Slope Stability

[easaw]

Highly saturated area was encountered at the toe while placing engineered controlled fill with a slope of 3:1 to a height of approximately 30 feet. It was noted that the location was a small lake, drained and filled about 50 years ago. According to the observation, the material mainly consists of dark brown fat clay with silt. Also, wetlands and adjacent tributary creek is identified within a previous lake bed (toe area).

Most probably, the above mentioned sub-surface area will be always highly saturated. I appreciate your suggestion to stabilize the ground/or the lake bed area before the placement of controlled fill material.

 

[fattdad]

It may be fine just how it is. You'd have to do further field exploration, assign shear strength, identify the position of the phreatic surface, and look at the safety factor for rotational and shear failure.

If safety factors are adequate, you could potentially address seepage by installing a toe drain.

(From reading your original post, I'm not quite sure whether the fill is in place or if you are considering the fill at some point in the future.)

f-d

¡papá gordo ain’t no madre flaca!

 

[easaw]

Thank you for the reply. I am curious to know how you came to a conclusion “it is fine just how it is”. I hope my first post did not give a complete situation of the location.

At the stated location, the placement of controlled fill is not done, but the developer likes to build the land ASAP. The developer is not interested to engage for further geotechnical explorations. According to the observation, the phreatic surface is at the toe surface level. The wetland and the adjacent tributary creek is very close (wet land -approx. 15’.00 & creek 100’.00 away from the toe/or lake bed) from the toe area. The wet land and the creek is under the core engineering authority and they do not want the developer to disturb the wet land to drain the current stagnant water.

Now without any further field exploration, the developer is looking for a suggestion to stabilize the saturated toe land/or lake bed area before the placement of controlled fill

 

[fattdad]

All I meant is a 3H:1V slope of controlled fill just isn't that steep. The presence of a near surface water table may also not be that much of a big deal, depending on nature of the saturated soils. Water alone is not that bad, it's when there are excess seepage gradients and soft soils where you can get into trouble.

Can you go in there with a hand auger and see just how hard/compact the saturated soil are. . ?

f-d

¡papá gordo ain’t no madre flaca!

 

[GeotechGeo]

Is the embankment going on top of the soft saturated soil? Are you worried about settlement or slope stability? If the deposit is from an old lake then likely normally consolidated clays which will increase in strength with time as consolidates. Slow, staged construction so that pore pressure dissapates before placement of the next stage so you don't get an undrained failure and expect a lot of settlement.

 

[easaw]

Thank you for the suggestions. The embankment will be placed at a slope of 3:1 on top of highly saturated silty fat clay (toe area). Probably field exploration & lab tests will give an idea about the characteristics of the material, but the contractor is not very keen to spend more money for further field testing.

What is your opinion to place large rocks (approximately 6-12 inches) into the soft ground after mucking out all highly saturated silty fat clay from the toe area, which will act like geopier to stabilize the soft ground?

 

[fattdad]

Please provide some sense of whether the "silty fat clay" is soft. If it's just wet there may not be a real issue at all.

Not sure how large rocks will help. There may be some benefit to a drainage blanket in the form of separation geotextile and open-graded aggregate for subgrade preparation; however.

f-d

¡papá gordo ain’t no madre flaca!

 

[RCEJD]

We have had good success in these circumstances (details are not available to be sure) with the use of a geosythetic stabilization fabric followed by a gravel layer (1 to several feet thick). Arches and easy to place to form a hard layer to compact against. We have used this approach in hillside areas (mostly landslides) with great success. Good luck.

 

[easaw]

The fat clay is highly saturated and soft. The PI of this material is 50.

RCEJD, I appreciate if you could please let me know the location of perventive treatement you applied. Was is at the toe zone? If the preventive treatment was at the toe zone, what was the depth of the treatment?