Secondary Compression Reduction - Peat
Secondary Compression Reduction - Peat
(OP)
Hi all,
I'm currently working on a highway realignment project. The general stratigraphy is peat (up to 2.8m thickness) overlying glacial clays. The contractor is keen for all peat to remain in situ, and therefore high settlements are anticipated. We are proposing preloading to remove post-construction consolidation settlements.
My question is regarding reducing or removing secondary compression effects post-construction. I have reviewed various literature including Alonso et al 2000, and Oliveira et al 2016, citing references such as Ladd 1971. These cover the effect of preloading up to 150% on the secondary compression coefficient ca (strain / log time), however are limited to oedometer testing up to approx. 15000 minutes. Particularly Oliveira indicates that the impact upon the ca coefficient reduces over time (approx. 95% ca reduction short term for up to approx. 500 minutes, to only approx. 50% reduction beyond this).
I was wondering if anyone has experience of preloading of peat and its impact upon secondary compression, or have any relevant references regarding the efficiency of preloading (or other pre-treatment measures) for removing or reducing these settlements.
Thanks,
J
I'm currently working on a highway realignment project. The general stratigraphy is peat (up to 2.8m thickness) overlying glacial clays. The contractor is keen for all peat to remain in situ, and therefore high settlements are anticipated. We are proposing preloading to remove post-construction consolidation settlements.
My question is regarding reducing or removing secondary compression effects post-construction. I have reviewed various literature including Alonso et al 2000, and Oliveira et al 2016, citing references such as Ladd 1971. These cover the effect of preloading up to 150% on the secondary compression coefficient ca (strain / log time), however are limited to oedometer testing up to approx. 15000 minutes. Particularly Oliveira indicates that the impact upon the ca coefficient reduces over time (approx. 95% ca reduction short term for up to approx. 500 minutes, to only approx. 50% reduction beyond this).
I was wondering if anyone has experience of preloading of peat and its impact upon secondary compression, or have any relevant references regarding the efficiency of preloading (or other pre-treatment measures) for removing or reducing these settlements.
Thanks,
J





RE: Secondary Compression Reduction - Peat
That's how I approach the problem.
f-d
ípapß gordo ainÆt no madre flaca!
RE: Secondary Compression Reduction - Peat
RE: Secondary Compression Reduction - Peat
Another thought, if the concern with secondary consolidation is differential rather than total settlement perhaps this can be addressed with layering geogrid in your fill and/or subbase.
RE: Secondary Compression Reduction - Peat
The question of organic decay is interesting, but my gut feel is this would pale in comparison versus the compression/consolidation settlements.
DirtGuy's second point is spot on- our design guys nearly always go the geogrid route for embankments over such soils...geogrids with pioneer rock for basal drainage, then monitor settlements. I suspect with the time-frames involved in achieving consolidation of your peat layer, it would be more economic to do a staged construction with geogrids than to surcharge and wait. The geogrids might facilitate steeper fill batters, saving on material costs.
I imagine case histories with monitoring data would be your best bet, laboratory data is unlikely to yield anything reliable - sorry I can't provide any sources.
All the best,
Mike
RE: Secondary Compression Reduction - Peat
Fattdad - I had heard this type of advice previously but was slightly uneasy with its adoption - my understanding would be that because primary and secondary compression are due to different processes (e.g. primary due to pore pressure dissipation and secondary perceived due to viscous effects/particle realignment etc.) that a level of secondary compression would still occur (albeit likely reduced)?
OG - We anticipate that surcharging for one month would be adequate to forego any further primary settlements, and 'live' monitoring of settlement data will be performed on site for verification. The surcharging height is up to only 2.5m above existing ground and therefore we hope that this can be performed fairly quickly, and reduce the issue regarding time of load application.
Mike/DirtGuy - In terms of differential and the use of geogrid - we are planning to adopt a geogrid layer at the top of peat, largely to act as a separation layer, but also to allow a steep batter where the embankment is adjacent to the existing, and live, carriageway which will then help to reduce the differential across the carriageway.
However, the proposed route requires an length of embankment, followed by a cutting, followed by another embankment, and therefore longitudinal differential settlement is more of a concern than lateral(also a maximum post-construction total settlement has been specified by the end client).
Thanks,
J
RE: Secondary Compression Reduction - Peat
Basically, the consolidation from the surcharge, "ages" the formation. Just as if the secondary did occur. When you remove the surcharge, you'd essentially be like 10 or 20 years of aging. To see what may develop during the performance life, you'd just make the adjustments in your secondary equation.
At least that's how I see it.
f-d
ípapß gordo ainÆt no madre flaca!
RE: Secondary Compression Reduction - Peat
At this point are you set on the pre-load approach or have you looked at other types of ground improvement? My understanding is in peat 50% or more of total consolidation can be secondary(depending on fiber content, etc.) And since it sounds like your peat layer varies in thickness under a range of cut/fill depths, it seems challenging to me to meet strict total and differential settlement criteria without extensive pre-load time and/or depth. But perhaps PVDs could accelerate the preload and/or lightweight fill in the embankments could attenuate the secondary?
What do you think about using aggregate columns or rigid inclusions to bypass the peat altogether under your embankments? Depending on you fill heights you might be able to use geogrid form a soil load transfer platform in these areas, so your column spacing doesn't get cost prohibitive. The approach would speed up construction considerably and help with your headache about the long-term settlement. :)
Just some thoughts! Interested to hear what you end up doing
RE: Secondary Compression Reduction - Peat
RE: Secondary Compression Reduction - Peat
"The common view of secondary effects as being simply settlement following primary settlement is an oversimplification. Secondary effects occur during primary consolidation, as indicated by permeability data, as well as afterwards. Secondary effects probably result from different mechanisms in different soils. Some simple mechanisms include:
1. Soils have void spaces of widely differing sizes. In some soils, water may drain from the larger voids in accord with primary theory and then water may more slowly squeeze out of smaller voids, producing a secondary effect.
2. In organic soils containing plant matter, water may similarly squeeze out of the voids in accord with primary theory and then water may squeeze slowly out of the individual plant cells, through the cell walls, at a slow rate, producing a secondary effect.
3. In more granular soils, the shearing stresses between particles may be of a viscous nature. When primary consolidation is occurring, the rate of movement between particles is a maximum and the shearing stresses will then cause a maximum resistance to volume change. The retarded rate of compression explains the observation of lower fitted hydraulic conductivity compared to the measured values. The fitted value must be lower than the real value to explain the retarded rate of consolidation. Creep following primary consolidation would also be expected.
5. Field cases that show large secondary effects sometimes involve the settlement of comparatively narrow embankments with low factors of safety against failure. In some cases, some part of the time-dependent settlement may be due to mass movement of subsoil out from under the embankment due to the high shearing stresses and overall soil viscosity.
6. Some case histories of settlement of wide embankments involve a shallow highly compressible soil and deeper less compressible soils. Apparent secondary settlement may actually represent delayed primary consolidation of the relatively incompressible soil which cannot drain until the overlaying, more compressible layer, has consolidated somewhat.
7. In the case of some organic soils, the hydraulic conductivity of the soil decreases by more than an order of magnitude during consolidation under a given load. Consolidation naturally proceeds more rapidly initially but then at a decreasing rate because of the reduction in hydraulic conductivity, thus producing an apparent secondary effect.
8. Highly non-linear stress-strain curves can produce settlement-time behavior that looks like primary consolidation followed by secondary consolidation.
The behavior of the laboratory sample and the field layer are apparently different in that there is a clear secondary effect in the laboratory but none in the field. The bottom line is that In the field, primary consolidation is slow because of the layer thickness. As a result, effective stresses build up so slowly on most soil elements that negligible resistance develops in the dashpot and thus the secondary delay is eliminated."