I read the old thread on the expected consolidation of clays compacted wet vs dry of optimum and found it interesting but not determinative of a situation we have encountered.
How would the expected consolidation parameters differ for an undersaturated stiff clay versus a saturated stiff clay, all other things being equal? The clays I am thinking about are calcareous old lake bed sediments, lean to fat, with saturations ranging between 80% and 95% (above a deep water table) and 100% (below the water table). Dry densities in the 80-90 pcf range are not uncommon. Despite the low densities, these clays are stiff to hard in terms of drilling behavior (in fact, they contain carbonate nodules at places)and SPT values are not uncommonly 25-50 blows/ft.
The OCR ranges up to 2, but we still see consolidation behavior in tests.
Our approach to testing has been to run conventional 1-D consolidation tests, saturating the samples (well, at least adding water...) at normal stresses ranging between 1000 and 4000 psf to try to see what is happening in the elastic and unsaturated regions. We typically get 2% to over 8% instantaneous deformation upon adding water. The shape of the load vs deformation curves above and below the saturation point seem identical so our assumption has been that the expulsion of pore fluid is what is important in terms of consolidation magnitude, and it makes little difference in terms of magnitude if what is being expelled is water or air. Differences in air vs water permeability might affect time-rate calculations, although even then, the overall time to an advanced state of consolidation should be the same because one has to get rid of the water sooner or later. From this logic, we have obtained consolidation parameters and used these in consolidation models conventionally.
I am seeking comment on whether the use of conventional consolidation curve analysis and conventional settlement calculations is appropriate in this case.
By the way, we don't know to what degree our consolidation coefficients are being affected by sampling disturbance. We suspect some, but perhaps not all that much. Because the material is so stiff and nodular we have not attempted to drive Shelby tubes; we use a 3-inch drive sampler. This probably introduces some disturbance, although the elastic tests we have run don't indicate much cracking or fissuring. We decided there is no practical way to quantify the effects of disturbance and have pretty much presumed that the load-deformation behavior we see in our tests is reflective of the in situ material, or at least that the assumption will be conservative as to actual settlement magnitudes.
We recognize that the undrained shear strength of these clays will reduce settlement when loaded by embankment, asserting that at least initially they would consolidate fully but for the rigidity of a calcareous soil structure. However, we know from our testing those bonds weaken or are lost entirely when the material is wetted, which we have to assume will eventually occur below new embankment to one degree or another.
How would the expected consolidation parameters differ for an undersaturated stiff clay versus a saturated stiff clay, all other things being equal? The clays I am thinking about are calcareous old lake bed sediments, lean to fat, with saturations ranging between 80% and 95% (above a deep water table) and 100% (below the water table). Dry densities in the 80-90 pcf range are not uncommon. Despite the low densities, these clays are stiff to hard in terms of drilling behavior (in fact, they contain carbonate nodules at places)and SPT values are not uncommonly 25-50 blows/ft.
The OCR ranges up to 2, but we still see consolidation behavior in tests.
Our approach to testing has been to run conventional 1-D consolidation tests, saturating the samples (well, at least adding water...) at normal stresses ranging between 1000 and 4000 psf to try to see what is happening in the elastic and unsaturated regions. We typically get 2% to over 8% instantaneous deformation upon adding water. The shape of the load vs deformation curves above and below the saturation point seem identical so our assumption has been that the expulsion of pore fluid is what is important in terms of consolidation magnitude, and it makes little difference in terms of magnitude if what is being expelled is water or air. Differences in air vs water permeability might affect time-rate calculations, although even then, the overall time to an advanced state of consolidation should be the same because one has to get rid of the water sooner or later. From this logic, we have obtained consolidation parameters and used these in consolidation models conventionally.
I am seeking comment on whether the use of conventional consolidation curve analysis and conventional settlement calculations is appropriate in this case.
By the way, we don't know to what degree our consolidation coefficients are being affected by sampling disturbance. We suspect some, but perhaps not all that much. Because the material is so stiff and nodular we have not attempted to drive Shelby tubes; we use a 3-inch drive sampler. This probably introduces some disturbance, although the elastic tests we have run don't indicate much cracking or fissuring. We decided there is no practical way to quantify the effects of disturbance and have pretty much presumed that the load-deformation behavior we see in our tests is reflective of the in situ material, or at least that the assumption will be conservative as to actual settlement magnitudes.
We recognize that the undrained shear strength of these clays will reduce settlement when loaded by embankment, asserting that at least initially they would consolidate fully but for the rigidity of a calcareous soil structure. However, we know from our testing those bonds weaken or are lost entirely when the material is wetted, which we have to assume will eventually occur below new embankment to one degree or another.
