Q. What is the permeability of ACBs?
A. ACBs are available in various percentage surface openings. The individual block's percentage opening will dictate its permeability factor.
Q. Which product is appropriate for my application?
A. The Hydraulic Engineering Circular 23 (HEC 23) determines block selection by analyzing the hydraulic conditions with the ACBs permissible shear and calculating the safety factor. The HEC 23 is available for download at www.shoretec.com.
Q. What determines block thickness requirements?
A. Typically, the design hydraulic conditions determine block thickness requirements. Other considerations include vehicle traffic, loading, etc.
Q. What sizes of cabled mats can be made?
A. The maximum mat width that can be produced is 8' due to shipping reasons. Mat lengths can be produced from 8' to 40' long. Smaller widths can be made if required.
Q. What is maximum velocity ACBs can handle?
A. Current Federal design procedures utilize bed shear stress for primary stability analysis. This approach takes into consideration: velocity, depth of flow, and energy slope gradient to determine how much hydraulic energy (bed shear) is incident on the ACBs. Block style and thickness are then selected to provide resistance to the design bed shear with appropriate safety factors based on the individual blocks maximum critical shear value as determined by, full scale, hydraulic flume testing.
Q. What are the advantages of the bi-directional system over the single directional system?
A. The single directional system just has the lifting cables in the longitudinal direction and has half-blocks. The Bi-Directional system has an extra transverse cable to connect the mats side by side forming one continuous system and also has all full blocks.
Q. Which geotextile filter fabric is best to use under ACBs?
A. 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 through the ACBs causing sub-structural erosion and consequential ACB failure. Premier Concrete offers a geotextile selection guide for determining which fabric is best depending on your soil conditions.
Q. Are cables necessary?
A. Cables can help facilitate installation as well as enhance the overall long-term performance and stability of the mat system.
Installation - Cabled mats are more easily placed in many cases including steep slope and wet applications. Utilization of cabled mats on large projects can significantly enhance speed of installation, where a 320 sq. ft. mat can be placed at a rate of up to four mats (1,280 sq. ft.) per hour.
Stability and Performance - The use of cables can help maintain long-term system integrity and significantly reduce the risk of failure do to either vandalism, flow of sizable debris, or short-term capacity exceeding hydraulic forces.
Q. How do you connect bi-directional mattresses at the sides?
A. The two transverse cables are held together by a crimp. Premier Concrete provides CAD details with various connection methods.
Q. Do I need to be concerned about cable deteriorating?
A. Cables are typically sized to carry five (5) times the weight of the particular mat, based on breaking strength many times more than the potential load the cable will see in service after mat placement. Cable deterioration rate will depend on cable material (galvanized steel, polyester, etc.) and the service environment. Except for severe chemical environments, cable deterioration should not adversely affect the long-term system performance.
Q. What is the maximum slope ACBs can handle?
A. 2:1 slopes are the preferred limit. However, steeper slopes, up to 1:1, can be designed providing a proper slope 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.
Q. How fast can ACBs be placed?
A. 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.
Q. Can mattresses be assembled on the jobsite?
A. Mattresses can be assembled at the 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.
Q. How much do ACBs cost?
A. 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 more cost effective than cast in place concrete.
Q. How do you handle upstream and downstream terminations?
A. If the ACB system does not start and end at an existing structure, typically the blocks (and underlying geotextile filter 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 riprap 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.
Q. How do you terminate at the top and toe of a slope?
A. Typically termination at the top of an embankment is achieved by tucking the Mattresses (and underlying geotextile filter fabric) into a top anchor trench. Top anchor trench depth can range from 12" to 36" (2 to 3 blocks) deep depending on the specific job characteristics and potential for undermining at the top of the embankment.
Q. How do you anchor the ACB System?
A. 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 mats 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. CAD details for anchoring are available from Premier Concrete.
Q. How do you determine when mechanical anchors are necessary?
A. Mechanical anchors are used on steeper slopes or when an extra safety factor is desired.
Q. What subgrade compaction is required?
A. 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.
Q. Can you drive on ACB mattresses?
A. When exposing a mattress to traffic, the cross section of the mattress, geotextile filter fabric, sub-base and or subgrade 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 subgrade material type and thickness of the sub-base or subgrade. ACBs subject to traffic in wet conditions must also be analyzed for pore pressure development and potential for loss of fines in the subgrade or sub-base.
Q. What is the maximum wave height that ACBs can handle?
A. Opposition to wave attack is primarily based on unit weight. Studies have been conducted by the Federal Highway Administration (FHWA) 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.
Q. How do you handle obstructions and curves? (How do you install ACBs around pipes?)
A. Several CAD details are available in our Technical Manual and available for download on our website. Typically, the ACB is placed in full units (full blocks with 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 pipeline are then filled with grout to a distance of 18" from the existing structure and struck flush with the block top surfaces. We can also make angle mats from the construction plans or most contractors will cut the mats in the field and grout the seams. Again, Premier Concrete provides CAD details explaining this method.
Q. Do you need to fill and seed the open cell block?
A. You can specify in the plans how you would like the mats to be filled, or let it fill naturally depending on the location and how quickly you are looking to get vegetation to grow.