rain seeping into soil on construction site

[tildeme]

I'm still an engineer in training so please excuse me if I sound like I don't know what I'm talking about. This is why I'm posting here asking for further insight and see if there any advice or professional resources that can be suggested to help me.

So I have a project where I'm designing foundation for various electrical substation structures. We have a soil boring report that indicates that plasticity index is 30 so we can safely assume that there are no expansive clayey soils and that the report indicates a silty sand/rock that is fairly strong. Which makes me believe that there should not be any issues with settlement differentiation based on soil reports. However, the guys in the field believes otherwise.

There has been massive amounts of rain in the last few weeks and they're very concerned with the strength of the soil for some of our pad and shallow foundations. (It hasn't been designed yet.) They believe the soil borings may not be accurate and that they the foundation will not hold based on what they see in the field.

I've been told that they have decided to do have a second company do a second boring report later on. But my boss still wants me go ahead to design the foundation. What I want to know as an EIT is how exactly does rain affect the soil mechanics? How do I take into account the rain that is soaked up in the soil after a soil boring report is done? Isn't plasticity of soil more important? If the bearing strength decrease, how do I design the foundation taking this into account?

FYI, I designed foundations using PLS-caisson and I just usually check to make sure the plasticity of the soil is ok and don't usually give rain and other weather conditions much thought. I also want to add that there is a licensed PE who checks over my work but hasn't been around to answer my questions since he's having surgery. So I've been trying to do research on my own. Any help would be greatly appreciated. Thanks in advance.

 

[oldestguy]

This is a condition similar to what is found in the spring of the year in Northern parts of US, when the frost leaves. I call it a super-saturated condition. The result is usually weakening.

If your weather conditions do not dry things to "normal" conditions at the time of the work, we usually call this a "change of conditions" as compared to what a soil report in dryer times would call for.

It then is common, assuming that you must go ahead with the work, to fix the site to meet what the designer had anticipated for normal weather. A common procedure is to dig out and waste a given depth of the wakened stuff and replace with competent compacted granular material, meeting the support characteristics that the design was based on. There are many different ways to fix these conditions, so the undercut method may not be best for you..

Of course this costs something more than the usual work and, assuming the owner OK's the work, this undercut and filling with compacted material or other site improvement method is done to keep the job moving and to properly support the structure.

The usual procedure is to have the geotech firm that did the boring report come out to the job and recommend the fix so that the result meets what the designer designed for. They usually are equipped to test the compacted fill, etc. and, if they did not send a geotech engineer to the job, the field guy is in contact with his office and that way can tell the contractor exactly what his boss recommends. It is done every day.

In your case, doing more borings may be a way to resolve this and then come up with a different "bearing capacity" or more probably recommend a site fix which would be similar to the geotech out on the job might be recommending.

If it were me, I'd not do any more borings, but plan for doing an on-site evaluation as the work goes on, using an experienced geotech engineer who comes on the job or uses his experienced technician to do the field evaluation and get his boss's recommendation. This way, the geotech or the technician can make use of the on site equipment to determine more precisely just what is needed to be done. Generally this is a back-hoe excavator that does "test pits" that the technician or geotech can use to see what the condition is.

This condition is a very common one and any experienced geotech firm ought to be able to guide you along as to what is needed.

 

[fattdad]

A PI of 30 does not guarantee there is no shrink-swell potential, 'cause we don't know the Liquid Limit (i.e., you may have an elastic silt or a lean clay - can't tell).

Saturation does not affect soil strength. Saturation affects the soils tendency toward disturbance. If the soil gets wet and you don't try to work on it or with it, it'll survive just fine. Heck how do all those soils below the water table end up with any strength?

Design the foundations based on the original geotechnical engineering study (fyi, I notice that you are a civil engineer, so you may want to confur with a structural engineer for this too?)

Use general notes to state the obvious: Foundations to be constructed on undisturbed natural soils or properly compacted structural fill. If there is disturbance, undercut and replace (with compacted fill), lower the footing, or scarify and recompact.

When you say, "I've been told" who's doing the telling?

Good luck.

f-d

¡papá gordo ain’t no madre flaca!

 

[oldestguy]

fatdad:
Sorry to differ on saturation and soil strength.

Take a clay at the plastic limit and change it to its liquid limit. All the way through that moisture change range it is saturated, with the voids at or near "saturation". Plenty of change in strength through that range. Take it a step farther beyond liquid limit to soup, same statement about strength change.

Same goes for moisture changes down to the shrinkage limit.
Saturation (or near that) holds all the way down to that place also, along with a strength change.

 

[fattdad]

for a given dry density, the range of moisture contents from well below "optimum" all the way to saturation will have no affect on dry density and little affect on strength. I know there are trivial distinctions, but let's not argue that.

Your case - changing soil from plastic limit to liquid limit assumes soil disturbance (i.e., the actual test) and has no control on dry density. These are two cases where saturation moisture content is varying, but dry density is also varying. Can't relate this to the field condition.

f-d

¡papá gordo ain’t no madre flaca!