Estimating Consolidation Settlement within Fill

[nbr1]

It is my understanding that 1D consolidation testing is generally best for sedimentary type soils; however, I'm not sure about engineered fills. Would consolidation testing of remolded samples (proposed fill) provide useful information? If so, what does one use for a preconsolidation pressure when estimating settlement WITHIN the fill?

 

[oldestguy]

I don't know about others as to their practice, but for me, I neglect any settlement that may take place within an engineered fill. The reason is the soil below usually is significantly more compressible and the accuracy of the estimating is not that precise anyhow.

 

[nbr1]

Thanks oldestguy....I was able to stumble upon a journal article that was of benefit; understanding that the settlements are probably relatively minor.

Nwabuokei, S.O., and Lovel, C.W., "Compressibility and Settlement of Compacted Fills"; 1986

 

[Ron]

As oldestguy noted, they can usually be neglected for a couple of reasons. First, if the soil is compactible and competent for fill, its settlement is most likely going to be elastic. Elastic settlements are usually quite small unless the layer is quite thick.

Secondly, and related, if the soil is fill and it is compacted, the compaction takes out most of the consolidation issues and leaves mostly elastic conditions for settlement.

If you have very thick fill (say 8 to 10 feet) that is simply dumped in with no compaction, some settlement can occur. You can consider this and compute it from the void ratio change between the loose state and the compacted state.

 

[emmgjld]

Always be sure of the definitions.
To Some - Structural Fill is always an imported Granular, usually non- or very low plastic material. The compaction spec and control should be commensurate with the thickness of fill and final use (i.e. what kind & type of structural loads are to be applied).

To others, myself included - Structural Fill is a pre-approved (by the Geotech & in full knowledge of the design/Structural Engineer) soil or material, may be native, placed under controlled conditions (compaction specs, appropriate testing & observation), all commensurate with the anticipated use.

There are also many areas between these extremes.

 

[JedClampett]

I have vastly different experiences with fill than either oldestguy or Ron. No matter what we specify or how close we inspect, we get long term settlement approaching 2% in fill (select or other). We design water and wastewater treatment facilities, so we routinely have deep tanks or basins next to at-grade structures. We always either fill the excavated volume with a non compressible material (like slurry), or support the at-grade structure from the native unexcavated material with piers or columns.
When we haven't respected this settlement we've been sorry, with cracked walls, misaligned stairs and other unfortunate events.

 

[hokie66]

My experience is similar to Jed's. For a whole building founded on a uniform thickness of inspected fill, I would agree that settlement of the fill is not usually a big issue for the building, but can be a problem for connecting services. For fill adjacent to basements and the like, time dependent settlement is invariably an issue, and these areas require any external slabs to be treated as suspended elements. Going back a few years later to fix settled pavements is not fun. As to estimating how much settlement will occur, I don't know.

 

[oldestguy]

The question dealt with lab (or other) testing of "engineered" fill as related to "consolidation". First off, "consolidation" usually only applies to squeezing out of water. However. most "engineered" fills in my experience are granular soils and I would not be doing that test on that material. I don't see where consolidation testing applies there. However, for the few cohesive soil compacted fills I know of for building structures (presumably fitting the definition of engineered), the main concern is to not over compacting and later having heaving from expansion. So the question of settlement within the fill usually has not been of concern for either type of soil used for "engineered" fill. In the above I do not consider roadway compacted fills as necessarily fitting the term "engineered", although I suppose they do. Having been the head honcho for WiDOT soils dept, we never tested any compacted fill for "consolidation" characteristics.

 

[moe333]

Thick engineered fills (granular and/or cohesive) are a different story, particularly if a slope is involved and with wetting/irrigation. Too much to discuss here without a focus. Try a search with "Noorany" + focus term.

 

[GeoPaveTraffic]

With dams and levees, we typically expect settlemnts of the embankment of at least 1 percent of the fill height and often closer to 3 percent of the fill height. Levees are typically constructed to a lower percentage of the max than dams.

Mike Lambert

 

[moe333]

I have seen USBR state design should be 1% of fill height as a conservative estimate for settlement of well constructed earth dams.

 

[nbr1]

In my original question, the term "engineered fill" was meant to indicate the fill was placed at a defined moisture and compaction. I've never heard of this term only pertaining to a granular material....not sure that everyone would agree with it being granular only????

Again, the reference I posted seems to have a rational approach to estimation of fill settlement (settlement within a lean clay, lacustrine, fill); however, I haven't mulled through it completely. Ref: Nwabuokei, S.O., and Lovel, C.W., "Compressibility and Settlement of Compacted Fills"; 1986

My gut feel is that the settlements would be on the order of 1 to 2 percent of the fill thickness as most have suggested.

 

[fattdad]

compression of engineered fills is likely. Consolidation is not likely. Compression is an elastic response. Consolidation is a response based on saturation and permeability. I'd find it hard to believe that the fill is saturated to the point where change in void ration would be influenced by saturation and permeability.

I'd use settlement plates and periodic survey to gauge the post-placement compression as much of what you think may happen will occur during placement.

f-d

¡papá gordo ain’t no madre flaca!

 

[hokie66]

I must have been using the terminology incorrectly all these years. When a fill settles under its own weight over time, I call that consolidation, but wouldn't consider it elastic. Am I wrong?

 

[JedClampett]

hokie, you and I agree on that.

 

[moe333]

As others have stated, consolidation involves void ratio reduction due to water being squeezed out of saturated material. What you are describing is elastic compression.

 

[oldestguy]

The terminology probably can be blamed on Terzaghi? Trouble is maybe those writing our dictionaries didn't get to take the courses back then. Somehow combining two businesses fits the term also.

 

[BigHarvey]

Terminology has to be used in the right way : when air is squeezed out of a soil, natural or engineered, the right word is compaction. When water is squeezed out, then it's consolidation. Consolidation is generally not associated to engineered fills constructed in layers above groundwater table, and are generally made of granular materials. The only fills I can think of which might experience consolidation are mine tailings and dredged sédiments but I wouldn't call them engineered fills. As a vertical drain contractor, I have been asked sometimes to install drains in materials above groundwater table by people who didn't understand the basics of consolidation theory ...

 

[hokie66]

I'll concede that in geotechnical terms, consolidation is not what happens in engineered fills which settle under their own weight over time. But this settlement, compression, volume change, whatever, is not elastic...it is time dependent and doesn't return to the original when unloaded. In structural terms, it would be creep deflection.

 

[fattdad]

When a fill settles under its own weight over time, I call that consolidation, but wouldn't consider it elastic?

I this example it could be either. If the mode of settlement is directly related to the relief of excess pore pressure over time, then it's consolidation. However, if the fill is not saturated and the settlement response is related to modulus (or C-sub Alpha), I'd call it compression. Bear in mind that secondary compression (i.e., the change of void ratio in the absence of changing effective stress) is called "compression" for a reason. Consolidation is not taking place!

But this settlement, compression, volume change, whatever, is not elastic...it is time dependent and doesn't return to the original when unloaded. In structural terms, it would be creep deflection.

It is pseudo-elastic. So, you are correct, it's not precisely elastic. I think geotechnical engineers use the term compression and think in terms of "elastic" behavoir, 'cause it's a way to approach the problem. In granular soils (i.e., where permeability is to great to really affect the analysis, we'd consider some "elastic response" to loading (modulus based) and then add (roughly) 20 percent for the anticipated long-term "creep" effects. You see in most instances, the compression that you'd calculate would occur during construction (i.e., as the fill is being placed). So, now the question is what would you expect post-construction (i.e., after delivery to the owner)?

f-d

¡papá gordo ain’t no madre flaca!