Civil Engineering/raft foundation

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Expert: - 11/10/2008


Question
How would u construct a raft foundationin in soft clay for a large structure(5 storey)? What are the advantages of raft foundation for large buildings?

Answer
Hi Keris,

The advantages of a raft foundation are that it distributes the loading from a building relatively evenly over the footprint of the building and also minimises differential settlement of the columns.

For your building, you will first need to work out the maximum pressure and ensure that the clay can safely take this without undue settlement. Then you dig down to the formation (if there are no basements, this should be at least 1.5-2 m below ground level depending on local conditions), place a 75mm of blinding/weak concrete, over which you may need to apply waterproofing, over which you put some protection boards, over which you place the rebar, then put up formwork around the perimeter and pour in the concrete.

For thick rafts you will need to control the heat of hydration for about 5-7 days. For a 5-storey building the raft will not be very thick (say 600-900mm), but this depends on punching shear from the heaviest loaded columns.

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Arshad Khan

Expertise

I can answer any questions to do with civil and structural engineering consultancy and construction industry in East Africa and the Middle East, and specifically with the analysis and design of reinforced concrete structures. My particular expertise is in the aseismic design and optimisation of tall buildings.

Experience

Employment history: 36 years in Construction and consultancy in the UK, Kenya, Tanzania, Uganda, Ethiopia, Eritrea, South Africa, Somalia, Zambia, Austria, Bahrain, Kuwait, Doha and the U.A.E.

Organizations
.Fellow of Institution of Structural Engineers (UK) .Fellow of Institution of Civil Engineers(UK) .Member of the Institution of Engineers, Kenya .Registered Engineer, ERB, Kenya .Member of the Architectural Association of Kenya (Engineers Chapter) .Chartered Engineer (UK)

Publications
•1984: International Conference on the Art and Practice of Structural Design, London •1994: 3rd Int. Kerensky Conference in Structural Engineering, Singapore •2008: International Conference on High-Rise Towers, Abu Dhabi

Education/Credentials
BSc, 1st Class Hons, in Building Engineering, University of Bath, UK MSc in Concrete Structures and Technology, University of London. Diploma of Imperial College, UK.

Awards and Honors
•Science Congress Special Award (for 2-seater Hovercraft - 1968) •Institution of Civil Engineers Award for outstanding performance at Bath University (1975) •Concrete Society Postgraduate study Bursary Award (1976) •Consular Representative for British High Commission, Nairobi. (1995 to 1998) •Examiner for Institution of Civil Engineers Professional Interviews, Nairobi. (1997 to 1998) •Branch Representative in Vienna for PI assessment for Inst. of Struct. Engrs. (1999 to date)

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