07-CULVERTS



CULVERTS

GENERAL:

Culvert is a cross drainage structure having a total length of 6 m or less between the inner faces of the dirt walls or extreme vent way boundaries measured at right angles thereto. Pipe and RCC slab/box culverts are discussed under this section. Construction of culverts shall conform to IRC: SP-13 and MoRTH Specifications or as directed by the Engineer.

PIPE CULVERTS:

Requirements:

  1. Reinforced concrete pipes NP-3 or NP-4 type conforming to IS: 458 shall be used. The internal diameter shall not be less than 900 mm except in unavoidable situations.
  2. Masonry work for pipe culverts shall conform to Section 3200 of this Manual.
  3. Concrete work including reinforcing steel for pipe culverts shall conform to Section 3300 of this Manual.
  4. Provide concrete cradle bedding for pipes of internal dia. 1000 mm or more and when height of fill is more than 4 m above the pipe. Make concrete cradle bedding having mix not leaner than M15 with shape and dimensions as indicated in the Drawing or as directed by the Engineer. The pipe shall be laid on the concrete bedding before the concrete has set.
  5. Provide first class bedding when height of filling is less than 4 m above the pipe. Make a continuous layer of well compacted approved granular material passing 5.6 mm IS sieve, suitably compacted/ rammed.  The compacted thickness of the granular bedding layer shall not be less than 75 mm.
  6. For expansive soils, provide a layer of approved granular material or non-expansive material of minimum 450 mm thickness under the bedding.
  7. The back fill of soil shall be clean from boulders, large roots, excessive amounts of sods and other vegetable matter and lumps which shall be approved by the Engineer.
  8. Backfilling upto 300 mm above the top of the pipe shall be carefully done and the soil thoroughly rammed, tamped or vibrated in layers not exceeding 150 mm. Approved pneumatic or light mechanical tamping equipment can be used.

Workmanship:

  1. For height of fill more than 3 m, or three times the external dia of pipe above bed level, construct the embankment to the level above the top of the pipe (equal to external dia of pipe) and width not less than five times the dia of pipe. Lay the pipe in trench after the construction of embankments.
  2. If spongy, soft or other unstable material is met with at the location of pipe culvert, remove the material to the required depth, width and length, back fill with approved granular material and compact to the required level.
  3. Where rock or bouldery strata is met with, take down excavation to at least 200 mm below the bottom level of pipe, remove all loose material and fill the space with approved earth.
  4. Make arrangements for lifting, loading and unloading    of pipes from factory/yard to site, such that no undue strain or damage occurs due to fall or impact.
  5. For manual unloading from trucks, roll down pipes on a pair of skids hooked onto the trucks and control movement with a rope passing round the pipes.
  6. Preload the areas to induce major portion of settlement before the pipe is installed.
  7. Provide a bedding surface of firm foundation of uniform density, throughout the length of culvert with specified bedding material depending on dia of pipe and height of fill above pipe.
  8. Lower the pipes in bed either by tripod pulley arrangement or by manual labour using chain pulley blocks in a manner to place them in proper position without damage.
  9. When two or more pipes are laid adjacent to each other, place them separated by a distance equal to half the dia of the pipe subject to a minimum of 450 mm.
  1. When pipes are laid in two layers, keep the centres of pipes such that when joined shall form equilateral triangles.
  2. Lay the pipes as specified in IS: 783 for different conditions such as positive projecting condition, trench condition etc.
  3. Lay the pipes on the prepared foundation, commencing from outlet and proceed towards the inlet. In case of pipes with bell mouth, keeps the belled end facing the upstream. Keep the invert of pipe minimum 150 mm below average bed level. However, if the invert level is more than 300 mm, a catch pit of size 1500 mm x 1000 mm up to ground level shall be constructed.
  4. Join the pipes either by collar joint or by flush joint. Place the collar such that its centre coincides with the joints and even annular space is left between the collar and the pipe.
  5. Choose either internal flush joint or external flush joint. Fill the jointing space with cement mortar 1:2, which remains in position when forced with a trowel or rammer.
  6. Fill the recess at the end of the pipe with jute braiding dipped in hot bitumen or suitable approved compound, while jointing pipe lines.
  7. Keep the width of collars 150 mm to 200 mm and caulking space between 13 mm and 20 mm according to diameter of pipes.
  8. Back fill trenches after the pipes have been laid and after jointing material has hardened. On top of pipe upto 300 mm, thoroughly ram, tamp or vibrate the soil in two layers. Thoroughly consolidate the materials under the ‘haunches’ of pipes using light mechanical tamping equipment.
  9. Carryout filling of the trench simultaneously on both sides of the pipe, such that unequal pressures do not occur.
  10. Ensure that the minimum height of fill above the pipe including road crest shall be 1000 mm.
  11. When minimum specified cushion cannot be provided over the pipe, encase the pipe in M10 concrete of specified thickness.

Tolerances:

The following tolerances are permitted for concrete pipes as per IS: 458.

i) Overall length : ±1% of standard length.

ii) Internal dia. : ±10 mm.

iii) Barrel thickness   :

For 80 mm to 95 mm: +5 mm/-2.5 mm.

 Over 95 mm  : +7 mm/-3.5 mm.

Sampling and testing:

  1. Quality control tests for masonry work shall be carried out by the contractor as per Engineer's instructions conforming to Section 3200 of this Manual.
  2. The Contractor shall submit to the Engineer the manufacturer's certificate for concrete pipes regarding the dimensions, manufacturing defects, tolerances (IS:458) and three edge bearing test (IS: 3597) before delivery at site.
  3. During construction of pipe culverts, the Contractor shall carry out the tests indicated in the table 3600-1 as per Engineer's instructions.

Table: Quality Control tests for pipe culverts

 Sl.

No.

Material/work

Test /check

Frequency

 

1

 

Bedding

-Materials (as per specification)

-Length, width and thickness.

-Preformation of cradle to lay pipes in bedding.

-Top and bottom levels.

 

While laying.

 

2

 

Laying and jointing

of pipe

-Invert level.

-Longitudinal gradient.

-Spacing when 2 or more pipes are laid in single

 or multiple rows.

-Jointing of pipes

 

Before back filling.

 

3

 

Backfill

-Filling of trench on both sides (simultaneously)

-Tamping around pipe.

During filling earth/

granular material

around pipe after

laying.

4

Cushion over pipes

Thickness.

While filling.

 

5

Brick/Stone

masonry for head

walls

 

Section 3200 of this  Manual.

As per relevant

clauses.

6

Side slopes on head

walls

-Slope.

-Stone pitching.

Before construction

of guard stones.

General:

The cross sections of foundations, substructures such as abutments, return walls, superstructure, drainage spouts, railings, parapets etc. and all other details shall be strictly in accordance with the Drawings or as directed by the Engineer.

Requirements/Workmanship:

  1. The requirements and quality control for various types of materials used for the construction of culverts shall comply with Section 3300 of this Manual.
  2. Take the minimum depth of foundation up to the stratum having specified bearing capacity shown in the Drawing but not less than 2 m below the scour level where no bed protection is provided or 1.5 m below the protected bed level.
  3. In case of rocky bed, ensure embedment of foundation into the rock below, the minimum depth being 500 mm for hard rocks and 1200 mm for soft erodible rocks.
  4. Provide 300 mm thick plain concrete M15 grade footing, unless otherwise specified in the Drawings. Provide a minimum offset of 150 mm for the base of substructure.
  5. Set out plan dimension of the foundation at the bottom of foundation trench and check with respect to original reference line and axis.
  6. Before laying foundation concrete, clean the earth surface of all loose material and sprinkle water to wet. Provide side formwork as per required dimensions and height.
  7. Lay foundation concrete continuously to the required thickness up to the level of construction joint proposed.
  8. Finish the concrete surface smooth with a trowel and ensure curing as specified in Section 3300 of this Manual.
  9. Carryout dewatering where necessary for laying of concrete so as to keep the water level below foundation level with adequate provisions/precautions.
  10. Remove loose sand laid on foundation before commencement of back filling. Refill all spaces excavated and not occupied by permanent work with earth upto surface of surrounding ground.
  11. In case of excavation in rock, fill the circular space with M15 concrete.
  12. Adopt coursed rubble/stone masonry or plain/reinforced cement concrete for piers, abutments and wing/return walls. For wings and return walls up to 3 m height, use random rubble masonry or plain cement concrete.
  13. Before commencing the masonry/concrete work, scrub the foundation with wire brush and remove all loose material and wet the surface.
  14. Make provision for weep holes in solid abutments and provide filter media.
  15. Provide vertical expansion gaps of 20 mm width between abutments and wing walls.
  16. Provide plain/reinforced concrete components as per Section 3300 of this Manual.
  17. Wearing coat, railings/parapets, approach slabs and drainage spouts shall be provided for the structure as per the Drawing or as instructed by the Engineer.

Sampling and Testing:

Quality control tests of the concrete work shall be carried out in accordance with Section 3300 of this Manual.

RELATED CODES:

1.IRC:SP: 13-Guidelines for the design of small bridges and culverts.

2.IS:458-Precast concrete pipes (with and without reinforcement) - specification.

3.IS:783-Laying of concrete pipes.

4.IS:3597-Methods of test for concrete pipes.

5.MoRTH-Specification for Road and Bridge works (5th revision).

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