Civil Engineering Reference
In-Depth Information
Soil parameters for design
18.1 Introduction
In previous chapters I described simple theories for the mechanics of soils covering
strength, stiffness and permeability. In later chapters I will describe analyses for the
behaviour of slopes, foundations, retaining walls and tunnels. These analyses may be
empirical, they may be solved by hand calculation or they may be solved by complex
numerical analyses but in any case they will require input of numerical values for design
parameters for applied loads and soil parameters. They will also need input of factors
of safety or load factors and these, too, are design parameters. The soil parameters
and factors required are different for slopes, for foundations and for retaining walls
and they are different for different methods of analysis. I will discuss the parameters
required for each type of problem in the appropriate chapter; here I will discuss the
general principles for selection of design parameters and factors for simple analyses.
Complex numerical analyses using the finite element or similar methods require
numerical models for soil behaviour, like Cam clay described in Chapter 12. The
parameters required for Cam clay are the simple critical state parameters but other
numerical models often require special parameters and I will not discuss these here.
Soils are collections of mineral or clay grains which are packed together loosely or
densely; they may also have a structure which is a combination of fabric and bonding,
as described in Chapter 16. There are some soil parameters which depend only on
the nature of the grains; these are called
material parameters
. There are some soil
parameters which depend also on the state; these are called
state dependent parameters
.
Soil parameters may be modified by structure.
Soil parameters can be measured in the laboratory tests described in Chapter 7 or in
in situ tests described in Chapter 17. They may also be estimated from classification
and descriptions of the nature and state of the soil. Natural soils in the ground vary
over relatively short distances. Even a set of tests on identical reconstituted samples
will give different results due to small variations in the test conditions. Consequently,
any set of determinations of a single parameter will show a statistical variation and,
in selection of design values, it is necessary to take account of these variations.
18.2 Principles of design
Engineers are required to design structures which are safe, serviceable and economical:
they must not fall down,
they must not move too much and they should not
