Gleaming white marble-clad high-rise buildings lining wide new expressways and interspaced with impressive monuments, gushing fountains, and lush green parks in an otherwise dry, desert-like landscape are the astonishing result of 15 short years of construction activity in Ashgabat, the capital city of Turkmenistan. Since independence in 1991, and as controller now of its own national wealth and destiny, this oil- and natural gas-rich former Soviet breakaway republic has a clear vision of where it wants to go and how quickly it wants to get there.
Beneath these visible signs of a modern metropolis is the construction of advanced EPB tunnels to create the first elements of underground infrastructure for the city. The Ashgabat City Administration is providing 30km of 6m and 3.5m diameter utility conduit for the collection of sewage and drainage water, distribution of fresh water, and the laying of electricity and telecommunication cabling.
The existing conurbation of Ashgabat is a legacy of Soviet influence, the oldest of the low-rise buildings constructed in the 1950s to replace a city almost destroyed by a devastating earthquake in 1948 and now the stuff of legend.
Turkmenistan is on the geological plate northwest of the Himalayas and is subjected to frequent earthquakes (Fig 1). While the exact magnitude of the 1948 earthquake is not known, it is believed that much of the devastation was caused by the amplification of the seismic waves by the high groundwater content beneath the city. Pushed up by tectonic forces over eons, the sandy barren land of Turkmenistan was once the sea floor. Now, though appearing a desert, it holds groundwater in extensive horizontal aquifers and sand lenses. A major strategy for protecting the high-rise buildings of the new city is to reduce and control this groundwater.
The water table is charged by snowmelt off the north-facing slopes of the Kopetdag Mountains behind Ashgabat that form Turkmenistan's southern border with Iran. It is also charged artificially by a system of man-made channels or käriz constructed in older times to gather and channel water from the mountains into the city for potable and irrigation needs. It was the construction of the 1,400km-long Karakum Canal, started in the 1920s from the Amu Darya River on the border with Uzbekistan and completed to Ashgabat in May 1962, that superseded the network of käriz and caused the system to be abandoned. Although no longer used, the käriz still channel water into the city area, adding to the groundwater content.
During the Soviet era, a system of dewatering wells and drainage channels was constructed to prevent groundwater flooding the city's low-lying areas, but this proved expensive and damaging to the environment, causing the rise of natural salts. The plan for the new project is to create horizontal wells that will drain the groundwater naturally. Plastic perforated pipes of 200mm diameter and up to 200m long will extend laterally from the main collector to drain water by gravity. With regular maintenance manholes, and managed from control centres, the system covers a greater area than did the system of vertical wells, and requires no electricity, pumps, or mechanical parts (Figs 2 and 3).
The city's plans for a system to replace the network of costly pumping dewatering wells had been discussed for several years. Japan offered to fund construction of a project, but charges were said to be prohibitively high. Russia also submitted proposals for construction of different elements of a project, but it was negotiations between the two new independent governments of Turkmenistan and Ukraine that launched the current project into construction in 2002.
Project realisation is based on a barter-type arrangement. During the Soviet era, Turkmenistan was the supplier of natural gas to the industrial powerhouses of the USSR in Ukraine. Supply continued after the breakup of the Union, and in 1993 to 1994 the new market economy of Ukraine was in natural gas supply debt to the new market economy of Turkmenistan. Instead of paying the debt in hard currency, Ukraine offered to build the drainage project for Ashgabat. Design and construction of the project is in the hands of the Ukrainian company CAI, Construction Association lnterbudmontazh. Information about the project value and the system of payments was withheld.
Under the directorship of Anatolij Bukan from the head office in Kiev, the tunnelling division of CAI, lnterbudtunnel, comprises a team of experienced tunnelling engineers and managers who worked previously with KievMetrostroy and two other tunnel construction companies in Ukraine. In 2001, and as a divisional chief engineer of KievMetrostroy, Bukan discussed the project during a visit by him to Ashgabat. Following that, he was charged with establishing the CAI tunnelling team and setting up operations in Ashgabat.
The installation in Ashgabat is a permanent development comprising a suite of offices for 317 Ukrainian engineers, a camp for 500 resident employees, an equipment workshop and stockyard, and a high production concrete segment casting facility.
The CAI workforce in Ashgabat totals about 3,980. Of this, up to 30% are Ukrainian and 70% are Turkmen. The current project contract is for a period of seven years from 2002 to 2008 and encompasses 30.32km of tunnel excavation - 16.5km of 6m o.d. utility tunnel beneath the city centre and 13.82km of 3.5m o.d. drainage and sewage trunk line to the 300,000m3/day treatment plant also in construction by a different division of CAI. In addition, there is construction of the access chambers to the utility tunnels and their internal structures.
The 6m-diameter utility tunnels are divided in half horizontally and the lower section divided again with two vertical walls. Groundwater and sewage flow in two lower compartments with the third in reserve, and the upper section houses a fresh water supply pipeline and electricity and telecommunication cables. A different department of CAI will use horizontal drilling to install the lateral drainage wells and construct their manholes. The CAI design department is working with the scientific institutions of Turkmenistan to complete final design and project construction criteria.
Once collected, drainage water will flow by gravity at about 12.5m3/sec under little pressure into the two compartments of the 3.5m-diameter transfer tunnels for discharge some 7km out into the desert. Treated effluent will be held in an artificial irrigation supply lake. High salt levels rule out cost-effective reclamation of the drained water.
The geology beneath Ashgabat is said to be quite complicated, comprising alternating layers of clay, sand, and gravel with the sand particles being smaller, smoother, and more round than river sand and held in compact formation by gypsum. Random boulders are also remnants of the previous sea-bottom habitat.
"For tunnel excavation, we are using three new EPBMs supplied by Herrenknecht of Germany," said Bukan. ''We have two 6m machines and a 3.5m-diameter machine. We discussed the Ashgabat job with several manufacturers, but there was doubt that a project of this size would go ahead in such a new young independent Turkmenistan. Herrenknecht took the project seriously from the start and worked with us on the design and supply of the machines. The geographical closeness of Kiev to Germany was another factor, and there is a direct flight between Ashgabat and Frankfurt, which helps with travel and transportation of spare parts."
The order was signed in 2001, and the three machines were delivered according to the agreed dates in 2002.
"From the geological data supplied, Herrenknecht engineers recommended EPB rather than slurry systems," said CAI Chief Engineer in Ashgabat Victor Bespalov. The machines have 9m-long screw conveyors and foam generating and injection systems for soil conditioning. Foam is used to bind the gravel and sands and protect against surface settlement.
"We are working largely beneath the main streets of the city and not close to any highrise buildings," said Bespalov, "but we are monitoring all buildings within a 40m corridor of the tunnel line. The average depth of the tunnel is 12m below surface and 15m to invert at the deepest. For the 6m diameter tunnelling we have completed to date, the maximum settlement has been 110mm and within the 150mm tolerance. We use less foam in ground that is more compact or contains more clay, but in loose sand material it is difficult to create the plug and control the face using foam alone. Working in a full face of loose water-saturated sand is our greatest challenge."
During the visit by TunnelTalk to Ashgabat, it was explained that settlement as a result of the system's drawdown of the natural groundwater was not expected to be a problem since only the groundwater and no sediment or soil fines are to be extracted. It was further explained that the initial drawdown, by the designed 8m or so, will progress slowly and take about five years. The system will then operate automatically to maintain the new level through seasonal fluctuations.
The tunnels are lined with 1m and 1.2m long rings of precast concrete segments comprising five segments and a key in both the 3.5m and 6m diameter tunnels respectively. Annular grouting is through the segments with each 6m diameter ring consuming 3.86m3 of grout. When TunnelTalk visited Ashgabat in September 2005, average production, working three schedules of 8hr shifts/day, 7 days/week, was 12 rings/day with a best of 15 rings in one day for the 3.5m machine and 9 to 10 rings/day for the 6m diameter machines.
Completion of the project is divided into phases. Excavation of the first phase of 8km of 6m diameter tunnel and 6.6km of 3.5m diameter tunnel is scheduled to be completed by July 2006 with the second and final phase completed in 2008. CAI Deputy Director General Vladimir Halin reported from Ashgabat in early January 2006 that 6.3km of 6m diameter tunnel and 8.4km of 3.5m diameter tunnel had been excavated to date.
To withstand future seismic events, the new utility tunnels are designed to 9.8 on the Richter scale. Engineer Igor Geletiuk explained that while the reinforced, bolted primary segmental lining will withstand an 8 Richter scale event, extra resistance is provided by a reinforced in-situ concrete inner lining. "During an earthquake, the wave cycle is, at most, 20m. Formulae calculations indicate that as seismic waves travel through, the snaking response of the tunnel can induce gaps of some 60mm in the circle joints of the segmental lining. To permit this necessary shear force response in the in-situ lining, we have incorporated 70mm seismic movement gaps at 20m intervals. To prevent tangential stress between the two linings, we have included a separating or sliding membrane between the 300mm thick reinforced primary segmental lining and the 150mm thick reinforced secondary lining."
An internal pipeline was considered, said Geletiuk, but the cost of a 1.2m diameter imported fibreglass pipe was about two times more expensive than an in-situ concrete lining. In construction, the bolt pockets of the primary lining will be filled with fast hardening, non-shrinking concrete to prevent water leaks during an earthquake. The 70mm wide circumferential seismic movement joints in the in-situ lining are filled with modified graphite strengthened with a geotextile that allows for a 100-200% expansion of the joint.
For casting the in-situ concrete lining of the tunnels, CAI examined the forms of the leading European manufacturers and placed an order with CIFA of Italy to supply two full round telescopic shutters for the 3.5m diameter tunnels and two shutters for the 6m diameter tunnels. The 20m long forms correspond to the 20m lengths between 70mm seismic gaps. The 70mm seismic gaps at 20m intervals are repeated also in the internal structures.
The 3.5m-diameter tunnel forms were delivered in May 2005 and are progressing at a steady rate of one 20m pour/36 to 48hrs for an overall advance of 600m/month for two forms. Cast into the inner lining of the smaller transfer tunnels is the key into which the central dividing wall is cast as a separate following operation. While discussing supply of the 6m diameter forms with CIFA, CAI also enquired after design and supply of a shutter that would cast the internal dividing walls of the larger diameter utility tunnels in a single operation. Supply of such a complex form was said to be a first for CIFA.
During the visit, it was yet to be decided how to use the separate parts of the 6m shutter; either together for a full round cast or in a staggered invert/arch process. The separate casting of the internal walls will progress as an independent operation.
The utility drainage tunnels project in Ashgabat is strengthening the bonds between Ukraine and Turkmenistan. Together with the engineers from Ukraine working on the project in Ashgabat, there are 30 Turkmen engineers studying in Ukraine to advance future phases of the project. There is talk of starting the development of an underground metro for Ashgabat. Following development of the utility drainage tunnels and evident expansion and modernisation of the city, a metro for a rebuilt Ashgabat will become a necessary public service facility. Having made close ties with major international tunnel equipment suppliers on the current project, CAI can be expected to design and construct a metro system to similar state-of-the-art, innovative, and progressive standards.
As a visitor to the project, it was evident that construction competence and expertise is available among the Russian-educated and Russian-experienced tunnel engineers. Once the three EPB TBMs were assembled and commissioned under initial supervision by Herrenknecht technicians, CAI electricians and mechanical engineers dismantled and reassembled the machines several times. The first 6m machine was used initially to drive two 2.5km long utility tunnel passes under the Karakum Canal about 30km from Ashgabat. After the first drive, the TBM was dismantled and reassembled for the second, and then again for transportation into the city and assembly for the first utility tunnel drive.
CAI is also managing operation of the segment casting yard with its Ceresola moulds after initial set-up assistance from German concreting engineer Thomas Steinfeld, and application of the CIFA forms by the CAI crews is producing a quality insitu lining finish. These are operations that follow excavation of the large rectangular boxes for construction of the underground control chambers and tunnelling access points for the job. Installed using Casagrande clam shell excavator machines from Italy, these large diaphragm wallsupported shafts resemble the open boxes of metro stations and exemplify the level of management competence and high standard construction being achieved in Turkmenistan.
TunnelTalk had a memorable and well managed visit to Turkmenistan and to the project works and thanks are extended in particular to Alyona Pazenko for her patient and professional skills as our Russian/English interpreter.