GEOTECHNICAL

Plasticity Index

Liquid limit minus plastic limit; the range of plastic behaviour of a soil

Also calledPIIpsoil plasticity
Related on InfraLens
CODES
IS 2720IS 1498
Definition

The plasticity index (PI = LL − PL) is the numerical difference between the liquid limit and the plastic limit — the range of water content over which a soil behaves plastically. A high PI means a clay that holds a wide plastic range, is highly compressible, swells/shrinks markedly and has poor strength when wet; a low PI (or non-plastic) indicates silts and sands.

PI plots against LL on the IS 1498 plasticity chart, where the A-line (PI = 0.73(LL − 20)) separates clays (above) from silts/organic soils (below) and gives the group symbol (CL, CH, ML, MH, OL, OH). PI also correlates empirically with swell potential, free-swell index and CBR, making it a quick screening number for subgrade acceptance and the need for soil stabilisation (lime, cement) or replacement.

Where used
  • IS 1498 plasticity-chart soil classification
  • Swell-potential + expansive-soil screening
  • Pavement subgrade acceptance (IRC 37)
  • Selecting lime/cement soil-stabilisation dosage
  • Earth-fill + embankment material control
Acceptance / threshold
Computed from IS 2720 Part 5 results. Subgrade/fill PI limits are project + IRC-37 specific; high-PI clays generally require stabilisation or replacement.
Frequently asked
How is plasticity index calculated?
PI = Liquid Limit − Plastic Limit, both expressed as percentage water content and determined per IS 2720 Part 5.
What does a high plasticity index indicate?
A highly plastic, compressible clay that swells and shrinks with moisture and has low wet strength — generally a poor subgrade/fill needing stabilisation or replacement.
Related terms
IS 2720 — Parts List (Methods of Test for Soils)
Complete list of all 19 parts of IS 2720 — methods of test for soils.
Soil Classification
Per IS 1498 / Unified Soil Classification System
Atterberg Limits
Liquid limit, plastic limit + shrinkage limit defining fine-soil consistency
Effective Stress
Stress carried by the soil skeleton — total stress minus pore water pressure
Cohesion & Friction Angle
Mohr-Coulomb shear-strength parameters. Clay: cohesive (c). Sand: frictional (φ). C-φ soil has both.
California Bearing Ratio (CBR)
Subgrade strength index for pavement design
Pore Water Pressure
Water pressure in soil voids; reduces effective stress and hence strength
Expansive Soil (Black Cotton Soil)
Montmorillonitic clay that swells when wet + shrinks when dry, cracking structures
Proctor Compaction Test
Lab test giving the soil compaction curve (OMC + MDD)
Consolidation Test (Oedometer)
Lab test giving compressibility + rate of long-term clay settlement