This article proposes a constructive solution for a protective combined geotechnical barrier to minimize the impact on existing buildings and structures from the construction of a subway tunnel. The barrier is utilized during the construction of a metro tunnel in weak soils on a developed area. The object of research is existing buildings and structures in the area affected by underground construction. The research subject is the additional deformations of the foundations and structures of existing buildings and structures. The aim of the study is to reduce the impact on neighboring buildings during the construction of the metro tunnel. Method. The research was conducted using numerical modeling. Three-dimensional and planar finite element models were developed in the Plaxis software package for analysis. Results. The values of additional foundation settlements and stresses in the building structures were determined. The effectiveness of the installation of a geotechnical barrier along the escalator tunnel is confirmed. The barrier reduces the maximum absolute foundation settlements of the surrounding buildings by an average of 40 % and the relative differential settlement by 66 %.
1.Introduction
The object of research is existing buildings and structures in the area affected by underground construction. The research subject is the additional deformations of the foundations and structures of existing buildings and structures.
Underground construction in dense urban areas causes additional deformations of foundations and structures of neighboring buildings. This issue is particularly relevant in the construction of metro systems in urban environments due to the risks of damaging surrounding buildings and the significant economic costs involved in their restoration. For example, the construction of the world's first London Underground was accompanied by emergency situations. In May 1861, during earthworks on Euston Road, sidewalks, gardens, telegraph lines, water pipes, and building facades were damaged [1]. The impact of tunneling on existing structures is discussed in works [2–6]. Protection of surrounding buildings from the effects of underground construction is addressed in works [7–10].
Large difficulties arise during the construction of tunnels in weak clay water-saturated soils. According to research [11], urban development on weak clay soils is heavily influenced by foundation construction technologies due to the possibility of disrupting the soil structure. This occurs as a result of changes in the stress-strain state in the soil mass during the excavation of a deep pit or tunneling. Disturbances from construction equipment are also transmitted to the soil. Vibrational and impact actions lead to compaction or vibratory loosening of soils. This results in the vertical deformation (technological settlements) of neighboring building foundations.
Weak clay soils have long-term creep. Creep is not studied during standard engineering and geological surveys. Accordingly, this property is not taken into account when calculating foundation settlements. At the same time, buildings constructed on such soils receive settlements during their existence that exceed permissible values. Additional foundation settlements and forces in the structures of existing buildings depend on the work technology, equipment, type of enclosing structures, etc.
Therefore, to obtain accurate values for additional deformations, it is necessary to consider the various impacts that occurred on the soil in the surrounding area prior to construction.
The dependency of the excavation locations relative to buildings on their stress-strain state was examined in works [12].
Damage to existing buildings during underground construction primarily results from the engineering-geological and hydrogeological conditions of the construction site, the construction technology employed, and the complexity of the project [13]. Consequently, ensuring the safety of buildings and structures within the zone of influence of underground construction is a paramount task for engineers. Research in the field of structural safety assessment is described in works [16–20].
The relevance of this work lies in the development of an optimal design for a protective structure (barrier). This barrier is intended to minimize the force factors that arise in the structures of existing buildings as a result of metro tunnel construction. Particular attention is paid to material efficiency. The study utilizes nonlinear models for the behavior of soil and building materials. It is expected that the research results will serve as a guideline for engineers involved in geotechnical calculations when assessing the impact of excavation in constrained urban environments.
The aim of the study is to develop a design for a protective screen to reduce the impact on adjacent buildings during the construction of a metro tunnel.
To achieve the stated objective, it is essential to accomplish the following tasks:
to analyze the influence of the excavation's location on the stress-strain state of the buildings' primary load-bearing structures; to determine the additional foundation deformations and the resulting forces in the load-bearing structures during tunnel excavation by performing a coupled analysis of the "soil-foundation-structure" system; to develop a methodology to mitigate the impact of tunnel excavation on existing buildings.
2.Materials and Methods
During construction (reconstruction) in densely built-up conditions, the safety of existing buildings and structures is affected by the following man-made impacts:
1) temporary change in the groundwater regime – change in hydrogeological conditions in the construction area due to local water decrease;
2) temporary technological impacts associated with the method of work.
The safety criteria for existing buildings are restrictions on settlements, their relative differences or tilts by limiting values:
(1)
(2)
where – the magnitude of additional deformations (their differences) of surrounding structures due to the influence of the -th group of factors on the foundation; – is the minimum distance between the foundations of a neighboring building; – are deformations from previous impacts in an unstabilized state of the foundation. This value is determined by calculation based on the results of assessing the degree of stabilization of settlements when monitoring the position of sensors on cracks.
The value of the maximum permissible additional settlement, relative difference or tilt of adjacent structures should be calculated for a specific building or structure in their actual deformed state, when performing a joint calculation with the foundation [19]. The obtained values guarantee the safety of existing buildings' structures during the development of additional deformations.
The study examines the construction of an escalator metro tunnel as part of an underground complex. The underground space is located in a historic urban area. For such buildings, strict requirements are imposed to limit deformations of their foundations [22–30]. A general view of the underground structure is shown in Fig. 1.
Figure 1. General view of the underground complex (the tunnel is not shown conditionally).
The physical and mechanical characteristics of the soils are presented in Table 1. The foundations of existing buildings are water-saturated sands. The groundwater level is located at a depth of 1.5–2.0 m from the day surface.
Table 1. Physical and mechanical properties of soils.
No.
Ground
γ, kN/m3
ν
С, kPa
φ, °
E50ref = Eoedref (Eurref), MPa
1
Made ground
18
0.30
–
–
–
2
Saturated sand
20
0.32
1
25
13.55
3
Silty clay of fluid consistency
19.3
0.37
13
16
2.4
4
Clay loam of fluid consistency
18.2
0.37
38
9
3.1
5
Plastic sandy loam
21.3
0.35
18
24
7.5
6
Clay loam of low plasticity
20.8
0.35
33
29
9.0
7
Semi-solid loam
21.5
0.35
31
19
12.4
8
Dislocated hard clays
21.1
0.2
69
22
12.7
9
Hard clays
21.8
0.2
273
5.3
30
The engineering and geological conditions of the construction site are shown in Fig. 2.
Figure 2. Engineering and geological conditions of the construction site.
The surrounding buildings consist of load-bearing longitudinal and transverse walls, which are united in height by horizontal floor discs. The walls, 0.55–2.0 m thick, are made of brick with lime-sand mortar. The ceilings are supported by metal or wooden beams. Some are vaulted from brick and concrete. The foundations are made of strip rubble stone on a lime-sand mortar. Roofs of complex shape are built on wooden rafter structures.
The calculation of deformations of the foundations of surrounding buildings was carried out using the finite element method in the Plaxis 3D software package [29]. The computational area has plan dimensions of 350×360 m. The total depth of the soil foundation is 70 m. The load-bearing soil layer model (Ground No. 9 in Table 1) is the Hardening Soil model. For soft soils, the Mohr–Coulomb model was used. The deformation and strength characteristics of the soils of building foundations are taken into account the results of dynamic sounding. These characteristics reflect the actual state of the soil. This approach involves changing properties. During the entire operational cycle, they changed under the influence of loads from the building and various man-made factors. The surrounding buildings are specified by volumetric elements with a Linear Elastic material model to take into account their rigidity in the formation of the stress-strain state of soils. The specific gravity of buildings is assumed to be 5 kN/m3 [30], which is an average for most buildings. The work of the soil in contact with structures is modeled using reducing interface coefficients These coefficients reflect a decrease in the strength characteristics of soils due to technological influences. The values of these coefficients depend on the soil, the material of the enclosing structure, the method of its construction, etc. and are determined from reference data. For concrete and reinforced concrete structures = 0.67 [31]. The design diagram of the underground complex and the metro tunnel is shown in Fig. 3.
Figure 3. General view of a three-dimensional design diagram in Paxis 3D.
The calculation of additional settlements was carried out in the following steps:
1) calculation of natural stresses of the soil mass;
2) zeroing out movements, activating surrounding buildings;
3) activation of the fencing of the pit and interfaces, changing the hydrological conditions of the soil within the pit;
4) stage-by-stage development of the pit;
5) stage-by-stage excavation of the metro tunnel.
The following options for enclosing structures of the starting pit were considered:
1) diaphragm wall made of reinforced concrete 0.6 m thick, made using trench technology;
2) a row of drilled tangential piles with a diameter of 0.6 m with injection of pile mates for waterproofness of the fence;
3) soil-cement barrier using Jet-Grouting technology.
Options for pit enclosing structures are shown in Fig. 4.
Figure 4. Options for pit enclosing structures.
The stiffnesses of the considered options are presented in Table 2. When determining the moments of inertia per linear meter of the fence for Option No. 2, areas of jet grouting of joints are not taken into account, and for Option No. 3 the conditional width of the fence is assumed to be 0.6 m.
Table 2. Rigidity characteristics of fencing design options.
Option No.
Material
Modulus of elasticity E, MPa
Sectional area A, m2
Axial stiffness ЕA, kN
Bending stiffness ЕJ, кNm2
1
Reinforced concrete
30000
0.60
1.8×107
5.4×105
2
Reinforced concrete
30000
0.45
1.35×107
3.0×105
3
Soil-cement
150
≈0.6
9×104
2.7×103
The calculation results for the pit fencing options are presented in Table 3.
Table 3. Calculation results for pit fencing options.
Option No
Maximum displacement
Umax, mm
Maximum bending moment Мmax, kNm/m
Maximum normal force in strut rail Nmax, kN/m
1
23
1050
1334
2
25
735
1387
3
56
192
2434
Figure 5. Comparison of pit enclosing wall options.
An analysis of enclosing structure options is presented in Fig. 5. For Option No. 1, soil displacements and forces in strut rail structures are minimal. This is due to the high stiffness of the structure. Also, Options No. 2 and No. 3 do not guarantee the tightness of the pit during its development. The entry of water into the pit entails the development of suffusion processes and additional settlements of buildings. Therefore, the enclosing structure of the pit is made diaphragm wall of reinforced concrete 0.6 m thick, made using trench technology.
The excavation of the escalator tunnel at an angle of 30 ° is carried out by an earth pressure balance tunnel boring machine with a starting pit with a diameter of 23 m. The fencing of the pit is made of reinforced concrete walls and transverse diaphragms for the railway tracks of the overhead crane. The lining of the productivity tunnel is made of prefabricated reinforced concrete blocks measuring 1.0 m and 0.4 m thick. The outer diameter of the tunnel is 10.5 m. The depth of the tunnel is about 65 m from the ground surface level. A general view of the starting pit and escalator tunnel is shown in Fig. 6.
Figure 6. General view of the starting pit and escalator tunnel.
3.Results and Discussion
Additional Settlements of Building Foundations When Tunneling a Metro Tunnel Without Protective Measures
The results of calculations of vertical and horizontal soil displacements without protective measures are shown in Fig. 7.
Figure 7. Isofields of soil mass deformations in the zone of influence of tunnel excavation without protective measures.
The foundation diagram of existing buildings that fall within the influence zone of tunnel excavation is shown in Fig. 8. The area of 130×60 m is taken as the boundary of the influence zone.
Figure 8. The foundation diagram of foundations of existing buildings and structures in the zone of influence of tunnel excavation.
To determine additional deformations of building structural elements along design sections 1-1 and 2-2, two-dimensional models were compiled in Plaxis 2D. The structures were modeled using rod elements. In the places where walls and ceilings meet, hinges are introduced in the design diagram. Additional structural deformations of existing buildings are shown in Figs. 9 and 10.
Figure 9. Diagrams of additional settlements of neighboring buildings (section 1-1 in Fig. 7). Maximum value Sad,max = –5.943 mm, (ΔS/L)max = 0.0006.
Figure 10. Diagrams of additional settlements of neighboring buildings (section 2-2 in Fig. 7). Maximum value Sad,max = –4.634 mm, (ΔS/L)max = 0.0005.
The limiting value of the additional settlement of the foundations of neighboring buildings is = = 5 mm, and the relative difference in settlement = 0.0004. Thus, the deformations of neighboring buildings exceed the permissible limits. Protective measures must be taken to ensure safety.
Additional Settlements of Building Foundations When Digging a Metro Tunnel with a Geotechnical Barrier
The protective barrier consists of diaphragm wall of reinforced concrete 0.6 m thick and 27–35 m deep. Different depths are made in order to save materials. The diaphragm walls are located parallel to the tunnel axis at a distance of 3 m from it. They limit horizontal displacements of the soil mass. The barrier also includes two diaphragm slabs 4 m thick. The diaphragms are made using single-component Jet-Grouting technology. This technology is discussed in [32–34]. Soil-cement slabs increase the rigidity of the structure in the transverse direction to the axis of the tunnel and prevent it from floating. A general view of the geotechnical barrier is shown in Fig. 11.
Figure 11. General view of the combined geotechnical barrier.
The results of calculations of vertical and horizontal soil displacements using a barrier are shown in Fig. 12.
Figure 12. Isofields of soil mass deformations in the zone influenced by tunneling during the construction of a geotechnical barrier.
Additional structural deformations of existing buildings are shown in Figs. 13 and 14.
Figure 13. Diagrams of additional settlements of neighboring buildings (section 1-1 in Fig. 7). Maximum value Sad,max = –3.469 mm, (ΔS/L)max = 0.0002.
Figure 14. Diagrams of additional settlements of neighboring buildings (section 2-2 in Fig. 7). Maximum value Sad,max = –2.831 mm, (ΔS/L)max = 0.0002.
According to the calculation results, additional settlements and their relative differences do not exceed the maximum permissible values. The use of a geotechnical barrier ensures the safety of existing buildings.
4.Conclusions
A significant amount of destruction of existing buildings occurs during construction and reconstruction near them [35, 36]. In dense urban areas, additional deformations of building foundations are subject to strict requirements. It is necessary to ensure safe work and eliminate the risk of emergency situations.
The total amount of additional settlement is affected by soil compaction processes, changes in hydrogeological conditions in the construction area, and various technogenic impacts during construction and reconstruction. The greatest danger to the structures of the surrounding buildings is represented by the technologies used for the work. Therefore, when designing, it is necessary to adopt the safest technologies. If necessary, protective measures must be developed.
This article examines the effectiveness of using a combined geotechnical barrier when excavating a metro tunnel on soft clay soils. The optimal design solution for the fencing of the starting pit has been determined. Calculations were carried out by numerical modeling in flat and spatial formulations. The effectiveness of the installation of a geotechnical barrier along the escalator tunnel is confirmed. The barrier reduces the maximum absolute foundation settlements of the surrounding buildings by an average of 40 % and the relative differential settlement by 66 %.
Utilizing the Linear Elastic model for adjacent buildings is a limitation that neglects the physical nonlinearity of brick masonry and crack development under differential settlements. However, this approach is justified by the primary focus on the global spatial stiffness of structures for predicting the ground settlement trough, while also reducing the computational cost of the coupled soil-structure interaction analysis in Plaxis 3D.