Transported Soils: Formation and Characteristics
Transported soils are formed when rocks undergo weathering at one site, and the resulting particles are relocated to another location. This movement occurs through natural transportation agents, such as gravity, running water, glaciers, and wind.
When weathered material is carried away from its original site and deposited elsewhere, it forms transported soils. The specific characteristics of these deposits depend on the transporting agent and the conditions under which deposition occurs. Understanding the nature of transported soils provides critical insights into soil formation processes, aiding in various fields such as geotechnical engineering and environmental studies.
Types of Transported Soils
Based on the mode of transportation, these soils can be classified into four primary categories:
- Alluvial deposits
- Gravity deposits
- Glacial deposits
- Wind deposits.
1. Water-Transported Soils
Water is one of the most influential agents in the transportation and deposition of soil. As it moves, it carries soil particles either in suspension or by rolling and sliding them along the streambed. The size of the particles transported depends largely on the velocity of the water. Swifter currents can carry larger particles, such as boulders and gravel, while slower-moving waters are more likely to transport fine silt and clay.
When the velocity decreases—due to widening, deepening, or a change in direction of the watercourse—coarser particles settle first, while finer ones remain in suspension. This process leads to the formation of various types of deposits, such as alluvial deposits, lacustrine deposits, and marine deposits, each with distinct characteristics.
I. Alluvial Deposits
Alluvial deposits are formed by rivers and streams that carry and deposit soil particles. These deposits are often found along riverbanks, floodplains, and deltas. The soil composition varies due to the mixing of particles transported from different locations.
- Natural Levees: These are formed when rivers overflow their banks during floods. As water slows down, coarser particles like sand and gravel settle near the riverbanks, creating elevated ridges.
- Floodplain Deposits: Finer particles, such as silt and clay, are deposited farther from the river, forming fertile floodplains.
- Alluvial Fans: When a river’s slope decreases suddenly, coarser particles settle into flat, triangular formations known as alluvial fans.
Rivers that meander can create unique features such as oxbow lakes. These are formed when a river changes course, leaving a U-shaped water body that eventually fills with organic and silty deposits.
II. Lacustrine Deposits
When rivers flow into lakes, the sudden reduction in velocity causes coarse particles to settle near the lake’s edge, forming lake deltas. Finer particles, like silt and clay, travel further into the lake and settle in quiet waters, forming lacustrine deposits. These deposits are often layered due to seasonal variations and can be weak and compressible, posing challenges for construction.
III. Marine Deposits
Soils carried by rivers into seas or oceans form marine deposits. These deposits consist of materials transported from terrestrial sources, such as eroded particles from riverbanks, and marine contributions, including organic remains of marine life. The composition of marine deposits can be influenced by chemical interactions between seawater, marine organisms, and the transported particles.
Marine clay deposits, especially those near the surface, are weak and compressible. If exposed above sea level and subjected to fresh water, leaching can make these deposits highly sensitive to disturbance.
IV. Glaciofluvial Deposits
In regions with melting glaciers, flowing water transports glacial soil, creating stratified deposits known as glaciofluvial deposits. These materials are sorted by size due to water velocity and can vary from fine silt to coarse gravel.
2. Gravity Deposits
Gravity deposits are formed when soil or rock fragments are transported short distances under the influence of gravity. These deposits are commonly found at the base of cliffs, steep slopes, or areas affected by landslides. The force of gravity causes disintegration and downward movement of materials, which often results in loose and poorly compacted deposits.
Examples of Gravity Deposits
Talus Deposits:
- Talus refers to the accumulation of coarse, irregular rock fragments and soil particles at the foot of a cliff.
- It is formed when rocks from higher elevations break apart and slide or fall down under gravitational forces.
- Talus material is typically loose, porous, and coarse-grained, making it a useful source of broken rock pieces and coarse soils for various engineering applications.
Landslide Deposits:
- Landslides occur when large masses of soil and rock are displaced and transported downhill.
- These deposits tend to retain the original characteristics of the materials, as gravity transportation does not significantly alter their composition.
Key Characteristics of Gravity Deposits
- Short Transportation Distance: Gravity limits the movement of materials to relatively short distances, ensuring minimal alteration to their properties.
- Loosely Compacted: Due to the nature of their deposition, these soils are generally loose and porous, posing challenges for construction stability.
- Minimal Sorting: Unlike deposits transported by water or wind, gravity deposits do not undergo sorting, resulting in a mix of particle sizes and shapes.
Gravity deposits play a significant role in understanding soil stability and are crucial in areas prone to landslides or rockfalls. Their engineering applications and challenges vary depending on the specific conditions of the site.
3. Glacial Deposits
Formation of Glaciers
Glaciers are massive sheets of ice formed by the compaction and re-crystallization of snow over time. These large ice bodies grow and move, scouring the surface and bedrock beneath them. As glaciers advance, they carry a wide range of materials, from fine particles to massive boulders, over long distances. When glaciers melt, they deposit these materials, leading to the formation of glacial deposits.
Types of Glacial Deposits
I. Till
- Definition: Till refers to deposits directly left by melting glaciers. It consists of unsorted mixtures of particles of all sizes, from clay to large boulders.
- Engineering Use: Well-graded and compactable, till often provides high shearing strength, making it suitable for construction materials in certain conditions.
Figure: Till
II. Moraines
- Terminal Moraine: Ridges of debris deposited at the glacier’s terminus during melting.
- Ground Moraine: A flat or gently undulating surface formed by till deposited beneath a glacier.
III. Eskers
- Definition: Long, winding ridges of sand, gravel, and boulders deposited by streams flowing within or beneath glaciers.
- Dimensions: Eskers can be 10–30 m high and stretch from 0.5 km to several kilometers in length.
IV. Drumlins
- Definition: Isolated, elongated mounds of glacial debris.
- Dimensions: Drumlins typically range from 10–70 m in height and 200–800 m in length.
V. Erratics
- Definition: Large boulders transported and deposited by glaciers far from their original location.
Vi. Outwash
- Definition: Deposits of sand and gravel carried and stratified by glacial meltwater. These materials are generally sorted and layered.
Engineering Properties of Glacial Deposits
- Heterogeneous Composition: Glacial deposits include a mix of clay, sand, gravel, and boulders. This variability can affect their suitability for construction.
- Compaction: Glacial pressures result in dense deposits, often making them excellent foundation materials, particularly where they contain sand and gravel.
- Challenges with Clay: Deposits high in clay content may pose compressibility and strength issues, making them less ideal for foundations.
4. Wind Deposits (Aeolian Deposits)
Wind, as a natural agent of soil transportation, plays a significant role in arid and semi-arid regions. Wind deposits, also known as aeolian deposits, are formed when soil particles are transported and deposited by wind action. The particle size of these deposits depends on wind velocity and other environmental factors.
Types of Wind Deposits
I. Sand Dunes
- Formation: Sandy particles are moved by wind through rolling or short-distance airborne movement. When the wind loses energy, sand accumulates to form dunes.
- Location: Found in arid deserts, coastal regions, and on the leeward sides of sandy beaches.
- Engineering Use: Sands from dunes can be used in certain construction applications but have limited utility due to their loose and fine nature.
II. Loess
- Definition: Wind-blown silt deposits often mixed with fine sand and clay particles.
- Key Characteristics:
- Low Density: Loess has a loose and porous structure.
- High Vertical Permeability: Facilitates rapid water movement in the vertical direction.
- Cementation: Often includes minerals like calcium carbonate and iron oxide, making it stable when dry but prone to collapse when saturated.
- Location: Prominent in regions such as the Mississippi and Missouri River basins in the USA, northern China, and parts of Europe.
- Engineering Challenges:
- High compressibility and poor bearing capacity when wet.
- Requires special attention for foundation design to prevent collapse or undue settlement.
III. Volcanic Ash
- Definition: Fine igneous rock fragments ejected during volcanic eruptions.
- Properties:
- Light and porous.
- Decomposes quickly into plastic clays.
- Notable Example: The Mt. St. Helens eruption deposited extensive ash layers.
Engineering Considerations for Wind Deposits
- Variability: Aeolian deposits exhibit significant variability in particle size and composition.
- Subsurface Investigation:
- Wind deposits may overlay other soil types, creating heterogeneous subsurface conditions.
- Detailed investigation is crucial for understanding the properties of underlying soils.
- Construction Implications:
- Loess and similar deposits demand careful design to address challenges such as low strength and high compressibility.
- Sands from dunes may require compaction or mixing with other materials for enhanced stability.
5. Swamp and Marsh Deposits
Swamp and marsh deposits form in areas with stagnant water and fluctuating water tables where vegetation can thrive. These deposits are characterized by:
- High Organic Content: A significant portion of these soils consists of decomposed or partially decomposed plant material.
- Soft and Compressible Nature: The soils are extremely soft and exhibit high compressibility, making them unsuitable for construction purposes.
- Unpleasant Odor: Organic decomposition often results in a noticeable and unpleasant smell.
The organic accumulation in such areas produces two main types of materials:
- Peat: Partially decomposed plant matter, light in weight, spongy, and highly compressible.
- Muck: Fully decomposed material with similar characteristics to peat but more degraded.
These deposits pose significant challenges for construction and require extensive ground improvement techniques for any engineering applications.
Conclusion
The nature and properties of soil deposits are influenced by their mode of transportation and deposition. This article explored various types of deposits, including:
- Gravity Deposits: Localized and loose deposits like talus.
- Glacial Deposits: Dense and variable soils formed by glaciers, ranging from coarse till to fine outwash materials.
- Wind Deposits: Aeolian soils such as dunes and loess, which exhibit unique characteristics but pose challenges in foundation design.
- Swamp and Marsh Deposits: High organic soils like peat and muck, unsuitable for construction without extensive treatment.
Understanding these deposits is crucial for geotechnical engineers, as soil variability directly impacts construction safety and stability. Comprehensive subsurface investigations and an appreciation of the depositional environment enable the development of effective solutions for foundation and construction challenges.
Frequently Asked Questions
The most widely transported soil is alluvial soil. It is formed by the deposition of soil particles carried by rivers and streams. These soils are typically found in riverbanks, floodplains, and deltas, making them highly prevalent and agriculturally significant.
Transported soil is soil that forms when weathered rock particles are moved from their original location to a new site by natural agents such as water, wind, gravity, or glaciers. The characteristics of transported soils depend on the mode of transportation and the conditions of deposition.
Aeolian soils, also known as wind-transported soils, are carried and deposited by the action of wind. Fine particles like sand, silt, and dust are lifted and moved over varying distances before settling when the wind’s energy decreases. This process forms features like sand dunes and loess deposits.
Transported soil differs from residual soil in that it forms at a location different from its source. Residual soil remains at the site where the parent rock underwent weathering, whereas transported soil is carried away by agents like water, wind, gravity, or glaciers before deposition.
Transport studies of heavy metals in soil are conducted by analyzing soil samples for metal concentration, mobility, and retention. Techniques include leaching experiments, modeling metal transport using hydrological simulations, and assessing soil properties like pH and organic matter content to predict heavy metal behavior.
The suitability of soil depends on its intended use. Transported soils like alluvial deposits are often more fertile and suitable for agriculture due to their diverse mineral composition. Residual soils may be more stable for construction because they are less likely to shift or compress.
