e. Sand Drains Plus Surcharge. A surcharge load is normally placed
above the final fill level to accelerate the required settlement. Surcharge
is especially necessary when the compressible foundation contains material
in which secondary compression predominates over primary consolidation. The
percent consolidation under the surcharge fill necessary to eliminate a
specific amount of settlement under final load is determined as shown in the
lowest panel of Figure 20.
f. General Design Requirements. Analyze stability against foundation
failure by the methods of Chapter 7, including the effect of pore pressures
on the failure plane. Determine allowable buildup of pore pressure in the
compressible stratum as height of fill is increased.
(1) Horizontal Drainage. For major installation investigate in
detail the horizontal coefficient of consolidation by laboratory tests with
drainage in the horizontal direction, or field permeability tests to
determine horizontal permeability.
(2) Consolidation Tests. Evaluate the importance of smear or
disturbance by consolidation tests on remolded samples. For sensitive soils
and highly stratified soils, consider nondisplacement methods for forming
drain holes.
(3) Drainage Material. Determine drainage material and arrangement
to handle maximum flow of water squeezed from the compressible stratum in
accordance with Chapter 6.
g. Construction Control Requirements. Control the rate of fill rise by
installing piezometer and observing pore pressure increase for comparison
with pore pressure values compatible with stability. Check anticipated rate
of consolidation by pore pressure dissipation and settlement measurements.
Section 7.
ANALYSIS OF VOLUME EXPANSION
1. CAUSES OF VOLUME EXPANSION. Volume expansion is caused by (a) reduction
of effective stresses, (b) mineral changes, and (c) formation and growth of
ice lenses. Swell with decrease of effective stress is a reverse of the
consolidation process. For description of swelling problems and suggested
treatment, see Table 8. Where highly preconsolidated plastic clays are
present at the ground surface, seasonal cycles of rainfall and desiccation
produce volume changes. The most severe swelling occurs with
montmorillinite clays although, in an appropriate climate, any surface clay
of medium to high plasticity with relatively low moisture content can heave.
For estimation of swell potential see Chapter 1, Section 6.
2. MAGNITUDE OF VOLUME EXPANSION. Figure 22 outlines a procedure for
estimating the magnitude of swelling that may occur when footings are built
on expansive clay soils. This figure also indicates a method of determining
the necessary undercut to reduce the heave to an acceptable value. Further
guidance for foundations on expansive soils is contained in DM-7.3, Chapter
3.
7.1-253
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