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Abandoned Mines Part II: Protective Closures

The first article of this series, “Abandoned Mines – Preserve or Destroy?” addressed a few issues regarding preservation versus destruction of abandoned mine workings. A recent example of destructive closure was provided where five shafts at the Mammoth Spar Mine, south of Wickenburg, were backfilled in January 2010 on BLM-administered public land. The objective of the Bureau of Land Management, the State Mine Inspector’s Office, and other agencies is to “proactively close abandoned mines that pose a risk of injury or death.” Although few people would argue against a policy that promotes safety, there has never been complete agreement with the methods used to achieve that objective.

Other voices have spoken for moderation, cautioning that indiscriminate closures of shafts and adits can deprive certain wildlife species of needed habitat. In fact, regulations require federal agencies like the BLM and National Forest Service to assess underground mine workings to ensure that habitats are protected.[1] Not all abandoned mines are suitable for wildlife, but unrelenting pressure on traditional habitats makes these mine workings increasingly important and – in some cases – they may be the last alternative for the long-term survival of some threatened species. Solutions for preserving abandoned mines include fencing, barricades and steel grates or cupolas that permit access for wildlife while keeping humans out. Mines that serve as habitats have been destructively closed in the past, trapping entire colonies of bats in the underground workings. In other cases, excessive human visitation and vandalism have forced bats and other wildlife to abandon some mines and caves. In April 2009, an unknown number of individuals shot and killed up to 90 bats in an abandoned mine in the Superstition Mountains.

The BLM’s policy handbook, Abandoned Mine Lands Policy Handbook states:

Abandoned underground mines provide significant habitat for bats-more than half of North America’s 47 bat species are known to use mines. Acquiring even a basic understanding of bat use of abandoned mines often requires repeated surveys during different seasons. Sealing mines without first evaluating their importance to bats may be one of the single greatest threats to North American bats.”[2]

There are two broad categories of closures: (1) Destructive closure, where the shafts or adits are permanently sealed by blasting or backfilling, and (2) Protective closure, where the shafts or adits remain intact, but access to the underground workings is prevented using some type of barricade.

Protective closures actually have two objectives that frequently operate concurrently: First, to keep people out of a dangerous underground mine without resorting to destructive closure; and second, to protect wildlife that might be using the mine for nesting, birthing, hibernation, night roosting or migration, etc. Protective closure means that the subterranean workings are effectively closed to human visitors. For wildlife, it remains a seasonal or permanent habitat – but with the danger of human disturbance greatly reduced or eliminated.

Simple Fence at Mine Shaft

Figure 1, Simple Fence at Mine Shaft

The simplest and least costly method of protective closure is a fence constructed with metal stakes and wire, as shown in Figure 1.[3] Although this type of structure can be defeated by a vandal with wire cutters, it provides several benefits, including:

  • It is a visual warning to visitors.
  • With proper signage it serves notice that entry into the workings is criminal trespass.
  • It can at least partially relieve potential liability.
  • Fences can be quickly installed on site.
  • With periodic maintenance these structures can last for decades.
  • The materials are not expensive.[4]

Warning signs like that shown above are generally effective for most visitors but unfortunately, are not a deterrent to the small minority of people who choose to enter an abandoned mine in spite of the risks. [5] Not surprisingly, very few fences have been erected – even around abandoned mine shafts that are difficult or impossible to see from a distance in daylight or darkness. In my experience, the mere mention of fencing is met with deprecating remarks from state and federal officials.

When underground mines serve as permanent or seasonal habitats for wildlife, more robust solutions must be considered. One type of barricade is a gate structure like the one shown in Figure 2. These purpose-built gates are installed inside the entrance of an adit and must be designed to conform to its dimensions as well as to the type and condition of native rock in the walls. The variability of these factors is almost limitless and requires customized solutions. There is no “shrink-wrapped” or “one size fits all” gate.

Vertical Mine Gate

Figure 2, Vertical Gate for Adits – Grandview Mine

Notice that the sides and top conform to the shape of the roof and side walls of the adit. Understandably, vandal-resistant gates such as this require the use of materials that are more costly. However, quality materials and extensive anchoring, using super-strong rods, ensures that this gate will remain in place for a very long time.

Variations of the adit design include gates that incorporate the use of culverts when the entrance to an adit is at risk of future collapse. This type of protective closure is particularly important for maintaining air flow in existing secondary mine openings, as shown in Figure 3.

Culvert Mine Gate

Figure 3, Culvert Gate

As with mine adits, shafts come in a seemingly endless variety of shapes and sizes. Surface conditions around the shaft entrance (collar) can further complicate design criteria and affect the choice and cost of materials. Shafts that are particularly large can limit the degree of off site construction; requiring that some assembly be performed at the mine site.

Figure 4 illustrates a type of protective closure known as a cupola that accommodates the requirements of bats and owls in the same mine. Clicking on the image will open a larger graphic.

Low Cupola Gate

Figure 4, Cupola Design for Mine Shaft

Protective closures with gates or cupolas are only used when populations of bats, raptors or tortoises have been confirmed. As we will see however, preserving these habitats is far from automatic.

In Part three we will examine a gating project that used the cupola design to protect a colony of California Leaf-Nosed Bats and Barn Owls at a mine shaft near Wickenburg. You will be introduced to two organizations that work with state and federal agencies to protect habitats in abandoned mines.

Footnotes and References:

[1] Access to BLM Abandoned Mine Lands policies and manuals can be reached via these web sites:
Abandoned Mine Land Program Policy Manual Section (MS-3720) http://www.blm.gov/nhp/efoia/wo/manual/manuals.html
AML National Strategic Plan http://www.blm.gov/nhp/efoia/wo/fy06/im2006-145attach1.pdf
AML Website http://www.blm.gov/aml
BLM Manual Sections http://www.blm.gov/nhp/efoia/wo/manual/manuals.html

[2] Bureau of Land Management H-3720-1 Abandoned Mine Lands Policy Handbook. Bat Surveys. Page 61.

[3] The shaft shown in this photo is approximately 200 yards west of the Mammoth Spar Mine. It is one of two shafts in the immediate vicinity that are awaiting external surveys to determine if they serve as habitats.

[4] The approximate retail cost of materials shown in Figure 1 is less than $80.00 (8 metal stakes at $7.00 each and 288 feet of wire at 6 cents per foot, plus signage.)

[5] Infrared sensors placed in abandoned mines in Nevada before and after warning signs were installed revealed no significant reduction in human visitation. Bat Conservation International. 2009. “Managing Abandoned Mines for Bats.” Page 79.

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