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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’