Temporary shoring question

[Vashon118]

Back in 2008 construction began on a mid-rise apartment (with ground floor retail) project in Seattle. Excavation was mostly completed when work was halted (and has yet to restart) near the end of 2008.

Pictures of the shoring system:









How long can this temporary shoring last until it weakens to the point that the hole would have to be filled back up?

[dchan]

It looks like a tieback system, but I'm not sure if it's merely a temporary retaining wall.

For one thing, the wall is made from concrete that took multiple pours to reach its final height. Pouring a concrete wall isn't cheap, and it certainly isn't a common option for temporary shoring, if it's an option at all. Also, what are you going to do after the temporary concrete wall is finished being used as temporary shoring? Knock it down? The demolition costs for that are going to astronomical for temporary shoring.

The one thing that initially confused me about this is the anchorage for the tieback system. For most tieback systems, the tendons that tie the wall to its anchors are anchored underground. But then again, most common tieback wall designs are meant to hold back earth for under-bridge crossings. Therefore, they have the luxury of having enough room to be anchored underground. With the tendons and anchorage hidden away from the surface, the result is somewhat aesthetically pleasing.

As you can see from your first picture, the amount of room doesn't appear to be a luxury here: there's a building directly next to the development. Therefore, the geotechnical engineers likely had to modify the tieback design based on the available space to anchor the tendons. Hence, the reason why the anchorage appear above ground (though the exposed tendon ends look kind of gnarly).

So if it's the case that this is actually a permanent tieback wall system, then it can last as long as the design lifespan dictates. I would say at the very least decades, then.

[Nowhereman1280]

That's not temporary at all. If you look at the first picture those cylinders in the ground with rebar coming out are big cement columns drilled down to act as the permanent shoring for the foundation. I suspect that is an early picture of the site before the hole was dug?

Those big columns ring the entire site and are what is holding back the dirt (probably by a tieback system). They aren't going anywhere for probably a couple hundred years even if the building is never finished.

[dchan]

Originally Posted by Nowhereman1280

That's not temporary at all. If you look at the first picture those cylinders in the ground with rebar coming out are big cement columns drilled down to act as the permanent shoring for the foundation. I suspect that is an early picture of the site before the hole was dug?

Those big columns ring the entire site and are what is holding back the dirt (probably by a tieback system). They aren't going anywhere for probably a couple hundred years even if the building is never finished.

I would imagine the OP took all of the pictures at the same time. In any case, the condition in the first picture appears 'undug' because anchorage columns are located a reasonable distance from the excavated hole. This is so that the anchorage is located a certain distance away from the tieback wall so that it can have the proper leverage to actually tie back the wall.

The anchorage columns do not necessarily have to ring around the excavation site. As you can see here, the anchorage columns are located at a very specific location: directly adjacent to a building in the neighboring property lot. The lack of space on this side of the development forced the engineers to design the anchorage columns like this. Had space been not an issue, they would have likely simply buried all of the anchorages neatly underground.