DESIGN OF IRRIGATION UNDER TUNNEL

PROJECT GUIDE: BY:SOWJANYA MAM V.PRIYA REDDY (1602-12-732-032)

Introduction of aqueductsBasic data required for design of UT.

TOPICS COVERED:

DESIGN CONSIDERATION: Shape Materials Type of UT Layout Barrel Drainage wings Carrier channel section and its fluming Transitions Joints Limiting velocity and loss of head

SHAPE:Precast RCC pipes are economical for syphoning small drainage discharges.For large drainage discharges, circular, square, rectangular or horse shaped

barrels are suitable. MATERIALS:For the construction of the syphon barrels, prestressed concrete, RCC or masonry

or a combination of these may be used depending upon the availability of materials, labour and relative economy.

Type of UT: In barrel type of UT, the entire carrier channel portion is taken as it is over the

barrel or it may be flumed to rectangular or trapezoidal section to reduce the length of barrels.

In trough type, carrier channel water is taken across drainage channel through a trough supported on abutments raised from drainage.

LAYOUT:The layout of UT shall be so fixed that the drainage channel crosses the carrier

channel preferably at right angles.

BARREL: At the outlet end of the UT, the top of the barrel may be kept slightly depressed

below the normal downstream flood level in the drainage channel. The amount of this depression may be equal to the difference In the velocity head at

the exit end of the barrel and that In the drainage channel on the downstream side. In case of barrel of RCC box type or RCC slab with masonry walls, a minimum

cushion of 300mm may be provided with the precaution that heavy vehicles do not ply over the barrels unless the cover is suitably increased and the structure is properly designed.

In case of RCC pipes and circular barrels, a minimum cushion of 900 mm should be provided. This Will protect the pipes and barrels against damage by the movement of construction equipment over them and also permits any future regarding of the carrier.

At the site, the drainage bed is generally depressed and provided With PUCCA floor On the upstream Side, the drainage bed is joined to the PUCCA floor by a vertical drop (when drop is of the order of 1 m or less) or by a glacis of 3:1(when drop is more then 1m).The downstream rising slope should not be steeper than 5:1.

The barrels under the carrier channel bed portion are generally lower than the barrels at the entry and exit. This difference is negotiated with the provision of sloping length of barrels under carrier channel banks.

DRAINAGE WINGS:The wings shall be provided straight or in a smooth curve giving a minimum

splay of 2:1 on upstream and 3:1 on downstream.The wing wall sections are checked for carrier channel full and drainage

channel dry condition, considering backfill as saturated. If the foundation wing wall requires to be taken deeper than 15m from

consideration of scour, a concrete cut-off of required depth is provided along the upstream face of it.

CARRIER CHANNEL SECTION: It should have adequate provisions to minimize the seepage.The outer slope of bank should have a clear cover of 600 to 900 mm over the

designed phreatic line.High banks (>6m height) should be checked for slope stability.FLUMING:The Contraction in the waterway of the canal will reduce the length of barrels or the

width of the aqueduct. This is to produce economy in many cases. The fluming of canal is not done when the canal section is in earthen banks.

The width at mid depth of canal is taken as 100% for the calculation of fluming ratioFluming ratio should not be less than 70%.

TRANSITIONS:The flumed portion is joined with the normal section with proper transitions to

minimize the head loss at entry and exit.The transition should corresponds to a minimum splay of 2:1 on upstream side

and 3:1 on downstream side. It should be ensured that the flow follows the boundaries of the wings.

JOINTS: In case of RCC barrels, joints with water stops are provided across and along

the barrel length.The maximum spacing of these joints in either direction is 20m. In case of multi barrel syphons, units of 3 or 4 barrels can be adopted side by

side with longitudinal joints between the units having water stops all around.

LIMITING VELOCITY:The vertical slope on approaches should not be steeper than 1 in 3 on the entry

side and 1 in 4 on the exit side.Maximum permissible velocities

S.NO TYPE OF FLOORS MAXIMUM PEMISSIBLEVELOCITY in m/sec

1 Metals face(steel & cast iron lined) 10 2 Face of concrete grade M30& above 6 -grade below M30 4 3 Stone masonry face with cement pointing 3 4 Stone masonry face with cement plaster 4 5 Brick masonry face with cement plaster 2.56 Brick masonry face with cement pointing 2 7 Hard rock 4 8 murum 15.2 9 Soil silt 7.1

LOSS OF HEAD:The total loss of head H = h1+h2+h3+h4

where, h1= losses at the inlet & outlet (for syphon),h2= losses at elbows or bends (for barrel),h3= losses due to transitions (other than syphon),h4= losses due to skin friction (for barrel & trough).