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Cohesive Backfill To A Structure
3

Cohesive Backfill To A Structure

Cohesive Backfill To A Structure

(OP)
We have excavated a 5m deep hole in slightly overconsolidated clay in order to construct a water retaining buried structure.  We have sub-let the design of the structure and our designer is specifying that we import selected granular material for backfilling to the structure.

We have identified a saving by using the excavated clay as a fill instead of importing material.  Our designer tells us that the swelling pressures in the fill would cause overloading to the walls.

If we  place the fill and compact it to a similar void ratio and moisture content to the in-situ clay then will it be able to swell?  What is more, in the long term I believe that the effective stress parameters for the drained clay will be c'=0 and phi'= approx 30 degrees (which is not unlike the parameters for a granular fill).

Does anyone have a good reference on swelling clays and the loads generated by this process.  Also, do you have a view as to whether the clay will actually swell.  Is this a minerology problem?

Regards

Andy Machon

 
 

RE: Cohesive Backfill To A Structure

You didn't give any details on the clay at your site(existing moisture content,fines content, PI, swell potential, etc.),so I can only generalize.  It is usually not desireable to use a CH classified clay as a wall backfill since it can shrink and pull away or create vertical ground cracks if it dries out, allowing debris to migrate in from above, which in turn keeps the cracks from closing when wet weather comes and causes the clay to swell and expand. Repeated cycles of opening and closing cause the clay to compress laterally, building up large lateral pressures. You can run a swell test in the laboratory to see how much swell potential the clay at your site has. CH clay is also difficult to compact due to its high plasticity. What if the water retaining structure leaks at some time in the future?  CL classified clays and clayey sands (SC) are OK for use as structural fill.  Are your clays CL or CH?

RE: Cohesive Backfill To A Structure

(OP)
KAM

Thanks for the advice.  I have posted the question on behalf of a colleague so I will ask him tomorrow about the classification of the clay. (I should encourage him to use this site himself - perhaps this will be the catalyst !!)

Regards

Andy Machon

 
 

RE: Cohesive Backfill To A Structure

Andy,
What about using one of the "compressible" geotextiles against the wall?  This would allow mitigation of lateral swell pressure, even though that is not really their intended purpose.  Even two layers of the material would likely be less expensive than importing select fill.

RE: Cohesive Backfill To A Structure

This forum points up the regional conditions which determine how each of us will approach these problems.

 I practice in a semi-arid to arid climate. The majority of clays I deal with are CL and even CL-ML and they are responsible for significant backfill pressures against structures. I believe the question of what the actual classification of the clay matter (CH CL or SC) is as important as to the actual climate you are dealing with. Significant dessication cycles of any of these clayey soils can result in overconsolidated clays.

KAM's discussion of shrinkage and reswelling is right on but, should not be restricted to CH Clays. I have found thatvery careful soil placement and compaction, involving both 'minimal' compactive effort to serve the required purpose and soil placement at 'optimum or slightly above optimum moisture content' can be very effective if - I repeat if the long-term backfill moisture content will not greatly fluctuate.

The use of a geotextile may be effective in some areas but, in my arid climate, I would be very concerned that the geotextile may contribute to the moisture fluctuations by creating a path for moisture to move.

RE: Cohesive Backfill To A Structure

Great discussion

Ginger: what did you end up doing. Did you use the geotextile?.

RE: Cohesive Backfill To A Structure

(OP)
Hi chaps

Many thanks for the excellent input.  In true bold contractor fashion, after much argument, we capitulated and pandered to the wishes of our designer!!!!

We were able to select a fill whch could be compacted to give a phi = 40 degrees.  There were some savings to be had in the wall rebar quantities based on the reduced earth pressures exerted by this top notch fill. That would offset some of the cost of importing the material.  It also reduced the risk to the client that swelling would take place around the structure (but at our expense - not the designer's you might note !!).

As an aside, I am still a little unclear as to the mechanism of swelling and how one calculates the stresses induced by the swellng process.  To swell does the void ratio have to increase or is it simply water filling the air voids?  Is it a function of porewater pressure?

Regards

Andy Machon

 
 

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