Settlement over peat on the A5 at Pant Dedwydd near Cerrigydrudion, North Wales

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Authors:Nichol, Douglas; Farmer, Ian W.
Author Affiliations:Primary:
Wrexham County Borough Council, Engineering Services, Wrexham, United Kingdom
Other:
Ian Farmer Associates, Newcastle upon Tyne, United Kingdom
Volume Title:Engineering Geology
Source:Engineering Geology, 50(3-4), p.299-307. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0013-7952
Publication Date:1998
Note:In English. 7 refs.; illus., incl. 2 tables, sketch map
Summary:The A5 London to Holyhead trunk road suffers from large and varied settlements at Pant Dedwydd where it crosses a geotrophic peat bog developed on the glacial drift that overlies bedrock of Silurian Denbigh Grits Group. Recent carriageway settlements have created an undulating profile over a 100 m stretch, restricting forward visibility and posing recurring and persistent difficulties for highway maintenance. The highly compressible peat underlies almost 400 m of highway, ranges up to 7 m deep and a typical profile consists of an upper sphagnum peat layer and a basal reedy, woody and sedge grass peat. The highway was originally constructed by Thomas Telford in 1819 as a low floating embankment over the organic terrain. Increases in traffic over the intervening period together with successive asphalt overlays have resulted in a history of major repairs and although records are incomplete it is likely that the carriageway is now over 2 m thick over parts of the bog. Comprehensive geological investigations carried out to characterize the peat deposit included dynamic probe holes, boreholes and a trial pit. No unusual subsurface conditions were identified. However, scrub willow or grey sallow (Salix cinerea) was observed growing in spiraea thickets alongside the deepest hollows in the highway. The willows may contribute to the road subsidence by extraction of water from the peat beneath the carriageway. Accordingly, removal of the scrub willow and conventional re-grading of the carriageway have been recommended to correct the existing problem. Abstract Copyright (1998) Elsevier, B.V.
Subjects:Bogs; Case studies; Compressibility; Controls; Cores; Embankments; Field studies; Foundations; Geologic hazards; Glaciated terrains; Land subsidence; Mires; Peat; Remediation; Roads; Sediments; Settlement; Soil mechanics; Vegetation; Denbighshire Wales; Europe; Great Britain; United Kingdom; Wales; Western Europe; Cerrigydrudion Wales; Clwyd Wales; North Wales; Pant Dedwydd Wales
Coordinates:N530000 N530500 W0033000 W0033500
Record ID:1998073290
Copyright Information:GeoRef, Copyright 2018 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands
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