Technical Info

Bedding & Backfill

Please refer to ASTM D 2321 for installation of Weholite

Buried Pipe Installation

 

In any below grade piping installation, the quality of the installation is one of the key factors in the long-term performance of piping materials. In many applications, Weholite offers several unique qualities that can aid in safer and faster installations. These qualities are enhanced by proper design considerations and installation procedures.

This section offers a guide to good underground installation of Weholite and should serve to supplement the specifying engineers knowledge of local conditions. Other guides to below grade piping installations are contained in publications such as ASCE/WPCF Manuals of Practice, and ASTM standard D2321, Standard practice for Underground Installation of Thermoplastic Pipe for Sewers and Other Gravity Flow Applications.

Weholite is a flexible piping system. The flexible nature of the pipe allows minor displacements to be easily accommodated without damage to the pipe. Vertical deflection of the pipe side-wall mobilizes the side support from the passive resistance of the surrounding soil envelope. The quality and compaction of the embedment materials is the primary factor affecting the load carrying capacity of the Weholite soil-pipe system.

 

Trench Excavation and Placement of Bedding

The trench should be excavated to the required grade and alignment as shown on the contract drawings or as directed by the supervising engineer. The depth of the trench must be adequate for the placement of bedding materials, and the bedding and foundation materials (where the existing native materials are unsuitable for bedding). The minimum trench width shall be the larger of ‘the pipe OD plus 16 inches’ or ‘the pipe diameter times 1.25 plus 12”. ’ The maximum trench wall should not exceed the minimum trench width by more than 18”. Trench side-wall bracing as required by site safety conditions and / or the local regulatory requirements must always be followed. When using a trench box, and where soil conditions are suitable, step the trench wall at a point ¾ of the pipe diameter above the final bedding grade. The trench box should sit on the step. The stepped wall facilitates the placement of the initial backfill in the haunch zone … movement of the box will not cause the compaction applied to the initial backfill in the haunch zone to be lost.

Where water is present in the trench, excess water should be removed before laying the pipe. During dewatering do not remove the fines from the foundation or bedding materials. It may be desirable to place a geotextile if the fines are being washed away during dewatering.

The minimum bedding thickness is 6”. The bedding materials should be Class I or Class II materials (per ASTM D2321). The bedding layer must be free from stones larger than 1.5 inches (40mm). Ensure that the bedding is ‘on grade’ prior to placing the pipe.

 

Placing Pipe

Short lengths of 18” or 21” pipe may be placed manually. Most pipe will require appropriate handling equipment during installation. Pipe must not be dropped, rolled or pushed into the trench.

When there is a significant temperature differential between the temperature of the pipe at the storage location, and the temperature in the trench bottom, allow the pipe sections to acclimate before joining them. Temperature changes less than 15 F° will not cause significant changes in pipe length, nor cause significant internal stresses when the pipe is restrained from moving.

Weholite pipe will experience service load deflections. Minor pipe deflection may occur during pipe storage.

All flexible pipes (PVC, CMP, PE etc.) possess out-of-roundness that must be controlled during pipe installation. In fact, management of this service deflection is the best way to ensure that a pipe has been properly installed. This advantage is not possible with rigid pipe systems. Rigid pipe systems will bridge backfill voids that occur during installation, delaying the failure or problems until after the construction has been completed and the pipe is no longer being actively monitored. Flexible Weholite pipe will respond to installation and backfill loads in a predicable manner. Monitoring the deflection allows the quality of the installation to be assessed.

In general, placement and proper compaction of the initial backfill will tend to increase the vertical dimension of the pipe. Placement of the final backfill and the long term prism load of the soil in the trench, will tend to decrease the vertical dimension of the pipe.

To take advantage of these naturally occurring deflections, Weholite should be installed with the largest diameter dimension in the vertical position. Placement and compaction of the initial backfill will tend to increase the vertical dimension (height) by a small additional amount. Placement and compaction of the final backfill will tend to decrease the vertical dimension.

At the start of the project, measurements should be taken after initial backfill (300mm above top of pipe) is complete, and again after the final backfill is complete. The difference between the two values is the initial deflection. Each project will have a theoretical deflection due to imposed loads. The initial deflection should be limited to 2/3 of this value. If the amount of initial deflection exceeds this amount, it indicates that there may not have been sufficient compaction of the initial backfill. Consequently compaction should be checked and increased if necessary for the balance of the installation.

 

Placing Initial Backfill

Voids in the haunch area are undesirable. Backfill, especially in the haunch area should be angular and well graded. Class I and Class II materials as defined in ASTM D2321 are suitable. The grading is to limit the potential migration of the soils that may cause voids. The backfill materials must be suitable to provide the level of soil support that is required by the design. Backfill should be placed evenly on both sides of the pipe in lifts of 6” to 12” (150-300mm) and compacted to the required density. The backfill in the haunch area should be ‘shoveled’ and hand tamped with a haunch tamping tool to ensure that no voids remain. No compaction should be done directly over the pipe until the backfill has reached 12” (300mm) above the pipe crown. The initial backfill should extend to a depth at least 12” (300mm) above the crown of the pipe.

Placement and compaction of the initial backfill may result in deflection of the pipe before the final backfill is placed. In some cases compaction of the side fill may result in negative vertical deflections (an increase in the pipe vertical dimension and a decrease in the horizontal dimension).

 

Final Backfill

Final backfill materials are not a factor in developing the soil-pipe interaction that is the basis for the design against deflection, and buckling loads. Consequently the selection of the backfill materials and the compaction effort required for the final backfill is based on other operating conditions. Select native materials would be suitable in trenches located in open areas where minor settlement may occur in future. Granular materials may be selected if the trench is located in roadway. The contract requirements will govern what is needed.

 

Buried Connections to Rigid Structures

To prevent point loads from occurring where the PE pipe enters the rigid structure (concrete manhole, tank or valve chamber), it is necessary to limit the movement of the flexible PE pipe. The most effective way to limit differential movement between the Weholite PE pipe and the rigid structure is to provide an appropriately rigid soil structure. This is easily achieved with well graded aggregates that are compacted to 95% SPD. Consideration should be given to extending this high compaction zone in the trench bedding and primary backfill for a distance of 2 or more pipe diameters from the rigid structure.

Where the native soils themselves are unstable, it may be necessary to excavate and replace the native soils with well graded and compacted granular materials for a width of 2 or more pipe diameters for a distance of 3 pipe diameters along the pipe, and extending down ½ pipe diameter below the bottom of the pipe.

It is necessary to provide a mechanical connection between the Weholite pipe and the rigid structure. Since the material has a high resistance to abrasion and has a very smooth surface, pouring a concrete sleeve around the pipe will not be sufficient. One of two methods are commonly employed to make the mechanical seal. Either an HDPE ring can be extrusion welded to the pipe OD and the ring can be encased in concrete, or, the external wall of the pipe can be removed for a minimum distance of 1.5 profile turns, and the ‘open’ section of the pipe wall can be encased in concrete.

Generally, it will be necessary to ensure that there is no leakage occurring through this mechanical joint area. Since the thermal properties of the two materials are different, and since there will be little or no adhesion between the pipe wall and the concrete, it is necessary to add a gasket to provide a hydraulic seal. This is most easily accomplished through the use of hydrophilic gaskets (gaskets that swell slightly when wet). The forces that result from the swelling of the gasket can be substantial. Contact a gasket supplier to determine the appropriate type and thickness of gasket.