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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 Broms
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2001-01-17 |
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It is entirely possible that the settlement calculated from e.g. the results from oedometer (consolidation) tests can exceed the height of the fill. The compression index Cc had to be high, 2.0 to 3.0. The submerged unit weight of the compressible soil had to be low, 2 to 3 kN/mcu and the ground water table had to be very high, close to the ground surface. For e.g. a 10 m thick layer with peat with a compression index of 2.5, a unit weight of 12.6 kN/mcu and an initial void ratio eo of 5 the estimated settlement for a 1 m high fill is about 1 m when the ground water table is located at the ground surface. (The stress increase is reduced from 20 to 10 kPa as the fill is submerged.) The net increase of the effective vertical stress is 10 kPa throughout the clay layer when the width of the fill is large compared with the thickness of the clay layer. The settlement caused by the lateral displacements of the soft soil is neglected as well as the long term settlements caused by secondary consolidation and creep. If the thickness of the fill is reduced to 5 m the estimated settlement is reduced to 0.8 m. In order to reach a settlement of about 1 m when the thickness is 5 m then compression index had to be about 3. It is thus possible that the calculated settlement of a fill can reach or even exceed the height of the fill. This is not uncommon for organic soils like peat and gyttja.
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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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Follow-up:
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Author:
Liew Shaw Shong
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2001-05-18 |
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This is real and it happens in many road construction in East Malaysia where there are many weak subsoil in swampy ground.
This can be explained through a simple analogy as follow:
1. Assuming putting a 100mm thick steel plate on a spongy mattress, then measure the settlement of the plate. The settlement may be recorded as 20mm.
2. Then remove the steel plate and stack up another mattress and place the steel plate on the top mattress again. This time, the settlement will recorded as 40mm.
3. Continue to increase the mattress and do the same measurement. The settlement can be 100mm when five mattresses have been stacked up. This settlement is same as the thickness of steel plate. If more mattresses are stacked up, the settlement will certainly more than the thickness of the steel plate.
So, this example shows the settlement can be more than what you have topped up on the soft compressible subsoil. If the thickness is sufficient, this will occur. Of course, you may consider to use higher fill to top up, but be careful on the stability of bearing capacity failure.
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Follow-up:
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Author:
MG
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2003-02-19 |
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It is possible to achieve settlement greater than the amount of fill placed. I have worked on an embankment where such movement has occurred. In this particular case, the subsoils consisted of approximately 40 feet of very soft soils with void ratios approaching 4.0 and compression indices near 3.0. The thickness of this soft stratum permitted remarkable settlement
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Return
to subject 'Geotechnical design and analysis'
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Follow-ups:
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Subject:
Consolidation Settlement 