Replacement

Conduit replacement is achieved by excavating the embankment, removing the existing conduit, and replacing with a new conduit. Common replacement pipe material types include Corrugated Metal Pipe, Concrete Pipe, and Plastic Pipe. Several important questions to ask include:

Is the existing means of control adequate? Is the gate / valve in a good condition, location, with no operational problems? If not, an evaluation of outlet configurations should be first.
The next choice should be conduit material. The design of the conduit is usually dictated by a standard. High hazard dams have strict requirements for conduits. The engineer should consult applicable standards before choosing a material.
Support and bedding depends on many factors and is a key to successful replacement.
Any time there is a penetration through a dam, consideration for seepage control must also be made.

Corrugated Metal Pipe (CMP)

CMP is fabricated from factory-produced sheet steel with corrugations added to provide stiffness and strength. The sheets are typically coated with polymers, zinc (galvanized), aluminum, or aluminized zinc alloy. Additional coatings, such as bituminous, have been applied for added protection against corrosion and abrasion. CMP was first used for conduits in the late 1890s. Generally, round pipe ranges from 6 inches to 26 feet in diameter. Other shapes and sizes of CMP are available, but have had limited applications in conduits through embankment dams. CMP is typically joined with coupling bands that extend over several corrugations on each end of two adjoining pipes.

CMP has a history of poor perfomance due to joint seperations and deterioration. Major federal dam-building agencies, including NRCS and USACE, limit their use of CMP to low hazard dams. USBR does not permit CMP to be used for conduits through their embankment dams. CMP is lightweight and easy to install, but there are many disadvantages. CMP is appropriate for certain dams, but correct corrosion protection and installation methods are imperative.

Coated CMP Example, Low-Hazard Dam

Concrete

Concrete pipe used in embankment dams may either be cast-in-place or precast. Each casting types has advantages and disadvantages. Cast-in-place pipe is advantageous because reinforcing steel and waterstops extend across conduit joints and it can be cast in a variety of shapes. Precast pipe is advantageous because it is manufactured with tight tolerances in a controlled environment, can be installed quickly, and the joints can accomodate varying settlement along the conduit.

Plastic

Plastic pipe is often used in the renovation of conduits (e.g., sliplining or lining of existing conduits). Plastic pipe that is used in the construction of new, significant and high hazard embankment dams should always be encased in reinforced cast-in-place concrete to assure quality compaction against the conduit. Use of plastic pipe in new, low hazard embankment dams is generally limited to small diameters (less than 12 inches).

Plastic Options

High Density Polyethylene (HDPE) is the most common plastic pipe material used in embankment dams. Polyvinyl Chloride (PVC) is also been used but it has inherent disadvantages due to concerns with lack of watertightness (bell and spigot joint connections).

Examples

Cautions

Plastic pipe is designed as a flexible pipe, which requires the soil surrounding the pipe to provide structural support. A high level of uniform compaction is required around the pipe, especially below the haunches. For significant and high-hazard dams, plastic pipe is usually encased in concrete in embankment dams to provide shapes that allow for good compaction of embankment materials against the conduit. Advantages vs. Disadvantages

Advantages of Plastic Pipe:

Lightweight material that facilitates installation.
Resists corrosion and is not affected by naturally occurring soil and water conditions. May be preferable in certain conduit applications where aggressivewater or soil chemistry would limit the life of concrete or metal pipe.
The smooth interior surface reduces friction loss. Also, due to the very smooth surface of thermoplastic pipe, adherence of minerals (e.g., calcium carbonate) is minimized.
The ability to heat fuse PE pipe joints provides a watertight joint.
Resists biological attack.

Disadvantages of Plastic Pipe:

High coefficient of thermal expansion relative to concrete can cause movement of the slipliner, requiring the use of end restraints.
Can easily be damaged or displaced by construction and compaction equipment unless it is encased in concrete.
Compaction of earthfill is difficult under the haunches of the pipes unless encased in concrete to provide good compaction of earthfill against the conduit.
Heat fusion of pipe joints requires special equipment and an experienced operator.
Requires a concrete encasement for significant and high hazard embankment dams to provide a favorable shape for compaction of earthfill against the conduit.

Bedding

Pipe Bedding

Filter Diaphragm

A filter diaphragm is a designed zone of filter material (usually well-graded, clean sand) constructed around a conduit. It is a standard defensive design measure to prevent problems associated with seepage or internal erosion in earthfill surrounding a conduit. The purpose of a filter diaphragm is designed to intercept water that can flow through cracks that may occur in compacted fill surrounding conduits or water that may flow along the interface between the conduit and the surrounding fill.

Erosion Protection

Revision ID: 1447
Revision Date: 06/24/2022