Presentation on theme: "Project details • New headquarters for HKSB"— Presentation transcript:
1Hong Kong Shanghai Bank Building – geotechnical aspects of sub-structure construction
2Project details • New headquarters for HKSB – Wanted finest building in the world• HKSB had occupied site since mid-19th Century• Cost= HK$5.4 billion• Design team proposed– 47 storey steel-framed superstructure standing 180m above ground level– 4 level basement varying from 16 m to 20 m deep, 54m by 70 m in plan– Sea water intake to provide sea water for cooling.
4Ground Conditions • Up to 7 m of loose fill • Sandy marine deposits up to 4 m thick – absenton south side• Completely decomposed granite (CDG)– Ranges in thickness from 15 to 27 m– Top 6 to 9 m has weathered to medium dense silty sand• Granite bedrock - highly undulating surface• Ground water level 2 to 3 m below ground.
5CDG – Completely Decomposed Granite • Reddish Brown in colour.• Originally hard granite rock but has been decomposed by kaolinization.Alkalies have been washed out of the rock leaving behind quartz mica and clay.It is not a weathered rock, but is formed by this chemical change.
13Typical Ground Movements Due To Ground Excavation
14Potential Sources of Ground Movements • Demolition of existing basement• Dewatering for wall excavation• Wall excavation• Caisson excavation• Access caissons• Basement dewatering• Basement excavation
15Alternative engineering approaches Soldier piles supported by soil anchorsDrill wells to lower water table or floodingInterlocking sheet piles to form cofferdamStruts and soil anchors often required for deeper excavationsDiaphragm (slurry) wallsSlurry pumped in during excavation then displaced by concrete upon completion
16Solution adopted Diaphragm Wall Water table outside the site only requires only slight draw downCan go to deep levelsPossible to control differential head between bentonite slurry and adjacent ground water levelUsed measurements from basement construction in similar ground conditionsHigh level of monitoringsettlements and ground water level
23Outcome Measured settlements generally much less than estimated Believed to be due to very good control of differential head between bentonite slurryand adjacent ground water levelGround movements during excavation about 60 to 70% of those predictedCDG stiffer than expectedChartered Bank and Bank of China settled a maximum of 15 mm with a maximum tilt of 1:2000.No distress or damage to any buildingMeasured Vs Predicted Settlement
24Lessons learnedThe most difficult aspect of basement construction in an urban environment is keeping ground movements within acceptable limitsWhen carefully calibrated using measurements obtained from basement construction in similar ground conditions, simple models can produce accurate estimates of ground movements as a result of complex construction schemes• Instrumentation monitoring during construction is crucial to allow early detection of problems and to provide improved understanding of the effects of instrumentation