Highways Agency - Safe roads, Reliable journeys, Informed travellers

A419 COMMONHEAD JUNCTION IMPROVEMENT

2. EXISTING CONDITIONS

2.1 Commonhead junction is located on the A419 to the south east of Swindon town centre and is located approximately 1 km north of Junction 15 of the M4 (see Figure 1.1). It is an important junction for access into and out of Swindon. In its present form, Commonhead junction consists of a signal controlled at-grade roundabout. The current layout allows for future grade separation.

2.2 The road gradients at Commonhead junction, its approach roads and the A419 to the north of the roundabout are relatively level. Within the central reserve is existing mounding of varying height (up to a maximum of 3m), with mature planting on the crown and slopes. In the north west quadrant there is a mound (up to approximately 6.5m high) with mature planting to the road facing slopes, behind which is the housing estate at Liden. The new Great Western hospital is in the south west quadrant. The landscape to the east of the A419 is predominantly farmland, with the villages of Wanborough and Liddington around 1.5km distant. Commonhead junction lies on the boundary of the North Wessex Downs Area of Outstanding Natural Beauty (AONB).

2.3 The land generally slopes in a south-west to north-east direction. As such, the western side slopes of the northbound carriageway of the A419 are generally in medium to high cutting (up to approximately 5m in depth) and the eastern side slopes of the southbound carriageway of the A419 are generally on low embankment (approximately 1 to 2m in height).

2.4 The capacity of the existing roundabout is inadequate during periods of peak traffic flow, and even with traffic signals the junction still suffers congestion. There are regularly extensive queues on the A419 during peak periods. Traffic flows range from 55,000 to 66,500 vehicles per day of which 16% are heavy goods vehicles (HGVs) and journey delays can be up to 25 minutes. There have been 38 personal injury accidents (PIAs) recorded over the past 5 years.

2.5 The area remained largely unchanged between the late 1800s and 1977 with most of the land comprising pasture land, farm dwellings, country roads and woodland. However, rapid urbanisation to the north west of the Commonhead junction scheme has since ensued, as Swindon expanded.

2.6 The published geology for the site has been confirmed by ground investigation as Gault Clay overlying Lower Greensand. The Gault Clay lies between 4 and 6m thick across the majority of the site. The Lower Greensand comprises strongly cemented coarse grained sandstone material. The raised mound of material in the central reserve areas comprises re-worked Gault Clay, which, in parts, contains gravel and brick fragments.

2.7 Groundwater was struck in some of the boreholes drilled during a recent ground investigation at depths equivalent to 6.4m below existing ground level (begl) and 7.4m begl. Groundwater monitoring undertaken during and subsequent to the July 2002 investigation indicates that the groundwater levels have tended to rise since the ground investigation was completed. Groundwater levels would be expected to rise even further during the wetter winter months.

2.8 There are two significant surface water features within 2km of the site, namely Coate Water located 1.5km to the west of Commonhead junction and an attenuation pond 750m to the north of the junction. There are two watercourses (both unnamed on Ordnance Survey Maps) (OS) within the overall study area. Both these watercourses, which originate as springs from the high ground to the south east of the study area, flow in a generally northerly or north westerly direction, and ultimately flow into the attenuation pond. 2.9 The flood depth information report indicates that the site is not situated within the 0- 1m estimated 100-year flood depth. However, the northernmost part of the site is situated within the floodplain of the watercourses supplying the River Cole.

2.10 The Gault Clay is classified as a non aquifer of negligible permeability but the Lower Greensand and the alluvium to the north west are classified as minor aquifers.

2.11 The site is not situated within a Source Protection Zone for groundwater abstraction. Groundwater flow is considered to be towards the north i.e. towards the River Cole.