All of the closure types presented on this website can (and often should) be modified depending on rock substrate, biology, and cultural sensitivity. Here are some common modifications (but not an exhaustive list):
• EXPANDED METAL SKIRTING — For sites with extremely fractured or weathered bedrock or unconsolidated fill, expanded metal (EM) skirting should be placed around the gate to prevent unauthorized persons from digging under or mining around your new gate to gain access to the mine. The EM is held down with footers welded to the gate. This can be used with any Basic or Cupola-style gate and, with some adaptations, can also be used with fencing projects.
• CULVERTS — Entrances occasionally need to be stabilized before gating, particularly collapsing portals or open shafts with crumbling collars. The most common approach is to first install a culvert. Metal, concrete, and plastic culverts have all been used successfully, with the choice typically dictated by availability, cost, and strength requirements. Using the largest diameter culvert that fits the opening wil reduce potential negative changes in airflow and microclimate, and provides the best opening for emerging bats. Culverts can be stabilized with expanding polyurethane foam (PUF) or earthen fill. Gates can then be attached directly to the outside of the culvert or placed over the culvert and fill (in the case of a shaft closure). Experts should be consulted if a culvert is needed, since improper engineering can cause problems with drainage, change interior microclimates, or create a predator problem.
• HYBRID GATES — In mines with very small numbers of bats, hybrid gates, sometimes called ladder gates, have been used successfully to close portals with minimum expense. These consist of a narrow section of a Basic gate bounded on either side by non-bat-friendly gratings. Hybrid gates provide some access for bats while maintaining airflow.
• VARIABLE BAR SPACING — For mines in high-traffic areas, particularly where small children and pets are likely, you may want to space the bottom bars of the gate more closely together to prevent accidental access. The spacing in the top, however, should be at least 5¾" to accommodate bats. The gate should have a larger cross-sectional area to compensate for reduced flight space and airflow, if possible.
• WILDLIFE MODIFICATIONS — “Tortoise Holes” are often built into portal gates within the range of desert tortoises (Gopherus agassizii) to ensure that tortoises can still access to the mine to escape the heat. In areas where barn owls use mine openings for nesting, owl perches are being built onto mine gates. Note, however, that having an owl perch on a mine used by bats is counterproductive.
• FLYOVERS, WINDOWS, and CHUTES — At mines with extremely large numbers of summer-roosting bats, gates require special modifications for the bats' nightly foraging emergence. Flyovers, also known as half-gates, can be used in very large (truck-sized) openings, if protected by overhanging shields. Windows, smaller openings than flyovers, are boxed by shields on both sides and the bottom. They can only be built into large cross-sectional areas. Chutes are essentially boxed-in tubes that extend outward at an angle from a Basic or Cupola gate. These provide maximum flight space for bats, while discouraging unauthorized entry. Chutes, and to some degree Windows, are challenging to design and construct and should only be attempted by experienced gate builders.