Remaining Questions in Early Career

[Mr. Skippy]

Hello, I have only been working a few years and most my experience is pretty basic. Now, I no longer work for this company, but I have a few remaining questions I never got to ask the boss. Any help on these questions would be great.

1. For piers - the reports say "the bearing values are based on a factor of safety of three, dead load only. The weight of the shafts below final grade may be neglected." I don't understand the dead load only comment and why the weight of shaft can be neglected.

2. Unsaturated soil mechanics - somehow has evaded me as I went through college. USC test on unsaturated clay was the only strength test my former company ran most of the time. This to me seems very limited. It gives no information of internal friction angle and the true cohesion {after becoming moist in future} is hidden in the strength by apparent cohesion and frictional strength. Is it common practice to use usc strength from unsaturated clay for design or are there correlations to Atteberg values that give more accurate strength.

3. Overconsolidation - {as we learn it in college} is when the soil has experiences insitu effective pressures in the past that are greater than are currently present. But I have heard engineers talk about dried out clay as overconsolidated. This confuses me. Is the connection that - unloaded clay expands and can have negative pore pressure giving it extra shear strength and that is similar to how dried clay has soil suction and capillary forces giving it extra strength?

4. Settlement - My former company didn't seem to have any consolidation equipment in the lab nor did any of the reports mention settlement or overconsolidation other than that settlement will be insignificant. Is it common practice to use correlations for OCR and Cc values.

Any insight into how real world geotechnical engineering is preformed in regard to these areas of confusion I have would be appreciated. Thanks

 

[oldestguy]

1. My past experience always considered both live and dead loads, but the live load sometimes was reduced in calculations for settlement to account for what really happens.

3. In glacial country there is a lot of over consolidated clay.

4. Your past company apparently never got into any significant situations where settlement conditions needed those consolidation tests, such as where surcharge loading and timing info needs to be known. However, I will state that most of my experience with this phase predicting timing was more conservative probably because drainage layers were not known in detail. At least three significant jobs had timing come out quite well as predicted and for those the settlements was in feet and it took years.

For sites over some soft organic deposits I found it necessary to test samples several inches high in equipment and gear I had to make for the job. Pretty much did the job for surcharges and needing to know when to remove the surcharge.