UNIAXIAL STRESS AND STRAIN - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 67

5 UNIAXIAL STRESS AND STRAIN

A STRESS AND STRAIN
Material behaviour in soils is expressed by effective stress . 19

Their ratio is called stiffness, and the slope of the stress-strain curve is the elasticity
modulus E (Fig 5.1). If the curve is independent of the loading history, the material
is called elastic, and when E is constant it is linear-elastic, characterised by
Hooke's law.

' and strain

' = E (5.1)

Sometimes a linear value is used for a certain stress range: the tangent or secant
modulus. Usually, soil is dependent on loading history. Unloading and reloading
differ from earlier behaviour (marked by the preloading or preconsolidation
pressure). Soil may show creep (less in sand, more in clay) and/or relaxation, gain
strength and stiffness with time (ageing, preconsolidation ratio), and the elasticity
may depend on the loading rate (stiffer for faster loading). For larger strains soil
behaviour may yield and develop hardening, plasticity or softening, or it may break
in a more brittle way (soft rock).

stress stress

!"# !"#

!$# !$#

compaction compaction

preloading
point

preloading
point

swell swell

unloading
reloading

unloading
reloading

Figure 5.1 Uniaxial stress-strain curve

In principle soil mechanics includes multiple dimensions. Triaxial strains are
defined as follows

= ( x , y , z ) and ' = ( x ', y ', z ' ) in the directions (x, y, z) (5.2a)

19 Common in geotechnics: strain is defined positive for compaction; stress is defined
positive for pressure, throughout this topic. As strain is related to porosity, this definition
leads to an extra minus sign: d n = (1 n )d v .