Measurement of Properties - Geotechnical Engineering Investigation - page 218

California bearing ratio (CBR)

2 3 4 5 6 7 8 9 10 15 20 30 40 50 60 70 80 90100

AASHO soil classification

A-1-b A-1-a

A-2-7 A-2-6 A-2-5 A-2-4

A-3

A-4

A-5

A-6

A-7-6 A-7-5

Unified soil classification

GW

OH CH GM-u GM-d

MH OL GC

CL SW

ML SM-d

SC
SM-u GP

SP

General soil rating as subgrade, subbase, or base

Subgrade Poor Medium Good Excellent

Subbase Unacceptable Acceptable Good Excellent

Base Acceptable

FIGURE 3.65
Approximate correlation of soil ratings based on CBR values for use in design of light-traffic pavements. (From
Asphalt Institute, Thickness Design , The Asphalt Institute, Manual Series No. 1, College Park, MD, 1970. With
permission.)

σ σ σ

σ f

FIGURE 3.66
Stress-strain relationships of elastic and
plastic deformation: (a) elastic; (b)
rigid-plastic; (c) elastic-plastic.

ε ε ε

Strain Strain Strain

(a) (b) (c)

Plastic deformation: Permanent and continuous deformation occurs when the applied
stress reaches a characteristic stress level (Figure 3.66b). Geologic materials often combine
deformation modes under stress (Figure 3.66c).
Viscous deformation: The rate of deformation is roughly proportional to applied stress.

Geologic Materials
Compression: Varies from essentially ideal to less than ideal, i.e., elastic compression under
relatively low stress levels followed by plastic compression resulting from the closure of
soil voids or rock fractures. Some materials may deform only plastically, others may
exhibit plastic-elastic-plastic deformation at different stress levels.
Creep: A time-dependent deformation at constant stress level below failure level.
Expansion: An increase in volume from swelling, or elastic or plastic extension strain.

Geotechnical Parameters Used to Define Deformations
Deformation moduli are quantities expressing the measure of change in the form of the
dimensions of a body occurring in response to changing stress conditions. The quantities
include the elastic modulus (static and dynamic), moduli from the stress-strain curve,
compression modulus from pressure-meter testing, and the modulus of subgrade reaction.
Consolidation parameters define various aspects of the slow process of compression under
applied stress that occurs as water is extruded from the voids of clay soils.