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