Investigation & Repair of a Small Landslide on the Torr Head Coast Road, North Antrim Mark Cassidy, Byrne Looby Partners (formerly Doran Consulting Geological Survey of Ireland Landslides Workshop 21 April 2009
Presentation Format Back ground Landslip Field & laboratory studies Repair options Preferred solution Construction
Project Background Roads Service Consultancy Framework Contract COMMISSION NO: F5/12 C82 Torr Road Slip ROADS SERVICE DIVISION: Northern
Site Location Site
Site Location Site Extract OS Discovery Series Map Sheet 5
Located on Coastal Route Site NITB Giants Causeway & Coastal Route Torr Head
Torr Head Road 4m wide road, 0.5 to 1.0m wide verges Road positioned on sloping ground that falls towards sea cliffs Sections with gradient steeper than 1 in 5 30km diversion if closed History of small landslides
Landslip Occurred Jan 2005 Pre-slip slope angle = 40 degrees Scarp height = 0.75m Length = 14m Width = 8m Estimated volume = 50m 3 Shallow planar (or translation) slip affecting the grass verge adjacent to the road, but not the road. Damp brown sandy clay with gravel and cobbles exposed along the sliding plane
Adjacent section of road Tilted road boundary fence Developing tension crack/settlement trough (40m long 1 to 2m into road) Tension Crack
Roads Service Previous works Four visible adjacent retaining structures Horizontal terraces along slope Rock Armour/Mass Concrete Wall Cracked Masonry Wall 2005 Slip Masonry Wall Repair 2003 Terraces
Clearly Problem Stretch of Road Next Step: Investigate inaccordance with HD22/02 Managing Geotechnical Risk (now HD22/08)
HD22/02 Systematic Certified Approach Initial review categorise site Procedural Statement Preliminary Sources Study (Desk Study) Geotechnical Report (summarising all investigations & providing design parameters Geotechnical Design Geotechnical Feedback Report
Published Geology Quaternary Glacial Deposits Solid - Metamorphic psammites and shists Site Extracts Solid & Superficial Geology Maps, Northern Ireland Site
Desk Study Roads Service carried out repair works to the masonry wall during 2003 Road present on OS 1 st Edition maps
Topographical Survey Complement to walkover survey 160m plan length of road 11 cross sections Points at approx 1m intervals
Ground Investigation 4 No window sample holes with ground water stand pipes by Geotechnical & Environmental Services Ltd Granular Fill (GL 0.4m) Glacial Till (reworked?) (0.4 1.8 to 3.6m) Completely weathered schist
Stability Back Analysis Limit State Equilibrium FoS dry 0.8 FoS saturated 0.4 Therefore solution needs to accommodate groundwater Simple planar analysis FOS = Tan Tan = slope angle (40 o ) =effective angle of shearing resistance (35 o )
Risk Assessment to HD41/03 Purpose to: Determine nature of feature Level of risk If any actions required
Risk Assessment Table
Repair Options Remedial options to be considered are: 1. Reinforced concrete L-shaped retaining structure under road 2a&b. Anchored embedded retaining wall (secant pile wall or sheet pile) in verge alongside road. 3. Reinforced soil 4. Reinforced concrete L-shaped retaining structure placed down slope at toe of slip with imported granular backfill. 5. Soil nailing.
Option 1 Reinforced concrete L-shaped cantilver reinforced concrete retaining structure under road Option 2 Anchored embedded retaining wall (secant pile wall or sheet pile) in verge alongside road.
Option 3 Reinforced soil slope using imported granular backfill. Option 4 Soil nailed slope
Option 5 Reinforced concrete L-shaped retaining structure placed down slope at toe of slip with imported granular backfill.
Comparison of Options Preferred Option: Embedded Contiguous Pile Wall (400mm dia piles at 500mm cc) with Passive anchors (tension piles)
Preferred Option
Construction April June 06 Key Issues: Access to site Working width Stability of Plant Duration of Road Closure
Completed June 06 Thank you
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