40
curred when re-driving piles already near full penetration and gradually- increasing hammer drops over the first 200 blows were required for such cases; even so it was often found that a set could be achieved with little further penetration. Test loading was used to verify any doubtful driving results (Fig. 4).
Raking piles in lengths of up to 90 ft. and with a maximum rake of one in four, were driven using a vertical pile frame specially adapted by increasing the base area, pivoting the leaders and providing variable length braced tubu- lar back-stays. Each pile was pitched with the leaders in the vertical position and then tilted to the correct raking position. Because of the large base area needed for stability, raking frames were tricky to manoeuvere within con- fined areas; this was particularly so at the Canal Road flyover where a fur- ther complication was introduced by working over the existing canal. In this case careful pre-planning was required, including consultation with the piling- gang foreman who would be respon- sible for the rig, and agreed details were set down on paper for the in- formation of police and other au- thorities.
A number of buildings adjacent to the Canal Road flyover site were in a dilapidated state and to minimise the possible effects of ground shock du- ring driving, a heavy hammer with a small drop was used. This together with shoring proved successful and there was no delay or damage resulting from piling except one instance of plaster being shaken loose inside a particularly dilapidated building.
Obstructions to driving were in- frequent and in such cases another pile was driven as close as possible, the actual position depending upon probing to determine the extent of the obstruction. Obstructions causing piles to move out of position, often when the pile was deep in the ground, were more common and for these cases de- tailed as-driven record drawings were prepared on site and pile-cap details revised by the consulting engineers.
Preloading
Where piling was impossible due to under-ground obstructions such as iso- lated rock or existing seawalls, founda- tions were preloaded with up to 1,400 tons of kentledge. This usually took the form of concrete blocks and re- mained until major settlement, if any, had taken place.
711
BUDBON
MON
BT.
Lifting 70ft. R.C. pile, 18in. x 18in. section prior to driving for Canal Road flyover
In-situ and precast concrete
Falsework
Contractors used either steel or timber proping but other than this all falsework was of timber construction, water-resistant grade plywood being used for wrought surfaces. Falsework designs were prepared by each con- tractor and modified if necessary at the instigation of the resident en- gineer's staff.
All new plywood was pre-coated with at least two applications of an approved timber sealer, polyurethane paint being used extensively with good results. In general the concrete sur- faces produced were of a high standard but darker areas have been noticed particulary on columns; this may have been due to the shutter coating not having cured properly because of in correct application.
Falsework was constructed with
allowance for slight settlement during concrete placing and for deflection of the structure after striking of the false- work. To minimise the possiblity of unequal unequal settlement of falsework, ground level areas were thoroughly compacted by rolling and sleepers of substantial section were laid below each line of props. Each prop sat on a pair of wedges, which were tapped home to correct any settlement dis- closed by check-levels taken during concreting.
Repetitive use of timber falsework for an in-situ structure might be con- sidered out-of-date but in the case of the flyover at Lai Chi Kok interchange this method was selected by the local contractor after consideration of alter- natives and taking account of the labour at his disposal. In the event, this decision proved to be correct since it was no problem to increase at short notice the falsework available on site
Far East BUILDER, January 1971