1-1. Purpose and Scope. This manual presents guidelines for calculation of
vertical displacements and settlement of soil under shallow foundations (mats
and footings) supporting various types of structures and under embankments.
a. Causes of Soil Displacements. Soil is a nonhomogeneous porous material
consisting of three phases: solids, fluid (normally water), and air.
Soil deformation may occur by change in stress, water content, soil mass, or
temperature. Vertical displacements and settlement caused by change in stress
and water content are described in this manual. Limitations of these movements
required for different structures are described in Chapter 2.
(1) Elastic deformation. Elastic or immediate deformation caused by
static loads is usually small, and it occurs essentially at the same time
these loads are applied to the soil. Guidance for tests and analyses to estimate
immediate settlements of foundations, embankments, pavements, and other
structures on cohesionless and cohesive soils for static loading conditions is
given in Sections I and II of Chapter 3.
(2) Consolidation. Time delayed consolidation is the reduction in volume
associated with a reduction in water content, and it occurs in all soils.
Consolidation occurs quickly in coarse-grained soils such as sands and gravels,
and it is usually not distinguishable from elastic deformation. Consolidation
in fine-grained soils such as clays and organic materials can be significant
and take considerable time to complete. Guidance for tests and analyses
to estimate consolidation settlement of foundations, embankments, pavements,
and other structures on cohesive soil for static loading conditions is
given in Section III of Chapter 3.
(3) Secondary compression and creep. Secondary compression and creep
are associated with the compression and distortion at constant water content
of compressible soils such as clays, silts, organic materials, and peat.
Guidance for tests and analyses to estimate secondary compression settlement
is given in Section IV of Chapter 3.
(4) Dynamic forces. Dynamic loads cause settlement from rearrangement
of particles, particularly in cohesionless soil, into more compact positions.
Guidance to estimate settlement for some dynamic loads is given in Chapter 4.
(5) Expansive soil. Expansive soil contains colloidal clay minerals
such as montmorillonite that experience heave and shrinkage with changes in
the soil water content. Guidance for calculation of soil movements in expansive
soil is given in Section I of Chapter 5.
The purpose of this manual is to supply pointers for calculations of vertical displacements and settlement of soil below shallow foundations supporting numerous varieties of structures and below embankments.