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How
do you determine block selection?
Through
a Factor of Safety approach, guidelines as per the Hydraulic
Engineering Circular 23 (HEC 23) determine block selection by analysing
the hydraulic conditions with the ACB's permissible shear and
calculating the safety factor. Free design programs and the HEC 23
document are available through the HydroPave Technical Service Center @
1 800 EROSION.
What
is the maximum slope you can lay block to?
2:1
slopes are the preferred limit. However, 1:1 slopes can be designed
provided a proper stability analysis has been performed.
Constructability also becomes an issue on steep (1:1) slopes with
vertical elevations over 10' and will likely necessitate cables for
placement.
What
is maximum velocity the product can withstand?
Many
Conlock projects have been exposed to velocities over 30' per second.
However, current Federal design procedures utilize bed shear stress for
primary stability analysis. This approach utilizes the combined forces
of: velocity, depth of flow, and energy slope gradient to determine how
much hydraulic energy (bed shear) is incident on the ACBs. Once Bed
Shear is determined, it is compared to the individual block's maximum
critical shear value as determined by, full scale, hydraulic flume
testing.
What
is the permeability of the product?
Both
the Conlock I & Conlock II products are available in various
percentage surface openings. The individual block's percentage opening
will dictate its permeability factor. For example, a CONLOCK II 4VM-30
Block will exhibit an approximate surface open area of 30%. A 6NVM-05
block will exhibit an approximate surface open area of 5%.
When
do you use cables?
Cables
are used for installation purposes only; such as, applications calling
for submerged placements or for steep slopes where the more economical
hand placed systems can not be installed. Cables will not add to, or
enhance, the block's ability to remain stable.
Do
cables deteriorate in use?
Since
cables are used for installation purposes only, the only criteria for
cable is the ability to lift the required load at a given Factor of
Safety and withstand abrasion and damage during the mattress
fabrication process. Typical break strength Safety Factors are 3:1 to
5:1; i.e., if the load is 1000 lbs., cable break strength should be a
minimum of 3000 lbs. or more typically 5000 lbs.
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How
much production can a crew lay in a day?
Production
rates vary depending on the type of system being installed, the
accessibility of the project and geometries of the bed slope, side
slope and overall configuration of the placement. In general, 4" and 6"
thick hand placed blocks can be installed at a rate of 40 to 90 square
feet per man-hour. Prefabricated cabled mattresses can be installed at
a rate of 3 or 4 mats per crane-hour.
What
determines block thickness?
Block
thickness is usually derived as a function of required block weight as
determined for hydraulic stability reasons. In some cases where the
project hydraulics dictate a 4" high unit, a 6" (or higher) unit may be
selected for other reasons such as; higher vehicular load bearing
requirements. 4" tall blocks are typically adequate for light, rubber
tired, maintenance equipment like mowers and skidsters.
What
is your product's Manning Roughness Coefficient?
Both
the Conlock I & Conlock II products are available in various
percentage surface openings and thus various Manning's n coefficients.
The individual block's percentage opening will dictate its average
Manning1s n value. The Manning's n values (for non-vegetated blocks)
range from .022 to .032. Utilizing alternating height blocks (such as;
1/3 6" and 2/3 4") Manning's Numbers can reach as high as 0.045. Please
refer to our technical binder or contact our technical service
department for specific information.
How
are the blocks terminated at the upstream and downstream ends?
If
the ACB system does not start and end at an existing structure,
typically the blocks (and underlying fabric) are tucked into an anchor
trench and backfilled and compacted with the appropriate material. In
some cases, the "appropriate material" may be: concrete grout, stone,
or grouted rip-rap instead or soil. Trench depth is typically 1.5 times
the potential depth of scour. If the ACB system starts or ends at an
existing structure, a reinforced steel grouted interface treatment is
typically used to secure the blocks and insure that hydraulic
undermining will not occur.
How
do you terminate the ACBs at the top and toe of a side slope or
embankment?
Typically
termination at the top of an embankment is achieved by tucking the ACBs
(and underlying fabric) into a Top Anchor Trench. Top Anchor Trench
depth can range from 12" to 36" deep depending on the specific job
characteristics and potential for undermining at the top of the
embankment. There are some cases where the blocks are not tucked into a
Top Anchor Trench; however, in ALL cases the underlying filter fabric
is anchored in a trench.
What
kind of filter fabric do you use?
Overall,
woven mono-filaments are preferred over non-woven geotextiles because
of their high hydraulic conductivity and durability. However, in all
cases, the soil's particle size, disbursement and cohesiveness (among
other factors) will determine fabric selection. In general, if a fabric
exhibits too tight of a weave (or clogs up) sub-structural hydrostatic
pressure will build up and force the blocks up and out of the ACB
matrix. If the fabric weave is too loose, soil particles will migrate
up and trough the ACBs causing sub-structural erosion and consequential
ACB failure. Most geotextile suppliers can offer assistance, and in
some cases, fabric selection software to enable proper selection of the
fabric.
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What
kind of anchors do you use?
Anchoring
ACBs can be accomplished by several different methods depending on the
design objective. Most common methods utilize galvanized steel helical
or duckbill anchors. Attachment to the ACB revetment will vary
depending on if the system is cabled or non-cabled. If the system is
cabled, then a typical method is to thread the cable through the
anchor's end eye and secure it by means of compression sleeves or U
bolts. The voids around the anchor attachment (whether cabled or
non-cabled) are then filled with grout and struck flush with the block
top surfaces.
What
kind of grass do you recommend?
Native
grasses to the area of ACB placement will always have a better chance
of success as they are "acclimated" to the immediate environment.
Hydroseeding is most common method of application. Use of a tackifier
in the application process minimizes loss of seeds prior to
germination. Other methods such as the placement of organic or
bio-degradable mats (with or without seeds) on top of the ACB revetment
can substantially accelerate and increase the likelihood of vegetative
cover.
What
density / compaction do you need on the subgrade?
95%
standard proctor within +/- 3% of optimum moisture content is the
normal requirement for fill embankments. Existing compaction of
undisturbed soils is sufficient provided they are stable soils and do
not exhibit "yielding" of soft areas.
Can
you drive on ACBs?
When
exposing an ACB revetment to traffic, the cross section of the ACB,
filter, sub-base and or sub-grade must be checked for load carrying
capability. Load bearing capability will depend on the axle load,
frequency of the traffic, the thickness of the ACB, the durability of
the fabric and the sub-grade material type and thickness of the
sub-base or sub-grade. ACB pavements subject to traffic in wet
condition must also be analyzed for pore pressure development and
potential for loss of fines in the sub-grade or sub-base.
Can
mats be assembled on the job?
Mats
can be assembled at the ACB place of manufacture, at a remote site, on
the job or even hand placed on the application (provided placement is
not submerged or on a steep slope) and then post cabled.
How
much does a mat weigh?
Mat
weight depends on the thickness of block and overall width and length
that is fabricated. Typical mat weight is 8,000 to 40,000 pounds.
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How
high of a wave can your system stand?
Opposition
to wave attack is primarily based on unit weight. Studies have been
conducted by the Federal Highway Administration indicating ACB
stability at wave heights of 11 to 10' depending on the weight of the
block. Typically a 4" (30 to 40 lbs) block can withstand wave attacks
of 4'. Special care and consideration should be taken when designing
ACB applications for wave attack. Wave attack stability data can be
obtained through the FHWA or by contacting our Technical Support Dept.
How
does your installed cost compare to boulders, concrete and gabions?
In
most markets, ACBs are competitive in cost to 12" diameter (or greater)
rock (or RipRap) placed in an 18" or greater blanket thickness. In most
markets, ACBs are competitive with gabion mattresses and ACBs are
typically cheaper than cast in place concrete.
How
do you detail around an outfall? Manhole? Concrete apron?
Several
details are available in our Technical Binder. Typically, the ACB
revetment system is placed in full units (full blocks w/ no miter
cutting) until the placement of an ACB will not fit due to an impeding
structure. The resulting voids between the ACBs and the structure are
then filled with grout to a distance of 18" from the existing structure
and struck flush with the block top surfaces.
What
is the oldest ACB installation?
CONLOCK
products have been in service since 1983. Articulated Stone Blocks have
been used for thousands of years dating back to the ancient Romans and
Myan Indians (some of these systems are still functional today).
Can
I get the product in different colors?
Conlock products are available in 6 mono-chromatic colors. Orders in
color are subject to minimum quantities.
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