stabilizing slide prone section over silty clay sub grade 4

[roadspecialist]

grayish silty clay subgrade was exposed during roadway excavation.a three meter embankment fill placed directly on the clay subgrade slipped over the steep side slope after heavy rain.saturation of the subgrade affected by the spring located on right side upper slope.any suggestion on how to stabilize the subgrade?

 

[GeoPaveTraffic]

Hard to give much of reply based on the information provided. If I understand correctly recent rains have 1. casued a section of embankment 3 meters high to slide and 2. exposed a previously unknown slide on the up hill side of the slide to become know.

Questions:

1. was the subgrade benched before the embankment fill was placed?
2. how much material moved? Just the 3 meters of embankment? more/less?
3. is the spring within the slide area?
4. was the spring treated in any way as part of the orginal construction?
5. how deep is the clay subgrade?

 

[Focht3]

A few additional questions:

What's beneath the slide material? How steep is the interface between the silty clay and underlying stratum? What studies have been undertaken - or are contemplated?

Drainage seems one obvious answer as a part of the reconstruction. Is this an isolated event? What does the geotechnical consultant on the project say?



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[gandersen]

roadspecialist:

We faced a similar problem recently and used a deep-pipe underdrain running parallel to the roadway on the upslope side of the roadway to dewater (as Focht3 recommends). We cut off any subsurface flow by the underdrain and prevented any surface water from infiltrating into the slope on the upslope side of the roadway.

I could provide more details if you like.

Glen Andersen
BBC&M Engineering
Cleveland, Ohio

 

[BigH]

Its too bad that sketches can't be given. I take it the embankment has a steep downslope - say on right side. Is the embankment butted-up against similar up-slope on left side?
I agree with GAnderson - if there is an upper slope, a french drain should be installed at the base in order to preven uphill water from entering into the fill - and to drain the spring. It may be that the fill placed got saturated? rather than just the clay subgrade. In mountainous areas, not much else to do for modifying geometrics except for retaining structures. You might have thought of putting in a gabion blanket underdrain at base of clay subgrade and abovelying embankment. This would help drainage considerably - at least not create any "dam" by the embankment.
You didn't say what the undrained shear strength of the clay was - but, geometrics plays a big part of slope stability in mountainous regions. Had many such situations in Laos - mountain areas. In one 15km stretch over one rainy season, we had some 100 slips (99 of small-volume slips) but one major one that had to do with uphill rock surface. We did some drains, but in mountainous areas you just have, many times, to live with slips.

 

[Focht3]

Sketches can be given - but they have to be located on a web site that can be referenced - check out

http://www.tipmaster.com/includes/tgmlinfo.cfm?w=450&h=450

for an example (the smiley face near the bottom of the page.)

I know, I know - why can't [green]Eng-Tips[/green] host the images directly? I understand that it has to do with storage requirements...



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[roadspecialist]

thanks guys for replying.
1. the "slip" section traverses along a 6% gradient with steep downslopes on the left and the spring on the right upslope.
2.after roadway excavation to widen the existing road the esposed side slopes showed this predominant grayish silty clay formation. on some sections, there are reddish rock outcrops with clayey silt texture.we have no data on the depth of the clay layer on the subgrade but we infer that it will not be less than 3m.
3.after the rain, the earth fill on top of the clay layer slid off the steep left downslope exposing again the clay layer.
4.the spring is located 7m from the right ditchline of the roadway on the upslope about 1.5m above the road grade.
5.we are thinking of a retaining structure to hold the fill on the left.
6.would a granular drainable layer of fill be sufficient platform for the earthfill.we are thinking of replacing 1m of the clay subgrade with this material then place the earthfill over it.
Also as recommended by Focht3, we are adressing the drainage problem. the infiltration from the spring discharge is saturating the clay subgrade such that with the reduced shear strength and increased surcharge it can not hold the high fill over it.

details of gandersen's deep pipe underdrain and how to prevent water from infiltrating into the slope would be helpfull.
additional info re:gabion blanket under drain of BigH can give us more options.

thanks again and please keep it coming.

 

[GabionGuy]

For information on gabion structures, go to

http://www.maccaferri-usa.com

 

[roadspecialist]

thank you gabionguy.but what would be of best use is BigH's feedback on the performance of the gabion blanket under drain that he used.
hope he replies again.

 

[Focht3]

He will - if not, I'll bug him!






Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[gandersen]

roadspecialist:

The deep pipe underdrain system is constructed with an excavator and trenching box system. The pipe is a perforated pipe (usually 6" to 8" in diameter but should be chosen according to expected flow rates). We usually use a double filter design to minimized siltation in the pipe. Standard procedures developed by the U.S. Army Corps of Engineers for design of filters in dams work just fine. The depth of the underdrain depends upon the local conditions, the ability of the excavating equipment and the desire to cut off a certain amount of flow in the soils on the slope. Outfall pipes are tied in at regular intervals and multiple outfall pipes are used for redundancy.

Surface water infiltration into the backslope is generally handled by a combination of surface grading to build water conveyance systems and placement of low permeability caps at the surface with sufficient surface erosion protection.

I hope these details are sufficient to get the general idea.

Others may have other methodologies that they use.

Glen Andersen
BBC&M Engineering
Cleveland, Ohio

 

[Focht3]

Nope. That's generally what we recommend - taking into account that the construction of the drain system, if done improperly, could create a dangerous and unstable condition. The designer must take the stability of the slope into consideration when designing - and constructing - the drain system.



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[BigH]

Sorry for the tardy!

The situation for which I was trying to push for the gabion blanket drain was:

In a tight 50m radius curve with the road sometimes on flat land and sometimes sloping road of 5% (in mountainous zones you many times go down the "contour&quot. However to the "left" we had an valley incision formed in a steep uphill slope. We also had a steep downhill slope on the "right" - these are the cross section transverse to the road. Water would collect in the valley and make passing the area very bad - especially during the monsoon and before the new road was built. I proposed to use a gabion blanket (they are about 300mm by 1000mm by 1000 to 4000mm). I could lay this down so that water would seep through the gabion blanket - planned on about 20m of road length across the 7m carriageway. Thought this would be better than culverts which might "incise" erosion downslope of the outlet. Of course you would need geotextile atop and below the gabion blanket. But it makes sense to me - sort of a French drain blanket - and since it is "contained" - not hard to build.

Somehow, I think this concept would be one to consider for your work - you could tap into the spring to let it seep through the blanket - and with the "wide" area for seepage, may not have to worry about ersosion at outlets.

Couldn't get the TL or client interested so we ended up to our "jewels" in mud every time we had to drive the road - about 8 hours or more for a 10m stretch. Once the upgraded road was built, it was all-weather surface dressed - but the blanket still would have been very useful, in my opinion. (ps saw one lorry go about 5 km in 8 days!).

 

[dirtsqueezer]

BigH,

This sounds like a great concept, if I'm imagining the gabion blanket correctly. Would that be something one would worry that would be clogged by silt or the like- over time?

 

[BigH]

As with most gabion installations - that is why you have to use geotextile cloth surrounding those sides that would have soil placed against them.

 

[roadspecialist]

thank you guys
your inputs will provide us with more options to consider.
the gabion blanket is very interesting. thanks again BigH.
yes we are waiting for the figures on soil data and will also try to look into providing side drains on the toe of slope
thanks again

 

[jheidt2543]

BigH,

What a great idea! I think you should sketch it up and get a patent for it. You could call it "The BigH Rock Culvert". The royalties could be used to support Eng-Tips and of course, whatever is left over, could be used for your retirement. You might even get to retire early!

Although, everyone in this thread might stake a partial claim to the proceeds. Then, pretty soon there would be a gaggle of those money grubbing, blood sucking attorneys chasing you. Nah, forget the patent idea.

But, I do like the sound of "The BigH Rock Culvert"!