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NABEEL S R
Abstract
Trench less technology is the science of installing, repairing and renewing underground Pipes, ducts and cables using techniques which minimize or eliminate the need for excavation. It can reduce environmental damage, Social costs and produce in alternative to open trench method of installation, renewal and repair it includes in, development of all kinds of underground napping techniques, tunneling devices and specialist materials and equipment.
1. INTRODUCTION
Trench less technology is basically making a tunnel below the surface and staling service lines like water or gas pipes, electric or telecommunication cables etc. without any disruption to the public. It also makes it possible to install the utilities under rivers canals and other obstacles with no disruption of flow and with minimum or no damage to the environment. The difference between trenchless and other subsurface construction techniques depends upon the size of the passage under construction. If a subsurface construction does not permit human entry, it can be termed as trenchless.
The method is many times used without considering soil characteristics, overburden and the loads coming on the surface where the method is used. in case, the soil is sandy, water table is at shallow depth and heavy loads like that of urban traffic is expected, the depth of trenching has to be at a depth that the pressure of the load on the surface does not affect the bore else surface caving in cannot be ruled out. Therefore, sufficient precautions should be taken in loose and sandy soil strata, particularly near the ends from where the trenching is started or ended. Traditional open trench method seems ineffective for present requirement like under roads, railway tracks, highways, sewers, drains, cables etc.Without any disruption to traffic as it is not necessary to dig up or cut across the road. Other obstacles encountered in adopting these old methods are described as follows:
Obstructions on roads, busy areas diversions have to be provided before start of digging of work and entailing avoidable expenditure.
The cost of fuel as well as additional time wastage is huge over the total period of construction though not directly attributable to project cost.
Cutting dip trenches in congested areas, cracks appear on the adjacent buildings resulting in stay orders.
Damage cost to other service lines/cables present in ground due to lack of application of modern tools.
Enormous amount of excavated earth has to be carted away regularly on a day to day basis Causing noise and dust pollution . the same earth has to be brought back for refilling .
Trees, shrubs, gardens may have to destroyed damaging environment.
The techniques which have been developed to overcome these obstructions are known as “No-dig “/trench less technology.
2. TRENCHLESS TECHNOLOGY METHODS
Trenchless technology methods system has been categorized in to two groups:
1) New installation
2) Rehabilitation and Renovation
2.1 NEW INSTALLATION
Methods for installation of a new pipeline or duct, including dealing with service connection are:
1) Microtunnelling
2) Horizontal directional drilling
3) Short drive system
4) Guided drilling
2.1.1 MICRO TUNNELING
Micro tunneling is a term which is used to describe remotely controlled mechanical tunneling systems where the soil is removed from the cutting head with in the new pipe line which is advanced by pipe jacking.
Micro tunneling machines have now been developed to work from drives hafts in almost all types of ground conditions. The cutting head has to be carefully selected to deal with the expected ground conditions, with the appropriate cutting tools and crushing devices for the range of gravels, sands, slits, and clays.
The only excavating required from the service is for drive and receptions shafts. Soil may be removed from the face by an auger running through the newly installed pipeline to a skip in the base of the drive shaft.
Alternatively, water or betonies may be used to convert the soil into slurry at the cutting face. The slurry is less then pumped to the surface where the solids are separated before disposal. Microtunnelling is used extensively for sewerage work where surface disruption has to be minimized. Machines are now available to drive 100mm or more in soft ground for sizes 100mm diameter upwards. From drive shafts of less than 3mm diameter.
Micro tunneling system has been developed in which temporary steel tubes are jacked in and removed at the next manhole position, the new pipeline following in the established bore. In Microtunnelling, the only indication on the surface is the presence of a control container with a hoist for lowering pipes into the drive shafts. Noise levels and traffic disruption are minimized.
2.1.2 HORIZONTAL DIRECTIONAL DRILLING
Horizontal drilling systems are nowadays widely used for installing pressure pipes under major obstacles such a motor way intersections, large rivers and airports runways.
A small rotating and steerable drill bit is launched from the surface at an angle 10-15 and is used to drill 90mm mud filled diameter hole. During the drilling operation a 125mm diameter wash over pipe is drilled over the pilot string and following some 100mm behind the head. Alternate drilling then continues on the pilot string is removed and the bore is enlarged by a rotating barrel reamer attached to and pulled back by the wash over pipe, drilling mud being used to blushed away the cuttings and to support the reamed hole. Subsequent coming continues until required diameter is achieved. The product pipe is less than attached to the reaming head and pulled through the bore drives of more than 1.5km and of up to 1200mm diameter have been carried out.
Directional drilling involves steerable tunneling systems for both small and large diameter lines. In most cases, it is a two-stage process. The first stage consists of drilling a small diameter pilot hole along the desired centre line of a proposed line and in the second stage, the pilot hole is enlarged to the desired diameter to accommodate the utility line and to pull the utility line through the enlarged hole. The pilot hole is of approximately 3 inches in diameter and is drilled with a specially built rig up with an inclined carriage typically adjusted to between 5 and 30 degrees, which pushes the drill rods into the ground. However the optimum angle is 12 degrees. As the pilot hole is being drilled, bentonite-drilling mud is pumped down the center of the drill rods. The drill head consists of either a jetting head or drill bit. In the case of a jetting head, small diameter high-pressure jets of bentonite actually cut the soil and facilitate soil removal by washing the cuttings to the surface where they settle out in a reception pit. In case of drill bit, the bit is driven by a down hole mud motor located just behind the drill bit from energy derived from the pumped drilling fluid. Before the start of back reaming the pipeline has to be positioned on rollers in line with the hole to minimize any axial load on the line.
Advantages
Speed of installation combined with the minimum environmental and social impact.
Method involves minimum surface damage
Save a lot of time and expenses.
Great importance in congested urban areas where the numbers of underground utilities are high often very little space is available.
River crossings, the effect of buoyancy and danger of river bed erosion are eliminated as the utility can be installed.
Disadvantage
Special equipment and very high degree of operation skill is required.
As the cost of the equipment and the operation are high, bore length should be sufficient in order for it to be economical.
Mainly steel pipe is being installed by the method.
2.1.3 SHORT DRIVE SYSTEMS
2.1.3.1 Auger Boring
`Utilizes a rotating head to excavate the soil, which is transported by auger flights operating in a casing to the drive pit. The head is recovered at an exit pit or in the trench cut for the adjacent length of pipe line. Auger boring is used in the range of 100-1000mm diameter
The auger horizontal earth boring is a process of simultaneously jacking casing through the earth while removing the soil inside the casing by means of a rotating flight auger. The auger is a flighted tube having dual functions, firstly it has couplings at each end that transmit torque to the cutting head from the power source located in the bore pit and secondly, it serves to transfer soil back to the machine.
This method requires bore pit both at the entry and exit points of the bore. The auger-boring machine consists of the boring machine, casing pipe, cutting head and augers as the major components. The power source creates the torque, which rotates the auger, which in turn rotates the cutting head. The cutting head cuts the soil and the soil is transported to the machine through the casing by means of the auger, which acts as a screw conveyor.
The pipe size that can be installed by this method ranges from 4 inches (100mm) to over 60 inches (1500 mm). However, the most common size range is 8 inches(200 mm) to 36 inches(900 mm) and the average bore length ranges between 53 meter and 68 meter, though with experience and the use of latest techniques up to 180 meter of boring is possible using auger boring.
Advantages
The major advantage is that the casing is installed at the same time as the borehole excavation takes place. This method can be used in a wide variety of soil types.
Disadvantages
This method requires different sized cutting heads and auger sizes or each casing diameter, which increases the investment in equipment. The investment in bore, pit construction, and the initial setup is also required. In case of soils containing large boulders, this method cannot be used advantageously.
2.1.3.2 IMPACT MOLING
In which a percussive mole is launched from a drive pit to displace the soil and from a bore is widely used. The new conduct is normally drawn in behind the mole. They are used to install services for all utilities. Moling is a method, which forms the borehole by compressing the earth that immediately surrounds the compacting device which is an underground piercing (mole) is propelled by a power source. The tool is streamlined into bullet or shape. The method is restricted to relatively small diameter lines in compressible soil conditions.
Compressed air or hydraulic fluid, transmitted to the toot through the flexible hoses, imparts energy at a blow frequency of 100 to 600 strokes per minute to a reciprocating piston located inside the nose of the tool. This action results in the tool propelling itself through the ground. It is applicable in most ground conditions from loose sand to firm clay. The method required the use of boring and receiving pit. After the operation the unit can be backed out of the borehole. The tool is removed and the cable is attached to the air hose and pulled back through the borehole. In the case of rigid pipe, it can simply be pushed through the open borehole. Any type of pipe or cable can be installed by the method.
Pipe size is generally limited to 6 inches or less. However, modern techniques in mole manufacturing have increased the ability to make bores of large sizes. Even though 200 feet (60 meter) bores have been successfully made by this method, the span lengths were limited to 60 feet (18 meter) with 40 feet being optimum. Again span lengths have increased with modern advances in mole design.
Advantages
It is a rapid, economic, and effective method of installing small diameter lines. Any type of utility line can be installed using the method. The stability of the soil around the borehole is increased due to compaction. The investment in equipment is minimum.
Disadvantages
Compaction methods are limited in their length by reliability because basic systems are unintelligent, unguided tools that tend to bury themselves, surface in the middle road or damage existing utility lines.
2.1.3.3 ROD PUSHING
Is a technique in which a bore of about 50mm diameter is formed by displacement A rod is advanced by a straight hydraulic push and the pilot hole may be back reamed to the required size. The technique is used for the installation of pipes and conduits up to 15mm diameter over lengths of 30-40mm.
2.1.3.4 PIPE RAMMING AND THRUST BORING
Similar processes where a casting, usually steel, is driven through the ground from the drive pit to the exit pit. Accumulating soil is removed by compressed air and water after completing the bore. Pipe ramming is suitable for most types of soil but not suitable where there are solid rock formations. It is said to be a cost saving alternative to open trenching, angering or pipe jacking methods. Usually pipes up to 2000mm diameter can be laid using this technique depending upon the equipment uses.
Pipe ramming is a simple technique utilizing a pneumatic hammer to drive steel casings through the ground from one pit to another. Rams of pipes up to and over 2m diameter and exceeding 70m in length have been achieved. Basic operation is as follows:
The correct size of rammer is chosen by assessing casing diameter, length of ram and the prevailing ground conditions. Positioned on the tracing or 'H' Beam in the launch pit, the rammer and first casing length are connected directly or via ram cones. Using adjustable cradles or air-bags, the assembly is aligned and levelled on target. The air supply is partly applied to start the rammer and slowly increased as the first casing enters the ground. Full power is applied when the skin friction on the casing is sufficient to overcome the back stroke of the piston. As each casing length is entered the rammer is removed, the next casing welded in place and the rammer replaced and restarted .On completion of the ram, the soil in the casing can be removed by one of several methods, pressurization and blow-out, pressure jetting, mini excavator or b y any combination
2.1.4 GUIDED DRILLING
Guide drilling employs an excavation or soil displacement with compact lightweight rig for rapid mobilization. Small diameter jets mechanized cutting tools or displacement heads attached to a flexible drill string are positioned to form a bore as the head is thrust forward. The drilling head is launched from the surface at an inclined angle. Controlling the orientation of a slant face at the head effects steering in both vertical and horizontal planes. Monitoring of the alignment takes place using a transmitter in the head and a locating device at ground level.
After established the pilot bore; back reaming equipment is drawn through the hole to enable it to accept the product pipe, duct, or cable using an impact mole.
2.2 REHABILITATION AND RENOVATION
Methods including are:
1) Pipe bursting
2) Pipe eating
3) Retaining the existing pipes
4) Localized repair
2.2.1 PIPE BURSTING
Pipe Bursting
Pipe bursting, as the name implies, uses a hammer to break the existing pipe and force broken fragments bursting head into the surrounding soil while anew pipe is pulled and/or pushed in its place simultaneously.
There are different variations of pipe bursting method:
• Pneumatic pipe bursting: a pneumatic hammer is used to break the existing pipe.
• Static pipe bursting: the energy to break the existing pipe is in the pulling with no percussion action. Compared to pneumatic method, this is a quiet operation and action preferable in clayey soils or when there is need to cut (split) cast iron or ductile iron pipe.
In this method an existing pipeline can be replaced with a pipeline of the same or larger dimension without opening up the ground. It is especially useful in areas where the load on the system is more than the existing pipe can handle and replacement is required. The method uses a mole as a bursting head that is drawn through the existing pipe crushing it as it moves forward and replacing it with a new PE (polyethylene) pipe. The main advantage of this system is that a small power source can be used to drive the mole with minimum time. Upsizing from 100mm diameter to 225mm diameter is now well established, and pipes of up to 600mm diameter have been replaced. The average speed of replacing pipe through pipe bursting is about 500 ft/day.
(7) Pipe bursting was used at Elmendorf Air Force Base, near Anchorage (Public Works 1996]. The base had to replace 4,000 ft of 6-in
2.2.2 PIPE EATING
Pipe eating is an online micro tunneled replacement technique. The existing defective pipeline is crushed and removed through the new pipeline. Lateral connections must be disconnected in advance and may be replaced by rider sewers or reconnected by angled drilling. The pipe removal process can potentially be executed by horizontal auger boring, a process that excavates the existing pipe and surrounding soil by using a rotating cutting head attached to an auger that continuously removes the excavated soil. High- and medium-density polyethylene (HDPE or MDPE), collectively called “PE”, have been the most widely used replacement pipes for pipe bursting applications.
2.2.3 RE LINING THE EXISTING PIPES
This method requires access, usually by manholes, at both ends of pipe. A flexible liner is places into the defective pipe and with the use of water under pressure finds its own way and can pass bends of 90o. In places where joints have moved or sections are missing, but the passage is available, the liner creates a smooth transition. When the liner is in place, it is heat cured to create a rigid, tough, and smooth inner surface. of rehabilitation techniques include Cured-in-Place Lining (CIPP), Close-Fit Lining, Slip-lining, and Spray Lining, all with their own-patented variations, as well as various other localized repair techniques. Variations relate to the material used, wall thickness provided to offset structural or physical defects, the rate of rehabilitation, and the minimum time of shut-down for the existing service.
The rehabilitation of small diameter underground pipes is a new area where the cost competitiveness of Trenchless Technologies is well recognized. Many utility pipelines,
sewage in particular, become defective due to the corrosiveness of modern effluents. They also suffer from overloading and loss of capacity. One of the advantages of rehabilitation is that the new lining materials have a much lower surface friction coefficient, thus it is often possible to increase the capacity of the refurbished pipe without increasing its diameter.
CIPP, a fabric impregnated with polyester or epoxy resin is inserted into the defective pipe and inflated to fit against the pipe wall. It is then cured by hot water, steam or ultraviolet light. The system has many variants and can be designed to provide different wall thicknesses to meet particular needs. One advantage is that the lining adjusts to variations in the size of the pipe. It is widely used for the rehabilitation of gravity sewers, including laterals, and usually results in no loss of capacity.
Close-fit linings take many forms. The lining is deformed through a swage (a metal die) or manufactured in a folded state so that it can be pulled into the host pipeline. Various methods can then be used to allow the lining to revert to its full size or to the shape of the host pipe.
Spirally wound liners are a form of close fit in which a PVC strip is fed though a small access into the defective pipe. The PVC strip is then helically wound into place against the pipe wall using a winding machine operated from within the pipe. This technique is particularly useful for emergency repairs and for adding strength to pipelines that have been weakened.
Slip-lining involves putting a pipe within a pipe and grouting the resulting annulus between the new lining and the old pipe. This causes a reduction in capacity and the process has now been modified using polyethylene to reduce the thickness of the liner and to minimize the size of the annulus.
Modified slip lining often called close fit lining utilize the properties of PE or PVC to allow temporary reduction in diameter or change in shape prior to insertion in the defective pipe.
The method includes Roll down, Swage lining and deformed lining. The inserted pipe is subsequently expanded to form a tight fit against the wall of the original pipe, thus avoiding the need for annular grouting as in conventional slip lining. For Roll down and Swage lining, temporary reduction in diameter is achieved either by mechanical rolling (Roll down) or drawing through a reduction die (Swage lining). For Deformed linings, the pipe is deformed and folded immediately after extrusion and is coiled on a drum. After insertion in the defective pipe, the lining is expanded using steam and a re-rounding device to form a close fit.
These systems are suitable where the existing line is of good shape. As compared to conventional slip lining , in this method there is little or no loss of hydraulic capacity.
Spray linings using cement or resin are widely used on water pipelines. Spray lining materials have to be used carefully and approved by regulatory authorities due to the potential for releasing solvents and residues. Spray linings are suitable for dealing with leaks but not where there are structural defects. Localized repair techniques make use of robots in conjunction with CCTV cameras to clean, prepare and fill cracks and voids with epoxy mortar. This is often a cost effective way of dealing quickly with an isolated problem in an otherwise sound pipeline. The ease of transport and mobilization of the equipment is an advantage.
2.2.4. LOCALISED REPAIR
Local defects may be found in pipes due to cracking or joint failures. Systems are available for resin injection to seal localized defects in the range 100mm-600mm diameter.
Chemical grouting with urethane and similar materials are used in sewer rehabilitation. Remote and man entry grouting of defective joints and cracks may prevent infiltration in pipelines.
This is an inexpensive method of rehabilitating existing systems up to several hundred meters of length everyday from manhole to manhole.
2.2.4.1 Coatings
The purpose of a coating is to repair cracks in the original pipe, or to protect it from corrosive wastewaters or mechanical abrasion. A coating is either applied to the interior of the deteriorated pipe manually, or with specialized equipment. Manually coating applications can only be done in pipes with a diameter large enough to accommodate the worker, usually a minimum of 48 in. Coatings are applied by spray, brush, trowel, or roller. Coatings must be applied to a clean, dry surface. Therefore, they must be applied above the waterline, or (if applied below the pipe’s waterline) the flow must be diverted until the coating cures. A smaller pipe running through the center of the original pipe can serve as a bypass. A significant advantage of coating technology is that no extra work or equipment is necessary to make connections at laterals or manholes.
2.2.4.2 Cement Mortar.
Cement mortar coatings can be applied to concrete, steel, or iron pipes, and can extend the service life of a pipe by up to 50 years. The coating is made of one part sulfate resistant cement to two parts sand. The coating is applied by hand or by an automated spreading machine. Cement mortar cannot be applied in pipe networks with many bends, or in very cold regions. PWTB 420-49-10
2.2.4.3 Reinforced Gunite
Gunite is also referred to as “shotcrete,” and is a mixture of cement, sand, and water. Steel reinforcing mesh or lattice is attached to the interior of the original pipe. Gunite is manually applied with a pressure sprayer to the mesh or lattice. This method improves the structural integrity of the system and can be used on varying cross sectional profiles.
Installing gunite will significantly reduce the internal diameter of the original pipe. However, the smooth surface typical of gunite will reduce the coefficient of friction of the pipe and may actually allow more flow. Increased flow following application of gunite is particularly prevalent in piping that is severely corroded or tuberculated. Therefore, pipelines should be assessed for the impact of reducing the diameter before selecting this coating.
The application of reinforced gunite is restricted to pipes large enough for worker access, usually greater than 4 ft in diameter. The original pipe must be thoroughly clean and dry. The system flow must be completely shut down or bypassed for an extended period.
2.2.4.4 Resin
Resin coatings are made of specialized formulations of polyurethane, phenol epoxy or Poly urea. They can solve problems of corrosion and erosion. Resins can be applied to steel or concrete pipes, and have excellent adhesion and impermeability. Poly urea
2.2.4.5 Grout
Grout is actually a variety of materials used to fill voids, stabilize soil, hold bricks in place, coat cement, and prevent infiltration. Grouts are applied under pressure after the pipe has been adequately cleaned to provide a good work surface. A typical grouting application in new construction is to seal the joints between segmented concrete pipe. A common use in rehabilitation is to seal off groundwater infiltration in non pressure pipelines. Since grouting can be placed wet, it adapts well to irregular surface. Table 1 lists limitations and advantages of grouting.
3. PRE-REQUISITES
Through surveys and site investigations are essential to the success and efficiency of trench less installations and repair techniques.
(a) The survey results help top determine the most appropriate systems.
(b) The investigations methods include geophysical surveys, boring and sampling, measurement of ground water table, test pits and trenches, penetration tests, examinations and investigations of existing structures using ground penetration testing etc.
© Closed circuit televisions, ground penetration radar, utility detection equipments leak detection techniques are available for below ground activities.
(d) Use of GIS,GPS, terrain mapping and satellite mapping should be done
(SEE) factor is rather difficult to quantify or
Estimate in $ value and frequently over looked in countries where occupational safety and health and environmental protection regulation are not enforceable. But in country like in India these things can’t be ignored .Because we know it says “ pay me now or pay me later a lot more!”So we should know that the true cost of a project must include allowances and contingencies to mitigate and minimize the adverse impact caused by ignoring see factor. It should be realized that no construction method is perfect accident free /environmentally friendly, so considering its advantages over other it should not merely
Based upon direct cost/lowest bid value.
In general, construction cost may be broken down into:
Direct cost
Indirect cost
See cost
Considering direct /indirect cost, see factor is more appreciable over the
two. Almost all the cost value of trenchless technique and open trench technique are nearly equal.
5. NEED FOR TRENCILESS TECHNOLOGY
(1) The disadvantages and difficulties encountered in conventional trenching methods have resulted in thinking of the need for trenchless technology.
(2) The advantages of the no-dig technology are also responsible for the need of this technology to be adopted in mainly urban areas
5.1 OPEN TRENCH METHOD
It is a traditional met6hod of trenching for laying the utility lines below the surface. In present days, there are many disadvantages and difficulties in adopting this method, mainly in urban areas. Those are described below:
a. As the open trench is going to create obstruction roads, busy areas, diversions have to be provided before start of any digging word.
b. As the obstruction is created, the traffic has to be rerouted causing traffic jams.
c. Original users of the road have to undergo hardships in the form of additional mileage as well as time.
d. Many a time, while cutting deep trenches in congested areas appear in the adjacent buildings.
e. Another difficulty, which is encountered very often is the damage caused to other service lines or cables present underground, providing temporary supports to these lines during the construction is cumbersome and costly affair.
f. Trenches left open overnight should be fenced and barricaded. Hand of mechanical signs should be used where necessary.
g. While cutting open trenches, trees, shrubs, gardens etc. may have to be destroyed damaging the environment.
h. If any rehabilitation or renovation is required, the trenches once cut and refilled should again be cut through out and refilled causing difficulties to the public; that is, cutting and refilling is required at frequent times
5.2 ADVANTAGES OF NO-DIG TECHNOLOGY
(a) It reduces damages of valuable surface.
(b) It reduces the danger of improperly compacted excavations.
© It saves resources.
(d) It is accident free.
(e) It avoids traffic jam.
(f) It makes the use of the line (track) of the old pipe possible.
(g) It saves underground space (pipe busting).
(h) It reduces the impact on the environment.
(i) It provides the hassle-free road surface.
(j) It is possible to lay service lines across railway track, narrow lanes etc. When open trenching is impossible.
(k) Presence of a canal, pond, river etc. across the root poses no problems to the trench less technology systems.
(l) Without disturbing the traffic and life on the surface, the lines can be laid below ground in a much shorter time by using this technology.
(m) For replacement, repair and rehabilitation of old water and sewer lines in cities, it is very helpful to use trench less technology without disturbing the normal life on the surface.
6. CONCLUSION
Planners need to understand the opportunities that the alternatives TT have to Offer. Designer must design with the knowledge that a wide range of TT and techniques are already available with continuing economic advantages likewise with trenchless installation refurbishment and rehabilitation techniques, a wide range of social technical and economical advantages will also be readily available. High level strategies within the utilities to utilize the available technology must be encouraged. This implies developing an understanding of the value of the application of technology and linking this closely with asset management strategies already in place. Once there is greater awareness of the impressive state of development that T.T industry has achieved when advantage is taken of the range of technique equipment and material that already exist and when the traffic authorities and utilities recognize the need for and advantage of T.T then the true potential of no-dig techniques will start to be realized.
