Soil Mechanics Tutorials

A complete engineering soil identification includes: (a) a classification of constituents, (b) the description of appearance and structural characteristics, and (c) the determination of compactness orconsistency in situ.

a. Field Identification. Identify constituent materials visually according to their grain size, and/or type of plasticity characteristics per ASTM Standard D2488, Description of Soils (Visual-Manual Procedure). (1) Coarse-Grained Soils. Coarse-grained soils are those soils where more than half of particles finer than 3-inch size can be distinguished by the naked eye.

The smallest particle that is large enough to be visible corresponds approximately to the size of the opening of No. 200 sieve used for laboratory identification. Complete identification includes grain size, color, and/or estimate of compactness.

(a) Color. Use color that best describes the sample. If there are two colors describe both colors. If there are more than two distinct colors, use multi-colored notation.

(b) Grain Size. Identify components and fractions in accordance Coarse-Grained Soils.

(c) Grading. Identify both well graded and poorly graded sizes as, under Supplementary Criteria for Visual Identification.

(d) Assigned Group Symbol. Use Table 3 for estimate of group symbols based on the Unified Classification System.

(e) Compactness. Estimate compactness in situ by measuring resistance to penetration of a selected penetrometer or sampling. If the standard penetration test is performed, determine the number of blows of a 140 pound hammer falling 30 inches required to drive a 2-inch OD, 1-3/8 inch ID split barrel sampler 1 foot.

The number of blows thus obtained is known as the standard penetration resistance, N. The split barrel is usually driven 18 inches. The penetration resistance is based on the last 12 inches.

1) Description Terms.
2) Compactness Based on Static Cone Penetration Resistance, q+c,. Reference 2, Cone Resistance as Measure of Sand Strength, by Mitchell and Lunne, provides guidance for estimating relative density with respect to the cone resistance.

 If q+c, and N values are measured during the field exploration, a q+c,-N correlation could be made, and is used to describe compactness. If N is not measured, but q+c, is measured, then use 7.1-N = q+c,/4 for sand and fine to medium gravel and N = q+c,/5 for sand.

(f) Describe, if possible, appearance and structure such as angularity, cementation, coatings, and hardness of particles.

(g) Examples of Sample Description: Medium dense, gray coarse to fine SAND, trace silt, trace fine gravel (SW). Dry, dense, light brown coarse to fine SAND, some silt (SM).

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