Building fills with SW-SM Material 3

[GrumpyG]

We are importing type SW-SM material for a building pad. The specification clearly states SW and SM material is satisfactory. Fills for the building pad are from 0' to greater than 20'. Maximum rock size for fill in the specification is 3".

The site plans indicate to use ML or CL material for the building pad, but the specification, like specifications for many other projects, says ML and CL is unsatisfactory. The civil engineer stated in writing that the imported material meets the written specifications, but has deferred to the Geotech for a final decision.

The Geotechnical engineer has rejected the material.

Why would this material have been rejected?
Could larger sizes of rock be placed within 8' of subrade?

 

[civilperson]

Ignorance or a mistake on the geotech's part. Rock is superior at any uniform gradation.

 

[GrumpyG]

civilperson:

When you mean "Rock", is that your term for any coarse-grained soil(soils "above the line" on the USCS chart)?

The rock we wish to use is blasted from the site. I think is should be okay to fill using a 12" size rock or more, 8' or greater below subgrade.

 

[msucog]

around here, i keep "rock fill" recommendation to 8" or less in structural areas but don't object to an occassional 12" piece or oddball shaped piece that exceeds the recommendation diminsion in one direction. (if you spec 12", then you'll get 18" pieces). the "rock fills" referred to around here are usually more dirt than rock and quickly turn ugly with the lack of quality grading contractors and eagerness of them to turn an "8 inch lift" in to 12-16" lifts. or they say "it's too rough on my machine if i don't place it thick". either way, the spec is the spec.

based on the description given on this thread, i picture the phrase being the generic spec for structural fill that uually says max 3" or 6" particle size (not necessarily intended for rock fill recommendations). and this is usually stated that way to avoid having plumbing contractors trying to install pipes in rocky as heck conditions.

the geotech might have a problem with the CL material...or maybe there's too many outsized pieces for their liking. i wouldn't dare to guess since i don't know anything about the loads, grades, site conditions, soil conditions, fill material, etc. why not ask them why the material is rejected?

 

[GrumpyG]

We proposed using SW-SM for fill.
100 % passes the 3/8".
11.3% passes the #200 sieve.
Onsite material is clay and is unusable due to moisture and freezing issues. Knowing the time of year for construction, we decided to not completely balance the site and import select fill for the project - SW-SM.

The comment was that the material is apparently "too sandy" for the the fills and slopes we are building. However, the slopes will be "mowable" upon completion.

The rock size is merely a desire to get rid of as much shot rock as possible with minimal crushing.

The phrase is the generic spec. The building is a two story hospital.

 

[DarthSoilsGuy]

is the Geotechnical Engineer who did the preliminary soils report the same as the Geotechnical Engineer running the show on the testing side during construction? Which of the two rejected it? And by rejecting it, was it based on field observations of what's in the trucks by someone on-site, or was it because you brought them a sample and they sieved it out and reported it as out of spec?

is there something else going on here besides slope stability? by mowable i assume you're talking about 3:1 max. what you've presented doesn't make sense to me. my spider sense is tingling that the buck is being passed.

i could see issues about the mixing of shot rock into the soil more than the matrix being unusable because it peaks in the sands (w/ 11% passing #200). But, if the testing company (without the prelim geot report work) is calling the shots, i would understand that they are not interested in giving design input and will report the facts.

speculation on my part, very much so.

 

[GrumpyG]

The same geotech that wrote the report is the onsite inspector. Geotech Report states that Borrow fill shall have less than 50% silt/clay by weight. Another contradiction. He knows the material we proposed and just "doesn't like it." It is too sandy.
We had the material tested by another Geotech at our expense(minimal)for this project.

We have found another source of material that he will approve. We may be able to break even cost wise using this material.

Did I just learn that SW-SM material is no longer usable for fill material? This is contrary to previous experience.

 

[BigH]

Many times engineers I have run into do not like "sandy" fills - i.e., where all sand is basically medium to fine. They argue that it needs to be confined. This is why when it has been used on projects I had in India, there was always a clayey outer layer for confinement (problems, here too - but not relevant to your case). I don't have a problem with the use of the sandy material. Why not suggest to the geotech that you will place a confining zone at the outer 500 to 1000 mm of the fill so that the sand is "confined"? For the interior, as the fill gets built up, the material below is confined and this is not a problem although I have seen, too, the requirement of "confining cells". Don't agree with them and don't agree with the need for outer confinement. The suitability of the sandy fill to the outside is a matter of the slope at which it is placed (remembering that "saturated" slopes are stable at about 1/2 that of "dry" slopes), the type of covering on the outside (presumably grass vegetation) and the prevention of water from washing down from the crest of the fill and eroding the slope.

 

[DarthSoilsGuy]

If confinement is playing a major factor, it will show itself by good density testing procedures.

if using a nuke gauge (a sand-cone would also work, but don't try to test sands with a drive tube Hahaha),

take density testing at different probe penetrations. With surface testing (no probe penetration which is generally only a good testing procedure for road base stone), you may be failing because there is no confinement during compaction. But, after 2", 4", 6" and 8" probe embedment you may see an increase in compaction to nearly passing the test. And if you gently remove 4" and give a very flat surface for the gauge to sit on and then do a 8" probe embedment test, you may find that you have passed after all.
The overburden provides confinement during compaction. For very sandy soils (especially for a regional material i know as "processed fill"), i consider the 8" lift of soil that is immediately placed as a confining blanket and the target soils for compaction to be the previous blanket.


It's easier for the tester if the soil just passes from surficial testing. Soils that have confinement issues sometimes get false failing compaction tests as you can imagine from what i've described above. if you get passing tests on the surface (with 6" or 8" probe embedment of course), then the soil is probably good enough that confinement is not an issue and you'll need to continue getting surface passes.

I think with 11% passing #200 that it won't play to be that much of an issue, but keep this in mind if it comes up.

 

[BigH]

agree with DSG above in general - for exigency have always taken in situ density tests (sand cone, rubber balloon, nuke) at the surface) but if it "fails", then the thing to do is to dig down 4 inches or so and take the test again. For sandy soils, one really should test the previous layer for is situ density (compaction) rather than the layer you are currently placing.

 

[Mickney]

Just some thoughts on placing shot-rock as structural fill. It must be done under strict observation of a qualified geotechnical engineer. Hard to test compaction with 12" or larger fragments of rock.

Some rock specs. I have used in the past include:

1) recommending an adequate amount of fine-grained materials in the rock to minimize the presence of voids.

2) capping the rock material up to 5 feet (pavements) and 10 feet (buildings) below final subgrade using filter fabric and structural fill.

3) be conscientious of future utilities that are deep. it would be very hard to trench smaller utilities that extend through rocky fill.

 

[GrumpyG]

Fortunately, the cost for the testing agency's preferred fill over the site contractor's quarry overburden was a slight savings. I had talked to another Geotech and he agreed to some extent that sand is not a good fill material. Possible sinkholes in the area also were cause for concern if the sandy material was used.

All is well.........until we start using the stuff.

 

[theCorkster]

GrumpyG,

"Possible sinkholes in the area ....."

Are the sinkholes limited in maximum size or diameter such that the shot rock will "filter" migration of the fill into the sinkhole? Out of curiosity, what's the source of the sinkholes?

 

[GrumpyG]

Gray laminated dolomite with dark gray chert in irregular bends, stringers and nodules, with band s of quartz and quartz sand grains.
Technically, Ordovician-aged, carbonate, Rickenbach Formation(Ori).

 

[Begone]

In the past, we have avoided using very coarse grained material in sinkhole prone areas as it also allows water to easily filter into the ground surface bringing with it Carbonic acid, which, over time will, cause the calcite in the dolimite rich soils to degrade. Usualy want to use something like stone dust which is mostly impervious or material with about 15-20% fines. I can see the geotech's concern with course grained material. Sometimes, they will let you go if you put a clay cap or some such material around the sands, similar to what you would do in Dams.

 

[dBasement]

Brand newbie here. Great site.

Is seismicity a concern in your area? Is there a high water table? If so, potential liquifaction may explain the geotech's concern.

That being said, we have used fills such as that described with a great deal of success here in the PNW (Vancouver Island). Keep your moisture contents slightly above optimum during placement and, as said earlier, test the lift below.

 

[bb29510]

must be a local deal, because we rather use sand than a cl type material. we will use 100% sand, like a beach sand in 95% of our jobs. If the sand has too much silt then it can go to pumping. Most of our clay material get undercut and hauled to landfill. Must be a location problem

 

[GrumpyG]

As it turns out, the site contractor used the alternative material at no additional cost. The geotech was happy, the Owner was happy, etc.

I just thought it was odd that the Geotech wanted a material with more clay and would not approve the SW-SM material.

With the 18-20'fills, the shot rock was easily used.

Thanks for everyone's input.