ANALYSIS OF SETTLEMENT AND VOLUME EXPANSION
1. SCOPE. This chapter concerns (a) immediate settlements, (b) long-term
settlements, (c) rate of settlement, (d) criteria for tolerable settlement,
(e) methods of reducing or accelerating settlements for saturated
fine-grained soils and (f) methods for controlling and/or estimating heave
in swelling soils. Procedures given are for fine-grained compressible soils
as well as for coarse-grained soils.
Guidance in other special cases such as collapsing soil, sanitary land
fill, etc., is provided in DM-7.3, Chapter 3. Monitoring of settlements is
discussed in Chapter 2.
2. OCCURRENCE OF SETTLEMENTS. The settlement of saturated cohesive soil
consists of the sum of three components; (1) immediate settlement
occurring as the load is applied, (2) consolidation settlement occurring
gradually as excess pore pressures generated by loads are dissipated, and
(3) secondary compression essentially controlled by the composition and
structure of the soil skeleton.
The settlement of coarse-grained granular soils subjected to foundation
loads occurs primarily from the compression of the soil skeleton due to
rearrangement of particles. The permeability of coarse-grained soil is
large enough to justify the assumption of immediate excess pore pressure
dissipation upon application of load. Settlement of coarse-grained soil can
also be induced by vibratory ground motion due to earthquakes, blasting or
machinery, or by soaking and submergence.
3. APPLICABILITY. Settlement estimates discussed in this chapter are
applicable to cases where shear stresses are well below the shear strength
of the soil.
ANALYSIS OF STRESS CONDITIONS
1. MECHANICS OF CONSOLIDATION. See Figure 1. Superimposed loads develop
pore pressures in compressible strata exceeding the original hydrostatic
pressures. As pore pressure gradients force water from a compressible
stratum, its volume decreases, causing settlement.
2. INITIAL STRESSES. See Figure 2 for profiles of vertical stress in a
compressible stratum prior to construction. For equilibrium conditions
with no excess hydrostatic pressures, compute vertical effective stress as
shown in Case 1, Figure 2.