Upgrading Yorkshire Water’s Watersheddles Reservoir spillway gave Mott MacDonald Bentley an opportunity to innovate both in project design and delivery.
Project: Watersheddles Reservoir Spillway Improvements
Client: Yorkshire Water Services
Project value: £1.8m
Main contractor: Mott MacDonald Bentley
Principal design consultant: Mott MacDonald Bentley
Watersheddles Reservoir holds 866 mega-litres of raw water and has served the local catchment area well since it was first constructed in 1877.
However, an inspection in November 2011 recommended an upgrade to the overflow system to ensure it was capable of safely conveying the reservoir’s probable maximum flood (PMF) of 29.8 cu m/s, in compliance with the Reservoirs Act 1975.
This led Yorkshire Water Services to enlist Mott MacDonald Bentley (MMB) as design-and-build contractor on the £1.8m scheme to improve the existing spillway.
The preferred solution was to line the existing masonry spillway with reinforced concrete – a more sustainable and practical solution than replacing the existing spillway.
A traditional approach would involve constructing the reinforced concrete channel lining in-situ. However, the existing spillway was very steep, with a maximum gradient of 1:2.5, creating significant risks in terms of working safely on the steep slope and limitations in terms of accessing the channel to facilitate the works.
“The use of precast components generated significant reductions in labour, time and material costs, saving an estimated £104,000 compared with a traditional in-situ concrete solution”
So instead of an in-situ solution, MMB developed a precast concrete (PCC) solution involving PCC units manufactured in factory conditions, with onsite work limited to placing and joining the components in position – thereby eliminating all in-situ concrete works.
Altogether the project comprised the placement of 30 PCC ‘u-sections’, each 1.52 m long, 3.3 m wide (externally) and 250 mm thick (walls and base).
These included 25 straight units to cover the 39 m of the spillway with a gradient of 1:2.5, and five splayed units to cover the upstream 7 m of spillway where the gradient becomes shallower.
The use of precast components generated significant reductions in labour, time and material costs, saving an estimated £104,000 compared with a traditional in-situ concrete solution.
In addition, zero health and safety incidents were recorded on site during the installation of the PCC u-sections.
Innovating with non-industry standard joints
The u-sections are joined using spigot and socket joints, with a specially selected butyl rubber sealant applied between units.
This sealant is commonly used in many tunnelling, manhole and culvert projects and has been shown to have a good bond to concrete, while its elasticity allows for movement of the joints due to expansion and contraction of the PCC units.
It is applied flush with the surface of the spillway, avoiding the use of a surface protective sealant such as polysulphide, which requires more frequent maintenance.
However, Yorkshire Water’s Reservoir Safety Asset Standard requires spillways to be designed as water-containing/retaining structures, which implies the use of water-retaining joints.
Deviating from the client’s standard required a vigorous design approval process involving Yorkshire Water and an external technical adviser to ensure the proposed joint system was robust and fit for purpose.
MMB worked with specialist subcontractors to test a mock joint piece under the maximum likely hydrodynamic pressures that could be experienced on the Watersheddles spillway under PMF conditions.
This proved the joint system’s performance as a watertight rather than water-retaining joint system and MMB gained the required approvals to proceed with the PCC solution.
Safer PCC-enabled installation
Part of the u-section design concept was to eliminate the need to access the steep section of spillway directly, which was impossible without constructing expensive temporary works.
Instead, a rail and winch system was designed to allow u-sections to slide into place within the spillway channel.
The rail system had to be strong enough to support the weight of each 5.5-tonne u-section and sufficiently durable to remain underneath the PCC units once they had been installed and grouted into place (for watertightness purposes).
The rail system composed steel channel sections 6 m long, bolted to the existing spillway prior to installing the u-sections.
Lifting points were cast into the base and walls of the PCC units so they could be winched into position. Steel running plates were bolted to the base of the units after manufacture to allow them to slide easily along the rails.
The first u-section was installed on 20 May and by the end of June all of the units had been installed. The overall construction work finished last month, ahead of the Reservoir Safety Act 1975 compliance date of 1 November 2015.
Although MMB has produced a number of PCC solutions for Yorkshire Water projects throughout its AMP5 programme, Watersheddles provided the company with an opportunity to innovate with non-standard approaches.
PCC solutions are gaining ground in the water industry, but challenging industry standards on joints and installation led to a reduced delivery time, lower project costs and an improved health and safety record for the site.
We are now working with Yorkshire Water to see if this solution can be adapted and applied to other spillway improvement projects.
Kate Speirs is project leader at Mott MacDonald Bentley