VEGETATED ARTICULATED
CONCRETE BLOCKS
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| VEGETATED ARTICULATED
CONCRETE BLOCKS
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1. CATEGORY
2.0 – Bank Armor and Protection
2. DESIGN STATUS
Level I
3. ALSO KNOWN AS
Vegetated block revetments, ACB channel lining, ACB armor layer
4. DESCRIPTION
An Articulated Concrete Block (ACB) system consists of durable concrete blocks that are placed together to form a matrix overlay or armor layer. The blocks abut or fit together to form a continuous blanket or mat. The blocks are placed on a filter course (typically a geofabric) to prevent washout of fines through the blocks. Articulated block systems are flexible and can conform to slight irregularities in slope topography caused by settlement. Lateral continuity and connectivity between individual blocks are provided by means of either cabling or some form of articulation (see Figure 1). Vegetation in the form of live cuttings is inserted through openings in the blocks into the native soil beneath the blocks or grasses and forbs can be seeded into a soil infill. The block tops should remain flush with one another to maximize stability as shown in Figure 2.
 Figure 1. Concrete block mat (with displaced blocks) to reveal method of articulation between blocks. |
 Figure 2. Concrete block mat with limited grasses becoming established in block openings. To be environmentally-sensitive, ACBs should be specified and constructed for vegetation, e.g., soil infill, seeding, live stakes. |
5. PURPOSE
Articulated block systems are used to armor streambanks and/or channel bottoms (Lagasse et al., 2001); they have also been used to protect the downstream faces of embankment dams against overtopping (Koutsourais, 1994). Vegetation that becomes established in a block revetment improves its engineering performance (viz., increased resistance to liftoff and sliding), in addition to providing environmental benefits such as improved visual appearance and riparian cover (see Figures 3 and 4).
 Fig. 3. ACB revetment immediately after construction. |
 Fig. 4. ACB revetment after vegetative establishment. |
6. PLANNING
Useful for Erosion Processes:
Toe erosion with upper bank failure Scour of middle and upper banks by currents Local scour Erosion of local lenses or layers of noncohesive sediment Erosion by overbank runoff General bed degradation Headcutting Piping Erosion by navigation waves Erosion by wind waves Erosion by ice and debris gouging General bank instability or susceptibility to mass slope failure
Spatial Application:
Instream Toe Midbank Top of Bank
Hydrologic / Geomorphic Setting
Resistive Redirective Continuous Discontinuous Outer Bend Inner Bend Incision Lateral Migration Aggradation Conditions Where Practice Applies:
ACBs resist long duration, high velocity flows or tractive stresses. Articulated Concrete Blocks and other hard armor systems are employed when flow velocities are high (Fischenich, 2001). These "hard armor" systems provide permanent erosion protection in areas subject to high waves and/or scour attack where site conditions exceed performance limits of "soft armor" systems such as turf reinforcement mats (TRMs). ACBs constructed without Revegetation considerations may never provide habitat enhancements. Biotechnical planting techniques (see Live Staking) or, at a minimum, grass establishment techniques should be incorporated into the design in order to have any environmental benefits.There is some concern that the geotextiles filter cloth may impede root penetration into the subsoil. The insertion of live cuttings through the block openings and the fabric could eliminate that concern. Permanent vegetation, woody cutting, grasses and forbs should not be designed or installed below the AHW as it will not persist.
Complexity:
Moderate. Vegetated ACBs are relatively simple to place and construct. Some site preparation may be required to smooth and grade the bank back to a stable angle. The blocks are typically pre-bonded to a filter cloth and placed as mat units as opposed to individual blocks. Some additional work is required to harvest and prepare live cuttings for insertion into the block revetment.
Design Guidelines / Typical Drawings:
Articulated concrete block systems, both unvegetated and vegetated, have been fairly widely employed to protect drainage ways, canal banks, and streambanks where velocities and associated shear stresses are high enough (see Figure 5) to preclude the use of soft armor systems.Several types of patented or proprietary blocks are available (see Figure 6), each with its own method of inter-locking or articulation. The blocks are placed on (or in some cases pre-bonded to) a geotextile filter cloth to prevent the loss or washout of soil through the openings in and around the blocks (see Special Topic: Role of Geotextiles). This requirement is particularly critical when the block system is subjected to a pumping action from flow reversals. Failure of an articulated block system is associated with loss of subgrade beneath the blocks. Loss of subgrade in turn can result in block-to-block installation discontinuity, viz., generally any adjacent blocks which protrude more than 1-inch (Lipscomb et. al, 2001), will induce failure of the subgrade and/or the blocks. Flanking or loss of blocks at the edges can be a problem with block systems; therefore, the system must be well keyed in at the edges.
Detailed design guidelines and specifications for ACB systems can be found in HEC-23 (Lagasse et. al., 2001). Interested users should consult these specifications for more precise guidance about block stability calculations. Although these calculations require individual block data that are supplied by the manufacturer, the analysis procedure is generic. Blocks should be analyzed using the "Projecting Block" factor-of-safety approach. See Lagasse et.al (2001) for spreadsheet analyses and sample calculations.The designer should specify the maximum block-to-block height projection as a basis for the construction inspector to accept or reject the installed ACB system. Suggested values for maximum acceptable block-to-block vertical displacement are:
13mm (1/2 in) for jobs where good inspection and channel preparation are anticipated
25mm (1 in) where subgrade preparation may be problematic
Suggested values for factors-of-safety (FS) are:
For channels, FS = 1.5
For bridge piers, FS may need to be > 1.5
Conceptual or schematic drawings for the placement and layout of an articulated concrete block revetment (see Figure 7).
Figure 5. Allowable velocity for vegetation and other types of streambank cover. (Thiesen, 1992)
Figure 6. Different types of articulated concrete blocks (from US Army, 1981).Vegetation (primarily grasses and forbs) can be grown in the openings/interstices between blocks, or within the blocks themselves. Live cuttings can likewise be inserted through holes or openings in the blocks into natural soil beneath. This vegetation mutes the harsh, unnatural appearance of the armor layer. More importantly, the plants improve the performance of the armor (Hewlett et al., 1987) by increasing both interlocking between structural units, and resistance to liftoff and local shear forces. The block mat should be keyed into the channel bed below the depth of anticipated scour or at least two blocks beneath the finished toe. In addition, the depth of the key trench should be a function of scour.
One of the common reasons that some floodway managers opt to remove woody vegetation from ACBs is concern that the stems, trunks and roots will deform and destabilize the blocks. On the contrary, it has been observed that the trunks and roots can actually grow through and greatly stabilize the ACBs (see Figure 10, 11, and 12). Unlike solid concrete, the ACBs have holes in the blocks that allow plant tissue (cambial material) to "engulf" the blocks (see Special Topic: Bio-Adaptive Plant Response).
Figure 7. Schematic or conceptual design drawing for articulated concrete block revetments or mats.
7. ENVIRONMENTAL CONSIDERATIONS / BENEFITS
Vegetation growing in the interstices of the blocks mutes the harsh, unnatural
look of concrete and improves the general visual appearance of bank armor.
Streambank vegetation also improves aquatic and riparian habitat in addition
to providing important functional benefits (Coppin and Richards, 1990). Hard,
stony surfaces, such as ACBs, are often colonized by attachment-type benthic
macro invertebrates, particularly when placed in channels with shifting, sandy
bed materials. Streambank vegetation creates cover, shade, and insect food
sources for fish and other aquatic organisms near the water's edge. Upper
and midbank vegetation provide cover and habitat opportunities for small mammals
and other riparian wildlife.
8. HYDRAULIC LOADING
The critical velocity and failure shear stress for ACBs are on the order
of 4.3 m/s (14 ft/s) and 226 Pascals (4.7 psf), respectively. Performance
studies (Lipscomb et. al., 2001) of articulated concrete blocks (Corps block)
under vegetated and un-vegetated conditions have shown that vegetation increases
the critical failure shear stress by about 40%. Failure of an ACB system
is defined as excessive loss of subgrade beneath the block mat resulting
in vertical discontinuities between adjacent blocks (see Design
Guidelines).
10. ADVANTAGES
Provides hard armor protection to streambanks where high velocities and stresses
preclude the use of vegetation "soft" armor, e.g., ECBs and TRMs,
but at the same time allows incorporation of vegetation, which provides environmental
benefits and improves engineering performance, e.g., greater liftoff resistance
and increased inter-block friction. Articulated block systems are flexible
and can conform to slight irregularities in surface topography caused by settlement.
11. LIMITATIONS
The main limitation of articulated block systems is the danger of flanking
or unraveling at the edges. Lateral continuity and connectivity between
individual blocks are provided by means of either cabling or some form of
articulation, but the edge blocks are more vulnerable. The following precautions
and limitations should also be observed:
For cabled ACBs, the channel side slopes should be no steeper than 1V:2H.
For non-cabled systems, side slopes should be no steeper than 1V:3H
Ascertain whether flume tests were conducted on light or heavy weight blocks, beware of extrapolated results.
Maximum velocity in flume tests tends to vary from product to product. Depending on the ACB system, stable channels may be possible for velocities of about 6 m/s (20 f/s) or more, and corresponding shear stresses form 1.44 to 1.92 kN/m (30-40 psi) with proper control of subgrade and with an appropriate geotextile (Clopper, 1989).
It is not reasonable to expect vegetation to become established below the
AHW or bankfull discharge elevation (see Figure 2). See Special Topic: Bankfull
Discharge.
12. MATERIALS AND EQUIPMENT
Articulated concrete blocks systems consist of various propriety designs
(see Figure 6 ). Connectivity between blocks is provided
by either cabling or some type of interlocking articulation.
A granular filter layer is recommended for final shaping and a geotextiles filter
fabric is required although some
commercial
block systems are pre-bonded to a filter cloth. A mechanical lifting device
is required to lower and place the blocks or block mats.
Live cuttings
that have been pre-soaked (see Special Topic: Harvesting
and Handling of Woody Cuttings) will be needed. A round steel stake
(pilot bar) to punch a pilot hole through the fabric and subsoil will make
inserting the live stakes easier and reduce mortality. A growing medium
or topsoil infill will aide the establishment of grasses and forbs.
13. CONSTRUCTION / INSTALLATION
Placement of the blocks or block mats requires preparation and smoothing of the bank surface. Directions and specifications for placement and installation of various types of proprietary block systems or mats should be obtained from the manufacturer.
Standard horticultural practices should be followed for the establishment of plants and revegetation of the ACBs.
14. COST
Likely to be most expensive of hard armor alternatives (compared to riprap and gabion mattresses), except where rock is not locally available. Material costs for concrete blocks range from $27 to $32 per m2 ($2.50 to $3.00 per ft2), depending on freight cost to jobsite. Geotextile cost is approximately $1.00 per m2 ($0.10 per ft2). Installation costs are approximately $16.00 to $21.50 per m2 ($1.50 to $2.00 per ft2), depending on project/site conditions. Accordingly, total installed costs can range from $43 to $54 per m2 ($4.00 to $5.00 per ft2). By comparison, riprap costs have varied from $22 - $44 per metric ton ($20 - $40 per ton) delivered and placed ($98 - $197 per linear m ($30-$60 per linear ft)).
15. MAINTENANCE / MONITORING
The main monitoring requirements are: 1) to check periodically for the
detachment and loss of blocks from the mat, and 2) to check for settlement
which indicates that the filter layer beneath the mat is not functioning
properly and that fines are washing out through openings. Individual blocks
can be replaced as needed. Vegetation also can be introduced as needed
by inserting live cuttings through openings or interstices in a mat. Installations
which are on slopes steeper than 1V:2H should have a monitoring program
with follow-up performance reports.
16. COMMON REASONS / CIRCUMSTANCES FOR FAILURE
Flanking or loss of blocks at the edges can be a problem with block systems;
therefore, the system must be well keyed in at the edges. The loss of blocks
from the center of the mat may lead to loss of articulation/interlocking
and compromise the effectiveness of the system. Cabling armor units together
and encapsulation (e.g., wire cages used with gabion (Reno) mattresses)
minimize this problem. Improper placement and sizing of filter fabric beneath
an ACB mat can lead to washout of fines beneath the mat and collapse or
settlement problems. The filter fabric layer can sometimes act as a sliding
surface. This problem is gradually ameliorated as vegetation becomes established
in the mat.
17. CASE STUDIES AND EXAMPLES
Examples of articulated concrete blocks used for a channel lining (unvegetated condition) and a bank protection (vegetated condition) are shown in Figures 8 and 9, respectively. Additional examples of the application of Vegetated ACBs are shown in Figures 10-13.
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Figure 8. Articulated concrete blocks being placed to protect a channel. |
Figure 9. Articulated concrete blocks (in vegetated condition) protecting a bank. |
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| Figure 10. ACBs used
on a side channel susceptible to high velocities, Guadalupe River,
San Jose, CA Photo by J. McCullah |
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| Figure 12. ACBs without vegetation were actually less stable. Guadalupe
River, San Jose, CA Photo by J. McCullah |
Figure 13. Vegetated
ACB has no vegetation established below AHW. Rouge River, Ford
Field, Dearborn, MI. Photo by J. McCullah |
Please visit the Photo Gallery for more pictures.
18. RESEARCH OPPORTUNITIES
Determine the extent to which geotextile filter fabrics placed beneath ACB
mats impede the growth, development, and penetration of roots through the
filter cloth into the native soil beneath.
19. REFERENCES
Clopper, P.E. (1989). Hydraulic Stability of Articulated Concrete Block Revetment Systems During Overtopping Flow. US Department of Transportation, Federal Highway Administration. Pub. No. FHWA-RD-89-199. Virginia.
Coppin, N. J., & Richards, I. (1990). Use
of Vegetation in Civil Engineering. Butterworths: Sevenoaks, Kent
(England).
Fischenich, C. (2001). Stability thresholds for stream restoration
materials. EMRRP Technical Notes Collection (ERDC-TN-EMRRP-SR-29),
U.S. Army Engineering Research and Development Center, Vicksburg, MS. (pdf)
Hewlett, H.W.M., Boorman, L. A., & Bramley, M.E. (1987). Design
of reinforced grass waterways. CIRIA Report No. 116,
Const. Indus. Res. & Info. Assoc., London, England, 118 pp.
Koutsourais, M. (1994). A study of articulated concrete blocks designed
to protect embankment dams. Geotechnical Fabrics Report, Vol.__,
No. 8, pp. 20-25.
Lagasse, P. F., Byars, M. S., Zevenbergen, L. W. & Clopper, P. E. (2001).
Bridge scour and stream instability countermeasures: Experience, selection
and design guidance. (Hydraulic Engineering Circular No. 23, FHWA
HI 97-030), Washington, D. C., pp. 1.3-1.11. (pdf)
Lipscomb, C.M., Christopher, Thorton, C.I., Abt, S.R., & Leech, J.R. (2001). Performance of articulated concrete blocks in vegetated & un-vegetated conditions. Land & Water, Vol. 45, No. 4, pp. 19-21
Thiesen, M. (1992) Evaluation of Biotechnical Composites Under High Velocity
and Shear Conditions. In Proceedings of Conference XXIII, International
Erosion Control Association, 1992, Reno, Nevada.
U.S. Army Corp of Engineers (1981). Low cost shore protection: Final
report on the shoreline erosion control demonstration program, Office
of the Chief of Engineers, Washington, DC
