Slide 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, 49 th Annual Meeting Lancaster, PA March 24, 2014 Slide 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, 49 th Annual Meeting Lancaster, PA March 24, 2014 Slide 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, 49 th Annual Meeting Lancaster, PA March 24, 2014 Slide 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 GSA Northeastern Section, 49 th Annual Meeting Lancaster, PA March 24, 2014 NEW BRIDGE 1952 BRIDGE RR BR. Slide 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, 49 th Annual Meeting Lancaster, PA March 24, 2014 Slide 6 And There are Many Sinkholes in King of Prussia GSA Northeastern Section, 49 th Annual Meeting Lancaster, PA March 24, 2014 Slide 7 Available Geologic Map: Fairly Simple, Right? Only 3 formations, no faults or folds GSA Northeastern Section, 49 th Annual Meeting Lancaster, PA March 24, 2014 Ref.: Berg, T.M., Geologic Map of Pennsylvania, Bureau of Topographic and Geologic Survey, Harrisburg 1980. TRIASSIC UNCONFORMITY TRIASSIC ERODED BEDS CAMBRIAN Slide 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, 49 th Annual Meeting Lancaster, PA March 24, 2014 Slide 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, 49 th Annual Meeting Lancaster, PA March 24, 2014 Slide 10 A Complicated Geologic Picture GSA Northeastern Section, 49 th Annual Meeting Lancaster, PA March 24, 2014 HARPERS PHYLLITE VINTAGE DOLOMITE (BLUE) LEDGER DOLOMITE SCHUYLKILL RIVER BRIDGE 2-SPAN BRIDGE SIMPLE-SPAN BRIDGE OVERTURNED ANTICLINE STOCKTON SANDSTONE LIMESTONE CONGLOMERATE THRUST FAULT NORMAL FAULT RR BRIDGE SCHUYLKILL RIVER Slide 11 Rock Conditions & Bridge Foundations GSA Northeastern Section, 49 th Annual Meeting Lancaster, PA March 24, 2014 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 Slide 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, 49 th Annual Meeting Lancaster, PA March 24, 2014 Slide 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, 49 th Annual Meeting Lancaster, PA March 24, 2014 Slide 14 Conclusions Dont 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, 49 th Annual Meeting Lancaster, PA March 24, 2014