Microtunneling is an accurate and reliable trenchless method that can bore through all types of soil, sand, and clay, and even through hard rock.
Pipeline diameters varying from 250 to 3000 mm can be installed using this method. The accuracy achieved is as good as +/- 10 mm which may be more when large diameter pipes are involved.
Microtunneling can be done on a straight drive or a curved drive. However; curved drives are more complicated than a straight drive and require special guidance systems to correctly steer the cutter head.
While a standard laser sight can keep the machine on course in a straight drive, curved drives require guidance systems such as those used on longer bores.
Curved alignments also help extend the drive length by avoiding obstacles and intermediate shafts.
The microtunneling process has evolved over time to allow curved alignments that allow extended drive lengths by avoiding obstacles and intermediate shafts.
This method has successfully completed horizontal as well as vertical curves. Horizontal curves allow pipeline projects to follow curved public rights-of-way without too many manholes, and short straight segments.
Vertical curves allow for shallow launch and reception shafts for installing non-gravity pipelines.
The first curved alignment microtunneling project in the U.S. was completed in 2010 by Northeast Remsco as part of the $1.6 billion USD “Clean Water Project” in Hartford, Connecticut.
Important Factors in Successfully Curving a Microtunnel
Usually, a microtunneling project is curved for reasons such as:
- When it is impossible to reroute the project.
- When the curve cannot be negotiated by short straight drives.
- When the social and environmental concerns demand a curved tunnel.
Pipe materials are rigid and therefore the deflection is concentrated on pipe joints and the jacking stresses are transferred across pipe joints in a curved alignment.
Tight curves are therefore only possible with rigid, thick-walled reinforced concrete or polymer concrete jacking pipes with articulating joints.
Safe Transfer of Jacking Forces Using Hydraulic Joints
The jacking forces should be safely transferred from pipe to pipe.
For this purpose hydraulic joints which are engineered pressure transfer rings are essential. These rings have predictable mechanical behavior and permit a proper understanding of stresses acting on the pipe.
They also provide full-face transfer of pressure without a peak in stress even at large angles.
The pressure transfer rings also minimize the eccentric loading on the pipe that is normally observed when transferring loads through articulated joints.
When the alignment is curved, axial loads are transferred with substantial eccentricity, leading to a reduction in its axial carrying capacity.
The jacking loads are significantly increased on a curve because the MTBM and the straight pipe sections behind it are deflected along the curved alignment by the reaction forces on the wall of the tunnel.
Pipe Joint Design
Since the jacking pipes have to withstand the jacking loads, they are designed to be structurally strong. For curved alignments, the pipe joint design is equally important because the articulation is concentrated in the pipe joints.
Though shorter pipe lengths reduce the joint articulation angle, it increases the number of joints. It is therefore recommended to use longer pipes unless unavoidable, and keep the number of joints to a minimum.
Too many joints can act as weak spots and also increase the cost per linear feet owing to slow advance rate as connection and disconnection are needed with every new pipe to be installed.
The joints have to provide a good seal against groundwater and soil even in the articulated state. The design of the joint therefore should be such that the area where the pressure is transferred is sealed against water and soil so that the behavior of the pressure transfer ring is not adversely affected.
A good joint design will consist of an external steel band or collar cast into the concrete and a spigot end with the gasket in a groove on the outside of the pipe.
Hydraulic joints allow tunneling with a much greater joint angle between the jacking pipes than conventional timber pressure transfer rings allowing for much tighter radii.
Relevant measurements such as fluid pressure in the hydraulic joint and the joint gap width between jacking pipes are continuously recorded and monitored and help in documenting the stress states applied to the jacking pipe.
With the knowledge of the stress state, the jacking pressure can be used such that the limit values are not exceeded.
Excess stress can compromise the integrity of the pipe.
To complete a curved microtunnel successfully, the guidance and navigation system should be able to communicate without a glitch. The guidance system required for a curved tunnel can be a Laser Total Station System or a Gyro Navigation System. (Also read: Horizontal Directional Drilling Methods: Gyro Vs. Magnetic Wireline Steering.)
While using a laser guidance system, either a signal relay is used or the laser is physically repositioned to maintain a line of sight with the advancing microtunnel boring machine (MTBM).
The gyro system uses instrumentation installed at the face to report the pitch and directional information to the operator.
The position of the MTBM is known at any given time and this helps the operator to steer the cutter head precisely in the desired direction.
The horizontal and vertical position, pitch and roll of the MTBM are determined using a motorized total station and laser target unit. The system can operate irrespective of drift or refraction and can also take automatic measurement of the excavated tunnel.