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The Hidden Dance of Water and Soil: How Do Pressure Changes Affect Soil Volume?
In soil engineering, soil consolidation is a crucial process. Soil consolidation is the mechanical process by which soil gradually changes volume under the influence of pressure changes. The complexity of this process lies in the fact that soil itself is a three-phase material composed of soil particles and pore water (usually groundwater). When water-saturated soil is subjected to increased external pressure, the volumetric stiffness of the water is high relative to the soil structure, meaning that the water initially absorbs all pressure changes without changing volume, creating excess pore water pressure.
Once water diffuses from an area of high pressure, the soil structure gradually becomes stressed and shrinks in volume due to osmosis.
The theoretical basis of soil consolidation is closely related to effective stress and hydraulic thermal conductivity. Early modern theoretical models were proposed a century ago by Karl Terzaky and Paul Fillenger. Terzaky's model is still the most widely used in engineering construction and is mainly based on the diffusion equation. It is important to note that in a narrow sense, consolidation refers only to delayed volumetric reactions due to the dissipation of excess water pressure.
Soil types rich in organic matter will show significant signs of creep over time. In this case, the process by which the soil gradually changes volume under constant effective stress is called "subconsolidation." In many construction projects, especially when the building is located on a soil layer of low stiffness and low permeability (such as marine clay), the effects of consolidation are particularly evident, resulting in large settlements over a long period of time.
Typical technical risks for soil consolidation include projects such as land reclamation, dam construction, and tunneling and basement excavations in clay soils.
Civil engineers often quantify the effects of consolidation using a device called a cylinder tester called a compaction test. In this test, the consolidation properties of the soil are quantified by subjecting a soil sample to a known series of pressure changes and recording the thickness of the sample over time.
The history and terminology of consolidation
The first modern theoretical model of soil consolidation was proposed in the 1920s by Terzaky and Fierenger, with substantial differences in approach between the two. Terzhaky was mainly based on the diffusion equation, while Fierenger considered local Newtonian laws for liquid and solid phases. Since then, Terzhaky's method has been widely adopted because of its simplicity and practicality, while Fillenger's rigorous model has long been ignored due to its abstractness. The debate between the two eventually led to tragic results. Fearenger's theory was ignored for many years.
Terzhaky said, "Consolidation is a process that meets pressure changes and involves the reduction of saturated soil moisture, which is not replaced by air."
Volume change
The volume change of consolidation depends on the progressive elimination or absorption of water and is affected by long-term static loading. The pressure exerted on the soil will cause the soil particles to pack closer together. When the soil is saturated with water, excess water will be squeezed out, affecting the soil's consolidation behavior. The magnitude of consolidation can be predicted according to different methods. Terzhaky's classic method uses a cylinder test to determine the compressibility of the soil.
When the applied pressure is removed, the soil will rebound, regaining some of the volume lost during consolidation. When pressure is reapplied, the soil will consolidate again on the reconsolidation curve. Clay usually consolidates and settles under its own weight and the pressure of the soil above it.
Time dependence
The consolidation process often takes a long time, especially in saturated clays, which have extremely low hydraulic and thermal conductivity and water penetrates out of the soil very slowly. This means that during the drainage process, the pore water pressure increases, partially bearing the applied pressure.
The exact time for consolidation depends on the nature of the soil and the load applied, so in some cases it may take several years to achieve full consolidation.
Soils with high organic matter content, such as peat soil, may experience creep. This phenomenon is due to the gradual and sustained impact of the soil on the pressure, which changes the volume of the soil at a rate that is significantly greater over longer time scales.
Reflection and future exploration
Through the study of soil consolidation, we can gain a deeper understanding of the interaction between water and soil, which not only affects the stability of structures, but also involves environmental sustainability. As urban development continues to accelerate, how to effectively manage soil consolidation, ensure foundation stability and reduce environmental impact has become an important issue. How can the discussion of these issues promote our thinking and practice about future construction?
