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7/27/2019 Presentation-J Binns Presentation- Micropiles
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Site Investigation – less is more?GSM-R Mast Micropile Foundation Design Case Study
James Binns
Yorkshire Geotechnical Group
Young Engineers Presentation Competition
November 2007
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GSM-R Mast Micropile Foundation
Design Case Study
• Scheme details
• Installation techniques and equipment
• Site investigation data
• How to design
• Risks
• Summary
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Scheme Details
• GSM-R = Global System for Mobilecommunications – Railways
• Skanska UK Civil Engineering
• C. 2000 No. GSM-R mast antennaeto be installed adjacent to railways
• ~110 installed to date• ~15% – piled foundations
• System running by 2012
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Scheme Details
• Loadings : – mast height (15-30m)
– site wind speed (20-30m/s)
• Compression – 75-200kN per pile
• Tension loadings – up to 100kN per pile
• Horizontal forces resisted by soil shear resistance via the r/c base being castdirectly on ground (AIP)
• Performance specification
– <15mm at SWL
– <25mm at 1.5 x SWL
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Scheme Details
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Installation Techniques & Equipment
• Previous design – CFA piles, screw piles – Both systems cannot penetrate bedrock
• Self drilling micropile ~ 5 suppliers in the UK
• Ischebeck Titan
– 40/16 hollow bar – 90/115/150mm Φ sacrificial drill bits
• Bar comes in 3m lengths and full length is installed usingfull strength couplers
• Centralisers provide minimum cover
• Grout flush
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Site Investigation
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Site Investigation
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How to Design
• Desk study basics – what is the geology
– BGS geology map
– BGS memoir
– BGS archive boreholes
• Design parameters from geology
– Literature e.g. BS8004, BS8081, 10,000 Anchorages in Rock
• Combine with data from SI for overall assessment
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Risk
• Low risk
• Minimum socket lengths detailed in design
• Self drilling technique, experienced driller can tell
competent ground. Additional lengths of bar availableon site
• Piles designed on shaft friction alone, end bearing
ignored
• All micropiles tension tested to 1.5 x SWL
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Risk
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Risk
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Why bother with SI
• SI used to determine if shallow foundations suitable
• Data can be useful for micropile design
• Howburn
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Why bother with SI
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Why bother with SI
• SI used to determine if shallow foundations suitable
• Data can be useful for micropile design
• Howburn
– SI sands & gravels to 8.2m where refusal was met – Design based on skin friction in granular soils and 3m rock
socket
– Design depth 12m
• Ground conditions confirmed in drilling
– Test results - >3mm axial movement at 1.5 x SWL
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Summary
• SI does not penetrate ground to depth that piles are to
be installed, ground conditions ‘unknown’
• SI supplemented by ‘desk study data’
• All data then combined to form design ground model• Micropiles are economic, quick to install, versatile and
can penetrate difficult ground
• Each and every micropile tension tested to 1.5 x SWL,
therefore designs are low risk and ‘self checking’
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