Trenchless Technology for Underground Services

D K Gautam, Scientist; Narendra Kumar, Technical Officer; S K. Panigrahi, Scientist; A. K. Sharma, Scientist, and Sameer, Technical Assistant, Central Building Research Institute, Roorkee.

The implementation of trenchless technology for laying utility services under the ground in urban areas, the various problems affecting the structures and traffic congestion by using traditional methods (open cut trench) can be substantially reduced. This technology seems to be friendly to all concerns including authorities, contractors, consultants, environmentalists, and many others.

The main objective of the paper is to provide basic information on this technology, specifically underground directional drilling, microtunneling, impact moling, and pipe jacking which have been rapidly accepted all over the world.

With the greater emphasis on infrastructure development projects for economic development in India, it is felt that the trenchless technology may poise for its increased adoption in the growing metropolitan cities. These cities in India can no longer afford to disrupt traffic and disturbance to public life using traditional open cut trenching for laying underground services. The question is, why should we dig the road when we can drill it for laying the underground utilities and also why do not adopt better alternatives and eco-friendly methods, readily available. This article, in general introduces and discuss in brief the various important methods under trenchless technology being used for laying underground utilities.

Introduction

The rapid growth of cities needs more and more underground infrastructures such as electrical and communication networks, water and sewer piping system and other small utility tunnels. In metropolitan cities, the jamming of road traffic is a severe problem for the society. The traffic problem get increased when the infrastructure is constructed using the traditional methods of open trench construction, because, it causes significantly disruption of traffic. In addition, unsafe trenches and risky conditions can cause danger to pedestrians and workers. Besides, the traditional methods always disturb the environment, such as, site clean up difficulty and soil movement problem. In recent years, making horizontal bores under the ground has become the preferred construction method for installation of the new underground infrastructure. The most of the above problems can be eliminated by using trenchless technology which can be performed at the same or even lower cost than the conventional open trench methods. Being a trenchless method for installation, underground horizontal boring is particularly suited for locations that are inaccessible by conventional open cut techniques. This may include situations where the proposed installation lies in an environmentally sensitive location, crossing of buildings, roads, highways and railway lines. The low environmental and social impact of this technology has made the underground boring extremely viable and desirable by various departments and municipalities where there is a high investment in surface infrastructure and the commutation of traffic makes open cut alternatives inconvenient. The trenchless methods allow installation of most underground infrastructure systems with minimum surface disruption. With the view of demand for trenchless construction practices, the horizontal directional drilling, microtunneling, impact moling & pipe jacking industry has grown substantially over the past few years.

Limitations of Open Trenching

Traditional open cut trenches are in general use for laying the underground services but have following inherent disadvantages, which can be eliminated / minimised by adopting the trenchless technology methods:

a) Traffic hindrance: Traffic obstruction, accidents, extra cost in providing diversions and their maintenance etc.

b) Environment pollution: Dust and air pollution by vehicles and machines, noise pollution to public, pollution of ground and surface water etc.

c) Citizens and society: Increased cost in repair and rehabilitating the roads, compensation for damages, advantage of early availability of the utility and avoidance of public hindrance etc.

Trenchless technology will prove competitive with its increased use considering the social cost aspect in the project. For a meaningful safe environment and public health in our cities the application and adoption of this advance technology may soon become imperative.

The paper describes the few important methods for installing utilities for infrastructural development under the ground.

Trenchless Technology

The trenchlesss technology is an innovative process of installing and rehabilitating the underground utilities like water, sewer & gas pipelines, electricity & communication cables and other underground facilities. This technique minimises or eliminates the need for excavation of the open cut trenches. It is also referred as ‘No Dig’ Technology which is an eco-friendly, hi-tech and speedy construction technology for construction industry.

The trenchless construction is defined as "A family of methods and equipment capable of being used for installation of new/ rehabilitation of existing underground infrastructure with minimum disruption to surface traffic and other activities at the surface."

Trenchless technology, the term itself includes many generic techniques which have been further custom developed or even patented by many advocates and developers in this field. The techniques, as we shall call them are not just one area of engineering and technology but integration and interfacing of various branches of engineering like civil, electrical, mechanical, electronics, instrumentation, metallurgy, etc.. Interaction of so many branches of engineering makes it more complex in the field of modern urban underground utility services management.

Trenchless Boring Methods

There are many trenchless methods of making horizontal bores under the ground. The few important and widely used methods are described here in brief:

I. Horizontal Directional Drilling
Trenchless Technology for Underground Services


Horizontal Directional Drilling (HDD) technology is evolved from the merging of oil field and water well drilling technologies. The main advantage of this technology over conventional services laying techniques is that it is a trenchless method that requires a minimum amount of excavation to complete an installation. This technique allows flexibility in installation of the pipes etc. as the path can be drilled straight or curved to avoid surface and subsurface obstacles.

The directional drilling industry has grown very fast and is being utilized by several users. It was first used in the oil and gas industries and then expanded to include utility installations, environmental remediation and the installation of sewers etc. The installation of pipe utilizing directional drilling is a multi stage technique completed in two phase operations which includes the drilling of a pilot hole and its subsequent reaming to install the product pipe. Installation of pipe is conducted from the surface and commences with the drilling of a pilot hole along the path of installation. The pilot hole is launched from the surface at a certain angle and then gradually becomes horizontal when the required depth is reached. The hole can be steered and tracked from the surface using a walk over or wire line locator system to direct the hole to the exit location. Once the drill string reaches the surface at the exit location, a reamer is attached to the drill string and pulled back to the entry point. This process enlarges the hole for the installation of the product line. To achieve the required size of hole, the several reaming operations can be performed, Figure 1. Generally, all reams prior to the actual product installation are referred to as pre-reams and the final ream to which the product pipe is attached is referred to as the back reaming.

The product line is installed once the borehole is enlarged to a diameter that is generally 1.5 times of the outside diameter of product pipe or conduit. This is necessary to allow for an annular void for return of drilling fluids & spoils and to allow for bend radius of the product line. There is a variety of different reamers for each type of soil. A blade reamer is used for soft soils, a barrel reamer for mixed soils and a rock reamer with tungsten carbide inserts is used for rock formations.

The wet drilling process requires large volume of drilling fluid which provides various functions like hydraulic cutting with jet, lubrication of cutting head, transportation of cuttings out of the bore, stabilization of the bore hole against collapse etc. The slurries most commonly used as drilling fluid are bentonite based.

The advantage of using this method is its low operational cost, effectiveness in congested urban areas for crossing of roadways, Low restoration cost as it requires very small entrance pit.

The possible application of horizontal directional drilling can be in soil vapour extraction, ground water extraction and infiltration wells.

II. Microtunneling
Trenchless Technology for Underground Services


Microtunneling is a process that uses a remote controlled Microtunnel Boring Machine combined with the pipe jacking technique to directly install product pipelines under the ground in a single pass. This process avoids the need to have long stretches of open trench for pipe laying which causes extreme disruption to the community. Cutting & excavation of the soil and pushing of the pipe is done simultaneously.

The equipment is basically in two parts–the first part comprises of a special piece of steel called "Boreshead" which moves at the head of pipe being installed. The second part is comprised of several small equipment which include pushing equipment, power units, mud preparation equipment and other auxiliary equipment. One of the two operators sits inside the Borehead and operates the cutting head, steering mechanism, water and mud jets etc. and ensures that the pipes are installed accurately in the given tolerance of line. The second operator operates the other equipment. Generally, the metallic pipe is pushed into the ground by simultaneously removing the equivalent (volume wise) quantity of soil. After one length of the pipe is pushed, the head of the second length of pipe is welded circumferentially to the tail of the leading pipe and is pushed forward, Figure 2. New lengths of pipe are added and pushed till all pipes are installed and the total length is completed. An another small pit called receiving pit is excavated on the receiving end to take out the Boreshead.

This technique under trenchless technology enables the construction of environmental friendly subsurface facility projects, mainly in piping systems.

Microtunneling is economically competitive with direct burial when the depths exceed six meters which is due to the increased cost of deep trench excavation and trench supports. This method is cost effective when faced with unstable soil conditions and the work below groundwater level. These conditions increase the risk of surface settlement during a direct burial or conventional tunnel installation and also increase liability for all parties with regard to the damage of property and personal injuries. Microtunneling is the safest tunneling option when faced with these conditions, because the workers and the public are not directly exposed to hazardous conditions.

It is very fast boring method and the main application is for large diameter but shorter bore lengths. The boring can be carried out under dynamic loading conditions. The installation of the pipes can be done accurately in given tolerances.

III. Impact Moling

Impact Moling is a trenchless installation method for placement of small diameter pipes, ducts and cables in which percussion or hammering action of a pneumatic piercing tool is used to create the bore by displacing the soil rather than removing it. The method typically is non-steerable, however, the steerable systems are also in use in the recent years.

When properly designed, the impact moling is the simplest, least expensive and the most widely used trenchless method for installation of service connections to gas, water & sewer mains usually under sidewalks, drive ways and other short crossings. General advantage of impact moling are low operational cost, relative simplicity in operation, minimum excavation for the installed product and minimum public disruption. The support equipment is limited to a small air compressor.

Feasibility of the method is restricted by its generic limitations (limited boring diameter & length) and by local ground conditions that can greatly affect its performance. The adverse ground conditions may include cobbles, dense dry clays and other non-compactable soils which may affect penetration rates.

Impact moles in general, consist of an enclosed steel tube containing an air-powered piston which strikes the nose of the tool to drive it forward. A bore is formed by displacing/compacting the soil laterally Figure 3. The friction between the ground and the mole body prevents the mole from rebounding backwards and therefore the repeated impacts of the piston advances the whole unit in the ground.

Steerable moles are also available which address the path deviation shortcoming of non-steerable moles and are poised to expand the usability of the method by virtue of being able to bore the curved path. Impact moles are also known as earth piercing tools, soil displacement hammers, impact hammers, percussive moles or pneumatic moles.

The application of this method specifically for very small diameter bores for laying transmission cables and small diameter pipes etc. It can be used comfortably in congested areas as it needs very small space for operation.

IV. Pipe Jacking
Trenchless Technology for Underground Services


Pipe Jacking is a technique for installing underground pipelines, ducts and culverts etc. The powerful hydraulic jacks are used to push the specially designed pipes through the ground behind a shield and at same time the excavation takes place within the shield. The method provides a flexible, structurally strong, water tight and finished pipeline, Figrue 4. The ground engineering considerations and economics of the work may impose restrictions on use of the process. A number of manual and mechanical excavation systems are available. Pipes in the range of 150 mm to 300 mm can be installed by this method.

To install a pipeline, the thrust and reception pits are constructed usually at manhole positions. The dimension and construction of the thrust pit may vary according to the specific requirement. The mechanised excavation may require larger pits than hand excavated drives. A thrust wall is constructed to provide a reaction against the jacks and the high pressure jacks provide the substantial force required for jacking the concrete/metallic pipes for underground services.

The method needs higher capacity hydraulic jacks and needs little larger size of pit for operation of the jacks. The use is limited to medium diameter and small length bores.

Conclusion

In a very short span of few years, the construction industry in India has adopted the trenchless technology to a great extent, specifically in metropolitan cities and has benefited the society by minimizing the disadvantages of environmental and social impacts. However, the technology is still to be adopted by the construction industry for small projects for general awareness in common public towards trenchless construction for environment friendly urban development.

The use of trenchless technology will play a vital role in laying underground infra structure, specially, in pipelines installation in difficult geological situations. As discussed, the underground construction by various methods used under trenchless technology are very fast in comparison to open trenching methods, however, being mechanized process of construction, requires trained technical man power and modern equipment.

Acknowledgment

The authors are thankful to Director, Central Building Research Institute, Roorkee for the permission to publish the paper.

References

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NBMCW May 2009