No_1_May_and_June__1951 — Page 47

ALEXANDRA

BUILDING

Foundation Work

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The Architects' Conception of the Completed Building

Work has commenced in earnest on the superstructure of the new Alexan- dra Building and the steel framing is beginning to rise rapidly from the foundations. It is hoped that the first section of the complete project will be ready for occupation within a year. The length of time it has taken to reach the present stage of construc- tion has been due to the many difficul- ties that were met with and had to be overcome during the excavation for the foundations and the driving of the Vibro piles upon which the entire structure will eventually rest.

Although it was not originally en- visaged, it was found that before pile- driving work could be safely carried out, sheet piles would have to be driven around the perimeter of the building area. The purpose was twofold. The foundations for the new building required excavation work to a depth of 18) ft, below street level extending into the pavements along both Chater Road and Des Voeux Road, thus close to heavy continuous traffic. To protect these streets from settling, steel sheet piling was chosen as a protection around the excavation.

The ground water level at the site varies with the tides, and daily reaches a level of about 7 ft. below street level. In order to be able to construct the foundations this water table had to be lowered by 12 ft.

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during the construction period. make this possible the entire site was surrounded by the steel sheet piling, thus forming a large cofferdam. two way's water could enter this cofferdam. Firstly by leakage through the sheet piling. The type of sheet piling used, with a continuous contact line in the interlocks proved to be practically watertight after a few leaks had been caulked. The second way water can penetrate into the cofferdam Is by underflow, i.e. water finding its way under the sheet piling. To prevent this, the sheet piling must be long enough to penetrate into some im- permeable stratum, or at least into a soil which offers sufficient resistance to the flow of water to make pumping a practicable proposition and to avoid "boils" in the floor of the excavation. These conditions have been success- fully met and pumping to keep the excavation dry from groundwater could be done with intermittent use of a 21" diameter pump.

Kind of sheet piling. The steel sheet piling used on the job was Larssen No. 2, 40 ft. long, supplied by the British Steel Piling Co., Ltd.

Driving. With the exception of the corner piles and some piles near the corners all the sheet piles were driven by a Vibro Pile frame using either a 2 or 24 ton single acting steam hammer. Corner piles, which could

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not be reached by the pile frame were driven by a No. 7 McKiernan-Terry Double Acting Steam Hammer, placed directly on the pile and held in posi- tion by a special frame. This latter hammer was not used for other piles than the corner piles on account of damage done to top of piles due to hard driving required and on account of the noise which this hammering produced

Obstructions. While excavating to remove the foundations of the old Alexandra Building the lower end of an old ramp was unearthed in the corner opposite the Gloucester Hotel. This ramp which was 20 ft. wide, and had rounded corners ended with 2 layer of dressed granite about 3 ft. inside the cofferdam, at about low tide level and was founded on a rubble mound. The bottom of the ramp was removed and rubble taken out to a depth of about 20 ft. below street level before sheet piles were driven. Due to the heavy flow of water it proved impossible to remove all the rubble and it was decided to drive the sheet piles through it. About sixty sheet piles had to penetrate this rubble mound which, according to driving records, varied in thickness from a few