Settlement / grade depression in a road 3

[fivestring]

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.

 

[BigH]

Can you draw a sketch - is this a side-hill cut and fill?

 

[fivestring]

yes, it was built on the side of a steep hill with basically one side of the road in cut and the other in fill. The side that is in fill has the grade depression and cracking issues. thanks for replying.

 

[TDAA]

What was the fill material?
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.

 

[cntw1953]

Is the road well drained? A hand sketch of a typical cross section will help.

 

[fivestring]

I was brought in after construction so I don't know about the fill material, other than knowing that we typically have clay soils in this area,or how it was constructed. I have noticed rock outcroppings as well in natural areas. The existing grade that the fill was placed on might be around 20-25% and the fill slope from the road to the existing ground looks to be steeper than 2:1. The road has a ditch on the cut side and looks to be adequately drained. I can not add a sketch at this time but I can attach one later. Thanks for your posts!

 

[fattdad]

You are likely dealing with settlement of the fill soils or settlement of the native soils due to the weight of the newly-placed fill. Maybe a combination of both.

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!

 

[BigH]

We get a lot of these about these parts since the fills here were "trail-blazed" in. The ones that I have drilled have N values in the order of 4 to 5. But, most slips are occuring during the rainy season - or slightly after so in our case, the slips, when they occur, are shallow translational slips. We do get cracks in the road - nice half-moons, then things go - but these are typically long term problems - some areas "slid" 3 times over some 10 years - I got them to dig out the sliding material and replace with rock fill and buttress. I'll try to find some snaps and post.
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.

 

[fivestring]

I would like to see the snaps if you have them. I wasn't thinking about the fill slope itself beyond the road sliding, I guess, because it is not visible that it is occurring. Could that type of shallow transitional slip be happening and not evident to the naked eye? The only thing that is noticeable is rutting on the slopes from erosion, and this is not occurring in some places where we do have the depressions and cracking. I was thinking that there was an issue with sub-grade settling. The cracks are the nice half moons as you described and this did happen after significant rains. Thanks for the help.

 

[BigH]

I'll look this Sunday for the snaps - but as I understood, the road embankment is a cut and fill so the fill might extend from the centreline to the downhill side? This might be causing your slips - the non-knitting (which is common in all specifications for placing fill against an existing slope.

 

[fattdad]

Movement you see on hardscape you may not see on the ground.

f-d

¡papá gordo ain’t no madre flaca!

 

[oldestguy]

Sounds like mainly sliding, quite possibly on the original ground inclined surface.

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.

 

[cntw1953]

If this road is only about year old, you should be able to dig out the design document including fill material & information on the original grade. That would help to pinpoint the problem areas, and bring about an feasible yet economical solution.

 

[fivestring]

I think everyone involved was hoping for a quick fix or economical solution and it sounds like that is not the case. The municipality will not accept the road until it is fixed. Another issue is that the road will only serve, maybe 10 residential lots. I'm not sure the appropriate repair will be worth it.

Oldestguy, What are the options to keep the water out of the fill? Thanks.

 

[oldestguy]

Fivestring:

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.

 

[fivestring]

Oldestguy, I am not familiar with mixing bentonite. Can you explain further "Use about 4 inches for the "seal""? Thanks for the post.

 

[fattdad]

to the OP. While some form of ditching or surface water control may be one way to address you problem, it's premature to implement any fix until you know the failure mode. At this point you don't know whether you're dealing with a failure surface at all (i.e., it could be a settlement issue afterall). If you are dealing with a failure surface (i.e., from shear), then it's critical to this whole process to know where the slip surface is located. Is it at the fill to native soil interface? Is it within the fill? Is it below all this on some underlying clay layer? You really need to have some context about this before you implement a repair offerred in cyberland.

f-d

¡papá gordo ain’t no madre flaca!

 

[oldestguy]

To address the question of slippage or settlement, generally slippage results in cracks parallel to the alignment as well as semi circular cracks for a local slip failure. You also may see the bulging of the toe of the fill, trees, if any, will lean down hill.

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.

 

[BigH]

Attached find some snaps of some "half-moon" slips that I have seen. Most of these appear to be translational slips. It was not determined if the slips were on the interface line. It is interesting to note that Sites 1 and 2 had downhill slopes in the order of 2.5H:1V. Site 3 was much steeper being a rock controlled slope heading down to a river. Hope you find these interesting. Neither Site 2 or Site 3 has yet to "fail."

 

[fivestring]

Thanks for the pics. That looks to be very similar to what I am experiencing. As I mentioned before, the fill slope looks to be steeper than 2:1. That might be part of the problem.