TM 5-818-4/AFM 88-5, Chap. 5
(d) Insufficient number of tests to define com-
paction curve accurately.
(e) Improper foundation for mold during com-
paction.
(f) Incorrect volume or weight of compaction
mold.
(g) Incorrect rammer weight and height of fall.
(h) Excessive material extending into the exten-
sion collar at the end of compaction.
(i) Improper or insufficient distribution of
blows over the soil surface.
(j) Tendency to press the head of the rammer
against the specimen before letting the weight fall.
(k) Insufficient drying of sample for water con-
tent determination.
(2) One-point and two-point compaction test. The
possible sources of errors for the one-point and two-
point compaction test are essentially the same as those
for the five-point method discussed in (1) above. In ad-
Figure B-6. Illustration of two-point compaction method
dition, appreciable inaccuracy in results may occur for
both methods if attempts are made to extrapolate
distribution of added water. Otherwise, the results
maximum density and optimum water contents from
may be erroneous, especially for highly plastic clays.
nonuniform families of compaction curves (fig. B-5).
In tough clays, it is likely to be inaccurate because of
B-5. Field compaction and test sections.
insufficient curing time for the specimens.
For most soils, laboratory densities, water contents,
(5) Grain-size gradation compaction control meth-
and strength characteristics can be satisfactorily
od. This test method developed in 1938 is applicable to
reproduced in a field-compacted backfill. However,
coarse, medium, and fine-grained sands. The method
during the initial stage of construction frequent
involves sieve analysis to establish grain-size grada-
checks of density and water content should be made
tion curves, whose shapes are then correlated with
for comparison with design requirements and adjust-
maximum dry density obtained from the standard
ments should be made in the field compaction proce-
five-point compaction tests or relative density tests.
dure as necessary to ensure adequate compaction.
For a given compaction effort, the maximum dry
density of cohesionless material (sand) is also a func-
tion of particle shape. Thus, the correlation between
foundation support for critical structures, or when
grain-size distribution and density would, by necessity,
other requirements, materials, and conditions are un-
have to include consideration of particle shape. It is
usual, the specifications may provide for the construc-
doubtful that this method would provide test results
tion of test sections. The test section is used to deter-
more rapidly than the one-point and two-point meth-
mine the best procedures for processing, placing, and
ods or the relative density method currently accepted
compacting the materials that will produce compacted
by the Corps since samples must be dried for sieve
backfill having engineering properties compatible
analysis. Therefore, this method is not recommended
with design requirements. Therefore, construction of a
for routine compaction control.
test section may involve using different types and dif-
ferent weights of compaction equipment, using differ-
d. Possible errors. All tests involving mechanical de-
ent lift thicknesses, using different amounts of com-
vices and human judgment are subject to errors that
paction applications (different numbers of passes or
could affect the results. In order to properly evaluate
coverages), processing materials differently with re-
test results, the inspector must be familiar with the
spect to water content control, and mixing to obtain
possible sources of such errors.
improved gradation. A discussion on test sections for
(1) Five-point compaction tests. The following er-
shale materials is presented in Appendix A of
rors can cause inaccurate results:
FHWA-RD-78-141 and illustrates a wide variation in
(a) Aggregations of air-dried soil not completely
test results, even for very carefully conducted field
reduced to finer particles during processing.
tests.
(b) Water not thoroughly absorbed into dried
material because of insufficient mixing and curing
b. By exercising rigid control over the water con-
time.
tent, processing, placement, and compaction proce-
(c) Material reused after compaction.
dures, by frequent density sampling, by keeping com-
B-6
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