Civil Engineering Reference
In-Depth Information
Fig. 1.1
Materials encountered during excavation.
Soil
The soft geological deposits extending from the subsoil to bedrock. In some soils there is a certain amount
of cementation between the grains which affects the physical properties of the soil. If this cementation is
such that a rock-hard material has been produced, then the material must be described as rock. A rough
rule is that if the material can be excavated by hand or hand tools, then it is a soil.
Groundwater
A reservoir of underground water. The upper surface of this water may occur at any depth and is known
as the water table or groundwater level (GWL).
1.2 Engineering definitions
Geologists class all items of the Earth's crust as rock, whether hard or soft deposits. Civil engineers con-
sider rock and soil separately.
1.2.1 Rock
Rocks are made from various types of minerals. Minerals are substances of crystalline form made up from
a particular chemical combination. The main minerals found in rocks include quartz, feldspar, calcite and
mica. Geologists classify all rocks into three basic groups:
igneous
,
sedimentary
and
metamorphic.
Igneous rocks
These rocks have become solid from a melted liquid state.
Extrusive
igneous rocks are those that arrived
on the surface of the Earth as molten lava and cooled.
Intrusive
igneous rocks are formed from magma
(molten rock) that forced itself through cracks into the rock beds below the surface and solidified there.
Examples of igneous rocks:
granite, basalt, gabbro.
Sedimentary rocks
Weathering reduces the rock mass in to fragmented particles, which can be more easily transported by
wind, water and ice. When dropped by the agents of weathering, they are termed sediments. These sedi-
