Installation Methods

Polyethylene encasement is inexpensive, easy to install on-site, and requires no additional manpower or equipment. And since it is a passive method of protecting ductile iron pipe in corrosive soils, it does not require the continual monitoring, maintenance, or supervision by trained personnel necessary with cathodic protection systems.

Although the polyethylene encasement should prevent contact between the pipe and surrounding backfill and bedding material, it is not intended to be completely airtight or watertight. All lumps of clay, mud, cinders, or other materials that might be on the pipe surface should be removed prior to installation of the polyethylene encasement. Care should be taken to prevent soil or bedding material from becoming trapped between the pipe and the polyethylene.

The polyethylene film should be fitted to the contour of the pipe to effect a snug, but not tight, encasement with minimum space between the polyethylene and the pipe. Sufficient slack should be provided in contouring to prevent stretching the polyethylene when bridging irregular surfaces, such as bell-spigot interfaces, bolted joints, or fittings, and to prevent damage to the polyethylene during backfilling operations. Overlaps and ends should be secured with polyethylene-compatible adhesive tape, string, or any other material capable of holding the polyethylene in place until backfilling operations are completed.

For installation below the water table or in areas subject to tidal actions, it is recommended that both ends of the polyethylene tube be sealed as thoroughly as possible by wrapping circumferentially with adhesive tape at each joint overlap.

As with all protection methods, proper installation is vital to the success of polyethylene encasement. The actual installation sequence, however, is less important than the quality and care taken during installation.

ANSI/AWWA C105/A21.5 INSTALLATION METHODS

Method A -- In this method, which is preferred by most utilities and contractors, one length of polyethylene tube, overlapped at the joints, is used for each length of pipe.

Method B -- Separate pieces of polyethylene tube are used for the barrel of the pipe and the joints. Note: Method B is not recommended for bolted-type joints unless an additional layer of polyethylene is provided over the joint area, as in Methods A and C.

Method C -- Each section of pipe is completely wrapped with a flat polyethylene sheet.

METHOD 'A' STEP-BY-STEP INSTALLATION GUIDE

Although ANSI/AWWA C105/A21.5 includes three different methods of installing polyethylene sleeving, most utilities and contractors prefer to use some form of Method A. Two popular forms are explained in detail below.

METHOD 'A' FOR NORMAL, DRY TRENCH CONDITIONS

Step 1. Cut a section of polyethylene tube approximately two feet longer than the pipe section. Remove all lumps of clay, mud, cinders, or other material that might have accumulated on the pipe surface during storage. Slip the polyethylene tube around the pipe, starting at the spigot end. Bunch the tube accordion-fashion on the end of the pipe. Pull back the overhanging end of the tube until it clears the pipe end.

Step 2. Dig a shallow bell hole in the trench bottom at the joint location to facilitate installation of the polyethylene tube. Lower the pipe into the trench and make up the pipe joint with the preceding section of pipe.

Step 3. Move the cable to the bell end of the pipe, and lift the pipe slightly to provide enough clearance to easily slide the tube. Spread the tube over the entire barrel of the pipe. Note: Make sure that no dirt or other bedding material becomes trapped between the wrap and the pipe.

Step 4. Make the overlap of the polyethylene tube by pulling back the bunched polyethylene from the preceding length of pipe and securing it in place. Note: The polyethylene may be secured in place by using tape, string, or any other material capable of holding the polyethylene encasement snugly against the pipe.

Step 5. Overlap the secured tube end with the tube end of the new pipe section. Secure the new tube end in place.

Step 6. Take up slack in the tube along the barrel of the pipe to make a snug, but not tight, fit. Fold excess polyethylene back over the top of the pipe.

Step 7. Secure the fold at several locations along the pipe barrel (approximately every meter).

Step 8. Repair all small rips, tears, or other tube damage with adhesive tape. If the polyethylene is badly damaged, repair the damaged area with a sheet of polyethylene and seal the edges of the repair with adhesive tape.

Step 9. Carefully backfill the pipe according to the AWWA C600 standard for backfill procedure. To prevent damage during the backfilling, allow adequate slack in the tube at the joint. Backfill should be free of cinders, rocks, boulders, nails, sticks, or other materials that might damage the polyethylene. Avoid damaging the polyethylene when using tamping devices.

ALTERNATE METHOD 'A' FOR WET TRENCH CONDITIONS

In wet, sloppy trench conditions, the pipe should be completely covered by the polyethylene tube before it is lowered into the trench. This alternate method is illustrated below.

Step 1. Cut the polyethylene tube to a length approximately two feet longer than that of the pipe section. Slip the tube over the pipe.

Step 2. Spread the tube over the entire barrel of the pipe, pushing back both ends of the tube until they clear both pipe ends. Make sure the tube is centered on the pipe to provide a one-foot overlap at each end.

Step 3. Take up slack in the tube to make a snug, but not tight, fit. (See Step 6 above.) Secure the fold at several locations along the pipe barrel (approximately every meter). Wrap a piece of tape completely around the pipe at each end to seal the polyethylene, leaving ends free to overlap the adjoining sections of pipe.

Step 4. Lower the pipe into the trench and make up pipe joint. Be careful not to damage the polyethylene when handling or jointing the pipe. Complete installation following Steps 4, 5 (taking care to seal ends of overlap by wrapping tape completely around the pipe at each end), 8, and 9 above. Note: When lifting polyethylene-encased pipe, use a fabric-type sling or a suitably padded cable or chain to prevent damage to the polyethylene.