Soil Mechanics: Atterberg Limits, Soil Classification & In-situ Density

40 vocabulary flashcards covering key terms from the lecture notes on Atterberg limits, soil classification systems, plasticity charts, and the sand-replacement field density test.

Liquid Limit (LL)

Minimum water content at which soil will flow; in the Casagrande test it is the moisture content that closes a 12 mm groove after 25 blows.

Plastic Limit (PL)

Moisture content at which a soil just begins to crumble when rolled into 3 mm diameter threads.

Plasticity Index (PI)

Numerical difference LL − PL; represents the range of moisture where the soil remains plastic.

Casagrande Liquid Limit Device

Standard apparatus with a brass cup lifted and dropped by a cam to determine a soil’s liquid limit.

Grooving Tool

Tool used to cut a standard groove in the soil paste placed in the Casagrande cup.

A-line

Empirical boundary on plasticity charts separating clays (above) from silts (below).

Plasticity Chart (USCS)

Graph of PI versus LL (

Unified Soil Classification System (USCS)

Engineering system that classifies soils by grain size distribution and plasticity symbols like GW, CL, CH.

British Soil Classification System (BSCS)

BS 5930 scheme grouping soils into coarse and fine categories with qualifiers L, I, H, V, E.

Well-graded Gravel (GW)

Gravel with Cu > 4 and 1 < Cc < 3 containing little or no fines (0–5 %).

Poorly-graded Gravel (GP)

Gravel not meeting GW gradation requirements; little or no fines (0–5 %).

Silty Gravel (GM)

Gravel with >12 % fines below A-line or PI < 4.

Clayey Gravel (GC)

Gravel with >12 % fines above A-line and PI > 7.

Well-graded Sand (SW)

Sand with Cu > 6, 1 < Cc < 3; fines 0–5 %.

Poorly-graded Sand (SP)

Sand failing SW gradation criteria; fines 0–5 %.

Silty Sand (SM)

Sand with >12 % non-plastic or low-plastic fines (PI < 4 or below A-line).

Clayey Sand (SC)

Sand with >12 % plastic fines above A-line and PI > 7.

Low-plasticity Clay (CL)

Inorganic clay with LL < 50 lying above A-line on plasticity chart.

High-plasticity Clay (CH)

Inorganic clay with LL > 50 lying above A-line on plasticity chart.

Low-plasticity Silt (ML)

Inorganic silt with LL < 50 lying below A-line (or PI < 4).

High-plasticity Silt (MH)

Inorganic silt with LL > 50 lying below A-line.

Organic Soil (O)

Soil containing significant organic matter; symbols OL (low LL) and OH (high LL).

Peat (Pt)

Highly organic, fibrous soil of very low density and high compressibility.

Fine-grained Soil

Material with >50 % passing 0.063 mm sieve (No. 200); may be silts or clays.

Sand Replacement Method

Field procedure to determine in-situ dry density by excavating a hole and replacing its volume with calibrated sand.

Pouring Cylinder

Device with cone and shutter used to pour standard sand into a calibration can or test hole.

Calibrating Can

Metal container of known volume used to determine the bulk density of standard sand for the sand-replacement test.

Bulk Density of Sand

Mass of calibrated sand that fills a known volume, needed for in-situ density calculations.

Dry Density of Soil

Mass of soil solids per unit total volume; computed from bulk density divided by (1 + M).

Moisture Content (M)

Ratio of mass of water to dry soil mass, usually expressed as a percentage.

Consistency Index (CI)

(LL − natural MC)/PI; indicates relative consistency of in-place soil.

Number of Blows (N)

Count of cup drops in the Casagrande device required to close the groove; plotted versus moisture content to find LL.

Speedy Moisture Meter

Rapid field device for estimating soil moisture using calcium carbide gas pressure.

Calibration of Sand

Process of determining cone weight and bulk density of sand before using the sand-replacement equipment.

Well-graded (W) qualifier

Describes soil with a wide, continuous particle size range meeting Cu and Cc limits.

Poorly-graded (P) qualifier

Describes soil that is uniform or gap-graded and fails well-graded criteria.

Uniform (Pu) qualifier

British term for soils with a narrow particle-size range (steep grading curve).

Gap-graded (Pg) qualifier

British term for soil missing certain size ranges, resulting in a flat portion on the grading curve.

Organic Modifier ('O' suffix)

Letter added in BSCS to show presence of organic matter in any soil group.