Arrangement of Multistory Underground Parking Garages in Complex Engineering and Geological Environment

This article discusses possible variants of underground parking lot in complex engineering and geological environment of St Petersburg, namely: soil excavation in open pit and excavation with solid supports. These methods were analyzed with regard to reasonability for preset construction conditions. A series of predictions were performed using Plaxis 2D software, two variants were described in this article: excavation with the bottom elevation of +0.560 m and excavation with the bottom elevation of -2,500 m and adjacent building (at the distance of 2.5 m). The formulated problems were solved using 2D models. The model was based on the Mohr–Coulomb theory which facilitated estimation of geotechnical environment in the vicinity of the construction. Predictions of retaining wall according to the two considered variants have demonstrated that maximum soil settlement near the excavation is 20 mm and the maximum additional settlement of building at the distance of 2 m from the excavation is 30 mm, which in both cases corresponds to predetermined specifications. KeywordsConstruction activities, Underground parking garages, Specialized methods, Sheet piling, Complex engineering and geological conditions.


Introduction
In well-developed countries underground garages become more and more popular due to shortage of surface parking lots and increasing number of vehicles. Some megalopolises in Asia already include obligatory arrangement of underground garages according to certain regulations under all newly erected structures. faculty of civil engineering, University of Minnesota, has seven underground stories, it was built in 1983.
The lack of parking places is accompanied by numerous problems: narrowing of driveways due to rows of parked vehicles, decrease in traffic safety, damaged lawns, environmental impairment, and others. In historical sites the best solution is the arrangement of multilevel underground garages, since in this case the most reasonable use of underground space under erected or retrofitted structures is achieved.
Construction of underground garages is the most difficult in megalopolises with complex engineering and geological environment, such as St Petersburg.
Architectural beauty and impressiveness of St Petersburg was created by prominent architects of Russia and Europe. Many buildings in historical part of the city are architectural masterpieces having no analogues in the World, which requires provision of their preservation, delicate and safe adaptation to modern conditions. Many old buildings after more than 100 years are at such stage when numerous failures are accumulated during long lifetime. For instance, loadbearing structures can be destroyed (walls, roof, foundations), decorations of facade and interior furnishings, structures and buildings are totally deformed with crack formation.
In addition, during long lifetime of residential buildings, requirements to standards of living also varied which led to moral depreciation of old buildings. Elimination of factors of physical and moral deterioration occurs during major repair and retrofitting. Regarding cultural heritage sites, this is adaptation for modern use. Arrangement of underground garages is requested and justified action improving investment attractiveness and quality of living in central districts, this measure should provide areas of comfortable existence due to elimination of parking lots in yards and adjacent streets. Specialized methods of construction are applied in complex engineering and geological environment (Carayannis et al., 2017;Cherepovitsyn et al., 2017;Cherepovitsyn and Ilinova, 2018).
At present, sheet piling becomes especially important because it facilitates construction activities in constrained urban environment without damages to surrounding buildings and structures.
This article discusses construction of two-level underground garage for 785 vehicles in the historical center of St Petersburg.
Underground waters, confined to silty (EGE-4) and fine (EGE-6.1 and 6) sands, positioned lower, produce pressure due to overlapping clay soils. These waters are opened at the depths of 5.20-12.70 m (absolute level: 0.35 --7.30 m). Underground waters of this water bearing stratum are hydraulically connected forming potentiometric surface with free-flowing waters at the depth of 0.90-3.00 m (absolute level: 2.40-5.33 m). Delivery head is 3.80-10.40 m. Opened levels are close to average annual values.
Construction site according to Specifications SNiP 23-01-99 is referred to climatic region II, subregion II V.0. Absolute minimum temperature of ambient air is -36°C, absolute maximum is +34°C. Soil freezing depth is 1.49 m.
All site area is surfaced by filling. Major portion of the area is asphalted. Absolute levels of site relief vary from 6.5 m to 5.3 m, smoothly declining eastwards. Northern and western sites are occupied with 4-5 story residential brick buildings from early 1900-s. The buildings were erected on rubble stone continuous subsurface foundation. Some buildings are characterized by significant deformations in the form of diagonal cracks. The buildings are in critical condition; their stability can be maintained by reinforcement of their foundations. Other part of the site was occupied by nonresidential 2-3 story buildings which were demolished. Parts of concrete foundations of these buildings remain on the site.

Methods
The project of multicompartment residential complex comprised of three buildings with integral rooms and underground garage is located on a site with the surface area of 26,085 m 2 . At present the site is free from buildings, the existing engineering service lines should be disassembled or removed. Three buildings of ordinary historic development without tenants are located adjacent to the site from the north: building 11, building 13, building 15, they should be retained and repaired. On this territory it is possible to perform complex retrofitting of quarters and construction of new buildings and structures, providing that there is conformity with specified number of stories and formation of construction front.
The residential complex ( Figure 1) is characterized as follows: three residential buildings with the height of up to 27.0 m, number of stories is 7 above ground and 1-2 underground (garage); 2D dimensions are as follows: two buildings -16×60 m, the third building is of complex configuration with pile foundation, load per pile is 120-150 t, load per cluster of piles is more than 300 t, pile penetration is 25-30 m from ground surface.
The underground garage ( Figure 2) located on the site under overall residential complex is characterized as follows: number of stories: 2 underground, depth: 8.0 m, pile foundation, load per pile: 120-150 t, load per cluster of pile: more than 300 t, pile penetration: 25-30 m from ground surface. Taking into account the depth of foundation frame and preliminary arrangement, the design excavation depth is from 5.2 to 8.0 m (for one-level and two-level underground garage, respectively). Excavation with solid supports or excavation by TOP-DOWN method assumes creation of single solid floor disc which provides excavation shoring. In this case it is supposed to arrange two floor discs at the covering levels of the first and the second story (or foundation frame in one-story parts of the garage). Excavation is carried out only after creation of respective support (floor disc).
Configuration of support takes into account geometry of facility, existence of above-ground rooms, technological openings for soil removal and for staircases and lifts, communications and engineering service networks, etc.
In the area of upper support, it is possible to construct integral supporting disc made of elements of steel concrete floor and metal spacers. Such combined floor structure facilitates creation of single supporting disc, as well as decreases complexity of vertical wall elements concentrated mainly in residential areas of the complex (Figure 3). In this case a possible sequence of excavation of two-level underground facility is as follows: at the first stage, two-level underground facility is arranged under protection of two levels of metal spacers in the first trench; at the second stage, the two-level underground facility is arranged in parallel trench. The produced elements of steel concrete structures are combined into single underground facility in the following sequence ( Figure 4): inserted spacers between the elements of two underground facilities in sections are cut off at the length of 3-4 m; in working site/section, the elements of sheet piling are removed; free segment between two sections is covered with International Journal of Mathematical, Engineering and Management Sciences Vol. 5, No. 5, 897-912, 2020 https://doi.org/10.33889/IJMEMS.2020.5.5.069 902 concrete; then the next section is processed where the described sequence of operations is repeated (Kazanin et al., 2019a(Kazanin et al., , 2019bSidorenko et al., 2019).  The advantages of the variant of underground facilities using separate excavations are as follows:  Possibility to perform excavation in required sequence;  Reduced penses for soil excavation;  Possibility to arrange deformation and temperature joints;  Unnecessary soil excavation on large sites.  Possibility of rapid shutdown in the frames of certain startup site, for instance, in emergency cases or insufficient funding.
The main disadvantages of the variant are as follows:  Necessity of embedment and vibration removal of numerous sheet piles;  Necessity to mount numerous spacers.
Metal sheet piling was designed using Plaxis 2D software.
Plaxis 2D is a powerful and convenient finite element software package designed for 2D calculations of deformations and stability of construction objects. PLAXIS is used worldwide by leading civil and industrial engineering companies and institutes for the calculation of pits, embankments, foundations, tunnels, deposits and reservoirs. The program features a large set of functions for modeling various geotechnical tasks.
The geometry is modeled using standard types of structural elements and loads, using CAD-based drawing tools, which allows quickly and efficiently creating finite element models.
The Staged Construction mode allows simulating the construction process by activating and deactivating soil clusters and structural elements at each stage of the calculation. The proposed types of calculations (plastic calculation, consolidation calculation, dynamic and seepage analysis, stability calculation) allow using Plaxis for solving complex geotechnical problems. PLAXIS offers a variety of soil and rock behavior models (from simple linear models to advance nonlinear ones). Verified and robust iterative computational procedures are used. With multithreaded computing and a 64-bit computational core, Plaxis can operate with the largest and most complex models.
The Output program provides various ways to display forces, displacements, stresses, and flows in the form of contours, vectors, and tables. Cross sections may be used to investigate areas of interest. Results from tables can be copied to other applications for further processing. Using the Curve manager option, graphs of various types of results can be created at different stages of the calculation. Using Python scripting, results can be imported into other programs that support APIs. In order to provide minimum impact of construction activities on existing buildings and structures, the following engineering approaches were adopted: prior to excavation activities, the bases, foundations, and above-ground structures of buildings Nos. 11, 13, and 15 adjacent to the project site had to be reinforced; excavation shoring was made of sheet pooling. Sheet pile length was 24 m. Sheet pile was not removable (except for certain internal sheet piles); excavation activities were performed using one or two rows of expanding metal structure made of pipes; prior to start of the construction, the piles for foundation reinforcement and main piles were tested, the sheet piling was embedded and pilot trench was excavated. These measures provided detailed data on impacts of embedment on adjacent territory. In the course of construction, settlements of surrounding territory and building were monitored, as well as vibrations and horizontal deviations of sheet piling (Rengach, 1970;Berendeev, 2009).
2D flowcharts were used for solution of the considered problems. A series of predictions were performed using Plaxis 2D software; two variants were described in this article: excavation with the bottom elevation of +0.560 m and excavation with the bottom elevation of -2,500 m and adjacent building (at the distance of 2.5 m).
The model was based on the Mohr-Coulomb theory which facilitated estimation of geotechnical environment.

Results
Prediction of excavation retaining wall at the bottom elevation of +0.560 m was performed using Plaxis 2D software. Soil properties used in Plaxis 2D model are summarized in Table 1. Absolute surface elevation was +6.000 m. One-level excavation is shored by Z-shaped sheet piles: AZ 42-700N. Its properties are as follows: cross section area S = 259 cm 2 /m, wall weight m = 203 kg/m 2 , moment of inertia I = 104,930 cm 3 /m, elastic moment of resistance W = 4,205 cm 3 /m, elasticity modulus E = 200,000 MPa.
Spacers are made of metal pipes 720×10 installed at the height of +4.700 m.
Prediction of retaining wall of complex excavation site at bottom elevation of 2,500 m was performed using Plaxis 2D software. Soil properties used in Plaxis 2D model are summarized in Table 2. Absolute surface elevation was +6.000 m. In this case combination of sheet piles AU-14 and pipe piles 820×10 reinforced by double T beam 40k5 and filled with V20 concrete is used.
Sheet pile properties are as follows: cross section area S = 132 cm 2 /m, wall weight m = 104 kg/m 2 , moment of inertia I = 28,680 cm 3 /m, elastic moment of resistance W = 1,405 cm 3 /m, elasticity modulus E = 200,000 MPa. Spacers are made of metal pipes 720×10, their first row is installed at the height of +4.700 m, the secondat the height of +2.250 m.
This problem was solved in Plaxis in the form of two plates positioned at minimum distance from each other with interspace filled with elastic soil which transferred load from one plate to another.  Vol. 5, No. 5, 897-912, 2020 https://doi.org/10.33889/IJMEMS.2020.5.5.069

Discussion
Predictions of excavation retaining wall according to variant 1 have demonstrated that horizontal displacements of sheet piling are 12 mm. In addition, soil is moved upwards at the bottom by 166 mm, which can be attributed to poor mechanical properties of soils (Tables 1-2).
Soil deformation occurs mainly in the active zone, behind the retaining wall, and only its small values are noted below the level of the pit bottom. It is known that the stiffness of the soil with small deformations is much higher than with the large ones. Accordingly, soil below the pit bottom has higher stiffness parameters than soil above the pit bottom.
The movement of the sheet pile occurs due to insufficient bending stiffness and an increase in the pressure of weak soil on the sheet pile after shear zone increase.
Maximum soil settlement near excavation pit is 20 mm, which conforms the specifications. Predictions according to variant 2 show that horizontal displacements of the wall are 6 mm. In addition, soil is moved upwards at the bottom by 32 mm. The maximum additional settlement of building located at 2 m from the excavation is 30 mm, which conforms the specifications.
Sheet piles are made of metal rolled profiles varying by shape of traversal cross section, lock design and its position with respect to the profile. Metal rolled profiles are characterized by moderate width and relatively high moment of resistance with regard to cross section area. Lock design in the form of hook or can provides simple installation of sheet pile for locking, required strength, tight connection of sheet piles between themselves. Foreign and domestic experience of the use of metal sheet piling made it possible to reveal areas of their efficient application as permanent elements of underground buildings and structures (subsurface and partially-in-ground). The application fields of shaped steel sheet piles as permanent structural elements cover all spheres of construction activities. Conventionally sheet piles are used for erection of hydrotechnical facilities upon construction of mooring, ports, etc. (Klein and Cherkasov, 1985;Konyukhov, 2005;Kovshov and Skamyin, 2017;Carranza et al., 2018;Zambrano et al., 2018) In recent decade steel sheet piles have been widely applied in rod construction (automobile and railroad) as shoring of tunnels, overhead crossings, embankments and the like; moreover, wide experience has been gained in the field of application of sheet piles in civil and industrial construction as retaining walls, shoring of excavations, trenches, mines and as cutoff curtains upon construction of underground parking lots, technical, sales, entertainment, and sporting rooms.
It should be mentioned that prior to submersion of sheet piles, it is necessary to check the following parameters: to check certificates for sheet piles; to examine visually curvature of sheet pile, state of locks, linear dimensions, state of metal. During submersion of sheet piles, it is necessary to check the following parameters: speed of submersion; during submersion of initial sheet piles (or packages), it is required to pay attention to their strictly vertical direction which is verified by tilt sensor in two planes. Vertical direction of sheet piles in two planes should be verified for each five sections of sheet piles. It is required to take measures preventing deviation of sheet pile from designed position in excess of specifications (Benderev, 2008;Benderev, 2011).

Conclusions
It should be mentioned that nowadays, upon global growth of megalopolises, even slight error in designing or construction of underground garages in the frames of greenfield projects or retrofitting of existing buildings can lead to destruction of remaining monuments of architecture as well as to rehousing of residents.
In order to provide reliability and safety of remaining monuments of architecture during construction of underground garages, as well as safety of neighboring structures which can be affected by the construction or retrofitting activities, it is required to provide integrated geotechnical assistance. Taking into account complex and multifactorial problems of geotechnical assistance, it is possible to achieve solution only by modern numerical methods based on physical and geomechanical nonlinear models.
The presented results are practical and can be applied by companies involved in designing and construction of underground garages in complex engineering and geological environment as well as performing monitoring of structures.