Settlement in Clay above water table 5

[geonet]

Calculation of settlement in clays above the water table is something I'm unfortunately still unclear on. I am currently calculating settlement in clayey soil above the water table. Total settlement, no matter how quickly it occurs, is of interest to me due to several of reasons. I have consolidation data for the clay layers above the water table. My approach has been to calculate the settlement as if it were consolidation settlement, not elastic, and then assume that settlement will occur relatively rapidly. Is this a reasonable approach in calculating the magnitude of settlement.

 

[BigH]

Two points:

1. Have anyone involved here ever tried to use the particulate particle theory rather than elastic theory for settlements? M.E. Harr (Purdue) wrote a book on the subject some years back. Whilst I was tad "overwhealmed(?)" on the probability theory, etc. there were some good thoughts on the differences between particulate and continuum computations.

2. I believe that sometimes we all must sit back and really take a look at the "accuracy" expected of calculations - especially involving settlement. It seems that "papers" all hit the settlement "right on" but that is doubtful. Most, I would surmise, would agree that if we are within 25% of the actualy settlement in our predictions we had a decent day. In 2002 there was an elaborate study published in the Canadian Geotechnical Journal regarding the use of electric cones (piezo with two pwp transducers) to estimate the preconsolidation pressure and comparison of this work with Mayne's work. These were on Quebec clays. In the end, even with the "sophisticated" cone, the scatter of data was up to 20%. So - are we sometimes getting "smarter" than we really are??

A side note on accuracy: When I was in Indonesia, I saw engineers and university professors taking a standard penetration test N value and determining the capacity of driven piles to the nearest 1/100th of a tonne. Talk about accuracy - some accuracy . . . . not.

- - - I am kind of keen to see your thoughts on the particulate theory - - -

 

[Focht3]

While I have heard of Harr's approach, I haven't read the book. I guess I'll have to find a copy...

On the issue of accuracy: I'd call a day with all my settlement predictions within 25% of the actual value as an outstanding day. To me, within 30% is a good day, and within 50% a decent one. At least with respect to movements. I'd go along with BigH's 25% range when talking about ultimate pile capacity and similar load capacity predictions. For me, movement is much harder to predict.