January_1965 — Page 21

Split-level Raft Supports New Tower Block

building rate of

six days per storey achieved

t

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USE of a split-level reinforced con-

crete raft foundation instead of piles, and marble slab facings instead of Portland stone are two features of the new $HK10 million Hang Chong Building, Queen's Road, Hong Kong, which make it unique among the towers of the colony's central

area.

The raft design was produced by the consulting structural engineer. Professor S. Mackey, University of Hong Kong. It not only reduced construction time, but made a con- siderable economy on the cost of the structure.

A preliminary inspection of the site and study of the foundation particulars of neighbouring build- ings indicated that, while good bear- ing subsoil might be expected rea- sonably close to the ground sur- face at the Queen's Road end, the depth to good soil would progres- sively increase towards the north end of the building.

The split-level raft eliminated the disadvantage of varying soil strata.

A full site investigation was car- ried out prior to demolition to ex- plore the need for strengthening the underlying soil by grouting pro- cesses. Should grouting have been found necessary this early investiga- tion would have allowed the grouting to be completed in one operation using the weight of the existing building as back pressure rather than in successive incremental stages as new construction pro- gressed.

The keynote of the construction planning operation was time-saving.

All time-consuming factors in design, drafting procedure, investigational work and construction operations were carefully considered and steps taken to ensure delaying factors were reduced to a minimum.

Ingenuity

In the design of the foundation raft consideration was given to the desire to avoid ground-loading-tests, and this involved restricting the maximum foundation pressure to a value acceptable to the Building Authority for the type of soil con- cerned. In this instance it was 3 tons per sq. ft.

Far East Architect & Builder January, 1965

CROSS SECTION

high this involved considerable in- genuity in the raft design to make the centroid of the subsoil reaction coincide with that of the column loads, while still keeping the soil bearing pressure within an upper limit of 3 tons per sq. ft.. and main- taining a raft design which was neither excessively deep nor grossly expensive.

The split-level raft was construct- ed 5 ft. thick which eliminated the necessity for piles allowed for in the original plans (see drawing above). Professor Mackey attributes this saving to early soil investigation, which he claims was a "most im- time-and- portant" factor in the For a building virtually 22-storeys money-saving construction.

Peripheral sheet piling close to an adjoining bank building

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