Encased Burial in concrete

It is recommended that manufactured plastic spacers be used.

Spacers are used to prevent excessive deflection from loading or buoyancy forces.

The use of bricks or wood is not recommended because these materials may deform   the conduit wall.

The bottom spacer is to provide sufficient clearance off the trench floor to permit the specified thickness of concrete to gather at the bottom.

Anchoring

An important consideration is to make sure that the entire duct bank is run as straight as possible from manhole to manhole.

Once the duct bank is straight, it is necessary to anchor it to prevent movement when the concrete is poured.

Movement may be caused by buoyancy, concrete churning and vibration.

Recommended methods incorporate reinforcing rods which are driven into the trench floor to secure the duct bank and to prevent movement during the concrete pour.

 

In areas where soil conditions make it impossible to drive a rod deep enough to gain an effective anchor, it is recommended that a trench jack be firmly placed directly over the spacer location and adjustable wedges be used to wedge down the duct bank.

                                    CONDUIT AND DUCT SEPARATION

Power Duct Banks

 

In power duct banks, individual raceways should be separated from one another for the following reasons:

To provide adequate dissipation of the normal build-up of heat from cables within the raceway.

To provide void space to allow the encasement material to fully surround each raceway.

To physically separate raceways in the event of a cable fault.

Communication Duct Banks

Since there is no appreciable build-up of heat or risk of a cable fault in communication duct banks, separation is intended only to allow the encasement material to fully surround each raceway.

Spacers

Spacers are used to prevent excessive deflection from loading or buoyancy forces.

The bottom spacer is to provide sufficient clearance off the trench floor to permit the specified thickness of concrete to gather at the bottom.

The placement of these spacers varies in accordance with the raceway material and installation specification. Typically, spacers are placed 5 feet (1.5 m) to 8 feet (2.4 m) apart.

Expansion/contraction due to Temperature Change

Raceways, when not restrained, expand or contract the following approximate distances per 100 feet (30.5 m) for every temperature change of 10° F (5.6° C) for the following materials:

– PVC: 3/8 inches (9.5 mm)

– HDPE: 1 inch (25.4 mm)

– RTRC: 1/8 inches (3.2 mm)