Guard Base Guide

Precast Concrete Bases

Concrete Bases are key for low maintenance grid span. The strength of the base, as well as the bearing pressure under the base must match the forces from the equipment going over the crossing. For added strength bases require an internal rebar cage for reinforcement to handle heavy vehicle traffic. Grid Span Bases, whether for creek crossing or animal management, can be cast in place - which often makes more sense. If your project involves a concrete driveway that will be poured up to the grid span you may choose to cast the bases in place. If you are cutting a grid span into an existing driveway and you would have to schedule a concrete contactor as well as concrete delivery, precast may be the better solution.

Base Requirements

Concrete

Internal rebar cage (reinforces the base to handle heavy vehicle traffic)

In Place Casting vs Pre-Casting

In Place Casting

Grid span bases can be cast in place (for both animal management and creek crossing)

Note: if your project involves a concrete driveway that will be poured up to the grid span you may choose to cast the bases in place.

Pre-Cast

Grid span bases can be purchased pre-cast to save time and money (for both animal management and creek crossing)

Note: Cutting a grid span into an existing driveway would require a concrete contactor, as well as concrete delivery. Ordering pre-cast bases would be economical and quicker.

Grid Pit Functionality

Animal Grid Pit

Pits on animal guards present a 3D effect so that the animal won’t try to cross. Without a pit on an animal guard, bolder animals may try to step between the rails and cross on the ground.

Permanent TrackOut Pit

The deeper the pit, the longer you can go between cleanouts.

Inlet Grate Pit

Pit mat be lined with a membrane to contain everything that gets washed into the pit.

Note: Site and use requirements often dictate which base size and type will work best.

Abutments

In a creek guard, the void is pre-determined by the creek, canal or ditch size. Our systems are meant to be set back into the bank, so that concrete will not be in direct contact with the water source. If abutments that contact the waterway are necessary, it is best to hire a local geotechnical engineer to provide a cast in place abutment design.

Associated Risks

There are varying risks associated with the engineering and installation of grid spans.

Uneven Ground Settlement

If a cattle guard base settle 2” on one side of the cattle guard into a pasture, the consequences are that there will be a bump when crossing. Not a huge deal. Because the “consequences for failure” of a cattle guard installation into a pasture are low, customers may just set the cattle guard on used railroad ties staked to the ground. This is an affordable option that provides a pit and will last a handful of years before needing to be re-set. Other crossings like a cattle guard on a high-speed county road need to be installed with great care to avoid any settling or heaving. The Big R Grid Span systems are engineered to handle specific loads, but proper site conditions and installation are crucial to the systems performance.

Soil Conditions

An important factor for installing the bases is the ability of the soil to handle the pressures transferred from the loadà grid à basesàdown to the soil.

Bearing Capacity

This is the capacity of the soil to support the load of the precast concrete bases before shear failure.

Note: When the “consequences for failure” is high, it is important to get a local soil testing company to test the site and provide a geotechnical report. Engineers use the results to determine the bearing capacity at the site. If the native soil does not have the necessary bearing capacity, you will need to over-excavate, frequently to the frost line, where the bases will go, and bring in compactable fill to the elevation of the bottom of the bases.

From there the compactable fill need compacted in lifts (layers) to the specified requirement (typically 3,500 to 4,000 psf).

Backfill Envelope

The required shape and dimensions of the compacted fill. While the backfill envelope is shown with angled edges, because that is how the forces are transferred, usually people just make them rectangular for simplicity.

There is additional information in both the Installation guide and glossaries. Our pre-engineered systems are designed to perform when installed correctly, and do not take into account specific site conditions. We recommend talking to a local geotechnical engineer to verify site specific conditions and requirements outside of a standard cattle guard installation.

The information on this website is not meant to be used for any specific application without an independent competent professional examination and verification of its accuracy, suitability, and applicability by a licensed professional engineer, he/she does so at his/her own risk and assumes any and all liability resulting from such use, Such information or materials may contain inaccuracies or errors and we expressly exclude liability for any such inaccuracies or errors to the fullest extent permitted by law. Your use of any information or materials on this website is entirely at your own risk, for which we shall not be liable.