It takes an experienced team to create a custom excavation protective system.
By Josh Chandler, PE
Some excavations are relatively straightforward and simply require choosing the right excavation protective system for the job. Others — deep or complex trenches — require a custom-engineered solution.
When contractors need a custom-designed excavation shoring system from United Rentals, they submit information about their trench to a sales associate at their local branch, including depth, width, length, soil conditions, general site conditions and any surcharges that are near the excavation (heavy equipment, buildings, etc.). Once approved and all drawings are completed, they receive their stamped and approved excavation shoring design plan.
Transforming the requirements into a stamped, professional engineer-approved design is a multi-step process and a team effort, led by a licensed professional engineer (PE).
The conceptual design
Before formal design plans are created, the shoring request goes to a team of design engineers who create a conceptual design of the proposed shoring system.
The site-specific conditions determine the loadings that get applied to whatever shoring system is being utilized for support. At United Rentals, we compare the strength of the materials of each individual member of the shoring system to the applied loads from the site. Shoring design engineers perform the necessary calculations at the conceptual stage to ensure the proposed system will meet structural requirements if accepted by the customer.
It usually takes a few hours to come up with this “rough draft” for the shoring design. The engineering department submits it back to the United Rentals branch, and the sales associate shares it with the customer.
The conceptual design is an important step in the process. Once contractors review it, they may realize they forgot to communicate some important piece of information about the project or the site. That happens about 50 percent of the time.
That’s one reason we want contractors to see what we’re going to put on paper before we actually do produce the formal design plans. We want to make sure up front that everybody is on the same page to help eliminate revisions later.
We can generally look at things and have an idea of what will work, but until you really run the numbers and get into the analysis you won’t know for sure. For example, if you’re putting in bracing levels, is it two bracing levels or three? Do you need crossing struts or can you go clear span without any interior support? Decisions like that you have to run through the formal analysis, checking to ensure all components of the support system pass applicable engineering design standards.
Designing a support system is comparable to designing a building, except that loads are applied laterally rather than vertically. Each structural member of a shoring system is a column or beam, and engineers have to consider the consequences of members undergoing bending, compression, or tension.
The biggest difference is in the loads. In the building industry, 100 psf (pounds per square foot) is typical for a commercial building load. In the shoring industry, when you’re working underground with dirt, you can be in the 1,000 psf range without any surcharges.
If you have additional surcharge loads nearby, such as roadways, waterways, buildings or railroads, you can be in the 2,000 to 3000 psf range, depending on the depth. So it’s a different economy of scale.
Completing the formal drawings
United Rentals uses a graphics communication package that enables drafters to design in 3D.
Customers have a better idea of what the shoring system will look like and they have a much better idea of how to be in compliance when they can look at 3D plans instead of 2D plans. That’s one way we are unique; to my knowledge, United Rentals is the only solution provider out there that produces 3D shoring designs.
Once the formal drawings are complete, the PEs seal and certify the entire shoring design package, which includes the drawn plans and the supporting calculations.
Another advantage of working with United Rentals Engineering Department is that it performs design services for other lines of business that offer support to the construction project.
For example, we work hand-in-hand with our Fluid Solution Division on accompanying construction applications like dewatering. In most cases, 99 percent of the time, contractors not only need to support the excavation structurally, they also need to dry out the excavation so they can do their work. United Rentals has Fluid Solutions engineers that can assist with the groundwater control portion of the design. The dewatering plans can also be included in the sealed design package.
Jumping regulatory hurdles
For public works projects like infrastructure installation or repair, the PE-sealed plans must be sent to regulatory reviewers, who may require changes.
We work across the United States and Canada, and we deal with a lot of different regulatory agencies from state to state and province to province. We get to know these agencies, but there are no universal codes like there are on the building side, such as the International Building Code that most in the building industry follows.
On the shoring side, it really depends who you are submitting to — a municipality, a department of transportation or even an engineering firm that was hired by the owner. They are all looking for different things, so there’s a lot of back and forth on submittals.
The United Rentals engineering team does whatever it takes to get the shoring plans approved and back to the contractors as quickly as possible.
Josh Chandler, PE, leads United Rentals’ Engineering Department staff. The department meets the engineering needs of contractors, industrial and commercial clients by developing, designing and implementing site-specific shoring, shielding and other structural support systems for heavy-construction trench and excavation projects nationwide. Chandler is registered as a Professional Engineer in 48 states as well as in five bordering Canadian provinces.