Hong Kong Shanghai Bank Building – geotechnical aspects of sub-structure construction

Project 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, 54 m by 70 m in plan – Sea water intake to provide sea water for cooling.

General Ground Conditions

Ground Conditions • Up to 7 m of loose fill • Sandy marine deposits up to 4 m thick – absent on 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.

CDG – 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.

Soil Conditions

Soil Conditions

Soil Stiffness

SPT N-value vs depth-DVR

SPT N-value vs depth-QRC

Design Objectives • Limit ground movements to prevent damage to adjacent structures and services • Develop a construction sequence that would be as fast and as cost-effective as possible

Site plan

Typical Ground Movements Due To Ground Excavation

Potential Sources of Ground Movements • Demolition of existing basement • Dewatering for wall excavation • Wall excavation • Caisson excavation • Access caissons • Basement dewatering • Basement excavation

Alternative engineering approaches Soldier piles supported by soil anchors Drill wells to lower water table or flooding Interlocking sheet piles to form cofferdam Struts and soil anchors often required for deeper excavations Diaphragm (slurry) walls Slurry pumped in during excavation then displaced by concrete upon completion

Solution adopted Diaphragm Wall Water table outside the site only requires only slight draw down Can go to deep levels Possible to control differential head between bentonite slurry and adjacent ground water level Used measurements from basement construction in similar ground conditions High level of monitoring settlements and ground water level

Clamshell for diaphragm wall construction

Diaphragm wall construction 1

Diaphragm Wall construction 2

Diaphragm Wall Construction 3

Construction sequence •Demolition •Diaphragm wall installation •Caisson foundations • Installation of temporary ground level propping system •Excavation and slab construction •Rock anchors

Monitoring Ground water levels Settlement

Outcome Measured settlements generally much less than estimated Believed to be due to very good control of differential head between bentonite slurryand adjacent ground water level Ground movements during excavation about 60 to 70% of those predicted CDG stiffer than expected Chartered Bank and Bank of China settled a maximum of 15 mm with a maximum tilt of 1:2000. No distress or damage to any building Measured Vs Predicted Settlement

Lessons learned The most difficult aspect of basement construction in an urban environment is keeping ground movements within acceptable limits When 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