Page 65 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soils That Are Sediments
60 Geotechnical Engineering

4.1.2 Transportation and Sorting
There are five common transporting agents for sediments: wind, water, ice,
gravity, and animals including man.

Sorting refers to a tendency for sediments to be separated into uniform grain sizes,
which varies according to the transporting agent. Landslide and glacial till soils
exhibit little or no sorting action, particularly when compared with water- and
wind-deposited soils.

A stream or a river is capable of excellent sorting action, with gravel being
deposited by rapidly flowing current, sand by a moderate current, silt in a slow
current, and clay with practically no current at all. Common sediments and
their degrees of sorting are listed in Table 4.1.

4.2 SEDIMENTS AND GEOGRAPHY

4.2.1 Location, Location

The fact that many towns and cities were founded near rivers or seacoasts
emphasizes the relevance of sedimentary soils in engineering. The reason of course
is accessibility for transportation and trade. Sediments in alluvial and coastal
areas often are recently deposited and relatively soft, having settled to an equilib-
rium density under their own weight. This means that if an additional weight
such as a foundation is added, it will compress the soil and settle. The amount of
settlement that can be allowed is the most common criterion used in foundation
design. The soil density and stiffness tend to increase with depth, a factor that
is utilized when founding structures on piles.

4.2.2 Settlement
Sediments that are in equilibrium with their own weight are said to be normally
consolidated. Venice is an example of a city that is situated on a soil that still is
normally consolidating, which explains why Venice is sinking. Both the amount
and rate of settlement are important considerations for any design.

The position of a groundwater table also affects settlement because of the buoyant
reduction in soil weight—the lower the weight, the less a soil will tend to con-
solidate under its own weight. For example, consider the weight of a bucket
of sand both in and out of the water. According to a principle first put forth
by Archimedes, the weight of the submerged object is reduced by the weight of
the water that it displaces. This difference is readily calculated by knowing the
density of the soil and of the water.

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