Understanding Geology To The Core: Three Site Inventory Investigation Methods

Site inventory sometimes involves investigating conditions that you just cannot see on the surface. For example, you just can’t see the underlying geology on your client’s site with the naked eye. Therefore, we need to use some different methods to investigate the ground beneath our feet.

Site reconnaissance sometimes requires more technical invasive investigation techniques. Three investigative techniques can help you gather site inventory information about the geology of your site.

Three Ways To Get To Rock Bottom

Trenching, probing, and core drilling are three investigation methods landscape architects use to explore the geology deep underground. Let’s look at each method and discuss their pros and cons.

As Easy As Cake

Layered carrot cake demonstrate the layers found in a site's underlying geology.

Geology of a site is like the layers in this cake.

Do you remember the last time you cut into a layered cake? The section where the cake was cut exposed the different layers of cake and frosting and showed you the structure of the cake.

Image of a geology trench to explore underground geology

Trenching is a classic method for exploring a site's geology. Photo by John Atherton.

Trenching a site is like cutting a cake. The trench cuts through the site’s soil and shallow geology to expose its strengths and weaknesses.

Site trenches can reveal the soil profile, geologic structure, man-made landfill materials, and depth to bedrock. Trenches are easy to dig with conventional construction equipment but offer only a limited view into the site’s geology unless several are dug in representative areas throughout the site.

Some other methods similar to trenching include test pits, shafts, tunnels. All of these surface excavations can provide excellent information for a site inventory.

Probing Deep To Rock Bottom

Probing a site is like sticking a toothpick into a cake. A probe is like a toothpick, but you are looking for the bottom of the pan which is a site’s bedrock.

Probing the soil and underlying geology is another geology investigation method that can be used during the site inventory phase of a project. Probing a site can provide you with information about the depth of bedrock on a site.

Construction engineers hammer a probe into the site’s soil till the probe meets resistance. This technique can estimate bedrock depth but is less reliable than trenching or core drilling. Penetration refusal may indicate bedrock depth or could just be a rock mixed in the soil. If concerns about the site’s geology are generated by probing, core drilling or excavations may be in order.

Knowledge To The Core

Core drilling is like the straw in this milkshake.

Have you ever tried to drink a really thick, cold milkshake? When you pull the straw out of the creamy treat, you can see a sample of the milkshake stuck in the straw. Sticking a straw in a milkshake is just like core-drilling a site’s geology.

Core drill bit image

Core drilling cuts through the underlying soil and geology to reveal what lies underneath a site. Photo by Graham Holliday.

Core drilling (also known as coring) is an accurate method for investigating a site’s geological substrate. A large diameter drill bit penetrates the soil and fills a hollow tube with a representative sample of the underlying geology. An engineer or geologist examines and interprets the core samples. Coring can reveal the depth to bedrock and the geologic strata of a site. Test cores usually penetrate twenty feet below planed building foundations.

Core drilling is a cost effective way to get reasonably accurate information about a site with minimal site disturbance.

Investigating The Underground

X-Ray Glasses

Because we don't have X-ray vision...

Probing, core drilling, and excavating are three ways that landscape architects can put on X-ray vision glasses and peak underneath the surface.

Questions about site inventory methods like these may be part of the California Supplemental Exam for landscape architects. Check out additional articles for more information on the exam and for study material.




John is a landscape architect who is currently preparing to take the California Supplemental Exam to become licensed in California. He is currently a licensed professional landscape architect in Georgia and Florida. John graduated from California State University, Pomona with a BSLA degree in landscape architecture in 1997 and has extensive practice experience in residential and commercial landscape design.

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