This document is the TN 5.09 Thermoplastic Pipe and Utility Crossings. Design, Excavation, Installation considerations for utility disturbances explored.

Underground utilities include storm sewer, electric, sanitary sewer, gas, water, telephone and cable. Utility conflicts are typically avoided during the design process. However, in some cases, existing utilities are not properly identified or located during the design process. In other instances, the location of the utility is known, but adequate construction controls are not in place. Both can result in a negative impact to an existing utility when installing a new utility.

The purpose of this Technical Note is to provide additional information on the impact of utility crossings. While the discussion centers on the use of thermoplastic (high density polyethylene, HDPE and polypropylene, PP, specifically) pipe, there are documented cases of all types of pipes being affected, and in some cases damaged, by subsequent utility activity nearby.

Design consideration depend on the type of utility that is installed, which may also dictate depth. Check with local ordinances on utility locations and separate distances to minimize conflicts.

Excavation considerations recommend that a utility survey be conducted prior to construction to locate all existing utilities. Once construction begins, confirm types of equipment that is utilized to avoid utility damage.

Installation considerations note to assess proximity of an existing utility when preparing a suitable trench or embankment. Existing utilities may prevent the use of traditional backfills, controlled low strength material or flowable fill can be used.

Effects of adjacent excavation and response of pipe materials are issues of soil stability until the excavation encroaches on the pipe embedment zone. The long term performance characteristics of any pipe material can be compromised in cases where slope failure occurs. The use of a protective system would limit the impact of an adjacent installation on an existing utility.

Methods of identifying utilities include backfill materials, locator tape and ordinances. Repair of utility cuts for HDPE & PP Pipe use split couplers and replacing large utility cuts with pipe sections. For small utility cuts, use internal seals.