CASE STUDY
Lingfield Gardens: Penstock Retrofit into a Surface Water Hydrobrake Manhole
Confined space retrofit of a manually operable penstock into a live SuDS hydrobrake chamber — delivered without disturbing the existing hydrobrake unit.
Project Overview
This project involved the retrofit of a penstock control mechanism into an existing surface water hydrobrake manhole on a residential development. The hydrobrake chamber, part of the site’s SuDS (Sustainable Drainage System) attenuation network, required an additional means of manual flow isolation to allow safe maintenance access and give the client control over discharge in the event of downstream issues.
The Challenge
The existing chamber was a below-ground precast concrete hydrobrake manhole with access via a double manhole cover, located within a residential garden close to an occupied property. The brief required:
- Installing a penstock without removing or replacing the existing hydrobrake unit
- Working within a confined space to industry safety standards
- Coring through the reinforced concrete manhole slab (biscuit) to fit the new spindle and frame without compromising the structural integrity of the chamber
- Minimising disruption to the surrounding garden and driveway
- Reinstating all surface covers and finishes to match existing
Our Solution
Our team carried out a full confined space entry and retrofit installation, completed in the following stages:
1. Site Setup and Safety
A tripod, winch, and fall-arrest system were rigged directly over the manhole opening to enable safe, controlled confined space entry and rescue capability throughout the works. Gas detection, ventilation, and a top-side attendant were maintained for the duration of the entry.
2. Access and Excavation
The surrounding turf and sub-base were carefully excavated to expose the chamber and adjacent access covers, with spoil neatly stockpiled for reinstatement.
3. Core Drilling
Using a diamond core drill, a precise opening was cut through the concrete chamber wall to accept the new penstock spindle guide, allowing the mechanism to be operated from above ground without needing to enter the chamber for routine operation.
4. Penstock Installation
The penstock frame and sluice plate were lowered into the chamber and fixed to the invert using anchor bolts. The threaded spindle was aligned vertically through the cored opening, secured with a wall-mounted guide bracket, and connected to a hand-wheel/operating nut at the surface.
5. Spindle Alignment
The threaded spindle was plumbed through the cored opening and restrained with a stainless wall bracket so the penstock can be raised and lowered cleanly from ground level.
6. Reinstatement
The excavation was backfilled, the ductile iron access covers and frames were reset flush with the surrounding ground, and the turf area was reinstated around the chamber.
The Result
The retrofit gives the client a fully operable penstock, controllable from ground level, integrated cleanly with the existing hydrobrake chamber. The works were completed with no damage to the existing hydrobrake unit, no disruption to the wider drainage network, and a neat, low-visibility surface finish that blends back into the garden.
Key outcomes
- Manual flow control now available at surface level without confined space entry for routine operation
- No compromise to the structural integrity of the existing chamber
- Full compliance with confined space safe working practices throughout
- Clean reinstatement of covers, hardstanding, and landscaping
- Site has now been adopted by United Utilities
- Client now has their bonds back
Have a similar chamber, penstock or hydrobrake issue holding up adoption?
Speak to our adoption specialists for a fast, practical quotation.