Soil mechanics is a specialty within the field of
civil engineering. Soil mechanics deals with soil response to physical stress. The stress
may be weight of a building, vehicle traffic, or various other forces. Various
classification systems have been devised to evaluate soil suitability for engineering
purposes. Those systems are complex and will be only briefly described here.
- Soil consistency refers to the soil's response to stress.
Stress is pressure applied to the soil. In physics, pressure is force per area. The force
applied to soil is normally weight. The response to stress is called strain. Engineers
often plot strain as a function of stress to determine the point at which a material
fails, called the yield point. Engineers are interested in both the deformability and
firmness of a soil. A system called Atterberg Limits is used to describe the shrinkage
limit, plastic limit, and liquid limit of a soil. As water is added to a dry soil, the
soil changes from solid to semi-solid to plastic to liquid. The moisture content in the
soil at the threshold between semi-solid and plastic is called the plastic limit. The
moisture content in the soil at the threshold between plastic and liquid is called the
liquid limit. Liquid limit is determined by forming a groove in a dish of soil and
impacting the dish until the groove closes. The test is done using the apparatus in Figure
3-15 of the textbook, following the ASTM procedure D-4318. The plastic limit is determined
by rolling a thread of soil on a glass plate until the 1/8-inch-diameter thread begins to
crumble. This technique is also explained in ASTM procedure D-4318. A large liquid limit
indicates high compressibility and high shrink swell tendencies. Subtracting the plastic
limit from the liquid limit yields the plasticity index. A large plasticity index
indicates low shear strength.
- Soil mechanics classification systems. The two main
systems in use are the AASHTO and the Unified systems (see Figure 3-14 in the textbook).
The Unified system is the older of the two. It is used by geotechnical engineers. Soils
are classified based on their Atterberg Limits and on particle size as determined by
sieving. The groups within the Unified system are explained in Figure 3-16 of the
textbook. In general, the Unified system assumes that course material is better than
fines, low liquid limit is better than high, a narrow range from the plastic limit to the
liquid limit is better than a wide range, and that well graded material is better than
poorly graded material. Well graded, means lots of sizes mixed together. This allows small
materials to fill the pores between larger materials, and therefore gives a denser mix
than does uniformly sized material.
The AASHTO system is newer and more widely used. It was developed by and for state highway
departments. It also is based on Atterberg Limits and particle sizes. However it
interprets these data differently than the Unified System. The AASHTO system is outlined
in Table 3-12 of the textbook.
- Soil compaction and consolidation. Soil compaction
means the removal of air-filled porosity. Consolidation means the removal of water-filled
porosity. For most engineering purposes, the more dense a soil is the better its
load-bearing capacity. To make a soil more dense requires removing as much of the porosity
There exists for each soil an optimum moisture content for compaction, i.e., the moisture
content at which a given pressure will create the most dense material. If the soil is
drier than the optimum, too much friction exists between soil particles, so compaction is
difficult. If the soil is wetter than the optimum, compaction may be easy but rather
fruitless because most of the porosity is filled with water, not air. In general, removing
water by compressing the soil is much more difficult than removing air, so the engineers
aim is to compact, but not to consolidate, the soil. Compaction of soil at the
construction site is accomplished through the use of rollers and vibrators.
Contractors and engineers often order a Proctor test (ASTM D-698) performed on the soil.
The Proctor test determines the optimum water content for the compaction of that
particular soil. The contractor then could dry the soil (or allow it to dry in the sun);
or could moisten the soil. At the ideal moisture content the contractor's rollers will be
Students are encouraged to look up the following vocabulary
words in the textbook glossary or elsewhere and to browse the following web sites.
Notes on Atterberg Limits from another course.
NASA's Soil mechanics findings from Mars Pathfinder.
Homepage of American Society for Testing and Materials
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