Select Fill Materials 5

[ONENGINEER]

Could someone help me in understanding the term "Select Fill" materials. I understand these fills are not expansive and can substitute the expansive soils in order to decrease the potential vertical rise of the sub-grade soil due to moisture variations. What are the typical ingredients of select fill materials. I presume they are sand and clay. How many percent clay do they have? Thank you.

 

[ONENGINEER]

Okiryu,many thanks for the link. Is there any upper bound for the uplift pressure as the PI of soil increases. The local consultants in Texas use much lower values for high PIs. I wonder if aeliantexan could also give me an opinion on this. Thank you.

 

[Okiryu]

I do not deal too much with expansive clays, so I would also like to see aeoliantexan's input.

 

[aeoliantexan]

Sorry to be slow; I've been distracted.

My associate confirmed that local practice (Dallas area) is some variation on about 1000 psf upward skin friction or adhesion applied over the depth of seasonal variation (commonly about 10 to 15 feet). He prefers a higher skin friction applied to about 2/3rds of the seasonal variation.

In dry weather, the undrained strength of our clays can be much higher than 1000 psf. But is the soil swelling and heaving at that time? Has the clay in the top few feet shrunk away from the wall of the pier? In wet seasons, will it both swell and soften from the top down?

Like a lot of rules of thumb, the practice generally seems to work most of the time. I would be cautious about applying it in other parts of the country.

 

[SEARCH2016]

Thank you aeoliantexan to open the blackbox. Trying to understand better, you referred to the undrained strength of clays in Dallas areas. Is there any rule of thumb between the uplift pressure and the undrained strength? The one discussed earlier was between the PI and uplift pressure, which I thought to be very conservative. I would appreciate your comments on this too. I have also seen that they use 1000 psf but is this uplift pressure the same in all clay soils in Dallas area? The question is still how to estimate the uplift pressure and based on what soil parameters (in Dallas area). Hope you can shed some more lights on this.

 

[aeoliantexan]

I don't have any handy relationships or rules of thumb for swell pressure. You can address it with swell testing in the oedometer, of course. One would think that there should be a relationship between unconfined strength and swell pressure, because both are related to relative water content. I just assume that if the soil is highly plastic, hard, and dry it will probably swell too much. Locally, the PVR, Potential Vertical Rise, is used a lot by practitioners and derided by academics. You can find it in the TXDOT Soil Manual. This just estimates the potential movement under overburden load, not pressure.