Operations Against Tunnel Complexes
The use of tunnels by the VC as hiding places, caches for food and weapons, headquarter complexes and protection against air strikes and artillery fire was a characteristic of the Vietnam war. The 'fortified village', usually underlaid by an extensive tunnel system containing conference, storage and hiding rooms as well as interconnected fighting points had also been frequently encountered. However, as operations progressed into the war zones subsequent to January 1966, an even more extensive type of tunnel complex had begun to be encountered which combined underground security of personnel and supplies with an integrated, tactically located defensive system of fighting bunkers.
The tunnel/bunker complexes encountered in the war zones were obviously the result of many years of labour, some in all probability having been initiated as early as WWII, with extension and improvement continuing throughout the war against the French and up until the time of their discovery by the US troops. These complexes presented a formidable and dangerous obstacle to operations that had to be dealt with in a systematic, careful and professional manner. Additionally, they were an outstanding source of intelligence, as evidenced by the several tons of documents found during the clearing of the Saigon-Cholon-Gia Dinh headquarters complex in Operation Crimp, January 1966.
The first characteristic of a tunnel complex is normally superb camouflage. Entrances and exits are concealed, bunkers are camouflaged and even within the tunnel complex itself, side tunnels are concealed, hidden trapdoors are prevalent, and dead-end tunnels are utilised to confuse the attacker. In many instances the first indication of a tunnel complex was fire received from a concealed bunker that might otherwise have gone undetected. Spoil from the tunnel system was normally distributed over a wide area.
Trapdoors were utilised extensively, both at entrances and exits and inside the tunnel complex itself, concealing side tunnels and intermediate sections of the main tunnel. In many cases a trapdoor would lead to a short change-of-direction or change-of-level tunnel, followed by a second trapdoor, a second change-of-direction and a third trapdoor opening again into the main tunnel.
Trapdoors were of several types; concrete covered by dirt, hard packed dirt reinforced by wire, or a 'basin' type consisting of a frame filled with dirt. This latter type was particularly difficult to locate in that probing would not reveal the presence of the trapdoor unless the outer frame was actually struck by the probe. Trapdoors covering entrances were generally a minimum of 100 meters apart. Booby traps were used extensively, both inside and outside entrance and exit trapdoors. Grenades were frequently placed in trees adjacent to the exit, with an activation wire to be pulled by a person underneath the trapdoor or by movement of the trapdoor itself. Typical trapdoor configurations are shown below;
Tunnel complexes found in the War Zones were generally more extensive and better constructed than those found in other areas. In some cases these complexes were multileveled, with storage and hiding rooms generally found on the lower levels. Entrance was often gained through concealed trapdoors and secondary tunnels. In the deeper complexes, foxholes were dug at intervals to provide water drainage. These were sometimes booby-trapped as well as containing punji-stakes for the unwary attacker.
Although no two tunnel systems were exactly alike, a complex searched by 1st Battalion, RAR, during Operation Crimp may serve as a good example. The complex had the following characteristics;
Another tunnel characteristic of note was the use of air or water locks that acted as 'firewalls', preventing blast, fragments or gas from passing from one section of the tunnel complex to another. Use of these 'firewalls' is illustrated below;
Recognition of their cellular nature was important for understanding these tunnel complexes. Prisoner interrogation indicated that many tunnel complexes were interconnected, but the interconnecting tunnels, concealed by trapdoors or blocked by 3 to 4-feet of dirt, were known only to selected persons and were used only in emergencies. Indications also pointed to interconnections of some length, e.g. 5 to 7-kilometers, through which relatively large bodies of men could be transferred from one area to another, especially from one 'fighting' complex to another.
The 'fighting' complexes terminated in well-constructed bunkers, in many cases covering likely landing zones in a war zone or base area. Bunker construction is illustrated below with examples uncovered by 1st Battalion, 503rd Airborne, during Operation Crimp;
Bunker raised three feet with four firing ports
Bunker raised approximately one foot with one firing port
Integration of these bunkers into a 'fighting' position is illustrated by the following diagram apparently used as a guide for VC construction in the CRIMP area;
The following experience of the 1st Infantry Division in the Di An and Cu Chi area is representative of tunnel operations;
Tunnel Exploitation and Destruction
As other entrances were discovered and plotted, they were marked in such a way as to indicate if the VC used them after discovery, but before destruction could be accomplished. In many cases tunnels were too exstensive to be exploited and destroyed in the same day and the VC mined entrances and approaches during the night after the tunnel team departed.
Upon completion of exploitation, forty pound cratering charges were placed fifteen to twenty meters from all known tunnel entrances and, where exstensive tunnel complexes existed, ten pound bags of CS-1 Riot Control Agent were placed at intervals down the tunnel at sharp turns and intersections and tied into the main charge. Where sufficient detonating cord was not on hand to tie-in all bags of CS-1 to the main charge, bags of CS-1 were dispersed in the tunnel by detonation with a defused M-26 fragmentation grenade fused with a non-electric cap and a length of time fuse. Sharp turns in the tunnel protected the demolitions man from the grenade blast, if the detonation occurred before he exited the tunnel.
Tunnel Flushing and Denial
The infantryman discovering a spider hole or tunnel entrance during intensive combat lobs an M-25 CS 'baseball type' grenade in the hole, followed by a fragmentation grenade. The bursting of the CS gas grenade places an instantaneous cloud of CS in the tunnel and the fragmentation grenade blows the CS through a section of the tunnel while killing any VC near the entrance.
The low level contamination resulting from this method would serve only to discourage rather than prevent future VC use of that tunnel entrance.
Hasty Tunnel Flushing and Denial
In some areas the combat situation would permit a hasty search for hidden tunnel entrances but either lack of time or VC occupation of the tunnel would not permit exploitation by the tunnel team as described above (Tunnel Exploitation and Destruction).
In this case the Mity Mite Portable Blower could be employed to flush the VC from the tunnels burning CS Riot Control Agent grenades (M-7A2). In addition, the smoke from the grenades would, in most cases, assist in locating hidden entrances and air vents.
After flushing with CS grenades, powdered CS-1 could be blown into tunnel entrances to deny the tunnel to the VC for limited periods of time. In either case, these methods were only effective up to the first 'firewall' in the tunnel.
Representative Equipment List;
Protective mask; TA-1 telephone; one half mile field wire on 'doughnut' roll; compass (x2); sealed beam 12-volt flashlight (x2); small calibre pistol (x2); probing rods (12-inch and 36-inch); bayonet (x2); M7A2 CS grenades (x12); powdered CS (as required); coloured smoke grenades (x4); insect repellent and spray (4 cans); entrenching tool.
Small calibre pistols or pistols with silencers were the weapons of choice in tunnels, since larger calibre weapons without silencers had been known to collapse sections of the tunnel when fired and/or damage eardrums.
Careful mapping of the tunnel complex often revealed other hidden entrances as well as the location of adjacent tunnel complexes and underground defensive systems. Constant communication between the tunnel team and the surface was thus essential to facilitate tunnel mapping and exploitation.
Coloured smoke grenades were often used to mark the location of additional entrances as they were found. In the dense jungle it was often difficult to locate the position of these entrances without smoke.
Dangers inherent in the above operations fell into the following categories and had to be taken into account by all personnel connected with these operations;
Mines and booby traps in the tunnel entrance/exit area
Punji pits inside entrances
Presence of small but dangerous concentrations of carbon monoxide, produced by burning-type smoke grenades after tunnels were smoked. Protective masks would prevent inhalation of smoke particles, which were dangerous only in very high concentrations, but would not protect against carbon monoxide
Possible shortage of oxygen as in any confined or poorly ventilated space
VC still in the tunnel who posed a danger to friendly personnel both above and below ground
A trained tunnel exploitation and denial team was essential to the expeditious and thorough exploitation and denial of VC tunnels since untrained personnel may have missed hidden tunnel entrances and caches, taken unnecessary casualties from concealed mines and booby traps and may not have adequately denied the tunnel to future VC use. To facilitate this, teams were trained, equipped and maintained in a ready status to provide immediate assistance when tunnels were discovered.
Tunnels were frequently found to be outstanding sources of intelligence and would therefore be exploited to the maximum extent possible. However, since tunnel complexes were carefully concealed, often extensive, and well camouflaged, search and destroy operations had to provide adequate time for a thorough search of the area to locate all tunnels. Complete exploitation and destruction of tunnel complexes was very time consuming and operational plans had to be made accordingly in order to ensure success.
The presence of a tunnel complex within or near an area of operations posed a continuing threat to all personnel in the area and no area containing tunnel complexes could ever be considered completely cleared.
US Army Military Assistance Advisory Group Vietnam (MAAG) - Lessons Learned No. 32 Operations Against Tunnel Complexes (April 1966)
The Tunnels of Cu Chi, Tom Mangold & John Penycate, Pan Books, 1985, ISBN 0-330-29191-2