If two footings each with the same net stress but with different base widths are placed on a sandy soil underlain by a normally consolidated clay subject to settlement, which footing will have a greater settling displacement? Essentially comparing influence values with settlement values of different base widths of footer.
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Comparing Influence value and Settlement of increased footing sizes 1
- Thread starter rondada
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It depends on a number of things - how deep is the "sandy" soil? What is the relative difference between the "E" of the sand and the "E" of the clay - since most stress distributions are based on elastic theory - this would be of major consideration in how the stresses are transferred down the substrata column. You have stated that the clay is normally consolidated - yet most clays show some quasi-preconsolidation due to aging effects. This then, will affect the consolidation settlements withing the clay regardless of the stress distributions.
Let's us assume, for the purposes of this discussion that the sand layer is very small compared to the width of the footing - so that, in both cases, the stresses imposed on the clay are the same. Also that the clays have OCR =1. Then obviously the settlement of the footing that is larger will be more as it stresses more clay to a greater depth.
On the other hand, assume that the sandy layer is more "rigid" than the clay and is significant in thickness compared to the smaller of the footings - say equivalent to the footing width. Then, of course, more stress is "attracted" to the sandy stratum and the stresses at the top of the clay for both cases would/could be much different - again, the smaller footing would have smaller settlement.
My suggestion - as this might be a homework problem, is to actually develop a chart of footing size vs bearing pressures and determine settlements for different sized footings say with no sand layer first. - on the other hand, you could develop a chart of bearing pressures permitted on a footing based on a specific settlement, say 25 mm, 40 mm, etc. This would be a good learning experience. After you do this, then play with a two-layer system for stress determination based on varying thicknesses of sand and using stress distributions as given in Poulos and Davis' Elastic Solutions for Soil and Rock Mechanics. Do a similar exercise. You will learn a lot more by doing the exercises yourself than hoping we will answer the problem for you.
Let's us assume, for the purposes of this discussion that the sand layer is very small compared to the width of the footing - so that, in both cases, the stresses imposed on the clay are the same. Also that the clays have OCR =1. Then obviously the settlement of the footing that is larger will be more as it stresses more clay to a greater depth.
On the other hand, assume that the sandy layer is more "rigid" than the clay and is significant in thickness compared to the smaller of the footings - say equivalent to the footing width. Then, of course, more stress is "attracted" to the sandy stratum and the stresses at the top of the clay for both cases would/could be much different - again, the smaller footing would have smaller settlement.
My suggestion - as this might be a homework problem, is to actually develop a chart of footing size vs bearing pressures and determine settlements for different sized footings say with no sand layer first. - on the other hand, you could develop a chart of bearing pressures permitted on a footing based on a specific settlement, say 25 mm, 40 mm, etc. This would be a good learning experience. After you do this, then play with a two-layer system for stress determination based on varying thicknesses of sand and using stress distributions as given in Poulos and Davis' Elastic Solutions for Soil and Rock Mechanics. Do a similar exercise. You will learn a lot more by doing the exercises yourself than hoping we will answer the problem for you.
Given that both foundations are located in the same soil stratum, I think the settlement of larger foundation is always high because the depth of influence of larger foundation is high. I couldn't imagine a situation where the settlement is other way around.
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