COASTAL ENGINEERING - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 284

D A LIQUEFACTION MODEL
The sudden change of constitution of a sand or clay layer into a heavy mud
usually occurs due to a dynamic trigger, an earthquake or a heavy vibration by
wave impact or human action (pile driving). Two specific situations are considered:
a slope and a horizontal layer.

cotg cotg

q q q q q q q q q q q

10 10 10 10 10 10

increases, q decreases increases, q decreases increases, q decreases increases, q decreases increases, q decreases increases, q decreases increases, q decreases increases, q decreases increases, q decreases increases, q decreases increases, q decreases

tested tested tested tested tested tested

STABLE STABLE STABLE STABLE STABLE STABLE

start metastability start metastability start metastability start metastability start metastability start metastability start metastability

5 5 5 5 5 5

h [m] h [m] h [m] h [m] h [m] h [m]

100 100 100 100 100 100

30 30 30 30 30 30

10 10 10 10 10 10

3 3 3 3 3 3

D r D r D r D r D r D r

METASTABLE
0.2

METASTABLE
0.2

METASTABLE
0.2

METASTABLE
0.2

METASTABLE
0.2

METASTABLE
0.2 0.4 0.4 0.4 0.4 0.4 0.4 0.6 0.6 0.6 0.6 0.6 0.6

Figure 16.17 Metastable submerged slopes tested and simulated by 3D numerical model

Static liquefaction
The situation of a slope is characterised by the shear resistance. The slope may
seem stable, but if the layer is loosely packed (contractant) then sudden failure
(bifurcation) may occur, since the residual shear strength is less then the actual
shear stress state. A trigger can cause the layer to liquefy (static liquefaction). If the
slope is in such a critical situation, it is called metastable. It is difficult in practice
to determine whether a slope is metastable. Sandy underwater slopes are at certain
depth metastable, and a trigger may cause an semi-undrained response of sudden
large excess pore pressures.

Figure 16.18 Large scale model test hydraulic fill
Advancing slopes during hydraulic fill (D 50 =135mu, U=1.5). Slides are noticed
at t1 > t2, t4 > t5, and t6 > t7; Excess pore pressures just before slide t6 (dotted line).