Gone are the days when pressure and gravity sewers had to be laid by open cut methods. Trenchless Technology has given utility owners alternative methods of constructing “on-grade” pipelines, the most common being microtunnelling. However more recently as technology and operator skills have been refined, HDD is becoming a real alternative for these types of pipeline projects. What limits the grade of HDD projects are the ground conditions; in ideal soil conditions it is possible to construct bores down to 0.5 per cent grade, but in rock due to the distance of the sonde housing behind the mud motor (approx 4.5 m on mid sized machines) the most achievable grade would be less than 2.5 per cent. The distinct advantages that makes HDD so attractive as an alternative to microtunnelling is its ability to be able to construct grade sewers on curved bore lines, achieve much longer bores in varying diameters and its adaptability through varying ground types.

Construction of a 1.2 km on-grade rising main

In 2004 the UEA Group’s Trenchless Division was contacted by Maunsell Australia to give an opinion on the construction of a 1.2 km sewer rising main in an environmentally sensitive area within the Parliamentary triangle in Canberra, ACT. The project was designed to accommodate increased sewer flows for a new ultra modern residential development sited on the foreshores of Lake Burley Griffin adjacent to the old Kingston Power Station and the city’s railway station.

UEA agreed that the project could be achieved if the grade was not too flat and there was minimal rock encountered, if significant volumes of rock were encountered - preliminary Geo-tech reports indicated some outcrops- the design could be significantly influenced. As the bore was grade critical and the start and finish points were at a similar elevation, it was necessary to design the project with a scour at the lowest point; UEA devised a project methodology for Maunsell to work to as a starting point for the design.

Like most trenchless construction projects using HDD, polyethylene (PE) pipe is the most commonly used product due to its strength for pulling in over long distances, flexibility when being dragged around bends and into bore pits and its long term durability.

This project also required the use of PE as HDD seemed the only option due to its proximity to the complexity of the bore, proximity to an open stormwater drain, numerous road crossings and heritage trees running along the route. Using PE posed another design and specification issue for Maunsell as the Australian Capital Territory’s water and power authority (ActewAGL) had not constructed similar projects using a PE pressure system in the past. Maunsell had the task of investigating an acceptable outcome that would meet ActewAGL standards before the project could be released for tender.

Initially UEA Trenchless were second choice on the project but were later awarded the contract when its bid complied with the stringent tolerances that Maunsell designers had called for. After a six month postponement, the project started in July 2005 with a contract period of twelve weeks. The “Kingston Foreshore Development Sewer Pressure Rising Main” consisted of a 355 mm PE main which started at a 1.5 m deep valve chamber on Wentworth Avenue and extended to an existing carrier main on “Windsor Walk”, Brisbane Avenue – a main thoroughfare which leads up to Parliament House.

The project involved the HDD construction of 1,180 m of 355 mm PE main “on-grade”; connection to a valve chamber on Wentworth Avenue; scour valve chamber and associated overflow pipe to an adjacent sewer and manhole construction (approx. 450 m from the start of the project) and final connection to a gravity carrier main via a manhole and fabricated stainless steel “oblique connector”.

UEA contracted a local civil company, Guideline, to construct the scour valve and build all the associated manholes.

The main risk with the HDD component of the project was the grade of the main. The bore path was designed to accommodate existing services and keep to a depth that would avoid known rock outcrops. As the project was only 100 m from Lake Burley Griffin, ground water was known to be a potential issue at depths below 4 - 5 m. To achieve these parameters the rising main needed to be constructed at -1.5 per cent grades for the first 450 m to the scour valve and then +0.9 per cent for the remaining 750 m. To compound the difficulty of the project the bore was designed on a curve of 30 degrees over 150 m during the first 450 m to the scour valve, then a further curve of approx 30 degrees after the scour valve which also added a challenge as the grade at this point was down to 0.9 – 1.1 per cent. To meet these specifications the client limited the project tolerances to +/- 150 mm in the horizontal and +/- 30 mm in the vertical.

Prior to starting the bores and to achieve the required tolerance and line, the route was surveyed at 5 m intervals to provide the exact depth to centre of bore for the drill crew to track the bore accurately. The survey also plotted the point at which the main grade went from – to +, this was to be the location for the intended scour valve.

UEA use DigiTrak bore location systems on all its HDD units, for this project we chose the Eclipse model. Like other locators it allows us to monitor bore depth, direction, inclination, sonde orientation and temperature as well as battery life. Additional features with the Eclipse locator is the ability to “look ahead” – which allows the operator to predetermine depth at a forward location, “Target Steering” – which aids in steering to a known point and more importantly for this bore is the accuracy of grade. The Eclipse sonde and receiver combination displays grade in 0.1 per cent increments.

The reason UEA originally purchased this location system was for its enhanced feature, the Eclipse can be set up for “wire-line” locating. This is an advanced locating facility which is used where normal “walk-over” location cannot be used due to excessive bore depth or other physical constraints such as salt water, service congestion or reinforced concrete – which will all distort signals at surface levels. UEA has used the Eclipse’s “wire-line” facility on a number of very difficult bores with more than satisfactory results, its ease of operation and dual use make it very attractive for “mid-sized” drill rig owners to own.

Bore construction

To undertake this project UEA used its 25 tonne Vermeer D50 x 100 HDD. It has a relatively small footprint for its power and its ease of operation made it ideal for the project where the drill crew needs to be focused on the bore accuracy rather than compounded site difficulties.

The bore could not be completed in a single shot due to design restraints, the accuracy and machine parameters, and so UEA developed a work method to break the project into four smaller achievable bores ranging in length from 220 to 400 m.

The site at which the bores could be positioned was restricted by the location of trees, roads and pedestrian access. All sites were limited in access but still needed to accommodate the drill rig, recycling system and a crane truck for rod pack changing. In most cases the open channel, although being a possible environmental risk, was used to accommodate welded pipe prior to the pull-back.

The bore pits at these sites varied from 3 m deep to 7 m at the scour valve, this meant that all holes required box shoring and the deeper ones required constant dewatering. The excavation of the holes revealed the ground conditions to be negotiated throughout the bore, at chainage 0.0. At the far end of the project the ground was hard dry clay. However further along the bore and as the entry and exit holes became deeper the effects of the nearby lake meant that the ground was water charged and therefore quite soft.

After a few days of site establishment the boring started, the first being just over 300 m long at -1.2 per cent grade with a gradual curve of 30 degrees in the horizontal. The target was reached and the drill head depth surveyed in the exit pit as part of the “Inspection Test Procedure” prior to any further up-sizing taking place.

As the ground at chainage 300.00 was so soft due to the water table to overcome a possible drop in invert of the pipe the drill crew, in conjunction with UEA Engineering’s team, developed a reamer that would support itself and not cut into the invert as the reaming process took place. This became typical for most of the bore where minimal pre-reaming was carried out and the product pipe was installed with few passes or conditioning of the bore hole.

The second bore was to go through the deepest point of the project which included the scour valve. From a timing perspective this section was the most critical as the project only had a twelve week construction period and there was a chance that the contractor would need most of that time to build and test the scour valve. The bore had to bottom out at exactly the right place and be on line as the scour valve area was constrained by the head wall of the open storm water channel, a road and a tree overhanging the site.

A 30 tonne excavator was needed to reach the depth required to construct the valve chamber and lift in the specially built box shoring. Once past the scour section the bore then had to negotiate a similar curve to the first bore but on a slightly flatter grade. The UEA drill crew achieved this bore in less than half the time of the first bore due to refined work methods and gained familiarity from the ground conditions.

Once the scour section of the bore was complete the remaining work was installed with few issues. Rock was struck where the geo-tech report had indicated but a small revision in the bore grade overcame what could have been an issue if it had been more extensive. All the boring was completed within nine weeks.

Commissioning the main

As each section of the main was installed it was pressure tested and CCTV inspected. This ensured that all sections of the main were not ponding and that the PE welds were structurally sound as time was critical. If the pipe did fail it would need to be extracted before the pipe had time to become a permanent fixture in the surrounding ground. Once each section was prior to the electrofusion joints being done, then once all the joints were complete and the scour valve commissioned the main went through final testing.

At this point we realised we had an issue with the transitional connections between the PE main and the scour assembly. As the main went under pressure the PE contracted, causing the joint to fail. A redesign and prefabrication of the connection to the valve chamber overcame the problem and the main was passed.

Initially the rising main was to be connected straight into a larger carrier main at Windsor Walk, but due to a bank of Telstra conduits being constructed on top of the main a further redesign was necessary before the final commissioning could take place. This involved the repositioning of a manhole and the fabrication of a stainless steel “oblique connector”. Again UEA’s engineering arm provided the skills to fabricate the stainless pipe connector. The final installation and cutting into the carrier main was carried out by ActewAGL staff.

The success of this project was due to a “can do” approach by all parties, the design was thought out, the job was well planned and the whole team worked well together to achieve a successful result from what was initially a very risky and technically difficult project: UEA would like to take the opportunity to thank all those involved with the project; UEA staff Mark Bouffler (project manager ACT), Nathan Davis and Peter Maher (site management and operations), the client LDA, Maunsell and ActewAGL.