COASTAL ENGINEERING - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 268

layer underneath. These excess pore pressures dissipate in accordance with the
sedimentation process of the liquefied layer. The expelled water due to
densification appears on top of the sand column (after 8 shocks it reached about 8%
of the height of the top layer). This test is performed by successive equal shocks,
and each time a liquefaction occurred in the top layer and the following
sedimentation proceeds each time about 25% faster than after the previous shock.
The increase of relative density D r and the decrease of porosity n of the top layer is
shown in the inset of Fig 16.2 (the critical density is 0.65). The lower layer had a
high density and did not liquefy.

100 0.47

6

n

0.65

n sedimentation

5

D r

100

100

D r

0.39

hydrostatic

90

4

90 0 0.34

1 2 3 4 5 6 7 8

shock

6

80

80

3

1

70

70

5

n = 0.47

2

60

60

2

4

3 50

50

time from start
liquefaction [s]

liquefied

3

40 40

4

1

2 30

30

0

5

16

n = 0.34

20 20

46

1

pressure
head
h [cm]

z [cm]

6

75

10 10

112

t [s]

u [kP a]

20 50 10

0 100 150 30

(a) measurements first shock (b) test set-up (c) actual pore pressures first shock
Figure 16.2 The effect of liquefaction to the direct surroundings

In multi-dimensional situations the induced excess pore pressures may dissipate
alongside the liquefied zone. The expelled water has also to dissipate through the
sides and above the liquefied zone, creating excess pore pressures there as well.
This process is governed by the transport and storage capacity of the direct
surroundings of the liquefied zone. The related consolidation is in the order of
minutes. During this period the shear strength of the soil is seriously reduced. It is
important to realise that the influence of liquefaction may reach far beyond the
liquefied zone itself.

Liquefaction of a land fill
The recent harbour enlargement in Barcelona included two new large
breakwaters and a wharf for a container terminal, covering 934 hectare and 1500
metres quay wall (Fig 16.3a). The quay wall of the wharf consisted of large pre-
cast reinforced sand-filled caissons (each 17.5 metre high, 18.5 metre wide and 40
metre long) based on a rubble mound sill in a dredged trench. Land reclamation of
the area behind the quay wall was performed by hydraulic fill.