NICET Geotech key terms

Water content boundaries that mark changes in the consistency of fine-grained soil: the liquid limit, plastic limit, and shrinkage limit. They are used to classify clays and silts and to predict how a soil behaves when moisture changes.

The water content at which soil passes from a plastic state to a liquid state, determined by the Casagrande cup or fall cone test. It is the upper boundary of the plastic range used in classification.

The lowest water content at which a soil can be rolled into a thread about 3 mm in diameter without crumbling. It marks the boundary between semisolid and plastic behavior of fine-grained soil.

The numerical difference between the liquid limit and the plastic limit. A large PI indicates a highly plastic clay that changes volume significantly with moisture; a small PI indicates a silty, less compressible soil.

The water content below which further drying does not reduce the volume of a soil mass. It helps estimate potential shrinkage and swelling of fine-grained soils during moisture loss.

The decrease in total volume of a soil expressed as a percentage of its dry volume when water content drops from the liquid limit to the shrinkage limit. It indicates how much a soil will contract on drying.

The mass of water in a soil sample divided by the mass of the dry solids, usually expressed as a percentage. It is measured by oven-drying and is fundamental to nearly every soil test and specification.

A test that separates a dry soil sample through a stacked series of wire-mesh sieves to determine the percentage of material retained on each size. Results produce a gradation curve describing coarse fraction distribution.

A sedimentation test that uses a hydrometer to measure the density of a soil-water suspension over time, allowing calculation of the size distribution of particles finer than the No. 200 sieve.

A plot of percent finer versus particle size on a semi-logarithmic scale, derived from sieve and hydrometer tests. It shows whether a soil is well-graded, uniform, or gap-graded.

The ratio of the particle size at 60 percent finer to the size at 10 percent finer on the gradation curve. Larger values indicate a broader range of particle sizes and generally better gradation.

A shape parameter calculated from the gradation curve using the D30, D10, and D60 sizes. Values between about 1 and 3 indicate a smoothly graded curve without gaps.

A soil containing a smooth, continuous distribution of particle sizes from coarse to fine. It typically compacts to higher density and has better strength and drainage than a uniform or gap-graded soil.

A standard system that groups soils by behavior using letter symbols such as GW, CL, and SM. Classification depends on grain size distribution, Atterberg limits, and the percentage of fines.

A classification system developed for highway subgrade evaluation that assigns soils to groups A-1 through A-7 based on grain size, plasticity, and the group index. It is widely referenced in pavement design specifications.

A numerical value in the AASHTO system that rates the quality of a soil as a highway subgrade, with higher values indicating poorer performance. It is computed from percent passing the No. 200 sieve, liquid limit, and plasticity index.

The ratio of the mass density of soil solids to the density of water. It is used in phase relationships and hydrometer calculations and typically ranges from about 2.6 to 2.8 for most mineral soils.

The volume of voids divided by the volume of solids in a soil mass. It quantifies how loosely or densely the solid particles are packed and relates directly to porosity, density, and compressibility.

The volume of voids expressed as a fraction or percentage of the total soil volume. Unlike void ratio, porosity is bounded between zero and one and is convenient for storage and flow calculations.

The percentage of void volume occupied by water. A value of zero means dry soil, 100 percent means fully saturated soil, and intermediate values describe partially saturated conditions above the water table.

The mass of dry soil solids per unit of total volume, including voids. It is the primary acceptance measure for compacted fill because higher dry density generally means greater strength and lower settlement.

A plot of dry density versus molding water content from a laboratory compaction test. The peak of the curve identifies the maximum dry density and the optimum moisture content for a given compactive effort.

The highest dry unit weight a soil can achieve at a given compactive effort, occurring at the optimum moisture content. Field specifications typically require a percentage of this laboratory value.

The water content at which a specific compactive effort produces the maximum dry density for a soil. Below it, lubrication is insufficient; above it, water displaces soil particles and density drops.

A laboratory compaction test using a 2.5 kg hammer dropped 305 mm in three layers of soil in a 1/30 cubic foot mold. It represents low-to-moderate field compactive effort.

A higher-energy laboratory compaction test using a 4.5 kg hammer dropped 457 mm in five layers. It models heavier field rollers and typically yields higher maximum dry density at lower optimum moisture content than the standard test.

The field dry density expressed as a percentage of the laboratory maximum dry density for the same soil and compactive effort. Specifications commonly require 90 to 98 percent depending on the application.

An in-place measurement of compacted soil density, commonly by the sand cone, rubber balloon, or nuclear gauge method. Results are compared to laboratory maximum density to verify compaction acceptance.

A field density method in which a calibrated sand is used to determine the volume of a test hole excavated from compacted soil. The recovered soil mass divided by that volume gives in-place wet density.

For cohesionless soils, the ratio expressing how dense a natural sand or gravel is compared with its loosest and densest possible states. It is a key indicator of liquefaction and settlement potential.

A borehole test in which a split-spoon sampler is driven 450 mm with a 63.5 kg hammer, recording the blows for the final 300 mm as the N-value. N-values correlate with soil density, strength, and settlement.

An in-situ test that pushes an instrumented cone into the ground while recording tip resistance and sleeve friction. It provides a continuous stratigraphic profile and is used to estimate soil type and strength parameters.

The maximum resistance of a soil to internal sliding along a failure plane, described by cohesion and angle of internal friction. It governs bearing capacity, slope stability, and lateral earth pressure.

The component of shear strength that is independent of normal stress, arising from interparticle attraction and cementation. It is significant in clays and essentially zero for clean sands and gravels.

The slope of the Mohr-Coulomb failure envelope, representing the frictional resistance between soil particles. Dense, angular, well-graded soils generally have higher friction angles than loose, rounded, uniform soils.

A laboratory test in which a soil specimen is sheared along a forced horizontal plane under a controlled normal stress. Repeating at several normal stresses defines the Mohr-Coulomb strength parameters.

The time-dependent reduction in volume of a saturated soil caused by squeezing out pore water under sustained load. It is the principal source of long-term settlement in compressible clay deposits.

The maximum effective stress a soil has ever experienced in its geologic history. Knowing it distinguishes normally consolidated from overconsolidated clays and predicts whether loads cause large or small settlements.

The ratio of preconsolidation pressure to the current effective vertical stress. Values above one indicate a soil that has been unloaded or eroded and will compress less than a normally consolidated soil.

A parameter describing the rate at which excess pore water pressure dissipates and consolidation proceeds in a clay layer. It is used to estimate how long settlement will take under a proposed load.

A measure of a soil's ability to transmit water through its interconnected voids under a hydraulic gradient. It controls drainage, seepage, and the rate of consolidation and is typically found by constant-head or falling-head tests.

The movement of water through soil pores driven by a hydraulic gradient. Engineers evaluate seepage quantity, uplift pressures, and the risk of piping or internal erosion beneath dams and excavations.

Internal erosion in which seepage progressively washes fine particles from beneath a structure, forming a channel that can lead to failure. It is a key concern for dams, levees, and dewatered excavations.

The moisture content boundary at which a fine-grained soil transitions from plastic to liquid behavior, typically measured with the Casagrande device or fall cone. It anchors the plasticity index and classification decisions.

The laboratory procedure that determines the relationship between molding water content and dry density for a soil under a specified compactive effort. It establishes the target density used for field acceptance.

The paired values of water content and dry density produced by compaction testing. The resulting curve defines optimum moisture content and maximum dry density for a given soil and energy level.

A laboratory procedure using a brass cup that is dropped repeatedly until a soil pat closes a standard groove over a specified number of blows. Results are plotted to determine the liquid limit water content.