The West Highland Line (WHL) is the main railway line between Glasgow and Oban on the west coast of Scotland, the line runs alongside Loch Lomond up to Crianlarich then on to Oban, Fort William and Mallaig.
On the section of track between Tarbet and Crianlarich, north of Glasgow, where it runs adjacent to the loch, investigations showed that six under-track culverts were in need of replacement including works to the associated drainage and inlet/outlet headwalls.
The contractor for the project was Carillion Civil Engineering, having been appointed to complete the necessary works by Network Rail. All works were carried out in-house by Carillion without recourse to ‘specialist’ sub-contractors.
Investigation of the various culvert sites indicated that all the culverts requiring replacement under the scheme were in remote locations. This resulted in all plant and materials having to be transported along the track to site using a Road Rail Vehicles (RRVs) from dedicated access points.
This meant that the pipe replacement works had to be undertaken using a combination of Rule of the Route Possessions, RETB daytime possessions and a five-day Blockade. Track monitoring regimes were required to monitor the settlement. The headwalls and associated works were undertaken during ‘fenced greenzone’ and RETB daytime possessions.
The original 450-mm ID and 550-mm ID fireclay single and twin piped culverts required replacement due to partial collapses, blockages and displacements in the pipes. The pipe depths ranged from 2.5 to 9 m below track level, with overall pipe lengths varying between 10 and 32 m. Planning for the replacement works required that two of the 450-mm ID pipes were to be replaced as size-for-size, two pipes were to be upgraded from 550-mm ID to 600-mm ID and two were to be upgraded from 450- to 600-mm ID to cope with projected future capacity requirements.
Ground conditions on the project, which was generally through railway embankments, consisting of sandy clay material with a loose rock fill core. This information was obtained prior to planning the works using a series of trail pits at the inlet and outlet sites of the existing culverts.
The limited access and space restrictions at the individual construction sites meant that a technique that could be used in such circumstances while providing the required outcome was needed. Careful investigation of the options available led engineers to select pipe bursting as the preferred method.
Not only did pipe bursting meet the requirements caused by the limited space and awkward site access, it also minimized the likelihood of track settlement that may have been caused by selection of an augered installation method. Pipe ramming was ruled out due to difficulties in setting up and welding pipes at the site locations, where the existing inlet level was on a rock head. The use of the bursting technique was selected due to the ability of the chosen bursting rig, to drill through collapses, as well as requiring minimal rock breakout and thrust pit preparation.
Given the pipe sizes on the project and the upsizing requirements laid down in the construction plan, investigations led Carillion to opt for using a Hammerhead Hydroburst HB125 static pipe burster with a P70 hydraulic pack, 750-mm diameter expander and a 550-mm diameter expander, all of which was purchased from U Mole (a division of Vp plc). In addition, the need to drill through some of the collapses within the existing pipe to establish the pullback rods in position meant that a Railhead Trihawk V drill head was also purchased from U Mole to place at the head of the rod stem to provide the cutting bit.
Further equipment used on the various sites comprised both TracStar 500 and TracStar 900 PE butt fusion pipe welders and a 650-mm receiving cradle.
In the event a Hydroburst HB125 was used, where necessary, to drill through collapsed pipes using the Trihawk V bit. Once the pullback rods were positioned, and the cutting head removed if required, the new 500-mm OD or 630-mm OD. HDPE pipes were pulled into place behind the bursting head and suitable expander in a single pass to leave the new culvert pipe in place.
The HDPE pipes used for the installations were 630-mm OD SDR41 and 500-mm OD SDR17.6 HDPE pipes that were supplied as 6-m long sticks. The sticks were then welded together to form the required installation length, using the appropriate TracStar unit, in railway sidings prior to transporting to work site using the Road Rail Excavator and Trailers coupled with extended tow bars.
Construction works started on Jan. 4, 2010, and was completed on May 11. Including where necessary the drilling works through collapsed pipes, the time to install the pull-in rods for the bursting machine varied between one to five hours. Similarly, the final bursting runs and simultaneous new pipe pull-in operations, which took place on Feb. 27, March6, 10, 13, 22 and April 24, took between one and a half to five hours.
While most of the replacement installations completed using the pipe bursting system were on average 20- to 30-m long, in one instance on a crossing of more than 20 m, the drilling facility was used to pass through a pipe collapse totaling some 17 m in length.
The HammerHead HB125 static pipe-bursting system offers contractors one machine that is versatile enough to replace a wide range of water, sewer and gas pipelines ranging from 150 to 500 mm in diameter. Designed with production in mind, the HB125 offers 113 t of pulling force.
A simple one-person operation, with on-board rod basket and 19 second rod payout, the HB125 has five hydraulic stabilizers for leveling and rear jack adjust to place the machine on grade and aligned with the existing utility. Adjustments can be made during the burst as conditions change without stopping the bursting process.
HammerHead uses a heat-treated alloy rod. The API-style joint handles thrust loads encountered when pushing around sweeping bends and encrusted or collapsed pipes compared to loose joint rod designs, which can buckle in such conditions.
On the WHL project the decision to use rotational torque assist was key to the success of the scheme. The HB125 is the only pipe-bursting machine that combines thrust and rotational torque during payout to allow operators to push and rotate a drill head or lead rod through collapsed or encrusted pipes.
Other features on the HB125 include a rod lock device which places constant tension on the pipe string eliminating rod rebound thereby increasing production. An auto rod spinner also quickly makes and breaks joints with and helps to increase overall production.
While the majority of the works ran relatively smoothly, given the limited access and other restrictions, however, on one occasion, due to unexpected bends and a severe blockage in one of the existing pipes, one of the culverts had to be excavated during a railway blockade. This involved the removal and re-instatement of approximately 400 t of embankment over a five-day, pre-planned closure of the WHL. During the excavation, the pipe alignment and level was changed to minimize the requirement for excavated material removal and a new brickwork outfall cascade was constructed to train the water back to the original outfall position.
In a further change to the existing pipe layout, a twin 450-mm ID. pipe was combined into a single larger 600-mm ID pipe to ease pipe bursting operation. This also provided a useful cost-savings to the project.
In addition to the site restrictions already mentioned, the stretch of railway line covered by the works also passed close to nearby SSSI Sites and lay within the Loch Lomond & Trossachs National Park. This meant that over-pumping of flows during the works had to be kept to a minimum to avoid unnecessary silt generation. As a further protective measure, oil protection booms were installed downstream of works to mitigate for any potential spillages that might occur. On completion of the works all site had also to be fully re-instated. In some instances, planning consent was also required for some elements of the works on the £800,000 project. Ultimately all six replacement installations were completed successfully without any incidents or accidents along the way. Disruption to train traffic was minimized by using trenchless technology on almost all of the sites which also protect the local picturesque environment.
Keith Robertson, sub agent for Carillion Civil Engineering Scotland, said of the project: “This has been an interesting project, which has seen Carillion Civil Engineering Scotland adding pipe bursting to its capabilities. The project has been challenging due to access restrictions and railway working, but has been a success. We look forward to undertaking further works on behalf of our current and other potential clients.”
Commenting on the project from the main Equipment manufacturer’s perspective Frank Gowdy, sales director for U Mole, said: “The built-in facility on the HB125, which enables operators to utilize rotational torque assist, was particularly useful on this project in addition to the pull-in pipe burst capacity of the unit. The lengths of collapsed pipe that the machine had to negotiate were up to 17 m in a single installation. We are very pleased that our equipment has been able to assist Carillion’s crews to complete this challenging project under difficult access conditions and look forward to future cooperation.”
U Mole, a division of Vp plc, is a direct factory distributor for Earth Tool equipment, offering the company’s full range of products for trenchless installation and pipeline replacement. This range includes impact moles (earth piercing tools), pipe ramming hammers, twin capstan winches, static rod pipe bursting systems and the portable cable-based, hydraulic pipe burster range.
U Mole also supplies a full and extensive range of trenchless and limited dig equipment including: Suction/vacuum excavation systems and surface coring equipment for the ‘Keyhole’ excavation of utilities and services with minimum disruption to the local area and minimum damage to the buried service. Other trenchless related equipment available includes the market leading Robbins SBU equipment, Pipe to Site coiled pipe trailer range, pipe fusion systems, Reed Tools and a complete range of accessories such as Cobra flexible rodding, towing heads, cable socks, pipe pigs, swivels, breakaway connectors etc.
This article was submitted by U Mole, a part of Vp Plc, based in Cambs, United Kingdom.