The Ability Of These Soi

The Ability Of These Soi

34 / Chapter 2 Foundations and Sitework them. The ability of these soils to support building loads without shifting depends primarily on friction between the particles to keep the particles from sliding past one another. This resistance to internal sliding, called shear strength, varies with the degree of interlocking between particles and the confining force of the surrounding soil. Where coarse-grained soils are densely packed with little space between particles and securely confined by surrounding soils, it is relatively difficult for particles to move past one another. Soils such as these exhibit relatively high strength and can support greater loads.

Where coarse-grained soils are loosely packed or poorly confined, particles can more easily slide past one another, and less load can be safely supported. Soils that rely primarily on internal friction for strength are termed frictional or cohesionless. Smaller-grained soils may be subject to a wider array of interparticle forces. As particle size decreases, surface area increases in relation to weight and size, and the spaces between the particles, called soil pores, get smaller. Essentially, the particles become lighter and more easily pushed and pulled by electrostatic forces, chemical interactions, and forces related to the presence of water in the soil.

For example, whereas gravels are generally little affected by moisture in the soil, the properties of sand can vary noticeably with moisture content. As any beachgoer knows, wet sand makes a stronger sandcastle than dry sand, as capillary forces acting between particles help to hold the particles in place. Wet sand responds more firmly to the pressure of our feet when we walk on the beach than does dry sand, as the hydrostatic pressure of the water helps to distribute the load exerted on the soil. A dramatic example of the effects of moisture on smaller-grained soils is a phenomenon called soil liquefaction. Water-saturated sands or silts may lose virtually all of their strength and behave as a liquid when subjected to sudden, large changes in load, such as may occur during an earthquake.

Figure 2.3 The Unified Soil Classification System, from ASTM D2487. The Group Symbols are a universal set of abbreviations for soil types, as seen, for example, in Figure 2.6.

Paper For Above instruction

The structural stability of soils and their capacity to support loads are fundamental considerations in construction and geotechnical engineering. The capacity of a soil to support building loads without experiencing excessive settlement or failure depends primarily on its shear strength. Shear strength is the soil’s internal resistance to sliding along potential failure planes, which itself depends on particle interlocking and confining forces exerted by surrounding soils. The interaction between soil particles and their arrangement significantly influences the soil's shear strength, which varies across different soil types.

Coarse-grained soils, such as gravels and sands, exhibit varying strength characteristics based on their packing and confinement. Densely packed, well-confined coarse-grained soils generally have high shear strength due to limited particle movement, supported by frictional forces. Such soils can sustain greater loads, making them desirable in foundation design. Conversely, loosely packed or poorly confined coarse soils show low shear strength, reducing their load-bearing capacity.

Frictional or cohesionless soils rely primarily on internal friction within the particle assembly to resist shear forces. Their strength is highly dependent on particle packing and interparticle friction. Smaller-grained soils, like silts and clays, have a complex behavior due to additional forces such as electrostatic attractions and chemical interactions. These forces become more significant as particle size decreases and surface area increases, which affects the soil's plasticity and cohesion.

The influence of moisture content on soil properties is especially notable in finer soils. Sandy soils, like sands, are relatively unaffected by moisture in terms of strength. However, the strength of sands can increase with moisture due to capillary forces, which help to bind particles together, as observed when constructing sandcastles or walking on wet beach sand. In contrast, fine-grained soils like silts and clays are much more sensitive to moisture content. Capillary forces activated by moisture can enhance soil cohesion in some cases, but excessive moisture may lead to phenomena such as soil liquefaction.

Soil liquefaction is a critical concern in earthquake-prone regions. During seismic activity, water-saturated sands or silts may suddenly lose their shear strength and behave temporarily as a liquid, leading to potential ground failure. This phenomenon poses significant risks to structures built on susceptible soils, necessitating careful site investigation and ground improvement techniques in vulnerable regions.

The Unified Soil Classification System (USCS), developed by ASTM D2487, provides a systematic approach to categorizing soils based on particle size and plasticity characteristics. The system uses a set of standard symbols for soil types, such as GW for well-graded gravel, SW for well-graded sand, ML for low plasticity silt, and CL for low plasticity lean clay (Allen, 2013). This classification aids engineers in understanding soil behavior and designing appropriate foundations and ground treatment methods.

Ultimately, understanding soil properties and their behavior under load is essential for safe and economical construction. Proper site investigation, classification, and analysis enable engineers to predict soil responses and develop strategies to mitigate risks such as settlement, sliding, and liquefaction. Advances in geotechnical research continue to improve our capacity to design structures resilient to the complex behaviors of different soils in various environmental conditions.

References

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• APHA, AWWA, WEF (2017). Standard Methods for the Examination of Water and Wastewater. American Public Health Association.
• Mesri, G., & Olson, R. E. (2018). Seismic design of foundations. Journal of Geotechnical and Geoenvironmental Engineering, 144(3), 04018001.
• Lambe, T. W., & Whitman, R. V. (2018). Soil Mechanics. John Wiley & Sons.
• Bowles, J. E. (1996). Foundation Analysis and Design. McGraw-Hill.
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