Material specification for backfill behind concrete structures

[Good_Guy]

would like to know if the material to be used as backfill behind concrete structure is OK

Specification requirement

Specification for backfill material


a) Native Backfill - Excavated materials such as clay and muck are not suitable as backfill material. Others that are free from grass, roots, brush, vegetation and boulders may be used. Rock pieces having maximum dimension greater than
75 mm shall not be used as native backfill.

(b) Imported Backfill - Any earth material except clay, muck and boulders`
satisfying the specification for native backfill and is capable of being compacted in the manner specified in the drawings.


(c) Soil materials that have a plastic index higher than 30 are not permitted. For granular fill materials the following grading shall be met



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Proposed material property : The geotech engineer says that a gradation report is not required as only the PI<30 is enough

The material to be imported is a CLAY/Silty CLAY, fine to medium gravel, yellow/brown in colour and PI<30


IS THE MATERIAL OK to be used as backfill behind concrete structures?
Thanks to all

 

[oldestguy]

Depends on what pressure wall is designed for, how placed(compacted, etc.) What is the drainage system? Cost of material?

 

[Good_Guy]

Thanks oldestguy

The design has been good and there is a drainage layer or single size aggaregate between the concrte wall and this fill separated by geofabric and a drainage pipe under it

The issue is that the native fill (excavated for the concrete structure) is not usefull as a backfill material and hence has to be imported from about 30 kms away

The information that I hope to get here is

1) is PI<30 acceptable as the only criteria
or
2) do i still need a gradation report to ensure that the bulk of the material I am hoping to get here is not clayey

 

[GeoEnvGuy]

Where I work we call it granular B, also known as coarse to fine sand with less than 10% fines, obtained from a natural sand and gravel deposit.

A PI of 30 can still hold water and not drain freely enough to the drainage layer. As for cost to the client for hauling suitable fill materials, with engineered materials you get engineered performance with less than suitable materials you get less than suitable performance.

 

[Mad Mike]

I'm confused- your specification for both native and imported backfill reads that CLAY is not suitable.

You then say that your proposed backfill material is CLAY/silty CLAY, fine to medium gravel. Which is it, a clay or a gravel?

A PI of 30 can be really rubbish material; so I would imagine you need to revert to the specified "No Clay".

How is it that you're hauling 30kms to bring in a CLAY/silty CLAY...is there nothing better within a 30km radius?

What is actually suitable on the project depends not only on the wall design, but also the settlement that can be tolerated above the back-fill, and it's stability if incorporating a back-slope. I wouldn't want material with a PI approaching 30 in my backfill unless there is no consequence whatsoever to its settling / saturation / potential instability if a back-slope.

You should describe your projects more carefully if you want good answers - start off by giving the details and height of the structure proposed, followed by the details of the proposed back-fill...are slabs to be constructed on it? Will it be formed to a substantial back-slope?

Often the project specification, and the willingness to bend it to accommodate shoddy soil conditions, depends on the cost of the structure- so if this is a 30' high reinforced concrete wall, the answer might be very different to that for a 4' high concrete wall.

All the best,
Mike

 

[r13]

Type and use of the structure is very important - basement wall, retaining wall, gravity wall. What is your structure and use?