Complete civil engineering design and construction support for a major residential development
We were initially appointed to provide highways & transportation services to support a Full Planning Application for a 60 unit housing development with Doctors surgery at this site.
We subsequently carried out the detailed engineering design and produced the Construction Specification and Drawings.
- The Havant Road development in Horndean, Waterlooville, Hampshire was Phase One of an overall site comprising up to 220 dwellings. Our client Sunley Estates, with Charles Church and Persimmon, proposed a development of 60 units, later replacing the proposed doctors surgery with a 15-unit apartment block.
- The main highway access to the development was to be provided by a priority junction with the adjoining B2149 Havant Road.
- A pumped, public foul sewer diagonally dissected the site requiring diversion. In addition, the limited capacity of the adjacent highway drainage system caused surface water flooding on the site.
Services covered by this project sheet:
The main highway access to the development was to be provided by a priority junction with the adjoining B2149 Havant Road.
Strategically located traffic surveys were organised to collect data from across the local highway network. This included personal injury data so that an analysis of Road Traffic Collisions could be included in the assessment.
TRICs software was used to estimate the likely traffic generation from the development. This was assigned to the existing peak hour traffic movements observed at nearby junctions. ARCADY and PICADY software was used to estimate the potential queues and delays on the highway network caused by the additional traffic. Percentage estimates for traffic growth in the longer term were also agreed with the Highway Authority, Hampshire County Council.
On site, using Autotrack swept path modelling analysis, it was demonstrated how unimpeded access for emergency and service vehicles could be provided around the development.
The TA identified and proposed a number of offsite highway improvements and traffic management measures that would minimise delays and improve road safety.
The TA considered the sustainability of the site; using public transport, walking, or cycling for journeys to and from local amenities. A Travel Plan for the development provided practical information to encourage the uptake of non-car journeys by residents and visitors. The Travel Plan proposed appointment of a Travel Plan Coordinator who would be responsible for monitoring and reviewing the uptake of Sustainable Transport options.
Many local authorities now require a Drainage Strategy to be submitted with the Planning Applications for all major developments. Building developments have an impact on many interrelated flooding and drainage issues: Highway and Land drainage, Groundwater and Watercourses, Foul Water drainage, Surface Water drainage and Sustainable Drainage.
A drainage strategy ensures an integrated approach is identified that addresses all the drainage issues specific to a development.
In effect, this requirement means that much of the detailed design work including site surveys, sewer records, geotechnical investigations, ground profiling, hydraulic modelling and calculations of the various drainage systems has to be carried out speculatively, i.e. before the Planning Application is Approved.
In this case a pumped, public foul sewer diagonally dissected the site. This sewer would require diversion so that the proposed housing layout could be provided. A diversion was agreed that the rising main would be laid to follow the main access road through the site. The sites foul drainage system would be gravity fed and connected to a manhole upstream of the pumping station.
The Flood Risk Assessment had identified localised surface water flooding near the adjacent highway caused by the limited capacity of the highway drainage system. Design improvements proposed re-profiling of the ground levels to significantly improve the highway drainage. The drainage ditch under the access road would be culverted, open ditches would be widened and re-shaped to improve the hydraulic performance.
Results of the geotechnical investigation had identified a free draining subsoil, suitable for soakaways. However, to prevent any contamination to a local aquifer, the Environment Agency only allowed run off from the house roofs to be drained directly to soakaways. Surface water drainage from the public highway or the developments roads and parking areas was to include bio retention measures to remove contamination.
The sustainable drainage system (SUDS) solution proposed permeable paving and an underground attenuation tank together with a maintenance plan to ensure long term system performance.
The site successfully obtained Full Planning Consent in 2015. Subsequently, the Stilwell Partnership was appointed to produce the detailed design drawings for the Discharge of Planning Conditions and Construction.
Section 278 agreement with the Hampshire County Council would ensure that the design and construction of all highway improvements, including the highway drainage system, were constructed to an adoptable standard. Long term maintenance would then be the responsibility of the Highway Authority.
Highway Authorities require Road Safety Audits (RSAs) of public highway improvements to be carried out. Skilled road safety engineers, following Highways England procedures DMRB Vol 5, HD 19/15 and the IHT Guidelines, critically analyse the proposals to identify potential hazards that could arise.
The RSAs were carried out by our independent, accredited road safety audit team. A combined Stage 1/2 RSA was produced at the Outline Stage of the Highway Design. Later a Stage 3 RSA was undertaken of detailed design. Recommendations identified at each stage were referred back to the designer and incorporated into the design drawings where appropriate.
A Section 278 Agreement Plan summarised the agreed design which supplemented by a series of drawings detailing specific items such as road construction, vehicle swept paths, traffic manmgent measures.
Drawings of the engineering proposals together with a Specification were issued for Construction.
The main site access would be culverted, drainage ditches would be widened and re-shaped. Concrete retaining walls, gabions, and a pedestrian footbridge also formed part of the landscaping.
Hydro Filterra Advanced Bioretention systems were included to capture hydro carbons washing from the carriageway surface towards the downstream watercourse or aquifer.
A full set of setting out drawings provided the coordinates and levels for buildings and site infrastructure.
The SUDs, surface water and foul water sewerage design layout drawings included pipe and manhole schedules, and standard details.
A number of agreements covering the public sewers were entered into with the water company, Southern Water. The foul sewer layout drawing, Longitudinal sections and standard details were produced for the Section 104 Adoption of Sewers agreement. Other agreements and drawings covered the adoption of sewers and connections on the site.
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