SLOPE INSTABILITY AND SMALL HYDROELECTRIC SCHEMES WITH
PARTICULAR REFERENCE TO PEAT SOILS
Ian Uglow SLR Consulting Ltd. iuglow@slrconsulting.co.uk
Courtesy of Kieron Hanson
Courtesy K Hanson & J Turnbull
Courtesy K Hanson & J Turnbull
Glen Kinglas 2004
With permission of J Turnbull
Is slope stability an issue? • What are the implications on the viability of a scheme that suffers from slope failure?
Derrybrien Peat Slide 2003 • • • •
Courtesy R Lindsay UEL
Large area affected Effects extended to 20km downstream Significant delay to construction programme Impact on subsequent renewable energy schemes
Peat Instability • What is it? • Is it different to any other slope instability • Can it be avoided?
Primary Impacts on Peat Stability Topography
Climate
Drainage Peat stability
Construction
History
Inter-relationships that impact on Peat Stability Topography
Drainage
Climate Peat stability
Construction
History (Stability Forestry etc)
Topography •
Gradient – slopes >4 degrees are considered to be at risk.
•
Peat thickness – stability related to thickness.
Topography
Drainage
•
•
Digital mapping in 3D to identify areas of potential instability. Adjacent areas above and downstream of the site must be assessed.
Climate Peat stability
Construction
History
Climate Changes in the short and medium term rainfall patterns can condition the peat for instability.
Topography
Drainage
Desiccated peat that becomes re-saturated appears to be particularly prone to instability. (Note potential from forestry and construction drainage).
Climate Peat stability
Construction
History
History: •
Forestry – – – –
•
Ploughing and drainage Root penetration Cracking Desiccation by evapotranspiration and interception of rain by the canopy.
Stability – The best indicator of potential instability are signs of natural, historic instability such as: • Relict slump features and mudflows, • cracking in the ground surface,
Topography
Drainage
Climate Peat stability
Construction
History
Failure in Forestry Plantation
Courtesy R, Lindsey UEL
Drainage Changes to the existing drainage pattern can initiate instability •Natural – Streams and other surface features – Peat pipes – water courses within Topography the body of the peat. •Construction – Access roads Drainage Peat stability – Hard standing and lay-down areas – Turbine bases Construction – desiccation •Post Construction
Climate
History
Effect of Artificial Drainage • Drainage ditch in peat which has closed due to peat movement. • How stable is this slope? • Implications for construction?
Courtesy O.M.Bragg, Dundee University
Construction • Pipeline and access roads – ‘Floating’ on peat or founded on subgrade? – Cut into side-long ground requiring embankments and cuttings Drainage – Drainage, new and maintenance of existing natural.
• Construction and lay-down areas • Heavy lift areas.
Topography
Climate Peat stability
Construction
History
Conclusions 1.
Planning • •
2.
Critical slope gradients Peat?
Peat Stability Assessment • •
3.
Identify areas at risk Investigate and survey whole area, not just route corridors
Pre-construction • •
4.
Prepare construction method statement Detailed assessment of individual elements as part of the detail design.
Construction • •
Compliance with the construction method statement Close liaison between construction and the geotechnical engineer to agree changes to the design and method statement.