Soil Mechanics: Properties and Concepts

Soil Mechanics

Soil Properties

Soil mechanics is a branch of soil physics and engineering mechanics that describes the behavior of soils. Soil is a complex mixture of various materials, including gravel, sand, silt, clay, organic solids, and other matter. The properties of soil vary depending on the predominant size of the particles. Soils are classified as gravel, sand, silt, or clay based on the sizes of the particles. The most comprehensive soil classification system is the Unified Soil Classification System (USCS), which divides soils into coarse-grained and fine-grained categories. Coarse-grained soils, such as gravel and sand, are characterized by particles larger than 0.075 mm, while fine-grained soils, like silt and clay, have 50% or more passing through the #200 sieve (diameter = 0.075 mm).

Soil Classification

The Unified Soil Classification System (USCS) is the most widely used soil classification system for most major construction projects. It divides soils into two broad categories: coarse-grained soils, which are gravelly and sandy in nature, and fine-grained soils, which have 50% or more passing through the #200 sieve. Coarse-grained soils are further classified into gravel and sand, while fine-grained soils are classified as silt, organic silts, and clays. The USCS uses symbols determined from sieve analysis and Atterberg Limit tests to identify soils.

Effective Stress

Effective stress in soil mechanics refers to the stress that contributes to the deformation of a soil mass. It is calculated by subtracting the pore water pressure from the total stress. This concept is important in understanding the behavior of soils and their response to various loading conditions.

Soil Compaction

Soil compaction occurs when soil particles are pressed together, reducing pore space between them. Heavily compacted soils contain fewer large pores, less total pore volume, and a greater density. Compacted soils have a reduced rate of water infiltration and drainage, as large pores are more effective at moving water downward through the soil than smaller pores. Gaseous exchange also slows down in compacted soils, increasing the likelihood of aeration-related problems. Soil compaction increases soil strength but also means roots must exert greater force to penetrate the compacted layer. Bulk density is a way to quantify the change in pore space and soil strength. Soils with a higher percentage of clay and silt have a lower bulk density than sandier soils.

Shear Strength

Shear strength is the resistance of a soil mass to shear forces. It is an essential parameter in geotechnical engineering, as it determines the stability of slopes, the bearing capacity of foundations, and the performance of retaining walls. Shear strength can be influenced by various factors, such as soil type, moisture content, and compaction.

Earth Pressure Theory

Earth pressure theory deals with the study of the forces acting on the surfaces of soil elements and structures, such as retaining walls, slopes, and excavations. It is based on the principles of soil mechanics and hydraulics and is used to predict the stability of earthworks and foundations.

In summary, soil mechanics is a critical field in geotechnical engineering that describes the behavior of soils. Understanding soil properties, classification, effective stress, soil compaction, shear strength, and earth pressure theory is essential for designing and constructing structures that can withstand the forces and loads imposed on them by the soil.