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Trenchless pipe installation
By Chris Breen and Eric Thorn of Opus International Consultants, and the National Certificate of Water Reticulation qualification.
In recent years there have been large advances in trenchless technology for installing new pipelines and more can be expected in the next few years. In addition to tunnelling, there are four other methods of trenchless technology used for new pipelines in New Zealand.
These are:
The limits and advantages of each of the technologies are discussed below. This is intended as a general guideline only as rapid advances are being made and some equipment and some contractors may be able to obtain more distance or better accuracy.
Horizontal Boring
This is boring with normal drill rods in a horizontal (and hopefully straight) direction. A drill bed is set up at the correct depth and aligned at the required direction and grade and a small hole drilled (typically about 70mm diameter). This hole is later reamed out to the required diameter. This can be done either with a cutter that back cuts a short length of the line and then pushes the cut material to the far end of the hole for removal or by pulling a pig or similar through the hole to expand the hole to the required diameter. This method can be used with pipelines up to about 900mm diameter or larger provided the ground conditions are suitable, and holes will remain open safely
Advantages
The advantage with this method is that the drilling equipment is relatively cheap and it can drill rock if required. It can also be used at any depth. Over short distances relatively accurate line and grade bores can be achieved. Claims of accurate grades of 0.5% over 30 – 50 m are common in the industry. My personal experience is that this is generally not obtained and that to get 0.5% lengths have to be less than 30m.
Disadvantages
The disadvantages are that the drill head cannot be controlled after drilling starts. The head generally slowly falls below the grade that it started at. It can however go in any direction if a hard layer is encountered at a shallow angle to the bore direction. Because of the tendency of the drill to slowly fall off the design grade a maximum length of about 60m is recommended for firm consistent materials. In hard materials where the drilling head is likely to be deflected by varying strengths of materials and in soft materials where it is likely to “fall” more rapidly this distance may have to be reduced to as little as 10m.
A drilling pit is required to the depth of the line and in the location of the line. The pit has to be long enough to fit the machine in. This is generally not a problem but can be on some confined sites.
As the original hole is reamed out after drilling the final hole can at times finish up with it’s centre not in the same location as the original and some variation additional to that caused by inaccuracies in the original drilled hole can occur.
Pipe Materials
With this method polyethylene pipes would be the most common material. However, concrete, steel, PVC or Hobas GRP can all be used. With these materials the pipe jointing needs to be considered. Rubber ring joint pipe should not be used as the installation will push the joints in past the witness marks causing failure of the pipe collars. If the pipe is not the same outside diameter over its full-length consideration needs to be made of if and how the annulus between the drilled hole and the pipe is to be filled.
Directional Drilling
In Directional Drilling drilling the initial pilot hole can be steered (within limits). Directional drilling typically starts from the surface some distance away from the start of the permanent line and drills down and onto the required line and grade. The driller can control the direction of the drill head both vertically and horizontally. The drill head position is monitored from the surface where both it’s position, depth and inclination can be measured. If it is not in the correct position a correction can be made to the drilling direction. Normally a pilot hole is first drilled and is subsequently reamed or pigged out.
This system is becoming increasingly common especially for telecom ducting and gas mains. Many watermains and sewers are now being installed with this method. It is suitable for pipelines up to about 900mm diameter, and ranges up to 500 metres or more, depending on depth and ground conditions.
 Surface launched guided boring machine with integral drill-pipe carousel
At least one 0.1% accuracy machine is now in New Zealand. I have not seen the results of this machine but I would expect that it should be able to drill to grade of about 1% at depths of up to 3 to 4m but some horizontal weaving would be expected.
Advantages
Many contractors have directional drilling equipment. It is relatively cheap and quick to install. Most soil materials can be handled provided they are reasonably consistent. With suitable drilling muds, pipelines can even be installed in running sands below ground water level.
Disadvantages
It has the disadvantage that the drilling head has to be off alignment before action is taken to correct it. There are also limitations with the accuracy of the detection equipment that is used to locate the drilling head. Most surface directional boring detection equipment becomes unreliable or does not record at all below about 3.5m although some claim to be able to operate at 5 to 6m. The final drilled hole is generally not very straight. At times this is improved when the hole is reamed as this can straighten out some of the wandering. With increasing accuracy of machines these limitations are decreasing.
The major risk is associated with inconsistent soil conditions where for example, interfaces between soft and hard soils cause the line to deflect, or buried boulders, rocks, tree trunks and the like might bring the drill to a complete halt. The back-reaming cutter is also selected depending on the material, so a clay cutter is unsuitable for back-cutting soft sandstone. This means it is often not feasible to drill from one type of soil into another of significantly different characteristic.
The other major aspect to consider is the space needed to lay out the pipe string before it is pulled into final position. Small diam pipes are quite flexible and can be threaded around surface obstructions, but as diameters increase this rapidly becomes more difficult.
Pipe Materials
Welded polyethylene pipes or PVC pipes with solvent cement or threaded joints may be used.
Pipe Jacking
With this method the pipe is pushed through the ground from a jacking pit. The material is excavated from inside the pipe and the pipeline advanced by pushing again. The line and grade can be controlled by the amount of excavation around the driving head. As the line advances new pipes are added at the start and jacked along the line.
Advantages
This method has the advantage that it can be used for large pipelines where the other methods discussed above are not suitable. This method has the advantage that good control of direction and grade is possible with a good contractor.
Disadvantages
Care is required in the design to ensure that the pipes have sufficient strength for the jacking forces required for installation. Typically special manufacture jacking grade pipes are used.
Simple “Pipe Jacking” is now generally considered to cover situations where excavation at the face is manually undertaken. This limits operation to pipelines with a minimum of about 1500mm diameter, but can offer a major advantage in that there is no large tunnelling machine that must be extracted once the end of the jacked pipeline is reached.
The method is not suitable for lines in running sands. The method is generally very expensive, but for large diameter deep lines may be the only available option.
Pipe Materials
The pipe material must be strong enough to take the jacking forces required. Concrete is typically used, but earthenware, steel or fibreglass can also be used.
Micro Tunnelling
Micro-Tunnelling is similar to Pipe Jacking but uses some form of mechanical excavation machine at the face. The machine can take several forms depending on ground conditions and how the spoil is removed. Two common styles are the “slurry” machine, where the spoil is turned into a watery slurry for removal, and the “partial open face “ machine where spoil in a relatively unchanged state is removed by skip, conveyor or auger.
This system requires a pit to be excavated to the proposed pipeline depth. The pit has to be large enough to fit the boring machine and/or pipes and the hydraulic jacks. The size required varies from machine to machine. For the smaller machines this may be as little as 1.5m. For larger machines it may be 10m or more.
The drilling machine is located in front of the first permanent pipe. Normally the pipeline is steel or concrete but other materials may be used. A steel pipe can be used as a casing with the service pipe installed inside the steel duct later. The drilling machine is steered from a control panel at the surface using remote CCTV and laser guidance.
Depending of the type of machine numerous services such as power, hydraulics, water and lubricant may need to be fed to the tunnelling machine. These services must de disconnected and then reconnected each time a new pipe is added to the string. Significant equipment and space is also needed at the surface for lifting pipes into the pit, storage of pipes and so on. For slurry machines large screens and tanks are also required at the surface to settle the muck out.
The larger machines generally drill a full size hole. Some of the smaller machines drill only a small hole (approx. 70mm) which is later reamed out to the required size. At least three of these smaller machines are presently in NZ. These smaller machines have a maximum range between pits of about 70 to 100m. After this it is difficult to see the laser guidance dot on the target. Larger machines can often tunnel for a few hundred metres between pits. The governing factor is often the pipe resistance in the hole, although this can be eased by lubrication. As the pipeline gets longer the forces become greater and the pipes may crush. In this case drive lengths can be increased with the use of intermediate jacking points.
At present it appears that this technology is economic at depths greater than about 6m and when it is necessary to lay a line very close to or under existing buildings or services that cannot tolerate the disruption caused by open cut methods. It should nevertheless be recognised that substantial surface disruption cannot be totally eliminated, and in particular the disruption that does arise will remain in the same location for a considerable period.
Advantages
These machines give very accurate bores. They are however very expensive, although the price is coming down as more machines become available.
Slurry machines can cope with virtually any type of soil including running sands and soft rocks. Open face machines can only cope with soils that are reasonably self-supporting, but require far less surface spoil processing equipment.
Disadvantages
The technology is relatively expensive and there are limited contractors with the equipment in New Zealand.
Pipe Materials
Pipe jacking pipes are usually earthenware and concrete but steel and Hobas-fibreglass are also used.
Trenchless pipe installation content is provided in our stormwater management course, delivered by Chris Breen and Eric Thorn of Opus International Consultants, and the National Certificate of Water Reticulation qualification. Chris has provided an edited version for this article.
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