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Excavations & pit sizes

For all trenchless methods, both new installation and rehabilitation, it is necessary to excavate at each end of the works to reach the working depth for installation or the host pipe for rehabilitation. The choice of location for such excavations should take into account plant access, other utilities and space for preparation and handling of the PE100 pipe string. In rehabilitation works there will always be some things such as valves and bends which cannot be lined through and which therefore need to be removed in advance. It is clearly an advantage if the working excavations can be located at such positions.

Another consideration, particularly with larger diameters, is that the liner pipe takes up a lot of space, and effectively precludes the deployment of cross strutting over a considerable length of the excavation. It is essential that the excavation support is designed with this in mind. Particularly with larger diameters and deeper installations great care should be taken so that lateral ground support is not reduced for any adjacent buried infrastructure.

All the methods also require some equipment placed at the surface adjacent to the excavations. This may be simply a compressor, generator or winch for certain techniques. For others the primary plant, for example drill rig or die/reducer, is located at the surface in the direct line of the installation or host pipe. For live gas isolations equipment will have to be installed onto the host pipe to carry out the isolation. This equipment can be removed once the pipe has been isolated and the excavation used for the insertion process. On completion the same excavation can be used for the reconnection equipment. Those techniques that install a full length pipe string will require space for this in the line of the installation or host pipe as well, including preparation and fusion into a single length. Refer to the individual method modules for details of the specific requirements of each method.

Mole ploughing needs no excavation for these purposes but does need them for connection to existing buried pipes at each end of the works. For diameters greater than DN90 the pipe is laid along the ground on or close to the line of the installation.

Pilot tube microtunnelling is generally undertaken in shafts that will be used as manholes in the finished works or in existing manholes in certain cases. The method uses discrete lengths of PE100 pipe that are fusion jointed at the base of the entry shaft prior to pushing or pulling into the line of the drive.

All of the other trenchless techniques that use PE100 pipe require excavations that can allow the pipe to enter, and in some cases exit, the works without being bent in a radius that is tight enough to damage or overstress it. The entry excavation must be long enough to allow the liner pipe to make a smooth transition from ground level to the depth of installation or the host pipe in the case of liner pipe. In order to avoid the risk of buckling, particularly when installing higher SDR pipes, the design radius for this transition may be up to 40 times the liner pipe diameter depending on SDR and whether or not there are joints. For larger diameter pipes and/or greater depths this may require excavations of considerable length for certain methods.

Small PE pipe up to DN180 can be delivered in coil form, which has a radius of only around 12 times the diameter, so that much smaller excavations can be used. As a result some techniques such as impact moling, can have quite small excavations.


Figure 1

Image courtesy : Exova Utilities

Figure 1 shows the excavation lengths necessary to achieve a radius of 40 times satisfactorily at 2 metres depth to invert of host pipe. These lengths will need to be increased by some 2 metres for every extra metre in depth Similarly the lengths can be decreased when the depth to the pipe is less.

Width of excavations is a function of pipe diameter and depth; deeper excavations may require heavier and more substantial support in terms of struts, props or walings and the excavation must be wide enough for these not to obstruct the path of the PE100 pipe. For smaller diameters width may be dictated by safe working space needed for personnel and equipment in the excavation.

Directional Drilling requires excavations at both ends of the works to the depth of the line. While the exit (for the PE100 pipe) end where the drill rig is located does not need to accommodate the pipe as shown in Figure 1 it does need to provide sufficient space for the drilling rods and for the PE100 pipe to emerge and be disconnected from them without bending. At the entry end, remote from the drill rig, an excavation length as per Figure 1 will be required. At this end it will also be necessary to lay the full length of the pipe string on the surface in the line of the installation.

Impact Moling is generally undertaken at shallower depths than other methods although not less than 1 metre, and is seldom used at diameters greater than DN160 so almost always uses coiled pipe. Unless coiled pipe is being used an excavation at the entry end as per Figure 1 will be required and it may be necessary to lay out the full length of the pipe string on the surface in the line of the installation, and to allow space for preparation and fusion The excavation must be long enough for the mole to be correctly aligned.

At the exit end a smaller excavation will be sufficient if the pipe is to be connected to existing pipes. It need be long enough simply for extraction of the mole itself and for the connection works to be undertaken properly.

Sliplining requires an excavation as per Figure 1 at the entry end. It will be necessary to excavate in order to expose and cut into the host pipe to provide access for cleaning, inspection, and insertion of the liner pipe. At this end it will also be necessary to lay the full length of the pipe string on the surface in the line of the installation and to allow space for preparation and fusion. At the exit end a smaller excavation will be sufficient if the pipe is to be connected to existing pipes. It needs to be long enough for the winch cable to pull the PE100 liner pipe straight right to the exit point from the host pipe. If there is insufficient space this can be achieved using a pulley wheel fixed at the exit point. But sufficient PE100 liner pipe must be pulled through to enable end connections to be made.(see Figure 2).


Figure 2

Image courtesy : Nowak

Where the insertion passes round bends there will be an increase in the frictional drag, and whilst this is generally small for the slow bends created by ‘kicking’ each pipe joint a degree or two during installation, the increase may become significant if too many bends are included in one insertion. As the excavations are a significant proportion of the overall cost, it is clearly an advantage to have as few as possible, and hence to make insertions as long as possible, so if the route involves a significant number of bends, it is sensible to have a flexible operations plan which starts with a relatively short insertion including several bends so that the effect can be accurately assessed without risk of getting stuck, after which longer insertions can be planned with confidence.

Pipe bursting/splitting requires an excavation as per Figure 1 at the entry end. It will be necessary to excavate in order to expose and cut into the host pipe to provide access for insertion of the liner pipe. At this end it will also be necessary to lay the full length of the pipe string on the surface in the line of the installation and to allow space for preparation and fusion. At the exit end a smaller excavation will be sufficient if the pipe is to be connected to existing pipes, but must be long enough for sufficient PE100 pipe to be pulled through to enable end connections to be made The exit excavation is where the winch or, more commonly, rod puller is located and must therefore be large enough for this and for safe working around it. This exit excavation also needs to provide reaction to the winch so will require a plate or surface against which the winch can be anchored. Sufficient PE100 liner pipe must be pulled through to enable end connections to be made (see Figure 2).

Close-fit lining (rolldown and die drawing) requires an excavation as per Figure 1 at the entry end. It will be necessary to excavate in order to expose and cut into the host pipe to provide access for cleaning, inspection, and insertion of the liner pipe. In order to keep the excavation length to a minimum, the optimum layout is to place the die/reducer at ground level at the entry end of the excavation, and to place another roller about half way along the excavation to guide the liner pipe into the host pipe, this latter roller being designed so as not to buckle the liner pipe or create drag. This arrangement effectively places half of the ‘swan neck’ transition outside the excavation, whereas if the die/reducer is placed in the excavation itself, the excavation will need to be to be considerably longer, and will leave virtually no surplus reduced diameter pipe available at the completion of the insertion, which may be critical in the case of Die Drawing which involves longitudinal reversion.

For several reasons, it is very important that the liner pipe does not approach or leave the die/reducer at an angle. Firstly, the liner pipe will adopt a curve during the die drawing process and this will increase friction inside the host pipe during insertion. Secondly it will result in asymmetric die drawing forces, and local yielding of the PE might be induced without this being apparent in the winch load. Thirdly, it will not be possible to obtain a seal at the lubricator, without which the die load will be substantially increased. It is also important that the liner pipe does not enter the host pipe at an angle, as this can cause damage to the liner pipe or in extreme cases displace the host pipe itself.

At this end it will also be necessary to lay the full length of the pipe string on the surface in the line of the installation and to allow space for preparation and fusion.

At the exit end a smaller excavation will be sufficient if the pipe is to be connected to existing pipes. It needs to be long enough for the winch cable to pull the PE100 liner pipe straight right to the exit point from the host pipe. In die drawing it also needs to be long enough for the longitudinal reversion of the PE100 when the tension is released, and sufficient PE100 liner pipe must be pulled through to enable end connections to be made (see Figure 2), after longitudinal reversion in the case of die drawing.

Where the insertion passes round bends the same issues as for sliplining must be considered: increase in frictional drag, optimum location of excavations, and flexibility in the operations plan.

Close-fit (folded): the entry excavation size depends on the method. A site-folded method will require an entry excavation as per Figure 1 based on the folded diameter. Advice on minimum radii should be sought from the system supplier and pipe manufacturer. With a factory-folded method, where the folded PE100 liner pipe is delivered on a drum or reel, it may be possible to insert the folded liner pipe through existing manholes as shown below. the limiting dimension is the bend radius at the point of insertion. Again, advice on minimum radii should be sought from the pipe manufacturer. At this end for site-folded methods it will also be necessary to lay the full length of the pipe string on the surface in the line of the installation and to allow space for preparation and fusion.


Image Courtesy : Wavin

At the exit end a smaller excavation will be sufficient if the pipe is to be connected to existing pipes. It needs to be long enough for the winch cable to pull the PE100 liner pipe straight right to the exit point from the host pipe. Sufficient PE100 liner pipe must be pulled through to enable full reversion and for end connections to be made (see Figure 2).

The same issues as for rolldown and die drawing also need to be considered where the insertion passes around bends. Although the folded pipe is less susceptible to increased drag at bends it can still occur and must be allowed for when calculating the winch capacity needed.

Pipe reaming should be considered in the same way as directional drilling. It will be necessary to excavate in order to expose and cut into the host pipe to provide access for insertion of the liner pipe. While the exit (for the PE100 pipe) end where the drill rig is located does not need to accommodate the pipe as shown in Figure 1 it does need to provide sufficient space for the drilling rods and for the PE100 pipe to emerge and be disconnected from them without bending (Figure 2). At the entry end, remote from the drill rig, an excavation length as per Figure 1 will be required. At this end it will also be necessary to lay the full length of the pipe string on the surface in the line of the installation and to allow space for preparation and fusion.

Pipe extraction requires an excavation as per Figure 1 at the entry end. It will be necessary to excavate in order to expose and cut into the host pipe to provide access for insertion of the liner pipe. At this end it will also be necessary to lay the full length of the pipe string on the surface in the line of the installation and to allow space for preparation and fusion. At the exit end a smaller excavation will be sufficient if the pipe is to be connected to existing pipes. The exit excavation is where the winch is located and must therefore be large enough for this and for safe working around it. This exit excavation also needs to provide reaction to the winch so will require a plate or surface against which the winch can be anchored. Sufficient PE100 liner pipe must be pulled through to enable end connections to be made (see Figure 2).




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