Forest Road Engineering Guidebook - Ministry of Forests

Forest Road Engineering Guidebook
Soil compressibility
A soil’s potential for compressibility is not easy to determine by visual examination
alone. This is usually done by laboratory tests. However, if it is suspected
that the soil will settle considerably under load, this should be reported.
There is a general relationship between consistency (as defined for cohesive
soils) or soil density (as defined for non-cohesive soils) and compressibility.
For example, the soft and loose soils are likely to compress considerably
under loads.
Reporting soil description
In forming a description, the predominant particle size is what is used to
describe the soil type. The relative content of other particle sizes modifies the
description (e.g., sand GRAVEL with some silt).
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A predominantly coarse-grained soil is termed either a gravel or a sand,
depending on which component appears to be the more abundant. The less
abundant component and the fines are used as modifiers; the least important
component is stated first. For example, a soil with 30% fines (silt), 45%
gravel, and 25% sand would be best described as sandy, silt GRAVEL.
The terms in Table A1.4 are used to indicate various proportions by weight
within the respective grain-size fractions.
Table A1.4. Soils description terms.
Example
Descriptive Term by Weight Proportion
NOUN GRAVEL, SAND, SILT, CLAY >50%
“and” and gravel, and silt, etc. >35%
ADJECTIVE gravelly, sandy, silty, clayey, etc. 20–35%
“Some” Some sand, some silt, etc. 10–20%
“Trace” Trace sand, trace silt, etc. 1–10%
For example:
• A “silty CLAY, trace of fine sand” would be >50% clay, 20–35% silt, and
0–10% sand.
• A “sandy GRAVEL with some cobbles” would be >50% gravel with
20–35% sand sizes and 10–20% cobbles.