Design of the Main Diversion Structure of a Barrage (Part - 5) - Civil Engineering (CE) PDF Download

Protection works 

Just upstream and downstream of the solid floor of the spillway apron, the river bed is protected by certain methods like block protection, loose stone apron, etc. as may be seen from Figure 17 showing a typical section of  spillway of a barrage. These protection works are discussed below: 

FIGURE 17. Section through a typical barrage spillway

Upstream block protection 

Just beyond the upstream impervious floor, pervious protection comprising of cement concrete blocks of adequate size laid over loose stone shall have to be provided. The cement concrete blocks of size around 1.5m x 1.5m x 0.9m are generally used for barrages in alluvial rivers. The length of the upstream block protection may be kept equal to a length D, that is, the design depth of scour below the floor level (Figure 18). 

Figure 13. Upstream Block Protection

Downstream block protection 
Pervious block protection is provided just beyond the down stream impervious floor. It comprises of cement concrete blocks of size 1.5m x 1.5m x 0.9m laid with gaps of 75mm width and packed with gravel. The downstream block protection is laid on a graded inverted filter designed to prevent the uplift of fine sand particles upwards due to seepage forces. The filter should roughly conform to the following design criteria. 

1. d₁₅ of filter / d₁₅ of foundation ≥ 4 ≥ d₁₅ of filter/d₈₅of foundation

Where d₁₅ and d₈₅ represent grain sizes. d_x is the size such that x% of the soil grains are smaller than that particle size. Where x may be 15 or 85 percent. 

2. The filter may be provided in two or more layers. The grain size curves of the filter    layers and the base material has to be nearly parallel. 

The length of the downstream block protection has to be approximately equal to 1.5D, where D is the design depth of cover below the floor level. Where this length is substantial, the block protection with inverted filter may be provided in part of the length and block protection only with loose stone spawls in the remaining length as shown in Figure 19.

Figure 19. Downstream Bolck Protection

Loose stone protection  

Beyond the block protection on the upstream and down streams of a barrage located on alluvial foundation, a layer of loose boulders or stones have to be laid, as shown in Figure 20(a). The boulder size should be at least 0.3m and should not weigh less than 40kg. This layer of boulders are expected to fall below at an  angle , or launch , when the riverbed down stream starts getting scoured at the commencement of a heavy flood [Figure 20(b)]. The length of river bed that has to be protected with loose stone blocks shall be around 1.5D, where D is the depth of scour below average riverbed. 

FIGURE 20. Section through downstream protection : (a) After initial laying ; (b) After scour of downstream nverbed due to passage of flood.

It may be mentioned that the loose stone protection shall have to be laid not only down stream of the barrage floor, but all along the base of guide bunds, flank walls, abutment walls, divide walls, under sluice tunnels, as may be observed from the typical layout of a barrage given in Figure 21. 

Figure 21. Typical Layout of a Barrage and Its Appurtenant Structures

Once the basic dimensions are fixed for all the barrage component, they are designed structurally, considering the forces estimated from the hydraulic analysis, The Bureau of Indian Standards code IS: 11130- 1984 “Criteria for structural design of barrages and weirs” specifies the recommendations in this regard, the important ones of which have been discussed below. 

As the major portion of the barrage structure including the raft floor, piers, divide walls, under sluice tunnels, etc. is constructed as reinforced concrete structures, accordingly the general principles specified in IS: 456- 2000 “Code of practice for plain and reinforced concrete” shall have to be followed. Since most of the construction is likely to remain underwater, the minimum cover may be kept at 50mm for safety. Some other items, notably sheet piles, gates, gate groves, etc. have to be made of structural steel made, conforming to relevant Bureau of Indian Standards specifications. The important components of a barrage are discussed below with the specific structural requirements.

Cut-off (Sheet-pile) 

The upstream and downstream cut-offs of a diversion structure may be steel sheet-piles anchored to the barrage floor by means of R.C.C. caps, or may be built of masonry or reinforced concrete. The sheet pile cut-offs are to be designed as sheet pile retaining walls anchored at the top. They shall be designed to resist the worst combination of forces and movements considering possible scour on the outer side, earth pressure and surcharge due to floor loads on the inner side, differential hydrostatic pressure computed on the basis of the percentage of pressure of seepage below floor etc. In case the effect of cut-offs is taken into account for resistance against forward sliding of the structure, the cut-offs shall also be designed to withstand the passive pressures developed. The R.C.C. pile caps shall be designed to transmit the forces and moments acting on the steel sheet piles to the barrage floor. 

Impervious floor (also called solid apron)

Generally there are two types of floors, the first being called the Gravity type and the second as the Raft type. In the former, the uplift pressure is balanced by the self weight of the floor only considering unit length of the floor, where as the latter considers the uplift pressure to be balanced not only by the floor but also the piers and other superimposed dead loads considering a span as a unit. Contemporary designs of barrages have also been of the raft- type, and hence this type of construction is recommended and discussed in this session. 

The thickness of the impervious floor shall be adequate to counter balance the uplift pressure at the point under consideration. The thickness of the downstream floor (cistern) shall also have to be checked under hydraulic jump conditions, in which case the net vertical force on the floor is to be found out from the difference of the vertical uplift due to sub-surface flow and the weight of water column at any point from above due to the flowing water. 

The design of the raft has to be done using the theory of beams on elastic foundations and the following forces, or their worst combination has to be taken:

Beyond the block protection on the upstream and down streams of a barrage located on alluvial foundation, a layer of loose boulders or stones have to be laid, as shown in Figure 20(a). The boulder size should be at least 0.3m and should not weigh less than 40kg. This layer of boulders are expected to fall below at an  angle , or launch , when the riverbed down stream starts getting scoured at the commencement of a heavy flood [Figure 20(b)]. The length of river bed that has to be protected with loose stone blocks shall be around 1.5D, where D is the depth of scour below average riverbed. 

Structural design of barrage components 

• Buoyancy
• Wind forces
• Forces due to water current
• Differential hydrostatic pressure with the gate of one bay open and the adjacent bay closed
• Seismic forces, if any
• Hydro dynamic forces due to seismic conditions

A  pier  shall  have  to  be  designed  as  a reinforced  concrete  column  and  IS:456-2000  may  be  followed  accordingly.

For  the  design  of  other  components  of  a  barrage  project,  like  Divide walls, Abutments,  Flank walls, Return walls, etc., IS: 11130-1984 may be followed.

Construction of concrete barrages

Barrages  are nowadays made  of  reinforced  concrete  and  designed  as  raft  type  structures  which are  light  in  weight  compared  to  storage  dams (designed as gravity-type structures). The  design  of  barrages  is  done  by  accepting  some  calculated  risks  and  hence  it  is  important  that  the  construction  of  such  a  structure is done with  great  care  and  there  is  no  room  for  construction  failure  to  occur. In  this  section,  the  important  steps  for  a  careful  construction  of  barrage  is  explained  and further  details  may  be  had  from  Bureau  of  Indian  Standards  Code  IS:11150-1993 “Construction   of  concrete  barrages – code  of  practice”.

Data required for construction activities 

For  planning  and  execution  of  construction  activities, a  number  of  data  is  required,  most  of  which  would  be  available  from  the  design reports. These include:

• Index map of the site
• Contour plan of the area
• Cross-sections of the river
• Bore-hole log charts
• Permeability coefficients
• Rainfall data of the location
• Flood discharges, minimum and maximum water levels
• Location and accessibility of quarry areas for coarse and fine aggregates
• Working drawings of barrage and appurtenant structures
• Sequence of construction of various blocks comprising of number of bays and abutments, etc.
• Requirements of inter-dependence of various items
• Necessary precautions to be taken
• Special features of construction, if any Construction planning 

Construction planning  

The  construction  planning  for  any  structure  can  be  broadly  classified  under  two  heads: a) Infrastructure planning,  and  b) Procurement planning  and  is  applicable  to barrage  construction  also. It  also  includes  the  finalization  of  a  programme  of  works  and,  intermediate  review  of  the  programme  vis-à-vis  the  actual  work  going  on in  the  field. These points are briefly explained below.

Infrastructure planning

This  aspect  of  planning  needs  to  ensure  approach  roads,  power  and  water supply,  workshop,  stores,  aggregate  processing  plant,  concrete batching  and  mixing plants,  camps  and  work sheds. It  also  requires  the  establishment  of  other  amenities,  such  as  market,  schools,  medical  facilities,  and  other social  and  cultural needs  of  the  field  staff  and  workers. The  planning  should  be  carried  out  to  the  extent  possible  before  the  work starts,  so  that  the  uncertainties and  delays  in execution  of  work,  and  precise  time  estimates  for  the  job  planning  could  be  evaluated.

Procurement planning 

This  requires  the  storage  of  various construction  related  materials,  like  explosives  for  blasting  rocks,  cement  for  construction,  steel  sheet  piles, structural  and reinforcement  steel,  aggregates, etc. The construction  equipment  necessary  to  execute  the  work  have  also  to  be  procured  along  with  adequate  spare  parts  and accessories.

Programme of works 

This should  be  prepared  at  the  start  of  the  construction  activities  and  consist mainly of Bar Chart  Programme  for  the  project  duration  showing  the  quantities  and monthly  progress  required  for  various  major  items  of  the  project. Another  master  network  plan  based  on  PERT/CPM  planning  may  have  to  be  worked out  for monitoring  the  project  work. Based  on  these  programmes,  the  planning  for  finance,  manpower,  equipment  required  for  various  activities  in  different  seasons  of work  have  to  be  prepared.

Review of programme and resources 

This  should  be  carried out  from  time  to  time  as  the  construction  work  progresses  and  should compare  items  such  as  the  budgeted  programme   of  work  and  the actual  programme  of  work  reviewed  at  intervals  of  three  or  six  months. Also,  the  actual performance  of  various  machines  have  to  be  compared  with  the estimated performance  and  recommend  necessary  corrective  measures  that  should  be  taken. Availability  and  procurement  of  essential  materials  like  cement,  reinforcement steel,  sheet  piles,  etc. have  to  be  reviewed  as  well  as  that  for  accessory  and  spare parts  of  plant  and  machinery  in  use  and  the  availability  of  skilled  and unskilled  manpower.

Sequence of construction   

This  important  activity  has  to  be  planned  perfectly,  since  mistakes  at  this  stage  would  be  difficult  to  be  rectified  later. The  major  items  under  the  sequence  of construction  are  as  follows:

• Layout  of  the  barrage  axis  as  per  the  approved  plan  by  constructing  short  pillars  called  axis  pillars  at  suitable  locations  along  the line  of  the  axis  across  the  river. 
• Benchmark  location  has  to  be  established  the  entire  project  area  to  help  site  the  various  components  like  floor,  crest,  piers,  etc. at proper elevation.
• Temporary  access  bridge  has  to  be  constructed  for  transporting  men, material  and  equipment  from  one  bank  of  the  river  to  the  other.
• Layout  of  cofferdams  have  to  be  decided  on  the  site  conditions,  nature  of  river  course,  and  programme  of  works  for  the  season. Coffer  dams  are temporary  structures  constructed  in  the  riverbed  to  provide  an  enclosed  area  where  the  actual  construction  might  be  executed. Details  of  the  design of  a  cofferdam  may  be  had  from  Bureau  of  Indian  Standards  Code  IS:10084-1982(Part1) “Criteria  for  design  of  diversion  works: Coffer dams”.
• Once  the  coffer  dams  are  constructed,  the  water  within  the  enclosure  has  to  be  dewatered. The  Bureau  of  Indian  Standards  Code  IS:9759-1981  “Guidelines  for  dewatering  during  construction”  may  be  referred  for  details,  but  the  main  points  are  noted  below:

1. After  completion  of  the  excavation  above  the  water  table, dewatering  of  the  foundations  have  to  be  commenced  by  well points  or  open  pumps  and  the  water  table  progressively  lowered.  Well  point  systems  may  be  suitable  for  sandy  soils  but  in  silty  clay  foundations  strata  open  pumps  and/or  deep  well  pump  may  be  preferred. If  an  impermeable  compact  shingles-coffle  layer  is  sand witched  between  sandy  layers  in  the  depth  to  be  excavated,  then  deepwell  pumps  with  strainer  throughout  its  depth  has  to  be  used.  
2. The  preliminary  requirements  of  dewatering  pumps  should  be  bored on  the  inflow  to  the  work  area,  calculated  on  the  basis  of permeability  of the strata and closeness  of  the  water source.  
3. During  dewatering  operation, care  should  be  taken  to  ensure  that there is  no  removal  of  fines  from  the  sub-strata  that  may  weaken  the  foundation.  
4. Any  seepage  of  water  from  the  foundation  at  local  points  or springs  have  to  be  taken  care  of  properly  so  that  there  is  no  piping  of  the  foundation  material.  
5. Excavation  of  the  foundation  to  the  barrage  profile  is  to  be  made either  manually  or  by  machines  in  reasonably  dry  conditions . During excavation, water table  should  be  maintained  at  a lower  level  at  which the excavation is being done. The excavated soil should be disposed- off  either  manually  or  by machines,  to  suite  the  site  requirements . In case  machinery  is  employed, the  final  excavation  of  the  lowest  layer should be done manually to the specified depth.   
6. Cutoff  walls  may  be  steel  sheet -piles  driven  from  riverbed  in  case of  nonbouldery  strata  of  riverbed  but  in  bouldery  strata,  either concrete  or  steel sheet pile  cut-offs  have  to  be  constructed,  both by excavating a trench and then back filling with sand. For a discussion  of  the  details  of  steel  sheet  pile driving  or  construction  of  cutoff walls in  trenches  the  code  IS:11150-1993 “Construction  of  concrete  barrages - code  of  practice”  may  be  referred  to.
7. Once  the  cut-off  walls  on  the  upstream  and  downstream  sides  of  the  barrage  are  installed  and  partially  covered  with  pile  caps,  the  foundation surface  of  the  raft  floor  has  to  be  properly  leveled,  dressed  and  consolidated. The  foundation  should not  contain  loose  pockets  or  materials  and they should  be  watered  and  compacted  to  the  specified  relative  density. Clay pockets should be treated as specified by the designer. It  has  to  be ensured   that proper  drainage  arrangements  in  the  foundation  according  to  the  designs  including  inverted  filter,  wherever  indicated,  are  provided  and concreting work  is  taken up.
8. Instruments  like  piezometers,  pressure cells,  soil stress meters, tilt meters  as  specified  should  be  installed  carefully  such  that  the  electric  or  mechanical connections  to  a central  control  panel  is  least  disturbed  during  construction.
9. The  batching,  mixing, placing  and  protection  of  concrete  has  to  be  done  in  accordance  with  IS:456-1978  “Code  of  practice  for  plain  and  reinforced concrete”. 
10. Where  mechanical  parts  like  gate  guidining  rails,  gate  seals  are  to  be  installed,  block outs  should be  left out   so  that  the  parts  may  be  embedded later. 
11. Dowel  bars,  or  if  necessary,  metal  sealing  strips  should  be  provided  for  the  joints  between  the  pile caps  and  barrage  floor.
12. The  sequence  of construction  of  barrage  bays,  silt  excluder  and  piers  have  to  be  done  in  lifts,  starting  from  the  downstream  end  of  the  barrage and with  continuous  pour  in  suitable  layers,  or  as  specified  by  the  designer.
13. Abutment  and  flared out  walls  may  be  constructed  on  pile  foundations  or  on  well foundations.
14. Divide  walls  have  to   be  constructed  on  well  foundations  and  the  wells  have  to  be  sunk  to  the  founding  levels  and  the  work  of  barrage  bays  on either  side  of  the  divide wall  should  be  taken up  after  construction  of  well  caps. 
15. The cement  concrete  blocks  in  the  flexible  apron  on  the  upstream and downstream  of  the  solid  aprons  of  the  barrage  floor  may  be cast in-situ with alternate blocks cast at a time. These  may  be constructed  with  form work  that  should  be  so  designed  that  when  it  is stripped  off,  the  required  gap  is formed  for  filling  the  filler  material  to  facilitate  speedy  construction,  pre-cast  blocks  may  be  used. 

 Care and diversion of river 

Since  a  barrage  would  be  covering  almost  the  entire  width  of  the  river,  and  it  would  take  quite  a  few  years  to  construct  the  whole  structure,  it  would  be necessary  to  construct  only  portions  of  the  barrage  at  each  construction  season,  when  the  flow  in  the  river  is  relatively  less. There  may  not  possibly  be  any construction  in  the  flood  season. During  the  construction  season,  the  river  has  to  be  diverted  from  the  area  enclosed   for  construction  by  suitable  flow  diversion works.

The  programme  of  construction  of  river  diversion  work  should  mainly  be  determined  by  the  availability  of  working  period,  likely  time  that  would  be  required  for construction  of  coffer dams,  associated  diversion  works  and  construction  capability.

The  period  available  for  construction  of  cofferdams  is  generally  limited  and  depends  upon  the  post-monsoon  pattern  of  the  river  course  and  quantum  of discharge and  programme  of  work  of  various  items  of   permanent  nature. Cofferdam  construction  for  the  portions  nearer  to  the  river  banks  where  velocities  may not  be high, may  be  of  earthen  type  cofferdams  and  when  the  work  advances  into  the  river  portion,  composite  type  cofferdams  consisting  of  single  sheet piles  backed  with earthen  embankments  may  be  provided. Suitable  protection  on  the  river side  has  to  be  provided  to  avoid  dislodging  of  sheet  piles  due  to  scour  of  soil  backing. For  details  about  the  choice  of  coffer  dam  to  be  adopted,  one  may  refer  to  the  Bureau  of  Indian  Standards  Code  IS:10084-1982 (part1)  “Guidelines  for  choice  of type  of  diversion  works: cofferdams”.