Selected BMPs: Trench Plug

Definition: Trench plugs are temporary or permanent barriers installed at regular intervals in pipe trenches.

Purpose: Trench plugs are used in two types of installations: 1) trench breakers, in which fill is placed for preventing erosion caused by the lateral movement of runoff in the open trench, and 2) trench saddles, used for minimizing the potential of groundwater seeping along the pipe and the associated piping of fines.

Application: Trench plugs are required if the slope of the trench exceeds 5 percent and if at least 500 feet of trench will be open at one time. Typical situations in developing areas that may require trench plugs include the installation, repair, or replacement of water or gas utility distribution lines.

Water can enter trench excavations from rainfall runoff or from infiltration in instances were the trench intersects the groundwater table. Trench breakers are used to prevent the unrestricted flow of water along the slope of the open trench excavation. Permanent trench breakers should be placed at:

• Regular intervals along sloping trenches
• Bases of slopes
• Adjacent to water bodies and wetlands and as needed to avoid draining wetlands
• Road crossings

Trench breakers also are used to isolate areas in trenches for dewatering.

After trenches are backfilled, the backfill material may become a conduit for groundwater seepage. Frequently, seepage flow will concentrate along the pipe. The seepage can cause unwanted impacts, including:

• Dewatering of wetlands
• Surface subsidence caused by the piping of fines

Trench saddles constructed from clay are used for minimizing the potential of groundwater seeping along the trench after it has been backfilled. Trench saddles generally are composed of granular or pelleted bentonite because it has very low permeability and excellent sealing properties.

When pipes are outlets from surface-water impoundments, trench saddles will not control seepage adequately. For outlet pipes, seepage collars or diaphragm drains are required (see Appendix I, Common Design Elements).

Recommended Design Criteria:

Requirements for Regulatory Compliance

(none specified)

Performance-Based Guidelines

The spacing of trench breakers and trench saddles will be determined by field conditions. However, the minimum recommended spacing is shown below:

If the laying of pipe will be delayed, then temporary trench breakers, consisting of earth berms or staked straw bales, should be installed. The breakers should be replaced by breakers composed of approved materials, such as sand bags, after the pipe is installed in the trench. If trench saddles are not required, synthetic foam sprayed in place can be used to construct trench breakers during the construction period.

Once the pipe has been installed, sand bags can be used to form the area where the clay saddle will be emplaced. The clay saddles should be a minimum of 12 inches wide.

Operation and Maintenance: Trench breakers should be inspected after construction and repairs should be made if necessary.

Considerations: For trench excavations exposed to overland runoff flow, diversions should be used to route runoff away from open trenches (see BMP description for DIVERSION). Where trenches cross perennial drainages, temporary culverts or coffer dams can be used to bypass flow. In streams having very low discharges, temporary impoundments can be constructed to capture flow, which then can be pumped around the open excavation.

Figure 1. Method for minimizing in-trench flow by conveying runoff across pipe trench.

Water pumped during construction dewatering must be treated to remove suspended sediments before the water is discharged to a natural drainage. As a general rule, if a vegetated filter strip of adequate dimensions is available, it should be used to accept water discharged from the trench (see BMP description FILTER STRIP). Vegetated filter strips must be selected carefully to ensure that water is not discharged directly into a natural drainage. An energy dissipator made of plywood or rip rap should be used to reduce the velocity and spread the flow onto the filter strip. If conditions are such that water cannot be safely discharged onto a vegetated filter strip, then a facility such as that described in BMPs FILTER BAG, PORTABLE SEDIMENT TANK, or STRAW BALE BARRIER should be used.

When constructing trench saddles, powdered bentonite is not recommended because of its potential for generating dust.

References:

Algonquin LNG, Inc. Soil Erosion and Sediment Control Guidelines: an industry guide. February 1996.

ANR Pipeline Company. Environmental Compliance Manual, Volume III: Related to construction and maintenance activities. An industry guide. Detroit, MI. Spring 1996.

Commonwealth of Pennsylvania, Department of Environmental Protection. Erosion and Sediment Pollution Control Program Manual. April 1996.

National Fuel Supply Corporation. General Erosion and Sedimentation Control Plan: an industry guide. Erie, PA. June 17, 1996.

Commonwealth of Pennsylvania, Department of Environmental Protection, Bureau of Land and Water Conservation, Draft Underground Utility Line Construction-Typical Erosion and Sediment Controls, 1996.

Specifications and Modifications: Plans and specifications for installing trench plugs should describe the requirements for applying the practice and should contain the following minimum requirements:

• Location where the trench plugs will be installed
• The minimum dimensions and spacing
• Acceptable materials to use in construction

Trench Breakers

• Trench breakers should be free of brush and stumps and other debris.
• Topsoil should not be used to fill sacks.
• Fit filled bags snugly around the pipe. At least one bag should extend underneath the pipe.
• Trench breakers can be sized according to the following recommendations:

Trench Saddles

• Each saddle should be constructed according to the trench dimensions shown in the following figure.
• Sand bags may be used to form the area where the clay saddle will be constructed.
• Excavate an area completely around the pipe. Dewater as necessary.
• Place the bentonite into the dry trench so that the pipeline is completely surrounded. The dry bentonite should extend to within 12 inches of the final grade. Use approved respirators and safety goggles.
• Do not add water. Backfill saddle to the finished grade. Infitrated water will hydrate the saddle in place.