How Can Groundwater Affect Soil Properties?

In IAM News’s CSI Infrastructure episode, Iphigenia covered the potential suspects in cases of infrastructure failure. One of the most notorious culprits is the effects of groundwater on underlying soil – so, how can groundwater affect soil properties?

Groundwater refers to the water that is present beneath the Earth's surface within the pores and fractures of the soil and rock. It is a crucial component of the Earth's hydrological cycle. When rain or snow falls, some of the water is absorbed by the soil, and some infiltrates deeper into the ground until it reaches a zone where the soil or rock is saturated with water.

Groundwater has a significant influence on the properties of soil. The presence and movement of groundwater can lead to various changes in soil, affecting its mechanical and chemical properties.

Saturation and Pore Pressure

The upper boundary of the saturated zone of the Earth is known as the water table, and when this water table rises, it saturates the area above it, filling those pore spaces with water. Saturation influences the pore water pressure within the soil; elevated pore water pressure can reduce the effective stress in soil, impacting its shear strength and stability.

Consolidation and Settlement

Changes in groundwater levels can lead to changes in effective stress and consolidation of soil. When groundwater levels decrease, the soil consolidates, which may cause settlement of the ground. Conversely, rising groundwater levels can lead to reduced consolidation and even heaving or swelling in certain types of soils.

Permeability and Seepage

Groundwater influences the permeability of soil (its ability to transmit water). In some cases, the flow of groundwater through soil (seepage) can erode or transport fine particles, affecting the soil structures and stability.

Chemical Reactions

Groundwater can introduce dissolved minerals and chemicals into the soil, leading to changes in its chemical composition. Certain minerals may dissolve or precipitate, influencing the soil’s strength and compressibility. For example, in some soils, the dissolution of gypsum can lead to subsidence or collapse of the ground.

Freezing and Thawing

In cold climates, groundwater can freeze, causing the soil to expand. The freezing and thawing of groundwater can lead to frost heave, where the soil is lifted as water in the soil freezes, expands and forms ice lenses.

Infrastructure constructed on soil affected by groundwater must consider the dynamic interaction between the structure and the subsurface water conditions. The influence of groundwater on soil properties can have significant implications for the design, construction, and long-term performance of infrastructure. Some considerations may include:

  1. Foundation Design: The design of foundations must account for changes in groundwater levels. Variations in water levels can lead to changes in soil strength, settlement, and lateral earth pressure. Engineers may need to design foundations that are resistant to both upward and downward movement of the ground caused by changes in groundwater conditions.
  2. Slope Stability: Groundwater plays a crucial role in slope stability. Elevated pore water pressures due to high groundwater levels can reduce the effective stress in soils, potentially leading to slope failures. Engineers need to analyse and design slopes, embankments, and retaining walls considering the effects of groundwater on soil stability.
  3. Seepage Control: For structures like dams, tunnels, and underground structures, controlling seepage is vital. Groundwater flow can erode soil and affect the integrity of these structures. Engineers implement measures such as cutoff walls, drainage systems, and grouting to control seepage and maintain the stability of infrastructure.
  4. Corrosion Protection: Infrastructure elements in contact with groundwater may be susceptible to corrosion. This is particularly relevant for structures with steel components, such as bridges, pipelines, and foundations. Engineers use corrosion-resistant materials, protective coatings, and cathodic protection systems to mitigate the impact of groundwater on metal structures.
  5. Ground Improvement Techniques: In some cases, engineers may employ ground improvement techniques to modify soil properties and enhance the performance of the foundation. Methods such as soil compaction, grouting, and the installation of vertical drains can be used to improve soil stability and reduce settlement.
  6. Monitoring and Maintenance: Continuous monitoring of groundwater levels and their impact on the infrastructure is essential. Regular inspections and maintenance are crucial to identify and address any changes in soil conditions, ensuring the long-term stability and safety of the infrastructure.
  7. Environmental Considerations: Groundwater conditions can also have environmental implications. Construction activities and changes in groundwater flow patterns may affect local ecosystems and water quality. Environmental impact assessments and mitigation measures may be necessary to minimize any adverse effects on the surrounding environment.

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