TM 5-818-4/AFM 88-5, Chap. 5
(4) Consolidation and swell testing. The rate and
less expensive than in situ pumping tests performed in
magnitude of consolidation under a given load are in-
the field; however, laboratory tests are less accurate in
fluenced primarily by the density and type of soil and
predicting flow characteristics.
the conditions of saturation and drainage. Fine-
(6) Slake durability of shales. Some clay shales
grained soils generally consolidate more and at a
tend to slake when exposed to air and water and must
slower rate than coarse-grained soils. However, poorly
be protected immediately after they are exposed. The
graded, granular soils and granular soils composed of
extent of slaking also governs the manner in which
rounded particles will often consolidate significantly
they are treated as a backfill material (para 3-3c).
under load but usually at a relatively fast rate.
Slaking characteristics can be evaluated by laboratory
jar-slake tests or slake-durability tests.
outlined in EM 1110-2-1906. The information ob-
(a) The jar-slake test is qualitative with six de-
tained in this test can be used in settlement analyses to
scriptive degrees of slaking determined from visual
determine the total settlement, the time rate of settle-
observation of ovendried samples soaked in tap water
ment, and the differential settlement under varying
for as long as 24 hours. The jar-slake test is not a
loading conditions. Consolidation characteristics are
standardized test. One version of the jar-slake test is
important considerations in selection of backfill mate-
discussed in FHWA-RD-78-141. Six suggested values
rials. The results of consolidation tests performed on
of the jar-slake index
are listed below:
laboratory compacted specimens of backfill material
can be used in determining the percent compaction to
Behavior
be required in the specifications.
Degrades into pile of flakes or mud
1
(b) Swelling characteristics can be determined
Breaks rapidly and forms many chips
2
by a modified consolidation test procedure. The degree
Breaks rapidly and forms few chips
3
4
Breaks slowly and forms several fractures
of swelling and swelling pressure should be deter-
Breaks slowly and develops few fractures
5
mined on all backfill and foundation materials sus-
6
No change
pected of having swelling characteristics. This fact is
Shales with values of 1 to 3 should be protected
particularly important when a considerable overbur-
when occurring in excavated slopes and compacted as
den load is removed by excavation or when the com-
soil if used for backfill.
pacted backfill with swelling tendencies may become
(b) The slake-durability test is a standardized
saturated upon removal of the dewatering system and
test that gives a quantitative description in percent by
subsequent rise of the groundwater level. The results
weight of material remaining intact at the conclusion
of swelling tests can be used to determine the suitabil-
of the test. Details of the test are presented in
ity of material as backfill. When it is necessary to use
FHWA-RD-78-141.
backfill materials that have a tendency to swell upon
(7) Dynamic tests for special projects. The dynam-
saturation because more suitable materials are un-
ic analysis of projects subject to seismic or blast in-
available, the placement water content and density
duced loading conditions requires special dynamic
that will minimize swelling can be determined from a
tests on both in situ and backfill materials. Tests re-
series of tests. TM 5-818-1/AFM 88-3, Chapter 7,
quired for dynamic analysis include: cyclic triaxial
and FHWA-RD-79-51 (app. A) provide further infor-
tests; in situ density measurements; and tests to deter-
mation applicable to compacted backfills.
mine shear wave velocities, shear modulus, and damp
(5) Permeability tests. Permeability tests to deter-
ing (ER 1110-2-1806).
mine the rate of flow of water through a material can
(8) In situ water content. The in situ water con-
be conducted in the laboratory by procedures described
tent, including any seasonal variation, must be deter-
in EM 1110-2-1906. Permeability characteristics of
mined prior to construction for materials selected for
fine-grained materials at various densities can also be
use as backfill. Natural in situ water contents will de-
determined from consolidation tests.
termine the need for wetting or drying the backfill
(a) Permeability characteristics for the design
material before placement to obtain near optimum wa-
of permanent drainage systems for structures founded
ter contents for placement and compaction. ASTM D
below the groundwater level must be obtained from
2216 discusses the test method for determining water
laboratory tests. The tests should be performed on rep-
content.
resentative specimens of backfill materials compacted
3-3. Selection of backfill materials. Selec-
in the laboratory to densities expected in the field.
(b) In situ material permeability characteristics
tion of backfill materials should be based upon the
for the design of construction excavation dewatering
engineering properties and compaction characteristics
systems can also be approximated from laboratory
of the materials available. The results of the field ex-
tests on representative undisturbed samples. Labora-
ploration and laboratory test programs should provide
tory permeability tests on undisturbed samples are
adequate information for this purpose. The materials
3-4
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