TM 5-818-4/AFM 88-5, Chap. 5
tial compaction water contents below optimum result
also be established by closer packing of soil particles, it
in initial high pore air pressures and pore water pres-
is evident that lower optimum water contents are pos-
sures, which reduce shear strength and allow soil
sible at higher compaction efforts.
particles to slide over one another displacing the pore
(2) The addition of water above optimum water
air to form a more dense mass. This process continues
content causes the voids to become completely filled
as long as the trapped pore air pressure can escape but
with trapped pore air and pore water and thereby pre-
requires increasing amounts of compaction effort to
vents the soil particles from moving into a more com-
achieve higher densities since the soil particles carry
pact arrangement no matter what the compaction ef-
increasing amounts of the compaction energy. For a
fort. Pore water pressure increases significantly with
given compaction effort, enough water may eventually
increasing water contents and causes increased reduc-
be added to the soil so that air channels become discon-
tion in shear strength. This fact is evident in the
tinuous, and the air is trapped. When the air voids be-
laboratory compaction mold when the compaction foot
come completely discontinuous, the air permeability of
sinks deeper and deeper into the soil as water content
the soil drops to zero; no further densification is possi-
increases past optimum. The same process occurs in
ble because at this condition transient pore air pres-
the field when sheepsfoot rollers sink into the soil un-
sures can develop that resist the compaction effort. At
til the weight is carried by the drum or excessive rut-
zero permeability the soil has reached its so-called
ting with rubber-tired rollers.
"optimum water content." Since zero permeability may
Section B-2. FIELD COMPACTION TEST METHODS
B-3. General. Laboratory test data obtained from
number of the compaction curves were developed dur-
laboratory-compacted specimens provide a basis for
ing the design phase, (2) when borrow material is ob-
design, and it is assumed that the engineering charac-
tained from a new source, and (3) when material simi-
teristics that will be built into the field-compacted
lar to that being placed has not been tested previously.
backfill will be approximately the same as those of the
In any event, laboratory compaction tests should be
specimens. Experience has indicated that for most
performed periodically on each type of fill material
soils, laboratory densities, water contents, and
(preferably 1 test for every 10 field density tests) to
strength characteristics can be satisfactorily repro-
check the optimum water content and maximum dry
duced in a field-compacted backfill.
density values being used for correlation with field
density test results.
B-4. Field compaction tests.
c. Quick field compaction tests. In addition to the
a. Compaction control tests. Compaction control of
standard compaction or relative density tests (para
soils requires comparison of fill water content and dry
B-2a), at least four relatively quick compaction test
density values obtained in field density tests with opti-
methods can provide good approximations of maxi-
mum water content and maximum dry density, or de-
mum dry density comparable to the standard methods.
termination of relative density if more appropriate for
The quick compaction methods include: one-point and
the fill materials that are cohesionless. For fine-
two-point compaction methods; the Water and Power
grained or coarse-grained soils with appreciable fines,
Resource Service (WPRS), formerly U.S. Bureau of
field results are compared with results of CE 55
Reclamation (USBR) rapid compaction control
laboratory (modified effort) compaction tests per-
method; and for granular cohesionless material, com-
formed according to procedures presented in
paction control by gradation. Since only the one-point
MIL-STD-621A and ASTM D 1557. For free-draining
and two-point methods are currently accepted by the
cohesionless soils, relative density of the fill material
Corps of Engineers for compaction control tests, only
is determined, if appropriate, using vibratory test pro-
these two methods will be discussed in detail. The
cedures prescribed in EM 1110-2-1906 and ASTM D
USBR and gradation methods are briefly summarized.
2049.
(1) One-point compaction method. In the one-
point compaction method, material from the field
formance of a standard laboratory compaction test on
density test is allowed to dry with thorough mixing to
material from each field density test would give the
obtain a uniform water content on the dry side of esti-
most accurate relation of the in-place material to opti-
mated optimum, and then compacted using the same
mum water content and maximum density, but this
equipment and procedure used in the five-point stand-
test is not generally feasible to do because testing
ard compaction test. The water content and dry densi-
could not keep pace with the rate of fill placement.
ty of the compacted sample are then used to estimate
However, standard compaction tests should be per-
its optimum water content and maximum dry density
formed during construction (1) when an insufficient
as illustrated in figure B-4. The line of optimums is
B-4
Previous Page
