Identifying Fill 1

[millerbrun]

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

 

[Ron]

Don't beat yourself up too badly. Identifying fill is sometimes hard even for experienced geotechs.

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.

 

[GeoPaveTraffic]

I agree with Ron. Fill from local site soils is very difficult to identify and is often missed. If not for the occational piece of gravel or debris (cans, glass, etc.)most fill would be missed.

Mike Lambert

 

[molerat2210]

Here is my mental list for identification of 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.

 

[millerbrun]

Thanks for the answers guys,

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.

 

[fattdad]

Never overlook the obvious. . . Uniform slopes, "flat" ground that's just too flat, landforms that don't fit in with the natural setting. No guarantee, but such observations warrant a second look for fill (well unles it's a cut).

f-d

¡papá gordo ain’t no madre flaca!

 

[molerat2210]

Identifying a fill based solely on soil structure is a little tricky. It would be hard to do just by grabbing a clump and saying "ah yes". Get a dictionary of geologic terms and read about words like slickensided, mottled, varved, laminated, etc. Understand the geologic processes. You need to be the expert for your local area. This is how we get the clients to always hire local geotechs.

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.

 

[theCorkster]

Old aerial photographs can be very helpful; you may have to request and pay for older photos.

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....

 

[Rottie1]

Similarly, can you assume that, if you have a 4m boulder retaining wall to the rear of a property which drops off further below to a creek, the land being retained is fill or could it be retaining natural ground?

 

[dirtsqueezer]

My suggestion would be to look at USGS geological maps for a region and getting to know your local geology that way. You could also go to your local city engineering office for a free look at some geotechnical reports for sites adj to yours, just to get started. That might give you more perspective as to what other people are doing, too.

 

[aeoliantexan]

You got a lot of good advice so far.

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.

 

[Riggly]

Interesting topic. As you see, there are several 'pointers' that we can use to help us identify fill. I think I learned this by looking at soils over and over- and yet, who knows exactly? Don't beat yourself up too much for this. Split spoon samples are not always retrieved as undisturbed - in fact they are disturbed samples. The soil structure is the most difficult. What I would advise you to do is to look at the patterns in the portion of the soils you know without doubt are native soils. Look at them closely- use a knife when possible and split them and look closely at the pattern. When you do that all the time, you will notice when something just don't look natural- it may take years. Sands can be tricky to when using split spoon, because they tend to be more disturbed and lose their pattern more easily, especially those within the groundwater. Sometimes- if you are not sure - just put 'possible fill'- you also have to review your log, as one of the example was pointed out where the log was found 12 feet below and change what you intitially thought was native. As fattdad pointed out, you have to look at geometry also. I was at a site not so long ago, and just looking at the topography I could see that one end was cut, and the other end was fill, and I was expecting reworked natural soil. I had to check what I observed in the drill cuttings with what I was seeing around me - based on my observation, I was right. The soil pattern varies depending on the physiographic province, depth, and parent material, and it may take a long time for you to see them all.

 

[dBasement]

Good discussion. In addition to the items noted above, a couple of other means I have employed:

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.

 

[dirtgirlhawaii]

I'm in the same boat as you, just working slightly longer as a bore logger. How I've learned to distinguish fill from other material are to ask yourself some questions:

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.

 

[reyv]

Identifying fill materials is very difficult even for most experience engineers. The right way to do it is to have knowledge of the local geology, or to do some literary research. I recently wrote a guest article on GeoPrac.net which provides some great tools for identifying geological conditions prior to performing an actual site investigation. The article can be accessed below.

How to Find Existing Geotechnical Subsurface Information

Rey Villa, MS, PE

http://geotech-apps.com