TECHNICAL | BTS MEETING/NORTH BRISTOL RELIEF SEWER
NORTH BRISTOL RELIEF SEWER
Tunnelling challenges on the North Bristol Relief Sewer (NBRS) Project were presented and discussed at the BTS Meeting in May by Dominic Barlow, Senior Project Manager, and Adam Evenden, Project Engineer, of J Murphy and Sons Ltd. This report on the meeting was prepared by Dominic Barlow and Adam Evenden
INTRODUCTION On the 25 May 2023 at the Institution of Civil Engineers, in London, Dominic Barlow, Senior Project Manager, and Adam Evenden, Project Engineer, with J Murphy and Sons Ltd (Murphy) gave a presentation on the North Bristol Relief Sewer (NBRS) Project constructed between 2019- 23 for client Wessex Water (YTL Ltd), and with Aecom as Lead Designer.
PROJECT OVERVIEW AND SCOPE Wessex Water had identified the need for a new attenuation sewer scheme located in the North Bristol area to alleviate flooding (see Figure 1). The scheme was required to relieve flows and alleviate flooding the existing Frome Valley Sewer and reduce future discharges from CSOs in the area. The new capacity in the network would also contribute to the significant local development of circa 85,000 new properties due to be built over the coming decades. The scheme was designed to future-proof the sewerage system in the area for the next 100 years. The outline scope of the project was to provide a
connection between the existing Frome Valley Sewer in the north and the Bristol Trunk Sewer in the south, as well as six other smaller sewer connections along
the route. The new sewer was required to have an attenuation capacity of 38,000m3
. At the tender stage, Murphy investigated 71 different
solutions and permeations on how this project could be built. The solution developed was for the following: 5.5km-long DN2.85m segmentally lined tunnel; 1.0km- long DN1.8m open-cut pipe; 450m of pipe-jacked tunnel; 115m of auger bored pipe; 2No 30m-deep 6m i.d. shafts; and, 14No access chambers.
PROJECT DESIGN DEVELOPMENT In the Instructions to Tenderers, Wessex Water provided indicative horizontal and vertical alignments of the proposed sewer. At that stage, Murphy sought to optimise and simplify the design by making some significant alterations. The horizontal alignment that Murphy proposed
removed kinks and smoothed curves, maximised straight sections of the tunnel, and reduced the disruption and risk associated with long lengths of pipe jacking under residential properties. Murphy proposed four significant changes to the original horizontal alignment: new location for the TBM drive site to move it farther away from residential properties; removal of two shafts (MH08 and MH11); and, to move the position
Above left, figure 1: Project Locality and Overview Above right, figure 2: Horizontal Alignment Development – Original route in green and Murphy proposed in red 18 | November 2023
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