Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance-Third Edition

Design Guideline 6 Wire Enclosed Riprap Mattress

6.1 INTRODUCTION

Wire enclosed riprap is commonly used in the state of New Mexico. The predecessor to this erosion control technique is known as rail bank protection and has been used in Arizona, Colorado and New Mexico since the 1970s. Wire enclosed riprap differs from gabions and gabion (Reno) mattresses in that it is a continuous framework rather than individual interconnected baskets. In addition, wire enclosed riprap is typically anchored to the embankment with steel stakes which are driven through the mattress. Construction of wire enclosed riprap is usually faster than gabions or gabion mattresses, and it also requires less wire mesh because internal junction panels are not used. Wire enclosed riprap is used primarily for slope protection. It has been used for bank protection, guide bank slope protection, and in conjunction with gabions placed at the toe of slope.

Successful long-term performance of wire enclosed riprap depends largely on the integrity of the wire. Due to the potential for abrasion by coarse bed load, wire enclosed riprap is not appropriate for gravel bed streams and should only be considered for use in sand-or fine-bed streams. Additionally, water quality of the stream must be noncorrosive (i.e., nonsaline and nonacidic). A polyvinyl chloride (PVC) coating should be used for applications where the potential for corrosion exists.

6.2 DESIGN GUIDELINES

Guidelines for the dimensions, placement, anchoring, splicing, and quantity formulas are shown on Figure 6.1. Design procedures for the selection of rock fill for wire enclosed riprap can be found in Simons et al. (1984), Maynord (1995), and Design Guideline 11. Guidelines on selection and design of filter material can be found in Holtz et al. (1995) and Design Guideline 16. The following guidelines and specifications reflect construction procedures for wire enclosed riprap recommended by the New Mexico State Highway and Transportation Department (NMSHTD).

1. Wire mesh fabric for riprap shall be hexagonal mesh or a "V" mesh meeting the requirements listed in the specifications.
2. Steel stakes may be railroad rails, not less than 30 lb per yard (14.9 kg/m), 4 in. (102 mm) O.D. standard strength galvanized steel pipe, or 4" × 4" × 3/8" (102 mm × 102 mm × 9.5 mm) steel angles.
3. If length of slope is 15 ft (4.6 m) or less, only one row of steel stakes 2 ft (610 mm) from the top edge of the riprap will be required unless otherwise noted on the plans.
4. Dimensions of the thickness, top of slope and toe of slope extents, and total length of protection shall be designated on the bridge or roadway plans.
5. The wire enclosed riprap thickness is usually 12 in. (300 mm) unless otherwise shown on the plans. Thickness is usually 18 in. (460 mm) at bridges.
6. Longitudinal splices may be made with one lap of galvanized 9 gage tie wire, 9 gage hog rings or 11 1/2 gage galvanized hard drawn interlocking wire clips.
7. In general, a minimum of 2 ft (0.6 m) of freeboard above the design water surface elevation should be maintained.

Figure 6.1. Wire enclosed riprap plans (NMSHTD).

6.3 SPECIFICATIONS

Wire Enclosed Riprap. Wire enclosed riprap shall consist of a layer of rock of the required thickness enclosed on all sides in wire fabric conforming with the details shown on the plans (Figure 6.1). The wire fabric shall be drawn tightly against the rock on all sides and tied with galvanized wire, locking clips, hog rings or connectors. When ties, locking clips, hog rings or connectors are used for tying mesh sections and selvages together, they shall be spaced 3 in. (76 mm) apart or less as shown on the plans. Galvanized wire ties shall be spaced approximately 2 ft (610 mm) on center and shall be anchored to the bottom layer of wire fabric, extended through the rock layer, and tied securely to the top layer of wire fabric. When indicated on the plans, wire enclosed riprap shall be anchored to the slopes by steel stakes driven through the riprap into the embankment. Stakes shall be spaced as indicated on the plans.

Filter. See Holtz et al. (1995) and Design Guideline 16 for selection, design, and specifications of filter materials.

6.4 INSTALLATION EXAMPLE

A typical example of wire enclosed riprap installed by NMSHTD is shown in Figure 6.2. A side slope of a guide bank at the I-25 crossing of the Rio Galisteo protected with wire enclosed riprap is shown.

Figure 6.2. Wire enclosed riprap used for guide bank side slope protection at I-25 crossing of Rio Galisteo, New Mexico (NMSHTD).

6.5 REFERENCES

Holtz, D.H., Christopher, B.R., and Berg, R.R., 1995, "Geosynthetic Design and Construction Guidelines," National Highway Institute, Publication No. FHWA HI-95-038, Federal Highway Administration, Washington D.C., May.

Maynord, S.T., 1995, "Gabion Mattress Channel-Protection Design," Journal of Hydraulic Engineering, ASCE, Vol. 121,7, pp. 519 - 522.

Simons, D.B., Chen, Y.H., Swenson, L.J., and Li, R., 1984, "Hydraulic Tests to Develop Design Criteria for the Use of Reno Mattresses," Civil Engineering Department - Engineering Research Center, Colorado State University, Fort Collins, CO, Report.