- Published on Monday, 09 June 2014 17:48
In the 1960s Leningrad (since 6th of September Saint Petersburg) city authorities decided on construction of metro station in the center of St. Petersburg, in the immediate vicinity of the world-famous attractions of St. Petersburg (the Hermitage, the Admiralty, and St. Isaac's Cathedral).
The initial project appeared in 1994, at the same time with the project of three more stations of Pravoberezhnaya Liniya – Sadovaya, Sportivnaya and Chkalovskaya.
Admiralteiskaya station was designed to be the deepest-lying station of then Leningrad subway due to purely geological reasons: at the site of the station dry and thick Cambrian clay layers convenient for underground tunnel construction lay at the depth of 90 meters, while above, on the ground, there stood unique old buildings, witnesses of important historical events and personalities.
Designers and builders understood that a construction project in such circumstances had to be of the highest degree of safety. A financial crisis did not allow the project to continue.
The construction of the inclined tunnel still continued, however. Lenmetrogiprotrans R&D Institute, Metrostroy and city officials continued searching together for optimal solutions. On June 20, 2007 the city signed a contract with Herrenknecht AG to deliver a mechanized tunnel construction unit. One of the two inclined tunnels (120 m and 25 m long respectively) was to be built with the use of Herrenknecht AG equipment.
This allowed for no-slump drilling. Before then escalator tunnels were built using low-temperature technologies, which resulted in considerable slumping during construction and for one or two years after the completion of the project (due to the ground melting).
When this method was used, the ground level slumped by 400-450 mm, which made it impossible to work in compact urban conditions. But with use of the Herrenknecht AG equipment the acceptable norms of day surface sedimentation did not exceed 25 mm.
Considering that the use of the Herrenknecht AG equipment provides no-slump drilling, the decision was made to vacate and take down just one building on the surface. This solution helped avoid resettlement of three more buildings, which could delay construction for a few more years.
The inclined tunnel was built using the cutting-edge tunnel-drilling equipment, the Herrenknecht tunneling shield, 10.4 m in diameter, with balanced earth pressure. The earth pressure inside the face zone was balanced to create the pressure corresponding to the pressure of the solid mass. At the time there had existed no experience of tunneling at an angle of 30 degrees with use of tunneling equipment.
A comprehensive system of geotechnical monitoring, designed by Metrostroy and Lenmetrogiprotrans, was used during the construction of the larger inclined tunnel of Admiralteiskaya. This system allowed to monitor in real time the stress strain behavior of the lining area, the tunnel, the host block, the day surface and the buildings on the surface.
The range of different monitoring methods included:
- Assessment of the state of soil in the tunnel lining and the quality of the filling between the lining and the ground with the help of ultra-wideband (UWB) georadiolocation.
- Ultrasound-assisted determination of voids and uniformity of grouting in the process of filling the gap between the lining and the ground mass;
- Determination of the stress strain behavior of the escalator tunnel lining in the process of drilling, with the help of vibrating-wire transducers installed in the lining;
- Measurement of settlement of the day surface and buildings, measurement of building tilt angles in the process of escalator tunnel drilling;
- Measurement of lining displacement in the process of tunnel drilling;
- Determination of the pore pressure in the ground mass with the help of vibrating-wire transducers installed in vertical wells at various depths before the drilling;
- Measurement of ground mass movements with the help of the extensometers, installed in vertical wells at various depths before the drilling;
All information was available in real time; the findings were compared to the criteria listed in the tunnel drilling normative documentation. When the values exceeded the normal range, various technological measures were used to solve the problem (changing the surcharge pressure, compensatory injection into the ground mass under buildings, etc.); For the first time ever, the available data allowed to measure the relationship between changes in stress strain behavior of the system: unstable quarternary lithological varieties, technological parameters of the drilling equipment and the lining of the escalator tunnel.
Comprehensive geological monitoring has allowed to make escalator tunnel construction safe and helped prevent damages to historic downtown buildings of St. Petersburg, federal architectural monuments protected by UNESCO.