backfill of native clay area with sand 1

[BLSPAM]

A client of mine is backfilling a 11 ft tall concrete basement wall with an 8 ft wide swath of good sand (about 30psf/ft). The native soil is clay (about 54psf/ft). How wide should be swath of sand be to use its propertied in the design of the wall? I originally designed the wall for resisting clay soils and the reinforcement is more than their previous engineer had specified on such a project. The backfill will be well drained with a drawing tile system.

 

[dik]

I would not backfill with sand... 1" clean crush with a geotextile liner... We are in a frost susceptible area and this would be a poor idea... specially for an 11' retaining wall.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[EireChch]

If by good sand you mean well graded with fine to coarse sand then frost shouldnt be an issue. I always assumed it was only silt and fine sand with silt that heaved?

Not sure what you mean about how wide should it be to uses its properties to design the wall?? can you show a sketch.

If the sand is with the active and passive wedges of the retaining wall then you can assume its properties, if its half sand and half clay withhin the wedge then you need to consider separately.

 

[dik]

It is... for frost heave. Sand may retain water due to the small particle size which may freeze. Our design frost depth is 6' and when they did the renovations at Polo Park about 30 years ago, in the parking lot area, they found frost at -12'. I still like free draining gravel, and 11' is a fairly high retaining wall.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[BLSPAM]

The backfill should drain well. It will also have a drain tile system and be adjacent to a heated space.

My question is "do you design the wall for the properties of the backfill or the native soils".

I once debated the use of geo-foam in lieu of soil backfill in order to reduce the load on an existing wall. In my mind, unless there is sufficient foam to resist the lateral load through friction to the soil below, the soil load against the geo-foam would be imparted on the wall. This is kinda a similar debate.

Another can of worms.... A retaining wall 2 feet from a large, adequately designed, building foundation wall, sees' what soil load? Both are the same height from footing to top of wall.

Thank you for entertaining these questions.

 

[BLSPAM]

"If the sand is with the active and passive wedges of the retaining wall then you can assume its properties, if its half sand and half clay within the wedge then you need to consider separately"

now it makes sense.

thank you.

 

[StrucPatholgst]

I once debated the use of geo-foam in lieu of soil backfill in order to reduce the load on an existing wall.

I was just at a very large retail property in NY State that had a large field of Geofoam blocks on one outer side of their cavernous basement. The non-foam foundation walls were cast-in-place concrete, the other once was tilt up concrete. They used it to improve conditions under the parking lot along the tilt-up wall. 300 truck loads of Geofoam.

I wonder when that recycled glass block product is going to put into that kind of use. The one that's currently supporting all six lanes of I-95 in North Philly.

 

[LOTE]

Use 45+phi/2 degrees from the foundation to determine the active wedge.

You can use bin pressures for determining the soil load when there is a stable feature (like a fully supported building foundation) a short distance behind your wall. Reference author Handy (I can lookout the full reference if you need it).

 

[DanielSgeo]

Hey LOTE I would be interested in that reference. Please provide. Do you mean like granular silo loads?

 

[LOTE]

I had a copy at one point, but am struggling to find it. Here is the article:

https://ascelibrary.org/doi/10.1061/(ASCE)0733-9410(1985)111:3(302)

 

[DanielSgeo]

Thanks LOTE!