Making use of temporary cohesion in clean sand

[oldestguy]

I have an engineer friend designing an addition to a town hall where an addition with basement will be built next to an existing building. The existing building sits on an old shallow depth stone foundation with 2 foot crawl space.
Those floor joists run parallel to the shallow foundation. The foundation therefore has very light load on it.

Soil is clean sand with maybe a 30 degree angle of repose when dry. When damp it stands at 45 degrees or better, but only due to that temporary cohesion.

He would like to place the new 8 foot depth basement wall about 6 feet out from the old foundation.

The client has very little in the budget for tie-backs, sheeting, etc. Short of placing the new basement wall well out from the old building, he is searching for a cheap way to do it closer.

This will be bid, so his plans may not be able to show "how to do it". The wall may be either CIP concrete or block.

Has anyone had experience where you make use of temporary cohesion in damp sand for short term construction with an unretained steep earth slope next to the forming for the basement wall? Admittedly OSHA has to be happy with it.

 

[PEinc]

I would not use temporary cohesion with sand. You never know when a job will be delayed or shut down after it has been sheeted and excavated. Also, what happens to this steeply sloped open cut if you get a heavy rain storm or downspout discharge?

Also, the weight of the wall itself may be significant even though the joists run parallel to the wall. Sheeting probably is not appropriate for supporting the building.

Even though the new wall is 8 feet away, you will still need extra space to build, backfill, and waterproof the new wall. Therefore, your 6 feet may be reduced to as little as 3 feet. It looks like you should underpin the existing building, if allowed.

Make sure that the wall along the excavation is properly supported against lateral movement. A free-standing wall could be trouble.

Just because the client has very little money in his budget for tiebacks, sheeting, etc., that does not mean you should accept his problem and risk.

 

[fattdad]

I would not account for cohesion in sand either. When it goes, it really goes!

f-d

¡papá gordo ain’t no madre flaca!

 

[lovethecold]

I don't recall the specifics, but, another engineer in the office was telling me about a similar situation where he did an EO. An excavation next to shallower footings. A company came in and made a grout wall to hold back some sand. I would guess the sand wasn't clean and probably extended into sandy lean clay. Perhaps this or something similar could work.