Geology is the foundation of soil mechanics. Without a proper understanding
of the underlying geologic processes that have formed the particular soil you
have to deal with, a soils or geotechnical engineer will be playing with fire.
This is where a well trained engineering geologist is so vitally important. They
can help other people involved in the design and construction of a facility or
project understand the geologic antecedents of the materials they have to use.
Geotechnical engineers classify soils, or more properly earth materials, for
their properties relative to foundation support or use as building material.
These systems are designed to predict some of the engineering properties and
behavior of a soil based on a few simple laboratory or field tests, though some
earlier systems were adaptations of soil-science classification systems. The
most common is the Unified Soil Classification System, with three major groups:
(1) coarse-grained, sands and gravels, (2) fine-grained, silts and clays, and
(3) highly organic soils (referred to as peat even when the soil is not truly a
peat). The first two groups are then subdivided as follows:
Coarse grain: gravels, sands, based on the grain-size of the
coarse-grained fraction. Fine grain: silts, clays, organics silts &
clays, based on plasticity and organic content.
Fine-grained soils are then subdivided according to their plasticity, while
coarse-grained soils are subdivided by the presence and properties of the fines
or the grain-size distribution of the soil.
A full geotechnical engineering soil description will include other
properties of the soil, including color, in-situ moisture content, in-situ
strength, and somewhat more detail about the material properties of the soil
than is provided by the USCS code.
The objective of a geotechnical investigation is to characterize the geologic
environment. This may include;
1) The lateral distribution and thickness of the soil and rock strata.
2) Groundwater Conditions and quality.
3) Ground response to changing conditions such as surface loading from
structures and unloading by excavations or extraction.
4) Physical properties of soils and rock formations.
5) Engineering properties of soils and rock formations.
6) Hazardous conditions such as unstable slopes, faults, flood plains, ground
subsidence and heave potential.
The magnitude of the site investigation depends upon the geological
variability of the site as well as the size (spatially and financially) of the