February_1967 — Page 8

ject to certain modifications which would not affect the cost of the work. The revised structure measuring a total length of 321 feet was designed using precast. post-tensioned concrete I beams each 106-10′′ long but with a cast-in-situ deck slab to form the of composite structure. Five rows lateral diaphrams were provided be- tween the beams in each span and post-tensioned to provide the lateral 13"-9" wide stiffness required. A

water main duct was designed to be independent of the main carriageway and to be covered with precast pre- stressed concrete channel units. The duct was constructed on the down- stream side of the structure and would later provide the central re- serve when the bridge is widened to form a future dual carriageway.

The bridge abutments and two piers were constructed in reinforced concrete supported on 14" square precast prestressed concrete piles.

Works Site

The contractor was instructed to start work on the 28th May 1964 and it was necessary for him to form his works area and precasting yards above flood level at an early stage. The material to be dredged from the river bed was found suitable for part of the filling and a borrow pit was opened on the Tai Po Road some 2 miles from the site.

Beam casting yards were set out on either side of the river on compacted filling that would

General view of bridge after deck slab was cast

loose in Tolo Harbour during typhoon 'Ida' and was blown into the works

considerable causing area damage to the contractor's temporary works and staging. Due also to cer- tain later modifications incorporated in the design of the superstructure the project was not completed until 22 November, 1966.

Piling

tractor to later form the

bridge approach embankments.

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A heavy timber piled trestleway was erected across the river slightly one side of the bridgeworks to facilitate constructional access and this was used initially for movement of the pile frames and was later strengthened for transporting the pre- cast beams into position.

Cofferdams with puddled clay backing were constructed to enable abutment and pier foundations to be concreted in the dry.

Although the contract period was 480 days the work was delayed by adverse weather conditions particular- ly during the typhoon season in 1964. In August 1964 a steel lighter broke

use

Approval was given to the con- 14" square precast. prestressed concrete piles in lengths varying from 50 ft. to 55 ft. which were fitted with conventional cast iron shoes. Piles were cast in the contractor's prestressing yard at Yau Tong Bay and transported to site by road. The long-line method of pre- tensioning with high tensile seven-wire strands of %" diam. was adopted and concrete controlled to give a 1:4.5 mix. (by weight) with 0.42 water/cement ratio including the moisture content in the fine aggregate.

The pre-tensioning method was divided into low and full tensioning stages. During tensioning, the locator plates, which were distributed throughout the whole length of the

Anchor blocks and stressing wires in precasting yard

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bed, were closely checked in order to ensure that all strands were maintain- ed in the proper position during con- creting. Tests on the moisture con- tent of the fine aggregate were car- ried out before actual mixing of the concrete and the amount of added water adjusted to compensate for any observed variation.

After concreting, the piles were cured by covering with a layer of sacking and kept constantly wet for at least seven days. Strands were released as soon as the results of test cubes taken gave 5,000 lb. per sq. in.

The length of the casting bed was 183 feet and three piles could there- fore be cast in each line. By using three beds and reuseable steel form- work the contractor could complete an average of two and half piles per day.

The two-point lifting method was adopted for transporting, handling and pitching for all piles. The pile driving was carried out with a 65 ft. high lattice steel piling frame which could be adjusted to give a rake of 1:3 and was fitted with a diesel driven twin drum winch.

Piles averaging five tons were driven using a 5.2 ton drop hammer with the free drop up to a maximum of 4 ft. 6 in. until the final set was obtain- ed. Despite heavy driving no hair. cracks or spalling of the pile heads occurred. The piling work was com- pleted in January 1966.

Of a total of 161 piles driven for the foundations, only one pile was driven out of the designed position and was later replaced. This was due to an unknown obstruction being met during driving. A test load equal to twice the working load was carried out on one pile selected arbitrarily. This load was sustained for a period of 48 hours with satisfactory results.

Pilecaps

Cofferdams measuring 88 ft. long by 11 ft. wide for the two pier pile- caps were started as soon as the pile driving for each pier was completed.

Far East Architect & Builder February, 1967