The Seattle “Big Dig” was a unique and interesting project for MacKay Landscape Services. The owner of an historic apartment building located at lower Queen Anne Hill wanted to expand the square footage he could rent to tenants. His idea was to build down, as in under the building, to create 2 new apartment units in this prime real estate location, increasing his commercial property value and monthly rental income.
Curious thing about building down: under this 1929 building existed a 75 ft x 48 ft x 8 ft space filled to the brim with native Seattle soil. The only access to the underground area was an 8 inch high crawlspace spanning from the entrance to the back wall. Our challenge? To excavate the entire space while maintaining the integrity of the building with the least disturbance to existing tenants. Let me say that again. We did this entire underground excavation while the building was fully occupied with tenants. A challenging prospect, but I knew my crew was up to the task.
As we had only 8 inches of vertical clearance to begin with, this was the definition of a tight quarters excavation. Creating a clear plan was the key. We started by jackhammering the hardened clay soil, loosening it from ceiling to floor, then removing it one 5-gallon bucket at a time. Once we’d gone in 3 feet, the concrete contractor installed temporary posts and beams on our left and right, to shore up the building as we continued to work forward.
Moving ahead, we cleared a hallway 6 ft wide x 6 ft deep, leapfrogging with the concrete contractor to install more posts and beams as we worked. The post and beam installation was repeated at specific intervals as we gradually excavated the length and width of the space by hand, one 5-gallon bucket at a time. No conveyor belts were used, so access to the building’s main entrance and hallways could remain clear for existing tenants.
Over 2-1/2 months, we excavated 309 cubic yards (8350 cubic feet) of soil in total, all hauled out manually. At approximately 1/2 cubic foot per bucket, a total of 16,700 buckets of soil were carried out by hand. A historical note, we discovered this entire space consisted of undisturbed native Seattle clay soil, meaning it has been part of the land in this area for decades, even centuries. I can only imagine the stories it could tell…
In case you’re wondering where all of that soil we hoisted out of the crawlspace went, we staged it in the back alley across several parking stalls. We periodically brought in dumptrucks and frontloaders to haul off the ever-growing pile of soil.
When the excavation was nearly complete, existing utility lines had to be rerouted 8 feet down to meet the base of the new floor. To prepare for this, our crew dug new trenches for the plumbing and sewer lines.
Once the utility companies did their thing, then the concrete contractor poured a new slab and walls, removed the temporary post and beams, and installed a permanent post and beam system to ensure the integrity of the newly-established space. Next up, the carpenters came in to frame, add window wells and begin new construction of the basement apartment units.
Overall, this was a complicated job which my crew carried out with skill and proficiency. The entire project required us to leapfrog, tagteam, and be flexible with other contractors on the site, as well as respectful and accommodating to the building’s existing tenants as they went about their daily lives. The entire project was completed in 2-1/2 months with little to no snafus, and a lot of soil.
Together, the excavation and new construction created an additional 1-bedroom unit and 2-bedroom unit, elevating the downtown apartments’ commercial property value and increasing the owner’s income potential for years to come.
You may also be interested in: