Overview
JNP Group were commissioned to assist with an outline planning application with all matters reserved except means of access for a development comprising of up to 270 dwellings (Use Class C3), a community building (Use Class E and/or F), Gipsy and Traveller pitches (covering up to 0.2 hectares), publicly accessible open space, landscaping, surface water drainage and all associated infrastructure at the site. The proposed development is located on a greenfield site, which has historically been and continues to be used for farming. The site sits adjacent to the M25 at Land South of Ashtead, Ermyn Way, Ashtead.
Project Solutions
To address the project’s challenges, JNP Engineers obtained utility asset maps that identified existing services. Where services crossed the site, and were not able to be diverted, an easement corridor was allowed for within the site plan.
As the site is underlain by chalk, the risk of ground dissolution was a concern. However, on further investigation by the Geoenvironmental department, it was found to be of very low risk. This allowed infiltration of surface water to be explored further to discharge the site’s surface water runoff.
To enable an infiltration-based solution to surface water runoff, several ground investigations were required. The investigations were to determine the infiltration rates across the proposed development as well as to monitor groundwater levels. Groundwater was not found to be of concern to shallow infiltration features, which allowed for a viable infiltration-based scheme.
Due to the site’s close proximity to the M25, JNP’s Geoenvironmental team completed a site stability report for National Highways, confirming minimal risk to the motorway. The risk was mitigated by ensuring the proposed infiltration features were set back from the site boundary, which looks to ensure no surface water from the development daylights onto the motorway from the basins.
Managing a large multidisciplinary team, JNP coordinated a collaborative design process to deliver a robust site plan for the outline application on behalf of two clients.
Summary
This project required a multidisciplinary approach to identify site-specific challenges and create solutions based on desk studies, infiltration tests and groundwater monitoring. JNP liaised with National Highways as well as the Lead Local Flood Authority to produce designs that suited both parties while producing an efficient sustainable drainage design. JNP Group’s synergetic approach has enabled them to incorporate both clients’ requirements along with the requirements of members of the design team. Through the design, JNP while able to address dissolution risk and infiltration offsets that meet the National Highways’ standards. In addition, JNP were able to provide a nature-based solution which has aided in an appealing development proposal.
Gallery
Client: Countryside Properties (UK) Ltd. (Wates Developments and Vistry JV)
Project Manager: Ryan Horngren
Project Team: Anthony Scoones, David Herrington, Joel Prestwich, Ben Thrift, Charlotte Grisby
Services Provided
- Flood Risk Assessment
- Liaison with Nation Highways
- Preliminary Sources Study Report (required by National Highways)
- Masterplan development
- Drainage strategy
- Existing utilities, capacity and diversion enquiries
- Input into the DAS
- Infiltration testing
- Ground water monitoring
Challenges
- Existing utilities crossing site
- Infiltration close to the M25
- Dissolution features
- National highway proximity
- Multi-client project
Solutions
- Obtaining utilities maps for the site
- Achievable infiltration rates and offsets to motorway
- Desk study reviews & reduce point infiltration
- Ground water monitoring
- Completing a PSSR
- Collaborative working with dual clients and design team to develop the site plan
