b. Elimination of Secondary Consolidation. Use the formula in the
bottom panel of Figure 17 to determine surcharge load and percent
consolidation under surcharge required to eliminate primary consolidation
plus a specific secondary compression under final load.
c. Limitations on Surcharge. In addition to considerations of time
available and cost, the surcharge load may induce shear failure of the soft
foundation soil. Analyze stability under surcharge by methods of Chapter 7.
5. VERTICAL DRAINS. These consist of a column of pervious material placed
in cylindrical vertical holes in the compressible stratum at sufficiently
close spaces so that the horizontal drainage path is less than the vertical
drainage path. All drains should be connected at the ground surface to a
drainage blanket. Vertical drains are utilized in connection with fills
supporting pavements or low- to moderate-load structures and storage tanks.
Common types of vertical drains are shown in Table 7 (Reference 15, Use of
Precompression and Vertical Sand Drains for Stabilization of Foundation
Soils, by Ladd). Sand drains driven with a closed-end pipe produce the
largest displacement and disturbance in the surrounding soil and thus their
effectiveness is reduced.
a. Characteristics. Vertical drains accelerate consolidation by
facilitating drainage of pore water but do not change total compression of
the stratum subjected to a specific load. Vertical drains are laid out in
rows, staggered, or aligned to form patterns of equilateral triangles or
squares. See Figure 18 for cross-section and design data for typical
installation for sand drains.
b. Consolidation Rate. Time rate of consolidation by
radial drainage
of pore water to vertical drains is defined by time factor
curves in upper
panel of Figure 10. For convenience, use the nomograph of
Figure 19 to
determine consolidation time rate. Determine the combined
effect of
vertical and radial drainage on consolidation time rate as
shown in the
example in Figure 10.
c. Vertical Drain Design. See Figure 20 for an example of design. For
a trial selection of drain diameter and spacing, combine percent
consolidation at a specific time from vertical drainage with percent
consolidation for radial drainage to the drain. This combined percent
spacing in the center panel of Figure 20. Selection of drain spacing
depends on the percent consolidation required prior to start of structure,
the time available for consolidation, and economic considerations.
d. Allowance for Smear and Disturbance. In cases where sand drain
holes are driven with a closed-end pipe, soil in a surrounding annular space
one-third to one-half the drain diameter in width is remolded and its
stratification is distorted by smear. Smear tends to reduce the horizontal
permeability coefficient, and a correction should be made in accordance with
Figure 21.
7.1-246
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