Types of Soil - Soil Mechanics - Civil Engineering (CE)

Soil Types

Classification of soils - broad categories

• Residual soils
• Transported soils

Residual soils

Residual soils are formed in situ by the chemical and mechanical weathering of the parent rock and remain at or very near the place of formation. The depth of residual soils typically ranges from about 5 m to 20 m, but may be shallower or deeper depending on climate, parent rock and geomorphic history.

Key points about residual soils:

• Chemical weathering is more intense in warm, humid climates than in cold, dry climates; consequently residual soils are commonly deeper and more mature in tropical regions.
• Accumulation occurs where the rate of rock decomposition exceeds the rate of erosion or transport of weathered material. Vegetative cover on the surface reduces removal of material by runoff and wind, encouraging residual deposit formation.
• Leaching by percolating surface water reduces with depth; therefore the degree of chemical alteration typically decreases downward until unweathered rock is encountered.
• Residual soils are often heterogeneous in particle size, mineralogy and engineering properties because weathering proceeds at different rates for different minerals and rock structures.
• Engineering significance: foundations and earthworks on residual soils must account for variable stiffness, permeability and strength with depth; the depth to fresh rock is an important design parameter.

Transported soils

Transported soils are weathered rock materials that have been moved from their original position by one or more natural agencies (water, ice, wind, gravity) and deposited elsewhere. Transported soils are classified according to the mode of transport and the depositional environment.

Common types of transported soils

• Alluvial deposits - material carried and deposited by rivers and streams. Alluvium is typically stratified and sorted into layers of sand, silt and clay, with coarser material deposited during high-energy flows and finer material during low-energy periods.
• Lacustrine deposits - sediments deposited in lake basins. These deposits are often fine-grained, with alternating layers formed seasonally or during variable inflow conditions; they may be silty or clayey and can be highly compressible.
• Marine deposits - sediments deposited in the sea. They contain particulate material derived from land and also organic remains of marine life. Marine clays are often soft and have engineering problems such as low bearing capacity and large consolidation settlements.
• Glacial deposits - material deposited by glaciers or by meltwater from glaciers. Glacial tills are usually poorly sorted, with a mixture of clay, sand, gravel and boulders; outwash from melting glaciers tends to be better sorted (sand and gravel).
• Aeolian deposits - soils transported and deposited by wind. Examples include dune sands and loess. Aeolian sands are typically well-sorted and porous; loess is a wind-blown silt that can be susceptible to collapse on wetting.
• Colluvial deposits - material moved and deposited by gravity (mass movement) such as scree, talus or slope-wash. Colluvium may be heterogeneous and form unstable slopes if not consolidated.
• Deltaic deposits - alluvial sediments deposited where a river enters a standing body of water (lake or sea). Deltas commonly show complex stratification and variable consolidation.
• Volcanic soils - materials derived from volcanic ash and ejecta (e.g., tuff). They can be highly porous and variable in mineralogy; some volcanic soils are very fertile agriculturally but may pose engineering challenges.
• Organic soils (peats) - soils with a high content of decomposed plant or animal matter. They are highly compressible, have low shear strength and poor bearing capacity; special foundation solutions are usually required.

Particle-size classification and texture

For practical engineering work it is important to know the particle-size distribution of a soil. A commonly used classification by particle size (approximate limits) is:

• Gravel: particles greater than 4.75 mm
• Sand: particles between 0.075 mm and 4.75 mm
• Silt: particles between 0.002 mm and 0.075 mm
• Clay: particles smaller than 0.002 mm

Soil texture (proportion of sand, silt and clay) strongly influences permeability, compressibility and shear strength. Textural classification is usually based on laboratory sieve analysis and hydrometer tests.

Physical and engineering properties related to soil types

• Permeability - coarse-grained soils (sands, gravels) have high permeability; fine-grained soils (silts, clays, organic soils) have low permeability, affecting drainage and consolidation behaviour.
• Compressibility and consolidation - clays, silts and organic soils are generally more compressible and consolidate over time under load; some deposited clays (marine and lacustrine) may produce significant long-term settlement.
• Shear strength - coarse-grained soils derive strength mainly from internal friction (angle of internal friction), whereas fine-grained clays may have apparent cohesion due to interparticle electrochemical forces; strength can vary widely with moisture content and degree of consolidation.
• Volume change behaviour - some clays (e.g., black cotton soil) are highly expansive and exhibit large volume changes with wetting and drying; loess may collapse on wetting; organic soils may shrink as they dry.
• Heterogeneity and stratification - transported deposits commonly display stratification and heterogeneity which control seepage, bearing capacity and slope stability; residual soils often show a weathering profile with gradual change in properties with depth.

Identification tests and simple field checks

Common rapid field checks and simple laboratory tests useful for initial soil identification:

• Visual and tactile examination (grain size feel, colour, plasticity).
• Thumb or ribbon test to estimate clay content and plasticity.
• Sieve analysis for particle-size distribution of coarse and medium fractions.
• Hydrometer test for finer fractions (silt and clay percentages).
• Atterberg limits (liquid limit, plastic limit) to classify fine-grained soils and assess plasticity.
• Simple permeability tests (e.g., falling-head, constant-head) for preliminary drainage assessment.

Engineering implications and common applications

• Site investigation must establish the type(s) of soil present, depth to bedrock, and the variation of properties with depth for safe foundation design.
• Alluvial and lacustrine soils may need dewatering or preloading to manage consolidation settlements for foundations and embankments.
• Expansive clays and collapsible loess require special foundation measures (deep foundations, moisture control, soil stabilisation) to prevent structural damage.
• Organic soils and soft marine clays typically need ground improvement (replacement, preloading, vertical drains, deep mixing) before construction or light-weight structures/use of piled foundations.
• Well-graded sands and gravels are preferred for drainage layers, sub-base and backfill due to high permeability and strength.
• Residual soils over weathered rock often allow shallow foundations where competent material is present; however, variable stiffness may require deeper investigations or special foundation design.

Examples and brief case notes

• Lateritic soils - formed by intense leaching in tropical climates; rich in iron and aluminium oxides; often used locally as construction material when suitably compacted and stabilised.
• Black cotton soil - a term used for expansive montmorillonitic clays found in certain peninsular regions; notable for high swelling/shrinkage and low bearing capacity when wet; requires design adaptations.
• Loess - wind-blown silt deposits that can be agriculturally fertile but may collapse when wetted; buildings on loess require cautious foundation design.
• Marine clays - soft, normally consolidated clays in coastal basins; pose settlement and stability problems for heavy structures and harbour works.

Practical guidance for engineers

• Always correlate field observations with laboratory test results before finalising design decisions.
• Determine groundwater conditions as they strongly influence effective stress, strength and consolidation behaviour.
• Document stratification, thickness of each layer and index properties (grain-size distribution, Atterberg limits, natural moisture content) in borehole logs.
• Consider suitable ground-improvement or foundation techniques depending on the identified soil type and the proposed structure loadings.

Summary

Soils are broadly classified into residual and transported types. Residual soils form in place by weathering, while transported soils are deposited by water, ice, wind or gravity. Each soil type has characteristic particle-size distributions, structure, permeability, strength and volume-change behaviour that influence engineering decisions for foundations, slopes, earthworks and ground improvement. Thorough site investigation, laboratory testing and recognition of the depositional history are essential to select appropriate design and construction measures.