TRENCHLESS INSTALLATIONS SUPPORT MAJOR ROAD IMPROVEMENT SCHEME
The A556 Knutsford to Bowden Improvement Scheme, being undertaken by Costain for its client Highways England and recently awarded the accolade of ‘Most Considerate Project’ from the Considerate Constructors Scheme, is a project valued at an estimated cost of between £165 million to £221 million. The road improvements are designed to cover a 4.5 mile (7.5 km) section which will transform the A556 trunk road between Knutsford and Bowdon into a modern dual carriageway. Costain will also be making the existing A556 into a single carriageway road with facilities for pedestrians, cyclists and horse riders.
The new road will address significant current congestion problems and is expected to reduce accident numbers within the area. Air quality locally will also be improved.
As part of the construction process, Chiltern Thrust Bore Ltd (CTB Ltd) has and continues to undertake both Guided Auger Boring (GAB) and Horizontal Directional Drilling (HDD) trenchless installations to install various crossings beneath the new road section or under existing live carriageways. Over the past year the company has been called in on a number of occasions to construct the crossings beneath the new and old roads as and when they become necessary and the road section under which they have to pass has been completed or remains part of the existing infrastructure. All works have been undertaken using Costain’s main site compound in Altringham as the works base.
To date CTB Ltd has utilised two European-manufactured GAB machines to install five crossings of varying diameter. Four of the crossings, for surface water drainage serving the new road utilised 300 mm, 450 mm and 600 mm diameter Naylor Vitrified Clay (VC) Denlok NC & NS jacking pipes as the product pipes. A further GAB crossing installed a 610 mm diameter steel sleeve for ducting purposes to accommodate other buried utilities. All of these crossings ranged in length from 40 to 80 metres.
The fourth crossing was completed using a Vermeer D36x50 HDD rig to install 140 m of 2-way 180 mm o.d. ducting in a single crossing.
Chiltern Thrust Bore’s selection of Naylor Denlok pipes for the installations using the GAB systems was partly based on past experience which has shown the product to be a proven tried and tested pipe with the durability and the diameter range required on the A556 project. The pipe also meets all the relevant British Standards relating to the project.
David Croot, Project Manager with CTB Ltd said of the pipe selection: “We have used the Naylor pipes all over the UK for many years and our decision to use the product on the surface water drainage installations was made for a variety of reasons, not least of which is good previous experience working with Naylor. The jacking pipes are available in the diameters we need when we need them and they meet all other project requirements in terms of the structural strength for pipes running beneath the new roadway and the flow characteristics required for the drainage system being installed. They marry well with the GAB systems we use and we have yet to experience any difficulties when using the Denlok pipe range which helps us meet our commitments to the main client Costain.”
The Denlok NS jacking pipe consists of a vitrified clay jacking pipe with parallel ends and a rebated accurate joint interface. All NS range of pipes consists of a coupling manufactured from 316 stainless steel, a LoknSlip seal and an MDF load transfer ring. Each pipe is supplied with all seals and a factory fitted stainless steel sleeve with the thrust ring supplied separate in a cover to protect the material from the weather elements. The NS range is available in diameters from DN400 to DN1200.
Denlok pipes are also available in the NC range from DN150 to DN300 which consists of a vitrified clay pipe with parallel end faces and a square accurate machined seal interface. All pipes have a factory fitted 316 stainless steel coupling which incorporates a moulded EPDM elastomer seal, each seal has an integral EPDM thrust ring, which transmits the jacking forces during pipe installation, all elastomer conforms to the requirements of EN681 1. By precision machining of the seal interface the coupling will with stand a 5 bar internal and external pressure allowing installation below the water table and protection from lubrication systems during pipe installation.
There is also the Revit range in diameters from DN225 to DN500, which is a special designed pipe for use by the ‘3’ Pass Guided Auger Boring Installation Technique, and the DenChem Range in diameters from DN150 to DN300 which is designed for pipelines needing to operate at high temperatures up to 130°C. The DenChem range comprises special ceramic pipes capable of withstanding higher temperatures and full resistance to alkaline and acid attack with applications in the chemical, pharmaceutical, food and dairy processing industries as well as brewing. High temperature chemical effluents resulting from process or cleaning operations can be handled with confidence.
However, ground conditions across the site varied greatly and included variations from sands through soft clays to very hard/dense clays containing cobbles and boulders. The ground investigation comprised various techniques including Trial Holes/Pits, Boreholes and Window Samples which included SPT Tests and Undrained Shear Strengths which provided CTB Ltd with N-values and Cu KPa respectively. These investigations were undertaken at each individual crossing site prior to the installation technique being decided upon and the information they provided ensured that the correct, most efficient and cost effective solution was applied, but even then the various installations proved challenging.
The varying ground conditions posed a challenge to CTB Ltd as every crossing needed careful consideration in its own right. Although the area of works in which CTB Ltd operated was relatively small, various types of tooling and ‘tweaks’ in the methodology needed to be considered and applied on an individual crossing basis. This was particularly pertinent at the most recent GAB installation location, named Culvert 9.
The expected ground conditions around Culvert 9 comprised soft clays according the ground investigation results, so soft in fact that the contractor originally had concerns regarding equipment support and whether this would affect performance, programme and achievable distances. However, once the bore commenced, it became apparent, at around the 20 metre mark during the advance, that the expected ground conditions did not actually exist. Instead very hard/dense clays containing cobbles and boulders were encountered as the bore head advanced during the first of the two crossings that were planned to be carried out from the one start shaft. This installation was to be a 610 mm diameter steel sleeve with a 590 mm i.d. with a vitrified clay drainage pipeline to be installed parallel to it.
By altering the installation methodology slightly, CTB Ltd managed to install the 610 mm diameter steel pipe without the requirement to retract the boring equipment and change the tooling, or even to make alterations to the actual rig itself.
Although this particular adaptation was successful, CTB Ltd had to be pragmatic in its approach to the second, larger diameter (762 mm o.d.) crossing and needed to react to the prevailing site conditions in a prudent manner. To this end bespoke cutters and lead augers needed to be made-up to suit the expected conditions, as well as developing a bespoke water injection system that had not previously been used on this type of machine. This manufacturing and system development was carried out whilst the 610mm diameter steel sleeves were being installed to reduce potential future delays on the drainage pipe installation.
Once boring commenced on the second bore, the ground encountered actually proved to be more aggressive than on the first bore but with the new equipment and adaptations the pipe was installed successfully.David Croot commented: “The unexpected variations in ground conditions did give us a bit of challenge but once we understood these ground variations and what was needed to overcome them our many years of experience enabled us to modify our methodology successfully to complete all the drives we have undertaken to date.”
For Naylor Drainage Simon Marsh, Denlok Product Manager, said: “We are always happy, whenever possible, to be able to support our clientele with the pipe products they need when they need them. Our ongoing relationship with Chiltern Thrust Bore has over the years enabled many successful project completions to be achieved using our Denlok jacking pipe systems.”