DIKE ENGINEERING - Introduction to Soft Soil Geotechnique: Content, Context and Application - page 301

A peculiar phenomenon is uplift, occurring when groundwater pressures are high
enough to lift the covering ground layer at the lee side slope toe. In some cases,
local erosion arises in the form of sand boils, which undermine the foundation
(piping). Another criterion is the overtopping, water gulping over the dike crest and
loss of stability by subsequent erosion of the lee side.
The bosom embankments are the smallest of all the four types of dike. They are
mostly constructed of clay and peat. The core below the crest often consists of a
great variety of materials as, in the course of time, local authorities have used sand,
rubble, and clay as available to add to the dike's height. The strain that bosom
embankments have to withstand differs from other types of dike. They are
subjected to a practically constant high water level. Their groundwater table is,
consequently fairly high. The subsequent saturation forms a hazard, especially
during periods of high rainfall, as earth-slides occurring along the inland slope
might inflict inundation.
Dike design
Already since prehistoric time, dams and dikes showed two distinct functions:
stability by the two parallel outer embankments of rock and mortar, and
imperviousness by the central filling of materials of the bed of the wadi (Jawa dam,
Kafara dam, Ma'arib dam). In ancient Netherlands, a consistent dike design was
developed by monks; it also consisted of the two separate functions: an
impermeable part by densified sea weed, and a stabile part by an earth body of
clay, reinforced by coupled rows of short wooden piles and a carefully placed stone
toe against wave attack. Until 1850 dikes were made symmetric, just like the
Kafara, with slopes of about 1:2.5. At present, a Dutch dike is faced with an
impermeable layer. This is mostly clay but sometimes supplemented by asphalt or
stone revetment. Apart from being impermeable, the layer must also resist
scouring. A grass cover with a healthy layer of turf is usually sufficient for dikes on
non-tidal river reaches. Dikes subjected to heavier attack are usually given a
revetment of stone or concrete blocks. The core of a dike 'bears' the dike. It must
provide support for the clay layer and give the whole dike sufficient volume and
weight to resist the pressure of the water piled against it.
It is hardly seen or felt, but a dike displaces horizontally for several decimetres
when subjected to high water. Shear forces in the underground provide a further
slip. Other than the clay facing, the core should be permeable. Any water that does
manage to percolate through must be allowed to flow away safely so the body of
the dike does not become saturated and weaker, as higher saturation significantly
decreases soil shear resistance. On the inland side the sand core is again faced with
clay to prevent it being washed away, either by rain or by water overtopping the
crest. Provision is made along the foot of the inland slope to allow water, which
has penetrated the core to drain away safely. Such drainage is vital for the safety of
the dike. It is to be said that many old river dikes have a clay core.
These general features may vary from place to place. Much depends on local
factors, which can differ widely such as the type of ground below the dike, the
materials available in the past, the pressure the dike must withstand, and the
traditions and customs of the area. The evaluation of the relative importance of the
various criteria of dike construction or dike improvement and subsequently the