Experiment Station. The program HEAVE is applica-

calculations require a measure or estimate of both sea-

ble to slab, long continuous, and circular shaft founda-

sonal wet and dry pore water pressure or suction pro-

tions. This program considers effects of loading and

files. It should be noted from figure 5-lb that perime-

soil overburden pressures on volume changes, hetero-

ter cyclic movement from extremes in climatic

geneous soils, and saturated or hydrostatic equilibri-

changes can exceed the long-term heave beneath the

um moisture profiles (equations (5-3) to (5-5)). Results

center of a structure.

of HEAVE using the saturated profile, equation (5-3),

(1) Soil-slab displacements. A slab constructed on

are comparable with results of manual computations

the ground surface of a wet site may in time lead to

described in figure 5-4.

downwarping at the edges after a long drought or

growth of a large tree near the structure (fig. 5-3a).

Edge uplift may occur following construction on an

Differential heave results from edge effects beneath a

initially dry site (fig. 5-3b). The AH in figure 5-3 is

finite covered area, drainage patterns, lateral varia-

representative of the maximum differential vertical

tions in thickness of the expansive foundation soil, and

heave beneath the slab, excluding effects of restraint

effects of occupancy. The shape and geometry of the

from the slab stiffness, but does consider the slab

structure also result in differential heave. Examples of

weight.

effects of occupancy include broken or leaking water

(2) Edge distance. The edge lift-off distance e of

lightly loaded thin slabs at the ground surface often

and sewer lines, watering of vegetation, and ponding

adjacent to the structure. Other causes of differential

varies from 2 to 6 feet but can reach 8 to 10 feet.

heave include differences in the distribution of load

(3) Deflection/length ratio. The deflection/length

ratio of the slab is A/L, where A is the slab deflection

and the size of footings.

and L is the slab length. The angular deflection/span

a. Unpredictability of variables. Reliable predic-

tions of future potential differential heave are often

5-3).

not possible because of many unpredictable variables

that include: future availability of moisture from

rainfall and other sources, uncertainty of the exact lo-

cations of heaving areas, and effects of human occu-

pancy.

(5-8)

(1) Potential differential heave can vary from zero

to as much as the total heave. Differential heave is of-

thickness of expansive soil layer, feet

swell index, slope of the curve between

ten equal to the estimated total heave for structures

supported on isolated spot footings or drilled shafts be-

points 3 and 4, figure 4-2

cause some footings or portions of slab foundations of-

swell pressure, tons per square foot

final vertical effective pressure, tons per

ten experience no movement. Eventually, differential

heave will approach the total heave for most practical

square foot

cases and should, therefore, be assumed equal to the

The final effective pressure is given by

total potential heave, unless local experience or other

(5-9)

information dictates otherwise.

(2) The maximum differential heave beneath a

lightly loaded foundation slab may also be estimated

by the procedure based on the moisture diffusion theo-

ry and soil classification data developed by the PTI.

Heave predictions by this method will tend to be less

4-2. A simple hand method and an example of predict-

ing potential total vertical heave from consolidometer

than by assuming that the differential heave is the to-

tal potential heave.

swell tests assuming a saturated equilibrium profile,

equation (5-3), are given in TM 5-818-1 and in figure

5-4. However, hand calculations of potential heave

Predictions of heave with time are rarely reliable be-

can become laborious, particularly in heterogeneous

cause the location and time when water is available to

profiles in which a variety of loading conditions need

the soil cannot be readily foreseen. Local experience

to be evaluated for several different designs,

has shown that most heave (and the associated struc-

(2) Computer applications. Predictions of poten-

tural distress) occurs within 5 to 8 years following con-

tial total heave or settlement can be made quickly with

struction, but the effects of heave may also not be ob-

the assistance of the computer program HEAVE avail-

served for many years until some change occurs in the

able at the U. S. Army Corps of Engineers Waterways

5-5