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Author:
Brandon
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2001-01-17 |
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Somebody can tell me is it logical to have a consolidation settlement so much so that the settlement (based on conventioanal Terzaghi formulae ) is almost equal or even exceed the the height of embankment placed (loading). i.e. there will be no net height gain even say a fill of 1m (about 20 kPa) is palced while the computed settlement is also 1m by calculation ? This sounds very unrealistic isn't it ?
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Follow-up:
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Author:
Bengt H. Fellenius
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2001-01-20 |
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Interesting query. Yes, it may sound unrealistic, but it is real. That is, perhaps less due to factors covered by the assumptions behind the conventional calculations and more to add-on effects.
It is easier to chat about the matter using Janbu modulus number in characterizing soil compressibility. Having to juggle two parameters, Cc and eo, is too hard. (The modulus number has a strict mathematical relation to the Cc and eo).
Assume a 5 metre thick, normally consolidated, saturated clay with a total density of 1,500 kg/m^3, a water content of 90% (void ratio of 2.4) and a modulus number of 5 (i.e., compression index, Cc, of 1.5). This would probably be a soft to very soft clay, incidentally. The groundwater table lies at the ground surface and the stress from a 1.0m embankment placed on the ground surface is 20 KPa. A settlement calculation applying conventional one-dimensional consolidation theory results in a settlement value of 1.1 metre. Of course, because the embankment settles below the groundwater table, buoyancy sets in, and the stress diminishes accordingly. When considering this effect. to return the same 1.1 metre calculation result, the thickness of the clay layer has to be 15 metre.
The modulus number does not have to increase very much for the calculated settlement to be only a portion of the embankment height. Already for a modulus number of 10, the thickness has to be unrealistically large to return a 1 metre settlement value even when the buoyancy effect is disregarded. Conversely, already for a moderate lowering of the modulus number and, also, the total unit weight, the calculated thickness will be much larger than the height of the embankment. At my cottage a few years ago, I wanted to straighten a road by cutting across a swampy area. I placed 2m of fill on the ground and lost 2.5 m of height! Of course, this is not settlement, and it is not one-dimensional consolidation.
Soils, be they clay or peat, having modulus numbers of 10 and smaller will in additional to the consolidation, be subjected to lateral flow. (Here, incidentally, consolidation is not just one-dimensional). The lateral flow can be a significant portion of the settlement of the ground surface. The lateral flow can cause considerable misery. Not because the calculated settlement is smaller than the actual, but because the lateral flow can adversely impact foundations in or adjacent to the filled area, e.g., basement walls and piles. If you want to see what lateral flow does to a pile, take a look at the photos shown in a short note I presented to 1972 5th European Conference on Soil Mechanics and Foundation Engineering in Madrid (Vol.2, pp.282-284).
When encountering conditions for which conventional analysis indicates settlement of half or more of the embankment height, the analysis may have to be performed by numerical analysis employing coupled calculations where time development of settlement, horizontal movement, and pore pressures are determined in the same analysis (not by superpositioning from several analyses). This is particularly important for the design of a approach embankment next to a piled bridge abutment. Incidentally, lateral flow can be reduced by means of wick drains.
Bengt H. Fellenius
PS. The cottage shortcut works fine. At times, there is a foot of water covering the road bed and you think you'd driving into a lake. Many have been faced with the test of proving their faith in my words by driving out into and across the lake, as it were. Funny, how many that arrive to the cottage with their feet wet to the knees, having walked the last part.
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to subject 'Geotechnical design and analysis'
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Follow-ups:
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Subject:
Consolidation Settlement 