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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