Civil Engineering/weir design

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weir design
weir design  
QUESTION: Dear Sir

A weir is to be constructed across a river for the purpose of store and distribute water for a village.

soil investigation was carried out across the river at the location of the weir.

according to above investigation competent bed rock is at 15-35 ft(varying) depth from the existing water level. river bed level is at 5-10 ft varying depth.

completely to moderately weathered rock layer about10ft was observed throughout the cross section.

due to the above deep weathered rock layer gravity wall anchored to competent bed rock was give up since it is difficult to anchor.


it is proposed to construct insitu piles. they were terminated at river bed level (mean value say 5'ft ) then wall is constructed  upto specified height.

to increase the seepage path cut off walls are to be constructed
upstream and downstream of the river

do u agree with the above proposal
your comments pl

ANSWER: Hi Priyantha,

In principle this looks fine, subject to all calculations, bearing capacities, etc being checked.

---------- FOLLOW-UP ----------

QUESTION: Dear sir
I tried to model above structure using ETABS

Pile ends are taken as fixed.
1)is it possible to get slab on river bed as on spring supports
2) is it required to apply spring supports along pile shaft to simulate soil strata
3) I consider following loads
    * self weight
   * water pressure (static force multiply by 1.5 to cater dynamic loads and impact loads
   * soil pressure below the river bed (wall is above the river bed only piles are below) is it required to consider it
   * upthrust on weir

Can u comment on above pl

Answer
Hi Priyantha,

It is difficult for me to give proper advice as there could be various local factors that influence the design. However, in principle:

1. Depending on how compressible the river bed is, yes you can incorporate soil springs. If it is fairly hard rock, there is no need.

2. Yes, you should use spring supports for piles- again it depends on what the geotechnical investigation report recommends.

3. Upthrust on weir may be relevant if the cut-off wall has leakage, leading to possible pressure differential between the higher water behind the weir compared with downstream.

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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.

Experience

Employment history: 40 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 •2013: IEK International Conference, Kisumu, Kenya

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 2010)

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