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

2. 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.

3. General Ground Conditions

4. 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.

5. 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.

6. Soil Conditions

7. Soil Conditions

8. Soil Stiffness

9. SPT N-value vs depth-DVR

10. SPT N-value vs depth-QRC

11. 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

12. Site plan

13. Typical Ground Movements Due To Ground Excavation

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

15. 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

16. 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

17. Clamshell for diaphragm wall construction

18. Diaphragm wall construction 1

19. Diaphragm Wall construction 2

20. Diaphragm Wall Construction 3

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

22. Monitoring
Ground water levels
Settlement

23. 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

24. 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