Retaining Wall for Landslip

[tclat]

Hi,

I am looking at a job on a moderately steep slope (1V:2H) where a road was cut along the contours along the boundary line. The cut apparently stayed unprotected and during heavy rains a land slip occured along a portion of the cut into the neighbouring property.

I'm looking at advising the following

1. Remove of all slipped and loose material
2. Cut a horizontal bench the across the base of the slip surface
3. construct a retaining wall at the base of the cut along the boundary to the height of the previous ground level
4. Reconstructing the land behind the retaining structure to its original slope of 1V:2H.

My logic being that we would essentially recreate what existed before the land was cut and the retaining wall would be designed to support the sloping backfill. Does this seem logical and practical?

Are there any other reasonable solutions?

From an overall stability point of view is it logical to conclude that the construction of the 1V:2H embankment against the existing slip surface will give an overall factor of safety similar to what obtained before the slope was the cut and before the slip occured?

Thanks

 

[hokie66]

You would need geotechnical advice from an engineer familiar with slip problems in your area. The one piece of advice I can give is to install a drainage ditch at the top of the slope to divert runoff around the problem area.

 

[tclat]

Hi Hokie,

I normally would but I am located in a very small island where geotechnical engineers really do not exist. Even engaging Civil/Structural engineering for a job like this is uncommon. (probably why the slip in the first place)

Because of the size of the slip....just over 7m along the cut, it would be difficult for me to advise to the client to fly in a geotechnical engineer from abroad. I would like to advise something conservative but not over the top that can't be explained logically.

Do you have any comments regarding construction of the embankment as I outlined in my original post?

Thanks

 

[hokie66]

Sorry, I don't, but some of our experienced geotechs will probably read your post in due course and join in.

 

[fattdad]

Just recall that now there's a failure surface you have less then the original strength in the overall slope. While it was marginally stable prior to failure and changes to the water table tripped the balance, the current state of stablity is even less. Cutting at the base of the slope (i.e., to enact repairs) may likely re-trigger the failure surface, which is now at it's residual strength.

Keep in mind also that you only know that the slope failed. You don't quite know whether it was a rotational failure or a wedge failure. For example, is there some underlying horizon of clay that is providing a horizontal base to the sliding surface?

There are several methods to stablize slopes. A retaining wall may be one choice, providing you can demonstrate that the post-corrective-action geometries provide a safety factor of 1.5 or such.

You can also consider soil nails or flattening the original 2H:1V slope (or other such modifications to the original grading plan. All this is make-believe stuff when there is no knowledge of the soil strength parameters that'll govern design. Also, at this point, there is limited data on the underlying geology (i.e., depth and attitudes of layers).

These are not shade-tree geotech problems!

f-d

¡papá gordo ain’t no madre flaca!

 

[BigH]

Can I enquire as to the nature of the material that failed - is it residual soil (derived from in-place weathering of rock)? Is it clay? Is it cohesionless? What kind of failure was the slip - was it a deep-seated rotational failure? Was it a thin translational failure? What was the precipitation history like say a month or so before and up through the time of the slip? We have, where I am now, many shallow translational failures along road slopes. Usually they are exacerbated by rainfall events. They are slow moving - not an "all at once" movement. If the failure zone is thin, you might consider digging out the slipped material and replacing with rockfill buttress. Do you still want to keep the road? There are a number of remedial measures, as indicated, and the ones to use depends much on the questions I asked. There is one novel - not so novel - but interesting one where "plates" are driven into the ground and through the sliding zone - the plates act to stabilize the downward movement. I'll try to find the references I have seen on this - but if you google translational landslides, you will find myriads of papers on what you might be looking for. For a slip only 7 m "long" I would suggest that a retaining wall is an overkill - unless mortar rubble is a very cheap (labour) commodity in your area.

 

[tclat]

Thanks Fattdad/BigH

The failure is not a deep seated rotational but more translational. The soil is residual but sits on a compacted rock of tuff consistency. Its not a very hard rock but is generally stable throughout the island and on the rest of the site. The cuts were not left vertical but where battered to about a 1:1 slope. It seems like a larger pocket of residual soil existed where the slip occured. I was not on the job prior to the slip so I can't tell what happened. I do know that it was torrential rains over several days that caused the slip.

Its not common to build retaining walls along roads but slip has gone into the neighbouring property and the owner is making a big deal.

I was hoping that reconstructing the slope behind the retaining wall to its previous slope 1:2 would be atleast result in a similar factor of safety of what existed before so that I would not have to do a detailed slope stability analysis. (tedious to do by hand and not really my expertise)

I would have thought that it would be essential to create benches across the slip surface to allow for the proper compaction of the backfill.

Any other thoughts?

TT

 

[kslee1000]

1V:2H is not a steep slop. If it is the case, a low retaining wall is a good solution with a bend of horizontal, or constructed like V, backfill to catch the potential slipping material in the future (some guess & geometric work required to determind the bend width). Your retaining wall design shall consider the effects from the slidding material (weight & thrust).

Simple slop stabilizing techniques shall be considered. also drainage is a critical aspect of success.

 

[BigH]

On the roads here where we have slips on slopes even as shallow as 2.5H:1V - they are shallow translational, we have dug out the sliding soil and re-established using rockfill. Ohio has a geotechnical note on this - I'll try to find the link and post later.

 

[kslee1000]

Amazing. What kind of soil it is?

 

[civilperson]

The neighbor making a big deal is your easy remedy source. Buy a grading easement or even buy the piece of land impacted. Some money in his pocket will speed the engineering repair process.

 

[Doc09]

Taking into consideration the nature of the site (oil over rock) and that the soil has already moved, I would propose the following as a quick and dirty soultion:

1. if water is still seeping / running between the layers you really do not have a whole lot of time to work as the soil is likely to move shortly after you expose it. If so, providing drainage at a distance upwards becomes necessary.

2. if you have access to rock blocks say 1m x 1m or 1m 2m, you can design a gravity wall which is quick to construct, stable and self draining. This approach has been proven valid on several jobs we have designed on of which is 11 to 19m high. for added stability, you can slope the wall back while providing a base about 70% (or better) of the height.

depending on the amount of water present bewteen the layers, you may still be able to apply this aproach but you'll have to do it strips of 3m or so just to limit the risk of new slippage.

 

[oldestguy]

The advice you got from DOC09 is good. The failure was due to water, as you note. Getting rid of the water entering the slope area will provide an amazing help. If water is not cared for, the wall may still fail later.

I'd suggest diversion ditches as high up the area as possible, even on the neighbor's land if possible. If that is done "properly" you may not need a wall after all. In your efforts, put most effort on this aspect.

In some cases a diversion of ground water seepage also is needed, with sub-drains in the soil above the slope, diverting that water slightly sloping, parallel to the contours. I use this method frequently for failing slopes and it works. Fill the trench above the pipe with concrete sand, not gravel or rock since they are not filters for soil and will plug up.