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Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance-Third Edition
Design Guideline 19 Concrete Armor Units
Concrete armor units are man-made 3-dimensional shapes fabricated for soil stabilization and erosion control. These structures have been used in environments where riprap availability is limited or where large rock sizes are required to resist extreme hydraulic forces. They have been used as revetments on shorelines, channels, streambanks and for scour protection at bridges. Some examples of armor units include Toskanes, A-Jacks®, tetrapods, tetrahedrons, dolos and Core-locTM(Figure 19.1).
The primary advantage of armor units is that they usually have greater stability compared to riprap. This is due to the interlocking characteristics of their complex shapes. The increased stability allows their placement on steeper slopes or the use of lighter weight units for equivalent flow conditions as compared to riprap. This is significant when riprap of a required size is not available.
The design of armor units in open channels is based on the selection of appropriate sizes and placement patterns to be stable in flowing water. The armor units should be able to withstand the flow velocities without being displaced. Hydraulic testing is used to measure the hydraulic conditions at which the armor units begin to move or "fail," and dimensional analysis allows extrapolation of the results to other hydraulic conditions. Although a standard approach to the stability analysis has not been established, design criteria have been developed for various armor units using the following dimensionless parameters:
The Isbash stability number and Shields parameter are indicative of the interlocking characteristics of the armor units. Froude number scaling is based on similitude of stabilizing and destabilizing forces. Quantification of these parameters requires hydraulic testing and, generally, regression analysis of the data. Prior research and hydraulic testing have provided guidance on the selection of the Isbash stability number and Shield's parameter for riprap and river sediment particles, but stability values are not available for all concrete armor units. Therefore, manufacturers of concrete armor units have a responsibility to test their products and to develop design criteria based on the results of these tests. Since armor units vary in shape and performance from one proprietary system to the next, each system will have unique design criteria.
Installation guidelines for concrete armor units in streambank revetment and channel armor applications should consider subgrade preparation, edge treatment (toe down and flank) details, armor layer thickness, and filter requirements. Subgrade preparation and edge treatment for armor units is similar to that required for riprap and general guidelines are documented in HEC-23 (see also NCHRP Report 568) (Lagasse et al. 2009 and 2006, respectively). Considerations for armor layer thickness and filter requirements are product specific and should be provided by the armor unit manufacturer.
Concrete armor units have shown potential for mitigating the effects of local scour in the laboratory, however limited field data are available on their performance. Research efforts are currently being conducted to test the performance of concrete armor units as pier scour countermeasures in the field.
Design methods which incorporate velocity (a variable which can be directly measured) are commonly used to select local scour countermeasures. Normally an approach velocity is used in the design equation (generally a modified Isbash equation) with a correction factor for flow acceleration around the pier or abutment (see for example, Design Guidelines 11 and 14). A specific design procedure for Toskanes has been developed for application at bridge piers and abutments and is described in Sections 19.4 and 19.5 to illustrate a general design approach where the Toskanes are installed as individual, interlocking units.
Another approach to using concrete armor units for pier scour protection has been investigated by the Armortec Company and involves the installation of banded modules of the A-Jacks® armor unit. Laboratory testing results and installation guidelines for the A-Jacks system are presented in Section 19.6 to illustrate the "modular" design approach in contrast with the "discrete particle" approach for Toskanes.
The Pennsylvania Department of Transportation (PennDOT) contracted with Colorado State University (CSU) in 1992 to investigate concrete armor units as a countermeasure for local scour at bridge piers. The purpose of the research was to develop guidelines for selection and placement of cost-effective armor unit sizes to mitigate pier scour (Fotherby and Ruff 1995, Fotherby 1995). A literature review of concrete armor units used in coastal and river protection works led to the selection of the Toskane as the primary concrete armor unit for which guidelines were to be developed. The Toskanes were modified from those used in coastal applications by removing the pointed corners from the hammerheads, increasing the length and cross section of the beam, and including reinforcing steel in the beam.
Hydraulic tests to evaluate the performance of Toskanes were conducted in an indoor flume and two outdoor flumes at CSU. Over 400 test runs were conducted. These tests included random and pattern placement of Toskanes tested to failure around piers and abutments, determination of protective pad radius, determination of pad height (comparing installations in which the top of the pad was level with the bed and installations in which the pad protruded above the bed), comparison of gravel and geotextile filters, number of Toskanes per unit area, and effect of angle of attack on Toskanes at a round nose pier. The data were analyzed, and using dimensional analysis the significant parameters were determined.
The design equation developed from regression analysis of hydraulic test data at CSU allows the computation of the equivalent spherical diameter of a stable Toskane size. The equivalent spherical diameter is the size of a sphere that would have the same volume of material as the armor unit as determined by the following equation:
Given the hydraulic conditions and dimensions of the pier or abutment, Equation 19.1 can be solved to select an appropriate size of Toskane for local scour protection. The design parameters and dimensions of Toskanes are illustrated in Figure 19.2.
The actual dimensions of the Toskanes are dependent on the size of unit constructed. Relative design dimensions are listed in Table 19.1.
The equivalent spherical diameter of the units constructed should equal or exceed the value determined from Equation 19.1. Custom sizes of Toskanes may be selected, but it may be more cost effective to use a standard size. Recommended standard sizes of Toskanes are listed in Table 19.2.
Tables 19.1 and 19.2 provide information necessary for construction of individual armor units once an appropriate size is selected. Design parameters for installation of a protection pad are provided in Table 19.3.
**Toskanes per unit area assuming a 2-layer thickness of 2Du.
The following design guidelines reflect the results of the research conducted at CSU (Fotherby and Ruff 1995, Fotherby 1995):
The discrete particle design approach illustrated by the Toskane design guidelines concentrates on the size, shape, and weight of individual armor units, whether randomly placed or in stacked or interlocked configurations. In contrast, the basic construction element of A-Jacks for pier scour applications is a "module" comprised of a minimum of 14 individual A-Jacks banded together in a densely-interlocked cluster, described as a 5x4x5 module. The banded module thus forms the individual design element. Figure 19.3 illustrates the concept. (Note that the photograph of Figure 19.3 shows that a module larger than 5x4x5 can be configured).
In late 1998 and early 1999, a series of 54 tests of 6-inch model scale A-Jacks was conducted at Colorado State University (CSU) to examine their effectiveness in pier scour applications. This program is described in detail in CSU's test report entitled, "Laboratory Testing of A-Jacks Units for Inland Applications: Pier Scour Protection Testing" (Thornton et al. 1999a and b).
The CSU tests were conducted in an 8-foot (2.44 m) wide indoor flume with a sand bed, and examined a variety of conditions, including no protection (baseline conditions), banded 5x4x5 modules arrayed in several different configurations, and individual (unbanded) A-Jacks armor units. Both round and square piers were used in the program. The results indicated that, when used in combination with a bedding layer (either granular bedding stone or a properly selected geotextile), the A-Jacks 5x4x5 modules reduced scour at the pier from 70 percent to more than 95 percent (scour depths were from 30 percent to less than 5 percent of that in the unprotected baseline condition).
Hydraulic stability of a 5x4x5 A-Jacks module can be estimated by setting the overturning moment due to the total drag force equal to the resisting moment due to the submerged weight of the module:
As a first estimate, the coefficient of drag Cd on an A-Jacks module can be assumed to be similar to that of a disc oriented normal to the flow velocity, with flow occurring over the top and around the sides. This value is approximately 1.2 (Venard and Street 1995). A conservative estimate for the location of the drag force would place it at the full height of the module, providing the greatest moment arm for overturning.
Tests were conducted at CSU in a steep (13 percent slope), fixed-bed flume to determine the hydraulic stability of the 5×4×5 A-Jacks modules in a typical pier scour configuration. Discharge was gradually increased until overturning of the module was achieved. Both submerged and unsubmerged conditions were examined.
Measuring hydraulic conditions at the threshold of overturning allows both the coefficient of drag, Cd, and the height of the drag force, Hd, to be determined directly from measured data. The other variables in Equation 19.4 are determined from the physical characteristics of the 5×4×5 A-Jacks module.
Using a drag coefficient Cd of 1.05 for the A-Jacks modules from the laboratory testing, and assuming that the drag force acts at the full height the module, the hydraulic stability of prototype scale A-Jacks modules can be determined. Table 19.4 provides the results of this hydraulic stability analysis (Clopper and Byars 1999).
Geometry. The movable-bed tests conducted at CSU indicate that a chevron-style A-Jacks placement around a bridge pier does not improve performance beyond that afforded by simple rectangular geometries. As the rectangular shape accommodates the basic 5x4x5 A-Jacks module design unit, this geometry provides the recommended style for layout and placement of the armor units. Figure 19.3 provides recommended minimum dimensions for the placement of modules around a pier of width "a" and having an unprotected depth of scour ys as determined by HEC-18 (Richardson and Davis 2001).
It should be noted that the CSU stability tests were conducted on a fully-exposed module; partial burial will result in a more stable installation. Also, the orientation of the modules in the stability tests exposed the maximum frontal profile to the flow (i.e., long axis perpendicular to the flow direction). Placement of the modules with the long axis parallel to the flow will result in a more stable arrangement than indicated by the recommended values in Table 19.4.
A-Jacks Placement. A-Jacks modules can be constructed onsite in the dry and banded together in 5x4x5 clusters in place around the pier, after suitable bedding layers have been placed. Alternatively, the modules can be pre-assembled and installed with a crane and spreader bar; this arrangement may be more practical for placement in or under water.
Bands should be comprised of cables made of UV-stabilized polyester, galvanized steel, or stainless steel, as appropriate for the particular application. Crimps and stops should conform to manufacturer's specifications. When lifting the modules with a crane and spreader bar, all components of the banding arrangement should maintain a minimum factor of safety of 5.0 for lifting.
Where practicable, burial or infilling of the modules to half-height is recommended so that the voids between the legs are filled with appropriate sized stone. Stone sizing recommendations are provided in the next section.
Bedding Considerations. The movable-bed tests conducted at CSU indicate that a bedding layer of stone, geotextile fabric, or both, should be included as part of the overall design of an A-Jacks installation. The purpose of a bedding layer is to retain the finer fraction of native bed material that could otherwise be pumped out between the legs of the A-Jacks armor units.
When bedding stone is placed directly on the streambed material at a pier, it must meet certain size and gradation requirements to ensure that it not only retains the bed material, but that it is permeable enough to relieve potential pore pressure buildup beneath the installation. In addition, the size of the bedding stone must be large enough to resist being plucked out through the legs of the A-Jacks by turbulent vortices and dynamic pressure fluctuations. In some cases, two or more individual layers of bedding stone, graded from finer in the lower layers to coarsest at the streambed, must be used to satisfy all the criteria. Figures 19.4a and 19.4b illustrate the bedding options discussed in this section.
Recommended sizing criteria for bedding stone (Escarameia 1998) are as follows:
In the above relations, Dx is the particle size for which x percent by weight are finer, and the designations Upper and Lower denote the respective positions of various granular bedding layers in the case when multiple layers are used. Each layer should be at least 6 to 8 inches (152 to 203 mm) thick, with the exception of uppermost layer which should be thicker, in accordance with Table 19.5. Note that the lowest layer of the system corresponds to the native streambed material.
In lieu of multiple layers of granular bedding, it is often desirable to select a geotextile which is compatible with the native streambed material. However, placement of a geotextile may not always be practical, particularly when installing the system under flowing water. If a geotextile is used, it is recommended that a layer of ballast stone, with characteristics in accordance with Table 19.5, be placed on top prior to installing the A-Jacks modules.
When a geotextile is used, selection criteria typically require that the fabric exhibit a permeability at least 10 times that of the native streambed material to prevent uplift pressures from developing beneath the geotextile. In addition, the Apparent Opening Size (AOS) of the apertures of the geotextile should typically retain at least 30 percent, but not more than 70 percent, of the grain sizes present in the bed. Design procedures for determining geotextile properties are provided in Design Guideline 16. Finally, the geotextile must be strong enough to survive the stresses encountered during placement of stone and armor units.
Limited field testing using a design layout similar to Figure 19.3 and the guidelines of this section has been conducted. Figures 19.5 a, b, and c show a demonstration site installation of A-Jacks for pier scour protection in Kentucky.
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