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Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance-Third Edition
Design Guideline 13 Grout/Cement Filled Bags
Grout/cement filled bags have been used to protect stream banks in areas where riprap of suitable size and quality is not available at a reasonable cost. Guidelines for the use of bags (sacks) as a streambank revetment can be found in HDS 6 (Richardson et al. 2001) and Keown (1983). Grout/cement filled bags have also been used as a countermeasure against scour at bridges. Historically they have been used to fill in undermined areas around bridge piers and abutments. As scour awareness increases, grout filled bags are being used to armor channels where scour is anticipated or where scour is detected. Whether they are implemented in a post- or pre-scour mode, grout bags are relatively easy to install and can shift to changes in the channel bed to provide effective scour protection.
A precise quantitative factor of safety design procedure is not normally completed for the design of grout filled bags. This type of design would be beneficial in determining the hydraulic stability of the bags, but historically this has not been done for grout filled bags. It would require a comparison of the hydraulic shear stress and the critical shear stress to uplift the grout bag as is done with riprap using discrete particle analysis. Information on hydraulic performance of grout bags at bridge piers can be found in Bertoldi et al. (1996) and Fotherby (1997). More often, engineering judgment is used to select a bag size that will not be removed by the flow. Installation practices are critical to the success of the system. Guidelines for the use of grout filled bags for bridge scour reflect information provided by the Maryland State Highway Administration (MDSHA 1996 with 2008 revisions).
(see attached Sheets 1 - 7)
PIERS AND ABUTMENTS (MDSHA):
Portland cement concrete shall consist of nine bags, 94 lb per cubic yard (55.8 kg/m3) Type II Portland cement, air entrainment, 6 ± 1% mortar sand aggregate, and water so proportioned to provide a pumpable mixture. The 28-day minimum day strength shall be 3500 psi (24,140 kPa).
Fabric bags shall be made of high strength water permeable material. Each bag shall be provided with a self closing inlet valve, to accommodate insertion of the concrete hose. A minimum of two valves shall be provided for bags more than 20 ft (6.1 m) long. Seams shall be folded and double stitched.
Reinforcing steel dowels, if specified on the plans, shall conform to ASTM A 615, Grade 60 and shall be epoxy coated.
The geotextile shall exhibit the following properties in both the machine and cross-machine directions, in accordance with MDSHA "Standard Specifications for Construction and Materials," 2008, Section 921.09, Application Class SE geotextile:
The bags shall be positioned and filled so that they abut tightly to each other and to the substructure units. Joints between bags in successive tiers shall be staggered.
Fabric porosity is essential to the successful execution of this work. Suitability of fabric design shall be demonstrated by injecting the proposed mortar mix into three 2 ft (610 mm) long by approximately 6 in. (150 mm) diameter fabric sleeves under a pressure of not more than 15 psi (103 kPa) which shall be maintained for not more than 10 minutes. A 12 in. (300 mm) long test cylinder shall be cut from the middle of each cured test specimen and tested in accordance with ASTM C 39. The average seven day test compressive strength of the fabric form shall be at least higher than that of companion test cylinders made in accordance with ASTM C 31.
Standoffs to provide a uniform cross section shall be used.
Ready mixed high strength mortar may be permitted by written permission of the Engineer. The ready mixed high strength mortar shall be furnished by a manufacturer approved by the Laboratory and the plan, equipment, etc., shall be subject to inspection and approval.
The concrete pump shall be capable of delivering up to 25 yd3/hr (19 m3/hr).
Bags should be designed and constructed as flat mats, 3 to 4 ft (0.9 m to 1.2 m) wide and about 0.3 m (1 ft) thick. The bag lengths should be on the order of 4 ft (1.2 m). Bags should not be filled to the point that they look like stuffed sausages, since they will be much more vulnerable to undermining and movement, and will not fit properly into the mat.
Both the designer and the installer should understand how the mat is expected to perform. Each bag should be independent of other bags so that it is free to move; however, the bag should be snugly butted against adjoining bags to minimize gaps in the mat. This concept will result in a semi-flexible mat that will be able to adjust to a degree to changes in the channel bed. The mat should not be constructed as a rigid monolithic structure. It would be helpful to have a pre-construction conference with the designer, contractor and the State inspector.
The bags should be sized and located in accordance with the SHA Standards for the particular type of foundation and condition of scour. It is recommended that the type of grout bag installation and its design be reviewed by an engineer with experience in evaluating scour at bridges.
Careful attention should be given to preparation of the bed on which the bags are to be placed. Where the bed is uneven, such as might occur in scour holes, best results will be obtained by planning for a sequence of placement of the bags so that each bag adds to the support of the other bags. This is particularly important in locations where several layers of bags are to be placed. It is unlikely that detailed plans will be developed for such locations, and the integrity of the installation will depend on the skill of the persons placing the mat. If the bed is highly irregular, appropriate modification of the bed and removal of obstacles should be accomplished prior to placement of the bags.
Each bag should butt up firmly against its neighbor to provide a tight seal and to minimize the occurrence of gaps between bags. Particular attention should be given to obtaining this tight seal between the foundation and the first row of bags.
For piers, the bags should extend to a distance of 1.5 to 2 times the pier width on both sides as well as upstream of the pier nose and downstream of the pier end.
For abutments, the best results are obtained for most locations by placing the bags the full length along the upstream wingwall, abutment backwall and downstream wingwall to form a solid mat. This arrangement provides for a smooth streamlined design that locates the ends of the mat away from the main stream current or thalweg. Of course, there are a wide variation of conditions at abutments and each location needs to be designed for the site conditions.
In some cases, it may be necessary to provide for both grout bags and rock riprap to provide the desired degree of scour protection. As a general rule, however, it is preferable to provide either riprap or grout bags but not both at any one pier or abutment.
For small structures such as bridges or "bottomless" culverts with spans in the range of 15 to 25 ft (4.6 m to 7.6 m), there are essentially two choices for the design of the bags:
If the center channel is unprotected, it can be expected to scour as the bed degrades or large dunes migrate past the protective pad. This may result in undermining and displacement of the bags next to the channel or possibly of the whole installation. As an interim guide, it is suggested that consideration be given to lining the entire channel if more than half of the channel would be covered by grout bags placed along the abutments. If the bags extend across the entire channel, attention needs to be given to the treatment of the upstream and downstream ends of the bag to avoid undermining and displacement.
The following interim guidance is provided with regard to use of filter cloth:
Filter cloth should generally be used at locations where the bags are placed in a single layer along a level plane on the channel bed or flood plain. The filter cloth provides for additional support and stability in the event that the bags are subjected to undermining or movement as a result of scouring and hydraulic forces.
Where grout bags are placed in layers in a trenched condition, such as might occur in a scour hole, there is probably less need to provide for the filter cloth. At this point, however, it is recommended that the decision to eliminate filter cloth be made on a case by case basis. The general rule should be to place filter cloth under the grout bags unless:
Grout bags provide for an efficient, cost effective means of underpinning foundations that have been scoured down below the bottom of the footing. General guidance on placement of bags and procedures for grouting the voids under the footing has been developed by MDSHA in standard drawings.
If grout bags are placed under water, they are barely noticeable. A well designed and installed grout bag mat exposed to view under a bridge can be expected to have a streamlined and pleasing appearance. At some sites, the mats become covered with silt and are barely distinguishable from the channel banks or bed. Grout bags placed along wingwalls are usually exposed to the sun. Bags in these locations are likely to be covered by vegetation, especially when they have been covered by silt during high water events.
There were a few sites visited where the bags had an ungainly appearance. In most cases, these were bags that were pumped so full that they looked like sausages. Other reasons for a poor appearance include inadequate attention to design, installation, preparation of the bed on which the mat is placed, or a combination of these factors.
Early installations included bags with lengths of 15 ft (4.6 m) or more. In some cases, the bags were too long to fit properly into a compact mat. Use of shorter bags should help to minimize this problem in future installations.
Specifications for grout bags for undermined areas at piers were also provided by the State of Maine Department of Transportation (1995) as follows:
The underwater grout bags shall be fabricated based on the dimensions of the existing voids to be filled. Bags should be on the order of 3 to 4 ft (0.9 m to 1.2 m) wide and 6 to 8 ft (1.8 to 2.4 m) long. Bags shall be securely placed to form a perimeter bulkhead to partially fill and enclose the substructure void. Grout shall be pumped to uniformly fill the secured bag with sufficient restraint so as to not rupture the bag. Consecutive bag placement shall be in accordance with the manufacturer's requirements. At a minimum this will require: placement of reinforcing bar between successive layers, stitching together adjacent bags with an overlapping splice (where accessible), and covering holes left by grout and other inserts.
NOTE: The State of Maine recommends stitching bags together for protection of undermined areas at piers. This procedure conflicts with the guideline provided by the State of Maryland in Section 13.3, Item 4.
Bertoldi, D.A., Jones, S.J., Stein, S.M., Kilgore, R.T., and Atayee, A.T., 1996, "An Experimental Study of Scour Protection Alternatives at Bridge Piers," FHWA-RD-95-187, Office of Engineering and Highway Operations R&D, McLean, VA.
Fotherby, L.M., 1997, "Footings, Mats, Grout Bags, and Tetrapods, Protection Method Against Local Scour at Bridge Piers," M.S. Thesis, Colorado State University.
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.
Keown, M.P., 1983, "Streambank Protection Guidelines for Landowners and Local Governments," U.S. Army Corps of Engineers, Waterways Experiment Station, Vicksburg, MS.
Maryland State Highway Administration, 1996, "Bridge Scour Notebook Supplement No. 1," MDSHA Office of Bridge Development, Bridge Hydraulics Unit (Revised 2008).
Maryland State Highway Administration, 2008, "Standard Specifications for Construction and Materials," July.
Richardson, E.V., Simons, D.B., and Lagasse, P.F., 2001, "River Engineering for Highway Encroachments - Highways in the River Environment," Report FHWA NHI 01-004, Federal Highway Administration, Hydraulic Design Series No. 6, Washington, D.C.
State of Maine Department of Transportation, 1995, "Supplemental Specifications for Underwater Grout Bags," Section 502, April.