Soil mechanics
Soil mechanics is a vital branch of civil engineering that focuses on understanding the properties and behaviors of soil, particularly its role as a construction material. It is essential for designing stable foundations and structures, as soil characteristics can vary significantly based on geographical location and composition. Factors such as moisture levels, erosion, and environmental conditions can influence soil dynamics, impacting a structure's safety and durability. Engineers must assess soil stability, bearing capacity, and the potential for issues such as settling or shifting, especially in areas prone to natural events like earthquakes or floods.
Different types of soil, composed of various mixtures of solid, liquid, and gaseous elements, have unique physical and chemical properties. These properties dictate the methods used in construction, from shallow foundations in stable soil to deep pilings in less supportive ground. Historical cases, such as the Leaning Tower of Pisa and the Millennium Tower in San Francisco, illustrate the consequences of inadequate soil mechanics understanding, highlighting the importance of this discipline in ensuring the safety and longevity of engineered structures. Overall, soil mechanics integrates knowledge from various fields to provide engineers with the tools necessary to predict and manage soil behavior in construction projects.
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Soil mechanics
Soil mechanics is a discipline of civil engineering concerned with the study of soil and its behaviors, particularly in regard to its use as a construction material. The discipline is important in understanding the role soil plays in setting foundations and other elements of construction. Soil requires special study because its properties may have different biological, chemical, and physical characteristics depending on its composition and location. The mechanics of soil at construction sites may be subject to varying conditions, such as regular changes in wind and moisture levels that can affect soil dynamics and its resulting behaviors.
The structural load of any construction project is supported by its foundation. In turn, the soil located directly under the structure helps brace it. Therefore, construction plans must take into account the composition of this soil and consider how it will settle under the enormous weight of a building. Engineers must ensure that the soil can support the weight of a structure without sinking or showing side-to-side movement. Understanding the characteristics of soil is particularly important in regions where the soil can shift due to earthquakes, debris flows, rockslides, sinkholes, or seasonal changes in the water table. The unique considerations regarding the weight and structural shape of a construction project may also demand certain soil requirements to guarantee long-term stability and safety. The discipline of soil mechanics requires knowledge in civil engineering, fluid mechanics, geology, soil physics, and hydraulic principles. When used in coordination, these disciplines provide engineers with the ability to predict the potential soil dynamics of a site.
Background
Soil mechanics has many civil engineering applications. In addition to the construction of sturdy building foundations, soil mechanics is a consideration in the building of mines that maximize the safety of miners, the designing of roads that are able to withstand heavy traffic patterns, and the erecting of bridges, dams, and levees.
Soil can take any number of forms, and the term may have different meanings depending on the particular discipline. Geotechnical engineers use a broad definition of soil when assessing the needs of an engineering project. In general, soil is defined as a mixture of earthen materials that are not permanently bound together. Soil is often composed of a combination of aqueous, gaseous, and solid elements. Aqueous elements usually consist of water, gaseous elements are primarily composed of air, and solid elements are made up of any composition of various soil grains. The solid portion of soil may include a mixture of rocks, volcanic ash, alluvium, aeolian sands (materials deposited by wind), silt, gravel, organic materials, and glacial materials. The physical, mineralogical, and chemical properties of soil varies from site to site due to its interaction with the atmospheric and hydrospheric conditions of each location. Wind, erosion, floods, and plate tectonics are all factors that can alter the overall composition of soil. Soil mechanics is also concerned with determining the cohesiveness, adhesiveness, and acidity of the soil at a site.
Therefore, soil in various locations may have different capacities for supporting the loads from buildings, dykes, and other projects. This concern is called bearing capacity. Engineers in soil mechanics must also take into consideration whether the soil at a site is stable enough to prevent injuries to the construction workers building the structure.
The foundation of a building may be either shallow or deep. The depth of a building foundation depends on the type of soil at the site. For instance, soft soils that form a thin layer over a firmer rock layer may only require small spread foot supports, while a thicker soil layer may need pillars that are embedded deep into the earth to maintain stability. In places where the surface layers of soil have poor bearing capacity, stabilizing pillars may have to be anchored several hundred feet into the ground to reach bedrock. Engineers must also account for the stress distribution of the project's weight load and the potential need to reduce the levels of groundwater (dewatering). Other considerations include the slope of the ground, water seepage, soil classifications, and the interaction between the foundation and soil particles over time.
Overview
Poor comprehension of soil mechanics can lead to a number of structural issues with buildings. For example, construction on the Palacio de Bellas Artes in Mexico City had to be suspended in the early nineteenth century when parts of the building began to sink into the city's spongy soil. Despite alterations to construction plans, the building has subsequently sunk 13 feet (4 meters) since its opening in 1934.
More recently, part of the foundation for the 58-story Millennium Tower in San Francisco, California, began to sink and tilt. By 2016, the building had sunk 16 inches (40 centimeters) into the ground, causing it to lean 6 inches (15 centimeters) at its top and 2 inches (5 centimeters) at its base. The developers used piles driven 60 to 90 feet (18 to 27 meters) into landfill rather than drilling into bedrock that was 240 feet (73 meters) beneath the foundation. This lack of stability—which could be partially attributed to a poor understanding of the soil mechanics of the site—has left massive cracks in the building's underground garage and has led to questions about its safety.
Perhaps the most famous case of structural issues related to soil mechanics is the Leaning Tower of Pisa in Italy. Started in 1173, the tower had already begun to tilt well before its completion in 1372. Despite several attempts to stabilize the building's famous tilt and prevent the complete collapse of the structure, the tower leaned at a dangerous 5.5 degrees from center by 1990. Between 1990 and 2001, an international team using the principles of soil mechanics was able to stabilize the tower by wrapping it in steel bands and placing a series of heavy lead weights on its north side. Workers then poured a concrete ring around the base and sank a series of cables into the earth that were anchored to the concrete. They removed large amounts of soil a little bit at a time, allowing the tower to retain a deliberate lean of approximately 4 degrees. By 2008, sensors showed that the building was stabilized in its new position. However, by 2022, the tower had a sudden and significant increase in its lean, so work on a project to completely straighten it began. This project was expected to substantially reduce tourism in the region. To combat this, the construction of a skyscraper-sized replica of the famous landmark, complete with its historic lean, was planned to house businesses and tourist attractions.
Bibliography
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