Introduction to Soil
Soil is one of the most important materials in civil engineering because almost every construction project is connected to the ground. Roads, buildings, bridges, pipelines, embankments, retaining walls, and foundations are all constructed either on soil or with soil as a construction material. In simple terms, soil is a natural material made up of small solid particles, with spaces between these particles. These spaces are called voids and may contain water, air, or both.
From a civil engineering point of view, soil can be considered as a three-phase material consisting of solids, water, and air. The solid phase represents the actual soil particles, while the spaces between the particles form the voids. When the voids contain both air and water, the soil is called partially saturated. When all the voids are filled with water, the soil is called fully saturated. When there is practically no water in the voids, the soil is considered dry. Understanding this three-phase system is very important because many important soil properties, such as density, moisture content, void ratio and degree of saturation, are based on the relationship between these three phases.
Soil is mainly formed from rocks through a natural process called weathering. Weathering breaks down rocks into smaller particles. Physical weathering breaks rocks into smaller pieces without significantly changing their chemical composition, while chemical weathering changes the minerals within the rock. Over a long period of time, these processes produce different types of soil particles. Depending on the geological conditions and the process of formation, soils can have very different characteristics.
The size of soil particles is an important property used in soil classification. In general, soil particles can be divided into gravel, sand, silt, and clay. Gravel consists of relatively large particles, while sand particles are smaller than gravel. Silt particles are much finer than sand, and clay particles are extremely fine. Therefore, in terms of general particle size, the order is gravel, sand, silt, and clay, from larger to smaller particles. However, it is important to understand that clay is not defined only by its particle size. Its mineral composition and plastic behavior are also important.
Gravel is a coarse soil material consisting of relatively large particles. It normally has good drainage and relatively high permeability because water can pass through the spaces between the larger particles. Gravelly materials are commonly used in civil engineering for applications such as selected fill, drainage layers, sub-base materials and other earthwork applications, provided they meet the requirements of the project specification. The presence of large particles does not automatically mean that the material is suitable; its grading, cleanliness, strength and other specified properties must also be checked.
Sand consists of smaller particles than gravel and is commonly found in natural deposits and construction materials. Sand generally has good drainage characteristics because water can move through the spaces between the particles. Unlike clay, sand normally does not exhibit significant plasticity. However, not every sand is suitable for construction. Its gradation, percentage of fines, cleanliness, density and other properties must be checked against the project requirements before it is accepted for use.
Silt consists of particles finer than sand but generally coarser than clay. Silty soils can be sensitive to changes in moisture and may have relatively poor engineering behavior compared with well-graded granular materials. When water conditions change, the strength and stability of silty soil can also change. For this reason, the presence of excessive fines or silt in construction fill may need to be controlled according to the project specification.
Clay is a very fine-grained soil that contains clay minerals. Clay is particularly important in soil engineering because it can show plastic behavior and can be strongly affected by changes in moisture content. When clay becomes wet, it can become soft and lose strength, while drying can cause shrinkage. Some clay soils can also swell when they absorb water and shrink when they lose water. These volume changes can cause problems in roads, foundations, and other structures if the soil is not properly investigated and controlled.
Another important basic concept is the difference between cohesive and cohesionless soils. Cohesive soils, such as clay, have significant interparticle attraction and can exhibit plasticity. Their behavior is strongly influenced by water content. Cohesionless soils, such as sand and gravel, mainly obtain their shear strength from friction between particles and particle interlocking. This difference is important because cohesive and cohesionless soils behave differently during excavation, compaction, loading, and water movement.
The spaces between soil particles are called voids. These voids are very important because they influence many engineering properties of soil. For example, if a soil contains a large amount of void space, its density may be relatively low. During compaction, external energy is applied to rearrange the soil particles and reduce the amount of air-filled voids. As the particles become more closely packed, the dry density of the soil generally increases. This is one of the main reasons why compaction is required during road and earthwork construction.
Water is another major factor controlling soil behavior. A change in moisture content can significantly affect the strength, workability, and density of soil. For example, if soil is too dry, it may not compact properly. If it contains excessive water, the soil may become soft and difficult to compact. This is why the Optimum Moisture Content (OMC) obtained from the Proctor compaction test is important. It provides an indication of the moisture condition at which a particular soil can achieve its maximum dry density under the specified laboratory compaction effort.
In construction QA/QC, soil testing is performed to determine whether the material is suitable and whether the completed earthwork meets the project requirements. Tests such as sieve analysis are used to determine particle-size distribution, while moisture content testing determines the amount of water present in the soil. Specific gravity helps determine the density relationship of the soil particles, and Atterberg limits are used to evaluate the plasticity characteristics of fine-grained soils. The Proctor test establishes the maximum dry density and optimum moisture content, while field density testing is used to check the actual compaction achieved at site.
For example, if the project specification requires a minimum compaction of 95% of the laboratory maximum dry density, the Material Engineer cannot determine compliance simply by looking at the completed fill. A laboratory Proctor test first establishes the reference maximum dry density. A field density test is then carried out on the compacted layer to determine its actual dry density. The field result is compared with the laboratory value to calculate the percentage of compaction. This demonstrates the important relationship between laboratory testing, field testing, and QA/QC acceptance.
Soil is therefore much more than simply "earth" or "dirt." It is an engineering material whose behavior depends on particle size, particle arrangement, mineral composition, moisture content, density, and other factors. A good material or QA/QC engineer must understand these basic properties before moving into advanced topics such as soil classification, compaction, permeability, shear strength, consolidation, and bearing capacity.
Chapter 1 — Key Concept
The most important concept to remember from this chapter is that soil consists of solid particles and voids, while the voids may contain water and air. The type and arrangement of the particles, together with the amount of water and air present, control much of the soil's engineering behavior. Understanding this basic concept will make the following chapters—especially soil classification, sieve analysis, moisture content, and compaction—much easier to understand.
Standards relevant to Chapter 1
When we introduce soil and its basic properties, the main standards you should know are:
ASTM D2487 — Standard Practice for Classification of Soils for Engineering Purposes (USCS). This is one of the most important standards for soil classification. It classifies soils based on particle-size distribution and plasticity characteristics.
ASTM D2488 — Standard Practice for Description and Identification of Soils (Visual-Manual Procedure). This is used for describing and identifying soil based on visual and manual examination. It is particularly useful for practical site work.
BS 1377—Methods of test for soils for civil engineering purposes. This is a major British Standard series covering soil testing. Different parts deal with different properties and tests, such as moisture content, particle-size distribution, density, compaction, permeability, and other soil characteristics.
BS 5930 — Code of practice for ground investigations. This is important from a broader civil engineering perspective because it covers ground investigation and the description and characterization of soil and rock encountered during investigations.
AASHTO M 145 — Classification of Soils and Soil-Aggregate Mixtures for Highway Construction Purposes. This is particularly important for road and highway projects because the AASHTO soil classification system is widely used for evaluating soils for highway applications.
The important point is that ASTM D2487/BS 1377/AASHTO M 145 are not interchangeable. They are different systems or standards used for different purposes. For example, USCS under ASTM D2487 and the AASHTO classification system can give different classifications to the same soil because they are designed for different engineering applications.