Supplemental Installation Guidance
This guide is intended for both the contractor and the engineer. It provides practical information for the installation of grid spans as trackout grids, creek crossings, and animal guards. It also provides the necessary considerations for proper design to achieve long term performance of the grid span.
It is the intent of this manual to suggest ways and means of improving installation practices. It is not intended to be used as a direct specification, but rather as a practical field guide for the installation of grid spans. OSHA safety regulations and guidelines must be observed during all phases of construction including foundation preparation, excavation, material handling, assembly and backfilling. Project plans and specifications provide the basic requirements for construction and installation. However, site conditions often vary from those anticipated during design.
The contractor and construction engineer must recognize these variations. Often, alternate or additional construction considerations are necessary. The following guidelines provide specific considerations and details for various conditions in a step-by-step construction sequence.
- Tools Required – Heavy equipment for unloading, excavating, compacting, setting and backfilling. Ratchet Set or wrenches. Ratchet extension may aid installation. Slings are preferred to chains which can chip the concrete and damage the coating on the grids.
- Unloading and handling - Carefully unload the truck and store for installation
- Gasket – Neoprene bearing pad can be placed on the precast concrete base to absorb shock from vehicle traveling over the grid. It is simply rolled out over the base step before setting the grids. Some specification instruct using a contact cement to hold the pads in place.
- Setting – Be sure your equipment can handle the weight of the grid it is lifting. Setting is usually done with straps positioned under the beams at four locations. If you require lifting lugs, they are available as a custom adder.
- Center Bolts – These bolts are used to connect grid to grid, on multiple grid installations.
The load carrying capacity of the concrete sills are largely dependent upon proper compaction. Therefore, to obtain maximum strength and prevent washing out and settlement, it is necessary that the backfill envelope under the foundation be made of good material, properly placed, carefully compacted and protected. If the native soils can’t be compacted to reach the necessary 4,000 pounds per square foot of bearing pressure, the installation will require bringing in adequate backfill.
All organic and foreign material shall be cleared from within the construction area. After clearing, the foundation shall be excavated to grade. Over excavation shall be avoided. Over excavated areas shall be backfilled and compacted. Be sure to have drainage within the area to prevent water from pooling within the excavation.
Placement of Bases, Grids and Accessories
Precast Concrete bases will likely be the heaviest individual pieces of your installation. Most bases have inset lifting eyes to pick and place the footings to the grade and location as shown on the drawings or specified by the precast footing manufacturer. Be sure that 100% of the footing base is in contact with the foundation soil to assure a proper bearing surface.
Place the grid on the step of the base and flush the ends of the base with the outside of the grid, as shown on the drawings or specified by the grid manufacturer. Attach the grid to the bases as specified, usually with hold-down clips over the flange of the beam or channel. The end wings shall be securely attached to the post and the cattle guard grid using hex bolts.
Selected, drainable materials achieve the best results. However, many local fill materials may do the job if carefully placed and compacted. Consult the design engineer or a soils engineer for proper backfill selection. Well graded granular material containing a small amount of silt or clay is ideal because is makes a dense, stable fill. Fill material must be free from rocks and hard earth clods larger than 3 inches (75 millimeters) in size. It must not contain any frozen material, sod, cinders or earth containing organic matter.
- 3" MINIMUM DEPTH LEVELING COURSE OF COMPACTED CLASS 5 AGGREGATE UPON WHICH THE PRECAST UNIT IS SET.
- The formal definition for Class 5 is a crushed and graded sand and gravel mix which has max grain size of 1 inch. This product has “binder” in it for shear strength.
Placing the Backfill
The backfill should be carefully compacted within the backfill envelope. Place the backfill equally on both sides of the bases in 6 inch lifts of compacted layers thoroughly compacting each layer to a 90% Standard Proctor density (AASHTO T99). Such compacted layers must extend to the limits shown on the plans on each side of the structure, or to the side of a trench, or to the natural ground line. Backfill must be placed and fully compacted on both the backwall and face of the bases.
- Face and backwall fill to be compacted in 6 to 8 inches (.15 to .20 meters) of compacted layers to density specified for adjacent embankment, but not less than 90% Standard Proctor Density (AASHTO T99).
- Hand Compaction - Hand tampers for horizontal layers should weigh not less than 20 pounds (9 kilograms) and have a tamping face not larger than 6 by 6 inches (150 x 150 millimeters).
- Mechanical Compactors - Most types of power tampers are satisfactory in all except the most confined areas. However, they must be used carefully and completely over the entire area of each layer to obtain the desired compaction. Avoid striking the structure with power tamping tools.
- Roller Compactors - Where space permits, sheepsfoot (recommended for clays and silts only), rubber tired and other types of rollers—with the exception of smooth rollers—can be used to compact backfill. But the fill adjacent to the structure should be tamped with hand or hand-held power equipment.
- Vibrating Compactors - Vibrating compactors can be used effectively on all types of backfill except heavy clays or other plastic soils. Small walk behind equipment is especially suited to trench installations.
Proper installation of your guard structure will result in longer and more efficient service. This installation manual is intended to call attention to both good practice and to warn against possible pitfalls. The principles apply to most conditions. It is not a specification but an aid to your own experience.
The following items should be checked to insure proper installation:
- Check alignment in relation to the plans as well as the actual site conditions.
- Make certain the guard widths, necessary appurtenances, etc. are correct.
- Excavate to the correct width, line and grade.
- Provide a uniform, stable foundation—correct site conditions as necessary.
- Unload, handle and store the guard correctly.
- Assemble the guard properly check alignment, follow special procedures for the grid connections, wings, gaskets, and other hardware used.
- Use a suitable (granular) backfill material as required in the plans and specifications.
- Maintain proper backfill width.
- Place and compact the backfill in 6 to 8 inches (150 to 200 millimeters) of thickness of compacted lifts.