The pros and cons of using underground and “in-the-slip” conduit installations
Many project owners have seen benefit from using underground and “in-the-slip” electrical conduit installations, which refer to a type of electrical construction where conduit is embedded within the concrete walls during the slip-form construction process. However, accommodating this type of electrical installation brings with it a unique set of challenges. Nevertheless, it’s possible to achieve goals for industrial hygiene, provide ease of access, and leave room for future expansion when conduit is installed underground (Photo 1) or within the concrete walls of a structure.
Take a look at the many advantages that can come with installing conduit in this manner.
Improved safety during construction — Underground installations keep craft professionals off ladders and pipe bridges, making safety during installation a key benefit for underground or in-the-slip electrical installation. When electricians install underground conduit long before the installation of process equipment and piping, congestion at the work site is minimized, also increasing safety.
Financial considerations — When open to the concept, owners often find they are able to save money with underground or in-the-slip electrical installations. While there are additional design and planning factors to discuss with the general contractor and engineer when considering either type of installation, it’s possible to make some generalized pricing comparisons. In the Table (click here to see Table), you see a pricing comparison for installation of different electrical solutions at a bio-fuels tank farm. The example uses three different electrical solutions: underground PVC conduit, overhead metal conduit, and overhead cable tray. The results show a compelling cost advantage for using underground PVC conduit.
What makes the cost estimate for underground PVC conduit so much less than overhead cable tray or overhead rigid metal conduit? Savings is achieved in materials and labor. Material costs are lower for underground conduit; the cost for underground PVC conduit is significantly less than that of rigid metal conduit. Labor cost is reduced in part because entire duct banks can be prefabricated off-site (Photo 2). This shifts expensive labor hours off the job site and minimizes project man-hour peaks. Using underground installation also eliminates the need for certain materials and installation for items such as overhead supports. Underground installation also reduces “crew stacking,” by allowing much of the electrical installation to take place early in the job. While each project is different, financially speaking, underground installation merits consideration.
Reduce cross-trade coordination — Fewer electricians at the work site as startup draws near means less coordination and congestion with other trades. With fewer workers on the same job site, it becomes more efficient for all trades. In addition, when more electrical infrastructure is installed underground or in-the-slip, more pipe and equipment space is left for other trades to work.
Improved industrial hygiene — This installation method impacts the cleanliness of a facility. In hazardous areas, for example, it reduces areas where dust can build up, reducing the maintenance needs for keeping the area clean. In food-grade areas, it reduces the nooks and crannies where bugs can congregate and grow — areas that typically require constant maintenance attention.
Let’s consider a facility in Victorville, Calif., which was constructed with electrical conduit installed within the walls of the concrete structure, as a case in point. At each floor, there was access to the main raceway to allow for wiring of instrumentation and controls. Because the client wanted an installation where all conduit was out of sight, an in-the-slip installation helped to achieve that goal. Simply speaking, it made for a cosmetically better installation.
Schedule — Using underground or in-the-slip installation methods can positively impact the job across the entire project schedule. With proper coordination, impact to others responsible for concrete or site work can be minimized, especially when prefabrication is used to accomplish a large portion of the work off-site. Consider the following example. A northwest Iowa ethanol plant used an underground duct bank installation. The duct bank was to be situated under the main truck and vehicle access roadway for the facility, which happened to be a very high traffic area. Asking other trades working at the location to shut down access to the job site for an extended period of time was not a viable option. An acceptable solution, however, was to use planning and prefabrication to build the duct bank off-site. With careful coordination, crews performed excavation, lowered the duct bank into place, made the necessary connections, and reconstructed the roadway in a single weekend — with little disruption to other activity at the facility.
This use of prefabrication and underground installation has significant impact on all trades and all parts of the schedule. Moving a large portion of the electrical installation to the early stages of the project greatly reduces the peak manpower required at the busiest time of the job as it nears completion.
While the financial and scheduling benefits for underground installation are more easily identified, there are certain obstacles owners, general contractors, and project teams must address before making an informed decision to implement an underground installation. Challenges range from identifying the location of underground utilities so that conduit is routed properly to the sizing of electrical wiring to address the extra heat caused by an underground “blanket of backfill” (see NEC Art. 310-15 and Informative Annex B).
Shifting coordination to different trades — An initial challenge is that electrical installation coordination shifts to different trades. Instead of the site manager coordinating with mechanical piping and equipment installation crews, coordinating underground and in-the-slip conduit installation requires working with the site grading, concrete, and slip-form crews. This effectively shifts much of the coordination to early in the project as opposed to later.
Backfill considerations — Underground conduit installation requires coordination with the general contractor and structural engineer to consider appropriate backfill selections. Having appropriate backfill ensures that trenches and duct banks do not become future sinkholes. Because certain situations call for different types of backfill, it’s important to be aware of the wide cost differences and applications for the different types of engineered fill and flowable fill as well as the potential costs of each, if the advantages are to be maximized.
What are some of the considerations for choosing backfill? If the underground installation serves as a primary feeder for power (main power service or power distribution), then flowable fill or concrete is preferred. Flowable fill is a mixture of wet, loose concrete that can easily settle itself into gaps and voids around the conduit. It dries hard, but not as hard as actual concrete. An advantage of flowable fill is that it can be covered or backfilled with dirt almost immediately, and it’s much less expensive than solid concrete. If the underground installation houses communications or low-voltage type wiring, then sand or dirt might be acceptable forms of backfill.
In addition to considering what is housed in the underground conduit, appropriate backfill materials also depend upon the location. If the underground installation is located in a high traffic area or where there is a high probability for digging in the future, then a solid concrete fill would be preferred to provide added stability.
Regardless of the use for underground conduit installation, it’s important to work early in the design process with the general contractor and the structural engineer to assure that all variables are addressed.
Settling foundations — The design and planning teams must work closely with the structural engineer to address the possibility of settling foundations so that conduit is not crushed. Different methods for accommodating settlement are used, depending upon where the conduit runs. If the conduit runs under a foundation and extends past the edge of the foundation, it is common to use rigid pipe at the sheer point and extend it 10 ft to 20 ft past the edge of the foundation. Crews may also choose to install a crush pad at the point where settlement is expected to occur. This might be either a Styrofoam pad or a pea gravel mat, which would allow the duct bank to move. If the conduit runs under a foundation and then comes up through the foundation at a 90° angle, there is not typically a problem. However, it is possible to sleeve around the stub-up in order for some settling to occur without taking the pipe down with it.
Flexible, just different — Some owners have concerns that underground installation does not allow flexibility for future expansion. However, with proactive planning, spare PVC conduit can be economically run in each duct bank from the onset of the project, allowing for additional space for expansion. The combination of spare conduit, along with prudent use of access points (handholes and manholes), makes underground conduit systems accessible and expansion ready. In addition, above-ground space not used during the initial installation remains available for future expansion.
It’s important to keep in mind that underground installations are not the only area where expansion limitations are found. Other installations also have their limits. For example, it’s common to find completely full pipe bridges and electric rooms with no room for additional overhead conduit or cable tray.
Importance of complete as-built drawings — While underground installation has advantages, it’s also something that can easily be forgotten. As-built drawings with coordinates, tracing means, visible markings, etc., become very important for any future digging or excavating. Maintenance workers or contractors need to be reminded of the location of underground electrical equipment.
Although accommodating an underground or in-the-slip electrical installation provides its own unique set of challenges, it is feasible for owners, general contractors, and trades to collaborate differently in order to realize project goals. Owners can achieve their desire for a visibly appealing installation with increased attention to industrial hygiene and ease of access while leaving room for future expansion.
Post, P.E., is president of Interstates Engineering, headquartered in Sioux Center, Iowa. He specializes in electrical engineering and design services for industrial and processing facilities, and can be reached at firstname.lastname@example.org.