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FAQs
Q:
How are slurry walls used to remedy seepage problems of levees?
(Click
for answer.)
Q: How is the thickness of soil-cement for
upstream slope protection of embankments determined? (Click
for answer.)
Q: What are “Poorly Reacting Sandy
Soils” and how could they be detected prior to use in soil-cement
applications? (Click for
answer.)
Q. What is the
difference between Roller-Compacted Concrete (RCC) and Soil-Cement
(SC)? (Click for answer.)
Q: How is cement content
determined for soil-cement slope protection applications? (Click
for answer)
Q: What factors need to be considered
in designing erosion-resistant soil-cement? (Click
for answer)
Q: What values of coefficient
of roughness “n” are typically used in the
Manning’s formula when designing open channels and spillways
with soil-cement (SC) or roller-compacted concrete (RCC)?(Click
for answer.)
Q: How does the use of aggregates affect erosion
and abrasion resistance in soil-cement applications?
Erosion of soil-cement in water resource applications may be defined
as the progressive disintegration of the material by water motion.
Abrasion erosion can be defined as the wearing away of a surface
by the action of water and waterborne particles.
 Laboratory
studies to assess the erosion and abrasion resistance of soil-cement
go as far back as 1942. The earliest tests were conducted at the
Civil Engineering Department of Oklahoma A & M College (now
Oklahoma State University). Additional studies were performed at
PCA in early 1970s and at three Universities in United States and
Canada in the early 1980s.
During these studies, soil-cement samples made using different
types of soils were subjected to:
- Standard ASTM D559 (wet-dry), ASTM D560 (freeze-thaw) and
ASTM C1138 (abrasion resistant) durability testing
- Compressive strength tests
- Water carrying gravel
- Breaking waves and impacting debris
- Water jet with varying water velocities
Generally, these studies concluded that the erosion and abrasion
resistance of soil-cement exposed to water carrying waterborne particles
can be significantly improved by using a coarser material as the
aggregate or adding gravel to a finer soil. The erosion resistance
can also be improved by increasing the cement content. These methods
improve the strength of the soil-cement. Similar to concrete, the
strength of soil-cement continues to increase with time. As a result,
soil-cement will continue to gain strength with age.
Because most soil-cement mixtures in field applications contain
very little coarse aggregate, the soil-cement erosion resistance
is in most cases controlled by the compressive strength of the cement
paste. PCA recommends that adequate cement content for soil cement
be determined based on durability tests or a minimum 7-day compressive
strength that correlates to durability. Depending on exposure conditions,
a typical design will require a minimum 7-day unconfined compressive
strength of 750 psi (5.2 MPa). Some agencies have specified that
an additional 2% of cement be added to account for construction-related
variations.
Laboratory test results and field performance show that properly
designed soil-cement can withstand the flow of clean water up to
a velocity of 20 ft/sec (6 m/sec) with little damage. Also, soil-cement
designed with adequate strength and durability, even without air
entrainment, can resist the long-term erosion affects caused by
wave action, flowing water and freeze-thaw cycles with little deterioration.
For higher flow velocities or water carrying abrasive waterborne
particles, the compressive strength should be increased or different
materials such as roller-compacted concrete be used. Means to increase
the strength of soil-cement exposed to more severe erosion conditions
include modification of the mixture proportions, including increasing
the cement content and/or changing to a coarser, more well graded
aggregate. Significant improvement in strength and erosion resistance
can be obtained by simply adding a gravel component of 20 percent
or more to a sand or silty sand soil.
For more information see Dam
Construction and Facing with Soil-Cement
(RD010) and Erosion
and Abrasion Resistance of Soil-Cement and Roller-Compacted Concrete
(RD126).
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Q:
How is cement content determined for soil-cement slope protection
applications?
 A:
For early soil-cement upstream slope protection applications an
adequate cement content was determined using the standard wet-dry
(ASTM D559) and the freeze-thaw (ASTM D560) durability tests. Both
tests take about a month to complete and tend to be expensive. Based
on experience and laboratory tests, researchers found that for granular
soils with the same cement content, the freeze-thaw test consistently
produces greater weight loss than the wet-dry test. Therefore, for
granular soils, good durability results could be obtained based
on the freeze-thaw test alone.
Today, however, most cement content determinations use compressive
strength as the basic design criterion. PCA developed a relationship
between 7-day compressive strength and durability based on more
than 1,700 sets of tests. On the average, this research concluded
that:
- Approximately 87 percent of sandy soil samples that achieved
600 psi (4.14 MPa) compressive strength at seven days will pass
the durability tests, and
- About 97 percent of the soils that achieved 750 psi (5.17 MPa)
compressive strength at seven days will pass the durability tests.
PCA’s compressive strength versus durability relationship
resulted in the adoption of a short-cut mix design method that requires
only a moisture-density test, sieve analysis, and a seven-day compressive
strength test. These tests can be completed in about a week and
cost significantly less than the durability tests.
 It
should be noted that the PCA methods were developed for soil-cement
pavement base applications. It is recommended that the cement content
determined from these methods be increased by 2 percentage points
to account for more severe conditions typically encountered in water
resource applications. It should also be noted that the short-cut
method is generally applicable for sandy soils that do not contain
more than 35 percent fines and are not highly plastic. Soils that
contain more than 35 percent fines or have plasticity indices higher
than eight may still be suitable for soil-cement applications, but
the cement content for these soils should be determined based on
the strength and durability tests.
Additional information on laboratory tests for determining cement
content in soil-cement can be found in PCA publication, Soil-Cement
Laboratory Handbook (EB052).
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Q. What is the difference
between Roller-Compacted Concrete (RCC) and Soil-Cement (SC)?
A: The American Concrete Institute,
Subcommittee 207.5 (ACI 207.5R) defines RCC as “Concrete compacted
by roller compaction; concrete that in its unhardened state will
support a roller while being compacted.” ACI 230.1R defines
SC as “a mixture of soil and measured amounts of portland
cement and water compacted to a high density.”
One can think of RCC as a no-slump concrete that is compacted by
a vibratory roller, whereas SC is a highly compacted mixture of
portland cement, soil (usually sandy soil), and water. With RCC
the cement, non-cohesive fines and water form a paste that coats
the coarser aggregates. With soil-cement not all the soil particles
are coated with a cement paste.
 The
main difference between RCC and SC for water resources projects
is the aggregate or soil used in the mixture and the resulting properties.
RCC usually contains controlled dense-graded aggregate with a nominal
maximum size aggregate averaging about 1-1/2 in. (38 mm). SC, on
the other hand, uses a pit run, non-selected sand with most material
no larger than ½-in. (13 mm).
PCA has published several documents covering mixture proportioning
methods to produce adequate durability and/or strength properties.
The adequacy of these design methods has been proven based on project
performance. Because RCC uses clean and larger, well-graded aggregate,
the compressive strength of RCC is usually higher than the compressive
strength of SC at the same age. For most exposed RCC projects, the
specified compressive strength at 28 days is 2000 psi or higher.
SC specified compressive strength is usually 1000 psi or less for
the same age.
The construction methods used on RCC and SC water resources projects
are quite similar. They both involve mixing, transporting, spreading,
compacting and curing.
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