What Does Duct Bank Mean?
A duct bank is an underground reinforced concrete or metal container used for laying utility lines such as electric and telecommunication cables. The cables themselves are enclosed in PVC to prevent them from coming in contact with each other, and at least two different types of conduits are arranged side by side. The conduits are bundled together with spacers for installation within a casing. The casing is installed using HDD and the conduit bundle is pulled in.
The PVC conduits within a duct bank are used to carry electrical or data cables as part of a utility network or for a specific building or commercial application. The advantage of using duct banks is that the casing protects the utility cables are protected, and the cables can be replaced easily by drawing a new cable through the conduit.
Pull wires are used to pull a new cable through the conduit without any excavation required. This applies to the first cables to be installed once the duct bank has been constructed, as well as replacement cables down the line. The duct banks are slightly sloped to allow condensation to drain off and provision is given for expansion and contraction due to weather changes.
Unprotected underground utility cables are vulnerable to damage due to excavation activities during construction. It is also difficult to repair single cables that are sealed underground. The advent of trenchless construction techniques has created the opportunity for different cable conduits to be housed within a single duct bank. It not only helps carry different cables in a safe environment, but it also makes future repairs easier to carry out.
Trenchlesspedia Explains Duct Bank
A duct back system consists of duct banks entering the ground, duct banks exchanging cables at manholes or pull pits, and duct banks emerging from the ground. The cable from one conduit in one duct bank can be fed to another conduit in another duct bank from the pulling pit. At certain points, the duct bank might need to branch out so that it can reach other destinations. These branching duct banks may not contain the same number of conduits as present in the original duct bank. The cable system within a duct bank helps route electrical and data cables under and within data centers, hospitals, manufacturing units, airports, highways, railway lines, etc.
Installing Duct Banks
Duct banks can be installed using open-cut excavation as well as trenchless methods. For the open-cut method of installation, the ground surface is mechanically excavated to reach the desired depth of installation. PVC conduit is assembled into the excavated trench and the area around the conduit is filled with high strength (3000 psi) thermal concrete. once the concrete has set, the trench is backfilled and the ground surface restored.
In the trenchless method of installation, especially for short crossings (under 400 feet) and where no bends are required, pipe jacking and jack and bore methods are commonly used. If soil conditions permit, the methods can be used for longer runs. For the pipe jacking method, a minimum casing size of 42 inches would be required so that workers can excavate the soil. The jack and bore method utilize a powered auger that removes spoil as the casing advances.
For longer crossings, horizontal directional drilling (HDD) is used as this method allows for bends. HDD is popular as the need for large pits is eliminated and the drilling can be performed from the surface. The cable system can be installed with the casing containing the conduits or by just installing the conduits as a bundle by themselves