Soils and Engineering

The two systems of soil classification

Pedological Classification - Soil Weathering, texture, chemistry, profile thickness, etc

Engineering Classification - Soil texture, degree of plasticity (Atterberg Limits)

Overview of Mechanics

As water content increases, the shear strength decreases

Plastic Limit

Water content at which the soil is a plastic

less water content than liquid limit

wide range of shear strengths at plastic limit

defined as the moisture content % at which the soil begins to crumble when rolled into 1/8 diameter threads.

Liquid limit

soil is practically a liquid

shows minimal shear strength

defined as the moisture content required to close a distance of 0.5 inch along the bottom of a groove after 25 blows of the liquid limit device

Plasticity index (PI)

difference between liquid limit and plastic limit

important measure of plastic behavior

PI vs Degree of plasticity

Unified system - classification scheme - symbol v soil type and index property

Unified system procedure

Determine the percent passing through the #200 sieve (boundary between sand and silt/clay)

if less than 50% passes, then it is a coarse grained soil (gravel and sand)

if greater than 50% passes, then it is a fine grained soil (silt and clay)

Father of soil mechanics

Carl Terzaghi

Voids lead to

lead to settling when pore space within the soil mass decreases under heavy loads - structural cracks or complete failure

objective of soil compaction

decrease void space and increase density - improves strength and volume stability