This article will provide a breakdown of 4 of the most common geotechnical field testing methods. If you would like more information on any of these specific field testing methods, take a look at some of our other articles. If you aren’t sure what kind of testing your project requires, feel free to contact our friendly engineers to discuss how we can help on your project.
1. Static Cone Penetrometer (Bearing Probe)
A bearing probe or static cone penetrometer is a tool used by geotechnical engineers or technicians to determine the bearing capacity of near-surface soils. Typically, a bearing probe is used during a bearing inspection to check the exposed subgrade soil and verify that the soil has enough capacity for the structure being constructed over it.
The most commonly found bearing probe has a 610 mm (2 ft) rod attached to a handle in a T-shape. At the end of the rod, a cone-shaped tip is used to press into the soil. When force is applied to the handle, the pressure from soil at the tip is transferred through the rod, to a gauge attached to the handle. A calculation must be used to take the reading from the gauge and determine the bearing capacity of the soil.
The tip on a bearing probe can be swapped out for one with larger or smaller surface area. Different tip sizes can be used for different soils. Softer soil may require a larger tip, while stiffer soil can utilize a smaller tip. The size of the tip must be taken into account when calculating the bearing capacity from the gauge readings.
While the gauge on the bearing probe provides important information, the experience of the engineer using the tool is even more important during field testing. Pushing the probe too quickly or failing to correlate to the correct soil conditions when calculating the bearing capacity, can create unreliable results. A simple piece of rebar, welded into a T-shape can be more effective in the hands of an experienced geotechnical engineer than a shiny new digital bearing probe in the hands of an inexperienced tester.
If you ever order a Soil Bearing Report from us for your own peace of mind or as a requirement of the Alberta New Home Warranty, you’ll see this piece of equipment on your site!
2. Standard Penetration Test
A standard penetration test (referred to as an SPT) is a tool typically used while drilling boreholes during a geotechnical investigation. By completing regular SPT’s in a borehole, a profile of the consistency or density of the soil can be generated. By understanding how the density of soil varies with depth, geotechnical engineers can provide valuable insight on how the foundation of a building should be constructed.
An SPT is commonly used down a borehole, through the center of hollow-stem augers, or into an open borehole where solid-stem augers have been removed. The following steps are taken to perform the SPT:
- Position a split-spoon sampler at the end of a steel rod and place the open end of the sampler against the soil (vertically)
- Hammer the sampler into the soil using a 63.5 kg (140 lb) slide hammer falling a distance of 760 mm (30″)
- Drive the sampler a total depth of 450 mm (18″) and record the blows required to penetrate each 150 mm (6″)
- The sum of the second and third blow counts recorded, will be the “standard penetration distance” or the “N-value”.
- If more than 50 blows are required to advance through any of the 150 mm (6″) segments, the hammer stops and the penetration depth after 50 blows is recorded
Many empirical correlations from the SPT N-value to other commonly used soil parameters have been made in the geotechnical world. Academic papers and textbooks can be used to explore some of these correlations.
3. Nuclear Density Gauge
The nuclear density gauge or nuclear densometer can be found around the world on construction sites big and small. Although the word “nuclear” can scare people off, when used correctly this device is perfectly safe.
Field testing using a nuclear densometer is very simple and it can provide results on the density and moisture content of soil within seconds. All you need to know is the soil’s maximum density (the standard proctor density, or SPD) and you can use this device to spit out a number indicating the density of the soil you are testing as a percentage of the maximum. Typically, a project will require a minimum SPD of 98%; although 95%, 97%, and 100% are also commonly found on project specification sheets.
To complete a field test with a densometer, all you need to do is find a flat spot, drive a metal pin into the ground to create a hole, insert the gauge’s pin into the hole, and press a button to run a test. After the specified test duration (15 seconds to 4 minutes) is completed, the gauge will tell you the soil’s wet density, dry density, and moisture content. By comparing the dry density from the gauge with the soil’s maximum density (aka the Proctor density, determined in a geotechnical laboratory), you can determine the soil’s compaction percentage and pass or fail the soil.
4. Pocket Penetrometer (Pocket Pen)
The pocket penetrometer, or pocket pen for short, is a field testing tool you are likely to find in the pocket of most geotechnical engineers. This small metal pen-like tool is one of the most simple, cheap, and useful tools available.
Using a pocket pen is also simple, you can grasp the handle end of the penetrometer and press the smaller metal end into the soil being assessed. A spring is used to measure the resistance from the soil and a reading of the pressure required to insert the penetrometer a specific depth into the soil can be used to calculate the soil’s strength.
During a geotechnical investigation, a geotechnical engineer will typically use a pocket pen regularly to check the strength of the soil, and document any changes as the borehole depth increases.
Conclusion
There are many geotechnical field testing methods available that weren’t covered in this article. Depending on your project and the soil type that needs testing you may require a different test. If you require geotechnical field testing within Calgary or anywhere in Alberta, give us a call or fill out our contact page and we’ll get back to you!
