Clay Pipe Installation Handbook

Chapter 4, Pipe Bedding

Types of Bedding

The engineer's specifications for providing uniform support of the pipe barrel on a firm foundation must be followed.

The pipe may be laid on a flat or unshaped trench bottom of suitable undisturbed native material or, in the case of over-excavating, on a restored flat bedding base. In either case the bottom of the entire pipe barrel shall have a continuous and uniform line bearing support.

When a bedding material is used, the bottom of the trench should be over-excavated. The proper amount of bedding material is then added to achieve final grade. The bedding material may be cmshed stone or other angular material placed on the trench bottom or by using the natural material providing it is properly compacted. The depth of the materiai should be at least one-eighth of the pipe diameter but in no case less than 4 inches. (Figures 10 to 17)

Rock Excavation

In rock excavation the pipe should be bedded on 6 inches of angular material. If other bedding materials are used, compaction is necessary to assure a firm base.

At this point the contractor can make a quick, simple check of the elevation of the bedding material using the grade rod, gradeliner or laser. This eliminates the need for further grading when the pipe is installed.

Bell or coupling holes must be dug so that the load is entirely supported by the pipe barrel, not the pipe bell. The bell or coupling must not support any portion of the load The holes should be no larger than necessary to make sure that the pipe barrel is resting firmly and evenly on the trench bottom or bedding material. (Figure 8)

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Load Factors

The engineering specifications will state the type C support for bedding the pipe in the trench. The engi neer uses a "load factor" based on a designated typE of bedding to compute the load bearing capacity of thi pipe. The field supporting strength consists of thi strength of the pipe itself increased by the support C the particular bedding. (Figure 9) ncpi009.GIF (6780 bytes)

The engineer has given careful consideration to the selection of a "load factor" to meet the calculated trench load. This is why the inspector must make sure that the design trench width is maintained and that thr contractor places the bedding material according tc the engineer's specifications. Remember, the pipr strength and the specified bedding foundation worl together to support the trench load.

The following drawings illustrate recommended classes of bedding with applicable load factors. (Figures 10 throuqh 17)

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*In concrete cradle construction, the percentage of reinforcement, p, is the ratio of the transverse reinforcement to the area of concrete above the top of the pipe and below the centerline of the reinforcement. Refererce ASCE #60, WPCF MOP FD-5 1982 pg- 204.

It is recommended that welded steel wire fabric be used in load factor design because of its uniformity and relative ease of installation.

Geofabric

Crushed rock or other coarse aggregate is recommended and used as a bedding material to improve the load bearing capacity of pipe. Deeper layers of these materials have been employed to stabilize the base of the trench. Loss of pipe support can occur when open-graded materials are used on sites having fine to medium sands at the base of the trench and a water table which fluctuates rapidly in the pipe zone. This is believed to be caused by water moving rapidly through the tine to the coarse material and carrying the tine sands with it. To prevent movement of the fine sands into the voids of the open-graded bedding material, the material should be encapsulated in geotextile drainage fabric. Overlaps should be provided and care must be taken to prevent entry of sands into the crushed rock or aggregate base. (Figure 18)

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NCPI - Clay Pipe Installation Handbook

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