maintained by a computer controlled system operated from the control container, and this pressure needs to be sufficient to displace the ground water, if present. The bentonite offers support to prevent ground collapsing into the annulus and causing settlement, it also provides lubrication to ensure jacking forces are kept to minimum.

The points of injection of bentonite slurry will differ depending on the diameter of the pipe. In the case of the 600 and 800 diameter Pipes, it is only possible to inject at the trailing
can and the pit seal as it is not practicably possible to enter the pipe. However, in the case of pipe diameters above 800 diameters, injection ports will be cast into the concrete
sleeve allowing injection along the pipeline during micro tunneling.

Micro tunneling is a repetitive process that is carried out like for like on numerous different projects. Whilst the pipe diameters will change from project to project, the process
remains the same.

Intermediate Jacking Stations (IJS) – If due to the length of the line the jacking loads will exceed the capacity of the main jacking frame or capacity of the pipe it may be necessary
to introduce into the pipe string an IJS. These allow the pipes to be pushed section by section thus reducing the jacking load on the main jacking station. Once the line is finished,
the IJS cylinder will be removed and the IJS closed.

On completion of the micro tunneling operation, it is necessary to attempt to displace as much of the bentonite slurry in the annulus with cement grout. Prior to all micro tunneling operations under roads, settlement calculation will be prepared, these calculations will estimate the amount of long terms settlement that will occur based on the amount of bentonite that is displaced by grouting. Grout will be either site mixed or delivered.

In the case of the small diameter pipes, 600 and 800, it is only possible to inject the cement grout from either end of the pipe into the annulus. The volume injected into the annulus will be measured and compared to the volume of assumed displacement given in the settlement calculation. This will indicate the long term settlement expectations. In the case of the pipe diameters of 1,000 Dia.

NOTE:
The settlement calculations do not assume that the annulus will be completely filled with cement grout. The calculations indicate that some minor settlements may occur as it is impossible to ensure the annulus is 100% full, however, the potential settlement should remain within the allowance for long term settlement as detailed in the specification.