CASE STUDIES
Sewer Diversion for Smart Motorway Construction
Established 1984 and now part of the Cappagh Group of Companies, Allen Watson Limited is one of the country’s leading specialist No-Dig trenchless contractor’s providing solution to a wide range of engineering challenges with a strong client base and excellent reputation.
The company was recently engaged on a project in southern England for the M27 J4 to J11 Smart Motorway Project for a Sewer Diversion at Gantry GY82 for which Southern Water was the client. The work involved a crossing beneath the M27 motorway at Eastleigh near Southampton, UK.
The Consulting Engineer for the works was Robert Walpole and Partners with Cappagh Browne Utilities Limited being appointed as the main contractor.
The project was required because an existing sewer passing under the M27 had become filled with concrete during a vertical bore piling operation. This concrete prevented any insitu repair options being viable. It was therefore decided that a trenchless bored project was required to install a new replacement sewer parallel to original pipeline under the motorway which could then be connected back into existing manholes on either side of the motorway. Allen Watson was appointed to undertake this installation.
DIFFICULTGROUND
Ground investigation boreholes were drilled at the drive and reception shaft locations which showed potential for Alluvial Deposits and/or Wittering formation (which is usually described as a Greyish brown laminated clay), wavy- to
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lenticular-bedded sand interbedded with clay in equal proportions and fine- to medium-grained sparsely glauconitic sand over a London Clay Formation. Water strikes indicate groundwater might also be present in the bore face.
Given the ground conditions and the length of drive involved, at 120 m, that a guided auger bore technique was most appropriate to achieve an accurate line and gradient for tying the new pipeline back in to existing levels. This was also coupled with the limited working area available for the new pipe installation. To complicate matters the alignment of the new replacement sewer had to pass through a restricted opening between the newly installed piles and existing piles while running parallel to and avoiding the existing damaged pipeline. Therefore, precise alignment was essential. This was one of the main reasons why Allen Watson opted to use its specialist Guided Auger system.
System used was a specially manufactured Guided Auger Bore built to Allen Watson Limited’s specification, allowing the boundaries of this technology to be extended within the prevailing difficult ground conditions. For the M27 crossing the 120mlong pilot bore was driven using Allen Watson’s GAB155v. Once completed and with the heavy wet swelling clay conditions experienced along the route, this machine was demobilised and replaced with Allen Watson’s American Auger 36/600 to directly install the sacrificial 610mmdiameter steel sleeve casing.
The works were initially designed as a one bore installation, installing 610mmo.d. steel casing
| July 2021 |
www.draintraderltd.com
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