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Pipe culvert is a tunnel structure constructed using pipes under roadways or railroads to provide cross drainage or to take electrical or other cables from one side to another. Pipe culvert, box culvert, and arch culvert are the common types of culverts used under roadways and railways.

backfilling in pipe culvert
Fig 1: Backfilling in Pipe Culvert

In this article, we discuss the excavation, bedding, laying, jointing, and backfilling procedure of pipe culverts.

1. Excavation for Pipe Culvert

  1. The foundation for pipe culverts shall be excavated to the lines and levels, as shown in the drawings. 
  2. The pipes shall be placed either in the shallow excavation of the natural ground or in open trenches made in existing embankments.
  3. At the places where the height of fill exceeds more than three times the external diameter of the pipe, the embankment shall be raised to a level above the top of the pipe, and to a width not less than five times the diameter of the pipe.
  4. The extra width of the excavation in trenches shall be minimum 150mm or one-fourth of the diameter of the pipe, whichever is more. At no place, the extra width shall be more than one-third the diameter of the pipe. 
  5. The vertical sides of the trench shall be as vertical as possible.
  6. If soft, spongy, or other unstable soil is encountered during excavation, such unsuitable material shall be removed to such length, width, and depth as directed by the engineer. 
  7. The backfilling of the excavation of the pipe culvert shall be done with granular material, which shall be properly shaped and thoroughly compacted up to the specified level.
  8. Where bed-rock or boulder strata is encountered, 200mm deep excavation shall be carried out from the bottom level of the pipe, and all rock/boulders in this area are removed and the space is filled with earth, free from stone or fragmented material.
  9. The backfill shall be shaped to the requirements and compacted to provide adequate support for the pipe.
  10. The excavated trenches shall be kept free from water until the pipes are installed, and the joints have hardened.

2. Bedding for Pipe Culvert

The purpose of the bedding surface is to provide a firm foundation of uniform density throughout the length of the culvert and shall conform to the specified levels and grade.

The bedding of pipe culvert can be carried out in the following two methods-

A. First Class Bedding

  1. In first-class bedding, the pipe shall be laid on on a continuous layer of well compacted granular material and shaped concentrically to fit the lower part of the pipe exterior for at least ten percent of its overall height.
  2. The bedding material shall consist of well-graded granular material or sand passing 5.6 mm sieve.
  3. The thickness of the compacted bedding layer shall not be less than 75mm.

B. Concrete Cradle Bedding

  1. When indicated in the drawings, the pipe shall be bedded in a cradle constructed of concrete having a mix not leaner than M-15 conforming to section 1700. 
  2. The laying of pipes shall be done on the concrete bedding before the concrete has set.

3. Laying of Pipes in Culvert

  1. The pipes shall be laid after the foundation is well cured and approved by the engineer-in-charge.
  2. In places where two or more pipes are to be laid adjacent to each other, the pipes shall be separated by a distance of a minimum 450 mm or equal to half the diameter of the pipe.
  3. Proper care shall be taken while lifting, loading, unloading, and lowering of concrete pipes at a factory or site so that the pipes do not suffer any undue structural strain, any damage due to fall or impact.
  4. The lowering and positioning of pipe shall be done with the use of a tripod–pulley arrangement or simply by manual labor to minimize damage to the pipe.
  5. The laying of pipes shall be commenced from the outlet and proceed towards the inlet and be completed to the specified lines and grades. 
  6. In case of the use of pipes with bell-mouth, the belled end shall face upstream.
  7. The pipes shall be fitted and matched so that when laid in work, they form a culvert with a smooth uniform invert.
  8. Any pipe found defective or damaged during laying shall be removed at the cost of the contractor.

4. Jointing of Pipes in Culvert

  1. The pipes shall be jointed by two methods, namely, collar joint or by flush joint. 
  2. In collar joint, the collars shall be of 150 to 200mm wide RCC and shall have the same strength as the pipes to be jointed.
  3. The caulking space shall be in the range of 13 and 20 mm depending on the diameter of the pipe.
  4. The caulking material shall be of 1:2 cement mortar, which shall be rammed with caulking irons. 
  5. In the flush joint, the flush joint may be an external flush joint or an internal flush joint. 
  6. In both cases, the ends of the pipes shall be specially shaped to form a self-centering joint with a jointing space 13 mm wide. 
  7. The jointing space shall be filled with 1:2 cement mortar, mixed sufficiently dry to remain in position when forced with a trowel or rammer.
  8. The pipes shall be so jointed that the bitumen ring of one pipe shall set into the recess of the next pipe. 
  9. The ring shall be thoroughly compressed by jacking or by any other suitable method.
  10. After finishing, the joint shall be kept covered and damp for at least four days.
Jointing of Pipes in Culvert
Fig 2: Jointing of Pipes in Culvert

5. Backfilling of Pipe Culvert

  1. The backfilling of the trenches shall be carried out immediately after the pipes are placed, and the jointing material has hardened.
  2. The soil used for backfilling shall be clean, free from boulders, large roots, excessive amounts of sods or other vegetable matter, and lumps.
  3. The backfilling up to 300 mm above the top of the pipe shall be carried out carefully, and the soil shall be rammed, tamped, or vibrated in layers not exceeding 150 mm, particular care being taken to consolidate the materials under the launches of the pipe thoroughly. 
  4. Proper pneumatic or light mechanical tamping equipment must be used for compaction.
  5. The filling of the trench on both sides of the pipe shall be carried out simultaneously so that unequal pressures do not occur.
  6. In case of the high embankment, after filling the trench up to the top of the pipe in the above-said manner, a loose-fill shall be placed over the pipe to a depth equal to the external diameter of the pipe and compacted.
Backfilling in Pipe Culvert
Fig 3: Backfilling in Pipe Culvert

FAQs on Laying of Pipe Culvert

?What is pipe culvert?

Pipe culvert is defined as a tunnel structure constructed using pipes under roadways or railways to provide cross drainage or to take electrical or other cables from one side to another. Pipe culvert, box culvert, and arch culvert are the common types used under roadways and railways.

?What is the purpose of bedding in pipe culvert?

The purpose of the bedding surface is to provide a firm foundation of uniform density throughout the length of the culvert and shall conform to the specified levels and grade.
The bedding of pipe culvert can be carried out in 2 methods
1. First Class Bedding
2. Concrete Cradle Bedding

?What are the methods used for laying bedding for pipe culvert?

The bedding of pipe culvert can be carried out in 2 methods
1. First Class Bedding
2. Concrete Cradle Bedding

Read More:
1. What is the Laying and Jointing Procedure of Concrete Pipes? [PDF]
2. What is Culvert? Its Types, Materials, and Location

Fasi Ur Rahman

Fasi Ur Rahman

EDITOR
Fasi is a Civil Engineer associated with Project Management Consultant for Tumkur Smart City Project in Karnataka, India. He is the author, editor, and partner at theconstructor.org

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