University of the Future, College of Engineering, Department of Civil Engineering. Prepared by: Lecturer Lina Hussein Ali. Geotechnical engineering studies the behavior of soil and rock as a construction material and as a medium that supports structures. It is a branch of civil engineering concerned with the physical, mechanical and hydraulic properties of soil and rock, and it aims to predict their behavior under loads and varying environmental conditions. This article reviews the theoretical foundations of soil mechanics, including the principle of effective stress, shear strength and consolidation, and links them to their applications in site investigation, foundation design, slope stability and soil improvement. It concludes that the safety of a structure depends on the accuracy of soil characterization as much as on structural analysis, and it highlights the challenges posed by gypseous soils and soft clay soils in Iraq. (Keywords: soil mechanics, effective stress, shear strength, bearing capacity, consolidation, soil improvement, gypseous soils.) Karl Terzaghi is regarded as the founder of this science in its modern form. He published his book Erdbaumechanik in 1925, in which he set out the principle of effective stress and the theory of one-dimensional consolidation. The importance of this discipline lies in the fact that every civil structure, including buildings, bridges, dams, tunnels, roads and embankments, ultimately transfers its loads to the soil, and a large share of structural failures can be traced to problems in the soil or the foundations. The article therefore aims to present the theoretical concepts governing soil behavior, link them to investigation and design practice, and review soil improvement techniques and recent research trends. Any geotechnical analysis is built on an accurate characterization of the soil. Soil is a three-phase system consisting of solid particles, water and air filling the voids between them. Phase relationships connect the void ratio e, the water content w, the degree of saturation S and the specific gravity Gs, the best known being S·e = w·Gs. The distribution of particle sizes is determined by sieve analysis for coarse soils and by hydrometer analysis for fine soils, and from the resulting curve the coefficient of uniformity Cu = D60/D10 and the coefficient of curvature Cc = D30²/(D10·D60) are calculated. The consistency of fine soils changes with their water content, so the Atterberg limits are used: the liquid limit LL, the plastic limit PL and the shrinkage limit SL. From these the plasticity index PI = LL − PL is derived, a key indicator of compressibility and swelling potential. For classification, the Unified Soil Classification System (USCS), standardized in ASTM D2487, is the most widely used for structural purposes, while the AASHTO system is used in road works. The USCS designates coarse soils, those with more than 50% retained on the No. 200 sieve (0.075 mm), with the symbols GW, GP, SW, SP, SM and SC. It designates fine soils, those with 50% or more passing the same sieve, with ML, CL, MH and CH, and highly organic soils are denoted Pt. Soil mechanics rests on several fundamental principles, the first of which is the principle of effective stress, its cornerstone, formulated by Terzaghi. It states that compression and shear strength are governed by the effective stress σ′, not the total stress σ, according to the relation σ′ = σ − u, where u is the pore water pressure. This principle explains why a rise in the groundwater table reduces soil strength, and why liquefaction occurs in saturated sands during earthquakes. Soil usually fails in shear rather than in compression or tension, and its strength is expressed by the Mohr-Coulomb criterion in terms of effective stresses: τf = c′ + σ′·tan φ′, where c′ is the effective cohesion and φ′ is the effective angle of internal friction. The engineer distinguishes between two conditions. The first is the short-term undrained condition, in which the undrained shear strength cu is used for clay soils immediately after rapid loading. The second is the long-term drained condition, in which the effective parameters c′ and φ′ are used after the excess pore water pressure has dissipated. Consolidation is the gradual compression of saturated clay soils over time as water is expelled from their low-permeability pores. The primary consolidation settlement of a normally consolidated clay is calculated from the relation Sc = [Cc·H/(1 + e0)]·log10[(σ′0 + Δσ′)/σ′0], where Cc is the compression index, H is the layer thickness and e0 is the initial void ratio. The rate of consolidation is governed by the coefficient of consolidation cv and the drainage path length Hdr according to Terzaghi’s one-dimensional theory: t = Tv·Hdr²/cv. Site investigation is the first step in any project. It aims to determine the sequence of soil layers and the groundwater level and to obtain the design parameters, and the quality of these data determines the reliability of every analysis that follows. Among the in-situ tests, the Standard Penetration Test (SPT) measures the number of blows N required to drive the sampler 300 mm and is used to estimate sand density and bearing capacity and to assess liquefaction. The Cone Penetration Test (CPT/CPTu) measures tip resistance, sleeve friction and pore pressure, and provides continuous layer profiling and data for pile design. The in-situ vane test measures the undrained strength cu of soft clay. The plate load test shows the load-settlement relationship and is used for the bearing capacity of shallow foundations. Laboratory tests include the triaxial test (UU, CU and CD), which gives c, φ and the stress path and is used for slope and foundation stability. The direct shear test gives c and φ on a predetermined plane and suits granular soils and interfaces. The consolidation (oedometer) test gives Cc, Cr, cv and the preconsolidation stress for calculating settlement and its timing. The unconfined compression test gives qu, from which cu = qu/2, as a quick estimate for saturated clay. The number and depth of boreholes are determined by the type of structure, its loads and the variability of the site, and the depth is usually required to reach the zone where the added stress becomes small compared with the existing effective stress. The foundation design must satisfy two conditions at the same time: adequate safety against shear failure, and total and differential settlement within permissible limits. The settlement condition is often the governing one in clay soils. Shallow foundations include isolated footings, strip footings and mat foundations. Their ultimate bearing capacity is calculated with Terzaghi’s equation for a strip footing, which Meyerhof, Hansen and Vesić developed by adding shape, depth and inclination factors: qu = c·Nc + q·Nq + ½·γ·B·Nγ. Here q is the effective stress at foundation level, B is the footing width, and Nc, Nq and Nγ are bearing capacity factors that depend on φ. qu is divided by a factor of safety, usually between 2.5 and 3, to obtain the allowable bearing capacity. Deep foundations (piles) are used when the surface layers are weak or highly compressible, or when loads are large. A pile transfers its load through end-bearing resistance Qb and shaft friction Qs: Qu = Qb + Qs = qb·Ab + Σfs·As. Shaft friction in clay is calculated by the α method (in terms of cu) or the β method (in terms of effective stress), and it is always preferable to verify the design with pile load tests on site. Modern codes are moving from a single factor of safety toward the limit state approach, examples of which are Eurocode 7 in Europe and the LRFD method in the United States, where separate partial factors are applied to loads and to soil resistance. Slope stability is assessed by a factor of safety, defined as the ratio of the available shear strength to the shear stress required for equilibrium along the critical slip surface, FS = τf/τd. The most common methods in practice are the slice methods, including the Fellenius method, the simplified Bishop method and the Morgenstern-Price method. Factors affecting stability include pore water pressure, rainfall, seismic loading, rapid drawdown in earth dams, and the progressive deterioration of the strength of fissured clay over time. Retaining walls are designed to resist the lateral pressure of the soil, and the magnitude of this pressure depends on the direction and amount of wall movement. These are the active pressure Ka when the wall moves away from the soil, the at-rest pressure K0 when there is no movement, and the passive pressure Kp when the wall is pushed toward the soil. According to Rankine’s theory, for a cohesionless soil with a horizontal surface and a smooth vertical wall, Ka = tan²(45° − φ′/2) and Kp = tan²(45° + φ′/2). A retaining wall is checked for three failure modes, which are sliding, overturning and exceeding the bearing capacity of the soil beneath the base, in addition to the overall stability of the site. When natural soil does not meet the project requirements, the engineer chooses between replacing it, using deep foundations, or improving it in place. The choice of technique depends on the soil type, the required depth, the cost and the available time. Surface compaction and dynamic compaction reduce voids through mechanical energy and suit granular soils and fill. Vibro-compaction and stone columns densify the soil or add stronger columns and drainage paths, and suit loose sands and medium-consistency clay. Preloading with prefabricated vertical drains (PVD) accelerates consolidation by shortening the drainage path and suits saturated soft clay. Chemical stabilization with lime, cement or fly ash relies on ion exchange and pozzolanic reactions and suits plastic and expansive clay and road layers. Grouting, deep mixing and jet grouting fill voids or form soil-cement columns and suit a wide range of soils, especially beneath existing structures. Reinforcement with geosynthetics adds tensile strength and provides separation and drainage, and is used in embankments, reinforced walls and roads. Microbially induced calcite precipitation (MICP) deposits calcium carbonate between particles and suits sands, although it is still mostly at the research stage. Post-construction verification remains an essential part of any improvement program, whether through SPT and CPT tests or through load tests and settlement monitoring. Among recent research trends, the finite element method is used with advanced constitutive models (Cam-Clay and Hardening Soil) to simulate soil-structure interaction. Probabilistic and reliability analysis treats the natural variability of soil properties as a random variable instead of a single representative value. Artificial intelligence and machine learning are used to predict soil parameters from in-situ test results and to estimate settlement. Fiber-optic sensors and remote sensing (InSAR) are used for smart monitoring of settlement and slope movement. In the field of sustainability, the use of cement in stabilization is being reduced and industrial waste such as fly ash and slag is being recycled. Gypseous soils are widespread across large areas of central and western Iraq. They show good strength when dry, but they lose much of that strength and undergo collapse settlement when saturated with water, because the gypsum dissolves. Structures built on them therefore require drainage control and prevention of water seepage, and may sometimes require deep foundations. Soft clay soils are found in the alluvial plain and southern Iraq, where the groundwater table is shallow, which increases consolidation settlement. As for salts and sulfates, the dissolved salts increase the risk of chemical attack on foundation concrete, which calls for salt-resistant cement and suitable insulation layers. In its conclusion, the article shows that geotechnical engineering rests on three interrelated pillars: accurate soil characterization, a theoretical understanding of soil behavior based on the principle of effective stress, and design practice that balances safety and cost. Soil remains a variable natural material whose properties cannot be specified in advance as they can for concrete or steel, so engineering judgment and monitoring during construction remain indispensable. This is what Peck expressed through the Observational Method. The article recommends strengthening site investigations in local projects, including collapse potential testing for gypseous soils, building a national geotechnical database to support probabilistic analysis and machine learning models, and encouraging research into sustainable improvement techniques suited to the Iraqi environment. References: Terzaghi, Peck & Mesri (1996), Soil Mechanics in Engineering Practice, 3rd ed., John Wiley & Sons; Das & Sobhan (2018), Principles of Geotechnical Engineering, 9th ed., Cengage Learning; Knappett & Craig (2019), Craig’s Soil Mechanics, 9th ed., CRC Press; Holtz, Kovacs & Sheahan (2011), An Introduction to Geotechnical Engineering, 2nd ed., Pearson; Bowles (1996), Foundation Analysis and Design, 5th ed., McGraw-Hill; Mitchell & Soga (2005), Fundamentals of Soil Behavior, 3rd ed., John Wiley & Sons; Peck (1969), Géotechnique, 19(2), 171–187; ASTM D2487-17 (reapproved 2025); EN 1997-1:2024 (Eurocode 7).