(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

Integrated Publishing, Inc. |