Digging Deeper into Cemented Soil

This article looks at what the excavation safety standard (Subpart P) and the Federal Register have to say about cemented soils. Specifically, we are referring to caliche and hard pan. Some parts of the country deal almost exclusively with this kind of soil, others only occasionally if at all.

In a previous article I explained how one of the intentions of Appendix A was to simplify what could be a complicated process when identifying and classifying all soils, including cemented soils. Additional information can be found at www.trenchandexcavationsafety.com

I am including in this discussion direct quotes from subpart P and the federal register for the reader’s benefit and further study. Some sentences are underlined by this author to draw attention to that information.

From Appendix A we find that cemented soil is either hard pan (cohesive soil) or caliche (granular soil). Either way this cemented soil is strong enough that it cannot be broken up into individual particles or powder by hand pressure (dry strength test). This type of soil could potentially be classified as Type A.

Here's a direct quote from the federal register about the characteristics of cemented soil which describes and helps us to identify it. The entire section is also provided at the end of this article for context and reference.

“Cemented soils are most commonly composed of granular, or coarse-grained particles. Carbonate salts—calcium carbonate being the most common—are the primary chemical agents that provide the cementation of the particles of soil. The action of the cementing agents adds to the strength of the soil by binding the particles together so that the soil can resist a greater degree of stress. The quantity of the cementing agent in a soil sample can be estimated by subjecting a soil sample to a solution of dilute hydrochloric acid and visually noting the intensity of the reaction. Unfortunately, this test does not give a good indication of the strength of the cemented soil. However, a satisfactory estimate of the relative strength of a cemented soil can be made by conducting a dry strength test. This is a test that is conducted by crushing dry soil samples between the fingers. The dry strength test is used primarily for estimating the strength of fine-grained soils, i.e.., clays which have cohesive qualities. Conducting this test on cemented coarse-grained soils, however, can give good estimates of relative strength that are equivalent to the strength estimates of fine-grained soils.”

FROM Subpart P Appendix A

Definition of Cemented Soil

Cemented?soil means a soil in which the particles are held together by a chemical agent, such as calcium carbonate, such that a hand-size sample cannot be crushed into powder or individual soil particles by finger pressure.

Definition of Type A soil

Type A?means cohesive soils with an unconfined compressive strength of 1.5 ton per square foot (tsf) (144 kPa) or greater. Examples of cohesive soils are: clay, silty clay, sandy clay, clay loam and, in some cases, silty clay loam and sandy clay loam. Cemented soils such as caliche and hardpan are also considered Type A. However, no soil is Type A if:

(i) The soil is fissured; or

(ii) The soil is subject to vibration from heavy traffic, pile driving, or similar effects; or

(iii) The soil has been previously disturbed; or

(iv) The soil is part of a sloped, layered system where the layers dip into the excavation on a slope of four horizontal to one vertical (4H:1V) or greater; or

(v) The material is subject to other factors that would require it to be classified as a less stable material

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Manual Test (2) The Dry Strength Test

(ii)?Dry strength. If the soil is dry and crumbles on its own or with moderate pressure into individual grains or fine powder, it is granular (any combination of gravel, sand, or silt). If the soil is dry and falls into clumps which break up into smaller clumps, but the smaller clumps can only be broken up with difficulty, it may be clay in any combination with gravel, sand or silt. If the dry soil breaks into clumps which do not break up into small clumps and which can only be broken with difficulty, and there is no visual indication the soil is fissured, the soil may be considered unfissured.

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FROM the Federal Register 45936 and 45937

The soil classification system that was proposed was developed by the National Bureau of Standards (NBS). The background of the system is explained in more detail in Exhibit 2-5. OSHA incorporated this classification system in the proposal based on the recommendations of NBS, after consultation with the ACCSH, and after a review by interested parties at the five industry-sponsored workshops. In addition, OSHA used several American Society for Testing and Materials (ASTM) Standards, as well as other sources, to obtain information that, in OSHA’s opinion, was needed to supplement and clarify the NBS recommendations. The ASTM Standards included: (1) Designation: D653-67 (Reapproved 1973)—“Standard Definitions of Terms and Symbols Relating to Soil and Rock Mechanics,” (Ex. 2-27); (2) Designation: D2487-69—“Standard Method for Classification of Soils for Engineering Purposes," (Ex. 2-28); and, (3) Designation: D2488-69—Standard Recommended Practice for Description of Soils (Visual—Manual Procedure),” (Ex. 2-29). OSHA used these sources to clarify and provide additional information in paragraph (b) of the appendix, “Definitions,” and in paragraph (d), “Recommended Visual and Manual Tests.” One example of the use of supplemental information involved the development of the definition of "cemented soil.” NBS made reference to cemented soil in its recommended definition of Type A soil, but provided no discussion as to what constitutes cemented soil other than to suggest that soils referred to as “hardpan” or “till” are examples of cemented soil. Cemented soils are most commonly composed of granular, or coarse-grained particles. Carbonate salts—calcium carbonate being the most common—are the primary chemical agents that provide the cementation of the particles of soil. The action of the cementing agents adds to the strength of the soil by binding the particles together so that the soil can resist a greater degree of stress. The quantity of the cementing agent in a soil sample can be estimated by subjecting a soil sample to a solution of dilute hydrochloric acid and visually noting the intensity of the reaction. Unfortunately, this test does not give a good indication of the strength of the cemented soil. However, a satisfactory estimate of the relative strength of a cemented soil can be made by conducting a dry strength test This is a test that is conducted by crushing dry soil samples between the fingers. The dry strength test is used primarily for estimating the strength of fine-grained soils, ie., clays which have cohesive qualities. Conducting this test on cemented coarse-grained soils, however, can give good estimates of relative strength that are equivalent to the strength estimates of fine-grained soils. ASTM D2488 (Ex. 2-29) states, in a description of the dry strength test, that: ‘The presence of high-strength water soluble cementing materials, such as calcium carbonate, may cause exceptionally high dry strength \” In the dry strength test, "high” strengths are indicated if “the sample cannot be crushed to powder by finger pressure, even though the sample may be broken.” “Very high” strength is indicated “if the sample cannot be broken between the thumb and a hard surface.” In another reference (“Handbook of Soil Mechanics”, Volume I, p. 98, by Arpad Kezdi) under a discussion of the dry strength test, it is stated: "If the sample resists crushing by finger pressure altogether, the soil is an inorganic clay of high plasticity, or a coarse-grained soil aggregate cemented by some high strength binder (e.g? calcium carbonate or iron oxide).” Based on the discussions of cemented soils in this literature, OSHA supplemented the NBS recommendations by developing a proposed definition of “cemented soil" with the intent of clarifying the NBS recommendations. The definition of “cemented soil” is intended to include those soils that exhibit strengths at least equivalent to the strengths required for Type A cohesive soil. The result of this was that some soils containing a slight amount of a cementing agent would not be considered as cemented soil, as such soil will not exhibit sufficient strength.