Building a High Rise Adjacent to Transit Tunnels - 02 Simpson
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Transcript of Building a High Rise Adjacent to Transit Tunnels - 02 Simpson
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BUILDING A HIGH-RISE ADJACENT TO TRANSIT TUNNELS IN SAN FRANCISCO
Lori A. Simpson, PE, GE
Treadwell & Rollo, a Langan company
Outline
• Project Description• Site Conditions• Issues & Constraints• Solution• Analysis• Construction• Conclusions
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Site Vicinity Map
Site Location
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Tower Plan
Tower ElevationNorth & South Tower
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Tower ElevationSouth Tower and Midrise
BART
MUNI
Basement Section
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Subsurface Exploration Plan
Stratigraphy
• Fill • Dune Sand• Marsh Deposits• Colma Formation
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Stratigraphy• Fill & Dune Sand
• Loose to medium dense sand upper 10’• Medium dense to dense sand below 10’• 26 to 34 feet thick• Uniformly graded• Liquefiable below GW
• Marsh Deposits• Loose to medium dense clayey/silty sand
• liquefiable• Medium stiff sandy clay
• compressible• 5 to 7 feet thick
Stratigraphy• Colma Formation
• Dense to very dense sand with silt/clay• Strong• Relatively incompressible
• Groundwater• Approximately 20 feet below ground
surface
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Cross section
Bay Area Fault MapSan Andreas Fault• 11 km from site• 7.9 mean characteristic
moment magnitude• 21% probability of
magnitude 6.7 or greater in just under 30 years
Hayward Fault• 18 km from site• 7.3 mean characteristic
moment magnitude• 27% probability of
magnitude 6.7 or greater in just under 30 years
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Seismic Hazards
• Strong Ground Shaking• DBE PGA = 0.49g• MCE PGA = 0.59g
• Cyclic Densification• 1/4” settlement
• Liquefaction• 2 to 3” settlement• Loss of bearing
BART Guidelines• The BART zone of influence is defined as the area above a line from subway invert
at a slope of 1-1/2 horizontal to 1 vertical.• Soil redistribution caused by temporary shoring or permanent foundation systems
shall be analyzed.• Shoring shall be required to maintain an at-rest soil condition; shoring structure
shall be monitored for movement.• Minimum predrilled depth for piles shall be approximately 10 feet below the line of
influence.• Vibration monitoring of piling operations closest to the subway will be required;
piles to be driven in a sequence away from the subway structure.• Tunnels, where affected, shall be monitored for movement and deformation caused
by adjacent construction activities to ensure structural and operation safety.• Dewatering shall be monitored for changes in groundwater level; a recharge
program will be required if existing groundwater level is expected to drop more than two feet.
• Where basements are excavated, the amount of loading (on subway) can be increased to the extent it is balanced by the weight of the removed material (120 pounds per cubic foot for dry soil, and 70 pounds per cubic foot of submerged soil); however, the effect of soil rebound in such cases shall be fully analyzed.
• All structures shall be designed as not to impose any temporary or permanent adverse effects, including unbalanced loading and seismic loading, on the adjacent BART subways.
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Issues & ConstraintsPoor soil within 33 to 39’
Cyclic densification
Liquefaction
No additional load on BART
Maintain GW within 2’ at BART
Limit vibrations on BART
Support loads in Colma formation
Ground Improvement
Ground Improvement
Extend loads below BART ZOI
Cutoff wall for the deep basement
No driven piles
Solution
Soil-Cement Mixing (SCM)• Mitigate liquefaction & cyclic densification• Transfer load to Colma formation• Deepen in BART ZOI• No vibrations• Use as shoring• Cutoff wall
Support building on a mat bearing on SCM
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Soil Cement Mixing Requirements• Liquefaction Mitigation
• 30% replacement ratio• Install in a grid to create cells
• Building Support• Extend at least 2’ into Colma formation • 400 psi minimum compressive strength• 550 psi under the tallest tower• 50% replacement ratio under tallest tower
• BART Requirements• Extend at least 2’ below the BART ZOI• Cut off groundwater at the deep basement
Peer Review Panel & BART Review
Concerns by review panels:• Pressures on BART tunnel• Pressures on basement wall during MCE• Compressive stresses in SCM elements under MCE
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FLAC Analysis
• 2-D Finite Difference Analysis• Mohr-Coulomb soil model• Analyzed existing, gravity, and MCE load conditions
MCE Load – North Tower
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FLAC Set Up
FLAC Stress – Existing Loading
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FLAC Stress – MCE Loading
FLAC Results
BART Tunnel• No increase in stress under gravity load• 4% increase in stress under MCE load
SCM Stress• 285 psi under MCE load• Design for 750 psi
Basement wall loads• Decrease in upper 2/3 of wall• Increase in lower 1/3 of wall
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Revised Basement Wall Pressures
Construction Observation• Tracked changing mix design• Obtained 2 to 4 samples of SCM slurry per shift• Confirmed depth of SCM panels• Confirmed consistency of installation method• Obtained core samples in representative panels to
evaluate low strength results
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Conclusions• Complicated project
• varying building heights• varying basement levels• BART & MUNI tunnels• poor soil conditions
• Peer review was helpful • confirmed project design satisfied requirements• reviewed results of loading analyses• provided input on SCM panel requirements• recommended performing FLAC analysis
Conclusions• Construction observation important
• tracked the mixes and their strength• took sufficient samples to verify strength• obtained cores when needed to address
low strength
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Thank You