Slope Stabilization Grandad Bluff Scenic Overlook La

Slope Stabilization Grandad Bluff Scenic Overlook La

Slope Stabilization
Grandad Bluff Scenic Overlook
La Crosse, Wisconsin
Bruce Weber, P.E., Sr. Consultant,
Brierley Associates, LLC
Robb Johnson, P.E., Vice President,
Engineering and Construction Innovations, Inc.
Site Location
Project
Location
http//mapper.acme.com/?ll=43.80146,91.22086&z=14&t=R
Physical Settings
Site Plan with Boring Locations
Problem Statement
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Serious bluff erosion
Limitations on site access
Safety concerns
Closure of the Overlook
Stabilize the bluff and enhance the site
Existing Conditions
Subsurface Conditions
Bidding Design
Challenges
• Site access
• Limited geotechnical data
• ? rock quality and depth
Bidding Requirements
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Contractor responsible for final design
Review and approval by Owner
Firm lump sum price
Extended warranty
Options
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Use preliminary soil nail design concept
Drilled soldier piles and treated wood lagging
Drilled tangent piles
Steel cased micropiles with shotcrete facing
Selected the last option based upon cost and
performed addition geotechnical investigation
Geotechnical Design Parameters
• Soil
– Unit weight
– Angle of internal friction
– Cohesion
130 pcf
26 degrees
0
• Competent Rock
– Unit weight
– Angle of internal friction
– Cohesion
148 pcf
0
8000 psf
• Rock
– grout bond stress
60 psi
Lateral Pressure Distribution
Design Theory
• Micropiles act as a simple beam pinned @
top/bottom
• Shotcrete facing spans between the micopiles
• Cap beam supported by micropiles and spans
horizontally between the anchors
Final Design Detail
Construction Sequence
1.
2.
3.
4.
5.
6.
Drill micropiles
Prepare the face
Place drain board and welded wire fabric
Spray shotcrete facing
Form and pour the concrete cap beam
Drill and tension the tie-back anchors
Construction Photos
Construction Issues
• Loss of rock socket during surface preparation
• Field welding shear studs on high strength
micropile casing
• Variation of rock depth in anchor bond zone
Anchor Proof Tests
• All tie-back anchors proof tested to 1.33 x
D.L. per FHWA guidelines
• Acceptance criteria less than 0.04-inch
creep in 10 minutes @ proof load
• All anchors met acceptance criteria
Lessons Learned
• More comprehensive geotechnical investigation
would have minimized uncertainties
• Most soil: structure interaction problems have more
than one solution
• The design-build approach consolidates
responsibility and minimizes differing site conditions
claims
• Good communications with the Owner and Owner’s
representative can effectively resolve issues and
minimize delays
Completed Project
Photos
Acknowledgments
• Owner: City of La Crosse, WI; Greg Kozelek, City Engineer
• Owner’s Representative: River Architects, La Crosse, WI
• Structural Consultant: Klein Engineering,
Whitefish Bay, WI
• Specialty Subcontractor: Midwest Drilled Foundations &
Engineering, Waukesha, WI