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Expert: - 11/12/2010


Question
existing ground condition is Sloping from 0.00m to -1.00m in span of 30-35m
Now at 0.00m foundation excavation up to depth of 2.5m is given (F1) and for all footings it has been given the same depth (F2 to F5).
Ground is fairly good and soil underneath consists of mixed  sand clay up to 1.4m and sand mixed gravel after that  however SBC is not tested at site. Overall SBC in the area is noted as 20-30t/m2 at depth not known. We have excavated at F1 location 2.5m and since the ground slopes downwards we have excavated  in decreasing order from the reference point of F1 and following table shows the foundation depths;
F2   2.2m
F3   2m
F4   1.8m
F5   1.5m
Is it right as per norms. Or we have to dig stepped footing. Plinth beam is required or not. Load per footing is 70t. Ground water is -2m from 0.00m.


Answer
Hi Saroj,

If you have been told to set the foundations at 2.5m depth, then I would suggest that you set the others at the same depth from the original ground level- in other words the bases would be at different levels, but maintaining 2.5m from ground level. Alrenatively, if the ground is fairly good, and in order to avoid problems with ground water, you could consider setting all foundations at 2m below the original ground level, but still stepping as you go down the slope?

There is no need for plinth beams unless there is a chance of differential settlement between adjacent bases.  

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Arshad Khan

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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.

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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)

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•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

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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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