Cross-linked polyethylene (PEX) is a flexible plastic tubing material frequently used for water supply systems due to its durability and resistance to scale and corrosion. PEX is generally suitable for direct burial underground, but this application is strictly contingent upon using the correct material type and adhering to stringent local plumbing codes and installation practices. Ignoring these specific requirements can severely compromise the longevity and reliability of the underground water system.
PEX Types Approved for Direct Burial
Not all PEX materials are manufactured with the same properties, and the type used is paramount for a successful direct burial installation. PEX tubing is categorized into three types—PEX-A, PEX-B, and PEX-C—distinguished by their resulting molecular structure. PEX-A offers the highest degree of cross-linking and is typically the most flexible option, providing superior durability and thermal memory to resist kinking and cracking.
PEX-A is widely regarded as the preferred choice for underground applications because its material properties make it highly resistant to temperature fluctuations and the mechanical stresses of soil movement. Some local plumbing codes may only permit PEX that meets a minimum Standard Dimension Ratio (SDR) of 9, which indicates a thicker wall necessary to withstand underground pressure. PEX-B is stiffer and may not be approved for direct burial in some jurisdictions due to a lower pressure rating compared to PEX-A.
PEX is inherently resistant to corrosion and scale, unlike metal piping. However, it is important to ensure the selected product is rated for the specific pressure and temperature of the application. Always consult the manufacturer’s specifications and local building codes, as these regulations dictate the acceptable PEX type and pressure rating for water service lines in your area.
Installation Standards for Underground PEX
A successful direct burial installation requires careful attention to the trench and the placement of the pipe itself, beginning with the necessary depth. The pipe must be installed below the local frost line, which is the maximum depth that ground water is expected to freeze in winter. Laying the pipe below this depth is the primary mechanism for preventing the water inside the line from freezing and potentially causing damage.
Proper preparation of the trench base, known as bedding, is necessary to prevent physical damage to the pipe from sharp objects. Before laying the PEX, the trench must be cleared of all rocks, debris, and sharp materials that could score or puncture the tubing. It is recommended to cushion the pipe by lining the trench bottom with a layer of sand or clean, screened soil, and then surrounding the tubing with the same fine-grained material before backfilling.
Fittings and connections are the most common points of failure in any buried pipe system and must be specifically rated for underground use. Any connections made below grade should utilize specialized, durable fittings, such as brass expansion or crimp fittings, designed to resist corrosion and leakage in soil contact. If brass fittings are used, they must be wrapped with a protective tape, such as a 10-mil pipe wrap, to shield the metal from corrosive soil chemicals and moisture.
To ensure the pipe can be located easily in the future, a tracer wire or detectable warning tape must be installed directly above the PEX tubing before the trench is completely backfilled. The tracer wire, typically a non-corrosive copper or stainless steel wire, allows the pipe’s location to be detected electronically from the surface. This measure is often mandated by local utility codes and helps prevent accidental damage from future excavation projects.
Protecting PEX from Long-Term Damage
While PEX is durable, several environmental factors can threaten the integrity of a direct burial line over its lifespan, requiring proactive protection measures during installation. Preventing the water from freezing is accomplished primarily through burial depth. In areas where the required depth cannot be achieved, such as under a driveway or at a connection point, the PEX should be sleeved with an insulating material or a larger PVC conduit to add a layer of thermal protection.
Rodents, particularly mice and rats, pose a genuine threat to buried PEX because they have powerful incisors that can chew through the plastic tubing. The most effective defense against this is encasing the PEX in a rigid physical barrier, such as Schedule 40 PVC or metal conduit, especially in areas known for rodent activity or where the pipe enters a building foundation. Using a protective sleeve that is one size larger than the PEX tubing provides an impenetrable layer against gnawing and simplifies any potential future repairs or replacements.
Installation near chemical hazards also demands caution, as PEX can be vulnerable to degradation from high concentrations of petrochemicals or contaminated ground. It is best practice to avoid running water lines near areas like fuel tanks or septic fields, which may contain chemicals that could compromise the long-term integrity of the polyethylene material.