Military studies have shown that adversaries have shifted to the use of underground bunkers to protect against the U.S. military’s precision strike capabilities that combine satellite navigation with guided missiles and bombs. To counter American precision strikes, China, Russia, North Korean, and Iran began burying facilities and hardening them with high-stress concrete and digging deep with advanced tunneling equipment that burrows hundreds of feet under ground and through solid rock.
The Pentagon’s recent nuclear posture review outlining the administration’s nuclear modernization noted the increasing use of hardened underground facilities by adversaries. The report said China and Russia “are fielding an array of anti-access area denial (A2/AD) capabilities and underground facilities to counter U.S. precision conventional strike capabilities.”
For North Korea, the report said, “North Korea relies on hardened and deeply buried facilities to secure the Kim regime and its key military and command and control capabilities. It uses underground facilities and natural terrain features to protect North Korean military forces. Consequently, the United States will continue to field a range of conventional and nuclear capabilities able to hold such targets at risk.”
The study found that underground facilities are used to conceal leaders, military and industrial personnel, weapons, equipment, and other assets. An estimated 10,000 underground hardened targets include about 20 percent that have a strategic function and half of those are in or near urban areas, complicating targeting.
Such facilities, called hard and deeply buried targets (HDBTs), are a serious challenge to U.S. national security objectives of maintaining the capability to hold such adversary assets at risk. Ranging from hardened, surface bunker complexes to tunnel facilities deep underground, HDBTs are typically large, complex, and well concealed, incorporating strong physical security, modern air defenses, protective siting, multifaceted communications, and other important features that make many of them able to survive attack by conventional weapons.
Potential adversaries are increasingly locating HDBTs in basements of multistory buildings located in urban settings, complicating attack planning and increasing the risk of serious collateral effects. This situation places a premium on achieving accurate target characterization so as to obtain the required lethality from precisely delivered weapons during a strike.
The main principles in modern bunker design are largely centered around survivability in nuclear war. As a result of this both American and Soviet sites reached a state of “super hardening”, involving defenses against the effects of a nuclear weapon such as spring- or counterweight-mounted (in the case of the R-36) control capsules and thick concrete walls (three to four feet for the Minuteman ICBM launch control capsule) heavily reinforced with rebar. These systems were designed to survive a near miss of 20 megatons
The Russian continuity of government facility at Kosvinsky Mountain, finished in early 1996, was designed to resist US earth-penetrating warheads and serves a similar role as the American Cheyenne Mountain Complex. The timing of the Kosvinsky completion date is regarded as one explanation for US interest in a new nuclear bunker buster and the declaration of the deployment of the B-61 mod 11 in 1997, Kosvinsky is protected by about 1000 feet of granite. One likely Soviet Union/Russian target, Mount Yamantau, was regarded in the 1990s by Maryland Republican congressman, Roscoe Bartlett, as capable of surviving “half a dozen” repeated nuclear strikes of an unspecified yield, one after the other in a “direct hole”.
Students at Georgetown University discovered thousands of miles of tunnels dug by the Second Artillery Corps, a secretive branch of the Chinese military in charge of protecting and deploying its ballistic missiles and nuclear warheads after translating hundreds of documents, combing through satellite imagery, restricted Chinese military documents and waded through hundreds of gigabytes of online data . The Chinese have called it their “Underground Great Wall” — a vast network of tunnels designed to hide their country’s increasingly sophisticated missile and nuclear arsenal.
Many underground command-and-control complexes and missile tunnels are hidden 328 feet and 1,300 feet below ground in rock or concrete with the majority less than 820 feet deep. Some are as deep as 1,640 feet to 2,296 feet in granite or limestone.
Weapons to destroy underground facilities
An earth-penetrating weapon (EPW) is designed to hit the earth at high speed and penetrate into the ground before exploding. Such weapons can be delivered by short-range missiles or aircraft, and are intended primarily to attack underground targets.
A National Academy of Sciences study in 2005 concluded fortified underground targets cannot be destroyed with conventional explosives. One or a few nuclear weapons will be needed. In response, the Pentagon is considering the use of two modified lower-yield warheads combined with precision guided missiles to target underground bunkers.
“Many of the more important strategic hard and deeply buried targets are beyond the reach of conventional explosive penetrating weapons and can be held at risk of destruction only with nuclear weapons,” the report said.
A nuclear bunker buster, also known as an earth-penetrating weapon (EPW), is the nuclear equivalent of the conventional bunker buster. The non-nuclear component of the weapon is designed to penetrate soil, rock, or concrete to deliver a nuclear warhead to an underground target. These weapons would be used to destroy hardened, underground military bunkers or other below-ground facilities.
An EPW only burrows a few meters into the ground before it explodes. Indeed, the earth slows the warhead so quickly on impact that it cannot penetrate very deeply. Rather, by exploding just a few meters underground instead of at or above the surface, a much larger fraction of the energy of the explosion is transmitted to the ground. The explosion creates a strong seismic shock wave that propagates and can crush or damage an underground bunker. Even a short penetration distance accomplishes this goal of “coupling” the energy of the explosion to the ground: penetration of a few meters increases the underground destructive effects by more than a factor of twenty for a wide range of warhead yields.
For example, exploding a 10-kiloton nuclear weapon at a depth of one meter would increase the effective yield by a factor of 20, resulting in underground damage equivalent to that of a 200-kiloton weapon exploded at the surface of the ground. But increasing the penetration depth to five meters would only increase the effective yield by an additional 60%, to 320 kilotons.
An underground explosion releases a larger fraction of its energy into the ground, compared to a surface burst or air burst explosion at or above the surface, and so can destroy an underground target using a lower explosive yield. This in turn could lead to a reduced amount of radioactive fallout. However, it is unlikely that the explosion would be completely contained underground. As a result, significant amounts of rock and soil would be rendered radioactive and lofted as dust or vapor into the atmosphere, generating significant fallout.
US weapons against Hardened Targets
The weapons the US military plans to use against hardened underground targets are the B83-1 gravity bomb, with a reported yield of around 1.2 megatons, or the equivalent of 1.2 million tons of TNT, and the B61-11 gravity bomb, that has a reported yield of 400 kilotons.
Nevertheless, even nuclear weapons have limited effectiveness at destroying the deepest or widely separated underground bunkers. For example, an earth penetrating weapon using the 1.2 megaton B83 warhead—the highest yield weapon in the U.S. nuclear stockpile—could crush underground bunkers to a depth of about 1000 feet. Deeper bunkers can be constructed with modern tunneling equipment, and are essentially invulnerable to nuclear attack.
“The B61 Mod11 penetrator was developed because the B-83 was not good enough against hardened deeply buried targets,” he said. The Mod 11 nuclear gravity bomb is said to have earth-penetrating capability. But nuclear experts say it does not have sufficient capability to hold at risk strategic targets that are protected by hardened layers of rock or concrete—often hundreds of feet below the surface.
One of the more effective housings, the GBU-28 used its large mass (2,130 kg or 4,700 lb) and casing (constructed from barrels of surplus 203 mm howitzers) to penetrate 6 meters (20 feet) of concrete, and more than 30 metres (98 feet) of earth.
While penetrations of 20–100 feet (6.1–30.5 m) were sufficient for some shallow targets, both the Soviet Union and the United States were creating bunkers buried under huge volumes of soil or reinforced concrete in order to withstand the multi-megaton thermonuclear weapons developed in the 1950s and 1960s.
The B61-12 has been in development since at least 2011 and will be the first in the series to feature a precision guidance capability. They will have a GPS and inertial navigation system (INS)-directed tail kit and strakes along the weapon’s main body similar in form and function to those on conventional Joint Direct Attack Munitions (JDAM) bombs.
U.S. military says the future B61-12’s precision guidance coupled with this low-yield warhead, which will also produces less radioactive fallout, will make these weapons more flexible and suitable for a wider range of contingencies than any of the existing types.
The new B61-12 bomb test took place June 9 at the Tonopah Test Range in Nevada. The latest B61 is supposed to consolidate and replace all existing B61s and will enter production fiscal 2020. The weapon is said to be able to be used for different kinds of attacks, such as low-yield strikes, surface detonations, high-yield bursts, and earth penetration.
But its earth-penetrating capability is in question. “The Mod 12 will have very little capability against hard and deeply buried targets,” said Mark Schneider, a former Pentagon nuclear policymaker. “Accuracy does not get you much against [hard, deeply buried targets], yield is critical.”
Kristensen believes a weapon does not need to reach the underground facility and can attack it by through “ground-shock coupling.” “You only have to get the explosion about 3 meters below to get the effect,” he said. “By doing that, a penetrating warhead with a modest yield will have the same effect as a much more powerful weapon detonated on the surface.”
“The 50 kiloton B61-12 would, if detonated 3 meters below the surface, have the same effect as a surface blast of 750 kiloton to 1,200 kiloton weapon,” he said. The B61-11 can reach to 400 feet down and the canceled RNEP, with a 1-megaton yield, would have reached up to 800 feet deep.
“In comparison, the 9 MT surface blast of the old B53 could have destroyed down to 750 feet,” Kristensen said. “It is always possible to build facilities deeper underground than can be threatened directly with these penetrators,” he said. “The real issue is how important it is to get to them.”
While the penetration depth increases with higher impact velocity, the weapon casing will be crushed—destroying the warhead inside—if it strikes the ground at too high a speed. Empirical and theoretical data show that the maximum impact velocity is roughly one kilometer per second and the maximum achievable penetration depth of such a projectile in concrete is roughly 10-20 feet.
In practice, the weapon must impact at lower velocity to reduce the deceleration experienced by the warhead, resulting in shorter penetration depths. Penetration depths will be larger in dry soil than concrete or rock, but one would have to expect that a hardened target would be placed below hard rock or concrete.
Another former government nuclear weapons specialist who spoke on background, said the Chinese and Russians have fielded “super hard” targets. Deployment of a future American hypersonic strike vehicle with precision strike capability may be able to hold superhard Chinese targets at risk, but current plans call for the missile to be non-nuclear tipped and thus unable to penetrate hardened targets.
China’s ‘Underground Steel Great Wall’ capable of defeating hypersonic weapon attacks: academician
China’s “Underground Steel Great Wall” could “guarantee the security of the country’s strategic arsenal” against potential attacks, including those from future hypersonic weapons, Qian Qihu, recipient of the country’s highest science and technology award, told the Global Times.
The “Underground Steel Great Wall” is a series of defense facilities located deep under mountains. While the mountain rock is thick enough to resist enemy attacks, entrances and exits of these facilities are often vulnerable and Qian’s work was to provide extra protection for these parts.
China’s nuclear strategy follows the principle of “no first use” and requires the country to have the capability of withstanding a nuclear attack before it responds with its strategic weapons.
Qian’s work guaranteed the safety of the country’s strategic weapons, launch and storage facilities as well as commanders’ safety during extreme times, said Song Zhongping, a military expert and TV commentator.
In an exclusive interview with the Global Times on Friday, Qian describes his work, the “Underground Steel Great Wall,” as the “country’s last national defense line.”
If other lines of defense including the strategic missile interception system, anti-missile system and air defense system fail to function against hypersonic missiles and recently developed bunker-busters, Qian’s work can still thwart such attacks.
“The development of the shield must closely follow the development of spears. Our defense engineering has evolved in a timely manner as attack weapons pose new challenges,” Qian said.
According to the academician, hypersonic weapons that move 10 times as fast as the speed of sound are capable of changing trajectory mid-flight and penetrate any anti-missile installations.
US media outlet CNBC reported that in March 2018 during a State of the Nation address, Russian President Vladimir Putin debuted new nuclear and hypersonic weapons, which he described as “invincible.”
The US is also trying to develop hypersonic weapons, as then US Deputy Secretary of Defense Patrick Shanahan, now acting secretary of defense, said in October. “We are going to fly sooner and more often than people have ever expected,” CNBC reported.
National defense challenges do not only emerge from the development of advanced attack weapons but are also a result of an unpredictable international environment, Qian said.
He cited the recent US stance whereby the Donald Trump administration is mulling lowering the threshold for nuclear weapons deployment.
The US is planning to loosen US nuclear weapons constraints and developing low-yield nuclear warheads, the Wall Street Journal reported in January 2018.
It is highly possible that US weapons with low-yield nuclear warheads are bunker-busters, with a higher surgical strike capability that may cause larger damage, military experts previously noted, warning that China should stay alert and upgrade its own national defense.
“Both China and Russia have drawn a ‘red line’ around their command-and-control facilities, and regard an attack on these capabilities as justifying a nuclear response,” the former official said. “Hence in a conventional conflict, both China and Russia are doctrinally prepared to use nuclear weapons first.”
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