During 1987, the Principal of the School asked me to assist designing a 2-storey extension block at the foothill of the small hill, which was the site of the old library. The new block would have six classrooms.
Being old student, it was my moral duty to help for providing me education.
Some excavation of soil was required to bring to the same level of the old block. This excavation varies from 0m to 2m. Was piling necessary for the foundation? This would require higher cost which the School was lacked of. Of course, the safety of the students was my main concern.
A simple Soil Investigation using Mackintosh Probe Tests were carried out. It revealed shallow firm to stiff residual soils up to 4m depth across the building with some variations. A bearing capacity of 150KN per sq. meter could be used at 1.0 m below the intended ground level. Spread footings or strip footings were all viable. The recommended footing location was 1.5m below ground level to reduce the effect of water level fluctuations.
Normally, the ground floor has ground beams tie to the columns and these beams were 230 wide x 600-750mm depth, with 300mm column stumps above the footings.
I decided to use the strip footings by providing 200-300mm thick base slab combined with 750mm ground beams longitudinally thus having inverted T-shape Footings with a total depth of 1050mm below designed ground level. This would help to stiffen the structure to distribute some variations of soils and fluctuation of water levels, at the same time cut down the reinforcements of the ground beams and foundation. Shallower foundation also meaned easier construction.
Cantilever beams and expansion joints were provided to join to the old blocks. It was expected there would be a little movement between the old and new block.
The extension building was completed in 1989 and worked well in accordance to the design intention at the minimum cost.
The strip footing looks like this picture from another project, but was not flooded.
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