Clay loss of strength from construction traffic

[RJ62]

At a recent site redevelopment, the thin AC pavement and base course of a used car lot were stripped and grading begun for shallow foundation structures. The soil under the base course was lean clay (CL), medium stiff to stiff (I don't have the blow count), but with a high moisture content (14 to 18%?). Shortly after grading began, the cyclic loading from the construction equipment remolded the clay enough to induce a loss of shear strength resulting in graders and backhoes creating huge ruts and depressions. To stay on schedule, the owner lime treated the soil.

Is there any research that would indicate that a loss of strength is likely, and how much, given a clayey soil, blow count, and moisture content? In other words, how do I communicate to the owner and contractor that the soil condition are such that cyclic loading from construction equipment travel may cause softening of the soil and the associated problems?

 

[TDAA]

ASTM D2166 - Unconfined compression.

Run both in-situ and remolded samples of the same material.

 

[conceng]

For what it's worth, I've dealt with a lot of contractors who think they are "pumping up water from below" when this loss of strength happens. It's rather difficult to explain the loss of cohesion of the soil particles for the now weakened soil when they think it just got wetter. The best way I've found to illustrate my point is to take a rather undisturbed, firm sample in my hand and start kneading it. It becomes softer the more you work it. I sweat a lot, but I don't think I even I could convince them that I'd get that much out of my hand that fast. This works specially well on silts.

Greg

 

[SirAl]

After all the removal and prepping operations including stabilization/modification that have taken place do date, I would evauate the insitu soil conditions and apply an appropriate bearing capacity using SPT, Qu, or other means and, of course, your professional judgement. You should have something to hang your hat on. I, personally, would have to probe below subgrade to determine the nature and condition of the subsoils as this is just as important as the bearing surface of the shallow foundation. A site specific geotechnical investigation would be appropriate for a permanent building.

 

[Ron]

Near surface clays may be underconsolidated, with an increased void ratio. You are experiencing instability as a result of this.

If you look at a Proctor curve of this material, you'll see that it is highly moisture dependent. A stability curve (CBR), will likely show a shallow slope on the "dry" side of optimum and then it drops steeply on the "wet" side.

It is likely that when the original construction was done, the moisture content of the material was lower, giving the contractor a false sense of stability and true compaction, but good constructability for the application. With changes in development, runoff, and the water table over the years (and considering that you might have had some rain after opening the soil surface for this construction), you now have a moisture content that is just right to show the instability that wasn't evident in the original construction.

The present construction methods, using rubber-tire vehicles, results in the kneading that Greg mentioned. The kneading removes air and provides more solid particle/water interaction than was there before, thus the "remolding"...in short, while the moisture content by weight doesn't change, the moisture content by volume goes up.

There are numerous treatises, articles and books that cover the structural interaction of clays and clayey materials. All of the classical geotechnical texts cover it to some degree (Terzaghi, Casagrande, Sowers, Lambe, Bowles...et al).

 

[oldestguy]

I'd keep in mind the conditions you found for future reference for the next time in that general area. It can be very common in certain areas and in certain seasons.

I think the discussion above about simple on the job hand tests to demonstrate this loss of strength is a good one, so you won't have to look in some reference book to predict what will happen. Another simple test is to stand in one spot and tromp your feet as if marchung in place. Soon you see the effect.

I have seen this many a time and I am not so sure it is related to recent rain, etc. I suspect your actual moisture content was at the high range of your numbers or higher.

In any case, if you have the chance to direct what goes on, when they call up and say they have the problem, you can give advice possibly without having to go there to see. A common reply I have given, is "Wait until tomorrow and then make sure the drivers take different routes".

For some reason truck drivers tend to follow the tracks of the guy ahead of them and get stuck.

Front end loader operators also have the same habit.

 

[Drumchaser]

3 cents/non scientific

A high PI clay, at opt. moisture and density will rut under a motorgrader significantly.

Stabilization using Lime (placed correctly) is a great way to start when only placing thin surfacing layers on heavy/medium clays.

Sounds like this subgrade may have been inferior at the start, with enough base/surfacing to hold up the loads (automobiles)

 

[cntw1953]

RJ62:

What is the end usage of this development? Were there/have you requested soil samples been taken and testing done?
Are you satisfied with the result of treatment by lime? I can sense you have some doubts in mind, and I knew where they coming from, but I couldn't tell the urgency, and what you are up to. I think we all can benefit by hearing more from you.

 

[RJ62]

Thank you for the replies. I've included some additional information about the site.

The final construction is for several two-story, shallow foundation buildings and an area of AC used for foot traffic and occasional maintenance vehicles.

A "standard" subsurface exploration was conducted before stripping the existing AC. Lab tests conducted on soil samples from 1' to 2' below grade(BG) indicated: Lean Clay, moisture content was approx. 18%, dry density 101 - 116 pcf, PI 9 to 19, LL <36, and an R-value of 11. Unconfined compression tests, at 3 ft BG, showed 0.8 to 1.1 ksf. Water Table at 43' BG.

In response to the replies, an unconfined was not done on surface samples (0.5 to 1.0 BG). The results on an in-situ sample and remolded one would provide boundary strengths but would not indicate the likelihood that loss of strength would occur. The rutting did not develop over the entire site.

The site layout was very tight so the contractor was limited in his traffic patterns and construction sequence. Kneading a piece of clay by hand is useful to instruct the contractor and developer, but it's difficult to translate the hand kneading into "number of passes of a scraper/grader before rutting occurs".

As Ron mentioned, any basic text on soil mechanics discusses soil-water interaction (mine include Bowles, Coduto, and Holtz and Kovacs). Holtz presents a graph showing strength as a function of compactive effort and moisture content (after Seed and Chan, 1959), it does not take into account cyclic or kneading action.

cntw1959, the lime treatment worked well, though expensive.

I may have confused things with my second question in the original post. If you are presented with the above lab data, and existing site conditions (the AC covering the site), what additional lab data and/or exploration do you need to indicate in the geotech report that "...loss of strength and severe rutting will likely occur if more than 100 passes by construction equipment is permitted".

I know the language is terrible and would give a lawyer fits, but this is the essence I want to reach. "Loss of strength is likely", with something solid to "hang my hat on".

RJ62

 

[TDAA]

Running the in-situ and remolded samples at the same moisture content will tell you more about the soil than just the range of values. Use the ratio of those values to get the sensitivity of the material. The effect is discussed in texts, and the calculation shown in the ASTM I noted (qu remolded /qu insitu).

Unless you have outside issues, like the material is uncontrolled, old fill on the site, or additional water coming in contact with the soil, you are getting a loss of strength due to the remolding process. This process is breaking down the clay particle structure developed during the sedimentation process. It may not hurt to see if the material is sensitive.

 

[fattdad]

having not read all the replies, apologies in advance.

Perfectly stable clays can have moisture contents that are close to saturation (i.e., too great for compactive effort and over "optimum"). This is not a problem under static loads.

When you have near-saturated soils and you impose a dynamic force (i.e., compactive effort), you will quickly reduce the soil to mud.

From my perch in the tree, projects should have a geotechnical engineering study that shows the nature of the soils in advance of construction. If you turn over to a contractor a site with stiff clay and his construction effort reduces it to mud, then there just may be something in the contractor's means and methods that was not o.k.

There are many natural soils and in particular natural clays that have perfectly natural moisture contents that are way over optimum. The simple act of construction leads to the problem. Just what steps the contractor takes to preserve this soil strength is very important.

Then again, did the geotechnical engineering study provide any reference to this at all?

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

Might want to look at this:

http://www1.agric.gov.ab.ca/$depart...Indicators_of_Soil_Compaction.pdf?OpenElement

 

[RJ62]

Thank you for your comments and suggestions. I have a few things to read.

RJ62

 

[Bob83]

Good post...... very informative, hope the government of all countries follow some steps to avoid the construction traffic and spare the citizens of the at peace.... as the work stress is increasing and the city like is becoming uncomfortable its butter to avoid such disasters...

Regards,

http://www.equipmentsjunction.com