The question asks for the approximate settlement of a piple group. The piles extend 30 feet into a 40 foot thick clay layer. The equation for settlement uses the initial effective stress at the midpoint of the clay layer (20 feet); However, the solution uses the pile length of 30 feet.
Is this a mistake in the 6 minute solution? I say you use the midpoint of the clay layer, as the equation states...
Thanks,
ktulu
I am not going to be able to get to this until later .. I have a working lunch today. :joke:
I did want to point out that I skimmed over the problem and wanted to provide a brief setup so you can properly understand the problem.
When looking at pile group settlement in clay soils, you can apply three simplified assumptions. Those simplified assumptions are:
1. Assume the pile group to be a solid foundation with depth of 2/3rd length of piles;
2. Effective stress at mid-point of clay layer is used to compute settlement; and
3. Consolidation Settlement Equation (general form as applied to typical consolidation settlement problems).
Please note that when you are evaluating the mid-point in assumption #2, you are basing that mid-point from converting the pile group to the solid foundation with depth of 2/3 length of piles. The mid-point is then re-evaluated as the 'mid-layer' of the clay located between the bottom of the solid foundation and the bedrock.
So, for Problem 72, the problem statement indicates 40-ft of soft clay such that the piles are embedded to a depth of 30-ft. If we apply the 2/3rd length rule, that means that the 'equivalent' depth of the foundation is 20-ft below the resevoir bottom. If we take the bottom 20-ft (e.g. the 'unembedded' depth of the clay) and split it into two then the re-evaluated 'mid-layer' is located at 30-ft beneath the resevoir bottom.
I know that is a whole lot to take in without the benefit of a picture - I will update once I have a chance.
Also, I would point out that if you are not taking Geotech depth - this is not a specified problem on the exam. In other words, I would not get wrapped around the axle on this problem if you don't have to :screwloose:
JR
