Geotechnical engineering

Geotechnical engineering is a branch of engineering science concerned with the mechanics, design, and construction of structures and systems made from or supported by soil or rock. Geotechnical engineering encompasses numerous fields such as geology, soil mechanics, and structural engineering. The science has its roots in the methods used by humans to build some of the most recognizable structures of the ancient world. In the modern era, geotechnical engineering has a wide range of applications. Some of the more common include engineering to protect against earthquakes, engineering dams, ensuring the stability of excavations, dealing with soil erosion, and cleaning up contaminated sites.

Background

From the early days of civilization, humans have tried to shape the world around them by altering the landscape and building structures from stone. To protect against flooding and provide water for their cities, the ancient Mesopotamians and Egyptians built flood walls, dams, and canals. Later generations constructed large stone temples and tombs. Some, like the pyramids at Giza in Egypt, are engineering marvels that have survived for thousands of years. Others fell victim to time or experienced engineering shortfalls that made them unstable or affected their construction. For example, the Meidum pyramid in Lower Egypt was plagued by construction errors and design challenges, leading to its collapse. The Dahshur pyramid south of Cairo has a "bent" upper section because its builders realized during construction its sides were too steep to remain stable.

During the early construction of the Leaning Tower of Pisa in the twelfth century, architects began to notice the structure's foundation was sinking into the soft soil underneath. War halted work on the project for nearly a century, but when it resumed, architects tried to compensate for the problems by building the upper stories slightly higher on one side. Eventually completed in the fourteenth century, the tower continued to subside and tilt until the late twentieth century. At that time, geotechnical engineers removed soil from under one end of the tower to stop its tilt and stabilize the structure.

Engineering mishaps such as the Leaning Tower of Pisa inspired architects during the later medieval and Renaissance periods to begin using scientific principles as a way to eliminate similar problems. Early work in the field of geotechnical mechanics began in the eighteenth century with French scientist Charles-Augustin de Coulomb. Coulomb's study of the effects of friction on soil led to a breakthrough in structural engineering. The birth of modern geotechnical engineering is credited to the work of civil engineer Karl von Terzaghi, whose 1925 book on soil mechanics revolutionized dam construction.

Overview

Geotechnical engineering is a large field with concerns spanning soil and rock mechanics, foundation construction, earthwork construction, and coastal and offshore engineering. A geotechnical engineer may examine the soil and rock conditions around a proposed project to determine the best way to design the foundation and proceed with construction. Engineers can evaluate a project site by simple observation and collecting soil samples, or may need more advanced tools such as subsurface drilling and photographic mapping.

Soil composition is a major element in determining the feasibility of a construction project. Soil stability varies by its origin, the size of its grains, the amount of air or water it can hold, and the amount of stress it can withstand. By examining the surrounding soil, geotechnical engineers can determine if an area can support the weight and stress of a structure and how much the soil will settle over time. Geotechnical engineering also takes into account the surrounding geography, such as the temperature of the soil, depth of the bedrock, and the stability of nearby slopes.

Bridge projects offer a different set of challenges than tunnels or dam projects, as do foundations built for above-ground structures compared with offshore or deep foundations. For example, shallow foundations for smaller buildings transfer the weight of the structure into the ground surface. Bigger buildings or those built on soil that is too weak may need deeper foundations. These foundations typically use steel or concrete support columns, piers, or drilled shafts to transfer the weight of the structure deeper into the ground.

One of the many practical uses of geotechnical engineering is in designing structures to withstand earthquakes. Earthquakes cause violent ground shaking that can quickly undermine the stability of structures in the affected area. In some cases, earthquakes can also "liquefy" the surrounding soil, causing it to act like a fluid. In 1971, soil liquefaction resulting from an earthquake nearly caused the catastrophic breach of a dam near San Fernando, California.

While the occurrence of earthquakes may be unpredictable, their location is not, allowing geotechnical engineers to design building foundations that can remain stable in earthquake-prone areas. Foundations must be built strong enough to withstand both the shaking ground and changes in soil pressure caused by potential liquefaction. At the same time, they must also be able to dissipate or absorb the kinetic energy from the seismic vibrations. In most cases, building foundations are designed to sway and move with the force of the earthquake.

Geotechnical engineering in the design and construction of dams must take into account not only the characteristics of the surrounding soil and rock but also the effect of the water pressure on the structure's stability. Arch dams are a type of dam that distributes the force of the water into the surrounding valley walls or floor. A gravity dam relies on the weight of the concrete to absorb the water's force. A buttress dam transfers the pressure into its foundation. An embankment dam made of soil or rock must have a strong central core to absorb the force.

The Hoover Dam, constructed in the 1930s on the border of Nevada and Arizona, is an example of both an arch dam and a gravity dam. The dam is 726 feet (221 meters) tall, 1,244 feet (379 meters) wide, and weighs more than 6.6 million tons. The water at the base of the dam exerts a pressure of about 45,000 pounds per square foot (219,709 kilograms per square meter). During construction of the dam, the sheer amount of concrete used generated heat that could have cracked the dam. Engineers working on the project devised a way to cool the drying concrete by circulating cold water through small pipes embedded within the structure.

Geotechnical engineers designing offshore structures, such as oil platforms or bridges over bodies of water, have to contend with different soil composition in addition to the force of water. Soil on the ocean floor is sedimentary, which is formed by deposits of fine particles accumulating over long periods of time. The soil in rivers and lakes may also be sedimentary. The thickness of the sediments may be affected by the dynamic nature of moving water and may vary depending on location. Watertight chambers known as caissons are typically used as support columns or piers for foundations built in water. The hollow column is embedded into the sea or river bed and the water is pumped out, allowing for a dry, air-filled workspace.

In addition to designing and constructing a stable foundation, geotechnical engineers must also take into account the effects of erosion and scour over the lifetime of a structure. Erosion is the wearing away of the ground surface caused by rain, wind, or ice. Scour is caused by the movement of water around a stationary object. Erosion may occur slowly over the course of years or more rapidly in the span of weeks or months. Some catastrophic events, such as hurricanes or floods, may cause serious erosion in a matter of hours.

Erosion or scour may eat away at the soil surrounding a building, leading to possible collapse of shallow foundations and decreased stability of deeper foundations. The changing nature of the soil from erosion or scour may also cause a foundation to settle prematurely. In coastal areas and areas prone to flooding, geotechnical engineers must often design structures with stronger and more anchored foundations to protect against possible erosion and scour.

In cases where the soil has been polluted with natural or human-made contaminants, geotechnical engineers are among those tasked with evaluating the situation and finding a cleanup solution. Engineers must analyze the composition of the surrounding soil to determine if the contaminants may seep into any nearby groundwater. They must also devise a strategy to safely remove the contaminants from the soil. In some cases, geotechnical engineers can remove pollutants without excavating the ground. With highly polluted areas, the engineers may be responsible for the safe removal and proper disposal of the contaminated soil.

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