earth retention, temporary

[classyhillbilly]

Hello, all,
I am an excavation contractor in the seattle wa area and i have a challenging situation. We are demoing a house with nine foot tall walls that once we remove them we will have a hole or void in the building lot, the problem is the earth/dirt is non structural and we cant slope it as the adjacent houses are very close to our lot and the dirt just isnt condusive to this. The question is what can we do that is both cost effective and temporary as we are going to build a new house in the same spot.

thanks, Dan

 

[PEinc]

How close? The adjacent homes probably also have deep basements. Therefore, you may not be undermining the adjacent homes. You may, however, need to protect their yards or lawns from falling into your sloped excavation.

How much distance is there between foundation walls of the homes? Where is the property line? Is it in the middle of the space between homes? Are there any underground or overhead utilities to worry about?

It may be cheaper and faster to slope your excavation onto the neighbor's property (with their permission only) than to shore your excavation. However, you may need to pay something to the neighbor(s) to let you dig on their property. You would also need to restore their lawn to original or better condition.

 

[PEinc]

P.S. Whatever you do, make sure you do pre-construction, during-construction, and post-construction surveys of the adjacent homes and properties to avoid any bogus claims for damages. Let the neighbors know you want to do this. Good communication with them is critical.

If you need to do any shoring, drilled-in, cantilevered soldier beams and lagging may be the most economical method if the soils and ground water conditions are suitable.

 

[classyhillbilly]

pe thanks for the response, sorry for the lack of details...the adjacent homes are approx 10 feet apart ( property lines are approx. 5 -8 feet a part)so it will be almost impossible to get in their property and slope.

We already did some demo and i tried to slope or build a bench to stabilize slope but the material is a glacier till/organic top soil material with no strength, and forget about it when it rains.

thanks again, Dan

 

[PEinc]

Shouldn't there be only one property line between the two homes? Usually, but not always, the property line would be centered between the 2 houselines.

If the other house is 10 feet away from your house, then you could make a sloped, open cut if the neighbor gives you permission AND if the neighbor's house has a basement as deep as your proposed basement. If not, you will need to install sheeting about 3 feet off the face of your proposed foundation wall.

Glacial till is not a very descriptive term for soils. It can mean a mixture of several different soil types. Usually, the till is very competent. Usually, the topsoil is very shallow in depth and should not be a factor.

 

[GeoPaveTraffic]

Recommend that you get a local geotech involved. I have worked the soils in the Seattle area, and they are nothing to mess around with. Shouldn't be too expensive to get a simple design from one of the local companies. Best if you sit down with them, show them the current conditions, and the plans for the house that you are planning to build. They should then be able to come up with 2 or 3 preliminary options of which you can choose the one for them to final design.

 

[civilperson]

Do partial demolition, (5'-6' horizontal portion), and backfill that portion immediately as high as possible. then remove the next five feet and fill that portion and raise the first portion as much as possible. With steep slopes, only 15' to 18' will be unsupported, and temporary braces may be used in that area to help support the removed wall portion.

 

[MSEMan]

Have you thought about NOT removing the underground walls and building your house on top of the existing foundation? It may mean less space in the basement, but given your situation it seems easier to improve/strengthen what you have below ground rather than demolish it and create new problems.