Expansive Subgrade below Pavement 4

[emmgjld]

I am looking for various methods of constructing flexible (asphalt) pavement structures on medium high to highly expansive soils. Accounting for the actual support of the traffic loading is not the issue. My issue is the total heave of the pavement/sidewalk section, with the associated pavement cracking and separation of the pavement and the curb/sidewalk. Over excavation of the expansive subgrade and replacement with gravel or wetted & moderately compacted clayey soils are the most common methods but, no real, consistent criteria is used, other than I have done this for 28 years. Anyone have published or unpublished methods and criteria? Discussion of other methods of dealing with the subgrade heave would be appreciated.

The particulars are:
Pavement Design Method is AASHTO 93. Light to Heavy Residential loading (EASL from 5 to 100, typical).
The area is arid to semi-arid. Soils and formations are dry to sl. moist. Soil Moisture Stabilization takes 5 to 10 years after development.
The soils are (or are derived from) Shale and Claystone, UCS - CL to MH to CH.
Hveem Carmany R Value from <5 to 16, Swells from 115 psf to 200 psf, Displacement of 3.8 to 5.1. Swell is critical, (Low to Very low CBR values).
Soil Swell, ASTM D-4546, method C Constant volume swells of 3,000 psf to 16,000 psf, with rebound upon unloading of 2.7% to 13.5%.
Soluble Sulfate Salts, may be disseminated throughout soil and as crystalline gypsum in fractures and bedding planes.

 

[Focht3]

Where is the site? How much pavement is involved? Or are you simply looking for 'a better way'?



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[gandersen]

emmgjld:

The only approaches that I know to minimize heave of pavements on expansive soils are related to either preventing the soil from experiencing large fluctuations in moisture content or decreasing the expansion potential (either by undercutting and replacing or some type of chemical treatment). In Jackson, Mississippi (USA) the highly expansive Yazoo clays are usually undercut and replaced by nonexpansive silts or silty clays with low expansion potential.

I'd be interested to hear what others have done.

Glen

 

[emmgjld]

The sites are in Western Colorado. Many miles of residential streets. I am looking for 'better' ideas, new ideas, old ideas that apparently were not as successful as desired and WHY they were not, to include any speculation. I feel that I have analized the problem fairly well, for my circumstances and climate but, looking for ideas which may be a little more cost effective.

As ganderson notes, preventing moisture fluctuations is important, I have the added problem that as an arid region is 'developed', soil moisture WILL increase, if for no other reason than evaporation is minimized by covering the ground with pavements, buildings and landscaping.

Removal and replacements works, but what criteria is applied to determine the amount of removal/replacment? As I noted, 28 years of experimenting and I have a fair idea. I would appreciate any thoughts on criteria. I remember my own frustration with my father when he would tell me 'This method seems to work, but I cannot put numbers to it'. Now I am in his spot as I deal with municipal development engineers who are asking for justification for my 'designs' and recommendations.

 

[Focht3]

I sympathize. In reality, the best evaluator is an experienced engineer. The various methods for performing calculations need to be tempered with a liberal dose of engineering judgment.

The Potential Vertical Rise method is pretty popular in central Texas; since it was developed here in San Antonio, it is essentially "calibrated" for our soils and climate. But you might find the calculations useful. Ghazzaly (sp?) and Vijayvergia (sp?) proposed two methods to predict swell, and used lab swell tests from around the world in their database. I'd read the paper, anyway.

I'm interested to see what others think are useful techniques -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[Baldie]

I also practice in Colorado, but mostly on the Front Range. In my experience, moisture treatment of the native soils to greater than optimum MC usually reduces the swell potential (Denver Swell test under 200 psf surcharge pressure at time of sample inundation with water) to levels considered "low" swell potential. 2 to 4 feet of overexcavation is typical. I have never recommended more than 4 feet of subgrade moisture conditioning for roads. (Foundations are another story.) I also prefer to lime treat or cement kiln dust (good CKD sources are limited so this may not be viable) treat the upper 12 inches. I give a structural coefficient of 0.12 to the stabilized layer which usually allows asphalt pavement thickness to be reduced sufficiently to offset the cost of stabilization. This assumes the contractor achieves 190 psi on unconfined compressive strength proctor pucks. Be careful - high sulfates contents can cause ettringite formation if lime is mixed with it. I would not overexcavate and replace with granular soils as this creates a "bathtub" to supply water to underlying expansive soils.

The CDOT pavement design manual provides a table (Table 2.1) relating PI of the subgrade to depth of treatment. Depth of 6 feet is recommended for PI greater than 50, and if PI is 20 to 30, depth of overexcavation is 3 feet. The CDOT table does not factor in the natural moisture content which could be high, resulting in lower swell potential than might be assumed based on PI only. The Metropolitan Pavement Government Engineers Council (MGPEC) provides moisture treatment depths based on % swell under 200 psf surcharge and the speed limit. Overexcavation depths up to 9 feet are recommended, and 2 feet (<35 mph) to 5 feet (>35 mph) would be recommended for swell of 4% to 5%. Jefferson County also has some guidelines.

We use nominal 2" diameter "California" samplers to run swell tests and get natural moisture content and unit weight. I like to have lots of swell test data. I also compare natural MC with PI. If MC is greater than PI, there usually is not significant swell potential, especially if MC is more than 1 or 2% greater. Many times, MC at depths below 4 feet are greater than the upper materials and the risk of swelling is lower. You might be able to show this type of data to the municipal engineers.

Good Luck,

 

[Focht3]

I have a major concern with moisture conditioning: it isn't permanent. The water will stay in place no more than a few months - and certainly less than a year. To me, that isn't worth the effort...

If you're going to inject water, at least add some KCl and "lignin" (wetting agent) to alter the clay's cation exchange characteristics. Use enough KCl and the clay becomes non-plastic -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[Baldie]

Focht3, I respectfully disagree with some of your last statements.

emmgjld states that these roads are for a new residential development. The moisture contents will increase due to irrigation of lawns. They will likely increase a lot based on my experience. Borings made in streets in established neighborhoods usually have moist to very moist subgrade soils.

Even if the roads were not in a development, the moisture content will not "disappear" within a year. Evaporation will be inhibited by the pavement structure itself, and moisture conditioning limits could be extended beyond the roadway edges to reduce the potential for drying beneath the pavement. Some (not me) might argue that the moisture contents would increase because of the reduced evaporation caused by the previously uncovered soils being covered with pavement. I think that over very long periods (decades) moisture conditioned soils would tend to dry out in areas that are not irrigated.

Focht3 brings up an interesting topic of water injection. My use of the terms "moisture treatment/conditioning" refer to overexcavation, adding water, and recompacting to 95% D698 at greater than optimum MC. I understand that water injection is used quite a bit in Texas, but it has just begun to be used in Colorado. Hayward Baker can provide information on this topic out of their Denver office. Are there any success stories with this methodology?

 

[BigH]

Focht3 - KCl - PI. Any luck?? mon ami.

 

[Focht3]

I understand that water injection is used quite a bit in Texas, but it has just begun to be used in Colorado. Hayward Baker can provide information on this topic out of their Denver office. Are there any success stories with this methodology?

Yes and no. Chemical injection was first developed in Colorado and the process was 'perfected' here in San Antonio. Hayward Baker bought that business about ten years ago; Art Pengelly in their Dallas office is HB's resident expert on chemical injection. Done properly - with the right chemicals - it's very effective. The original mix included KCl, NaOH, ammonium lignosulfonate ("lignin"), NaCl and water. The NaCl was dropped - it was a relic of the formula's origins in the oil field. NaOH may or may not be used to raise the pH for a particular application. Lignin is a wetting agent and isn't involved in the chemical reaction per se. Potassium chloride (KCl) is the main active ingredient in the chemical mix -


But water injection is a different animal. Most geotechnical engineers in central Texas won't even consider it - it's too risky. It has been used in the Dallas / Ft. Worth area, but with limited success. Anecdotal evidence suggests the failure rate is much higher than remove & replace or chemical injection - maybe 10 times higher!

Even if the roads were not in a development, the moisture content will not "disappear" within a year. Evaporation will be inhibited by the pavement structure itself, and moisture conditioning limits could be extended beyond the roadway edges to reduce the potential for drying beneath the pavement. Some (not me) might argue that the moisture contents would increase because of the reduced evaporation caused by the previously uncovered soils being covered with pavement. I think that over very long periods (decades) moisture conditioned soils would tend to dry out in areas that are not irrigated.

I understand your position, [blue]Baldie[/blue], on moisture migration. But data collected by one of my competitors and published by Dr. Jean Louis Briaud at Texas A&M University clearly shows that the moisture content underneath homes is affected by irrigation and seasonal effects over a much shorter time period than one year. The lag time for soil moisture migration due to seasonal effects is at most a month - certainly not decades. And the data also suggests that the common wisdom that moisture collects underneath slabs and roadways isn't correct, or at least isn't the dominant phenomenon in heavily irrigated areas. (This is consistent with your comment on streets in established neighborhoods.) The data shows that the soil moisture underneath floor slabs is actually lower than the soil moisture in the "adjacent" lawn for 9 or 10 of 12 months in both Corpus Christi and San Antonio, Texas. (Our summers can be quite hot and dry, resulting in very dry soil for at least a part of the July - October period.) Of course, we use St. Augustine grass for our lawns, which requires 1 to 1[&frac12;] inches of irrigation per week. This "artificial rainfall" outstrips any changes in the soil moisture that may be caused by the presence of the slab.

[blue]BigH[/blue] - my buddy's moving his office, so his files are in disarray. I'll bug him again today.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[Baldie]

Thanks for the comments Focht3. I agree that moisture content of soils varies seasonally. However, the overall trend over several years (say 5 to 10 years) in new developments is an increase in MC in at least the upper 20 feet. A competitor firm in the area that does a great deal of residential construction has compared pre-development suction profiles and MCs versus post-construction profiles, and they have found this to be true. Colorado Association of Geotechnical Engineers is currently investigating the post-development depth of wetting so there should be additional data in the next few years.

I think that specific depths of wetting and the degree of seasonal moisture variation will be specific to a locale. Texas' climate is different from Colorado's. For example, shrinkage of expansive soils is not a common problem in Colorado, but I understand that it is a problem in the wetter parts of Texas. One of my college buddy's regularly waters his foundation near Houston to keep the soils from shrinking. Nobody does this in Colorado, or at least they shouldn't.

 

[dirtguy4]

This is my first time answering a thread. I've been reading lots of your threads and find the advice and help that is provided excellent. I appologize for coming in so late on this thread. I'm from the northern parts of Canada and we have alot of problematic, expansive and compacted clay soils.

Surprisingly, some of the best research that I've seen regarding the problem of expansive soils is the book produced by D.G. Fredlund and H. Rahardjo called "Soil Mechanics for Unsaturated Soils" which where I believe your problem lies.

The good news is, is there are various ways to control your problem.

1. Compaction - Compaction of the highly plastic clay subgrade should be performed at least 2% above optimum and no more than 95% SPD. (To use an old saying - the wetter the better for high plastic clays) Don't over compact high plastic clays. Kneading the soils with a sheepsfoot roller on high plastic clay does two things.
A) It dries out the soil which is what we are trying to eliminate, and;
B) It forces air into the soil.

2. Subgrade soils - Try to use low plastic clay back fill instead of high plastic, that's pretty straight forward.

3. Positive Drainage - properly designed ditches to eliminate water from penetrating into your unsaturated soils is a must.

4. Flux Boundaries - Try to control the amount of precipitation, evapotranspiration, evaporation the subgrade soils.

Others previously mentioned Chemical Injection. I'm not that familiar with chemical injection, but I'm assuming it has some controlling mechanism (salt) on the osmotic suction and permeability of the clay soil and this is why it works(or doesn't)?

Take the money you would use on chemical injection and one suggestion I might make is after the road has been constructed place a lift of 6 to 8 inches of sand along the side slopes and ditches.(if sand is readily available). The sand will act as a catch and release system for any moisture infiltration that may occur, very similar to what they are using for landfill cover designs these days. The sand will act as a sunscreen for your clay below, not allowing it to crack and weather and at the same time not allow moisture to penetrate your subgrade eliminating the heave that is associated with highly plastic soils.

Pavement surface - every other year perform rubberized crack sealing of the pavement surface to help prevent moisture penetration.

Also, it is important to control the vegetation along the roadway. Don't allow trees or plants with large and deep root systems. Plants are capable of applying 1-2 MPa (10 - 20 atm) of tension into the pore water prior to reaching their wilting point (Taylor and Ashcroft, 1972). Seed grasses with shallow roots. You can also place topsoil or really any sacraficial layer of cover over the sand and the system should still function quite well.

Because you live in an Arid region, I'm guessing your seasonal water table is sometimes 5 to 10 meters below ground surface. Most of your design will be in the unsaturated soil zone so I hope this post will be of some use to you and I hope I was able to help.

 

[emmgjld]

The discussion has been good and I hope to stimulate some more!! I apologize in advance for this long post.

Baldie I began on the Front Range (Colorado Springs, semi-arid & Pueblo, arid) before opening a branch office on the Western Slope in ‘74. Overexcavating/Replacement with over optimum clays was common. We just dealt with the extreme conditions, before they were defined as Steeply Tilted Bedrock and therefore of ‘State Interest'.

The overexcavation table in the CDOT Manual has always seemed pretty excessive (I have never seen it accomplished on CDOT Projects over here), unless the soils are desiccated or there is a potential of extreme differential heave (Steeply Tilted Bedrock or very rapid rock lithology change). I rank this as almost as bad as the 70'+ penetration for drilled piers, advocated by the Ft. Collins Professor.

Baldie & Focht3 In my experience, Moisture Conditioning has proven to be effective in some instances and apparently is necessary if the soils are desiccated. I originally noted "Soil Moisture Stabilization takes 5 to 10 years after development". I have learned that in desiccated soil/soft rock, moisture collection will occur as more water is added to the system than leaves by evapo-transpiration. The added moisture accomplishes a initial heave, which appears to be unavoidable, baring any deep chemical treatment. I believe this initial heave has naturally occurred in the upper, weathered rocks and soils of the semi-arid Front Range of Colorado. After this initial heave has occurred (5 to 10 years), a ‘seasonal equilibrium' is achieved. The soils heave and shrink on a seasonal basis.

I have found one method to minimize the seasonal heave and shrink is if some of the additional moisture should remain trapped beneath structures, pavements and landscaping. I use good perimeter compaction beneath sidewalks and somewhat beyond, If possible. The key seems to be to prevent desiccation from the pavement subgrade. Surface desiccation usually means fissuring and rapid moisture loss from the subgrade (see dirtguy4). I use the same general concept for house foundations on low expansive soils.

If desiccation begins, the process can be very fast. I have observed extreme drying and associated fissuring in alluvial/debris fan deposits extending more than 40' deep, in less than 2 years, in my climate. Dense shale and claystone are about the same in the upper weathered zone but, seem to be a slower, at depth. I have observed that if proper protection is taken (again, see dirtguy4) the lag time for moisture penetration/loss can be easily extended from less than 1 month to 5 to 10 months or more.

Focht3 Chemical Stabilization. Is it normal for the chemicals/water to be added and mechanically mixed (discing). I have seen the HB ‘injection' method and heard that some of our hard, desiccated shales/claystones were too hard for proper penetration of the injection tubes. Could you point me to information on dosages, effectiveness, cautions, etc..

Dirtguy4 I agree with your post. Fredlund & Rahardjo and others are well read. The most difficult issue in an irrigated, urban environment is controlling the Flux Boundaries. Too many utility ditches, both alongside the pavement prism and penetration of the prism. Proper compaction is the logical solution but, accomplishing it is something else. Also, as we are seeing Xeric type vegetation, deep grass roots are encouraged to reduce the irrigation water requirements.

 

[dirtguy4]

emmgjld

Just a quick comment on the Xeric type vegetation. I understand the reason agronomists are wanting to use this type of vegetation, however, this type of vegetation makes life difficult for geotechnical and pavement design engineers because of it's suction properties. Have you come accross any studies on vegetation, particularily with respect to wilting and root depth? I'm wondering if there is vegetation that has a shallow type root system, but is very resilent in dry climates.

 

[inaz]

Just another quick comment. I've been involved in several projects where the clay subgrade is well compacted to optimum +. However, after AB trucks, water trucks, and other construction equipment run over the subgrade, it gets over-compacted, reducing the optimum moisture value and causing lots of pumping. Now I recommend compaction at under optimum, so additional compaction doesn't cause a rolling grade surface.

 

[VAD]

Hello emmgjld:

This is no doubt a topic of interest and based on your comments you have experienced the problems and successes obtained with a variety of solutions. I have done work with swelling soils and have tried several approaches and seen the types of distress reported by many in this forum.

I will throw out to the forum a concept which may be hair brained but as requested ideas are required. We know that there is difficulty in applying surcharge to a pavement section in many instances because of geometric constraints. Where geometrics is not an issue, the option of raising the grade with better material above swelling soil subgrades may be an option after careful study. If we resist the swelling pressures after doing some conditioning then we should be able to allow for better performance.

How about the concept of providing a resistance to uplift by the use of screw anchors. If we took a steel plate and placed it across the grade and provided anchors that were embedded deep enough belowthe depth of seasonal moisture variation and connected these to the plate to resist the uplift force can we not provide the counter action against heave. I fully well realize that the pressures would be great, we can also use strips at intervals rather than a complete monolithic plate. Use of Other materials ideas are possible.

This is just some thing to think about although one may say that it is going to be uneconomical. However, some other ideas may be conceived. Just a thought ouside the present box. A combination of a few things may forthcome.

Interesting comments by all on this interesting topic.

 

[Focht3]

How about the concept of providing a resistance to uplift by the use of screw anchors.

Your concept has been considered in other forms - most recently with soil nails (i.e. friction transfer of force.) In "my" geology, this approach isn't very practical - we have swelling pressures in the 4 to 8 tsf (0.38 to 0.77 MPa) range. If the anchors are spaced on 10 foot centers, the force per anchor would be 400 to 800 tons (3.6 to 7.1 MN)! While a closer spacing would be needed (for many reasons), it would probably prove to be uneconomical -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora. See faq158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

 

[VAD]

Yes, I have encountered swelling clays in Trinidad, West Indies where the pressure was as high as 8 tons per square foot nad observed a grade beam broken by this force like a toothpick. the concept of soil nailing was thought of as well.

If we use this in combination with moisture conditioning to allow the swell to occur the pressures may not be as high. Yes, this concept may be uneconomical as I mentioned. Something to think about is the intent.


Cheers

 

[dirtguy4]

VAD

Helical screw piles are used quite extensively up in northern Alberta, Canada. The oil and gas sector prefer this type of foundation...easy to install and uninstall in cohesive soils (clays and glacial tills) for light loads. I've seen them installed in concrete driveways, but I haven't gone back to check their long term performance. Interesting concept, had any luck?