1.1 Pipe Materials & Selection

Pipe Materials

The selection of pipe material is one of the most critical decisions in the design and operation of a water distribution system. The most common materials in use today include Polyvinyl Chloride (PVC), Ductile Iron Pipe (DIP), High-Density Polyethylene (HDPE), and historically, Asbestos Cement (AC).

Polyvinyl Chloride (PVC) is a flexible pipe material that has become the standard for many new distribution systems, particularly in sizes up to 12 inches, though larger sizes are available. PVC is favored for its excellent corrosion resistance, smooth interior (which results in a high C-factor and low friction loss), and relatively light weight, which makes it easy to handle and install. However, PVC must be protected from prolonged exposure to ultraviolet (UV) light, which can degrade the material over time. It is also susceptible to damage from certain organic solvents.

Ductile Iron Pipe (DIP) is a rigid pipe material known for its exceptional strength and durability. It is an evolution of the older cast iron pipe, with the addition of magnesium during the casting process making the iron more flexible and less brittle. DIP can withstand high internal pressures and external loads, making it ideal for large transmission mains and areas with heavy traffic or unstable soils. A primary disadvantage of DIP is its susceptibility to internal and external corrosion. To mitigate this, DIP is typically manufactured with a cement-mortar lining to prevent internal tuberculation, and it is often installed with a polyethylene wrap (bagging) to protect against corrosive soils.

High-Density Polyethylene (HDPE) is another flexible plastic pipe that is gaining popularity due to its extreme durability, flexibility, and leak-free joints. HDPE sections are typically joined using heat fusion, which creates a monolithic continuous pipe with no mechanical joints. This makes HDPE particularly well-suited for trenchless technologies like horizontal directional drilling (HDD) or pipe bursting, as well as for installations in seismically active areas or unstable soils where pipe shifting might occur. Like PVC, HDPE is resistant to corrosion but requires careful handling to avoid deep scratches or gouges that could concentrate stress.

Asbestos Cement (AC) Pipe, historically known by the trade name Transite, was widely installed in the mid-20th century. It is a rigid pipe made from a mixture of Portland cement, silica, and asbestos fibers. While no longer installed due to the health risks associated with asbestos fibers, operators will still encounter AC pipe in many older systems. AC pipe is generally durable and resistant to internal corrosion, but it can become brittle over time and is susceptible to deterioration in aggressive, low-pH soils. When repairing or replacing AC pipe, operators must follow strict safety protocols to prevent the release of airborne asbestos fibers.

Joints

Pipes must be securely joined together to maintain system integrity and prevent leaks. The type of joint used depends on the pipe material and the specific application.

• Push-on Joints: These are the most common joints for PVC and DIP in straight runs. They consist of a bell end with an elastomeric gasket and a plain end (spigot). The spigot is lubricated and pushed into the bell, compressing the gasket to form a watertight seal. They are quick to assemble and allow for slight deflection.
• Mechanical Joints (MJ): Common in DIP and for connecting pipes to valves and fittings. MJ consists of a plain end, a bell with a flange, a rubber gasket, a follower gland, and bolts. Tightening the bolts compresses the gasket, forming the seal. MJs provide a strong connection and are standard for fittings.
• Flanged Joints: Used primarily in above-ground installations, pump stations, and treatment plants where rigid connections are required. Flanges on adjacent pipe sections are bolted together with a gasket in between. Flanged joints do not allow for deflection and are generally not buried due to the risk of uneven settlement causing the rigid joint to break.
• Restrained Joints: Designed to prevent pipes from pulling apart under pressure. They are used where thrust blocks cannot be installed or where extra security is needed. Restrained joints can be mechanically locked together using specialized gaskets with metal gripping teeth, or bolted assemblies.

Bedding and Backfill

Proper bedding and backfill are crucial for the structural integrity of buried pipes, especially flexible pipes like PVC and HDPE, which rely on the surrounding soil for support against external loads. The American Water Works Association (AWWA) categorizes trench conditions into different classes:

• Class I: Angular, well-graded crushed stone or rock. Provides the best support and requires minimal compaction.
• Class II: Coarse sands and gravels. Good support, requires moderate compaction.
• Class III: Fine sands and clayey gravels. Requires careful compaction to achieve adequate support.
• Class IV: Silt, silty clays, and clays. Generally unsuitable for bedding flexible pipe directly unless heavily compacted under strictly controlled conditions.
• Class V: Organic soils, peat, highly expansive clays. Completely unsuitable for pipe bedding or backfill; must be removed and replaced with imported material.

Depth of Bury

The depth at which water mains are buried is dictated by two primary factors: the frost line and external loads.

1. Frost Protection: The pipe must be buried below the maximum expected frost penetration depth for the region to prevent the water from freezing, which would block flow and potentially burst the pipe. In northern climates, this can require burying pipes 5 to 8 feet deep or more.
2. External Loads: The pipe must be buried deep enough to distribute the weight of surface traffic safely. A typical minimum cover is 30 to 36 inches, but this may increase under heavy roadways.

Thrust Blocks

When water flowing under pressure changes direction or hits a dead end, it exerts a dynamic force on the pipe system known as thrust. If not properly counteracted, this thrust can push joints apart, causing catastrophic leaks. Thrust forces occur at tees, elbows, crosses, reducers, and dead ends.

Thrust blocks are masses of concrete poured in place between the pipe fitting and the undisturbed trench wall. They distribute the thrust force over a larger area of the soil, preventing the fitting from moving. The design of a thrust block depends on several factors:

• System Pressure: Higher internal pressure generates greater thrust force.
• Pipe Size: Larger pipes have a larger cross-sectional area, resulting in greater thrust force for the same pressure.
• Fitting Angle: A 90-degree bend generates more thrust than a 45-degree bend.
• Soil Bearing Capacity: Different soils can withstand different amounts of pressure. Solid rock has a high bearing capacity, while soft clay has a low bearing capacity. The thrust block must have enough surface area against the soil so that the applied pressure does not exceed the soil's safe bearing capacity.

Concrete should only be poured against the fitting itself, ensuring that bolts, joints, and flanges remain accessible for future maintenance. As an alternative to thrust blocks, restrained joints are increasingly used, transferring the thrust force along the pipe barrel via skin friction with the surrounding soil.