No_4_April_1969 — Page 34

N

Works yard

LAI CHI KOK BAY

To TSUEN WAN ?

Plan

ROAD

13 No. 93'-0" spans

15 No. 63'-0" spans

Existing Ground Level,

New sea-wall

+10°

Imean] sea

level

Existing sea bed

-40°

-80

Assumed line for pile tip levels

L85

LB6

Works yard

462'-0"

:

TO KOWLOON

FIRE STATION

BUS DEPOT

Actual line for pile tip levels

Key:

E

i

Top soil or road surface

Brown silty clay with sand, at san places mixed with brick, rubble, d cayed wood etc.

(Reclamation filling materials)

Dark grey silty clay with shells (Marine deposit)

Brown clayey sand

Yellowish brown and white

silty sand (Decomposed granite) Yellowish grey weathered and fractured granite (Bed rock)

-120

Fig. 2. Site investigation

MBI

Gradient of the main ramp is 1:20 and that of the intermediate ramps 1:14. Where the intermediate ramps reach the deck level the bridge widens to 130ft. and this dimension tapers down over a length of 380ft. to 92ft. the standard width of the marine section (Fig. 3). A longitudinal ex- pansion joint is provided in the wider sections of the bridge.

-

The structure is supported on piled foundations having precast concrete piles on land and prestressed concrete piles in the bay. The superstructure is simply supported on rigid reinforced concrete frames, each consisting of circular columns and an inverted T capping beam.

The deck is of composite beam- and-slab construction. All beams ex- cept those in the intermediate ramps were precast and prestressed before erection. The in-situ concrete deck

slab cast on the beams was screeded to falls and two layers of hot-rolled asphalt provide the final carriageway surface.

Lighting on the bridge is provided by twenty-eight 200 watt linear sodium lamps mounted on double-arm lamp posts 35ft. high spaced at approximate- ly 180ft. centres.

Space is provided in the central divide and under pedestrian pavements to carry utility services across the bay.

Ground conditions and foundations Site investigation

The bridge extends across the entire width of Laichikok Bay about half of which was reclaimed nearly 40 years ago. It was expected that the strata

below the original sea bed would be soft marine mud overlying decomposed granite resting on the parent rock. Site investigation was carried out during investigation was carried out during the early design stage to determine the level of a firm stratum.

Borings confirmed that a firm layer of decomposed granite above the bed rock was overlain by marine mud of variable thickness. The filling in the reclamation consisted of decomposed granite from the Kowloon foothills. The profile of the firm layer (Fig. 2) rose to a peak of about -10.0ft. P.D. (10ft. below Principal Datum) at the eastern end of the original bay, fell sharply to a low of about -90.0ft. P.D. near the existing sea wall and rose evenly to about -20.0ft. P.D. at the west end of the bay. Standard pene- tration (n) values within the firm stratum were generally above 100. Design of foundations

To avoid excessive differential set- tlement, it was decided to support the bridge on piles. An economic analysis of piling systems suitable for the land and marine conditions showed that there was not much to choose between proprietary systems and con- ventional precast piling.

Contractors were given the oppor- tunity to propose different systems but the successful tender incorporated precast piles.

Two sizes of land piles were used 16in. x 16in., each capable of safely carrying 75 tons, supported the shorter spans and 18in. x 18in., each able to carry 100 tons, were used under the longer spans. These piles were cast in

4500/%in. grade concrete and rein- forced with mild steel bars.

All marine piles were designed to carry 100 tons each and were 18in. x 18in. in cross-section. To eliminate troublesome cracking during handling and driving, the piles were cast in 6500/%4in. grade concrete and then prestressed.

Each of thirty-two 0.276in. dia- meter wires per pile was tensioned by a force of 9400 lb. to give a residual stress in the piles of some 700 lb/sq. in. At transfer, the required strength of the concrete was 4500 lb/sq. in.

Pile testing

The specification required that one test pile be driven at each grid position to determine the lengths of the other piles at that grid. A test load of 200 per cent. of the working load was applied to each test pile by means of a hydraulic jack reacting against ex- ternally supported kentledge. The test load was applied in increments to give 50, 75 and 100 per cent. of the total load, each increment remaining undis- turbed for 24 hours before the appli- cation of the next incremental load.

The progressive settlements were measured and recorded accurately throughout the whole period of the test, readings being taken immediately before each change of loading and at about 1⁄4, 1⁄2, 1, 2, 4, 8 and 24 hours after each change. The settlement of the pile was measured by means of dial gauges (accurate to 0.001 in.) on each side of a pile and bearing against fixed datum unaffected by the load- ing of the pile.

32

Far East BUILDER, April 1969