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Old 05-27-2009, 06:54 PM
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Default Trenchless Technology, also known as No Dig method

Trenchless Technology, also known as No Dig method
is an innovative method of laying underground utilities without digging the ground or with minimum excavation. It has become an effective and economical alternative to Open-Cut construction.

The common practice of laying underground utilities through Open-Cut construction, particularly in urban areas, leads to Traffic disruption, causes Pollution , is time consuming and above all is extremely People unfriendly. When Roads, Railways, Rivers needs to be crossed , the Open-Cut system becomes difficult, time consuming and expensive.
The Tenchless construction is the preferred method of construction in fast developing urban areas. The benefits of trenchless construction include:

1. Environmental Friendly
2. Negligible disruption to traffic flow
3. Faster and neater construction
4. Greater reliability

Trenchless Technology systems for underground services fall into three broad categories:

1. New Installations
a. Installation of new pipelines or duct, including service connections. Methods for this include Impact Moling, Guided boring and Directional Drilling; Micro Tunneling and Pipe Jacking.

2. Rehabilitation Methods
a. Online replacement of an existing pipeline or duct. An old pipeline is replaced with a new one. It can be used for size to size replacement or for upsize capability. Methods for this include pipe bursting and pipe eating.
b. Renovating or an existing pipeline or duct. It is used where performance of pipeline is unsatisfactory, but fabric has a residual value either structurally or as a lining support. It is often more economic to provide on entirely new one. Methods include slip lining, online replacement and spray online etc.

Various specialized trenchless technology construction methods are described briefly as under:

PIPE JACKING :

Pipe Jacking is a system of installing pipes behind a shield machine by hydraulic jacking from a drive shaft in such a way that the pipes form a continuous string in the ground. It requires the workers inside the pipes. The pipes are prefabricated and jacked into position from a jacking pit. The excavation can be either mechanical or manual. The pipes form continuous string on the ground and are usually joined by flush fitting collar. This could use reinforce concrete pipes, steel pipe, Hobbs(fiber glass) pipe, ductile iron pipe or corrugated metal pipe.

Excavation is normally accomplished inside the articulated shield. This shield can be guided with individually controlled hydraulic jacks. The shield is designed to provide a safe working environment for workers and to allow the pipe to be jacked in place through the thrust power of hydraulic jacks.

The process requires a simple, cyclic procedure of using the thrust power of hydraulic jacks (rams) to force the pipe forward. In unstable conditions the face is excavated simultaneously with the jacking operation to minimize over excavation and risk of face collapse. In stable ground conditions, excavation may precede the jacking process, if necessary. The spoil is removed through the inside of the pipe to the jacking pit. After a section of pipe has been installed, the rams are retracted and another joint is placed into position so that the thrust operation can be started again.

Because of the jacking forces required to push large diameter pipe through the ground, the jacking pit design and construction area are critical. The pit supports must be properly designed and constructed. The pit and thrust reaction structure must be designed to handle the repeated process of supporting extremely heavy sections of pipe as the operation is executed.
Guide rails are prepositioned so that the jacking pipe is properly aligned when it ism placed on guide rails. Much success in operation is based on ensuring proper alignment. The application of bentonite slurry to outside skin of pipe reduces the frictional forces.

The process is ideally suited for installing pipelines under railway tracks, highways and shallow canals.

MICRO TUNNELING :
Micro tunneling equipment have now been developed to work in almost all types of ground conditions. It is however essential to know the conditions as the choice of type of machine and cutting head is dependant on these. The ground condition also determines the type of spoil removal system and magnitude of jacking forces likely to be required.

There are 2 pits, launch pit and reception pit. The Launch Pit must be structurally stable. Its requirement vary, greatly depending upon the machine being used., ground conditions, pipe length and material, length of drive and type of exit installation. They may be round, rectangular or oval. Sheet piled, segmentally lined or even unsupported depending upon the ground conditions. There has to be some form of reaction face for the jacking frame to push against and a thrust wall is provided. The dimensions of the reception shaft should be such that micro tunneling shield can be recovered. As this shaft is not normally used for jacking operation, there is no need for abnormally high strengths or thrust walls. Hydraulic jacks and jacking frames are launched in launch pit and the micro tunneling machine starts operation from launch pit, goes underground and till the reception pit.

For spoil removal, an auger chain is established in an auger casing within the jacking pipe. The auger feeds spoil to a muck skip positioned beneath the jacking frame in the start shaft and when it sis full, it is hoisted to the surface. However in difficult soil conditions with high ground water head, recirculating slurry system may be used. The slurry system requires a suspension of bentonite as specially designed man made polymer to be prepared at the surface. This suspension is pumped to the cutter chamber via a system of pipes within the jacking pipes. The slurry is pressurized to a level required to maintain the face support. In the cutter chamber the slurry mixes with the excavated ground and this mixture normally passes through an in-built crusher with an eccentric radial motion to ensure that no ground particle larger than the slurry system can handle enters the return side of the system.


The mixture is pumped to the surface where the soil particles are removed from suspension by simple gravity decantation or by centrifugal forces. The newly cleaned slurry ism monitored and reconditioned by the addition of further chemicals. The slurry system has an advantage of being continuous as compared to auger based or pure jacking based which require hoisting of spoil.

HORIZOTAL DIRECTIONAL DRILLING (HDD) :
HDD is a method to install pipelines below a river bed without entering the river or disturbing the river banks. The HDD work is carried out from dry ground, well away from the river banks. The pipeline to be installed is welded in a straight line on one side of the river. The HDD equipment (drilling spread) is positioned on the opposite bank of the river. Using a proprietary drilling method, drill pipe is advanced along a predetermined arc. The drill pipe is steered along this course under the river bed until it reaches the predetermined exit surface location near the pipe string. The borehole is enlarged and conditioned. The product pipe ( pipe material can be steel or HDPE or rigid PVC as the situation warrants) is then pulled through the borehole.

While the HDD technique was developed to cross rivers and canals, it can also be used to install pipelines below railroad crossings, highways, congested city locations and other protected areas where the cut and cover method is impractical or undesirable. It can also be used for shore approaches.

Beside transporting fluids, HDD installed pipelines can be used as DUCTS through which Telecommunication cables, Fiber optic cables or High Tension power conductors/cables can be pulled.

The Trenchless construction provides a People friendly, Environment friendly and fast process for laying public utilities. It causes minimum traffic disruption during construction. More and more construction agencies and public utilities are now moving towards using this method as awareness is growing.
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