Lecture 7

Chapter 3

Soil Physical Properties--Mechanics

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.

1. 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.

2. 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.

3. 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 as possible.

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 most effective.

Students are encouraged to look up the following vocabulary words in the textbook glossary or elsewhere and to browse the following web sites.

Vocabulary

 liquid limit plastic limit plasticity index atterberg limits consistency compaction consolidation stress strain

Web sites

Notes on Atterberg Limits from another course.
URL: www.egr.msu.edu/~wolff/ce812/handouts/notes_atterberg.htm

NASA's Soil mechanics findings from Mars Pathfinder.
URL: www.ksc.nasa.gov/mars/science/surface.html

Homepage of American Society for Testing and Materials (ASTM).
URL: www.astm.org