Highways Agency - Safe roads, Reliable journeys, Informed travellers

Related pages

• 2.1 General
• 2.2 Mainline Layout
• 2.3 Junctions
• 2.4 Traffic Flows
• 2.5 Engineering Standards
• 2.6 Highway Features
• 2.7 Structures
• 2.8 Ground Conditions

2.8 Ground Conditions

2.8.1 Geology and Earthworks

Published geological maps and memoirs and site investigation records indicate the section of the A2 under consideration to be underlain by the sequence of materials shown in Table 2.1. A brief description of the geology of the site is given in Volume 1 of the Environmental Statement.

Table 2.1 Geological Sequence

The earthworks slopes along the A2 have been modified several times since the original construction to facilitate widening that has taken place since then. Cut slopes along the Phase 1 section have been formed in London Clay, Lambeth Group deposits and Thanet Sand. The slopes are generally formed at a grade of 1 vertical to 2 horizontal and appear reasonably stable. Embankment slopes similarly have been formed at 1 vertical to 2 horizontal and appear reasonably stable. Persistent instabilities of the embankment slope adjacent to the Northfleet Substation occurred during the 1970s and 1980s. However the north-facing slope was widened and stabilised during the 1990s.

2.8.2 Surface Water

Surface water features include a highway drainage pond adjacent to the coast bound carriageway at CH700 near Bean Junction and a further pond approximately 400m south of Bean Junction. A surface drain flows northwards towards the south side of the carriageway at CH1550. There are also lakes around Swanscombe, located several hundred metres to the north of the A2.

The only watercourse in the study area is the River Ebbsfleet. It arises from a spring approximately 50m north of the A2 carriageway at Springhead. From here the river flows northwards and discharges into the River Thames at Northfleet.

There are no surface water abstraction points for public water or any other use within 1km of Phase 1.

2.8.3 Groundwater

The Environment Agency (EA) has a duty to monitor and protect the quality of groundwater and to conserve its use for water resources as set out in their Policy and Practice for the Protection of Groundwater (1998). This document uses a series of vulnerability and source protection zone maps.

Groundwater vulnerability depends upon the presence, depth and nature of the overlying soil, drift and strata and is defined in terms of the aquifer and soils. The London Clay is a non-aquifer; the Thanet Sands are a minor aquifer and the Chalk is a major aquifer. Soils ranging from high to intermediate leaching potential are present along the route.

The A2 route between Bean Junction and Pepperhill Junction is underlain in parts by Chalk deposits, the principal aquifer in south-east England. The water table in this aquifer is generally tens of metres below the level of the western section of Phase1. The distance between road level and water table becomes a minimum of approximately 10m at CH4000 near the River Ebbsfleet.

Eight groundwater abstraction licences exist within 1km of Phase 1 and include a public water supply at Southfleet and industrial or agricultural supplies. The EA policy protects abstractions by defining Protection Zones. The road passes across Zone 1 (inner), Zone 2 (outer) and Zone 3 (catchment) Protection Zones as shown on Figure A6.1 in Appendix 6.

There are existing discharges to soakaways in Source Protection Zone 1 (Bean Junction Soakaway), Zone 2 (Sandy Lane) and Zone 3 (Swanscombe Slip Road). It should be noted however that the thickness of the unsaturated zone between the base of the soakaway and the existing level of the groundwater is more than 50m, providing some protection to the groundwater.

Groundwater protection and mitigation measures are covered in more detail in the Environmental Statement.