Abstract
In various fields of practice that deal with soils, from soil science to geotechnical engineering, the accurate, detailed, and universal description of the soils encountered at a site is key to understanding how those soils will behave and how they will affect the proposed construction. Typically, this is done by classifying the soils into established types based on characteristics such as gradation and plasticity properties. However, rarely are laboratory tests to establish the soil’s gradation and plasticity characteristics performed on every sample of soil that is collected. Instead, classifications are initially developed through visual and tactile assessment of the soil by an experienced technician, and laboratory tests are performed only on select samples to verify the initial classification and provide additional details. Furthermore, field personnel will provide descriptions of the soils encountered that go beyond the type of soil to include their color, moisture level, and more. These field observations are critical to understanding the conditions at the site beyond what can be assessed through quantitative laboratory testing.
Visual classification and soil descriptions are a matter of experience and acquired skills. Knowing how to estimate the gradation and plasticity characteristics as well as knowing what additional features need to be noted takes time and practice; however, this lab exercise will introduce a few basics of this process and offer the opportunity to develop descriptions of a few soils similar to those one might encounter in practice. The method presented in this exercise will provide a framework for describing most any kind of soil and promote the thought processes necessary to effectively describe soil conditions based on sensory observations.
Required Standards
ASTM D2488 – Standard Practice for Description and Identification of Soils (Visual-Manual Procedures)
Introduction
Visual-manual classification and description of soils is a process by which the engineering properties of a soil can be estimated and qualitatively described using only visual and tactile assessments.
First, a skilled technician will gather a soil sample in their hand and examine it visually. Items often described based on visual examination include angularity and shape (for coarse sand, gravel, or larger sizes), color (when moist, typically as compared to standard colors), odor, moisture condition (dry, moist, or wet), structure, and particle size information (range of sizes, maximum estimated size, and fineness as applicable).
Next, a series of manual tests can be performed which may include molding, rolling, squeezing, moistening/drying, or the application of a weak hydrochloric acid solution. Manual tests are particularly useful for fine-grained soils (silts and clays) and may be used to assess the consistency (in-place stiffness), plasticity, dried strength, and dilatancy (how readily the sample releases and absorbs water). The application of the acid solution is beneficial for identifying calcium carbonate, a compound commonly found in limestone and dolomite rocks and a common cementing agent in a wide range of soils (coarse and fine-grained soils).
Finally, all of the relevant information gained from these observations is summarized in a soil description and is used to assign a classification (name and group symbol) to the soil. This final description provides a wealth of information to other engineers and scientists without a single lab test being performed. In fact, these descriptions are helpful in the selection of samples for further testing as well as for grouping of similar materials into layers. This procedure is not a substitute for lab tests, but an experienced technician can accurately and effectively describe and classify soils using these means. With time and practice, you may be surprised to find how well your descriptions will coincide with laboratory test results1! Make sure to remember what your soils looked and felt like when you get the results from classification lab tests in the next few weeks.
Objectives
At the completion of this lab exercise, you will have satisfied the following objectives:
1. Visually assess a series of soil samples
2. Assign a color designation to each of the soil samples based on comparison with standard color palettes
3. Estimate the maximum particle size, size range, and shape (as applicable)
4. Perform a series of manual manipulations to assess the plasticity and dilatancy characteristics of fine-grained soils
5. Develop a description of each soil that includes the applicable information
Learning Outcomes
At the completion of this lab exercise, you should be able to:
–understand the typical behavior and characteristics of the major types of soil (gravel, sand, silt, and clay),
–perform the necessary visual and manual assessments to classify and describe soils in the field,
–write clear and detailed soil descriptions using the appropriate engineering terminology, and
–understand how to read a soil description and comprehend the relevant information.
Procedure
The visual classification procedure is divided into three parts: visual assessment, manual assessment, and classification and description development. Depending on the type of soil that you are working with, some visual or manual assessments and description items will not be applicable, so make sure you are thinking through each of the assessments and what type of information you are looking for. Furthermore, this procedure does not include each of the assessments that are included in ASTM D2488 (for instance, you will not perform the HCl reaction or identify organic materials). This is because it has been previously determined that some assessments are not applicable to the soils being considered. In practice, you should review the ASTM standard prior to classifying soils and incorporate as many of the visual and manual assessments as are needed to effectively describe your soil. Remember, the ASTM standard is a prescriptive specification that licensed engineers and engineering firms must follow.
Visual Assessment
To begin, consider the pan of soil before you that you wish to describe. Visually assess the particles for size first. In particular, start by estimating if the majority of the particles are “coarse or fine”. The standard dividing line between coarse-grained soils (which include sand and gravel) and fine-grained soils (which include silt and clay) is a particle diameter of 0.075 mm. To discern this visually, ask yourself if you can see the individual grains with the naked eye or not. As a rule of thumb, the majority of the particles that make up a coarse-grained soil will be visible to you as individual grains. A couple of example samples are provided in Figure 1 and 2 to show the difference between a sand (coarse-grained soil) sample and a silty clay (fine-grained) soil sample. Estimate if your sample consists of a majority (more than 50%) coarse or fine particles and thereby designate the soil as “coarse-grained” or “fine-grained” soil.
Fig. 1: Example of a predominantly fine-grained (silty lean clay) soil sample. Note how the majority of the particles are too small to see individually. The quarter also offers a quick scale comparison. 
Fig. 2: Example of fine silty sand sample. Note how here we can see many of the individual particles, but they are still quite small. This is a good indication that they are fine sand particles.
Now assess the particle sizes in a bit more detail. Estimate the maximum particle size that you see, then consider about what percentage of the particles can be sorted into the specified size ranges. Particles will fall into the categories described in Table 1 and ASTM D2488 Section 3.1. The size categories that are presented in the standard are defined by measured particle sizes (in inches or millimeters). These measurements are typically performed using sieve analysis or hydrometer testing, but for our visual classification, we will have to estimate the particle sizes.
| Name | Size, mm | Size, in |
|---|---|---|
| Boulders | >300 | >12 |
| Cobbles | 75 to 300 | 3 to 12 |
| Coarse Gravel | 19 to 75 | 0.75 to 3 |
| Fine Gravel | 4.75 to 19 | 0.19 to 0.75 |
| Coarse Sand | 2 to 4.75 | 0.079 to 0.19 |
| Medium Sand | 0.425 to 2 | 0.0170 to 0.079 |
| Fine Sand | 0.075 to 0.425 | 0.0030 to 0.0170 |
| Silt and Clay (fines) | <0.075 | <0.0030 |
Estimate the percentage of the particles by volume that fall into the categories of gravel, sand, or fines to the nearest 5% and write down these values. Identify the material that makes up the largest percentage of your material; this will be referred to as the “primary component”. Any other components will be included in the description as either a modifier (silty, sandy, gravelly, or clayey), as a “with” component (e.g. sand with silt), or as a “trace” component (sand with trace of clay). Whether the material type is a modifier, “with”, or “trace” depends on both the percentage of the other component as well as whether the overall soil is a coarse-grained soil or fine-grained soil (refer to the first paragraph of this section). If your overall soil is a majority coarse-grained soil with some fine-grained soil component, refer to Table 2. If your overall soil is a majority fine-grained soil with some coarse-grained component, refer to Table 3.
| Description Term | Percent Fine-Grained Soil |
|---|---|
| Modifier | 15 or more |
| “with” | Between 5 and 15 |
| “trace” | Up to 5 |
| Description Term | Percent Coarse-Grained Soil |
|---|---|
| Modifier | 30 or more |
| “with” | Between 15 and 30 |
| “trace” | Up to 15 |
The process described above can be a bit confusing. For a visual guide, refer to the flow charts provided in ASTM D2488 Figure 1 for majority fine-grained soils and Figure 2 for majority coarse grained soil. In addition, below are a couple of illustrative examples.
Example
Consider a soil that is 65% sand, 25% gravel, and 10% silt. Because of the combined 90% sand and gravel, this is a coarse-grained soil, so we will use Table 3. Based on the percentage of silt, we would describe this soil as “sand with gravel and silt”.
Example
Consider a soil that is 65% clay, 25% sand, and 10% gravel. Because of the 65% clay, this is a fine-grained soil, so we will use Table 2. Based on the percentages of sand and gravel, we would describe this soil as “clay with sand and trace of gravel”.
If you refer to the flow charts you will see that fine-grained soils are designated as CL, ML, CH, or MH. “C” is the symbol for Clay, and “M” is the symbol for Silt2. CL is a low plasticity or “lean” clay while CH is a high plasticity or “fat” clay. ML is silt while MH refers to an “elastic” silt. These designations are based on plasticity behavior which will need to be determined using the manual tests in the next section of our procedures.
The flow charts also make a distinction between “well-graded” and “poorly-graded” sand or gravel. “Well-graded” sand or gravel contains a wide range of particle sizes and the particles are relatively evenly distributed across this spectrum. “Poorly-graded” sand or gravel will have particles that are all of similar sizes. For visual classification, this is more of an experience based estimate (do the particles range from fine to coarse or are they mostly all fine, medium, or coarse for instance). When we perform our sieve analysis, we will determine the grading using numerical benchmarks.
The next part of our visual assessment is the visible moisture content of the sample. If the material is dusty and dry to the touch, we will describe it as “dry”. If the sample is damp but there is no free water visible, we will describe it as “moist”. If the sample contains visible free water (that is, water that has not adhering to the particle surfaces), we will describe it as “wet”.
The final visual assessment that we will need to perform is describing the color. Compare your soil to the standard color swatches and select the color description that matches best. If your soil has a mix of multiple distinct colors, list the names of each color in order from most prevalent to least (e.g. “brown and yellow” for a soil that is mostly brown with some yellow).
Manual Assessments
Identification of fine-grained soils (those with more than 50\% “fines” or particles less than 0.075 mm) or fine-grained portions of coarse-grained soils requires a series of manual manipulations designed to assess the plasticity and moisture response behavior of the soils. These tests are outlined in this section.
First we will assess the dry strength of the sample. For this we can use either naturally occurring dry clumps of soil or we can form roughly 1 inch diameter balls and allow these to dry. Make sure that when you select a naturally occurring clump that you do not accidentally select a large piece of gravel! Take each clump or ball into your hand attempt to crumble them. Note how easily or difficultly they break apart. Table 9 in Section 14 of ASTM D2488 provides a guide for how the dry strength should be described.
Next we will assess the plasticity of our fine-grained materials. To do this, collect a portion of the soil and roll it into a thread in your hand. Think of trying to make a long skinny snake with Playdoh. Attempt to roll the thread until it is about 1/8 inch in diameter. If the soil easily rolls to a diameter of 1/8 inch or less without crumbling, ball up the material and roll it again, repeating the process until the material starts to break apart as the thread reaches a diameter of 1/8 inch. Take note of how easily this process goes and how the thread behaves once formed. Compare your observations to the criteria in Table 12 in Section 14 of ASTM D2488 and thereby designate the material as non-plastic, or low, medium, or high plasticity.
Now you should have sufficient information to classify your fine-grained soil. Silt (ML) will have low to no dry strength meaning that it will easily crumble when dried. Silt will also be difficult to roll into a thread, crumbling readily when attempted. Elastic silt (MH) will have somewhat higher dry strength and may have low to medium plasticity. Low plasticity or “lean” clay (CL) will have a moderate plasticity and dry strength (higher than either type of silt), while high plasticity or “fat” clay (CH) will have quite high dry strength and plasticity.
If the soil sample is a fine-grained soil (i.e. more than 50% of the material has a diameter smaller than 0.075 mm), the full name and symbol for the fine-grained portion will be used in the description (e.g. sandy elastic silt with gravel, ML). If the soil sample is coarse-grained (i.e. more than 50% of the material is larger than 0.075 mm), only silt or clay will be used irrespective of the plasticity behavior (e.g. clayey gravel, GC).
Classification and Description Development
After performing our assessments, we now want to develop a full description of our material. Figure 3 presents a couple of example descriptions with the parts numbered. The items we need to include in our description are as follows:
- Moisture Content
- Color
- Soil Classification (Name and Symbol)
- Fineness (fine, medium, coarse for sand or fine, coarse for gravel)
Summary
Now we have completed the process of visually and manually evaluating our soil samples and developing a thorough description based on the results. With these descriptions, other engineers will have a good idea of how our soils look and behave including color, gradation, plasticity, and as-sampled moisture level. We have also learned the basics of how to evaluate soils through visual and tactile means, and how to coherently describe the soils to others. We are now poised to expand this knowledge to describe more complex conditions and to assess soils on a full range of properties as described in the ASTM standards.
- Believe it or not, a friendly soil classifying competition can be a good time, though it doesn’t make a great party trick unless you routinely bring dirt to parties.
- If you’re wondering why silt is not “S”, it’s because that letter was already taken with “sand”, so Casagrande selected “M” because Moh is the old German word for silt.