«Deeply Buried Facilities Implications for Military Operations Deeply Buried Facilities: Implications for Military Operations Deeply Buried ...»
One of the lessons of the Persian Gulf War was the effectiveness of using air and space power in military attacks. The corollary, which has been learned by adversaries of the United States, is that it is necessary to go deeper underground if they want to survive U.S military attacks. Underground facilities represent a serious military challenge because it is very difficult to determine their location, and perhaps more difficult to characterize the activities that are being conducted inside. The future military capabilities of the United States will depend in part on the ability to find critical enemy targets with standoff sensors, characterize their functions, and destroy them with precision guided conventional munitions.7 The remainder of this study focuses on locating and destroying these facilities.
In reality, underground facilities frustrate both of these requirements Underground facilities are difficult to find, are resistant to revealing the physical details that are critical to effective targeting, and in many cases are fundamentally beyond the reach of most conventional weapons. To complicate matters, the most difficult problem may be to characterize the contents of these hidden facilities and their military function. Unless one has high confidence in the nature of these facilities, military attacks may be counterproductive, as exemplified by the potential existence of weapons of mass destruction in these sites. To understand the nature of this problem, it is useful to address the construction of underground facilities and their likely configurations.
There are two basic classes of underground facilities. The "cut-andcover" facilities are constructed by digging a hole, inserting a facility, and then covering it up with dirt and rocks. These cut-and-cover facilities can be just below the surface of the ground or may reach a depth of perhaps 100 feet, and represent the vast majority of underground facilities today.8 In the case of contemporary cut-and-cover facilities, there is no question that conventional munitions can defeat them. There is a self-generating competition between those who design facilities and weapons designers that seek to defeat those facilities. While facilities can be built deeper, this increases the cost of the facility. At the same time, the weapons designer must consider the increasing cost of developing penetrator weapons that can destroy targets.9 The second class of underground facilities, which are constructed with tunneling operations, are located deep below the surface or deep within mountains. These deeply buried underground facilities may be hundreds of feet below the surface of the earth and be surrounded by solid rock. This class of underground facilities may be more difficult to locate and destroy, and will be emphasized in this study.
Siting and Design Considerations
The commercial world has long recognized the value of underground facilities for storage or industrial purposes. Abandoned mines, naturally occurring caverns, and rock cavities offer many advantages, including low humidity and little variation in temperature. For example, frozen food companies have used such facilities for decades to store their products Furthermore, creating such facilities is becoming technically easier for many governments. Modern tunnel boring machines can drill through solid rock vertically, horizontally, or at any angle, and are able to tailor the inside of a rock cavity to support the construction of a facility. For example, the machines used to dig the English Channel project were huge, encased drilling machines whose digging face consisted of a 95-ton, twenty-eight and onehalf foot diameter disc that is divided into numerous cutting blades. At its maximum efficiency, the tunnel was dug at a rate of about 50 meters per day.10 As the machine drilled through the rock, teams of workers would follow behind to line the cavity of the tunnel with concrete and guide the scraps of rock and material from mining down the track for disposal. In a single, continuous operation, the machines drilled a tunnel, removed the earth, and paved the inside of the tunnel with precast concrete segments.11 In the case of smaller facilities, tunnels with a diameter of 6 meters can be dug at a rate of 200 meters per day and larger cavities can be created at any number of locations along the tunnel.
When considering the vulnerability and survivability of deeply buried underground facilities that are designed for military applications, an important factor is having an adequate depth of cover on all sides of the facility. Common sense dictates that the deeper a facility is placed beneath the surface of the earth, the more survivable it will be against attack. Studies by the RAND Corporation and MITRE Corporation suggest that facilities located at depths of 2,000 feet beneath the surface are essentially invulnerable This does not mean merely 2,000 feet of overhead cover, but a 2,000 foot minimum distance to any surface point (on all sides), including the sides of a mountain. The material located between the underground facility and the surface of the earth is commonly known as "overburden". Naturally, more overburden between the underground facility and the surface of the earth is preferable, and the depth of overburden should be a prime consideration when selecting the location for a deeply buried facility.12 There are several other important design factors in addition to overburden that increase the survivability of deeply buried facilities. One is the use of dry, impermeable rock stratum at the required depth (i e, no imbedded water). Another is to ensure that the rock stratum is nearly horizontal and at least 100 feet thick to take advantage of the self-supporting mechanical properties of the rock. The use of overburden above (and on the sides of) the rock stratum should be broken in order to help attenuate ground shocks. And the underlayment below the cavity should act as a "mattress" to attenuate shock waves. An aquifer would be ideal since it could also be tapped for water and contribute to the self-sufficiency of the facility.
Furthermore, the rock stratum from which the cavity is carved should be selfsupporting and not require artificial support or lining, such as reinforced roof and walls, to be structurally sound Limestone and granite are desirable rocks for these purposes.
Access Tunnels and Internal Cavities
Access to deeply buried facilities can be accomplished through either horizontal or vertical shafts or tunnels, which must be large enough to allow equipment and material to enter and exit the tunnel. At the same time, the size of the tunnel has significant implications for the survivability of the facility, principally because a smaller diameter tunnel is less detectable than a larger diameter tunnel. Tunnel entrances also can be designed to collapse at predetermined lengths in order to attenuate the blast, shock, and overpressure of an explosion and thereby prevent those effects from reaching the critical functions or personnel in the underground facility. This can be accomplished either by the construction of blast doors, which reduces the cost and time associated with building long tunnels, or more simply through the use of long entry tunnels.
If tunnel length is the design feature that is used for attenuating blast, studies have shown that there is an ideal tunnel design ratio of 500 between the length and diameter of the tunnel.13 Tunnels that are constructed according to this relationship should fully attenuate the blast waves that travel down the tunnel, but this requires a long tunnel, which thereby increases the cost of construction.14 For example, if the entry tunnel were 16 feet in diameter, an 8,000-foot tunnel would be necessary to meet the blast attenuation ratio of 500. A 16 foot diameter is reasonable because virtually all construction equipment is designed to fit under 15 foot bridges and is not wider than 8 feet to meet highway standards.15 Another advantage of using long tunnels is to increase uncertainty about the location of the underground facility. As an example, if the entrance to an underground facilities were known, and an 8,000 foot tunnel was used to gain access to the facility, the radius around this known entry point creates more than 200 million square feet of surface area, or roughly 4,600 acres of possible locations for the facility. Even if one assumes that the tunneling activity does not reverse itself and travel in the opposite direction, an area half this size represents a tremendous area in which to conduct detailed surveys for determining the location of an underground facility. When the exact location of a deeply buried underground facility is unknown, it significantly decreases the ability to locate and neutralize them, and thus increases their survivability.
According to a study conducted by the MITRE Corporation, the internal dimensions of the rock cavity within which an underground facility may be constructed should not exceed 40 feet in width and 45 feet in height. These measurements were made in the case of a 2,000-foot overburden on all sides for maximum survivability.16 A series of chambers with these dimensions can be connected with a matrix of tunnels, as shown in Figure 1.
Figure 1. Illustration of Deeply Buried Facility
An example of a tunnel boring operation is the Boston, Massachusetts area water supply improvement project. This project involves boring 16-foot diameter underground tunnels for a distance of roughly 17 miles at depths that range between 200 and 400 feet. Furthermore, two underground chambers will be hollowed out to hold a total of 20 million gallons of water.
The estimated amount of material to be removed from the operation is approximately 850,000 cubic yards of rock for the tunnel, and an additional 170,000 cubic yards of material for the two storage tanks. All of these tunnels and storage tanks will be carved out of bedrock. This project is significant because modern tunnel boring operations are capable of digging extensive tunnels without providing any indication on the surface of the direction(s) that the tunnel may take. Short of actually entering the tunnel, the only evidence of the facility is the amount of material removed from the mining operation that must be disposed of as well as the ventilation and elevator shafts that may connect with the surface at arbitrary locations.
The difficulties of characterizing the direction and size of tunneling operations have significant implications for military operations, which is addressed in the next chapter.
The earlier description of an underground facility and its design considerations are derived from a report published in the early 1960s as well as a tunneling operation in progress during the time this report is being written. The earlier report recommends an overburden of 2,000 feet, which clearly was related to surviving a nuclear attack. While a 2,000-foot overburden may not be practical or necessary to achieve survivability against most potential threats, this level of overburden can be easily achieved by tunneling directly into the side of a mountain range. Whether digging down into the earth or digging into the side of a mountain, deeply buried facilities can be placed at such significant depths that these facilities are immune to attack by most weapons. Furthermore, the vast amount of land area under which it is possible to locate such facilities affords even greater survivability because it is difficult to detect the exact location of the facility. The degree of survivability is limited principally by the resources available to the state that constructs these facilities.
Deeply buried facilities are used by governments and industry to protect their civilian and military leadership, and by industry to protect vital equipment, which are central to prosecuting a war or maintaining vital commercial or industrial capabilities. Today, with improvements in tunneling capability, these facilities can easily be constructed to move troops and equipment as well as manufacture, store, and transport munitions, including weapons of mass destruction. During the Persian Gulf War, ten percent of the more than 18,200 sorties flown by coalition aircraft were aimed at the critical war making capabilities that are typically contained in underground facilities.