Soil Testing: What’s Under the Surface in Chicago


Before we can design the foundation for a new building of any size, it is important to discover what’s happening under the ground.  The location of water and the soil conditions determine what kind of structural system and foundation a building will need.  Today we explore … exploring the world beneath, with Subsurface Soil Testing.  

What’s Down There?

We’ve talked a little about what’s going on underground here in Chicago before.  Read about how the combination of the local water table and regional frost depth mean that NEARLY EVERY CHICAGO HOME HAS A BASEMENT.  What happens under the surface is intricately connected with the buildings that stand on the surface (and delve under it), with the water that flows out from underneath into our waterways, and with that surface itself – and how permeable it it.  We’ve discussed the vital importance of preserving permeable surfaces here in Chicago (and elsewhere) to PROTECT CHICAGO’S RIVER AND LAKE FROM SEWAGE OVERFLOW.

A geologist approaches the question of what’s underground differently than an engineer or an architect.  For today we’ll focus only on the top most layer – the soil.  Our practical interest ends when we hit bedrock – always a good foundation to build up from.  Everything above that layer is “soil,” from a foundation engineering point of view.  More specifically, we divide up the different soil types like this:

Boulder – a unit of “soil” requiring two hands (or more) to lift

Cobble – takes a whole hand to lift

Gravel – can be lifted easily with thumb and forefinger

Sand – can be seen with the eye but can’t be picked up individually

Silt – round particles that range from .002 to .00008 inches in diameter

Clay – flat, plate like particles even smaller than silt

For a good building foundation, you want to have good – stable – soil conditions.  These soil types tend to be found in layers – or strata – and we dig down to find the best layer for building foundations that is closest to the surface.  Rock, gravel, sand and some silt make for reasonable building foundations.  Clay tends to swell when its wet and shrink when it dries out – that makes it very unstable.  So how do we find the good strata?

A Solid Foundation Starts with Soil Testing

Every building foundation will settle (sink lower as the weight of the building is constructed on it, and as time passes) a bit.  What we want to watch out for is differential settlement – meaning that one part of the building sinks more than another.  You can probably imagine why that’s not a good thing.  To avoid that we have to have detailed information about what kind of sub-surface conditions we are dealing with and we get that information from soil testing.

How We Find Out What’s Under the Surface

The best way to find out what’s under the surface is to dig.  For shallow foundations (smaller buildings) we can use test pits – basically digging a hole in the ground.  We can run tests on the soil at the bottom of the pit and determine its bearing capacity.

drilling segments

When we need to know whats going on deeper than 8 feet or so, we use test borings taken with trailer, truck or bobcat mounted drilling rigs.  To be able to show you this, Chris stopped by the site of one of our upcoming projects to check on soil testing in progress, being undertaken by Pioneer Engineering and Environmental Services.

He watched as the crew used their ATV mounted drill rig using a series of hollow stem augers.  You can see the technician hooking up another 5′ auger segment to the one already drilled into the ground.  From left to right, the rig drills it down to near ground level and then is detached so that another segment can be added. A sampling tool is dropped down the hollow center of the auger to collect soil samples every 2.5 feet.

core camples

Above, we show the collection of soil samples – slated for the lab and further testing.  A few tests can be done on site.  The top right image shows the technician using a pocket penetrometer – which measures how easily the sample can be penetrated (hence the name).  As you can imagine, this is a useful metric when we are planning to set down a building on it.

The carefully labeled samples from each depth are sent back to a testing lab where they are tested for moisture content, compressive strength and density.  A final report will give the results of those tests and made recommendations for the design of the foundation system with bearing pressures and estimated settlement of the building.

Designing and constructing good buildings is a always collaborative process.  For new construction, we depend on accurate information from sub surface soil testing and effective designs from our engineering consultants.  This recent testing sets us up to but a strong foundation under our newest project!