Prepared and presented by: Asst. Eng. Lina Hussein Ali Rapporteur, Department of Civil Engineering, College of Engineering, Al-Mustaqbal University When we look at a soaring bridge, a tall tower, or a massive dam, our attention usually goes to the structural elements that are visible. Yet the safety of these structures begins below the ground surface, where soil and rock serve as the foundation that receives loads and distributes them. Hence geotechnical engineering emerges as a fundamental branch of civil engineering, concerned with studying the behavior of soil and rock and their interaction with structures, and with designing solutions that ensure both safety and economy. This discipline became established as an independent science in the first half of the twentieth century through Karl Terzaghi, who is regarded as the founder of modern soil mechanics. He formulated the principle of effective stress, which explains the behavior of water-saturated soil and is among the most important principles in this field. The principle holds that the strength and deformation of soil are governed not by total stress alone, but by the stress carried by the soil grains after subtracting the pore water pressure. Analysis of settlement, stability, and shear strength in most geotechnical problems is built upon this principle. Any geotechnical problem begins with site investigation, which includes exploratory drilling, sampling, and field tests such as the Standard Penetration Test and the Cone Penetration Test, in addition to laboratory tests to determine soil properties, including classification, compressibility, shear strength, and permeability. These data form the basis on which the engineer relies in selecting the type of foundation, whether shallow, such as isolated and mat footings, or deep, such as piles, and in estimating bearing capacity and expected settlement. Any neglect at this stage may result in a design that is unsafe or excessively costly. Perhaps the clearest illustration of the importance of this specialty is what history has shown in cases of failure and rescue. The Leaning Tower of Pisa remained for centuries an example of the effect of differential settlement in a soft clay layer, and it was stabilized only after a precise geotechnical intervention carried out at the beginning of this century by extracting part of the soil from beneath the higher side. In the 1964 Niigata earthquake, entire residential buildings tilted without their structural elements collapsing, because of the phenomenon of liquefaction, in which saturated sandy soil lost its strength. In the 1985 Mexico City earthquake, soft clay deposits amplified ground vibrations, causing damage to buildings far from the epicenter. These examples show that soil behavior can be the decisive factor in a structure’s performance. The fields of geotechnical engineering extend to include the design of retaining walls and deep excavations, the stability analysis of slopes and earth dams, tunnels and underground structures, and the study of geotechnical earthquake engineering. They also include improving weak soils by various methods, among them grouting, dynamic compaction, stabilization with additives, and reinforcement with geosynthetic materials. These techniques make it possible to use sites that were considered unsuitable for construction at a reasonable cost. In Iraq, this discipline is of particular importance, owing to the diversity and challenges of the country’s soils. Wide areas in the central and southern regions consist of soft clayey and silty deposits with a high groundwater level, and these soils are prone to large settlement over time. In other regions, gypsiferous and saline soils are widespread; these may be affected by moisture and lose part of their strength, in addition to the harmful effect of salts on concrete and reinforcing steel. This requires that design decisions be based on careful site investigation, not on assumptions taken from other sites. Geotechnical research today is moving toward sustainability, investigating ways to reduce the carbon footprint of earthworks and foundations, using industrial waste in soil improvement, and developing thermal foundations that harness geothermal energy. There is also increasing reliance on numerical modeling using the finite element method, and on modern technologies such as remote sensing and artificial intelligence in analyzing data and predicting soil behavior. There is also a growing need to study the effect of climate change on slope stability and the behavior of expansive soils, as a result of changing patterns of rainfall and drought. In conclusion, geotechnical engineering is the first link in the structural safety chain, and the success of any engineering project is closely tied to understanding the ground on which it stands. This calls for strengthening education and research in this specialty, giving site investigation the attention and funding it deserves, and encouraging collaboration between the geotechnical engineer and the structural engineer from the earliest stages of design. General References: Terzaghi, K., Peck, R. B., & Mesri, G. Soil Mechanics in Engineering Practice. Wiley. Das, B. M. Principles of Foundation Engineering. Cengage Learning. Kramer, S. L. Geotechnical Earthquake Engineering. Prentice Hall.