(1) Rigid Surface Layer Over Weaker Underlying Layer. If the
surface layer is the more rigid, it acts as a distributing mat and the
vertical stresses in the underlying soil layer are less than Boussinesq
values.
(2) Weaker Surface Layer Over Stronger Underlying Layers. If the
surface layer is less rigid than the underlying layer, then vertical
stresses in both layers exceed the Boussinesq values. For influence
diagrams for vertical stresses beneath rectangular loaded areas, see
Reference 8, Stress and Displacement Characteristics of a Two-Layer Rigid
Base Soil System: Influence Diagrams and Practical Applications, by
Burmister. Use these influence diagrams to determine vertical stress
distribution for settlement analysis involving a soft surface layer
underlain by stiff material.
(3) Multi-Layer (Three or More) Systems. See Reference 6 for a
discussion of the use of various approximate solutions for multi-layer
systems.
c. Critical Depth. If there is no distinct change in the character of
subsurface strata within the critical depth, elastic solutions for layered
foundations need not be considered. Critical depth is the depth below the
foundation within which soil compression contributes significantly to
surface settlements. For fine-grained compressible soils, the critical
depth extends to that point where applied stress decreases to 10 percent of
effective overburden pressure. In coarse-grained material critical depth
extends to that point where applied stress decreases to 20 percent of
effective overburden pressure.
3. RIGID LOADED AREA. A rigid foundation must settle uniformly. When
such a foundation rests on a perfectly elastic material, in order for it to
deform uniformly the load must shift from the center to the edges, thus
resulting in a pressure distribution which increases toward the edges (see
Figure 16). This is the case for clays. In the case of sands, the soil
near the edges yields because of the lack of confinement, thus causing the
load to shift toward the center.
4. STRESSES INDUCED BY PILE LOADS. Estimates of the vertical stresses
induced in a soil mass by an axially loaded pile are given in Figure 17
(Reference 9, Influence Scale and Influence Chart for the Computation of
Stresses Due, Respectively, to Surface Point Load and Pile Load, by Grillo)
for both friction and end-bearing piles. (See DM-7.2, Chapter 5 for further
guidance on pile foundations.)
Section 4.
SHALLOW PIPES AND CONDUITS
1. GENERAL. Pressures acting on shallow buried pipe and conduits are
influenced by the relative rigidity of the pipe and surrounding soil, depth
of cover, type of loading, span (maximum width) of structure, method of
construction, and shape of pipe. This section describes simple procedures
for determining pressures acting on a conduit in compressible soil for use
in conduit design. For detailed analysis and design procedures for conduits
in backfilled trenches and beneath embankments, consult one of the
following:
7.1-181
Previous Page
