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California Desert District

Addressing Public Safety in the California Desert

The remnants of abandoned mining sites tell an alluring story about an adventurous time in the Nation’s past.  Unfortunately, they can also be the subject of distressing news about injuries and fatalities.  One of the main goals of the BLM and the State of California AML programs is to address the health and safety hazards that exist in these formerly mined areas.  While surface water contamination presents a common challenge on AML sites, addressing physical safety issues is another major priority reclamation work, as dozens of people every year are injured or killed in and around abandoned mine facilities.  Over the years, the BLM, and their partners have worked together to safeguard the lives and wellbeing of individuals who come into contact with these sites.  The following describe remedial actions that are taken to help meet that goal.

BLM staff and bat biologist evaluate closure options for a hazardous abandoned mine opening in the Cargo Machacho Mountains, Imperial County, California. 

 Fencing and signing shafts, adits, and other abandoned mine openings is a quick way to provide a basic level of safety and give the BLM with its partners time to design a permanent solution and address historical and wildlife concerns.   The addition of net fencing is also helpful in protecting desert tortoises from falling into abandoned mines.

A decline mine is one that has an inclined shaft usually following the downward slope of the orebody or vein.

Before any actions are taken on an AML site the site itself must first be characterized.  During this visit the BLM’s field personnel will determine if remedial activities will impact historical, cultural, or wildlife resources or habitat.   The type of AML site activities to be performed determines which regulations guide the site activities.  In general, BLM follows National Environmental Policy Act procedures when conducting activities to address physical safety issues and for those sites where activities have historic, cultural, or wildlife impacts.  When an imminent physical safety emergency occurs, the BLM should conduct NEPA procedures concurrently or as soon as possible after the fact.  Physical safety risk emergencies may be categorically excluded from NEPA compliance.



AML Fence around open pit


Bat fence







Turning a Hole in the Ground into a Home

Bats in the desert choose roosting sites that might surprise you.  They often roost in abandoned mines and associated structures.  The BLM in partnership with the State of California – Abandoned Mine Lands Unit has done many projects in recent years to protect bat habitat, as well as humans from the hazards of abandoned mines.  These projects consist of constructing bat gates.   A bat gate has openings big enough for bats to fly out and hunt for insects at night, but small enough to prevent people from entering the mine.

Scientific knowledge of the response of mine dependent bats to gates has increased over the past 25 years resulting in the evolution of gate designs from simple round bar gates to the current angle-iron standard gate.  This evolution has resulted in substantial improvements in the security and effectiveness of closures for bats.  Options include polyurethane foam plugs (PUF) with a horizontal 48” pipe.  This process will continue using the Bat Conservation International design standard which may be modified in the future as we learn more effective methods of protecting bats while we protect the public from the hazards of abandoned mines. 

Bat cupolas are constructed over vertical and nearly vertical mine openings in order to provide access for bats while protecting the public from the usually extreme physical hazards of such openings.  Provision of a proper foundation for a bat cupola is structurally the most important design consideration and is highly site specific.  Options include polyurethane foam plugs (PUF) with a 48” or larger vertical pipe.  Rock conditions and the size of the opening are often the most important parameters in choosing a foundation design solution.  Other factors such as: the presence of timbers in the opening, the need to preserve historic structures at or near the opening, accessibility by construction equipment, the location, size, type of use by and species of the resident bat population, the need to preserve airflow conditions in the underground mine, and other factors may also play a role.  Design guidelines for the bat cupola itself are less well defined than they are for grated closures in horizontal openings, but many of the same basic principles apply.  Cupolas are normally built using concrete for the foundation with angle-iron crossbars on the sides and steel crossbeams with expanded metal on top of the cupola.


Bat cuploa





Other Closure Methods


Wildlife net over and open pit

Prior to closing an abandoned mine Bureau biologist will conduct winter and spring surveys to determine presence or absence of wildlife that may be using the structure.  Outcomes of each survey will then determine if the structure should be closed or not. 

After the determination is made a team led by a biologist will take precautions to exclude any wildlife that may be present.  Some skill is required to identify all entry points and to apply netting material to openings.

This process, which takes several days, allows any bats or birds to leave the mine at dusk.  A small opening through which bats can exit the structure is left open which is monitored by staff after sunset using night vision equipment.

Long-term Closures

Long term closure methods seal and prevent entry into the mine opening, but preserve the general condition of the opening.  This method of closure is applicable to openings which present a hazard but which may also have potential future economic and/or historic value or where a permanent closure is not practical.  Appropriate long-term closure methods for mine openings include:

  • Pre-cast or cast-in-place concrete caps;
  • Polyurethane foam plugs and native fill; and
  • Native rock or material to block bulkheads.


Man pouring polyurethane foam in to open mine

Bulldozer backfilling

Permanent Closures

Permanent closure methods completely close off an abandoned mine opening and eliminate access.  This option is used when the hazard is readily accessible to the public, and has no economic, biological, or historic value.   Appropriate methods to permanently seal an abandoned mine include:

  • Backfilling; and
  • Closure through blasting.