sary. Corrections to improve performance compared
ture interaction effects in swelling soil are complicated
with earlier structures may prove difficult to devise
by the foundation differential movement caused by
and implement and may require evaluation of the be-
soil heave. Sufficient samples should be available to al-
havior of the subsurface foundation soils and ground-
low determination of the representative mean of the
swell and strength parameters of each distinctive soil
water conditions.
stratum. The lower limit of the scatter in strength
b. Distribution and depth of borings. The distribu-
parameters should also be noted.
tion and depth of borings are chosen to determine the
a. Sampling requirements. The design of lightly
soil profile and to obtain undisturbed samples required
loaded structures and residences can often be made
to evaluate the potential total and differential heave of
with minimal additional subsurface investigations and
the foundation soils from laboratory swell tests, as
soil testing if the site is developed, if subsurface fea-
well as to determine the bearing capacity and settle-
ment. Consequently, greater quantities of undisturbed
tures are generally known, and if the local practice has
samples may be required in swelling soils than nor-
consistently provided successful and economical de-
mally needed for strength tests.
signs of comparable structures. Additional subsurface
investigation is required for new undeveloped sites,
(1) Borings should be spaced to define the geology
multistory or heavy buildings, structures with pre-
and soil nonconformities. Spacings of 50 or 25 feet and
viously untested or new types of foundations, and spe-
occasionally to even less distance may be required
when erratic subsurface conditions (e.g., soils of differ-
cial structures that require unusually limited differen-
tial movements of the foundation such as deflec-
ent swelling potential, bearing capacity, or settlement)
tion/length ratios less than 1/1000. Where the local
are encountered. Initial borings should be located close
practice has not consistently provided satisfactory de-
to the corners of the foundation, and the number
signs, a careful review of the local practice is neces-
should not be less than three unless subsurface condi-
3-2
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