Slope and Ground Mov't Repairs

[gingerbread]

Hello!

I am working on a slope and ground movement assignment caused by fat clay and high groundwater. The toe of the slope is adjacent to a concrete sidewalk which has uplifted (buckled) and the ground movement even extended into the streets. Due to the amount of underground utilities beneath the sidewalk and the heavy rain during the winter, excavating the area by removing the clayey soil and replacing with gravel and installing a drainage system is not what I want to do at the moment.

Are there better alternative temperary repairs that someone could recommend to stablize the toe for the time being without having to excavate during the wet weather season?

Thank you,
-gingerbread

 

[EireChch]

Is the movement in the sidewalk due to the fat clay swelling/expanding with the high GW or due to a slope failure causing bulging at the toe? Maybe a combination of both?

A temporary fix maybe to increase the toe support by placing a surcharge load on the toe (I.e placing 1m x 1m x 1m cocncrete blocks). The amount of blocks required could required could be calculate by modelling a surcharge loading in SLIDE. A FOS >1.2 would be appropriate for temporary support

 

[BigH]

How high is the slope? Are there any signs of distress in the slope or at the crest such as tension cracks, scarps, etc.? is the high groundwater level a seasonal thing or is it fairly stable throughout the year (i.e., is it the "equilibrium" level?). To show uplift of the sidewalk (and how long as it been in place?) the slope movement would have to go to some depth below the toe location - and since it seems to be manifested in the street as well, it may be deep - in other words it appears not to be a shallow planar translational movement. Is there anyway to unload the crest of the slope? Putting on blocks at the toe - assuming you have room, may or may not work as it depends on if the location is a driving (very shallow driving slope on any distress plane) or resisting. Since the effects are heading out into the street, it might not really do any good at all. You state that there are a number of underground utilities under the sidewalk which leads me to ponder if this is recent construction? Else, why didn't this happen in earlier years?

Again, if you can, try to quickly unload the crest of the slope and if you are having heavy rains, we have seen in the tropics the use of membranes placed on the slope to keep the water out of it. This will reduce the driving force. Are there any "springs" breaking out into the slope face? Would horizontal drains into the slope be useful? (short term might consider drains under vacuum to pull down the phreatic surface quicker - if the phreatic surface is high at equilibrium).

Just some thoughts - sorry if it seems a bit disjointed . . . but as I was writing, things from past projects just seemed to be popping into my mind . . .

 

[EireChch]

I also did consider drainage to improve stability however I thought that would be a more long term solution. Nonetheless, Horizontal bored drains maybe an idea. I'm not sure how familiar you are but, these would be typically be 100mm dia perforated pipe drilled upslope from the bottom of the slope in a fan arranagment in plan view. Counterfort drains could also be considered, these are trenches excavated perpendicular to the slope, typically to 4-5m depth (depending on the depth of drawdown required. A preferated pipe is placed in the base of the trench and surrounded with drainage metal. The outlet of the pipe could daylighted or be connected to the reticulation system.

I think a starting point would be some boreholes to asses parameters such as shear strength, cohesion and current ground water level. I would then do a stability analysis to asses various stability solutions.

If you could supply a cross section with borehole information it could help develop the geotechnical model. I'm sure you would get some better feedback then

 

[gingerbread]

Dear EireChch and BigH:

Thank you both for contributing solutions to my question. I Do have some laboratory testing data from the soils collected from the slope that I can use for a stability model(s). I will supply some more information later such as a cross section with soils paraneters to better illustrate the condition in the field.

 

[BigH]

The main question - how much time do you have? Many projects I have been on over the last decade - we have no time for studies and need to use judgment. Studies would be great and are needed if you have the time . . . but you may need to think of short term "improvements".

 

[EireChch]

Another thought following BigHs comments on deep seated and shallow failure. If it is deep seated failure, I don't think there are any short term methods that will really stabilise the slope? Membranes to stop surface infiltration will help however it appears that the high GW level is a problem. An in ground palisade wall, soil nails, re-grade the slope and drainage are the options, however these are long term fixs. I can't think of a quick and dirty fix that will solve deep seated failure? It seems as if the OP wants enough time to get through a wet season. In this case it could just be a case of closing the sidewalk and providing alternative sidewalk and monitoring the slope.

If it is shallow type failure. Blocks could be an idea. An improvement in this could be to place 1.5m high blocks right at the toe and place some hsrdfill behind to add further surcharge. These blocks can be rented for relatively cheap. Sliding and overturning failure should be ok in the short term

 

[gingerbread]

Gentlemen:

After studying the slope and sidewalk issue further, it appears water from the hillside (maybe even naturally occuring springs) is a major problem. The uplift in the sidewalk is most likely due to the water from the hillside mixing the fat clay. The plan is to install a drainage system on the slope and at the toe (underneath the sidewalk) and connecting the drains to the existing city's drainage inlet to discharge the water. By removing the water, I think, will eliminate much of the problems.

 

[BigH]

But why did it happen this year - we have no idea of the time frame - has the sidewalk and underground services been in for only one year - or ten or twenty?