Civil Engineering: Soil Mechanics and Hydraulics Vocabulary

Fundamental Soil Properties and Weight-Volume Relationships

Void Ratio is defined as the ratio of the volume of void space to the volume of solid substance. Porosity is the ratio of the volume of voids to the volume of the soil sample or specimen; it represents the open space between the soil grains. The Degree of Saturation refers to the ratio of the volume of water in the void spaces to the total volume of the voids. This represents a measure of the void volume that is filled by water and is expressed as a percentage ranging from $0\,$ to $100\,$. Moisture Content is the ratio of the weight of water to the weight of solids in a given volume of soil.

Unit Weight is the weight of soil per unit volume. This concept is further categorized into several types: Dry Unit Weight is the weight per unit volume of soil excluding water. Saturated Unit Weight is defined as the bulk unit weight of a soil when it is $100\,$ saturated. Bulk Unit Weight, also known as total, wet, or moist unit weight, is the total weight divided by the total volume. In terms of hydraulic stability, the Critical Hydraulic Gradient is the gradient that brings a soil, essentially coarse-grained soils, to a state of static liquefaction.

Density is the ratio of the total mass to the total volume of a unit of soil, and in many applications, it is expressed as a unit weight where weight is interchanged with mass. Dry Density is the ratio of the mass of the solids (soil grains) to the total unit volume of soil. Submerged Density, also known as buoyant density, is the difference between the total density and the density of water. Density Index, or relative density, is the density of a granular soil relative to the minimum and maximum densities achieved for that particular soil.

Atterberg Limits and Soil Consistency

Consistency is a term used to describe the degree of firmness of soil. This behavior is influenced by Cohesion, which is the attraction of one water molecule to another resulting from hydrogen bonding (water-water bond), and Adhesion, which involves the attraction of a water molecule to a non-water molecule (water-solid bond). Stickiness describes the capacity of soil to adhere to other objects and is estimated at the moisture content that displays maximum adherence between the thumb and forefinger. Rupture Resistance serves as a field measure of the soil's ability to withstand applied stress or pressure as applied using the thumb and forefinger.

Atterberg's Limits are the limits of water content used to define soil behavior. The Liquid Limit is defined as the moisture content, expressed as a percent, required to close a distance of $12.7\,mm$ along the bottom of a groove after $25$ blows. It is the point where soil begins to behave as a liquid material and begins to flow. The Cup Method is used to determine the Liquid Limit; this device consists of a brass cup and hard rubber, where the cup is dropped onto the base by a cam operated by a crank.

Plastic Limit is the moisture content at which the soil transitions from a semi-solid to a plastic state. The Shrinkage Limit is the moisture content at which the transition from solid to semi-solid takes place; it is defined as the moisture content at which no further volume change occurs even with a further reduction in moisture content. The Plasticity Index is the difference between the liquid limit and the plastic limit of a soil. The Shrinkage Index is the difference between the plastic and shrinkage limits.

Soil Consistency Indices and Origins

Liquidity Index is a ratio that signifies the relative consistency of a cohesive soil in its natural state. Plasticity is the degree to which a soil can be molded or reworked, causing permanent deformation without rupturing. The Consistency Index is the ratio of the difference between the liquid limit and water content to the difference between the liquid limit and the plasticity index. Skempton defined Activity as the ratio of the plasticity index to the percent of clay-sized fraction by weight.

Soils are classified based on their mode of formation and transportation. Residual Soil consists of soils formed by weathered products at their place of origin. Lacustrine Soil is formed by deposition in quiet lakes. Alluvial Soil is transported by running water and deposited along streams. Glacial Soil is formed by the transportation and deposition of glaciers. Marine Soil is formed by deposition in the seas. Aelian Soil refers to soils transported and deposited by wind. Gravel Soil is characterized by occasional particles of quartz, feldspar, and other minerals.

Soil Classification and Particle Size Analysis

Sieve Analysis is used to determine the grain size distribution of coarse-grained soil, while Hydrometer Analysis is used for soils passing the No. 200 sieve. Effective Size is the grain size diameter in the particle size distribution curve corresponding to $10\,$ finer ($D_{10}$). It is used to measure hydraulic conductivity and drainage. The Particle-Size Distribution Curve is used to determine four parameters for a given soil.

The Coefficient of Uniformity ($C_u$) is the ratio between the grain diameter corresponding to $60\,$ passing ($D_{60}$) and the grain diameter of the $10\,$ passing ($D_{10}$). The Coefficient of Gradation (or Coefficient of Curvature, $C_D$) is defined as the ratio between the square of the grain diameter corresponding to $30\,$ passing ($D_{30}$) divided by the product of $D_{60}$ and $D_{10}$: $CD = \frac{(D_{30})^2}{D_{10} \times D_{60}}$. Poorly Graded soil is a type where most soil grains are the same size.

Major classification systems include the AASHTO System, originally proposed by the Highway Research Board’s Committee on Classification of Materials for Subgrades and Granular Type Road. It classifies soil into eight major groups, A-1 through A-8, based on grain size distribution, liquid limit, and plasticity indices. The USCS System (Unified Soil Classification System) uses a two-letter symbol to describe texture and grain size. Under USCS, Coarse-grained soils have more than $50\,$ by weight retained on the #200 sieve ($0.075\,mm$). Fine-grained soils are silts and clays containing particles smaller than the No. 200 sieve. Cobbles are particles larger than $75\,mm$. The USDA System is also called the Textural classification system. Clay is defined as soil particles finer than $0.002\,mm$.

Soil Compaction and Permeability

Compaction is the volume change in soils where air is expelled from the voids while the water content remains constant. It can occur due to vibration or self-weight, and in construction, it is achieved by rolling, tamping, or vibrating. Soils are compacted to a dense state for four reasons: 1. To increase shear strength; 2. To decrease future settlements; 3. To decrease permeability; and 4. To increase the stability of slopes of embankments. Standard field procedures to determine unit weight include the Nuclear method, Sand cone method, and Rubber balloon method. The Proctor Compaction Test is the laboratory method used to obtain the maximum dry unit weight and the Optimum Moisture Content.

Permeability is the property that permits the flow of water or other liquids through the soil. Hydraulic Conductivity (Coefficient of Permeability) is the constant average discharge velocity of water when the hydraulic gradient equals $1.0$. Clays are considered relatively impervious, while sands and gravels are pervious. Factors affecting the coefficient of permeability include fluid viscosity, pore size distribution, grain size distribution, degree of saturation, and the roughness of soil particles.

Specific tests for permeability include the Constant Head Test for coarse-grained soils and the Falling Head Test for fine-grained soils. Transmissivity (or Transmissibility) is the ability of an aquifer to transmit water through its entire thickness. Discharge Velocity is the quantity of water flowing in unit time through a unit gross cross-sectional area at right angles to the flow. Drawdown is the magnitude of the lowering of a water table, usually near a pumped well.

Stress, Flow Nets, and Stress Distribution

Effective Stress is the sum of the vertical components of the forces developed at the points of contact of solid particles per unit cross-sectional area. Total Stress is the sum of effective stress and neutral stress (pore water pressure). Pore water pressure is the stress induced by water pressure, also known as neutral stress. Intergranular Stress results from particle-to-particle contact. Capillary Rise is the height water rises above the water table due to negative pore water pressure. Capillary Stresses are pressures less than atmospheric values produced by surface tension acting on the meniscus in void spaces. A Piezometer is an instrument used to measure in-situ pore water pressures.

Flow Nets are combinations of Flow Lines (the path a water particle travels from upstream to downstream) and Equipotential Lines (lines along which the potential head is equal at all points). In terms of stress distribution, the Boussinesq equation is used to determine the increase in vertical pressure at a particular depth caused by a point load at a given surface.

Soil Compressibility and Consolidation

Compression in soil layers is caused by the deformation of particles, relocation of particles, and the expulsion of water or air from void spaces. The three categories of settlement are Primary Consolidation Settlement (volume change in saturated cohesive soils due to water expulsion), Secondary Consolidation Settlement (plastic adjustment of soil fabrics), and Immediate Settlement (elastic deformation of soil without moisture content change).

A Normally Consolidated clay is one where the present effective overburden pressure is the maximum pressure it has ever been subjected to. Over Consolidated clay is one whose present effective overburden pressure is less than the maximum pressure it has been subjected to in the past. The Overconsolidation Ratio (OCR) is the ratio of preconsolidation pressure to present effective overburden pressure. The Compression Index is the logarithmic slope of the primary consolidation curve. The Swell Index is smaller in magnitude than the compression index. The Coefficient of Consolidation generally decreases as the liquid limit of the soil increases.

Shear Strength of Soil

Soil derives its strength from its capacity to resist shear. Laboratory tests include the Direct Shear Test (the oldest and simplest method) and the Triaxial Shear Test (the most reliable method). Triaxial tests include Consolidated-Drained (CD), Consolidated-Undrained (CU), and Unconsolidated-Undrained (UU) tests. The Unconfined Compressive Strength Test is performed on plastic soils like clay; the undrained shear strength ($c_u$) is calculated as $1/2$ of the unconfined compressive strength.

Coulomb's Equation relates shear strength to the normal effective stress on the failure plane. Deviator Stress is the difference between axial and radial stresses in a triaxial test. Undrained Shear Strength is the strength of saturated soil under conditions where no drainage of pore water occurs and is independent of applied stresses if the void ratio remains constant. The Angle of Shearing Resistance is the ratio of effective shear and normal stresses mobilized prior to failure. Mohr’s Circle is used to analyze stresses. Normal Force acts normal to the plane, while Shear Stress acts tangentially. Principal Stresses act in the direction of the principal axes.

Lateral Earth Pressure and Retaining Walls

Earth Pressure is the force per unit area exerted by soil on a retaining wall. The Active Earth Pressure Coefficient is the ratio used when a wall moves away from the soil, while the Passive Earth Pressure Coefficient is used when the wall is forced against the soil. Coulomb Earth Pressure Theory includes friction between soil and structure and assumes failure along a flat plane. Tension Cracks appear at the surface near a wall or slope; the tension crack depth is the point where horizontal effective stress is zero. Failure modes include Overturning (toppling or rotating due to lateral pressure) and Sliding (moving away from the soil).

Retaining walls are designed to prevent lateral movement. Types include:

• Gravity Retaining Wall: Depends on its own weight for stability.
• Semi-gravity Wall: A gravity wall with a wider base (toe or heel) and some reinforcement.
• T-shaped Wall: The most common cantilever wall where backfill weight contributes to stability.
• L-shaped Wall: Used when property line restrictions prevent a heel.
• Counterfort Retaining Wall: Uses intermittent vertical ribs (counterforts) acting as tension ties between the base and stem.
• Buttressed Wall: Similar to counterfort but ribs are on the front face and act in compression.
• Bridge Abutment: Short wall typically accompanied by wing walls.

Bearing Capacity of Foundations

A Foundation transmits building loads to the underlying soil. Footings consist of small slabs for this purpose. A Shallow Foundation is one where the ratio of embedment depth ($D_f$) to width ($B$) is low (typically $D_f / B \leq 2.5$ or $D_f \leq B$ according to Terzaghi). Types include Circular Footings, Strip (Continuous) Footings which support walls, and Spread (Isolated) Footings which support a single column. Mat (Raft) Foundations are structural slabs encompassing the entire building footprint, advantageous for compressible soils.

Ultimate Bearing Capacity is the maximum pressure the soil can support before shear failure. Ultimate Net Bearing Capacity is the maximum pressure above the current pressure. Allowable Bearing Capacity (safe bearing capacity) ensures a margin of safety. The Factor of Safety is the ratio of ultimate net bearing to allowable bearing capacity. Overburden Pressure is the effective stress of the soil removed to place the footing. Meyerhof proposed correlations using standard penetration resistance, while Hansen proposed a general bearing capacity equation. Skin-friction capacity refers to the resistance found on the shaft of a deep foundation system.

Slope Stability and Piles

Slope Stability Analysis involves identifying the Critical Circle (the slip circle with the lowest factor of safety). Critical Height and Critical Ground Slope Angle correspond to a factor of safety of $1.0$. Failure types include Base Failure (sliding surface below the toe) and Slope Failure (sliding surface intersects the slope or above the toe). The Angle of Repose is the maximum angle of a slope of granular material before failure. Angle of Slip Plane is the angle where a rupture may occur.

Piles are slender members of a deep foundation system (timber, steel, or concrete) driven, drilled, or jetted into the ground. A Friction Pile derives most of its load-bearing ability from skin friction. Pile Efficiency is the ratio of the average ultimate load in a group to the individual ultimate load on a pile. Braced Excavation and Sheet Piles (steel panels) provide lateral support for temporary trenches or cuts. Liquefaction is the process where saturated soil temporarily loses strength and acts as a fluid, often during seismic events.

Questions & Discussion

Question: Which of the following is not a component of the soil mass? Answer: None on the list. The components listed (Gas, Organic Matter, Minerals) are all potential components of a soil mass.

Question: What are the three categories of soil compression? Answer: Primary consolidation settlement, secondary consolidation settlement, and immediate settlement.

Question: What does the coefficient of permeability depend on? Answer: It depends on fluid viscosity, pore size distribution, grain size distribution, degree of saturation, and the roughness of soil particles.

Question: What defines a shallow foundation according to Terzaghi? Answer: A foundation is considered shallow if the depth of the foundation is less than or equal to the width of the foundation ($D_f \leq B$).