Subsurface Exploration with a split spoon sampler

What are subsurface conditions, and why do they matter? 

Subsurface conditions are the characteristics that determine the behavior of soils as they exist in the ground. These characteristics are sometimes referred to as 'in-situ' because they refer to the undisturbed conditions in the ground. Generally, these conditions are quantified using elements of particle mechanics, fluid mechanics, rock mechanics, and geologic principles. 

A successful subsurface exploration program will provide engineers with information about the water table depth, soil classification, relative density, and physical characteristics of soil conditions. Many of the data points collected during the exploration will be generated by visual observations made by engineers during the process and verified by laboratory testing at a later date. 

There are many ways of collecting soil samples, but we're focusing on split spoon samples collected using an automatic hammer. Similarly, there are several systems of classifying soils; for this post, we'll only be considering the Unified Soil Classification System (USCS) as it is the most widely used system by geotechnical engineers in North America. 

The USCS chart based on ASTM D-2487. This chart can be followed in a left to right fashion when conducting a field classification

For the uninitiated, the USCS is a protocol that enables the classification of soils based both on visual-manual methods and laboratory sieve analysis. Classifications completed in the field will be visual-manual classifications or, simply, field classifications. The USCS field classification system relies on the engineer to estimate the percentage of the sample, that is, gravels, sands, and fines, as well as noting any organics or other relevant details observed. For the purposes of USCS classifications, gravel is defined as anything retained on the number 4 sieve, in more practical terms, anything larger than 3/16 of an inch or 4.75mm in diameter. Sands are any particle between 4.75mm and 0.074mm. Fines are anything smaller than 0.074mm, thus passing the number 200 sieve. 

In addition to the grain size distribution, engineers will also note any odors detected on the sample. These odors can provide important insights into the chemical composition of the soil. An organic, swampy smell is a strong indicator of organic deposits or peat. A chemical or petroleum odor can be an indication of contamination contained in the soil. The color of the sample is also important as it can provide insight into the geologic conditions that created the soil. Soil is typically comprised of broken-down elements of rock, and color and angularity combined with known geologic history of the region can inform engineers about the origin of the soils. 

A split spoon soil sample showing stratification between soil layers. The dark, rich colored material seen in this sample is coal

The sample is collected using a split spoon sampler. The split spoon sampler is a 2-foot section of pipe that is split in half lengthwise with an inner diameter of 1.375". The design allows for the sampler to be opened while preserving the integrity of the soil sample inside. The split design enables engineers to observe and document any stratification present in the sample and see an approximation of in-situ conditions. 

The density of the soils can be inferred from data recorded during the sampling process. This data comes from the standard penetration test (SPT). SPT is a measure of how many blows from a 140-pound hammer dropped from 30 inches to drive the sampler 6 inches. A standard sampler is 24 inches long and has an inside diameter of 1 3/8 inches. This means that there will be four blow counts per sample. The SPT is then calculated by adding the middle two blow counts together. In practice, this means that the sum of the blows from the 6-12" increment and the 12-18" increment is the SPT. While SPT can be a useful data point, it is important to note that in most cases, it is measured by engineers listening to the sound of the hammer impacting the sampler and watching the sampler move to estimate the 6" increments, because of this SPT is often considered an approximation. Several corrections can be made to SPT data in an attempt to calibrate the metric to a specific environment. These include corrections for hammer efficiency, influence of overburden stress, length of the drill rod, and various borehole diameters. Despite a degree of uncertainty, SPT will still provide information about how tightly compacted a soil is. Dense soils will have higher SPT, meaning it took many blows to drive the sampler into the ground. In contrast, loose soils will have very low SPT values, sometimes even sinking into the soil under the weight of the hammer alone without any blows from the hammer being dropped.   

This image shows a sample of a cohesive soil recovered from 75-77 feet below ground. This was a sample that was very dense and had very high blow counts

Throughout the sampling process, it is important to make note of the moisture content of the soils (are they dry, moist, or wet), as well as any other relevant observations. In regions that experience permafrost, it's critical to document any ice inclusions in the soil sample. As noted previously, this is only one sampling method, and this process has many adaptations based on regional needs. In areas with particularly dense, coarse-grained glaciofluvial deposits, a larger diameter sampler is often used. Alternative tests such as the cone penetration test (CPT) can be used in regions with more fine-grained cohesive soils. Regardless of the sampler you're using, the core advice remains the same: document, document, document. 

If you're looking for a more visual explanation of the USCS visual-manual classification, take a look at our latest video walking through the process with course-grained soil!