Geotechnology of restoration of culverts in complex ground conditions without their decommissioning

. In the article, on the basis of extensive theoretical and experimental research carried out, innovative geotechnology for repair of culverts is presented, which provides an improvement in the conditions for interaction between the body of the structure and the soil massif with increasing technogenic impacts. The analysis of the technical condition of the culvert transport and engineering structures under special operating conditions is given. Factors determining their premature emergency condition and interfering with stable functioning under increasing loads and impacts have been studied. On the basis of many years of experimental and theoretical and experimental design work, geotechnology «Saturn» has been proposed, suitable for repairing all types of culverts on railroads and highways, engineering infrastructure facilities, and especially for repair and reconstruction of pipes under conditions of a constant intensive watercourse with nonstandard (atypical) constructive solutions, as well as located in hard-to-reach and difficult for the organization of the construction site sections of the route. The experience of successful application of the developed technology "Saturn" on the railroad during the repair and reconstruction of long-maintained culverts of stone and concrete pipes under the conditions of a constant intensive watercourse and at water disposal facilities during repair of deep tunnels without their decommissioning is described.


Degree of wear and the particularities of operation of the tunnel sewage collectors
Nowadays the total network of the tunnel sewage collectors in large cities is up to 600 km, and in St. Petersburg is about 270 km. The system of sewage tunnel collectors represents pipelines of the diameter of 1.5 -4.7 m excavated by the mining excavation method using tunneling machines at the depth of 6 -90 m. The major part (up to 75%) of the tunnel sewage collectors is located in the central historical part of the city in very difficult geotechnical and hydrogeological conditions in terms of construction and it does not have any doublers.

Analysis of the influence of anthropogenic factors on reliability of the long-operated sewage tunnels
There have been analyzed the archive data of technical inspections of the tunnel collectors belonging to the "Vodokanal-SPb" by the maintenance service. In the period from the late 1970s to nowadays the authors have conducted the tool inspections of the technical state of the tunnels (the total length of about 15 km) located in the historical city center (the depth of 12-15 m) and beyond it, within the approaches to three main sewage pumping stations collecting all sewage outflows of the city, embedded at the depths of 53-71m (figure1). Uniqueness of the data on observations of the state of the tunnels is that the technical inspection of the structures has been conducted during the long period since the late 1970s to nowadays (the end of 2016). During this period the same parts of collectors have been inspected several times. As a rule, each inspection was followed up by the tunnel state monitoring performed for several years. Therefore, there has appeared a possibility to track the dynamics of defects development.
The observation period was divided into three sub-periods: а) the 1970-80s; b) the 1980-2000s; c) 2000-2016. The identified defects which are more typical and influencing the exploitation reliability and bearing capacity of the tunnel were divided into 7 groups: d1-shrinkage cracks in the concrete liner; d2-gas corrosion features; d3-dripping leakages; d4-force cracks in the arch and along the lateral surface of the tunnel; d5-concrete biological corrosion features; d6-reinforcement corrosion, conduit dusting; d7-the presence of heading leakages.
The Figure 2 shows the dynamics and structure of defect manifestation during the operation period of the tunnel sewage collectors from the late 1970s to 2016. The nature of the defects prevailing in the first 15-20 years of the tunnel exploitation and their influence on the bearing capacity and exploitation reliability can be considered as negligible; their technical state can be regarded as serviceable according to the existing Russian State Standards and Construction Codes.
The subsequent 20 years of the tunnel operation are characterized by manifestation of force cracks and considerable increase of the defects in concrete due to its gas and biological corrosion. However, their technical state was described as partially serviceable.
The analysis of the defects appeared in the period of 2000-2016 is of particular interest. It is noteworthy that in this period the amount of the defects influencing the tunnel bearing capacity has increased: almost 40% growth in the number of force cracks in the arch and along the lateral surface of the tunnel; the features of reinforcement corrosion and dusting of the conduit; the presence of heading leakages in the tunnel body; the increase in the proportion of the defects associated with biological and gas corrosion. The technical state of the tunnel structure is characterized as partially capable. The tunnel sewage collectors require monitoring of their technical state, taking actions on reconstruction of the bearing capacity and exploitation reliability of the structure. The comprehensive long-term technical inspections of the tunnel sewage collectors allow conducting a retrospective analysis and identifying factors, which condition manifestation and development of the defects. The factors can be divided into internal and external ones according to the nature of their influence.
The nature of the internal factors is explained by the processes linked to transportation of sewage outflows [3,4].
The external factors are characterized by the complex interaction of the tunnel structure and the embedding soil bulk, which form a natural-technical system under the influence of the anthropogenic factors.
The evaluation of this interaction and definition of the area of the ultimate acceptable values of mutual influence of the natural-technical system elements is a task of geotechnical predictions. Other papers of the author is dedicated to the issues of optimum management of the natural-technical system elements [5,6].
In the framework of the given inspection we have a task of geotechnical support of the safe level of external anthropogenic impacts on the tunnel structure with account of its residual bearing capacity.

Geotechnical measures of protection of the tunnel sewage collectors
Geotechnical and structural measures to ensure reliability and safety of the long-operated tunnel sewage collectors should be selected basing on comparison of competing options, but the main requirement they should meet is a possibility of preventive use substantiated by geotechnical and structural calculations. The Table 1 presents examples of some geotechnical measures successfully taken in the projects of protection of the tunnel sewage collectors in St. Petersburg in 1998-2016. Table 1. Geotechnical measures of protection of the tunnel sewage collectors.

The nature of technogenic impacts Geotechnical and structural measures
The object for protection

Supernormative static and dynamic influences on the brickwork arch of the collector
Construction of an unloading screen and reinforcement of the structures; monitoring.
The brickwork sewage collector XVIII century under Konnogvardeysky boulevard The increase in static load applied to the tunnel sewage collector due to the weight of the building being constructed above it Construction of an unloading bridge with load transfer to the soils below the tunnel bottom, monitoring.

The sewage collector in Nalichnaya street
Unloading of a soil bulk during the construction of the deep transport tunnel above the tunnel sewage collector Construction of a protective screen of reinforced soil to prevent the uplift of the pit bottom, monitoring.
The transport tunnel above the tunnel sewage collector along the Obvodny Canal and the bridge of Alexander Nevsky Prevention of horizontal displacement of the tunnel sewage collector at unilateral unloading of a soil bulk during reconstruction of the embankment.
Construction of a geotechnical barrier, monitoring.
The tunnel sewage collector along the highway in the embankment of the Fontanka River Expansion of an area of ultimate acceptable displacements of the tunnel sewage collector at unilateral unloading of a soil bulk during reconstruction of the embankment.
Structural reinforcement and sanation of the tunnel sewage collector using the spiralcoiling method in order to increase the ultimate acceptable tunnel displacements, monitoring.
The tunnel sewage collector along the embankment of the Kryukov Canal and the Moyka River The increase in static load applied to the tunnel sewage collector due to the impact of heavy road transport and trams.
Structural reinforcement and sanation of the tunnel sewage collector using the coiling method in order to restore the bearing capacity of the tunnel up to the design level, monitoring.
The 840-m-long tunnel sewage collector in Tovarischesky prospect The calculation substantiation of the geotechnical protection measures was made according to the algorithm: collection of loads and impacts, definition of the physicalmechanical properties of soil and tunnel materials; creation of a geo-mechanical calculation model and calculation of displacements using the software Plaxis3DTunnel; creation of a calculation model using the software Robol; selection of the parameters of tunnel reinforcement. The calculation example is shown in the figure 3.

Application geotechnical measures during repairing of the tunnel sewage collectors in conditions of sewage outflows transportation
The tunnel collector in Tovarischecky prospect with the intervals 359m and 485m diameters D=2.5 m and D=1.5 m at the depth of 1274 m has been operating for more than 40 years. According to the inspection results the wear degree was more than 79%, the subsidence at the crossroads reached 25 mm. It was necessary to ensure protection of the tunnel sewage collector against the enhanced external static and dynamic transport loads and impacts. According to the georadar scanning there was identified overall peeling of the concrete liner from the tubing lining with occurrence of heading leakages ( Figure 4). The calculation sequence followed the algorithm above mentioned in section 4 of the paper, the calculation fragments are shown in (Figure 2), the ultimate acceptable displacements before the protection measures and after them are shown in the graph ( Figure  5): 1-D=1.5 m, 3-D=2.5 m -tubing lining with defects of the concrete liner; 2-D=1.5 m, 4-D=2.5 mtubing lining with reconstruction of the concrete liner and refractory lining reinforcement of the structure.  6. The state of the tunnel sewage collector: -а) before the repair and b) after the reinforcement of the structure and its sanation using the coiling method.
The sanation technology by coiling was applied for repairing and restoring bearing capacity of the tunnel. The scope of working operations included: cleaning the tunnel and preparing the surface; structural gluing of the concrete liner and the tubing lining by injection of SikaDur; reinforcing the arch surface by structural reinforcement with SikaWrap carbon fiber; coating the tunnel surface with PVC coiling profile; injecting polymer-cement mortar (Pcomp.=65MPа) into the intertubular space for structural gluing of the PVC lining with the tunnel structure. The operation fragments are shown in ( Figure  6). According to the conducted monitoring it was found out that vibro-dynamic tests of the tunnel before and after the repair demonstrated changes of the period of the collector's own oscillations from 0.54 sec. to 0.19 sec. i.е. by 58%, the amplitude of its own oscillations decreased from А= 300 micron to А= 15 micron, i.е. almost by 2 degrees of order. It shows provision of the integrity of the structure and joint operation of its layers (Figure 7). After repairing the guaranteed period of reliable work of the tunnel sewage collector is minimum 50 years. a b Fig. 7. The results of the measurement of the oscillations of the tunnel structures at applying the exterior vibro-dynamic transport load: а) the oscillogram of the fluctuation process of the tunnel structures with the peeling defects А=300-800 micron and) the same after reinforcement with "the structural gluing" and sanation using the coiling method А=15-20 micron.

Conclusions
Due to long operation, the tunnel sewage collectors in Russia have a wear of 55 ÷ 66 %. Complicated soil conditions in St. Petersburg have led to the wear of the tunnel sewage collectors up to 83% with the fast dynamic of development 1.5 ÷ 2% because of the action of different technogenic factors. The proposed geotechnical methods of protection of the tunnel sewage collectors including the methods of structural reinforcement and sanation in conditions of transporting sewage outflows accompanied by the monitoring system provide enhancement of the bearing capacity of the structure of the tunnel sewage collectors and their exploitation reliability.