Can PVC and CPVC pipe be installed underground? How deep does it have to be buried?

Underground piping must be installed following any application's regulations, ordinances, and codes. Since piping is installed in a wide range of sub soils, attention should be given to local pipe laying techniques which may provide a solution to a particular pipe bending issue. The following information is applicable to PVC and CPVC piping joined via the solvent cementing method and may be considered as a general guide.

Inspection: Before installation, PVC and CPVC piping products should be thoroughly inspected for cuts, scratches, gouges, or split ends that may have occurred to the product during shipping and handling. Do not use damaged sections. Damaged sections must be cut out and discarded.

Trenching: The trench should be of adequate width to allow convenient installation, while at the same time being as narrow as possible. Minimum trench widths may be utilized by joining pipe outside of the trench and lowering it into the trench after adequate joint strength has been achieved. (Note: Refer to manufacturer's instructions for recommended set and cure times for solvent cement joints.) Trench widths will have to be wider where pipe is joined in the trench or where thermal expansion and contraction is a factor.

Trench depth is determined by intended service and local conditions. In all cases, thermoplastic pipe should be installed at least below frost level. Pipe for conveying liquids susceptible to freezing should be buried no less than 12" below the maximum frost level. Permanent lines subject to heavy traffic should have a minimum cover of 24". For light traffic, 12" to 18" is normally sufficient for small diameter pipe (typically < 3" diameter). With larger sizes, bearing stresses should be calculated to determine cover required. Reliability and safety should always be considered, as well as local, state, and national codes.

• Water filled pipe should be buried at least 12" below the maximum expected frost line.
• It is recommended that thermoplastic piping be run within a metal or concrete casing when installed beneath surfaces subject to heavyweight or constant traffic, such as roadways and railroad tracks. Piping systems must be designed and installed to ensure they can handle anticipated loads. Refer to critical collapse pressure ratings.

The trench bottom should be continuous, relatively smooth and free of rocks. Where ledge rock, hardpan or boulders are encountered, it is necessary to pad the trench bottom using a maximum of four (4) inches of tamped earth or sand beneath the pipe as a cushion and for protection of the pipe from damage.

Sufficient cover must be maintained to keep external stress levels below allowable design stress. Reliability and safety of service are of major importance in determining minimum cover. Local, state and national codes may also govern.

Snaking of Pipe: For smaller diameter piping systems (typically < 3" diameter), snaking pipe is critical to compensate for thermal expansion and contraction of the piping when installing pipe in hot weather. (This may also apply to larger diameter piping under specific applications and site conditions).After the pipe has been solvent welded and allowed to set properly, it is advisable to snake the pipe according to the following recommendations besides the trench during its required drying time (cure time). BE ESPECIALLY CAREFUL NOT TO APPLY ANY STRESS THAT WILL DISTURB THE UNDRIED JOINT. This snaking is necessary in order to allow for any anticipated thermal contraction that will take place in the newly joined pipeline. Refer to the section on thermal expansion and contractions for additional information.

Snaking is particularly necessary on the lengths of solvent welded during the late afternoon or a hot summer's day because their drying time will extend through the cool of the night when thermal contraction of the pipe could stress the joints to the point of pull out. This snaking is also especially necessary with pipe that is laid in its trench (necessitating wider trenches than recommended) and is backfilled with cool earth before the joints are thoroughly dry.

Note: Expansion and contraction could become excessive in systems operating at near or at the maximum allowable temperature ranges with intermittent flow and buried lines. In these cases the lines should not be snaked. The use of properly installed expansions joints installed within suitable concrete pit is recommended for PVC and CPVC systems operating at or near upper temperature limits. A section of larger diameter PVC pipe or other suitable sleeve should be used over the carrier pipe to pass through the wall of the concrete. This will minimize the potential for damage (scratching $ scarring) to the carrier pipe as the result of movement caused by thermal expansion/contraction. Expansion joints should be suitably anchored independently of the carrier line. Axial guide should be used to direct movement into the expansion joint.

Backfilling: Where possible, underground pipe should be thoroughly inspected and tested for leaks prior to backfilling. Ideally, backfilling should only be done early in the morning during hot weather when the line is fully contracted and there is no chance of insufficiently dried joints being subject to contraction stresses.

The pipe should be uniformly and continuously supported over its entire length on firm, stable material. Blocking should not be used to change pipe grade or support pipe across excavated sections intermittently.

Pipe is installed in a wide range of subsoils. These soils should not only be stable, but applied in such a manner so as to physically shield the pipe from damage. Attention should be given to local pipe laying experience that may indicate particular pipe bedding problems.

Backfill materials free of rocks with particle size of 1/2" or less should be used to surround the pipe with 6" or 8" of cover. It should be placed in layers. Each soil layer should be sufficiently compacted to uniformly develop lateral passive soil forces during the backfill operation. It may be advised to have the pipe under pressure, 15 to 25 PSI during the backfilling.

Vibratory methods are preferred when compacting sand or gravels. Best results are obtained when the soils are in a nearly saturated condition. Where water flooding is used, the initial backfill should be sufficient to insure complete coverage of the pipe. Additional material should not be added until the water flooded backfill is firm enough to walk on. Care should be taken to avoid floating the pipe. Sand and gravel containing a significant proportion of fine-grained material, such as silt or clay, should be compacted by hand or, preferably by mechanical tamper.

The remainder of the backfill should be placed and spread in uniform layers in such a manner to fill the trench completely so that there will be no unfilled spaces under or about rocks or lumps of earth in the backfill. Large or sharp rocks, frozen clods and other debris greater than 3" in diameter should be removed. Rolling equipment or heavy tampers should only consolidate the final backfill. Under certain conditions, it may be advisable to have the pipe under pressure during the backfilling operation.

Note: Use of threaded connections should be avoided in underground applications. Where transition to alternate materials is require the use of a flange component with suitable gasket I recommended. At vertical transitions from below-ground systems to connections above ground, follow above-ground installation procedures regarding compensating for thermal expansion/contraction, weatherability, and proper support recommendations. Valves and other concentrated weight loads should be independently supported. Avoid excessive bending of pipe; excessive deflection of pipe and joints can reduce pressure bearing capability and cause failure.

Additional information on underground installations is contained in ASTM D2774, "Underground Installation of Thermoplastic Water Pressure Piping Systems," and ASTM F645, Standard Guide for "Selection Design and Installation of Thermoplastic Water Pressure Piping Systems," and ASTM D2321' Underground Installation of Flexible Thermoplastic Sewer Pipe."