SPT sampling and logging fill materials 1

[jamoca67]

I'm a newbie to the forum! I've always had some difficulty interpreting the difference between fill materials vs. native soil materials when performing and logging SPT tests. I will appreciate people's thoughts, opinions, experiences and any online references/presentations addressing this matter. It is sometimes not as easy as it may seem. Is there anything specific to watch out for as far as soil fabric, contents, etc. inside the spoon?

Thank you for reading and for your interest.

 

[oldestguy]

Yes it can be difficult, but if in doubt say "Possible fill" and you are safe. Sometimes "Probable fill" will do it. Not logging it as fill and if it really is, there may be legal problems when the job turns out show it as fill. Same goes for some dolomite bedrock. Split spoon sample may look just like sand. On the job they find it is slightly weathered rock. On one job where the Wisconsin DOT decided to save some money for the state colleges building costs and do the drilling with state crews. On the job the inspectors classed that excavation as bedrock. Boy did that job ever run over on costs. Damned embarrassing for the state DOT geotech section.

 

[BigH]

Agree with oldestguy - if there is any thought that it might not be natural (a material out of the ordinary from nearby jobs - or the area's geological history) - best to alert those that it might be fill. I usually used "Probable Fill" rather than possible - semantics.

 

[aeoliantexan]

It is like investigating a crime - use all the evidence available. If you drill through a layer of grass and black topsoil, most likely everything above that is fill.

Natural soils at shallow depth may have roots in place, worm holes, root holes, or thin stratifications etc that tend to get smashed shut or disrupted during placement of fill.

Loess and some alluvial soils may have root holes that tend to be predominately vertical. In fill, the root holes may be closed up or going in random directions.

Poorly compacted fill may have visible clods. Compacted fill may have sheepsfoot roller impressions you can find.

Fill often has a mixed-up quality, especially random mixtures of color, whereas natural deposits have a more uniform appearance.

Someone will laugh, but I find that when you dig into some natural soils, say silty clays, there is a "crunchy" feel and sound that doesn't occur in fill. Think of biting a Kit-Kat bar as opposed to a Milky Way. I attribute it to a weak cementation that develops with age.

Finding a gum wrapper or pop can helps too.

In short, you are right, it is not always easy, and what helps in some soils doesn't help in others. It is an art worth developing, however.

 

[hoobr]

As a fairly new geotech, I struggle with this as well.

I always look at the site grading, sometimes its obvious an area has been filled. If there isn't a topsoil layer under the fill, there is usually a colour change. The fill will be darker, and often the SPT counts will jump bit after the colour change. The senior guys in my office always break split spoon samples to look at how the soil structure breaks. I think the idea is that fill is very disturbed and usually won't break with smooth planes or other structure. They also smell the soil a lot. I don't know what for. They also look for rocks that don't belong, either shape or type. Often fill will contain little bits of organics which don't belong at at that depth. If I'm not sure, I still have a descriptive log and a sample for someone else to look over.