TM 5-852-4/AFM 88-19, Chap. 4
point lowering this corresponds to only about 1/40F. As
force. Of course, if foundation members are isolated
indicated in figures 2-12, 2-15 and 4-53, the potential
from the annual frost zone by methods as described in
strength and deformation properties of ice are such that
paragraph 4-31 such requirements are reduced. Since
strata of hard, high-density ice in the foundation may
negligible lateral support may be provided in the
exhibit better bearing characteristics than may frozen
(12) Innumerable
variations
and
soils, especially at temperatures above about 25F.
combinations of structural designs of foundation
However, porous ice may be compressible.
members are possible. Piers may be stepped, tapered
(19) Design procedure for predicting the
or made hollow, footings and columns may be combined
resonant frequency and displacement under vibratory
in the form of pedestals, etc. It is not possible to
loads is given in EM 1110-345-31020.
anticipate and give guidance for all such variations.
However, the principles outlined herein may be
(20) Design guidance with respect to other
combined as needed to fit any situation.
aspects of footing, raft and pier design are presented in
(13) Allowable bearing values for shallow
other paragraphs of this manual as follows:
footings and sills supported on well-drained granular
Settlement analysis, paragraph 3-4f
mats may be based on unfrozen strengths of these
Thermal stability analysis and control, paragraph
materials. The thickness of granular material between
4-2
footing and underlying natural soil must, however, be
Control of movement and distortion from freeze
sufficient to reduce concentrated stresses on the natural
and thaw, paragraph 4-3
soil to tolerable levels.
Estimation of creep deformation, paragraph 4-5
b. Illustrative examples for the design of
(14) Allowable bearing values for footings,
footings in permafrost.
rafts and piers supported on frozen materials should be
(1) Steps required for design of footings on
determined using analytical procedures and factors of
safety outlined in TM 5-818-1/AFM 88-3, Chapter 7 5
permafrost. Determine required depth of footing (a
above); determine the temperature distribution in the
(F.S. = 2.0 for dead load plus normal live load and 1.5
permafrost with respect to the base of the footing, for the
for dead load plus maximum live load) and the design
critical period of the year; check the bearing capacity of a
strength values determined as described in paragraph 4-
trial footing size in this critical period of the year using
4 for ultimate strength analysis, assuming that control
standard soil mechanics theory (a above) with properties
against degradation of permafrost has been carefully
of the frozen soil determined by field or laboratory tests
provided in the design. Since most footings supported
(see chap 3); adjust the footing dimensions to obtain the
on permafrost are placed near the top of permafrost
desired factor of safety; and make a settlement analysis.
where ground temperature may rise fairly close to 32 F
in summer and fall, soil design values within 1 to 3 ƒF of
Determine the distribution of the imposed vertical stress
32 ƒF are usually applicable. Stresses in the weakest
with depth beneath the footing.
Compute the
deformation of a free standing column of frozen soil,
underlying strata must be kept within tolerable levels.
using creep equations and constants obtained from
Creep deformations estimated in accordance with
laboratory unconfined compression creep tests (para 4-
paragraph 4-5 may also exert overriding control over
5). If the estimated settlement is excessive, enlarge the
allowable bearing values.
footing or otherwise revise the design to reduce the
(15) Those of the analytical bearing
estimated settlement to a tolerable amount.
capacity analyses presented in TM 5-818-1/AFM 88-3,
Chapter 75 which are most typically applicable for frozen
(2) Example of design of an isolated
square footing.
materials are illustrated in figure 4-61, covering shallow
and deep foundations.
(a) Assumptions:
(16) For computation of bearing capacity of
Permanent structure, 25-year life
unfrozen soils and for detailed general guidance,
Soil conditions as shown in boring
reference should be made to applicable procedures
log (fig. 4-62)
5
outlined in TM 5-818-1/AFM 88-3, Chapter 7 .
Properties of the 2-feet-thick
organic sandy silt layer (fig. 4-
(17) Design bearing pressures of 2500,
62) essentially the same as for
3000 and 4000 psf on frozen soils have been used
the underlying sandy silt
satisfactorily in the designs shown in figures 4-15, 4-16
Footing design of general type
and 4-27, respectively. A bearing pressure of 12,000 psf
shown in figure 4-63
has been used satisfactorily on very bouldery till with
Column load 150 tons
estimated highest soil temperature at the base of the
86
Required factor of safety with respect
footing of 25 F, near Thule AB, Greenland .
to ultimate bearing capacity =
(18) Settlement of footings on permafrost
2.0
from pressure melting or extrusion of ice from soil voids
Air space provided between building
should be assumed negligible under normal footing
floor
loadings. A footing bearing value of 3600 psf, for
example, corresponds to 25 psi. In terms of melting
4-95
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