Geotechnical and ecological foundations of the sustainable life cycle of the long-operated unique underground structures of the water disposal system in difficult ground conditions

Intensive development of megacities causes the need for sustainable operation of underground urban infrastructure facilities. In the special protection against man-made impact, long-operated unique underground structures of the water disposal system, which are related to facilities with an increased level of responsibility and danger, are needed. In the report, based on the experience of studying the operation of underground wastewater facilities in conditions of weak soils and intensive external static and dynamic impacts, there are proposed new methods for diagnosing their technical condition, modeling and monitoring of the joint work of the "underground structures aggregate of soil" system are implemented, forecasting and shaping conditions for its sustainable life cycle. Point and linear objects of the water canal of St. Petersburg are developed and implemented at unique (to a depth of 70 m.) geotechnology, providing simultaneous increase of bearing capacity, operational reliability and ecological safety of longoperated underground drainage facilities in difficult ground conditions with increasing man-caused impacts.


Introduction
General information about the object of deep engineering infrastructure in large cities. With long-term operation and intensive development of engineering infrastructure of megalopolises increase the requirements to the ecology and efficient usage of land resources. During engineering development of underground spaces of such a megalopolis, design of integrated measures for protection of town-planning environment against negative anthropogenic impact is of special actuality. Thereupon there must be introduced special safety requirements for the sewage and water treatment facilities. Sewage (transportation) of waste waters is done through the city sewerage system and tunnel collectors. In the general drainage system these facilities account for up to 60% in large cities and up to 70% in difficult hydrogeological conditions by construction volumes and costs.
Sewerage system objects data for the most typical Russian cities with the population over 1 million people is given in table 1. By now around 88% of all sewage collectors are made of ferroconcrete, around 7% -of metal (steel, cast iron), around 3% -of bricks, plastic, ceramics. Tunnel sewage collectors diameter is from 1.2 to 5.6 m, they are buried from 3 to 60 m underground. For example, in St. Petersburg all sewage network is divided into three basins that serve three main pumping plants up to 70 m deep and up to 66 m in diameter, with productivity of 1.5 mln m3 of sewage per day. For such conditions the main constructive solution for the tunnels are the ferroconcrete tubings with inner ferroconcrete jackets.
Transportation volumes of waste waters in some sections of the tunnels reach 20 m3 per sec, and in case of decrease of their operational reliability or failure will inevitably lead to a technospheric catastrophe. "Lengiproinzhproekt" institute together with the St. Petersburg State Transportation University has been providing scientific and engineering maintenance, design, construction and rehabilitation of St. Petersburg sewerage system objects for more than 30 years: more than 70 pumping plants, including those with depth of 45 m, 59 m and 71 m, and with diameters of 47 m, 59 m and 66 m; more than 15 km of tunnel sewage collectors with diameters of 1.85, 2.5 and 3.4 m and with depth of 16 m, 24 and 37 m [1]. Table 2 shows the most typical defects of long-term operated pumping plants and deep tunnels. Analysis of the materials of the investigations shows that at the moment 60% of gravity sewage tunnels and 80% of pressure sewage tunnels require repairs and sanitation.
Instrumental probing (with geological radar) shows that 70-75% of inner surface of pumping plants wells and sewage tunnels have continuity violation and cracks which require strengthening of construct and renewal of waterproof shell. The stresses in the reinforcement and concrete were measured using primary device string type PSAS and PLDS. To determine the soil pressure membrane load cells with a range of 0 to 12MPa and measurement error of 5-7% were used. The measurement was done both in the discrete and continuous mode using the local electronic switches (LEC), and data storages (END).
Analysis of monitoring data of large sunk wells with diameters from 50 to 66m and a depth of immersion of 55 to 71m shows that in the process of immersion in the soils with different strengths and asymmetric structures deviation from the vertical axis (careen) is observed, with a shift of the center up to 1.5-1.8 m. In this case, according to an automated continuous monitoring, as a result of abrupt landings (breakdowns) stresses in the reinforcement, concrete and soil pressure may exceed the calculated and the average values (according to the discrete measurements) 12-15 times.
To estimate the stress-strain state of the well shell with a sharp landing (breakdown) numerical modeling was conducted. In the calculations the finite element method (FEM) and the software package Robot Professional 2010 were used. The calculation is carried out for spatial shell with diameter of 66m and a height of 71m (with the number of three-dimensional finite elements equal 50828), falling under its own weight at an angle of 15 ° from a height of 140 cm on the compliant soil (average coefficient of elasticity for multilayer soil is taken K = 16500kN/m3). In the model because of the inclination angle the friction forces on the lateral side of the well were applied in the upper part of the shell on one side and in the bottom part on the opposite side [3].
The results of numerical modeling have shown (see Fig. 1) that in case of a dynamic blow (if the well is dropped from a height of 140 cm) equivalent von Mises stresses in the construct equal Sdin = 256MPa at the top of the shell and Sdin = 1538MPa in the area of the bottom rest, which respectively exceeds the limiting strength of concrete class B30 [Spred] to 14 or more times, and the changes in the geometry of the shell are observed. Thus, already in the process of the well immersion the construction of the well is damaged and the concrete is disintegrated due to breakdowns. Later during operation micro cracks lead to leakage, seepage and corrosion of concrete. To further ensure the safety of operation of facilities of this type it is necessary to strengthen and waterproof the construct by highpressure injection of polymer resins [4].

Geotechnical analysis of technical condition of the sewage tunnels under intensive anthropogenic impact and long term operation
Geotechnical analysis of the sewage tunnel was carried out for the most typical section located in a zone of intense dynamic impact of transport and the impact of new construction.  Uneven tunnel compressions, modified on the arch axis range from 5 to 276 mm. Comparative analysis of engineering and geological section on the tunnel route and its placement on the plan relative to the traffic junction showed that the greatest compressions up to 276 mm are located in the area of the tunnel under intense dynamic effects of the traffic, passing the layer of thixotropic quaternary deposits. Evaluation of the dynamic impact of the transport was carried out by the study of the oscillatory process with a set of manifold gauges CM TSP installed in the arch and blocks of the recording equipment [5].
The frequency of the oscillations of the collector during various traffic loads from 15 to 35 Hz, and the vibration amplitude to 35-70 microns was recorded. According to the research [6] for this type of ground deposits and the appropriate level of the dynamic effects the decrease of strength characteristics C and φ is up to 35% and 17%, respectively. To ensure the operational reliability of tunnels vibration protection measures, such as the use of spiralwound technology for internal lining the tunnel are suggested. For this section of the collector the numerical modeling was carried out to determine the maximum allowable axis displacement of tunnel lining. The criterion for the safety of the construct is the maximum allowable tensile stress of the concrete in the typical points of lining. Maximum allowable deformation and displacement values are presented in Figure 3.

Conclusion
Data received from long-term field observations for continuous operated embedded constructions being a part of megapolis water discharge system as well as the results of calculation and modeling allowed to conduct geotechnical analysis of the residual bearing capacity and to develop measures to ensure the safe operation of facilities of such type under the conditions of intensive external influences.
As experience shows, the presence of geotechnical tracking of environmentally hazardous facilities of such type for the entire period of their life cycle, including design, construction and long-term operation (even under intensive man-induced impact) provides the safety of the stable functioning of the megapolis utility infrastructure.
The carried out researches are a basis for geotechnical maintenance of steady functioning and ecological safety of underground constructions of a water removal at all stages of their life cycle.