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Detailed Geologic Mapping: An Essential Step in Foundation Design for Karst Sites

Detailed Geologic Mapping: An Essential Step in Foundation Design for Karst Sites. Eric J. Seksinsky, P.G., P.E. Bashar S. Qubain, Ph.D., P.E. Jianchao Li, P.E. GeoStructures, Inc. King of Prussia, PA. GSA Northeastern Section, 49 th Annual Meeting Lancaster, PA March 24, 2014.

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Detailed Geologic Mapping: An Essential Step in Foundation Design for Karst Sites

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  1. Detailed Geologic Mapping: An Essential Step in Foundation Design for Karst Sites Eric J. Seksinsky, P.G., P.E. Bashar S. Qubain, Ph.D., P.E. Jianchao Li, P.E. GeoStructures, Inc. King of Prussia, PA GSA Northeastern Section, 49th Annual Meeting Lancaster, PA March 24, 2014

  2. Outline • Geologic mapping for geotechnical investigations • Pennsylvania Turnpike widening project • Site geology and sinkhole setting • Preliminary and final data • Rock conditions and bridge foundations • Conclusions GSA Northeastern Section, 49th Annual Meeting Lancaster, PA March 24, 2014

  3. Geologic Mapping for Geotechnical Investigations • Confirm published geology • Focus test drilling • Translate relative dips of rock structure in cores to actual orientations • Project test boring profiles along the strike of beds when drafting subsurface sections • Explain faulted contacts and inconsistencies in published maps GSA Northeastern Section, 49th Annual Meeting Lancaster, PA March 24, 2014

  4. Pennsylvania Turnpike Widening Project • Valley Forge to Norristown: 3-mile widening project built in late 1990s • 1 major bridge + 2 smaller bridges • Pennsylvania Turnpike Bridge over the Schuylkill River • New, independent, 6 span bridge 1,225 ft long, 70 ft high • Alongside original (1952) 11-span bridge and 19th c. RR bridge NEW BRIDGE 1952 BRIDGE RR BR. GSA Northeastern Section, 49th Annual Meeting Lancaster, PA March 24, 2014

  5. Site Geology and Sinkhole Setting • Site is along eroded “feather edge” of the Triassic Unconformity • Triassic Stockton over Cambrian Ledger and Antietam-Harpers • Cambrian carbonates: • Altered, fractured and deformed • Subjected to several geologic time periods of exposure, intense erosion and karst development • Sinkholes! GSA Northeastern Section, 49th Annual Meeting Lancaster, PA March 24, 2014

  6. And There are Many Sinkholes in King of Prussia… GSA Northeastern Section, 49th Annual Meeting Lancaster, PA March 24, 2014

  7. Available Geologic Map: Fairly Simple, Right? CAMBRIAN TRIASSIC ERODED BEDS • Only 3 formations, no faults or folds TRIASSIC UNCONFORMITY Ref.: Berg, T.M., Geologic Map of Pennsylvania, Bureau of Topographic and Geologic Survey, Harrisburg 1980. GSA Northeastern Section, 49th Annual Meeting Lancaster, PA March 24, 2014

  8. 34 Preliminary Borings Found Ledger dolomite and Harpers phyllite but also hit... • Un-mapped phyllitic dolomite of the Cambrian Vintage Fm. • Un-mapped Triassic basal limestone conglomerate • Large subsurface voids and remnant dolomite ledges GSA Northeastern Section, 49th Annual Meeting Lancaster, PA March 24, 2014

  9. Exploration for Final Design • Assistance from Structural Geology Dept. at Franklin & Marshall • Insight from technical publications by Cloos and Pettijohn • 87 more borings + mapping in quarries and road cuts • Primary geologic objective: differentiate Vintage from the Ledger • Why?...Not just a geologic exercise! • Assess karst hazards affecting each bridge • Vintage is not cavernous like the Ledger GSA Northeastern Section, 49th Annual Meeting Lancaster, PA March 24, 2014

  10. A Complicated Geologic Picture STOCKTON SANDSTONE LIMESTONE CONGLOMERATE THRUST FAULT SIMPLE-SPAN BRIDGE VINTAGE DOLOMITE (BLUE) NORMAL FAULT 2-SPAN BRIDGE HARPERS PHYLLITE NORMAL FAULT LEDGER DOLOMITE OVERTURNED ANTICLINE SCHUYLKILL RIVER BRIDGE SCHUYLKILL RIVER RR BRIDGE GSA Northeastern Section, 49th Annual Meeting Lancaster, PA March 24, 2014

  11. Rock Conditions & Bridge Foundations • 6-Span Bridge • Sloping rock with voids and fingers • For foundation engineering  top of rock is top of Vintage not Ledger • Rock socketed caissons in the Vintage w/permanent casing through Ledger remnants to avoid load transfer to voids and fingers GSA Northeastern Section, 49th Annual Meeting Lancaster, PA March 24, 2014

  12. Rock Conditions & Bridge Foundations • Simple-Span Bridge • Cavern in Ledger below Triassic Unconformity • Spread footings on rock- limestone conglomerate • Cave grouted (3,000 CY) GSA Northeastern Section, 49th Annual Meeting Lancaster, PA March 24, 2014

  13. Rock Conditions & Bridge Foundations • 2-Span Bridge • Sloping rock, no voids • Driven H-piles • Pre-drilled into the Vintage and Harpers where rock surface slopes > 26° GSA Northeastern Section, 49th Annual Meeting Lancaster, PA March 24, 2014

  14. Conclusions • Don’t assume accuracy of available maps; work by private firms can have a role in updating the state geologic maps • Detailed geologic mapping is essential to develop accurate subsurface sections and assess karst risk • An understanding of complicated karst features such as the steeply inclined Ledger remnants and voids would not have been possible without geologic measurements • Correlation of boring data with detailed mapping optimized foundations GSA Northeastern Section, 49th Annual Meeting Lancaster, PA March 24, 2014

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