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Slope Stability - Saturated Sediments

Slope Stability - Saturated Sediments

Slope Stability - Saturated Sediments

I have a slope stability issue I was hoping someone could provide some insight for.  We are excavating river sediment (medium to high plasticity silt and clays with good amounts of organics).  I am determining the properties these sediments would have placed in a landfill without much dewatering (a worst-case scenario).  Essentially without free-standing water.  These sediments will be tracked into place at a 4:1 slope about 40' max height.  If these sediments are placed fully or nearly fully saturated,

1) Is this condition similiar to a submerged embankment that has seen a rapid drawdown of the water level, where the piezometric surface will be located along the face of the embankment along the entire geometry, which will greatly reduce the stability of the slope?

2) What effect will the moisture content (nearly or fully saturated) have on the internal friction angle?

3) What would be typical unit weights, phi angles to be used for the compacted sediment or references that list these?

Any insight or references to stability of saturated embankements would be usefull, also let me know if im way off base with anything.  The purpose of this analysis is to determine the requirements these sediments will need to meet (Mc%, Phi, etc...) in order to be stable starting at the worst case scenario and working back.

Thanks in Advance

RE: Slope Stability - Saturated Sediments

I would recommend a method of analysis similar to what you describe in item number 1.  As for the impact of moisture content on internal friction angle, for the short term (undrained shear shear strength) the water conten will greatly effect the shear strength.  In the long term, (drained condtions) the initial water content will have very little impact on the shear strength.  Lastly, only laboratory testin, and a lot of it, will answer these questions.

While the situation you are describing may appear simple, it is very complicated and will require significant field exploration, laboratory testing, and analysis.  My intial opinion, based on what you have provided, is that 4:1 with a 40 foot slope height and the material handling you are describing, is dreaming.  The plan will likely need to be modified to make this work.

If you provide some additional information on the overall plan and constriants, additional input is likely.

RE: Slope Stability - Saturated Sediments

First of all..its not a good idea to place saturated soils in a landfill due to leachate problems...most landfills have paint filter test requirements to control wet soils from being placed

as far as your bullet items:

1) No it is not the same as a rapid drawdown of a submerged embankment...a rapid drawdown results in an increase in total stress with the effective stress remaining the same (short term) your situation most likely will see a gradual increase in effective stress as the soils drain and subsuquent increase in shear strength...although not much since you say they are organic silts and clays

2) if you have saturated silts and clays with organics you better assume you have no friction angle...you only have the cohesion to count on for stability

3) typical saturated unit weights would be 100 to 130 pcf and friction angle of zero for saturated organic silts and clays

Also...you better look at it from a global stability issue

RE: Slope Stability - Saturated Sediments

The material is tyically muck. The moisture contents of dredged material can be quite large. Since it is a silt, it will have very low draied friction angle. Since it is distubed, it will have no cohesion. If you try to pile it, it will act similar in the short term to rapid draw down, in the sense the material will be saturated on the inside with low effective stress. Long before you get to 40 feet, the material will liquify and flow. Best bets are to build a walled bin to allow the material to drain or mix with dry material such as dead sand.

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