21 Boston – 02 – Site Preparation

The previous Safeway Store sat back from Queen Anne Avenue behind a large surface parking lot, with only a kiosk out at the street.

It represented a typical, somewhat suburban, approach to site planning, based on the automobile. As Seattle continued to grow, the inefficiencies in the use of the site became more and more obvious. A development of a Safeway property on Lower Queen Anne made it clear that combining housing with a supermarket could provide a much better return on investment. Through an exploration of a variety of design approaches the developers, CG21Boston and barrientos RYAN, conceived the current 21 Boston project.

The new approach required underground parking and a much larger Safeway store plus housing above. The site would need to be cleared – and the store went first.

It didn’t take long.

Contractor Compass Construction posted some general information about the project and their process.

And soon they were moving serious amounts of dirt out of the site.

The basic plan was to dig a giant rectangular hole 2 stories deep, build two floors of parking space, put the new Safeway on top of them at street level, and then build a variety of housing buildings on top of all that.

Here they were at the dig-the-giant-hole phase. This view is along QA Avenue, looking south. The first step was to drill a series of deep holes along the edge of the building site and then insert a vertical steel beam column into each one. If you look at the lower right hand corner of the photo, you can see the top of one. Between the columns wood lagging gets inserted behind the beam flanges. This holds the soil around the project in place so the sidewalk and street don’t collapse.

The trick is that you have to start at the top and build down, so it’s a step at a time. The soil will hold itself in place for a few feet as the digging is done, but after that it will start to slump; so the boards get inserted before that happens.

The boards are roughly attached to the steel columns. Then more digging happens and more boards get inserted below the ones already there. That’s tricky and timing matters.

The guys below are setting the boards in place. They can work up from the bottom for a short distance, say 5 or 6 feet; but then they have to shovel dirt behind the boards (man in background) before wedging the last board in place (guys in foreground). It’s both crude and amazingly accurate at the same time – those columns are in a straight line.

Eventually the hole gets deep enough that the concern is that the soil weight behind the board and steel wall could start to push over the whole wall; To prevent that, holes are drilled through the boards; and tubes and cables are inserted (you can see them sticking out of the wall below). A long hole is drilled through each one, including a pocket way under the street, cables and attachments are inserted, the tubes and pockets are filled with concrete, and the cables are tensioned to hold the wall in place. They’re called ‘tie-backs’.

It’s all a bit of magic, but it’s not designed to last forever. The idea is to hold the walls in place until the garage walls and floors are built, at which point they will hold back the soil.

Of course a hole this big and this deep holds a lot of dirt; and it all has to be removed. As the digging proceeds around the perimeter so the walls can be built, the dirt is all pushed by bulldozers into a giant pile in the middle. A regular train of dump trucks (singles and doubles) drives into the NE corner of the site and circles around the pile where a font end loader sits on top and fills them as they come by. The bulldozer in the middle of the picture keeps moving the pile towards the loader.

Up at the corner of 1^st N and Boston the site is not yet excavated. That’s where the ‘haul ramp’ is located to be able to get the trucks in and out. The heavy stones provide a temporary sturdy roadway.

They work like gangbusters to take advantage of the good weather. You can imagine what this process would be like in the cold and rain.

Next: Tower Crane