The Unseen Asset
Large-scale civil infrastructure projects often require extensive planning to pre-think challenges and complexities from structural loading to material selection. In heavy highway work, which generally involves building roads and bridges, problem-solving is essential, and frequently experience is the best teacher. In 2010, North Texas heavy highway and municipal contractor, Ed Bell Construction, was working on a highway rehabilitation for the Texas Department of Transportation (TxDOT) when they were asked to take on an ancillary scope of work.
The perimeters of the add services contract were two-fold: 1) rehabilitate a bridge with deteriorating headers and 2) conduct a side-by-side comparison of two different alternative fill materials. TxDOT had already spent several years trying to rehabilitate the failing embankments ramping up to the bridge using traditional soil stabilization methods, but moisture issues continued to cause settlement. As the embankments settled, the pavement where the road connects to the bridge on either end was bunching up, creating an unsafe driving condition.
In preparing for the material comparison scope of work, the project manager at Ed Bell Construction began to research several regional molded polystyrene geofoam manufacturers in preparation for the purchase order. In his research, the project manager found that while key variables of cost and production schedule were roughly the same, there was a significant difference between Atlas Molded Products and the competing manufacturer in their approach to problem-solving.
Just outside of Cleburne, TX, the US 67 bridge over SH 174 was deteriorating, as the embankments leading up to the bridge on either side continually suffered from soil erosion and instability. After several attempts to remediate the deficiencies using established practices without success, TxDOT began looking for alternatives. General Contractor Ed Bell Construction was asked to add the work to an existing contract to rehabilitate another nearby section of the highway and found that the challenges were less than typical.
The scope of work for the additional services was to compare two very different types of alternative fills by building one side of the bridge’s embankments using a lightweight, kiln-processed clay aggregate and the other side using molded polystyrene geofoam. Once the damaged bridge was rehabilitated, the next step was to excavate the embankments on either side of the bridge to roughly six feet deep and 120-feet long to give builders a fresh, wide trench to fill with the selected alternative materials. Electronic pressure monitors were to be installed beneath the fills on both sides of the restructured embankments so any future settlement could be monitored independently and remotely. Once the two trenches were refilled, the road installation plan called for a crushed limestone subgrade, covered with a hot asphalt mix, and topped with 10 inches of concrete paving.
While the clay aggregate material was only available regionally from one manufacturer in North Texas, molded polystyrene geofoam blocks were readily available from several different manufacturers. Having never worked with geofoam before, the project manager for Ed Bell Construction wanted to know more about how the large foam blocks would fit together and support structural loading before placing such a large purchase order.
Molded polystyrene is a closed-cell plastic that is block molded into materials that are well suited for a wide range of structural applications and Earth-facing insulations. Atlas Geofoam is manufactured in standard block sizes as large as 40” x 48” x 96” and custom-sized blocks as large as 40” x 72” x 288”. Atlas Geofoam also spans a range of compressive resistance that begin with EPS 12 with a load-carrying capacity of over 500 pounds per square foot and extend up to EPS 46 with a load-carrying capacity of over 3,500 pounds per square foot. One of the conundrums commonly encountered in specifying geofoam as a building material is that contractors who have not worked with it often have a hard time believing in geofoam’s performance capacity.
In construction, confidence comes from certainty, and astute builders will do everything in their power to eliminate unknowns before working with unfamiliar materials. In the case of the embankments being built in Cleburn, TX, the project manager at Ed Bell Construction reached out to two different geofoam manufacturers to learn as much as he could about the material before deciding on what to order and from whom. In the information-gathering phase, the project manager found that the material cost and production and delivery schedule was roughly the same for both Atlas Molded Products and the competitor, however, the willingness to be a solutions provider was very different. Between the various sizes and compressive resistance of materials, as well as options that included termite-resistant treatments and the possibility of many different engineered configurations for stacking the blocks there is a lot to know. While Atlas worked hard to understand project specifics and engineer a purchase order, the other vendor suggested simply filling the embankment with the highest resistance / most expensive material available. A team of Atlas’s in-house drafters produced shop drawings for the ideal block configuration pattern to maximize structural integrity while minimizing waste material and then numbered each block in the entire order to ensure it was easy to place precisely according to plan.
On both sides of the bridge, the trenches were lined with a filter fabric before being filled with the new materials. On the lightweight aggregate side, a dump truck simply backed up to the hole and dumped in the fill, which had a consistency like kitty litter. The aggregate was graded flat, performing much the way soil would under the grader. On the geofoam side, first, a leveling course of sand was laid, and the geofoam block configuration was stacked on top of it by hand. The 48” x 96” blocks were easily able to be put in place by a two-man crew without any specialized knowledge or equipment. While the majority of the blocks didn’t require any further modification, workers were able to customize the blocks that abutted the bridge’s superstructure using a hand-held hot wire cutter.
Once the two sides of the embankment were filled, it was time for the builder to lay down the road and despite their extensive experience, things went very differently than expected. The road plan called for a subgrade base of crushed limestone, covered with a hot asphalt mix and topped with 10” of concrete paving. While builders were initially skeptical of the geofoam-built embankment, they found that the clay-aggregate side turned out to be much more difficult to work with. While the aggregate material was very squishy under the weight of the grader and took a long time to get a smooth surface, on the geofoam side everything was rock solid. Once both sides were graded to specifications, a lightweight truss screed bridge paver was used to put on the final concrete topping.
Now nearly a decade since the embankments were built, the electronic sensors placed beneath the two sides of fill materials tell a story of success. On the lightweight aggregate side, the fill settled slightly more than originally anticipated but remained within TxDOT tolerances. On the geofoam side, there was a quick initial settlement and then no further movement at all, with the geofoam overall performing better than expected.
More than merely a building material manufacturer, Atlas Molded Products strives to be a project partner committed to engineering an appropriate solution as part of the construction team. While other suppliers may take the path of least resistance, selling their products like a commodity, at Atlas, we strive to be an integral part of the decision-making process with a commitment to doing things the right way. While there are many options in molded polystyrene geofoam from block sizes, compressive resistances, and configuration patterns to manufacturers, there is never a substitute for high quality, reliable service, and a willingness to collaborate to achieve the best value in any construction program.