COASTAL ENGINEERING - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 267

the structure's malfunctioning. The occurrence of these phenomena is covered by
the so-called filter rules, which defines limits of adjacent coarse and fine granular
grain sizes. For uniform materials the non-migrating filter rule states D 50 f / D 50 b < 5
to 10, and for wide graded materials three conditions are to be satisfied: (1) D 15 f
/ D 85 b < 4 to 9, and D 50 f / D 50 b < 12 to 18, and for preventing clogging D 20 f / D 20 b > 5.
Suffix f stands for the coarse filter and b for the base material. For wide graded
material suffusion or colmation is likely to happen.
If a filter is subjected to dynamic pore pressure gradients (a gradient of 20% is
considered significant) an extra norm should be considered: N f = n f D 15 f /D 50 b < 1 to
5. Artificial armour and rock fill degenerate with time, breaks at intergranular
contacts and fines are washed out or accumulate (blocking). This process is
characterised as material fatigue. Water may convey free sediments that settle in
the pores and change the hydraulic permeability. It may become a serious problem
under critical loading conditions. Growth of marine organisms in pores may have a
similar effect.
An adequate thickness of a filter layer is at least 3 times D 85 or 20 cm, or 30 cm
when placement occurs under water. In literature, refined filter rules can be found
for other materials (geotextiles).

Liquefaction
The sensitivity of granular material becoming liquefied is expressed by the so-
called liquefaction potential, which can be determined by special laboratory tests
(cyclic shear or cyclic triaxial test) on samples from the site at various
manufactured densities. Results should be calibrated with the in-situ density, which
is difficult to measure, unfortunately. Under critical dynamic loading excess pore
pressures may arise depending on the slope angle, the wave period and the drainage
capacity. When the excess pore pressure reaches the actual effective stress level,
the granular structure collapses into a mud: liquefaction. Particularly fine loose
sands ( D 50 < 300

) are sensitive to this excess pore pressure generation. When
sands are densely packed, negative pore pressures may arise, which create a
temporary additional strength. This can be of significant influence in dredging
operations.
When locally the soil liquefies, it behaves suddenly as a heavy liquid.
Immediately, induced additional pore pressures in the surroundings will arise (Fig
16.2), jeopardising structures by drastic reduction of required shear strength. When
a liquefied sand is covered, sand fountains at the surface may occur, sometimes
observed during earthquakes. Also fresh deposited hydraulic fill may suddenly fail
due to liquefaction. The air content is crucial for the liquefaction potential. Small
air content in the pore water (> 2%) may drastically reduce the likelihood of
liquefaction, because it damps the excess pore pressure generation significantly. A
light cementation of loose sands is a remedy against liquefaction, and here is a
potential for the application of biotechnology (see Chapter 13).
In Fig 16.2 a column test shows that after a shock the loosely packed top sand
layer suddenly liquefies and behaves like a heavy fluid. The pore pressures have
been measured by 6 sensors, equally spaced along the vertical. The weight of
liquefied mass is directly felt as excess pore pressures in the densely packed sand