Soil Bearing Capacity & Types

Soil Bearing Capacity

• Soil is defined, in a construction context, as the part of the earth below topsoil and above rock, formed by erosion, water, atmospheric conditions, and intense pressure over thousands of years.
• Organic matter, though present in small quantities, is usually disregarded in soil analysis.
• Mineral matter, the majority component, is classified as rocks or soils.
• Rocks have firmly bonded mineral particles, while soil particles are either individual particles or conglomerates of easily separable particles.
• Soils are classified as cohesive and non-cohesive types.

Non-Cohesive Soils

• Gravel: A natural deposit of rock fragments in a matrix of finer, typically sandy material. Particles are mostly larger than 2mm.
• Sand: A granular, mainly siliceous sediment from rock weathering. Gritty, lacks plasticity. Particles range between 0.06mm and 2.00mm.
• Well-graded sand: Contains a proportion of all sand particle sizes, with a prevalence of coarser grades.
• Compact gravel and sand: Requires tools like picks for removal due to high resistance to penetration.
• Loose gravel and sand: Easily removable by hand tools.
• Uniform or poorly graded sand: Particles mostly within a restricted size range.

Cohesive Soils

• Clay: A deposit mainly of fine, siliceous and aluminous products of rock weathering. Smooth, greasy texture, sticks to fingers, and dries slowly. Clay shrinks on drying and gains strength.
• Stiff clay: Requires tools (like picks or pneumatic spades) for removal, and can't be molded at its natural moisture content.
• Firm clay: Excavated with a spade, and moldable under substantial pressure at natural moisture content.
• Very soft clay: Extrudes between fingers when squeezed at natural moisture content.
• Soft clay: Readily excavated and moldable with fingers at natural moisture content.
• Boulder clay: Unstratified, containing sub-angular stones of varying sizes.

Silt

• Silt: Finely graded material, smaller than sand. Most grains will pass through a 75-micron sieve. Exhibits some plasticity and notable cohesion when dry.

Uniqueness of Clay

• Clay particles are non-equidimensional (flat, plate-like), differing in shape from gravel, sand, and silt particles, which are mostly equidimensional.
• Chemical weathering produces clay, contrasting with mechanical weathering that forms gravel, sand, and silt.
• Clay's flat shape leads to a significantly higher surface area-to-volume ratio compared to other soil types.
• Electrostatic forces between plate-like clay surfaces influence behaviour greatly. These forces are repulsive when water is present, causing swelling. As water decreases, clay shrinks.

Organic Soils

• Soil consisting primarily of organic matter is referred to as organic soil.
• Organic soils primarily contain decomposed plant matter (peat), having little or no mineral matter.
• Organic soils are highly compressible and are unsuitable for building foundations due to compressibility.
• They are usually dark in color and have a distinctive decaying odor.
• Additional characteristics may include a fibrous texture due to plant debris (bark, leaves, branches).
• Organic soils often form the top layer of soil and are frequently removed before construction starts.

Soil Identification and Classification Tests

• Several tests are used to identify and classify soils.
• Particle size distribution: Determines the proportion of gravel, sand, and clay in a soil sample.
• Soils are dried, then sieved through a set of sieves.
• The weight retained on each sieve is recorded to determine particle size distribution.
• Liquid limit test: Determines moisture content at which soil transitions from plastic to liquid state.
• Plastic limit test: Determines moisture content at which soil ceases to be plastic (can be rolled into a 3mm diameter thread without breaking).
• Plasticity index: Difference between liquid and plastic limits.
• Data from these tests is graphed. A graph defines the range of sand grain sizes (0.60mm-2mm), silt grain sizes (0.60mm-0.002mm), and clay grain sizes (less than 0.002mm).
• Results are plotted on graphs to establish properties.

Casagrande Classification Chart

• The chart uses two-letter symbols to categorize soils.
• The first letter indicates the particle size (e.g., G for gravel, S for sand, M for silt, C for clay, O for organic silts and clays).
• The second letter describes the soil's properties (e.g., H for high compressibility, L for low compressibility, P for poorly graded, W for well-graded).

Other Tests

• Dry density tests: Determine dry soil density in a soil's natural position.
• Standard penetration test and consolidation test: Determine soil compressibility and shear.
• Triaxial compression soil test: Determines shear strength parameters, used to establish allowable bearing pressure.

Test methods

• Test Pit Method: Involves digging pits/trenches, allowing for visual soil inspection and sample collection for laboratory tests (considered reliable). Limitations include cost and depth restrictions by a high water table.
• Test Boring Method: Uses a power-driven drilling system (hollow stem fitted with a bit) to collect soil samples. A split-spoon or Shelby tube is used to remove the sample core.

Explore the concepts of soil bearing capacity and its different classifications in construction. This quiz covers both cohesive and non-cohesive soils, including gravel and sand types. Test your knowledge on how these soils affect construction practices.