Relative Density vs Relative Compaction and in-situ testing
Relative Density vs Relative Compaction and in-situ testing
(OP)
I have a project site with 30-100 feet of non-engineered fill. I've conducted hollow-stem soil borings and have collected samples with SPT and California Modified samplers. The blow counts were recorded and corrected to (N1)60 blow counts with an additional correction for the Cal. Mod. samples. I reported the results as relative densities. I am now being told that I need to determine the relative compaction of the various layers by getting the lab dry densities of the rings and SPT tubes and report their percentages of the maximum dry densities (as determined by ASTM D 1557). The source of this request went on to say that blow counts should be used in support of these lab testing comparisions and not the other way around.
I've always thought relatively disturbed/undisturbed samples should not be used to determine relative compaction, especially with this much varied overburden. Could someone please clarify?
I've always thought relatively disturbed/undisturbed samples should not be used to determine relative compaction, especially with this much varied overburden. Could someone please clarify?
"Information is not knowledge" Albert Einstein





RE: Relative Density vs Relative Compaction and in-situ testing
IMHO, you have the field data. Use correlations as best you can...you're chasing an accuracy that you won't find. 30 to 100 feet of fill without any control is not a good thing. Probably the best remediation you can provide is to recommend surcharging and monitor the settlement of the surcharge.
RE: Relative Density vs Relative Compaction and in-situ testing
To add some information, I did collect some bulk samples, but they were of course collected from the hollow stem cuttings. The cuttings are pretty representative of discreet sampled materials at shallow depths, but as the bulks are collected from deeper depths (>10 feet) the soil is being mixed from the cutting head all the way to the ground surface. Since I only have "windows" of observation at 5-foot intervals, I cannot say for sure what is happening between these windows (engineering judgement at it's finest). To be honest, the material throughout the depths is a pretty uniform brown silty sand with little (<15%) to no clays or gravels.
My big question still remains, so I will rephrase it. Does testing of discreet relatively undisturbed samples (i.e. Cal Mod sampler rings) truly give reasonable relative compaction values? Is this method better (or more reliable) than using the established SPT correlations to relative density? Ok, so I guess technically that's two questions still remaining :).
"Information is not knowledge" Albert Einstein
RE: Relative Density vs Relative Compaction and in-situ testing
RE: Relative Density vs Relative Compaction and in-situ testing
Didn't these forums at least once some time back go over the term "relative compaction" and the majority didn't recognize that term as a commonly used term or calculation?
RE: Relative Density vs Relative Compaction and in-situ testing
The question still remains though, do relative compaction values(relative undisturbed/disturbed ring sample dry densities as a percentage of lab max dry densities) more accurately define the soil stratigraphy than corrected blow counts (correlated to established relative densities).
Please note that Relative Compaction does not equal Relative Density and these two terms should never be used interchangeably (e.g. 80% relative compaction equals about 0% relative density), see Lee, K.L., and Singh, A. (1971) "Compaction of Granular Soils," Proceedings of the Ninth Annual Symposium on Engineering Geology and Soils Engineering, Boise, Idaho, pp 161-174.
"Information is not knowledge" Albert Einstein
RE: Relative Density vs Relative Compaction and in-situ testing
RE: Relative Density vs Relative Compaction and in-situ testing
My usual term is "percent compaction," or "percent of lab max," although the term "D value" is used some places (limited to western US?).
RE: Relative Density vs Relative Compaction and in-situ testing
I think it a bit disingenuous of the person asking the OP to estimate the relative compaction from the N values. The N values are reflective of many things such as angularity of the particles, particle size vs the spoon's diameter and the like and this doesn't always translate to a general correlation.
The question is whether this is to show that there was no control on the fill being placed (a contractual thing) or the desire to build on the land (an engineering thing). if the former, one is probably way out of luck except in a qualitative sense that shows a very large COV for the N values obtained. This would indicate uncontrolled fill. If for the engineering, one would look at it in the sense of any soils investigation and apply design principals accordingly fully aware, though, of the large variations that are likely (but not necessarily) inherent in an uncontrolled fill.
RE: Relative Density vs Relative Compaction and in-situ testing
First I agree that the California sampler (i.e., with the brass rings, etc.) is not undisturbed. While you can get a density, it will be somewhat approximate. Further what maximum dry density will you use for comparison - some surface-obtained bulk sample? Likley to be just another approximation.
Let's say you use all these approximations and "calculate" some value like 92 percent relative compaction. So what? I mean would that mean you can't build on the stuff? Would that mean you'd have to use piles? No it wouldn't.
I'd jam a dilatometer or a cone in the ground and directly measure the material properties and assess the settlement/stability potential using those data. I don't tell you that as some glib response. I tell you this 'cause that's what I'd do and have done in the past. After all, the relative compaction is moot if you can show some measure of subbase improvement in conjunction with the in-situ properties of the fill are such that the structural loads and foundations can function for the intended development.
f-d (back from vacation)
¡papá gordo ain't no madre flaca!
RE: Relative Density vs Relative Compaction and in-situ testing
A few test pits can be excavated, if space allows. Nuke density tests can be performed at various depths to about 10 feet to compare with a few tube samples. I like test pits when using an existing fill for some engineering purpose to make sure there are few surprises like rubble and organic matter although I understand 100 feet of fill is extreme. Maybe a deep exploratory shaft is warranted with incremental density tests?
Drive cylinder samplers can be used at the bottom of the test pit instead of Shelby tubes if it's clayey enough.
CPTs are also a good way to collect lots of data from a fill that can be used with Schmertmann's method to calculate settlement.
Correlations of density with SPTs is a little weak. One recent dinky structure we worked on was on a rubble fill of blast rock. It needed 400 cy of grout and a mat slab to get things under control. The SPTS were high from the rubble but full of voids. The voids were narrow seams between boulders and not well shown by the borings. Test pits painted the picture more clearly.
RE: Relative Density vs Relative Compaction and in-situ testing
"Information is not knowledge" Albert Einstein
RE: Relative Density vs Relative Compaction and in-situ testing
If the soil is unsaturated,the disturbance will tend to be densification and strengthening of the sample, therefore unconservative.
Whatever the properties of this soil are today, they can be drastically changed by future increases in water content. Re-grading, irrigation, and climatic rainfall cycles can cause big changes in strength and compressibility. Even a well-compacted fill that thick can compress 1 or 2 percent when wetted.
I would be very cautious about predicting the long-term performance of anything supported on this fill, even after preloading.