Civil Engineering Reference
In-Depth Information
Weathering is essentially the opposite of development of bonding. It involves phys-
ical and chemical changes at approximately constant effective stress with increase or
decrease of water content. The effect of weathering is normally to decrease the strength
of bonding and to reduce the yield stress ratio as the state boundary surface collapses
towards the state boundary surface for reconstituted soil.
16.7 Changes in pore water salinity
The fabric of a fine grained soil is influenced by the salinity of the water throughwhich it
was deposited. Clays deposited through sea water are often flocculated and have higher
water contents than the same clay deposited through fresh water. If the original saline
pore water subsequently loses salinity the state boundary surface becomes smaller.
Figure 16.8(a) shows normal compression lines for the same soil sedimented in saline
water and in fresh water. Soil deposited in saline water is normally consolidated at A
and, with time, becomes lightly overconsolidated at B due to creep. If the soil is loaded
it will yield at
Y
s
for soil in saline water where the yield stress is
σ
ys
and the state will
move down the normal compression line for saline water. Notice that
σ
ys
>σ
b
and so
the yield stress ratio at B is greater than 1.
If the pore water becomes fresh, while the state is at B the normal compression line
moves to the left. However, the original fabric, created during deposition in saline
water is preserved. If the soil is loaded it will yield at
Y
s
for soil in saline water where
the yield stress is
σ
ys
but it will then move towards C on the normal compression line
for soil in fresh water. The relatively large compression from
Y
s
to C is similar to that
for bonded soil shown in Fig. 16.7. Notice that
σ
yf
<σ
b
and so the yield stress ratio
at B is less than 1 and B is outside the current normal compression line.
Figure 16.8(b) illustrates the state path A
C corresponding to the
loading path in Fig. 16.8(a) with stresses normalized with respect to the critical stress
σ
c
for soil in fresh water. The soil with fresh pore water but with fabric created in
saline water yields at
Y
s
and ends at C on the state boundary surface for the soil with
fresh water. The state at B is outside the state boundary surface for soil with fresh pore
water.
→
B
→
Y
s
→
Figure 16.8
Influence of changes of pore water salinity.
