Flood Alleviation Works at Hexham

  • Project Flood Alleviation Works at Hexham
  • Main Contractor Volker Stevin at Hexham
  • Key Benefit Working alongside the Environment Agency
Image taken inside the excavation at Hexham

Introduction

In August 2005, Volker Stevin were working for the Environment Agency on a Flood Alleviation Scheme at Hexham, Northumberland. As part of the installation of a new culvert system, a thrust pit was required to pass underneath an existing heavy goods railway line.

A 3.5m deep cofferdam, 23.0m by 7.5m was required approximately 3.0m away from the railway line. A clear span of 15.5m was required to allow easy installation of the 12.5m long, 900mm diameter pipe

sections, but movement was critical, with the maximum allowable deflection of 10mm due to the close proximity of the rail.

The ground conditions consisted of granular fill, loose sand and dense gravel overlying a band of silt. Groundwater was encountered at 4.96m below commencing level. Additional loading came from surcharges for plant and the railway, and haystacks were placed along the side of the excavation to reduce noise pollution to the nearby neighbourhood.


The Solution

A propped cantilever style cofferdam was designed by MGF utilising 6.5m long 4A Sheet Piles toed 3.0m into the dense gravel with a frame at 0.5m below ground level. In order to withstand the high load whilst maintaining a large clear span with minimal deflection, MGF’s new Super Tank Brace with 120Te Hydraulic Powerpacks was used.

A high capacity 120Te Mechanical Strut was positioned 7.5m from one end in order to keep within allowable deflection limits, while sustaining enough working room to lower in the 12.5m long pipe sections with ease. The sheet piles were pre-driven to full depth prior to any excavation taking place.


The Verdict

The excavation was quick and easy to install as the sequence was un-complicated with no need for re-location of struts.

The high bending capacity of Super Tank Brace meant that the clear opening required was

easily achieved and only a single frame was required to withstand the loadings.

Movement of the nearby railway was monitored closely throughout the excavation and defections were kept within allowable limits.