Chapter 4: Soil and Bedrock Logging


Section 1: Logging

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Material Order of Description

Keep core descriptions as simple as possible. The order of description is as follows:

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  1. Material
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  3. Density or consistency, hardness
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  5. Moisture
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  7. Color
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  9. Cementation
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  11. Descriptive adjectives
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  13. Unified Soil Classification System
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  15. Rock Quality Designation (RQD), percent recovery
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Keep the number of strata to a minimum. Remember that every small variation in a soil—such as a change in clay from “slightly sandy” to “sandy”—does not necessarily warrant a strata change. The logger must define strata that have significance to designers and contractors who will use the core log information. Designers and contractors are mainly interested in the primary and secondary soil or rock constituent and whether ground water is present.

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Density or Consistency, Hardness

Use the following charts to determine the density or consistency and hardness of material encountered.

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Density (Cohesionless)

Consistency (Cohesive)

TCP Values

Field Identification

Very loose

Very soft

0 to 8

Core (height twice diameter) sags under own weight



8 to 20

Core can be pinched or imprinted easily with finger

Slightly compact


20 to 40

Core can be imprinted with considerable pressure


Very stiff

40 to 80

Core can be imprinted only slightly with fingers



80 to 5 in./100

Core cannot be imprinted with fingers but can be penetrated with pencil

Very dense

Very hard

0 in. to 5 in./100

Core cannot be penetrated with pencil

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Mohs’ Hardness Scale




Approximate TCP Values

5.5 to 10

Rock will scratch knife

Sandstone, chert, schist, granite, gneiss, some limestone

Very hard

0 in. to 2 in./100

3 to 5.5

Rock can be scratched with knife blade

Siltstone, shale, iron deposits, most limestone


1 in. to 5 in./100

1 to 3

Rock can be scratched with fingernail

Gypsum, calcite, evaporites, chalk, some shale


4 in. to 6 in./100

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If any moisture exists, note the extent present. The samples will be assumed dry if the degree of moisture is not indicated. If free water is present, describe the soil as wet or water-bearing.

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Describe the primary color, and restrict description to one color. If one main color does not exist in a sample, call it multicolored.

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Identify the degree of cementation if any is present.

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Descriptive Adjectives

Use any descriptive adjectives that might further aid in the description.

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Unified Soil Classification System (ASTM D2487)

This soil system is based on the recognition of the type and predominance of the constituents considering grain size, gradation, plasticity index, and liquid limit. It contains three major divisions of soil: coarse-grained, fine-grained, and highly organic. See ASTM D2487, Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System), for the procedure for determining soil classification. TxDOT test procedures, Tex-141-E, Manual Procedure for Description and Identification of Soils and Tex-142-E, Laboratory Classification of Soil for Engineering Purposes may also prove useful in the determination of soil type.

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Rock Quality Designation (RQD) and Percent Recovery

Determine the RQD for rock core samples following ASTM Test Procedure D6032, Standard Test Method for Determining Rock Quality Designation (RQD) of Rock Core. Always note the RQD and percent recovery on logs of borings where rock is encountered.

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Log Form

For uniformity, use the standard log form 513, Drilling Log. Group the materials encountered into strata consisting of the same or similar constituents.

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