Foundation Design and Construction in Northwest NT and Ma On Shan
Superficial deposits overlying metamorphosed sedimentary strata (siltstones, sandstones and marbles) as well as igneous rocks were discovered in the northwest New Territories area in 1987 and in Ma On Shan in 1989. The marble usually has a karstic upper surface with solution features. Large cavities are present within the marble in some locations. In the investigation, design and construction of projects, which could involve ground investigation, excavation, foundation or groundwater pumping works, significant difficulties may be encountered as a result of such ground conditions.
Geotechnical control of foundation works in these areas starts with auditing of the ground investigation (GI) works. To ensure adequate information for foundation design, it is required to drill at least 20m into sound marble if marble is encountered. The GI works are required to be supervised full-time by a suitably experienced person, and periodically by a qualified geotechnical engineer responsible for the foundation design.
The foundation design is required to be supported by a geotechnical assessment prepared by a qualified geotechnical engineer. The assessment interprets the GI results, discusses the geology and karst morphology of the site, and justifies the foundation design proposed.
The foundation works are required to be supervised by a suitably experienced person full-time on site and a qualified geotechnical engineer, similar to the requirement for GI works. The supervising engineer is required to carry out a performance review of the as-built foundation works. The review needs to cover the ground conditions encountered during pile installation and an assessment of pile construction records and pile load tests.
For developments exceeding about 20 storeys and founded on marble with cavities, settlement monitoring of the building at monthly or close intervals is also required during and after building development.