roadbed stabilization

[schreibs]

I am a mech engineer, building a home with 1500 ft lane in from the blacktop over mostly sandy but also clay and silty soil. I have an area on the lane that is soft and spongy and allows the dump truck to sink down and make big ruts. I had already planned to use some Propex separation fabric(200ST) but certain areas are so bad I can stand on them and they act list a tarp laying on water-- almost! Digging down I find clay on top covering a deposit of saturated and mushy sand. I can dig down with my bare hand and remove huge handfulls of the wet snotty sand with little effort, and deeply. There is about 100 yards of lane with this problem. Please offer ideas for remediation. thanks!

 

[oldestguy]

If you do use some sort of reinforcing layer upon which you place a typical base course,be sure to check out the stresses imposed on that reinforcement. Once you over-stress it, it is too late. I'd make use of an experienced geotech engineer who has worked with reinforcing the roadway and who can design the thickness of base course needed to carry given axle loads. That advice takes much more input and work than might be tried here by well intended members. In any case never go whole hog with one design without representative trial areas that prove the worth.

 

[Ron]

Spread a layer of graded aggregate (gravel, recycled concrete, asphalt tailings, etc) about 4 inches thick. Mix it in with the existing material. You'll see a marked improvement in the stability. If not stable enough, add a few bags of portland cement and mix in. Spread the cement over the area to a depth of about 1/4" and mix in 6" deep. Compact all with compactor if you can get, heavy vehicle if not.

 

[schreibs]

thanks for the input guys.
Oldestguy: It might be too late in this one area(overstressed?), smaller about 3 x 10ft . . . however, it is representative of much of the 100 yards where I cut this lane out of the side of a hill(very low grade). I believe I harmed (stressed?) the underlay by cutting out the topsoil-- not sure?? Anyway, based on what I hear I will use this as a test case as you suggest and dig out the muck, fill with broken concrete (which I have available in the gravel pit next door), fill further with class 5 and asphalt tailings.

Ron: thanks Ron. I will try a mix of your and oldest guy's suggestions on this test area and relate how it goes. 100 yards is a big area to be dumping bags of Portland, as you suggest, I will wait till last on that one!

I am surprised that NEITHER of you had any suggestions for stabilization or separation fabric as that seems to be "popular" with some builders in this area and norther Minnesota. Is that mostly because you have never used it and are not familiar with it? It's about $350 to $600/ 300 ft roll depending on what you get-- Propex is the company name.

Location: Sherburne County Minn, near Big Lake & Elk River, on acreage on the Elk River, edge of glacier tailings, esker nearby, mucky black / sandy soil in hay field very near.

I will try not to lose my track loader in this pot of muck!

 

[SlideRuleEra]

schreibs - See the link below for guidelines to build a forestry road using geotextile fabric. This is likely comparable to a residential access road. Note that the document recommends gravel or base course at least one foot thick on top of the fabric. We built several short roads in conditions as soft as you have described at electric generating stations in coastal South Carolina - take the one foot thick material on the fabric seriously. Put traffic on the road, add gravel or base course as dips or depressions appear - don't just regrade the existing material. If you plan to pave the road, wait at least one year before doing so. Natural compaction is your friend.

Geotextile For Forestry Road Construction

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[oldestguy]

SlideRuls's reference is good. Note that the width of the fabric is as important as the length. 3 ft. seems just far too narrow. There are oddles of fabric types and uses out there. My preference would be high strength woven, since it has little stretch before being useful. However, sufficient thickness of base type cover is needed to avoid overstressing it. Preferably I'd want some strength tests (such as CBR) or good estimates on the subgrade before doing any undercut and replacement. Also, some replacement materials must be considered as not totally useful if the voids then become filled with frost susceptible material (as your area must consider). Broken concrete falls into this category. My stabilization experience is in northern states, mostly Wisconsin, including highways and regular area pavements, some of which involved fabric. On that score, I've seen many a fabric job fail and so I'd envision that your roadway probably is not a good place for successful usage, doe to width and length restrictions. I'd go more for undercut and replacement with something other than what you have used. In doing this I usually experiment with a tapered thickness of undercut and replacement test on representative area so as to find the working thickness that then can be applied to the whole site.

 

[oldestguy]

Forgot one thing. This undercut and replacement is for improving the subgrade. There still is needed a base course on top of that for the run-of-th-mill areas, generally kept the same over the improved areas.

 

[ACtrafficengr]

Before you start digging, is it a designated wetland that will require a permit first?

 

[schreibs]

OK folks here is what I did:
-- used my track loader to open up a sample area and removed soil as deep as the loader allowed-- about 18 to 24", but not uniformly.
-- re-filled partially with sand
-- placed broken concrete pieces like jigsaw puzzle and covered with sand
-- smoothed, added class 5 on top, final smoothed.
-- filled bucket with class 5 and ran track loader back and forth for some compaction.
RESULTS: most areas appeared to have increased support and stabilization; areas that I missed showed stress cracking. This method most closely resembles what oldest guy suggested as "undercut and replacement". See attached photos. Last one shows stress cracking on surface after job "complete", before second attempt at removal and refill. I tried to upload three and it looks like max is one photo?!
Two things of note: 1) for a home lane and 100 yards by 12 ft following this route seems like a pretty big deal; 2) new information: in recent weeks the gravel pit started pumping and dumping their water in ponds on north side of my property, higher in elevation than this lower hay field which abuts the Elk River. Recently they stopped pumping and are supposedly finished. However, the ponded areas remain and, I believe, are serving as high head(20 ft) hydraulic reservoirs for the lower water table in the swale of the hay field adjacent to my lane. If so, it seems reasonable that until the water table drops, it won't matter WHAT I do up above the sub-grade because it will ALL be just floating on this bed of fluidized material. What say you?!

I'm wondering if I waited a few months until the water table pressure is relieved maybe the problem will be relieved. The farmer who cuts my hay has never seen the water standing in the field like it is now.

I have put the fabric on back burner mainly due to the cost being excessive due to 12" min aggregate. . .

Finally, I'm considering Ron's advice in a new sample area(mix and compact) to see if that can be as effective as the concrete. However, I am hesitant due to the sub-grade being so deficient, so deep, that I would expect a continuous slab of concrete to float back and forth. Really, simply standing, I can rock my weight back and forth and watch a wave between my feet!

 

[emmgjld]

Where I am, the fabric is the common cure. That said, it has been used in the area for almost 40 years and there is a lot of experience, especially bad experience. There are many things I recommend which are somewhat contrary to the supplier recommendations. Actually, I think I have come close to driving the Tensar & Mirafi engineers a little mad.

The test sections is very good advice or find a couple of local engineers who have a lot of experience with what does & does not work for your conditions.

 

[oldestguy]

Schreibs: Sounds like progress to me. However, your use of the term "floating" probably is not technically correct. What I think you have is a deep deposit of silt or very silty sand, or silty clay. These gradations tend to not drain water easily and, when disturbed the pore pressure (water in the pores)goes a little above the gravitational effect of saturated water filled voids, causing a weakening of the temporary shear strength of the soil. Commonly you can make one pass over the area with the vehicle and no problem. Make repeated passes and the subgrade zone appears to be rubbery and weak, like a mattress. I have had contractors call and ask me how to stop this action. I tell them to take their front end loader off the area. They then say how did I know that the used that? Well, it is so common I am really surprised when experienced contractors don't know what is going on. The cures for the problem probably are not related to the nearby gravel operations and the ponds, other than to provide plenty of ground water.

The remove and replace is one option to spread the traffic loads down through the new fill to the sensitive lower stuff. Use of fabric is also OK, but designed, not just thrown in and hope it works. Stabilizing the sensitive stuff also works, but again needs some design, as with test areas. Mixing in lime, such as quick lime works, but not just a thin layer. Cement mixed in works, but not a thin layer. This stabilized layer has to function to spread the load, as the replacement fill does. However, being that there is some "beam strength" a thinner layer of stabilized soil is possible. I'd guess a foot of stabilization is not sufficient. The percentage to use should be selected from recommendations of lime or cement institute studies, if possible. You can mix with your loader, but a grader is better. Best is a rototiller.

If there is a ready mix plant nearby that may have a lot of old cement, a deal might work out.

Check out this paper.

http://www.graymont.com/sites/default/files/pdf/tech_paper/lime_treated_soil_construction_manual.pdf

 

[oldestguy]

One em about lime stabilization. The principle of this method is to change expansible clay minerals to less plastic material. So, clay content is important. That is why even lab trial mixes may be needed to be sure lime will do any benefit. Portland Cement, on the other hand, is a form of glue, not dependent on what it is mixed with for strength.

 

[schreibs]

Interesting pointers about both cement and lime. thanks, I will read up on the lime manual. The problem I see immediately with the lime, and less so with the cement, is that it will be hard to 1) find the mix ratio that works and the depth to treat, and 2) to be able to do any mixing of these materials with my only equipment being a track loader-- not a big articulated front end loader. There is no easy and cheap answer -- not that I was expecting one. I just have to find one that is reasonable for a simple farm road fix!

Tomorrow I have a civil engineer / sales rep from Tensor stopping by! (thanks for the pointer emmgjld) His fix will likely include product sales of course not to mention class 5 over the stabilization fabric.

 

[msucog]

for non-commercial, best approach is fix at least the upper 3-5 feet...3' probably sufficient for application you mention. if soft down below, then mix in cement at perhaps 3-5% depending on gradation of material...sandy would be lower and clayey/silty higher...this will stabilize (not soil-cement) most soft soils and aids in drying back while "gluing" it. at about $10/ea, you can get the 80-90lbs bags of cement (not concrete). I would suggest fixing the upper 1' as you did everywhere else so you don't have a hard patch up top performing different than rest...leads to more cracking. can use biaxial grid like a BX11/SBX11 (Tensar/Syntec) as more cost effective than TX for this application...then construct compacted base over this. if unstable at subgrade, grid will help but likely not completely fix so target a sufficiently compacted subgrade. often, simply scarifying and allowing the material to dry back then replace as properly compacted fill does the trick depending on location and time you have to allow to dry. remember good fill placement practices...in other words, don't scarify 2' then try to compact the top as this will not properly compact the full layer.

 

[oldestguy]

Schreibs: Before I'd do any full scale "fix up", and to help us here, it would be helpful to know a few things. What kind of traffic, loads and frequency? What are the soil strength numbers? Any freezing weather there?

As to getting the info like we use, that may be difficult. But for a rough test, how deep can you shove a 1/2" rod with average body push? How easy was it? How far in can you shove your thumb? All this before any disturbance by vehicles on it at least for over night.

The Tensar grids are strong stuff and it may be over kill.

With regard to Msucog's recommendation "for non-commercial, best approach is fix at least the upper 3-5 feet...3' probably sufficient", I'd say major over -kill.

 

[schreibs]

Thanks for joining in the fray msucog. It appears you offer 3 potential remediations. The first being a high end, deep, fix with the last (turn it over, let it dry, compact in lifts) being the one I may consider. I can appreciate what you're saying on the deep fix, but adding 3% cement to 100 yards x 12 ft x one yard deep seems a bit beyond what my resources can address. The one foot deep fix(imitating my initial work) is not attainable either as it would deplete my resource for broken concrete in the next 20 ft of roadbed. That was a trial I used to see if I could make "something" work with what I had available . . . not an overall fix for the full 100 yards. If I understand your thinking is that the grid will work, so to speak, to allow traffic but it is not fixing the subgrade. Your final suggestion for simply uprooting the subgrade and letting it air dry is somewhat reasonable to do-- but not at this time. I have to have use of this road for the time being and winter is coming soon. Besides the gravel pit owners pumped in reservoir, standing in the upper part of my land, I am convinced, is feeding the water table below to keep the area around the roadbed wet. Until that drys out and the water table is back to "normal" I cannot tell. It's not worth drying and re-compacting the existing soil materials just to have the water table mess them all up again. I will provide details for what the Regional Manager / PE Civil engineer recommended. ..

 

[cvg]

in recent weeks the gravel pit started pumping and dumping their water in ponds on north side of my property, higher in elevation than this lower hay field which abuts the Elk River. Recently they stopped pumping and are supposedly finished. However, the ponded areas remain and, I believe, are serving as high head(20 ft) hydraulic reservoirs for the lower water table in the swale of the hay field adjacent to my lane. If so, it seems reasonable that until the water table drops, it won't matter WHAT I do up above the sub-grade because it will ALL be just floating on this bed of fluidized material. What say you?!

suggest you also investigate and mitigate the water problem as well. it sounds like there is poor drainage. it could be from the gravel pit or it could be from rain or both. Improving drainage may require some fill and a culvert to handle surface runoff or perhaps some sort of hydraulic barrier / cutoff wall or subdrain to handle groundwater

 

[schreibs]

OK Oldestguy, here are some more details: I expect only 5 to 10 heavy trucks to pass across this roadbed in the next six months, mostly my little Isuzu Rodeo 4x4. When home building starts next spring there is the well drillers truck, cement truck loads, and home building truck supplies over this road. After the home is built I plan to final TUNE the road and use only my vehicles across it, say October next year. My plan is to get the road to about 70% "good" and do the final 30% once the home is near complete. It does not make sense to make it "perfect" just to have the cement trucks rut it all up. Cement truck weighs up to 65,000# I understand, but that does not mean I can't pay more and limit their loads to half, still they could be 40 or 50,000 #. The well driller, not sure, but guessing 30,000# max. The point is these are all nearly "one time" events. . .

It is hard to answer your question that appears to be asking for something that can be related to CBR without taking a half inch rod and trying it. Maybe I will do that, but here is what the Tensar Engineer and I concluded:
-- without using an penetrometer and just walking on it, some of the worse areas appear to be on the order of 1.5 CBR
-- two remediation suggestions based on my readily available sand and his product fixes:
1) crown subgrade materials down roadbed, use triangle grid TX160 under 9” of class 5 aggregate, compact each 3 to 4” lift of the build if possible.
— 2) use a woven textile fabric first after crowning, place 3 to 4” lift of sand, wet, smooth, contour, and compact if possible, place TX160, use 5 to 6” of class 5 aggregate.

1) was his first recommendation and 2) was suggested as a way to “utilize” my own readily available sand resource and minimize amount of class 5 to buy. However, it requires two "fabrics". I have to analyze the costs to see if it is actually cheaper.

Whatever I do I will do a test area first. . . I am hoping I can convince the local Brock White dealer to let me buy short sections of grid / fabric for my experiment!

 

[schreibs]

Here is a link to my dropbox that shows the photos of my first subgrade re-work, hopefully "Dropbox" routing will work.

It is very cumbersome to place images on this forum I see, and my earlier attempt allowed only one to be placed at a time.
Going to Dropbox you can access the whole folder:

https://www.dropbox.com/sh/423o1rll20i9lyk/AADx0kP2KHUjpnglCXOAobnHa?dl=0

 

[oldestguy]

Your traffic is a very low count of meaningful loads.. Repeated loads within short time intervals would be the main concern, with the heaviest being the worst effect. Later on, very light traffic and thus less need to get too elaborate now.. Asphaltic concrete paving with some base course later on a reasonable stable road will help significantly, but I like to see that subgrade not yielding to to more than a one inch rut remaining from a truck pass.

The 1/2 inch rod test is mentioned because around here in Wisconsin we use that a lot to check for low compaction of fill, or the need for improvement. A full shove by one hand to 2 feet is real bad. I'd have to guess you might get one foot. Not the worst, but would explain your situation better, not requiring the deep fix some recommend..

An estimate of CBR at 1.5 means ulta weak. The lowest I've ever designed for is 3 and that is mainly because of weak subgrade expected after thawing of silt soil that gained lots of water in frost lenses. The "design" by the sales person with reinforced by fabric seems light, but that is where a few test runs will tell what works. If it fails early on, I don't suppose there is any "guarantee".
Again taper the test run upper layer thicknesses. The test needs one of those loaded trucks, because that is the truck you want to support. A car or tractor not likely to mean much. The most useful layers are those on top of the fabric, spreading out the load on the fabric.

Those gradation numbers don't mean much to this OG, but any granular material may work with the proposed design. I don't quite see the need to crown the subgrade, other than removing ruts and humps. Surface crown later helps drain surface water. Too much crown and snow plowing will change that.

Your link shows a lot of the situation. Spot fixes with the broken concrete seems pretty deep, but a heck of a lot of work per foot of roadway. I assume you tried to vary the thickness of treatment. I'd estimate the concrete helps some, but not a significant difference from well graded sand and gravel. If broken up so no largest chunks were about 6 inches and well graded, maybe better yet.

The link works good, but try the "upload image" from the group up above for multiple shots.