SuDS REPORT – Natural Stone Permeable Driveway Project

1. BACKGROUND

1.1 The Site

The site, a large driveway approach and parking area to a private house in Beaconsfield, is located at the top of hill in Forty Green, Beaconsfield, Bucks. At 365m2, the driveway & parking area is formed of self-binding gravel.

The driveway slopes by 4% (1:25), causing problems with water run-off, which collects the fines from the bound gravel and creates severe silting of drainage gulleys/pipes/inspection chambers, and sunken hard landscaped garden.

Our brief was to replace the 365m2 bound gravel area with a natural stone solution, aesthetically complimentary to the existing vernacular, be more practical to walk and park on – particularly during wet conditions – and would also prevent overloading of the surrounding drainage network with stormwater.

1.2 Site Investigation

A series of trial holes were dug at various points across the site to establish the depth of bound gravel and to assess the condition and makeup of the sub-base and subgrade . Generally, the bound gravel layer was formed to a depth of 35-70mm. Below this – 110mm Type 1 Granular Sub-base. Below the sub-base, *sandy clay forms the subgrade.

*Although no formal testing was carried out, by analysis through sight and texture, it is estimated that the coefficient of permeability of the subgrade is between 10-6 – 10-9

1.3 Views of Site Prior to Implementation

The driveway and parking areas with bound gravel surface

2. IMPLEMENTATION

2.1 Groundworks Phase

The 365m2 bound gravel layer and existing sub-base was excavated to a depth of 100mm and removed from site.

2.2 Construction Phase

SPECIFICATION

PORPHYRY CUBE PAVING

Quantity: 365m2
Stone Description: Italian (Trentino) Porphyry Cubes
Material Technical Data: Refer to document ‘PORPHYRY CUBES DATA’
Finish: Natural split face walking surface, guillotined sides
Size(s): 60 – 80mm (6/8 size)
Bedding method: SteinTec tuffgrit bedding aggregate
Nominal Thickness of bed: 70-90mm
Jointing method: SteinTec tuffgrit bedding aggregate
Joint Widths: 10mm nominal
Sub-base: Existing Type 1 Granular Sub-base

2.3 Photos of Construction Phase

3. COMPLETION

3.1 Summary of Project with regards to SuDS

Total potential water attenuation by volume: 30-40m3 (over 365m2)
BS 7533-13 : 2009 Infiltration System: Type B – Partial Infiltration

Observations of performance in heavy rainfall:

Prior to the installation of the SuDS permeable driveway, severe overloading of the surrounding drainage network was observed, with heavy siltation of gulley, pipes and inspection chambers (see image below) as well excessive surface water run-off causing localised flooding to surrounding hard and soft landscape areas.

Following installation of the permeable paving, further heavy rainfall has affected the site. However, due to the new Sustainable Drainage System, water run-off has been reduced by approximately 80-90%. Any water now reaching the existing drainage system is clear – having been filtered through the pavement aggregate bedding layer – and is free of fines.