SLOPE STABILIZATION IDEAS NEEDED

[oldestguy]

I've been asked to come up with suggestions as to how one can stabilize the current situation against a potential land-slide.

The original ground is a sandstone bedrock controlled slope on the order of 32 to 39 degrees above horizontal. Dumped on this slope is over a hundred yards of sand fill that is on the silty side, basically weathered sandstone with glauconitic sands. The fill contact to bedrock is weathered sand with some wind-blown silty clays and probably cut-down trees.

Angle of loose fill repose is 38 degrees. Top fill surface is relatively level, sloped back to the hill side, presumably for preventing run-off over that loose slope and to divert run-off to a culvert.

Thickness of fill perpendicular to the bedrock is on the order of 20 feet. Vertical elevation difference from toe of loose fill to top of fill about 30 ft. (estimated).

The main risk here is heavy run-off coming down the slope and saturating this fill. Up-hill watershed is at least 1,000 ft. Length of fill is roughly 300 feet.

There is a culvert extending from the up hill side to the toe of fill slope, but it potentially can be plugged by debris causing a pond to occur on top.

I need ideas as to alternatives for stabilizing this situation against land-slide in event of heavy rainfall.

Been thinking: pave the top, bentonite or concrete filled trench to bedrock to keep water out of the fill, chemical stabilization of the sand, re-shape things to flatter slope on downhill side.

This is in a developed area with business and residential lots on the down-hill areas potentially in the way of a landslide.

Contractor will get a court order soon to remove the fill. However, he may balk and some fixing may be needed at least on an emergency basis.

Your thoughts?

 

[BigH]

oldestguy- can you do a rough sketch and post it?

 

[jdonville]

Drilled shafts, maybe with a grade beam and tiebacks?

Gonna be expensive for the contractor when he gets the bill. Probably drive him into bankruptcy.

Jeff

 

[oldestguy]

Ok Guys:

Here is a very very rough sketch, since so far no actual dimensions have been measured other than slope angles.

http://files.engineering.com/getfil...2-9c1d-66063e1cf3d2&file=sand_fill_sketch.bmp

The up-hill area is all woods and this general area has had many a problem with culverts plugging shut with debris. No one would be around to maintain this culvert in a heavy storm and then a pond would develop on the sloped fill. Paving may be an option, but that natural soil uphill also may allow infiltration to saturate things.

The sandstone is a source for water wells in some places, but is unlikely to be so porous as to be of any use in a storm. Here the sandstone is not saturated to a degree to bleed water.

When it comes to alternatives as to what to do, the court order will say remove the whole thing, at probably the least total cost. However, the site is worth something with that fill there and so it may be worth while to make it safe.

This would not be built upon with a building, but would serve as parking lot.

Not yet addressed is that culvert. 24 inch diameter. It discharges onto private property (an office building). A significant storm water handling system would be needed in the down-hill area if that culvert stays.

Its a typical example of what some dummies can do without local construction permits to stop them.

 

[palmahouse]

Does not look stable. But, perhaps convince the courts that you can do some surface (and possibly subsurface) drainage improvements to reduce the immediate risk and monitor the slope with slope indicator casings and/or TDR? I like your idea of paving, but, I would suggest that an upslope subdrain or drainage gallery may do more work compared to a cutoff, and, may likely help reduce the demand on structural elements for any future permanent repairs.

Boy, structural solutions would be extremely costly. Perhaps a better structural solution to drilled piles would be battered drilled micropiles and grade beams.

Good luck to your client on this one!

 

[oldestguy]

To clarify some: This is mostly within a dedicated street right-of-way and the community officials want me to act as their engineering representative, possibly doing some engineering for saving it, if possible. No permission was given to the guy to do this, so the court request will be to order it removed.

It is not likely the guy will do anything. Fortunately he has property in town and that could be attached to pay for what ever the community officials decide to do.

Current plans are to do a topog survey, some soil tests and a global stability analysis as it sits and if it becomes saturated.

The option to leave it there is why my question to all you experts.

It likely will be a court case some day, so data is needed now in case of that also.

 

[dgillette]

A fill is better than a canal, but steeply sloped saturated soil above the houses may not be terribly different from this:

http://www.sltrib.com/news/ci_12830658

 

[PEinc]

Can you drill and concrete some soldier beams into rock at the toe of slope, place some precast lagging, and then fill behind the lagging to stabilize the fill? The precast lagging/soldier beam wall could extend just high enough to allow a cantilevered design while allowing a flatter slope above and behind the wall.

 

[theCorkster]

Whatever the final solution is, it had better be maintenance free. I'd set stability based on fully saturated conditions since it doesn't sound like surface or subsurface drainage measures will be maintained.

I'd also be concerned about what assumption you'd make about the potential for pore pressure build up associated with seepage through the sandstone.

 

[oldestguy]

A question about softening due to saturation.

The fill is placed on disturbed topsoil. I ran a quick on-site test on topsoil at a moist condition and got a friction angle of 38 degrees and zero intercept.

Taking a sample home I saturated it making a very thick slurry and ran it again, getting friction angle of 20 and intercept of 10 psf. Test is on a one ft. square plate.

How likely will the topsoil under this fill get that weak in a saturated condition? No doubt it will have degree of compaction.

If this topsoil gets that bad, this fill sits on a greased layer.

 

[Prepakt1]

Might be advisable to drill in some ground anchors to bedrock and chemically grout the sand. Just grouting will not work as the whole mass could move. You would need some good anchoring into the underlying bedrock. The grout would keep the sand from being saturated only.

Intrusion Prepakt /marineconcrete.com

 

[Doc09]

oldestguy, this would be primary concern "filling sitting on a layer of grease", added to it the slope towards the culvert which could turn into a disaster should any water seep through (e.g. culvert not adequate enough to cater to the volume of water, cracked culvert, etc...)

We dealt with a situation like this back in 1994 where a road was built on top of layer (1 to 3ft thick) of medium to high plasticity clay overlaying limestone rock. Every winter the road would slide down hill, and be replaced again and again. We were hired to do the soil investgation and recommend a solution. The best we could do to provide as close as possible to a maintenance free solution, was to build a gravity retaining wall founded about a 1.5 feet into the rock, which provided the required stabilty and allowed for free drainage. Don't know if you have the room to build a gravity wall, but it would nice as ours has been stable since 1994.

 

[oldestguy]

Doc9:

Great idea. As you know those forces can be major stuff.
The main problem is building anything with that fill sitting there at its current angle of repose.

All of these responses are being read also by the current local officials, so they are seeing the difficulty in similar examples and what they are faced with for expense and other things.

 

[dirtydude]

Along the same lines of what Doc9 said, a riprap buttress fill could work as well. It would be free draining and could be keyed into the bedrock just the same as a gravity wall. The only limit would be if the space allows it to be installed.

 

[GabionGuy]

If you are looking for a structure to retain the soil once it gets saturated, what about a gabion? It will allow for drainage through the entire structure.

 

[oldestguy]

Thanks to all. My rough guess is the cheapest alternative is to remove the fill. However, all ideas are helpful, because eventually this area will require a lower elevation fill for a road within the right of way. Current fill is too high in elevation for long term use as a road and mostly on private land where it is level.

 

[SLL78]

This might be a crazy idea but could there be potential here for landscaping the area with plants whose roots would stabilize the area. This would be a question to discuss with a very experienced landscape architect but it could be a solution. As this is a populated area and it sounds like minimizing construction as well as maximizing aesthetics there could be a plant based solution for this problem.