Introduction

Shallow tunneling induces both lateral and vertical surface movements [14]. Ground settlement (surface vertical movement) is a critical threat to both the surface [43] and subsurface facilities [56]. Containing significant infrastructure in and around the proposed tunnel location, the assumption of an undeveloped site may lead to a significant error in the predicted ground surface settlement [28]. Various studies have considered the case of tunnel construction interaction. It has been recognized that the building stiffness should be taken into account in the assessment of tunnel-structure interaction since it generally tends to decrease the structural distortions and risk of damage with respect to the greenfield case [20, 25,26,27, 29, 40]. On the other hand, the tunnel-single pile and pile group interaction problems have been widely analyzed using field trials, physical modeling, and numerical simulations, leading to some confidence in the assessment of pile group displacements [18, 19, 26, 34, 35, 41], internal forces [32, 37, 44, 53], and pile failure due to tunnel excavation [42]. However, a great majority of these cases are urban tunnels in weak ground conditions. Although ground conditions are good, there can be some serious engineering problems.

Some tunnel–structure interaction problems in karstic regions were reported (i.e. [3]. The design and construction of tunnels in karst terrains are extremely difficult due to the problems associated with the unexpected location, irregular geometry, and unpredictable dimensions of the karst structures [3]. Karst formations are characteristic of strong heterogeneity and anisotropy due to complex void structures, typically consisting of three levels of voids, i.e., primary porosity, secondary fractures, and tertiary conduits [9, 49]. The karst conduits, faults, and fractures provide the main channels for groundwater flow in karst formations, and the flow may deviate significantly from the laminar condition described by Darcy's law as the flow rate or hydraulic gradient increases [16, 17]. This unpredictable nature of karstic structures requires special research and approaches in tunnel design studies. Although filling the karstic caves is the first solution that comes to mind, the complexity of the filling type and construction methodologies should be solved. Chemicals and cement-based (mixtures such as water-cement, sand, bentonite, fly ash, etc.) are used as filling material. However, the most economical solution is still cement-based injections. As stated by [16, 17], due to poor geological conditions and unsymmetrical tunnel pressure, bridge stability is of great concern during tunneling. Consequently, depending on the increasing population, the need for underground rail transport is increasing in densely populated areas. Constructing tunnels in areas with dense settlements and various infrastructures is extremely difficult compared to virgin areas. Because there is a possibility that excessive deformations and/or failures that may occur during tunnel construction may also affect the structures in their close vicinity. However, relatively low deformations are expected when tunnel excavations are performed in good-quality rock masses. However, the presence of karstic caves whose dimensions cannot be estimated exactly can cause unexpected sudden failures.

Some parts of the shallow T5-1 tunnel were constructed in thin-bedded karstic dolomitic limestones. The T5-1 tunnel is located between the approaching viaducts of Osman Gazi Bridge. Osmangazi Bridge or Izmit Körfez Bridge is the fourth longest span suspension bridge in the World, with a middle span of 1550 m and a total length of 2682 m, built between Dilovası and Altınova in the Gulf of Izmit, Türkiye. The T5-1 tunnel, which is located in the middle of the Osman Gazi Bridge approach viaducts, is extremely critical with this feature. The presence of karstic caves and this special location constitute the main problems. Therefore, taking into account the possibility of serious effects of excessive deformations and/or failure on the Bridge that may occur during the construction of the T5-1 tunnel, a certain part of which is located in the rocks with karstic features, it has become necessary to take precautions before excavations. Therefore, this study aims to evaluate the performance of the grouting performed before the excavation of the T5-1 tunnel, which was built in a karstic and complex environment. For this purpose, within the scope of the study, a case that is thought to be important in terms of tunneling literature is presented, considering the interaction of the T5-1 tunnel with the Osmangazi Bridge. In this context, the geomechanical properties of the geological unit were determined by interpreting the hydraulic properties and laboratory test data to put forward the bridge-tunnel interaction. Then, the situations before and after the injection of the karstic caves were investigated with the 3-Dimensional Finite Element analysis. The injection applied with a mixture of water, cement, and bentonite from the surface was evaluated by comparing the numerical results and in-situ measurements.

Project description

The majority of the population in Türkiye lives in a narrow region between Istanbul and Kocaeli provinces. The presence of intense industrial and port facilities in this region reveals the need for freight and passenger transportation. By increasing the capacity of the existing 2-line railway, which was built in 1873, 1 main and 1 connection railway line was designed next to the existing line to meet the need. The T5-1 Tunnel, which is the subject of this study, is located on the Gebze-Köseköy railway project route and has a length of 230 m (Km: 53 + 930 and 54 + 160). The northern approach viaducts (P-02 and P-03) of the Osman Gazi Bridge, located on the route of Gebze—Orhangazi İzmir Highway and built on the Gulf of Izmit, is located between the side piers (Figs. 1, 2).

Fig. 1
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Gebze-Köseköy project and T5-1 Tunnel location (Google Earth)

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Fig. 2
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Relationship between T5-1 Tunnel and Osmangazi Bridge north approach viaduct

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The T5-1 Tunnel is designed as a single-track railway tunnel and is 7.45 m wide and 6.25 m high. It is planned to be excavated with the classical excavation method, New Austrian Tunneling Method (NATM), and has an excavation area of 64.32 m2 (Fig. 2). The thickness of the overburden varies between 7 and 27 m When considering the location and geotechnical conditions, the T5-1 tunnel is a scientifically very interesting case for tunnel engineers. In addition, with the development of infrastructure, new tunnel crossings below existing bridges are becoming increasingly common [16, 17].

Geological and geotechnical settings

The tunnel route passes through thin-bedded, gray dolomitic limestone (Hereke Formation, [4] with interbedded Triassic marl (Figs. 3a, b). Karstification is frequently observed in the unit [24]. The unit has a moderate to slightly weathered structure in other places along the weak zones. The strength of the unit varies from weak to very solid.

Fig. 3
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Geological units observed around the tunnel (a: dolomitic limestone, b: marl, c: boundary between limestone and marl, black line: layers, blue line: shear zone)

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When the foundations of Osman Gazi Bridge Approach Viaducts are examined, there is siltstone and mudstone interbedded brecciated limestone under the P-02 pier, while dolomitic limestone with clay intermediate level is found under the P-03 pier. These two unit boundaries are transitional with the shear zone (Fig. 3c). Although the shear zone is not encountered on the tunnel route, the presence of the shear zone is extremely important in terms of the interaction between the structure on the piles and the tunnel. The site locates in one of the most active seismic zones of Türkiye (Fig. 4).

Fig. 4
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Seismotectonic map of the Marmara Region of Türkiye [23]

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In order to understand and examine the geological and geotechnical conditions of the tunnel route, in addition to the surface observations in the viaduct and tunnel area, the geological cross-section along the tunnel route was obtained by examining 7 drilling data (Fig. 5), and related tests were carried out in the laboratory and in-situ to determine the physicomechanical and elastic properties of the lithological units (Table 1).

Fig. 5
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Geological section of T5-1 Tunnel

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Table 1 Physico-mechanical and elastic properties of rock samples belonging to geotechnical units [22]
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Clay bands are observed in most of the tunnel overburden and the groundwater level is approximately 5.5 m below the tunnel base. During the drilling, karstic cavities were encountered 2.5 m below the tunnel base. The permeability of the bedrock forming the tunnel route was performed using the Lugeon test procedures recommended by Yihdego [57] in the drillings and evaluated with the Lugeon graphic method (Fig. 6).

Fig. 6
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Determination of Lugeon value in SK-36 borehole by graphical method

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At the exit portal of the tunnel, a highly permeable structure between 1 and 22 m from the surface, permeable between 22 and 37 m, and low permeable between 37 and 40 m was detected. When the drilling data and Lugeon values were evaluated together, it was understood that the rock conditions on the tunnel route included very fractured-cracked and in places karstic cavities. During the excavation in the tunnel (outside the tunnel-viaduct interaction zone), the karstic cavity encountered at a depth of approximately 2–3 m and in the form of a cony was filled with concrete (Fig. 7).

Fig. 7
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Karst cave encountered during tunnel excavation in dolomitic limestones

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Contour diagrams of the discontinuities were drawn by measuring the locations of 53 discontinuities from the dolomitic limestone units known to contain karstic voids, which have been encountered in the interaction zone of the T5-1 Tunnel and Osmangazi Bridge Approach Viaducts. Accordingly, the bedding dominant orientation is 12/106 (dip/dip direction) and the bidirectional discontinuity system is 69/228, 75/277. It also has irregularly oriented discontinuities (Fig. 8).

Fig. 8
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Pole points (a) and contour diagram (b) of discontinuity and layer measurements

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When the regular and irregular discontinuities (discontinuities and beddings) in the dolomitic limestones are evaluated together, it is thought that they can control the formation of karstic caves that cannot be followed within the unit due to their long continuity and cross-cutting positions.

Methodology

The permissible value for vertical deformation in the Osman Gazi Tunnel approach viaducts has been reported by the Turkish Highways Authority as 25 mm, and the deformation limit value in the direction parallel to the highway axis is 5 mm. In order not to exceed these limit values, tunnel support works are divided into 2 stages in order to minimize the structure-tunnel interaction in the T5-1 Tunnel. In the first stage, the injection was projected to fill the cavities and increase the strength of the rock mass. Then, the tunnel excavation and rigid support system were planned.

Grouting

Grouting is critically important in tunnel engineering. The compaction grouting mode or hydro-fracture grouting mode can be used depending on the local geological conditions of the given project [52]. To date, the majority of grouting applications have adopted the soil fracture technique [47]. However, as grouting is a complex process, both permeation grouting and compaction grouting can be observed in a fracture grouting-dominated process. Many factors may affect the eventual grouting modes used, such as the grout material, grouting pressure, soil type, and stress state of the ground [36].

Superficial injection method was chosen for the injection process to be applied to fill the discontinuities and karstic voids, which are quite complicated, due to the shallowness of the tunnel instead of the tunnel. In an area of 87 m in length and 27 m in width where the T5-1 Tunnel interacts with the Osman Gazi Bridge approach viaducts, injection wells with a depth of 33 m to 55 m were determined in a 3 m grid (278 wells) (Fig. 9).

Fig. 9
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Layout of injection wells on the T5-1 Tunnel route (Prohit [46]

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Equation 1 was used to calculate the pressure to be applied in consolidation injections while Eq. 2 was employed in order to prevent negative effects in loose material and shallow parts of wells. Equation 3 was used for pressure monitoring during the injection process.

$${\text{Pt = 2 + 0}}{\text{.33 H}}$$
(1)
$${\text{Pt = 2 + 0}}{\text{.23 H}}$$
(2)
$${\text{Pm = }}{{{\text{Pt}-(\text{w}} \times {\text{H}} \times \cos \,{\text{a)}}} \mathord{\left/ {\vphantom {{{\text{Pt\_(w}} \times {\text{H}} \times \cos \,{\text{a)}}} {10}}} \right. \kern-0pt} {10}}$$
(3)

where, Pt: Total effective pressure applied to the stage (kg/cm2); Pm: Pressure that should be read on the manometer (kg/cm2); H: The distance of the midpoint of the injected stage to the well mouth (m); w: Specific gravity of injection material (gr/cm3); α: Angle of the well with vertical.

In the pressurized water tests to be performed in the control wells to be opened after the consolidation injection is completed, the water leakage amount (injection pressure) was checked with Eq. 4 and the permeability was determined.

$${{\text{Q}} \mathord{\left/ {\vphantom {{\text{Q}} (}} \right. \kern-0pt} (}{\text{P}} \times {\text{L}} \times {\text{t) < 1 (10 Lugeon)}}$$
(4)

where Q: Total amount of water leakage in the stage (l); P: The total pressure applied in the test stage (kg/cm2); L The length of the test stage (m); t Test duration (min).

The determination of the injection mixture to be used in filling the karstic voids and the follow-up of the process are extremely important in terms of both cost and void filling efficiency. Adjusting the appropriate viscosity value and setting time according to the state of the voids in the mixture increases the injection efficiency. For this reason, 7 different mixtures were formed to be adjusted in the field according to the amount of flow in the injection wells (from more fluid to less fluid) (Table 2).

Table 2 Injection grout mixing ratios (CEM II –A of cement, gradation of TSE EN ISO 13500 sand: 95% must pass through sieve no 16, 50% sieve no 50 and 5% < sieve no 200, specific gravity should be > 2 gr/cm3; Prohit [46]
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Before starting the injection process, bentonite was mixed with water at a ratio of 1/10 and allowed to rest for 24 h, and then added to the mixture. The prepared injection mixture was used within 2 h. Hole diameters are 60 mm, injection process was started with grout number 1 and 1 m3 was given first. Then, the pressure values in each well were monitored in a controlled manner and the mixtures numbered 2, 3, 4, and 5 were passed. However, when the expected reflux could not be obtained in mixture number 7, the injection process was interrupted and the material was expected to set. In total 22,567 tons of mixture were injected into 278 wells.

Drilling and geophysical studies were carried out in the field in order to measure the success of the grouting methodology applied before the start of tunnel construction works.

Drillings

After the completion of the grouting processes, 3 core drillings with a depth of 50 m were drilled and a PWT test was carried out at 3 m. According to the PWT results, all wells were measured as impermeable. It was observed that the injection spread and filled the voids in the 50 m deep well VA25 at 26.80 m, 28.00 m, 29.80 m, 33 m, 33.30 m, 34.30 m, and 35.70 m (Fig. 10).

Fig. 10
figure 10

Injection spread in borehole VA25 (areas within the red lines indicate injection, the tunnel is between 28–35 m)

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Electrical resistivity tomography (ERT)

Electrical resistivity tomography (ERT) method was used to determine how effective the injection studies on the T5-1 Tunnel were. For ERT studies, 4 profile resistivity tomography lines were determined and these lines were measured with 110 m profile length, 56 electrodes and dipole–dipole expansion. In Fig. 11, the resistivity inversion section of the ERT-1 line is given.

Fig. 11
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ERT-1 Line Resistivity inversion section

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As seen in Fig. 11, the red-colored area indicates that the injection has fully penetrated, and the blue and green-colored areas show the areas where the injection has not penetrated. Areas, where the grout does not penetrate, were detected locally between 4 and 52 m of the profile length, between 2 and 10 m depths, between 5 and 18 m depths at 58–70 m of the profile length and between 78 and 96 m of the profile length at 5–28 m depths. It turned out that these areas should be filled by opening injection wells again.

Seismic measurements

Seismic studies by using the MASW method between the P-02 and P-03 piers of the northern approach viaduct were performed, and hence, seismic refraction and profiles were prepared (Fig. 12).

Fig. 12
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Distribution of the MASW measurement point over the tunnel

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The elastic parameters of the units forming the study area, the dominant period of the ground, and the ground amplification value were determined by seismic measurements. 2 seismic lines were determined, these areas are above the T5-1 tunnel where the injection was performed and between the piers of the Viaduct P-02 and P-03.

In the first seismic refraction data, the section length is 39.0 m, and a low-velocity unit was detected in the section between 27.0 m and 39.0 m and up to 7.0 m in depth. When the MASW measurement along the profile was evaluated, a joint or discontinuity system was detected between 4.50 m and 7.50 m at a depth of 13.0 m. In the second seismic refraction data, the section length is 52.0 m and it was seen that there is a low-velocity unit between 40.0 m and 52.0 m and at a depth of 7.50 m. In this part, when the MASW measurement was evaluated, no discontinuity or joint system was detected. In this part, it was understood that the grout penetrated the rocks and filled the voids.

For the first profile, the average shear wave velocity (Vs30) was obtained as 1066 m/sec for the 30 m depth, and the ground dominant period (T0) is 0.17 s at the measurement elevation in the field. For the Second Profile, the average shear wave velocity (Vs30) is determined as 1034 m/sec for the 30 m depth, and the ground dominant period (T0) is 0.18 s at the measurement elevation in the field. Considering these data, it was understood that the effective ground acceleration (amax) in the field should be used as 0.592 g.

Injection propagation in tunnel excavation face

By using the time optimally, the excavation and support works in the part of the tunnel that will be passed without injection were performed from the tunnel exit to the entrance. In the tunnel excavation faces at the entrance of the injected area, the injection spread is mostly in the form of filling the karstic spaces (Fig. 13a), there is no order or discontinuity tracking. Injections filled large karst cavities. As the tunnel approaches the region of the viaduct piers, the injection spreads along the karst cavity and discontinuity surfaces (Fig. 13b). In the tunnel viaduct region (Fig. 13c) and its continuation (Fig. 13d), injection appears to be completely under discontinuity control. In this case, it shows parallelism with the research studies carried out both before and after the injection procedure. In addition, it has been understood that the methodology applied for the grouting process in the excavations in the tunnel yielded very successful results.

Fig. 13
figure 13

Injection dispersions encountered during tunnel excavation (a-Km:54 + 049,00, b-Km: 54 + 028.25, c-Km: 54 + 004.25, d-Km:53 + 990.75 excavation face, injection are the areas within the yellow lines)

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Support system design at viaduct region

The tunnel interacts with the viaduct piers between Km: 53 + 980 − 54 + 060 (80 m). In this region, the distance from the Bridge side piers to the nearest bored pile is 22.90 m for the P-02 pier and 21.27 m for the P-03 pier (See Fig. 2). Determining the tunnel support to be applied after the injection process is important in terms of minimizing the viaduct—tunnel interaction and getting the highest performance from the support.

There are basically 3 main approaches to determine tunnel support systems such as empirical methods, analytical methods, and numerical methods [6]. Empirical methods include the rock mass classification systems. Several rock mass classification systems have been developed since the pioneering study by Terzaghi [55] involving a rock load factor classification [61]. The rock mass rating (RMR) and NGI tunneling quality index (Q) are the most applied and accepted systems worldwide. The RMR classification was proposed in 1973 as a jointed rock mass classification system [12], and major revisions were performed in 1989 and 2014 [13, 15]. The Q-system was developed in 1974 [11], and major changes in characterization and classification were proposed in 1993 and 2002 [10, 30]. The Q-system was developed based on the tunneling cases for hard and jointed rock masses [45]. Both classification systems are inappropriate for the tunnel support design in highly stressed jointed rock mass, and this is the limitation of the data on which these systems are based [48]. Analytical methods were developed by various researchers [21, 31, 54, 62]. In analytical methods, the medium is considered as homogeneous and isotropic. In other words, all studies are performed in an idealized environment. However, the units tunnel passes through during the application phase are not isotropic and homogeneous. For this reason, analytical solutions have limitations [5]. Numerical analysis methods are the main methods used to determine tunnel support systems [1, 2, 6,7,8, 38, 50, 51, 58, 59]. Decisions based on practice and experience are essential in determining support systems, and numerical analyzes can be considered as a guide to practical decisions. Some numerical modelling methods are the finite element method, discrete element method and finite difference method. Among these methods, the finite element method is one of the most successful methods for anisotropic and nonlinear environments [60].

Numerical modeling

As mentioned before, an interaction between the bridge and the tunnel is possible. To understand this interaction, 2 and 3 dimensional modelings are carried out. The NATM class of the interaction zone is B3. In the interaction zone, the excavation stage at upper bench is selected as 1.25 while that at lower bench is considered as maximum 2.5 m. As the support elements, NPI140 steel shoring, f28 SN (Store Norfors) rockbolts with 1.5 × 1.5 m pattern and 1.5′′ forepooling with 40 cm interval are used. In addition, double layers Q589/478 steel mesh and 20 cm shotcrete are applied. When selecting these elements, the principles of the New Austrian Tunneling Method (NATM) are employed. The 2D (Rocscience Phase 2D V8.2) 3D numerical analyses (Midas GTS NX) were performed by Emre Özcan Engineering [22].

Results of the numerical analyses

The maximum stresses created by the viaduct piers is 1000 kPa. The parameters of the rock mass and support elemens are given in Table 3. 2 and 3 D model prepared by [22] is shown in Fig. 14. 2D models for 3 different critical sections (Km: 54 + 013, 54 + 024, 54 + 035) in 6 steps (1-location of stresses in place, 2-excavation of the upper half, 3-installation of the support system, 4-installation of the lower half, 5 -installation of the support lower half, 6-tunnel lining construction and earthquake situation) are performed. The dimensions of the 3D models are 154 m × 147 m × 76 m, and the analyses are performed for all construction stages (96 stage). In the last stage, the peak ground acceleration is considered as 0.488 g. In this stage, as the support elements, only the lining concrete is used.

Table 3 Material parameters used in the numerical analysis [22, 46]
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Fig. 14
figure 14

Numerical mesh model for T5-1 Tunnel and Osmangazi approach viaduct interaction (a: 2D model (Rocscience Phase 2D V8.2), b: 3D model (Midas GTS NX) were performed by Emre Ozcan Engineering and Prohit Engineering [22, 46]

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By fininte element analyses, not only tunnel deformation, shoring effects and deformation of viaduct foundation but also maximum effects on foundation piles of the viaduct, deformation of tunel lining and axial forces on rockbolts are assessed. The total deformations for the seismic conditions are shown in Fig. 15. According to the results of the analyses, the maximum deformation of the tunnel lining is 2.91 mm while that of the viaduct foundations is 0.9 mm.

Fig. 15
figure 15

Total displacement in 2D and 3D numerical analysis for pseudo-static conditions (a: 2D Rocscience Phase 2D V8.2), b: 3D (Midas GTS NX) were performed by Emre Ozcan Engineering and Prohit Engineering [22, 46]

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According to the results of the pseudo-static analyses, the maximum defomation in the interaction zone is obtained as 4.2 mm for ceiling of the tunnel and 6.8 mm for invert of the tunnel. The maximum deformation for the viaduct foundations is calculated as 3 mm. All results obtained from the numerical analyses are summarized in Table 4.

Table 4 2D and 3D Numerical Analysis Results [22, 46]
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When Table 4 is examined carefully, according to the results of the 2D numerical analysis, deformation of 10 mm in the vertical direction (5 mm in 3D calculations) is calculated in the tunnel due to excavations in static conditions under the grouted rock conditions. It is observed that the effects of these deformations on tunnel and viaduct foundations are in the order of 1–2 mm in the static condition. In case of earthquake, 5–6 mm vertical deformation and 12–14 mm horizontal deformation values in the tunnel lining are calculated. It is calculated as 2 mm due to more representative conditions in 3D calculations. In addition, deformations of 4–5 mm of the viaduct foundation and piles are obtained in the earthquake conditions. Consequently, all deformations obtained from the numerical analyses are negligible and the grouting provides an important improvement on the rock mass. In addition, it is almost impossible to describe the distribution and the dimensions of the karstic voids along the T5-1 tunnel route. The karstic caves are filled by grouting and hence, the possible negative effects of the caves are eliminated. For this reason, during the analyses, the caves are not considered.

Monitoring results

Due to the interaction of the T5-1 tunnel and the Osman Gazi Bridge's approach viaducts, it is obligatory to monitor the deformations in the tunnel and on the viaduct. For this reason, a continuous monitoring system is set up with precise measuring instruments inside the tunnel and on the viaduct during the tunnel construction.

Measurements inside the tunnel

In-tunnel measurements are made on the basis of monitoring the deformations of the points placed on the supporting elements [39]. Tunnel monitoring data (convergence) gives serious information about tunnel conditions [33]. In order for the T5-1 Tunnel to be excavated safely and, progress can be achieved, the critical basic value was determined as 5 mm, the indicator level 7 mm, and the alarm level 10 mm with numerical analysis. The optotigonometric method is preferred to determine the deformations in the tunnel and 5 optical prisms are placed every 5 m in the tunnel (Fig. 16). By monitoring the daily deformations in the tunnel, weekly and monthly measurements are decided, and it is understood that the measurements are slightly better than the threshold values predicted for the tunnel (Fig. 16).

Fig. 16
figure 16

Deformation monitoring points and measurement graphs

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Measurements on the viaduct

Inclinometer (tiltmeter) and optical deformation sensor (Optical Displacement Sensor-ODS) monitoring system are installed for instant data recording on the Osmangazi Bridge North approach viaduct P-02 and P-03 piers. This setup is followed instantly via the cloud system.

On the bridge piers, the pier on the right side is 4,5 and 6, and ODS 4,5 and 6 are located on the P-02 pier, and the left pier is the P-03 (with Tilt 1, 2 and 3 and ODS 1, 2 and 3). The graph of these oscillations are read and evaluated on tiltmeters is given in Fig. 17.

Fig. 17
figure 17

Readings dated (01.06.2022) after the completion of the tunnel from tiltmeters and ODSs placed on Viaduct P-02 and P-03 legs (Orange dots show Tilt Meters, green dots show ODS points, yellow arrows show the direction of movement)

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There are 25 mm limit values for vertical deformation in bridge piers and 5 mm limit values for deformations parallel to the highway axis. In the measurements performed, values of 4.9759 mm in Tilt 1 and 4.3102 mm in Tilt 4 are read, and it is understood that the tunnel-viaduct interaction remains within the limit values by following the effects of vibration during excavations and the oscillations caused by the traffic on the Bridge (Fig. 18).

Fig. 18
figure 18

Tiltmeter and ODS graphics placed on the Viaduct P-02 and P-03 piers

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Conclusions

Structure-tunnel interaction is extremely important in tunnels excavated in rocks containing karstic cavities as well as weak rock or soil tunnels. In this study, the T5-1 Tunnel passing between the Osmangazi Bridge North approach viaduct P-02 and P-03 piers in the Gebze-Köseköy Railway Project is investigated in terms of structure-tunnel interaction.

In the T5-1 Tunnel design phase, research and test procedures are established to investigate the karstic cavities before starting the construction process and to estimate the extent of these cavities. Since these cavities pose a risk during tunnel construction, it is decided to apply injection, and injections are performed in 7 different injection mixtures and 4 different pressure phases.

The consolidation injection for filling the karstic cavities contributed to the acceptance of the rock environment as homogeneous and isotropic in numerical analysis. In addition, by filling the karstic voids, it prevents possible asymmetrical loads on the tunnel lining.

The injection creates a shell cover for the tunnel construction excavation and filled the discontinuities in the rock mass, eliminating the negative conditions that could cause excessive breakage, and allowing the tunnel to be excavated safely.

As another way to minimize structure-tunnel interaction, a fast-forming and relatively rigid tunnel support were modeled with 2D and 3D FEM instead of a flexible shell. Measurements taken from the tunnel and viaducts and, deformations calculated with 2D and 3D numerical models were compared. As a result of this comparison, it is understood that the measurement results give close results to the 3D numerical analysis results. For example, the maximum deformation is measured as 2.54 mm while this value is obtained as 3 mm from the 3D numerical analyses.

Finally, the tunnel was completed without any problems.