Settlement / grade depression in a road
Settlement / grade depression in a road
(OP)
I have been asked to come up with a solution to what looks like settlement in a road. This road is less than a year old and already has severe cracking and grade depressions in the majority of the 'fill side' of the road. My instinct tells me that the fill might not have been compacted properly, and we did receive a severe amount of rainfall a few months ago. What would be a good solution to this issue? I thought about removing the asphalt/aggregate and re-compact the subgrade; but to what depth? Would it be better to do something like specify a geotextile on the subgrade? Thanks.





RE: Settlement / grade depression in a road
RE: Settlement / grade depression in a road
RE: Settlement / grade depression in a road
Was the subgrade benched? How wide/high were the benches?
What was the approximate grade the fill was placed on?
Pretty vague so far. This could range from expansive fill to settlement to slope failure.
RE: Settlement / grade depression in a road
RE: Settlement / grade depression in a road
RE: Settlement / grade depression in a road
You may also be dealing with a slope stability issue, where it's not "settlement" per se, but rotational slope failure (like a landslide).
I am currently working on a project where the original ground was sand to a depth of 20 to 30 ft. Below the sand, there is hard clay. The site development required a fill slope that is 20 ft tall and goes 2H:1V back to the native grade, which heads and down to a drainage. In response to the newly-placed fill, the water table raised. Additionally, there is the load of the newly-placed fill.
Well, next thing you know (about a year later) in the Spring, there are big cracks, settlement and the like. We installed slope inclinometers and the movement is a wedge of soil that is sliding on the clay.
You can't always blame these things on consolidation or compression. Then again, sometimes you can. . .
f-d
¡papá gordo ain't no madre flaca!
RE: Settlement / grade depression in a road
Too, perhaps (likely) the fills were placed without stepping the new fill into the existing slope to "knit" them together - and, as a result, a sliding plane is occuring at the interface of the two.
RE: Settlement / grade depression in a road
RE: Settlement / grade depression in a road
RE: Settlement / grade depression in a road
f-d
¡papá gordo ain't no madre flaca!
RE: Settlement / grade depression in a road
Solution likely can be costly, but a common fix is to fill in the dips and live with annual maintenance issues.
Keeping the water out of the fill another treatment, but difficult to do in many cases.
I have fixed a condition like this by overloading it with a surcharge, pushing things on their way. The case was sitting on a layer of soft material, weak in shear strength. However that may not fit in your case, depending on what is down the slope.
RE: Settlement / grade depression in a road
RE: Settlement / grade depression in a road
Oldestguy, What are the options to keep the water out of the fill? Thanks.
RE: Settlement / grade depression in a road
Your up-hill ditch may be bleeding water into the soil.
If that is the case, some form of ditch paving may help. Do not pave with concrete. Asphalt mix may work, but it has to be flexible. Rigid stuff like concrete will bridge over depressions that later contribute to erosion underneath. Grouted rip-rap is just as bad.
A mix of bentonite with the on site soil also is a good seal, but it is likely to be eroded with strong flows. The blend I use is about 5 percent bentonite mixed in thoroughly. Use about 4 inches for the "seal". Too much bentonite and you get mush. Use powdered (driler's mud) bentonite, not granulated. Some suppliers sell a trde name Volclay. It might be sufficient to control slippage until the community takes on the road (Ha).
If there is water seeping under all of this, possibly trapped on top of the bedrock, a sub-drain, up-hill of the area to be protected also is done. This is like a moat around a castle. However, where? That can difficult to do in these cases. You might have to dig that trench right under your up-hill ditch, but then seal the top of the trench as was mentioned above.
The base of a cut-off trench ideally would be into the rock, since otherwise, water can flow right past the ditch.
In the ditch base you place a perforated pipe leading off to the areas where no damage occurs and divert it to the down hill side. Put some slope on this trench.
USE NO GRAVEL IN THE TRENCH. SOME ADVISERS HERE LIKE GRAVEL WITH GEOTECH FABRIC SURROUNDING THAT, BUT IT IS DIFFICULT TO DO PROPERLY. Can cost more than what I recommend.
My recommendation is fill the trench with concrete sand, meeting ASTM C-33 fine aggregate (NOT COARSE AGGREGATE). (They both are listed in that spec) This is an ideal filter for most soil types. If the perforated pipe is black plastic, with slots, that works well even if some sand gets in the pipe. Seal the top of the trench with clay. No point in introducing more water into the soils than necessary.
To be effective, the up-hill cut-off trench is best placed into impervious stuff, but lacking that, it sure does help, even if some leakage goes past it.
How do you know if water is flowing over the rock surface? Some test pits to rock will help answer that question, before you commit to a big fix.
RE: Settlement / grade depression in a road
RE: Settlement / grade depression in a road
f-d
¡papá gordo ain't no madre flaca!
RE: Settlement / grade depression in a road
Slippage down hill will usually result in a syrface scarp or a series of them, along with settlement. Cracks open perpendicular to the alignment and of course will let surface water in, but that is almost impossible to remove. So you prevent water getting to the fill to begin with.
Settlement due to no compaction or similar may show cracks not necessarily showing a pattern. A simple way to check on poor compaction is to drive rods or pipes into the earth and evaluate how difficult it is to do that. Lacking any experience in that, try driving them in undisturbed earth up hill from the cut to serve as a guide as to a standard.
A farmer's 4 ton 36" jack and a chain wrapped around the rod will get them out.
Bentonite is a name for a volcanic clay, the mineral montmorillonite. Most plumbing supply houses sell it to well drillers. It consists of very small platelet particles that are held together by electrical forces (a rough way to describe it) and there is great affinity for water into these platelet arrangements. This affinity for water, combined with the small size makes for a rather water tight material. It works great for sealing ponds, etc.
When you mix it with ordinary earth (usually particles shaped like rough balls, not plates), it fills the voids between them and seals the mix. It does not take a lot to fill these voids for a well graded material.
To mix with earth, almost any earth, but usually a sandy type, you can use any of many ways, such as s concrete mixer, rototillers in place and even hand mixing. For this job you might do mixing at an open area placing down a layer of silty sand, sand, etc. adding the bentonite on the top, and running roto tillers through the area until the mix is uniform Then haul to the job and spread. Compaction can be done, but not needed unless you are fearful of immediate wash away with storm water flow in the ditch. I'd tamp it manually at least. A 4 inch layer shuld help a lot. This also holds water so vegetation grows well there. Don't add water to this mix since it will gather it from nearby.
Like the posts say, you might want to check out the cause or type of failure before fixing it.
If the job was done by digging into the slope and just dumping down hill, then both settlement as well as slippage can be in the picture. More than likely settlement is present due to the loose stuff consolidating (although that word is not correct usage here). That's where I'd over load it rather than waiting a few years for things to stabilize.
Is there any way you can get the observations of neighbors, etc. to hear if there was any compaction equipment there? If all they saw was a dozer, a grader or maybe a back-hoe, then you know.
Of course you could hire a firm that tests these things.
RE: Settlement / grade depression in a road
RE: Settlement / grade depression in a road
RE: Settlement / grade depression in a road
It is a bit off-toppic, but I'd like to ask for something. I am in the UK as an abroad student and I am working on my masters project about geo-enginnering problems of road structures - taken generally, but I am also trying to collect different examples from practise, preferably those which weren't published yet. Because of lacking some cetral repository of geotechnical reports in the UK this is quite difficult.
Would you mind me to use this case in my work?
RE: Settlement / grade depression in a road
RE: Settlement / grade depression in a road
RE: Settlement / grade depression in a road
RE: Settlement / grade depression in a road
Year's back I heard about "California horizontal drains" and if you Google those words up comes info on them. Apparently it has been a successful way to stabilize sliding situations where water is doing the mischief in California.
They are drilled in from the toe of the slope and the hope is to tap sources of water.
I only used this procedure once. It was for a 30 ft, high lake laid clay bank that was sliding towards a valley.
First vertical holes were drilled and filled with sand. Then the horizontal drilling was done intersecting the vertical sand filled holes.
Vertical holes were 18 inches diameter and about 30 feet deep and the horizontal holes were 12 inches diameter maybe 60 feet long with sand blown in as casing was withdrawn.
Everything worked to stabilize the hill side, some 30 feet high, but in the winter the weeping drains caused a messy icy condition. I suppose we could have directed that discharge to a storm drain, but didn't think of that at the time.
Check out the references from Google and that may be an idea.
RE: Settlement / grade depression in a road
Could you give me some email or so? I dont want to spam here...
Thank you a lot.
RE: Settlement / grade depression in a road
Heat and humidity have a way of fostering microbes that oxidize iron, which in turn clogs up the screens of the horizontal drains. They'll work for a spell, but after a few years, your on you own as they become clogged.
I considered this approach for a job in Georgia and that's what the contractors told me.
f-d
¡papá gordo ain't no madre flaca!
RE: Settlement / grade depression in a road
Maybe those in the business of water supply wells might wade in here.
My single job use of the ideas did not clog.
The only problem with sub-drains that I have run into is use of galvanized perforated pipe in peat. Rather acid stuff. There the steel rusts out and leaves the galvanizing....Not much strength left.
RE: Settlement / grade depression in a road
RE: Settlement / grade depression in a road
RE: Settlement / grade depression in a road
f-d
¡papá gordo ain't no madre flaca!
RE: Settlement / grade depression in a road
RE: Settlement / grade depression in a road
I play the mandolin, but forever the amature.
f-d
¡papá gordo ain't no madre flaca!
RE: Settlement / grade depression in a road
I was at a Highway type conference in Denver several years ago and Dr. Paul Santi presented a short paper exhibiting Wick Drain Material inserted into short lengths of small pipe and pushed into a hillside. He had an article in Civil Engineering—ASCE, Vol. 71, No. 6, June 2001, pp. A10-A16.
In the Western Colorado area, I have had a fair amount of success with 'push drains' around residential structures and road cuts. Most of my experience has been pushing these into new to old slope failures, oftentimes with a significant amount of small to large boulders. The local county road & bridge has been using these fairly extensively in the higher elevations around Grand Mesa.
They are fairly inexpensive and if a boulder is encountered and more penetration is desired. The equipment moves a foot or two and tries again.
Here are a few links. The real long one is the basic presentation I heard. Hopefully this gives a few ideas.
http://inside.mines.edu/~psanti/
http
http://www.sloperepair.com/news_developments.php
http://trb.metapress.com/content/g50265m5067558j9/
RE: Settlement / grade depression in a road