NT - Longshaw Estate - Drinking Water Supply Work

Project: Longshaw Drinking Water Reservoir – Emergency Repairs and System Upgrade

The Problem

The 24,000-litre, stone-built, in-ground drinking water reservoir at Longshaw was leaking severely, holding only around half of its capacity. Escaping water was flooding the surrounding ground, leading to ground instability and collapse.

The reservoir supplies drinking water to the residents of Longshaw as well as several thousand daily visitors. Maintaining uninterrupted water supply was critical throughout the works.

Previous short-term solutions relied on daily water tanker deliveries, which were expensive, logistically difficult, and limited to a maximum of 24–48 hours.

The Challenge

We were originally tasked with repointing the stone reservoir. However, the requirement to guarantee continuous water supply — combined with restricted access, historic construction methods, and legacy defects from previous works — turned what appeared to be a simple repair into a complex civil engineering project.

The reservoir is located within woodland adjacent to the moors, accessible only by a narrow footpath crossing several small stone bridges. Since the original tank installation in the 1980s, surrounding trees and rhododendrons had expanded significantly, making access extremely difficult.

Our Solution – Temporary Bypass and Supply Continuity

To eliminate reliance on water tankers, we designed and installed a temporary bypass system using the existing pipe network.

Water was diverted through the woodland and connected into the system via a fire hydrant. From there, we installed a new 10,000-litre storage tank, plumbed directly into the filter room and connected to the extensive filtration system, complete with overflow provision.

This system maintained full water supply and pressure throughout the project, allowing the reservoir to be fully drained and repaired without service disruption.

Scope of Works (6-Week Programme)

As the project progressed, significant hidden defects were uncovered, expanding the scope to include:

• Repointing of the large stone reservoir blocks using specialist lime and Roman cement mortar

• Repair and refurbishment of the timber gazebo, including roof repairs where rot was present

• Resetting and repointing of the reservoir coping stones

• Installation of a new overflow and controlled discharge into the adjacent stream

• Construction of a new, fully accessible valve chamber with upgraded pipework and valve extensions

• Installation of a new 8” land drain to replace a blocked perforated system

• Repair and sealing of the incoming spring silt trap

• Clearance of self-seeded trees and invasive rhododendrons to restore safe access

• New pipework from the silt trap to the tank, including a secondary valve chamber

• Installation of a new reservoir exit penetration with 90mm HDPE electrofusion-welded pipework

• Rationalisation and logical reconfiguration of pipework and valves

Key Discoveries and Remedial Works

Previous works carried out in 2007 by a specialist water contractor were found to be a major contributor to the failure:

• An unsuitable plastic manhole system had been installed, cut into, and structurally compromised

• The valve chamber was undersized, too deep, and unsafe to access

• The tank stop valve was unusable, positioned directly beneath the overflow pipe

• The original puddle clay bund had been removed and replaced with weak concrete, allowing water to escape freely

Excavation revealed collapsed chambers, silt ingress, corroded cast iron pipework, and a chaotic arrangement of buried connections.

Engineering Solutions Implemented

• Removal of failed concrete and reinstatement of a puddle clay bund, enhanced with bentonite clay

• Installation of a new 2.4m deep concrete valve chamber, allowing safe access and long-term maintenance

• Relocation of the overflow away from the valve chamber

• Installation of valve extenders and brackets for ground-level operation

• Creation of a new tank penetration with HDPE pipework sealed using rubber seals and non-shrink underwater grout

• New land drainage around the chamber connected to a nearby stream

• Replacement of historic perforated drains with new 225mm twin-wall drainage

• Repair of the incoming silt trap, including replacement of failed connections with polyfusion-welded joints

• Careful pipe bedding and consolidated backfill to accommodate ground movement

The works required progressive escalation of plant, from a 1.5-ton excavator to 3-ton and 6-ton machines, along with multiple dumpers, a 3” pump, and trench sheeting for deep excavations.

Completion and Outcome

The reservoir now holds its full 24,000-litre capacity, with water correctly overflowing to the adjacent streams rather than saturating surrounding ground. All systems are sealed, accessible, and functioning as intended.

To future-proof the installation, we produced a comprehensive work instruction and valve operation manual for National Trust maintenance staff, detailing system layout, valve locations, and operational procedures.

What began as a simple repointing task evolved into a full civil engineering project — successfully delivered using our in-house machinery, expertise, and problem-solving capability.

Another complex project completed safely, efficiently, and to a long-term standard.