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# Lateral pressure coefficient - taking into account level of saturation2

## Lateral pressure coefficient - taking into account level of saturation

(OP)
I never paid much attention to this before.
So far my understanding was that if you have a saturated soil, the lateral pressure was based on the modified (reduced) density of soil and the additional hydrostatic pressure, see image.
The lateral pressure coefficient, I thought was unchanged.
Yesterday I read on a forum, possibly this one, that when soil is saturated, it loses it's shear strength and the lateral coefficient was equal to 1. I did not pay too much attention to it. I can't remember the type of soil, or whether there was a potential difference allowing water to flow, such as retaining walls subject to tides, I don't know if this is relevant.

Today I came across a Table from California Trenching and Shoring Manual, which shows a much different Ko value for clays, depending on the level of saturation.

Firstly, my question is: Is this relevant for clays only?
Secondly, is there a good reference with tabulated values, showing how Ko changes based on the level of saturation.

### RE: Lateral pressure coefficient - taking into account level of saturation

Very few temporary or permanent sheeting walls, trench boxes, slide rail system, etc. use Ko for calculating lateral earth pressure. Mostly Ka is used for these usually flexible wall system. For rigid basement walls, Ko is most often used. I am not aware of any requirement for using a lateral earth pressure coefficient of 1.0, saturated soil or not, unless designing for undrained clay where the internal friction angle of the clay equals 0 degrees (therefore, Ka = 1.0) and cohesion > 0 psf is considered. For a drained condition, cohesion would be considered to be zero psf but the friction angle would be greater than 0 degrees. There are graphs in most soils books that relate the phi angle of a drained clay soil to the clay's plasticity index. These graphs may show a phi angle of 20-couple degrees to 30-couple degrees. Therefore, Ko and Ka would be less than 1.0.

### RE: Lateral pressure coefficient - taking into account level of saturation

Note that Ko = 1-sinØ, for Ko to be 1, Ø must be zero, which can only occur in clay at undrained condition, as indicated by PEinc above. I don't think such tabulation exists in any of geotechnical publications. For project dealing with clayey soil and ground water, don't skip the need of consultation from a geotechnical engineer.

### RE: Lateral pressure coefficient - taking into account level of saturation

(OP)
OK thank you for elaborating.

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