Shear Strength of Soils - Elements of Soil Mechanics - page 124

Clays

Owing to the low permeability of these soils, any excess pore water pressures generated within them will
not dissipate immediately. The first step in any design work is to determine whether the clay is normally
consolidated or overconsolidated.

Soft or normally consolidated clay

A clay with an undrained shear strength, c u , of not more than 40 kPa is classified as a soft clay and will be
normally consolidated (or lightly overconsolidated). Such clays, when subjected to undrained shear, tend
to develop positive pore water pressures (Fig. 4.33) , so that during and immediately after construction,
the strength of the soil is at its minimum value.
After completion of the construction, over a period of time, the soil will achieve its drained condition
and will then be at its greatest strength.

Overconsolidated clay

With these soils any pore water pressures generated during shear will be negative. This means simply
that the clay is at its strongest during and immediately after construction. The weakest strength value
will occur once the soil achieves its fully drained state, the operative strength parameters then being c ′
and φ ′ .

4.14 The critical state

Critical state soil mechanics is a specialised topic and if a deep understanding of the subject is to be
gained, reference to specialised texts is required. This section of the topic merely offers a simplistic and
short introduction to the topic of the critical state. Readers interested in developing a thorough knowledge
of the subject are referred to the texts by Muir Wood ( 1991) and Atkinson ( 2007 ).
In Section 4.8.2 we saw that during a drained test, the void ratio of a soil changes during shear. If several
samples of the same soil are tested at different initial densities it is found that, if the rate of shearing is
constant, the samples all fail at the same void ratio (see Fig 4.14d ). If the deformation is allowed to con-
tinue the sample will remain at the same void ratio and only deform by shear distortion. This condition is
referred to as the critical state .
If a saturated, remoulded clay is subjected to a loading that creates a constant and low rate of increas-
ing strain, the clay will reach, and pass through, a failure point without collapse and will then continue to
suffer deformation as both the void ratio and the relevant stress paths follow a yield surface until a critical
void ratio value is achieved.
At this critical void ratio value, the values of the void ratio, the pore water pressure and the stresses
within the soil remain constant, even with further deformations, provided that the rate of strain is not
changed.
This important concept has led to the theory of critical state, an attempt to create a soil model that
brings together the relationships between its shear strength and its void ratio, and which can be applied
to any type of soil. The theory has been established as a research tool for several years and is now widely
used in geotechnical limit state design.
Critical state theory uses three parameters: p, q, and v, to describe the mechanical behaviors during
shear and compression.
p and q are defined as:

1
3

p = ( σ + 2 σ ) (1)

1 3