Importance of moulding moisture content for compacted sand fill

[TailignsEddie]

Hi all, I have a project where the contractor struggles to meet the moisture content specification for a compacted sand fill. The specification is 95% of SMDD, -3% to +3% of OMC. The contractor easily meets the density ratio, but is often out of the OMC range on the dry side. I know that moulding moisture content is important for clay fills like dam cores or landfill liners because there is a relationship between permeability and moulding moisture content, and with dam cores you want to avoid generating excess pore pressures in very tall dams etc. etc.

My question is, since the required density has been achieved for the non-plastic sand, is it OK to ignore the contractor’s failure to meet moisture content spec? For reference, the worst results I have seen is they are dry by about 4.5% of the OMC (which they have determined from MODIFIED density tests, not the Standard method, which the spec requires). I did not develop the spec; I have inherited this from another consultant. The contractor usually hits >100% Modified Max Dry Density. I realise this means they are a pretty long way (on the dry side) of the SMDD OMC (whatever that may be).

I’m struggling to find a reason to hold them to the moisture content specification. Any help would be much appreciated!

 

[BigH]

Question: Is the moisture content determined "at the time of the compaction" or is it determined, say 2 days later after it has been allowed to sit in hot sun?

 

[TailignsEddie]

BigH, I had the same thought. I don't know the answer but will be asking first thing Monday. I'm pretty sure it's same day testing, the moisture content is an oven dried sample (i.e. not taken from the nuke gauge).

 

[swazimatt]

my understanding of OMC is that it is the moisture that "assists or lubricates" material to achieve its optimum density. If you are achieving your required density i don't see the moisture content being an issue. there are multiple factors to achieve the density. if they used less compactive effort (eg lighter roller or fewer passes) they may have needed the additional moisture)

 

[dik]

I'm not a geotekkie... Sand is often difficult to compact, depending on the fines, gradation and granular shape. Moisture is often critical. How were the SMDD results arrived at?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[TigerGuy]

What is the purpose of the sand fill? Is there a reason to have such a tight moisture spec?

 

[cvg]

what is the OMC? what is your relative density?

in my experience optimum is 8 - 11% or higher. lower than that it is tough to get the density.

Moisture is usually added to the stockpile and again on grade just prior to compaction. this needs to be tested immediately or the moisture drains out and the sand is dryer than when it was compacted.

 

[TailignsEddie]

Thanks all. I’ve answered your questions below.
My response to the contractor was I would accept the failed test results. However, they are to continue moisture conditioning the sand, and must also continue to test the moisture content of the compacted lifts. I don’t want them placing totally dry sand that could collapse on wetting.

How were the SMDD results arrived at?
In this case the Australian Standard AS 1289.5.2.1 “Determination of the dry density/moisture content relation of a soil using modified compactive effort.”

What is the OMC? what is your relative density?
OMC is about 14 to 15%. The maximum dry density is about 1.88 t/m3 to 1.93 t/m3. These OMCs and MDDs are very consistent across about the 10 tests I have sampled. A typical result is shown below.
Relative density (density ratio?) is usually >98%. Density Index (relative density in terms of emax, emin etc) is unknown.

What is the purpose of the sand fill? Is there a reason to have such a tight moisture spec?
The fill is an embankment for an upstream raise to a tailings dam (yes, I know u/s raising is generally verboten). The raise height is about 4 metres. I don’t consider +/- 3% OMC a tight spec; it is pretty typical for embankment fills in Australia.

 

[dik]

I'm used to our granular stuff... that moisture content appears to be a bit high... don't deal with sand, around here.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[cvg]

you are obtaining the required density. MC just needs to be adequate to obtain the desired results. i think you should be ok.

we usually aim for 70% relative density (per min/max tests) which should correspond to about 100% of Standard Proctor.

 

[GC_Hopi]

I have run into this issue in the field. My approach for sand has been to not be so stringent on the moisture content if they meet the density requirements in the field. Sand does not have the same long terms concerns as say a clay when it comes to moisture.

My musings on why this happens with sand is that in the lab test the moisture does not escape the sample the same why it does in the field so you measure every bit of moisture. In the field the moisture moves more freely and can drain out of the top layer which is the portion you are testing so the field measurements show meeting density but not moisture. The reality is the sand probably had high moisture at one point to get density but that moisture quickly migrates as more lifts of sand and compaction effort is added.

 

[Kalamboogle]

Our typical soils contain less than 12% passing the 200 sieve, so either SP or SP-SM. For these soils, we recommend +/-5% from OMC. We have also waived moisture restrictions on a true SP if compaction can be achieved (Our state DOT does not restrict moisture contents). The bigger issue with these soils when loosening the deviation from OMC is constructability/stability. Even at 100% compaction, when dry, SP's tend to rut very easily and can cause concrete trucks, dump trucks, etc... to get stuck.

 

[dik]

...frost heave susceptible?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Kalamboogle]

I am in a "humid subtropical" climate so there is no concern of frost heave (maybe 5 days a year we hover around freezing temperatures).

 

[EireChch]

I agree with CVG, if you are getting the reqruied MDD then that is good enough in my opinion.

 

[GeoEnvGuy]

A little late to the discussion, 16% moisture content in a silty sand or sand doesn't happen out in the field it just drains out. I have the similar proctors on compacted tailings beach's and in the field they don't get over 10%, typically in the 4 to 8% range. I believe the proctor is retaining water because they seal the base of the mold.

 

[geotechguy1]

I think it's quite common with sands that density can be achieved significantly dry of OMC. Especially if it's gap graded or uniform graded and thus very permeable. The compaction energy in the field is likely significantly larger than the proctor test. in Alberta we would always hit it in the field at 3-5% and it seemed like a rite of passage / canon event for geotech EITs to have a big fight with a contractor over moisture conditioning sand.

 

[emmgjld]

Many of the Silty Sands I have encountered are fine-grained. Often interbedded with Sandy Silt. The higher moisture contents can be disturbing to the uninitiated contractors, to say nothing of Testers & Engineers who are used to the coarser sands and gravelly sands.
Depending on the type of compaction equipment, the true optimum moisture content in the field can be quite different from the laboratory. I have found it useful to compare the Standard vs Modified (D698 vs D1557) OMC and comparing with the Maximum Densities.