After the United States and the Soviet Union signed the Limited Test Ban Treaty on 5 August 1963, Soviet nuclear testing moved underground. The Soviets built the world’s largest nuclear testing facility, the Semipalatinsk test site (Polygon), in what is now north-east Kazakhstan. Tests were conducted both in vertical shafts at the Balapan test site, and in horizontal tunnels at the Degelen mountain complex.
The Degelen mountain complex covered an area of 300km² and included 181 tunnels. The last test planned for August 1991 was cancelled with the device in place, and it was destroyed in 1995.
Under the Elimination of Infrastructure for Weapons of Mass Destruction agreement between the Republic of Kazakhstan, and the US, these 181 tunnels were closed between 1996 and 2000.
The selection of a closure method for each tunnel was based on the rock type and condition, the portal condition, the groundwater conditions, the tunnel size, and the level of radioactive contamination.
This paper discusses the development and implementation of the tunnel closure methods, as well as the performance of the tunnel closure mechanisms, which included combinations of concrete plugs, surface and underground blasting, and portal backfill. The successful closure of these tunnels has not only denied physical access to the tunnels, but has reduced the radiation levels outside the tunnels by nearly 90%.
First tests
Nuclear weapons testing in the former Soviet Union started in 1949 following the establishment of the Semipalatinsk test site by CK KPSU and CM USSR decree in August 1947, for development of the First Mining Seismic Station (Objective 905). This was later renamed the USSR Armed Forces Ministry Training Ground, and later the State Central Scientific Research Test Area No 2 (SCSRTA No 2).
At this site, the first Soviet nuclear test was completed in August 1949, the first thermonuclear test was completed in August 1953, and the first hydrogen bomb was tested in November 1955. The last Soviet nuclear test was in August 1989.
Over this period of 40 years, 521 nuclear tests were conducted – 116 above-ground atmospheric tests, and 405 underground tests.
Historical background
After the Test Ban Treaty, 196 underground tests were conducted in vertical shafts at the Balapan site and 209 in tunnels at the Degelen mountain complex, where nuclear events were undertaken in 163 separate tunnels. An additional 18 tunnels had been excavated, but not yet used for underground tests.
In 1989, underground nuclear testing was suspended at the Semipalatinsk test site following a huge Soviet environmental protest. The test site was officially closed on 29 August 1991. In 1995, the Republic of Kazakhstan and the United States Department of Defense jointly signed a Cooperative Threat Reduction agreement, under which the nuclear weapons testing infrastructure at the Semipalatinsk test site would be eliminated.
This programme of elimination was jointly implemented by the National Nuclear Center, Institute of Radiation Safety and Security, Republic of Kazakhstan, in Kurchatov, Kazakhstan (formerly the nerve centre of the Polygon), and the United States Defense Threat Reduction Agency in Washington, DC, US. Under the terms of this agreement, all underground nuclear testing infrastructure was eliminated by physical closure, including the vertical test shafts at Balapan, and the tunnel complex at Degelen mountain.
Formal implementation of this programme started with the closure of tunnel No192 in April 1996, and was completed with the closure of tunnel No 160 in July 2000.
Degelen mountain complex
The Semipalatinsk test site is located in grass covered steppe lands with generally rolling hills, but the Degelen mountain complex is a large granitic massif, rising 500m above the surrounding steppe, and is approximately 18km x 20km in area, oriented approximately north-east/south-west.
The approximate locations and orientations of the 181 tunnels, which were all excavated before the 1989 suspension of testing. These tunnels ranged from 9-25m² in cross-section, and from 350m to more than 1000m long.
Geological setting
The test site covers an area of approximately 18,500km² in the eastern territory of the Central azakhstan Highlands, within the limits of the Caledonian Central Kazakhstan folded system. The area includes a series of north-westerly trending major tectonic faults, which divide the bedrock into a system of regional structural blocks. This block structure is a major geomorphological feature of the area, with isolated blocks being detached fragments of the upper part of the earth’s crust, forming detached mountains and hills. The Degelen mountain complex, where the underground nuclear test complex was located, represents one of these detached blocks.
Degelen mountain itself is an intrusive massif of nearly circular shape, with a diameter of about 18-20 km. In the western, larger portion of this block massif, the basic rock types are granites, syenites, and granite porphyries. In the eastern portion of the site, rock types are mainly quartz porphyries and tuffs.
Major tectonic joint systems often extend over several hundreds of metres, and may be present over the entire length of any particular tunnel. These joints are usually relatively tight, but can often be open locally over short distances. There are usually three to four predominant joint systems present at any particular location, represented by both steeply dipping and shallow dipping sets of joints. Open joints are usually calcite filled, sometimes iron stained, and less frequently filled with clay gouge material. Joint surface conditions vary widely, varying in waviness amplitude, waviness wave length, and roughness due to local asperities.
Rock mass quality is generally good, with RQD values measured in the tunnels generally ranging from about 80-100%, with local exceptions being somewhat lower in shear zones and faults, and in near surface areas at tunnel portals. Associated rock mass classifications were developed by both the RMR and the Q systems, with RMR typically ranging from about 60-80, and Q typically ranging from about 4-40. These classification ratings generally would classify as ‘good’, but in many cases may be better, depending upon local rock conditions. Conversely, there are also local exceptions where the rock mass quality is lower due to the presence of shear and/or fault zones, intense jointing and local weathering in the intensely jointed zones.
Intact rock strengths are generally quite high, usually ranging from approximately 100-160MPa. Rock mass strength would naturally be somewhat lower, but still quite high, with in situ P-Wave velocities usually ranging from 3-6km/s, with local exceptions being either higher or lower.
Groundwater conditions in the tunnel excavations were generally good, with the relatively tight joint systems presenting a relatively low rock mass permeability. In most cases the rock exposed in the tunnels ranged from damp or seeping to dripping conditions. In many cases the seepage conditions varied with the seasons, with wetter conditions evident in the spring after the snow melt, and becoming drier as the ground water recharge rate diminished.
Tunnel construction methods
All tunnels at the Degelen mountain complex were excavated in a horse-shoe shaped configuration by conventional drill and blast methods. In the early tunnels (1960s) ground support in the portal area (and at other areas as required for tunnel stability) was timber post and beam, with timber lagging as required. Timber was usually logs or very rough cut timber as opposed to dimension lumber.
Later on, in the 1970s and 1980s, ground support was changed to structural steel, initially with a steel post and beam system, and later to yieldable steel sets, with sheet steel lagging behind the ribs to prevent ravelling of material into the tunnel in between the ribs. Set spacing was usually between about 600 and 1200mm.
Occasionally, a cast-in-place reinforced concrete lining system was used. In areas where systematic ground support was not required, occasional cement grouted steel pins (dowels) were used to hold up wire mesh for rock fall control in the crown. These initial ground support systems were generally confined to the portal area and usually extended into the tunnel between about 20-100m, but sometimes further depending upon local rock conditions.
In the tunnels, no controlled blasting was apparently used to control overbreak, and the actual excavated tunnel cross-section was sometimes considerably larger than the ‘nominal’ diameter. Large overbreak was produced by blocks falling out of the crown and sidewalls where intersecting discontinuities formed ‘key’ blocks.
Tunnel closure methods
After a thorough evaluation of each individual tunnel, including rock type and quality, portal condition, ground water conditions, radiation contamination, tunnel size and condition of the initial ground support system, closure schemes were developed for each individual tunnel, tailored to the site-specific local conditions.
These closure schemes were developed jointly by NNC and its construction sub-contractor, Degelen Mining Enterprises. They were then reviewed technically by Gosgortekhnadzor (State Mining Technical Inspection Agency), and environmentally by the Ministry of Ecology, both agencies of the Republic of Kazakhstan. In addition, the US Defense Threat Reduction Agency and its underground consultant, Lachel & Associates reviewed the proposed closure schemes for acceptability before final approval was given.
Closure schemes
Almost all tunnels had the initial ground support in the portal area removed as part of the closure procedure. This was generally done after other closure mechanisms had been completed. If tunnel safety conditions did not permit access for removal of this lining, the liner was demolished by blasting. In addition, all tunnels had the portal area backfilled with broken rock, and graded to match existing surrounding topography. These two elements of closure were not really a closure variable, and will not be discussed in detail further.
The main tunnel closure mechanisms were blasting the tunnel closed using either an inside or outside drilling pattern, or construction of a concrete plug inside the tunnel. In the majority of cases, these two closure elements were used together. If access to the tunnel, either inside or outside was restricted because of radioactive contamination, safety hazards, topography, or access limitation for other reasons, applied explosive charges were placed at the surface above the portal to achieve closure, without drilling a systematic pattern of blast holes.
Most of the tunnels were in the range of 3-4m in diameter, but a few were up to about 5.5m in diameter. These size ranges allowed drilling and blasting from inside the tunnel if tunnel safety conditions permitted. In cases where tunnel plugs were constructed of mass concrete, they usually had scrap structural steel embedded as a deterrent to future demolition. If used in conjunction with closure by blasting, the concrete plugs were usually only 5m in length, but if used as the only closure structure without blasting, they were usually 15m in length.
If a concrete plug was used, the blasted zone inside the tunnel was normally about 15m in length, but if no concrete plug was used, the length of the blasted zone extended as far as 50m. If surface blasting above the tunnel was used, the blasted zone was normally about 20-30m in length, but if used alone, without a concrete plug, it was sometimes as long as 50m.
A typical composite tunnel closure scheme employing both a concrete plug and underground blasting. Underground blast holes were normally spaced in a pattern of about 0.7m, and were about 2m long. This produced a typical powder factor of about 2.4 kg/m³.
A typical tunnel closure scheme employing a concrete plug and surface blasting is shown in figure 4. For this type of scheme, drilling was normally vertical or slightly inclined, at a pattern spacing of about 2m. Occasionally, if topographic (or other) constraints prevented access over the tunnel, horizontal drill holes were used in the brow above the portal. For this blasting configuration, a typical powder factor was about 1.1kg/m³.
The various closure mechanisms used for the 181 tunnels in the Degelen mountain complex. A concrete plug used in association with blasting was the preferred closure mechanism. The concrete plug was always at the portal side of the blasted zone. In cases where there was a consistent effluent of groundwater from the tunnel, a drainage mechanism was built into the closure scheme, consisting of either a crushed rock drainage blanket/filter, or a drainage pipe penetrating through the plug and the portal backfill.
Closure performance
With the closure of the last tunnel in July 2000, all 181 Degelen mountain tunnels have been physically closed, achieving the Cooperative Threat Reduction mandate to eliminate all underground nuclear weapons testing infrastructure at the Semipalatinsk test site.
These tunnel closures have not only denied physical access, but have also substantially reduced the levels of radioactive contamination emanating from the tunnels. Based on both pre-closure and post-closure radiation monitoring, tunnel closures have reduced the radiation levels by about 90%.
All tunnel closures were achieved on schedule and within budget. The elimination of nuclear weapons testing infrastructure at the Degelen mountain complex of the former Soviet Semipalatinsk test site can therefore be considered a success from all aspects, technical, economic, environmental and political.
Related Files
Layout of Degelen mountain complex of tunnels
Schematic tunnel closure with surface blast concrete plug
Regional map
Radiation levels before and after tunnel closures
Schematic tunnel closure with underground blast and concrete plug
