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Related pages

• Summary
• 1 introduction
• 2 traffic, congestion and safety issues
• 3 geotechnical issues
• 4 environmental constraints
• 5 option development
• 6 options considered further
• 7 engineering comparison of options
• 8 economic comparison of options
• 9 environmental appraisal of options
• 10 waste management
• 11 public open day
• 12 responses from statutory and non-statutory consultees
• 13 conclusions
• 14 recommendations
• Environmental study findings
• Environmental study findings
• Environmental study findings
• Environmental study findings
• Environmental study findings

A417 Cowley to Brockworth Bypass Improvement

3 GEOTECHNICAL ISSUES

Background Information on Soils and Geology

3.1 The geology of the study area has been obtained from the British Geological Survey of Britain Sheet 234 (Solid and Drift).

3.2 Excerpts from the relevant 1:10560 geological mapping, Sheet SO 91 SW, have also been studied as part of this assessment.

3.3 The British Geological Survey borehole database has been consulted.

3.4 The records of Gloucester County Council were searched regarding this section of the A417. Some data was recovered regarding the improvements (provision of the third lane below the escarpment) carried out during the 1960s. However, the drawings are not dated.

3.5 A number of records were examined from the Local Studies Section of the Gloucester Library.

Previous Studies

3.6 Reports from previous studies in the area have been reviewed as part of this study. The reports studied include the following:

• Gloucester County Council: Brockworth Bypass. Preliminary Soil Survey Addendum: Crickley Hill proposals. April 1981.
• Ward, Ashcroft & Parkman: A417 (T) Crickley Hill Scheme Identification Study Report. August 1986.
• EJ Wilson and Associates: Report on the Geomorphology at Crickley Hill (A417) for the Highways Laboratories Gloucestershire County Council Report No 918, September 1988.
• EJ Wilson and Associates: Addendum Report to Geomorphological Survey at Crickley Hill (A417) Gloucestershire for the Highways Laboratory, Gloucestershire County Council. Report No 918A, December 1988.
• Gloucester County Council: A417 Crickley Hill Widening proposals. Preliminary Site Investigation Report. December 1988.
• Gloucester County Council: Hungerford to Hereford Trunk Road A417 Crickley Hill Improvement. Technical Appraisal Report - Volumes 1, 2 & 3. January 1989.
• Gloucester County Council: Hungerford to Hereford Trunk Road A417 Crickley Hill Improvement. Geotechnical Certification Procedural statement. April 1989.
• Gloucester County Council: A417 Hungerford to Hereford Trunk Road Birdlip Bypass. Geotechnical Feedback Report. July 1989.
• Fugro McClelland: Static Cone Penetration Tests at A417 Crickley Hill, Gloucestershire. Ref 89/0700. October 1989.
• Gloucester County Council: A417 Crickley Hill Improvement. 1989 Soil Survey Interim Interpretative Report.
• EJ Wilson and Associates: Interim Report on Slope Stability for A417 Crickley Hill Offline Improvement Scheme for the Highways Laboratories of Gloucestershire County Council. Report No 1101/1, March 1990.
• Gloucester County Council: A417 Crickley Hill Report on Northern Widening Options. December 1990.
• Gloucester County Council: A417 Crickley Hill Improvement. 1989/90 Survey Interim Factual Report.
• Gloucester County Council: A417 Crickley Hill Improvement. Geotechnical Investigations and Schemes for Road Widening on the Northern Valley side by Professor J.N. Hutchinson. August 1991
• Frank Graham Consulting Engineers: A417 Brockworth Bypass Geotechnical Interpretative Report. October 1991.
• Frank Graham Consulting Engineers: A417 Brockworth Bypass Earthworks Design Report. May 1994.
• WSP Group: A417 Brockworth Bypass; Construction (Design and Management) Regulations 1994, Health and Safety File, As-built Record, Construction Report, Part 4, Geotechnical Feedback Report. April 1999.
• WSP Group: A417 Crickley Hill Improvement. Preliminary Sources Study. January 2002

Topography and Outline Geomorphology

3.7 Within the study area the Cotswold Escarpment runs approximately SSW to NNE. To the east of the escarpment the land is formed by a plateau at an approximate elevation of between 240 and 290m AOD, whereas, to the west, the land drops away to an approximate elevation of 100m AOD. The lower slopes are generally at a shallower angle, ranging from 6º to 16º, while the higher slopes below the escarpment itself rise at between 30º to 45º.

3.8 In many places the slopes below the escarpment can be seen to exhibit evidence of past ground movements with features such as soil lobes, back scars and springs dominating. Many areas are hummocky in appearance indicating the possible existence of secondary landslipping masking the deeper-seated historic failures.

Geology

3.9 The study route commences at the top of the Cotswold Escarpment where the Inferior Oolite Group outcrops and descends to the Vale of Gloucester within the outcrop of the Lower Lias. In the south-east part of the study area further landslip material is encountered together with some deposits of Fullers Earth.

3.10 Almost all of the route below the escarpment to is within landslipped material (colluvium). Over this section of the study area the Upper and Middle Lias Formations, together with the upper horizons of the Lower Lias, are mantled by the colluvial deposits.

3.11 For details of the geological structure and information about soil types, see Appendix A.