Coat Grove, Martock

Project: 125 new homes on greenfield site
Location: Martock, Somerset
Client: Barratt Homes
Services: Infrastructure design, drainage design, Local Water & Highway Authority Technical approvals, LLFA engagement, Planning condition discharge, Site support & coordination, Client technical support & guidance

The Project

Following our initial success at helping the client gain a planning consent for the development of 125 new two, three and four-bedroom houses (marketed at £265,000 to £380,000) on a greenfield site in Martock, Somerset, we were retained to add the meat to the bones of the project and prepare all onsite detailed design work to enable the site to be constructed to the clients desired specifications.

The Problem

Most open market residential schemes seek to offer the main infrastructure to the local drainage and highway authorities through a process known as adoption. This gives residents peace of mind that any maintenance issues down the line will be funded by their water and council tax bills, as opposed to paying additional contributions to a site-wide management company.

Complex legal mechanisms exist for vesting such infrastructure into public ownership and are commonly referred to as Section Agreements. These Section Agreements have associated minimum design and construction standards that must be met to ensure the adopting authority is satisfied that the infrastructure they are adopting is built to last. Developers then pay the adopting authorities a monetary sum (typically a percentage of the total cost of the infrastructure) to attend site to inspect the construction as site works progress, to maintain the highest standards.

In this case, it was identified at an early stage that the strength of the existing ground was below the minimum value required by Somerset Highway Authority to support the road foundation, based on their standard road construction guidance. As such, an alternative construction proposal would have to be agreed upon if they were to agree to adopting the highway in those areas as public highway.

The officers responsible for approving the design and construction drawings for the highway adoption agreement felt were different to those that had commented on the scheme at the planning stage, and requested localized changes in the design. This subsequently led to detailed discussions with the officers to agree a revised road layout that met the requirements of the clients, the Local Planning Authority as well as the Highway Authority.

The drainage for the site was also complex, as the site was naturally bisected by a small stream (called a rhyne), and the majority of the site lied lower than the existing public drainage network. As such, a foul water pumping station needed to be designed to allow drainage from both sides of the ryhne to be pumped back up to the existing public sewer at the site entrance.

A highway culvert was also required so that both sides of the site could be accessed by a public highway. It was our job to prove that the proposal for a culvert would not impede flows and cause the water to back up in a severe storm, and that it would meet adoption criteria to support a highway structure.


Our Infrastructure & Drainage team successfully steered the Coat Grove development through a varied and complex approvals process to achieve designs that could be constructed and vested within public ownership, protecting the salability of properties. Drawing upon the in-depth knowledge we hold of the adoption and approval process allowed us to anticipate problems before they arose, and provide sensible, and innovative but buildable solutions when challenges presented themselves; all of which gave our PLC client confidence to start building and selling much needed homes.

The solution

We needed to pull together the conflicting priorities of a leading housebuilder: maximising building plots and amenity space; and those of the adopting authorities in meeting their standards.

Given the change in officer, we understood that early dialogue was key to achieving a timely and positive outcome for the client. We consequently prioritised discussions with the client and highways officer to work through the competing comments, and ensure that subsequent variations to the approved layout would meet with approval, without need for further iterations and unnecessary delay.

In this way, we were able to agree upfront that we could address the substandard road formation bearing capacity by reinforcing the road foundation with a geo-grid system to ‘stiffen up’ the road foundation. Working with the Goetechnical engineers and a specialist geosyntheitic manufacturer, we proposed two layers of geogrid were used within the capping layer – an innovative solution that was subsequently approved by the highways department. This solution meant that the overall construction thickness was no greater than that of a standard road, and avoided alternative more onerous/costly solutions such as localized ground improvement techniques.

In the context of the pumping station, our experience shows that no two situations are the same and our approach of engaging early with multiple parties ensured we were able to resolve complexities around telemetry, internal pumps and pipework, floatation and ballast calculations, emergency storage requirements and septicity calculations, right through to legal and landscape plans. We provided the focus to understand the requirements of each stakeholder to get a successful design that didn’t leave the client trying to tie multiple lose ends together.

Whilst the work undertaken by our in-house flood risk experts provided the evidence required to demonstrate that a culverted road crossing of the existing rhyne watercourse was acceptable in principle to the local authorities drainage officers, it was time to drill down into the detail. We therefore undertook a detailed 1D/2D hydraulic model of the rhyne utilizing topographical data in combination with datasets from Environment Agency monitoring stations, along the rhyne network in the locality of the site. This provided the evidence needed to demonstrate that the proposed culvert wouldn’t have a detrimental effect on either the upstream or downstream catchments during extreme rainfall events.

What’s more, we produced a culvert design solution that also met the requirements of the highway and drainage authorities, so that there was sufficient space to get the proposed utilities over and under the culvert, and be structurally sound to be classified as a highway supporting structure.

Calibro are responsive, commercial and always ready to help. They have formulated complex arguments and presented them in a digestible fashion that has ultimately helped to elicit positive consultation responses from the Authority

Phillip Kirby
Northland PM

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