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June, 2000 DIGGING out trenches for laying pipelines or cables along or under the roads already open to traffic is a nuisance but common part of construction jobs. The traffic gets diverted leading to frequent jams and impatient blaring of horns. In most of the cases, the back-filling of soil after the laying of pipes or cables is not done in a proper manner. Resultant pits and bumps make the vehicles jump and leave the drivers cursing. Even the cool breeze gets laced with the dust left on the roads. In a nutshell, the journey doesn’t remain a pleasant one. A solution to this wearisome problem is now in sight. Trenchless technology is all set to arrive in India — on a moderate scale for the present. As the name suggests, it is the technology that allows laying of pipes or cables along or under the roads without disturbing the road surface. The work continues below the ground while the traffic runs as usual. Trenchless laying of pipes and cables has become possible due to the invention of directional boring machines. Driven pneumatically or hydraulically these machines drive a soil-displacement-hammer through the soil and toe the pipe or cable along the underground bore thus created. The hammer has a moving chisel head that makes its way through the soil along the designed alignment. A small pit is required at the point of commencement of laying the pipe-line or cable. This is called launching pit. The Directional Boring Machine begins its work from this pit. Similarly, an exit pit is required at the point of termination of cable or pipe. The entry and exit angles of the pipe or cable are carefully decided keeping the stress parameters and bending radius in view. A directional boring machine consists of a boring unit, an electronic tracking equipment and a drill component. The pull force, torque and pump pressure of the boring rig depend upon the soil strata through which the pipe or cable is to pass, the diameter of the pipe or cable and its depth below ground. Unforeseen increases in pull load due to ground conditions are also taken into account. The electronic tracking equipment enhances the trencher’s ability by automatically adjusting track speed and force with respect to ground conditions. An electric stroke limiter increases control and doesn’t allow the engine to stall whenever tough conditions are encountered. The machine operates at low speed and high torque. This helps the cutting bit to penetrate even the toughest rocks. Numerous boom options are available with different types of cutters and drill-bits. Right choice can be made depending upon the job to be handled. Cables and pipes of polyethylene or steel of diameter varying from 75 mm to 2 metre can be laid below ground, rivers, canals, roads and buildings using trenchless technology. For paying PVC pipes, a soil displacement hammer is utilised. When a steel pipe is to be laid, a steel pipe ramming hammer is used. Underground drilling can be done even through a solid rock. Not only the laying of new pipes and cables but replacement of existing but damaged pipelines laid for supply of water or gas or to discharge sewage can also be done by this technology and that too without digging these pipes out. For this, heavy duty steel rods of three feet length are thrust into the old pipe these create a full string of rods. A force is then applied to fracture old pipe into small pieces. The machine also toes along a new pipe which keeps on occupying the position of old pipe being broken. Thus the dismantling of old pipe and laying of new pipe are done in a single activity. This brings saving of time and cost. It is important to carry out soil investigations of the area to be drilled before a job is taken in hand. The soil strata and individual physical properties of each stratum, the particle size distribution curve and the position of water table are important to be known. Cone penetrometer test to find out the detail of erratic soft clay deposits should also be done. In case of rivers, the scour level needs to be known. The entry and exit angles are to be decided after these studies are completed. In most cases, these angles are kept between 6° and 15°. The minimum bending radius, the drill-trace-axis, the distance between entry or exit point and the obstacle such as river to be crossed are also decided after these studies. Necessary permits, where required, may be from River authority, B and R department, Pollution control department and individuals should be obtained before a job is taken in hand. A drawing of the work area showing location of drilling machine, method of anchoring it, setting of drill fluid preparation unit, storage of pipes or cables to be laid, office cabin, generator and entry pit should always be prepared before starting a big job. Project execution Figure shows the drilling profile of a river across which a pipeline is to be laid by trenchless technology. To run a machine, generally a crew of two to three persons is sufficient. One person acts as the boring operator, one as the track equipment operator and third, if required, helps these two in carrying out their activities. Drill fluid is an essential requirement of all drilling operations. Normally, it consists of a suspension of water and bentonite. It makes the job of hydraulic cutting with the jet easier. Noise is when the machine is working and the vibrations are minimal. In case of working below a road, the overhead traffic doesn’t come to know that a pipe or cable laying operation is going on below the ground. Foreigners are producing machines of different sizes, equipped with a variety of hammers and bits. Choice of hammer is decided with respect to the type of job, quantum of work and the working conditions i.e. whether a fluid of drilling medium can be used or dry conditions are to be maintained. Sensing the great potential in trenchless technology, Indian manufacturers are also taking up production of directional drilling machines. One company has already set up a large unit in Rajasthan and is now producing such machines of different capacities. A couple of companies in India have imported directional drilling machines and are handling a few jobs in India and abroad. A 400-metre-wide and 15-metre-deep river in Thane near Mumbai, having hard rock strata at its base has been successfully crossed by an Indian firm and three layer polyethylene coated pipes of 450mm diameter have been laid by this firm 10 metre below the base of the river. Two more such jobs have been completed in West Bengal where 457mm dia HDPE conduits imported from China were laid below river Harohar and river Rupnarayan crossing more than 350 metre length in each case. It is also learnt that cabling work of MTNL in New Delhi is now being done using trenchless technology. In addition to having the advantage of non-disruption of traffic, trenchless technology is clean and non-destructive. It can be used in restricted widths, may be as little as 1 metre only. It is fast becoming popular in advanced countries. In India, only a few jobs have been undertaken and full benefits of the technology are yet to be reaped. It is time that we make use of this technology more often.
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