Gura Small Hydro Power Project – Kenya Design notes of Head Race Canal Structures

Ground Supported Canals

VS Consulting ( Pvt) Ltd.

April, 2013

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Design notes of Head Race Canal Structures , Gura SHP Project

1. Types of canal structures used in Head Race Canal

Rectangular open canal Rectangular Box canal Rectangular open canal as Aqua duct

2. Limit state design of Head Race Canal structure

For the Head Race Canal , the Ultimate Limit State ( ULS) design shall be used, with the structure being designed to resists all of with the following general load combinations:

The Loads which act on Head Race Canal structures are ,

1) Internal Water load ( Q1 ) 2) Dead Load ( G) 3) Submerged Soil Pressure ( Q2 ) 4) External Water Pressure ( Q3) 5) Traffic load ( T ) on road crossings 6) Live load surcharge ( L) on service road 7) Soil Surcharge ( Q4 ) 8) Wind load ( W ) on aqua ducts

2.1 Loads Combinations

The loads given above are combined and applied to the structure to generate maximum forces during the service life of the structure.

1. Case - 01 , Head Race Canal ( Cantilever walls) in operation 2. Case - 02 , Head Race Canal ( Cantilever walls) empty 3. Case - 03 , Head Race Canal acts as Box culvert

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4. Case -04, Head Race Canal as Aqua Ducts

1. Load combinations for Case -01

The worst load combination for the vertical internal wall face is generated when the water pressure act on the internal wall and when there is no soil or water load from outside of the wall.

In this case, the Ultimate load to the wall is given when the water level at the top of the wall level, as the canal is free to spill any additional water over the canal walls freely. Hence , taking a Factor Of Safety ( FOS) of 1.0 on the loads for design of cantilever wall for internal water pressure is sufficient.

1.0 Q1 The bearing pressure under the canal should be checked for the canal full operation condition.

The load combination is , 1.0 Q1 + 1.0 G

2. Load combinations for Case – 02

The worst load combination for the vertical wall from external side and on the base slab is generated when the canal is empty. The loads which can act on the external wall are water pressure, soil pressure from the backfilled materials and the traffic load Surcharge acting on the service road.

In this case, the Ultimate load to the wall is given by the load combination as given below

On the wall by the side of service road, 1.2 Q2 + 1.4 Q3+ 1.4 L

On the base, 1.4 Q3 + 1.0 G

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3. Factors of safety

The following FOS was used in checking the stability of the HRC structure.

I. Structure is not buoyant when dewatered fully. Provide a FOS of 1.2 II. Factor of Safety for Gravity Type Walls:

i) Overturning : 2 ii) Sliding: 2.5

III. Factor of Safety Cantilever Type Walls and foundation:

i) Overturning : 1.5 ii) Sliding: 2.0

4. Selection of section thickness of HRC

The main considerations are,

a). Stresses level of the section due to applied forces.

b). Deflection

c). Constructability,

d). Steel reinforcement density

c). Cover to steel.

5. Structural design of the canal section

The canal is considered as water carrying structure and the structural design

should be done according to,

• Steel Reinforcement design of the sections for bending and shear for ultimate limit state as per BS. 8110.

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• Limit of Crack width is 0.2 mm, as given in BS 8007. • Check the section of canal for Crack control, durability and deflection as per BS.8110

and BS 8007. • Uplift, Sliding, bearing pressure and over turning check for the canal

6. Construction Materials & Standards applicable.

Use Gr. 25 (20) concrete with a minimum cement content of 325 kg/m3

for severe exposure conditions.

The constituent materials of the concrete have to be complied with,

1. Natural aggregate of density 2000-2500 kg/m3 - BS. 882, BS. 5328

2. River sand as fine aggregate - BS. 882

3. Ordinary Portland Cement - BS. 12

4. Steel reinforcement - of fy = 460 N/mm2

5. Steel reinforcement - of fy = 250 N/mm2

7. Geology of the canal trace

The HRC on ground is proposed to built on cut ground. The canal which is supported on the ground is having a pressure of 25 to 30 KN/m^2 bearing pressure under the canal base. As the canal is supported on the excavated ground, there is no a problem of exceeding the allowable bearing pressure of the soil under the canal base. Hence, there will be no settlement of the canal in most parts of the canal trace.

However, if soft ground is present necessary ground improvement techniques should be employed to improve the soil to prevent settlement of the canal structure.

Under the foundations of the aqua ducts, the bearing pressure is high and soil investigation should be carried out to determine the safe bearing pressure.

General Manager / Senior Hydro power Engineer

VS Consulting ( Pvt) Ltd.