Foundation Studies, Analyses and Design in a Highly Fractured Aquaferous Zone
Department of Civil Engineering, Kaduna Polytechnic, Kaduna, Nigeria.
Foundation studies, analyses and design work for a proposed office development in the central area of the Federal capital city (FCC) of Nigeria is presented herein. Extensive field works, laboratory tests and analyses as well as analyses of the deduced data were involved in the work. Specifically, a total of three ( 3 Nos.) rotary drillings with rock coring was carried out in addition to twenty (20 Nos.) mechanised percussion drilling with standard penetration test (SPT). The former were done up to 15- 19.5m while the latter were carried out to maximum depths of between 8.15 – 9.65m. The materials proved from both sets of borings include silty sand, sandy silt, laterite, lateritic clay and concretionary laterite. Weathered and fresh basement rock were recovered between 9 – 19.5m. Geophysical survey results show extensive fractures which are water bearing. Consequently, water table was high (between 0.9 – 2.1m) compared to the proposed foundation level of 2.1m. In order to take care of both the bearing pressure ( 48 – 400 kN/m2 , using a factor of safety of 2.5) Raft foundation with ground beams were proposed to be placed on boulders. The boulders are to serve as drainage medium for the underlying water.
Keywords: foundation, bearing capacity, standard penetration test (SPT), water table, aquifer, weathered basement
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