Identifying Fill
Identifying Fill
(OP)
Hi all,
I'm an EIT and have been working as a geotechnical engineer for about a year now. Today I went to a site which I presumed was all native soil, but when I got to site I noticed a layer of topsoil about 0.6 m below the existing grade. I had it excavated and chased down the fill. However, I missed some because the original ground surface undulated and the topsoil re-appeared where I didn't expect it and I only stumbled upon it by chance.
The fill was reworked native material so I didn't even notice that it was fill other than by the topsoil. Some days I feel like I'm in over my head and lacking a lot of knowledge that I should have by now.
Can anyone suggest a good article/book or anything really that would help me with identifying fill on site. I know the basics of what I've been told to look for, but until I can really see it for myself and get the practice I'm not going to be very good at it. So something with pictures or diagrams would be very useful.
Cheers and thanks in advance
I'm an EIT and have been working as a geotechnical engineer for about a year now. Today I went to a site which I presumed was all native soil, but when I got to site I noticed a layer of topsoil about 0.6 m below the existing grade. I had it excavated and chased down the fill. However, I missed some because the original ground surface undulated and the topsoil re-appeared where I didn't expect it and I only stumbled upon it by chance.
The fill was reworked native material so I didn't even notice that it was fill other than by the topsoil. Some days I feel like I'm in over my head and lacking a lot of knowledge that I should have by now.
Can anyone suggest a good article/book or anything really that would help me with identifying fill on site. I know the basics of what I've been told to look for, but until I can really see it for myself and get the practice I'm not going to be very good at it. So something with pictures or diagrams would be very useful.
Cheers and thanks in advance





RE: Identifying Fill
I would suggest that you study the USDA Soil Conservation Service (though I think it is called something else now) classification procedures and their manner of identifying soils. Learn to use the Munsell Color charts and to identify various soil horizons. As you do more field work, this will become a bit easier, particularly if you get to use observation pits (test pits) in your work.
Imported fill is easier to spot. Native soils on the site subject to cut and fill onsite are more difficult to spot as fill.
RE: Identifying Fill
Mike Lambert
RE: Identifying Fill
1) Presense of organic material below about 5 feet provided its not part of a geologic layer. Peats and lignite are usually easy to associate with a geologic layer.
2) Presence of made-made materials: concrete, slag, garbage, debris, etc.
3) Jumbled structure. This is the hardest one to understand without a few years of looking at spoon samples. Helps to know your local geology and have a few years of looking at spoon samples in the field.
4) Erratic SPTs or low SPTs in an area where a consolidated formation is expected.
5) Misplaced rounded gravel or anguluar rock fragments.
In any case review the geologic map before going to the site. Have an idea of what natural formations you should expect. With a re-worked soil it can get tricky. You may only get one or two indicators above.
RE: Identifying Fill
For most situations I would keep a look-out for most of those indicators noted above. The one I'm most interested in is the structure of the soil. I don't really know what to look for. I have seen people grab a clump of soil and break it and from that they can determine whether or not it's fill. I assume that they are looking at the structure in this case.
This is where pictures would be helpful to see what structure is a natural deposit and what fill looks like after being placed and compacted.
Thanks for the replies everyone.
RE: Identifying Fill
f-d
¡papá gordo ain’t no madre flaca!
RE: Identifying Fill
Here are few more clues to my list.
6) Look at the terrain and landforms at the site. Think about the geologic processess and decide in your mind if it looks consistent or not with something natural. If not then you might be expecting to see other clues that a fill is present.
7) Study old topographic maps. Get familiar with the history of your area.
RE: Identifying Fill
Spend some time in the field with a good geologist and learn about soil formation, especially for soils in your area.
A driller I used said he could smell the difference between fill and native soil. A colleague says this works for him, even on old fill, provided it was placed with moisture - he says it smells like fresh rain on pavement or soil - especially when you're in a test pit.
However, some years ago a well respected senior engineering geologist was helping log a fault trench looking for the Vaca fault at an airbase. After opening up and shoring about 300-linear feet of trench to a depth of 12 feet, and logging the soils as native material, a decomposed 4"x4" post was encountered in the bottom of the excavation.
Not know to us (by historical working knowledge in the area or by review of old topographic maps), or by client knowledge, about 13 feet of fill, several hundred thousand yards, had been placed as engineered fill during the original base construction sometime in the late 30's.
Opps....
RE: Identifying Fill
RE: Identifying Fill
RE: Identifying Fill
Cohesive fill usually starts as a bunch of clods large and small. They don't fit together well until smashed together by the compactor. Uncompacted, the fill may retain that look for a long time. Compacted, you can still look for mixed-up colors, especially some topsoil chunks. Roller footprints can sometimes be found when you break a chunk.
Natural sand is usually alluvial and often displays thin bedding. If the sand is very uniform, be suspicious.
Loess is special. It often contains tiny root holes that are predominately vertical. If the holes are absent, it may be compacted. If the holes run various directions, in may be uncompacted fill.
This one is very subjective, but works in some locales: natural soil often feels crisp and crunches a little when you dig it with a shovel or trowel, but the same soil placed as fill lacks that feel and sound.
Keep at it. Undocumented fill causes lots of foundation problems, so identifying it is crucial.
RE: Identifying Fill
RE: Identifying Fill
Any twigs, roots, rootlets found in the matrix should be carefully examined. Are they broken? Are they characteristically sized according to where they would be if grown from a nearby tree?
Colour and insitu density can sometimes be good indicators too. Is the density and colour continuous or spotty?
Experience is really your best teacher and don't be scared to say you aren't sure if it is fill or not. You don't always have to have an answer.
RE: Identifying Fill
1) Does it REALLY make sense for fill to be here? Look at the vegetation in the area for some clues.
2) Is the sample uniform throughout? (BIG clue!) Natural soils rarely are.
3) What do the larger particles look like? Rounded gravel has never appeared in fill as far as I know it...but angular gravel is.
Reworked native soils will probably show significantly different blow counts as well.
I hope this helps somewhat.
RE: Identifying Fill
How to Find Existing Geotechnical Subsurface Information
Rey Villa, MS, PE
http://geotech-apps.com