Oversize material in engineered fill 4

[pa24pilot]

I'm a relatively new project engineer and I'm having trouble convincing an owner on one of my projects that rock over 3-4 inches in dia. should be removed from over-ex material before replacement as engineered fill. I tried explaining the difficulty/impossibility of proper compaction to him, but nothing seems to convince him. Does anyone know any other reasons I could explain that might enlighten him??

 

[Ron]

Beside the obvious reason of not being able to appropriately test compaction, there are several other reasons for minimizing large pieces of rock (cobble size) in the engineered fill.

Accumulations of these cobbles can create relatively large amounts of voids, into which sands can ravel. This leads to subsidence in some cases.

Another issue is punching in subgrade or directly loaded applications. The larger stones will initially carry the load placed on them, and will "punch" into surrounding soil. This is moreso a problem with pavement applications.

 

[Focht3]

Search the literature for documentation of problem fills with oversize material and show it to him. Or find the UBC code on oversize material and tell him that the contractor will have to follow those procedures. He'll choke on the cost!

Oversize material isn't a problem if the contractor has the right procedures and equipment; they bury VW-size boulders in the Los Angeles area in deep fills (a 30 million yd³ fill is "normal" for that area.)

 

[g7mann]

g7mann[geotechnical]
The inclusion of "large" or "oversize' materials in an engineered fill is relatively common and is not necessarily a bad situation. Whilst not desirable the presence of larger materials in a fill matirx is often practically and/or economically unavoidable. However, if oversize materials are contained in the fill soil matrix it is crucial that the larger materials not be placed or deposited in local concentrations. If they are clumped together they will then not only [potentially] develop void spaces into which the overlying soils can sift leading to development of surficial voids or depressions, but will make in-situ density testing difficult to impossible to perform. Raking of the loose fill can be helpful in preventing this "clustering" of larger materials.

I have found that burial of large materials, such as broken concrete and boulders of even several feet in size, is acceptable providing there is sufficeint space around each such "chunk" of material to allow for placement and adequate compaction of the surrounding fill [soil] matrix. I have also found [through bitter experience] that the surficial roughly 18 inches of fill should be free of ALL such oversize materials so that they do not have the potential to develop "hard spots" over which a structural element may drape. Draping can subject the structural element to greater stresses than it was designed to accommodate and can lead to distortion and damage.

Regardless of specification and rule we often have to deal with what we get in the field. If it's overize materials they must just be dealt with.

 

[ashjun]

Hi,
Sorry to bug in this late.
I think the key issue would be the use of standard procedure. The maximum dry density and optimum moisture content that may have been adopted for the particular fill, was probably based on the test when the over sized materials were removed (size 20mm plus is removed- standard procedure) from the sample. If your field rest results are even remotely to be compared in proportion to the lab results, I believe the same procedure should have been used for the fill material at site. Otherwise who would lay guarantee to the compressibility of the fill - the contractor, the soil lab.?

Regards

 

[dicksewerrat]

If you are laying pipe on this fill, the larger rock may punch holes in the pipes when the backfill is compacted.

 

[cvg]

most responses assume that a structural element such as a building, pipe or pavement is to be placed directly on the fill. If this is an engineered dam, levee, dike or large embankment on which a road or bridge will be constructed, placement of larger rock could be allowed (following some of the procedures mentioned) with no loss of performance. G7mann is correct that the top 1 or 2 feet "under structures" should be rock free.

 

[dirtsqueezer]

Alright, I couldn't help but weight in to the revived topic.

I assume we're talking about levelling fill for a structure. Large rocks can punch holes in infrastructure, though I almost always see either pea gravel around pipes, and for slabs or pavement, it's manufactured fill.

I see two ways to look at this from a testing perspective. According to ASTM D1557 Modified Proctor, a sample containing 0-5% over the 3/4" sieve does not include that oversize in the testing. That's right- it's just not accounted for. Knowing this, as an inspector, when I take the proctor report into the field, I look for compaction on the high side of 95%, as opposed to, say, 94.5% for low-oversize content structural fill. First, the field inspector has a certain level of competence when testing for compaction beyond what the gauge reads. If I'm testing bank run, and I get 4 tests at 91% and one test at 99%, I'm going to assume I've just tested over a large rock, as it is just plain obvious. The more large rocks you have in your fill, the harder it will be to make this call, and if you have a lot, it is impossible. Secondly, I think this rule is just like all other rules; to prevent the extreme cases of large rock content from going uncontrolled. It is a case where you have to make a judgement call. Though I think here, 3-4" rock may be a de minimus problem compared with others you might experience during the course of a project.

Hope this helped.

 

[GeoPaveTraffic]

Since we are all comming late to this party, here is my two cents worth.

I don't like oversized material in the fill due to its effect on the compaction of the remaining (undersized) portion of the fill. If there are large rocks in the fill then the weight of the roller may not effectively compact the soil around the rock. This can create a situation where large portions of the fill has not been compacted. As has been pointed out, it is between difficult and impossible to know if this is happening.

This also brings up another point, the defintion of oversized material is dependent on the type of equipment being used to compact the fill. For a walk behind plate compactor and a 6-inch lift, a 3-inch rock is oversized. For a 20 ton sheepsfoot roller and a 10-inch lift, a 6-inch rock is not significant.

 

[Focht3]

That's right, [blue]GeoPaveTraffic[/blue] !

You can't really talk about "acceptable material size" without talking about the equipment to be used to compact it. After all, an 8 ounce claw hammer is more than sufficient for small finishing nails, but is pretty useless driving railroad spikes...



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

 

[iandig]

I know its late to add my comments to this, but perhaps a better arguement to the contractor is to instruct him to undertake a site compaction trial, to full depth and with the proposed plant on the material. Then, upon completion of this, you can then undertake a slit trench/trial pit exercise through the compacted material and assess the sucess of the compaction. In other words, "words fail, see sketch".
This has come up in the UK, where weak sandstones material had been proposed for use as general fill, and the argument from the Contractor was that the oversized material would break down during compaction, however the site of large voids at the bottom of each compacted layer convinced him otherwise.