TM 5-818-7
one- or two-story buildings, warehouses, residences,
piration of moisture from heavy vegetation into the at-
and pavements are especially vulnerable to damage be-
mosphere. Figure 1-2 schematically illustrates some
cause these structures are less able to suppress the dif-
commonly observed exterior cracks in brick walls from
ferential heave of the swelling foundation soil than
doming or edgedown patterns of heave. The pattern of
heavy, multistory structures.
heave generally causes the external walls in the super-
(1) Type of damages. Damages sustained by these
structure to lean outward, resulting in horizontal,
structures include: distortion and cracking of pave-
vertical, and diagonal fractures with larger cracks
ments and on-grade floor slabs; cracks in grade beams,
near the top. The roof tends to restrain the rotation
walls, and drilled shafts; jammed or misaligned doors
from vertical differential movements leading to addi-
and windows; and failure of steel or concrete plinths
tional horizontal fractures near the roofline at the top
(or blocks) supporting grade beams. Lateral forces may
of the wall. Semiarid, hot, and dry climates and deep
lead to buckling of basement and retaining walls, par-
water tables can be more conducive to severe and pro-
ticularly in overconsolidated and nonfissured soils.
gressive foundation soil heaves if water become avail-
The magnitude of damages to structures can be exten-
able.
sive, impair the usefulness of the structure, and de-
(2) Cyclic heave. A cyclic expansion-contraction
tract aesthetically from the environment. Mainte-
nance and repair requirements can be extensive, and
rainfall and evapotranspiration may be superimposed
the expenses can grossly exceed the original cost of the
on long-term heave near the perimeter of the struc-
foundation.
ture. Localized heaving may occur near water leaks or
(2) Example of damages. Figure 1-1 illustrates
ponded areas. Downwarping from soil shrinkage (fig.
damages to a building constructed on expansive soil
1-2) may develop beneath the perimeter during hot,
with a deep water table in the wet, humid climate of
dry periods or from the desiccating influence of trees
Clinton, Mississippi. These damages are typical of
and vegetation located adjacent to the structure. These
buildings on expansive soils. The foundation consists
edge effects may extend inward as much as 8 to 10
of grade beams on deep drilled shafts. Voids were not
feet. They become less significant on well-drained
provided beneath the grade beams above the expansive
land. Heavy rain periods may cause pending adjacent
foundation soil, and joints were not made in the walls
to the structure with edge lift (fig. 1-3) and reversal of
and grade beams. The floor slab was poured on-grade
the downwarping.
with no provision to accommodate differential move-
(3) Edge heave. Damaging edge or dish-shaped
ment between the slab and grade beams. The heave of
heaving (fig. 1-3) of portions of the perimeter maybe
the floor slab exceeded 6 inches. The differential soil
observed relatively soon after construction, particu-
movement and lack of construction joints in the struc-
larly in semiarid climates on construction sites with
ture aggravated cracking.
preconstruction vegetation and lack of topographic re-
lief. The removal of vegetation leads to an increase in
14. Causes and patterns of heave
soil moisture, while the absence of topographic relief
a. Causes. The leading cause of foundation heave or
leads to ponding (table 1-1). A dish-shaped pattern can
settlement in susceptible soils is change in soil mois-
also occur beneath foundations because of consolida-
ture, which is attributed to changes in the field envi-
tion, drying out of surface soil from heat sources, or
ronment from natural conditions, changes related to
sometimes lowering of the water table. Changes in the
construction, and usage effects on the moisture under
water table level in uniform soils beneath uniformly
the structure (table 1-1). Differential heave may be
loaded structures may not contribute to differential
caused by nonuniform changes in soil moisture, varia-
heave. However, structures on a deep foundation, such
tions in thickness and composition of the expansive
as drilled shafts with a slab-on-grade, can be adversely
foundation soil, nonuniform structural loads, and the
affected by a changing water table or changes in soil
geometry of the structure. Nonuniform moisture
moisture if the slab is not isolated from the perimeter
changes occur from most of the items given in table
grade beams and if internal walls and equipment are
1-1.
not designed to accommodate the slab movement.
(4) Lateral movement. Lateral movement may af-
b. Patterns of heave.
fect the integrity of the structure.
(1) Doming heave. Heave of foundations, although
(a) Lateral thrust of expansive soil with a hori-
often erratic, can occur with an upward, long-term,
zontal force up to the passive earth pressure can cause
dome-shaped movement that develops over many
bulging and fracture of basement walls. Basement
years. Movement that follows a reduction of natural
walls and walls supporting buildings usually cannot
evapotranspiration is commonly associated with a
doming pattern of greatest heave toward the center of
tolerate the same amount of movement as free-stand-
the structure. Evapotranspiration refers to the evapo-
ing retaining walls. Consequently, such walls must be
ration of moisture from the ground surface and trans-
designed to a higher degree of stability.
1-2
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