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
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
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
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, 49th Annual Meeting Lancaster, PA March 24, 2014
NEW BRIDGE
1952 BRIDGE
RR BR.
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
And There are Many Sinkholes in King of Prussia…
GSA Northeastern Section, 49th Annual Meeting Lancaster, PA March 24, 2014
Available Geologic Map: Fairly Simple, Right?
Only 3 formations, no faults or folds
GSA Northeastern Section, 49th 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
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
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
A Complicated Geologic Picture
GSA Northeastern Section, 49th 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
STOCKTONSANDSTONE
LIMESTONE CONGLOMERATE
THRUST FAULT
NORMAL FAULT
NO
RM
AL F
AU
LT
RR BRIDGE
SCHU
YLKILL
RIVER
Rock Conditions & Bridge Foundations
GSA Northeastern Section, 49th 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
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
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
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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