This issue, Precast Solutions hears from Clay Tappan, P.E., BCEE, with CDM Smith.
What is your background and area of focus?
I specialize in water and wastewater pipelines and conveyance systems. I’ve planned, designed and/or constructed more than 1,300 miles of pipe ranging from 4 inches to 10 feet in diameter on four continents.
What projects do you typically focus on?
My primary focus is outside-the-fence pipeline systems as opposed to inside-the-fence plant/site work. These include major septic-to-sewer programs totaling more than 8,000 precast manholes and pump station wet wells.
What are some unique projects on which you specified precast concrete?
One of the more unique projects featured curved alignment box culverts, headwalls and very deep wet wells installed as caissons with tremie plugs at the bottom. This process involves placing the precast concrete structure (circular or square) that you are installing on bare ground. The interior is then excavated to essentially undermine the structure in a controlled fashion. Additional sections are stacked on top of the previous sections, and the excavation continues to the target depth. At that point, concrete is tremied into the bottom and keys into the structure to create the necessary anti-flotation mass and seal out groundwater from infiltrating or wastewater from leaking out. Sections are plant-cast with the desired lining system, which significantly improves quality control. This method is very helpful in congested areas where loose, difficult-to-dewater soils would result in a larger excavation affecting nearby structures or utilities.
In the case depicted in the photos, Pinellas County Utilities was faced with replacing an outdated lift station located on a tidally influenced creek. The lift station was undersized and leaking. Because of site constraints including the creek, adjacent homes and a major county thoroughfare, as well as silty sands next to the water, it was important to limit construction impacts while keeping the existing station in operation. The caisson/tremie method using precast concrete sections is the go-to way to address these situations.
What benefits does using precast concrete afford you?
The biggest things are consistency on the product, plant quality control versus field quality control and weather being much less of a factor.
How do you see the market for precast concrete changing?
The largest, most significant change in the concrete industry is the introduction of alternate reinforcement materials such as FRP bars and polymer or steel fiber. How to design and approve these alternate systems will need to be based on an integration of industry standards to develop a level playing field. As with any new technology or approach, there can be a gap in time between introduction and general acceptance while performance is being proven. If inadequate standards or inefficient enforcement result in failures or performance issues, the entire industry can feel the blow.