All the Easy Sites Have Been Taken
How does a contractor figure out what they will encounter when they dig underground for construction? What are the most common things left behind by past development? And what steps does the contractor take to mitigate the environmental, public and worker safety, and project schedule impacts when something is discovered underground?
We start with research at the University of Washington’s Suzzallo and Allen libraries on the history of a site to know what types of structures, companies, or buildings were there in the past. Using this historical information, we hope to reduce the "unforeseen conditions" that may occur during the excavation phase of a project and reduce our risk. Going into the project we want to have better knowledge of what may be in the dirt rather than relying on boring samples that are taken randomly around the site.
Our next step is a utility survey during the schematic design phase of preconstruction. At this stage we dig test pits, have a survey completed, research as-builts with the utility companies and/or agencies, work with a locate company, and document existing conditions using a sewer camera. Finally, we look to see if the project’s civil engineer has verified all existing underground utilities for conflicts.
And in the design development phase of the preconstruction we turn our focus to hazardous materials (for example: contaminated soil, underground fuel tanks, and asbestos materials.). Has a geotechnical report been completed? Has a hazardous material survey been completed? And have the locations of hazardous materials been shown or mentioned on the plans?
It is amazing how often, after all of that, we still find surprises underground. Here are stories from 3 of our current projects that talk about what we still uncovered while digging underground and how we worked to remediate them.
Renovation in Seattle
Our work in this almost 100-year-old building involves pouring concrete shear walls from the basement up through the 8th floor in 6 locations. We have to excavate below grade inside of the building to pour new footings to hold this new weight. For the first two shear walls, we added 34 micropiles after finding that existing footings were 18 feet below grade as opposed to the expected 9.5 feet.
During excavation for the fourth shear wall, we uncovered a buried oil tank. To remediate, we had the tank pumped out, decommissioned, and filled with gravel so that when buried again it will bear weight just like native soil.
Remediation in Tacoma
Our project site in Tacoma had previously been the location of several gas stations over the years, but had undergone two thorough clean ups and was granted a Department of Ecology “No Further Action” required letter.
Unfortunately, on our second day of excavation for the new building, we discovered an underground fuel storage tank. Ten days later, following an expedited permit process, the first tank removal was started under the watchful eye of no less than 15 government officials, environmental consultants, and specialty subcontractors. What we thought to be one tank turned out to be two tanks with a small pressure tank as well; all removed that day.
The very next day, after all the officials, consultants, and subcontractors had demobilized and left the site and we began excavating again, we discovered another underground fuel storage tank. This required us to start the entire remediation process again. After obtaining the proper permits and rescheduling all the government officials, environmental consultants, and specialty subcontractors, the entire clean up and removal process was repeated.
On the day of the second fuel tank removal, we discovered an old septic tank in our excavation area. And the following day we discovered another. We removed these tanks from the site and properly disposed of them, excavated and exported the surrounding contaminated soil, and imported and placed structural fill material for work to continue.
Remediation in Seattle
As we began to excavate under the existing parking lot for the new building, we discovered contaminated soil located mainly as expected, however the quality of the soil (sometimes clay, sometimes fill material or sand) was so soupy and the unrelenting rain created such unsuitable soils (akin to quick sand at times) that much overexcavation was required by the Geotechnical inspector.
During this activity, we uncovered a buried fuel tank which was not expected. The soils report indicated that all tanks previously on site had been decommissioned and removed. We immediately called in the required city inspectors and began to decommission the tank, including pumping out and properly disposing of the liquid in the tank, and had it removed from the site.
We then continued excavation of the contaminated soil found below the tank which required us to excavate farther down than originally planned. Finally, we backfilled the area with suitable soil for construction of the new building, per the direction of the geotechnical engineer.
It is said that prior planning prevents poor performance, but in the case of hidden objects buried underground, sometimes all the analysis available won’t avoid the ascertainment of accidental articles. That’s a stretch, but really, despite thorough planning, potholing, and preparation, sometimes we discover items in the ground that just shouldn’t be there. Sometimes several. And we must take the necessary steps to remediate them before continuing with our project.