Railway and highway projects include several engineering structures, such as tunnels, bridges, viaducts, cuts, and fillings. Among these structures, a tunnel, as the primary bottleneck, is the most important structure affecting project timelines. For this reason, rapid and safe construction of tunnels is extremely important for transportation projects. Yu et al. (2022) stated that underground space had been continuously utilized and developed with the settlement of humans, the formation of cities, and the development of societies, from the Karez and underground tombs in BC to the London metro in 1863, from the Tokyo declaration in 1991 to the Shanghai declaration in 2019. However, methodologies employed in tunnel construction have changed during the development of technologies. The speed and safety of tunnel construction have been developed and increased with the development of machines used in conventional tunneling (New Austrian Tunneling Method [NATM]). However, in the NATM tunneling, the construction process is applied on limited excavation faces and the advancement procedure is still lengthy. On the contrary, tunnel boring machine (TBM) tunneling has been applied successfully in metro tunnels (Barzegari et al., 2018; Bazargan et al., 2021; Bilgin, 2016; Ercelebi et al., 2011; Filho et al., 2022; Mahmoodzadeh et al., 2022; Marinos et al., 2008, 2009; Namatollahi & Dias, 2022; Rezaei et al., 2019; Zhang et al., 2022). Over the past two decades, besides subway construction, high-speed railway projects were initiated and completed successfully in Türkiye. Along these railway routes, challenges have been encountered because of the complex geological and geotechnical features of Türkiye (Aygar, 2022; Aygar & Gokceoglu, 2020, 2021a, 2021b; Aygar et al., 2023; Bilgin, 2016; Can et al., 2022; Gokceoglu, Aygar, et al., 2022). Despite difficult geological geotechnical features, a 10-km-long Bahçe-Nurdagi Railway Tunnel was successfully constructed using TBM technology (Gokceoglu, 2022), where the diameter of these tunnels was 8 m. The selection of appropriate TBM is crucial for TBM tunneling. Therefore, the geological and geotechnical conditions of tunnel routes must be investigated carefully. Otherwise, failure of TBMs becomes unavoidable, and research has been conducted on the jamming of TBM (Bilgin & Algan, 2012; Farrokh & Rostami, 2009; Hasanpour et al., 2017; Huang et al., 2019; Koizumi et al., 2016; Liu et al., 2023; Shang et al., 2004; Xu et al., 2021).

It is difficult to excavate relatively shallow tunnels in weak ground conditions compared to those in high-quality rock mass conditions. In addition, the diameter of tunnels is important for TBM tunneling. Consequently, the purpose of this study is to present a successful application of TBM in weak ground conditions and to explain the engineering measures in problematic sections before the excavation of tunnels. The T1 tunnel of the Ankara-İzmir High-speed Railway Project is a large and shallow tunnel, and it is the longest tunnel of the Esme-Salihli Section. The construction time of the tunnel is extremely important for the project timeline. This study presented the detailed geological–geotechnical characteristics of the tunnel route, and a detailed comparison between NATM and TBM for methods, analyses, and construction stages of tunneling. The comparison of NATM and TBM involved the engineering measures taken before the excavation and the construction phase, which were the scientific novelty of this study. The T1 tunnel is large with a diameter of 13 m, and it is the fastest completed tunnel with this size of diameter (13–14 m) in terms of both the penetration rate and overall tunnel completion of TBM (Robbins Inc, 2023). Therefore, this study has great scientific value as an important case study in the tunneling literature.

2 GEOLOGICAL AND GEOTECHNICAL CONDITIONS

The T1 tunnel, part of the Ankara-İzmir High-speed Railway Project within the Esme-Salihli Section, is a large and shallow tunnel located in the western part of Türkiye (Figure 1). Türkiye is one of the active seismic regions of the Earth, and the Anatolian Plate is located between the North Anatolian Fault Zone (NAFZ) and East Anatolian Fault Zone (EAFZ). The western part of the Anatolia plate consists of the Aegean Horst-Graben System. The T1 tunnel is in the Southwest Anatolian graben–horst system described by Koçyiğit (2015). Consequently, the project area has an important seismicity. In the first planning stage, eight boreholes were drilled and the geological cross-section was drawn (Figure 2). Along the tunnel route, gneiss, mudstone, and sandstone exist with the dominant lithology of weak mudstone. During the second planning stage, five additional boreholes were drilled (Figure 2). Based on the data obtained from the boreholes and laboratory tests, the geotechnical parameters of the sectors along the tunnel are summarized in Table 1.

           

           

As can be seen from Table 1, the tunnel route had very weak intact rocks and rock masses. The uniaxial compressive strength (UCS) of the intact rocks varied between 1.8 and 8.0 MPa while the GSI (Hoek et al., 2005) values changed from 12 to 30.

The tunnel is approximately 3 km long with overburden thicknesses changing from 8 to 14 m at portal areas and with a maximum overburden thickness of 80 m at the central part (Figure 2). In addition, at 376 + 500 km, there is a very low overburden thickness with a minimum of 3.75 m. At portal areas and near the low overburden section, the surface topography had profiles with slopes resulting in asymmetric loading.

The section of the T1 tunnel is given in Figure 3. Regarding the characteristics of the alignment, two types of segments (standard and heavy rings) were proposed along the tunnel due to its variability of geotechnical conditions (Table 2). The heavy rings were installed along portal areas, low overburden slope sections, and poor geotechnical sections. While the standard rings were installed at the rest of the alignment (Table 2).

           

3 COMPARISON BETWEEN NATM AND TBM

As stated previously, the T1 tunnel is the longest tunnel of the project section and the construction time of the T1 tunnel is hence extremely important for the overall project duration. At the planning stage, the T1 tunnel was planned to be constructed with NATM. However, the tunnel route had weak lithology types and low overburden. For this reason, the TBM was considered as an alternative.

As stated by Karahan et al. (2022), tunnel geology generally consists of Neogene terrestrial sediments, such as sandstone, mudstone, and siltstone. These sedimentary units at the tunnel level are generally moderately and heavily weathered (W3–W4), and of medium or low strength (R3–R4). As a result of these investigations, C4 in NATM class (umbrella) support system was proposed along the T1 tunnel. Before starting the tunnel construction work, additional boreholes, laboratory experiments, and additional research were carried out and it was observed that the results obtained were in line with the research results in the design phase. However, according to the reports at the project stage and in the tunnel support classifications, the geomechanical conditions of the previously completed T2-A tunnel were generally described as slightly to moderately weathered (W4–W5) with a medium-weak strength (R3–R2). These conditions mainly consisted of mica schists and local gneisses. It was determined as 59 m C4, 17 m C3 Slab, and 147 m B3 in NATM classes. As can be seen from these data, although the geological conditions of the T2-A tunnel were superior to the T1 tunnel, the support system during the project phase was insufficient where high deformations occurred, heavier support was used and the tunnel construction was completed. Hence, there was a clear indication of a strong likelihood of encountering similar or potentially more significant issues in the T1 tunnel. However, tunnel construction with NATM necessitates the construction of additional security tunnels. With the reconstructed excavation support system, only progress of 1.2 m per day could be made in the T-2A tunnel until the tunnel was completed. The cross-section of the support system classified as C4 and its excavation stages are shown in Figure 4a,b. As shown in Figure 4a,b, support class C4 had extremely heavy support and a rather slow progress. However, given the extensive length of the T1 tunnel and the challenges faced in tunnels with comparable geological characteristics in the region, there is a high probability of encountering safety, advanced speed, and weak zones during the excavation process using the NATM. This may require additional improvements and structural support measures.

           

Since potential problems can be encountered if the tunnel is constructed using NATM, the option of TBM for the construction of the T1 tunnel is taken into consideration. In case the tunnel is constructed with TBM, the inner diameter is 12.5 m (Figure 3) with a cross-sectional area of 122.65 m2. In other words, the TBM tunnel is 18.8 m2 wider than the NATM tunnel (Figure 4a,b). In addition, since the tunnel has two stories, there is no need for the construction of security tunnels. However, if a suitable TBM is selected, the tunnel will be completed faster, and no additional support improvement is required during excavations. A detailed comparison between TBM and NATM for the T1 tunnel is given in Table 3.

If the tunnel is constructed with NATM, 270 personnel are required. While this number drops to 118 for TBM. Keeping the number of personnel as low as possible in underground excavations reduces the risk of loss of life in possible failures or accidents. From this point of view, TBM has an indisputable advantage. One of the most important issues affecting the cost of a tunnel project is the amount of concrete and steel used in the support. The steel of the inner lining to be used in the tunnel is 10 076 tons in NATM, which may decrease to 5596 tons in TBM. The amount of concrete to be used in the inner lining of the NATM tunnel will be 98.600 m3, and this amount decreases to 56.000 m3 when the tunnel is constructed with TBM. In addition, the temporary structural elements used in the NATM tunnel, such as face supports, bolts needed during excavation, umbrella pipes, shoring, steel mesh, and shotcrete are not necessary for TBM as they are covered by the machine. On the other hand, although the initial investment in the TBM tunnel is high, it is highly cost-effective during the excavation and support phases. Considering all these issues, it is vital to reveal the detailed geological-geotechnical conditions of the tunnel route in sufficient detail and to choose the right type of TBM. Incomplete or incorrect data can lead to improper selection of TBM, resulting in delays and substantial cost escalations of projects (Karahan et al., 2022). As mentioned before, tunnels are very decisive in the duration of railway and road projects. It was estimated that excavation using a TBM for the T1 tunnel would take approximately 188 days, whereas the duration would extend significantly to 1270–1524 days for NATM. This estimation indicates a time difference of 6–8 times, even if everything progressed as intended. Taking into account all these factors, TBM was chosen as the construction method for the T1 tunnel.

4 ANALYSES

Along the T1 tunnel route, five sections were described as problematic. These are the worst geotechnical parameters, maximum and minimum overburden, high groundwater level, and maximum slope (Table 4 and Figure 5). In the problematic sections, heavy-type segments were designed while the standard-type segments were used in the other sections. The length of tunnels using the standard segments is 2240 m, while that using the heavy segments is 811 m. The segment design was carried out by TMD Engineering (2017a, 2017b).

           

The results of 2D numerical analyses on the heavy-ring design sections for the excavation stage are summarized in Table 5. When applying the numerical analyses, the recommendations of International Tunneling Association (ITA) (2000) were considered. According to the results of the heavy-ring sections, the maximum deformation in the y-direction (uy) was obtained as −7 mm while the maximum deformation in the x-direction (ux) was calculated as 3 mm (Table 5). Hence, the heavy segments designed for the problematic sections were found to be sufficient and the deformations were negligible.

Table 5. Total settlements at the excavation phase for heavy-ring sections (TMD Engineering,         2017b).

			Max surface displacements at tunnel axis (mm)
Section	The excavation phase	Displacement (mm)	uy	ux
S1			−7.0	3.0
S2			−6.5	0
S3			−5.5	0
S4			−3.3	2.0
S5			−4.5	2.5

In the lowest overburden region (km: 376+580), the overburden thickness decreased to 3.75 m (Table 5 and Figure 6). For this reason, to prevent a possible failure and TBM jamming, additional special engineering measures were taken into consideration. In this region, the reinforced concrete piles with a diameter of 100 cm and a length of 25 m as well as a reinforced concrete slab on the top of the tunnel were constructed (Figure 7).

           

           

The other critical section is the portal region, and hence additional engineering measures were considered. The performance of this region was checked by numerical analyses. The engineering design of the inlet portal (km: 376+914) is shown in Figures 8 and 9. The soil nails with 12 m and different patterns shown in Figure 8 were used as the support elements in the inlet portal. The details of the portal support are summarized in Table 6. Before applying for support, the slope was unstable because its stress reduction factor (SRF) was found to be 0.93 while the SRF was calculated as 1.44 after applying soil nails (Figure 10).

           

           

           

The stability of the face is one of the most important factors for the whole stability of tunnels and for selecting the correct excavation method of tunnels. This is particularly true for mechanized tunneling and specific boring machines (TBM). For example, the earth pressure balanced (EPB) shield and Slurry shield have been developed in recent decades for managing the instability of the excavation profile in unfavorable geotechnical and hydrogeological conditions, with challenging external constraints (Russo, 2003). Yang et al. (2022) stated that during tunnel excavation by EPB shield the earth pressure in the soil chamber was controlled by adjusting the tunneling speed and rotational speed of the screw conveyor. Consequently, EPB directly affects the selection of TBM type. In this study, the methods proposed by Anagnostou and Kovari (1996) and Gonzales (1986) were considered. According to the results of face pressure proposed by Anagnostou and Kovari (1996), the maximum face pressure was obtained as 1.8 bar, and hence an EPB type of TBM was selected (Figure 11a,b). The main characteristics of the TBM used are given in Table 7.

           

5 APPLICATION

After applying additional engineering measures, the TBM started to excavate the tunnel from the inlet portal on March 22, 2021 and completed it on October 19, 2021. The T1 tunnel was successfully constructed with the selected TBM. The measured average advance rate was 16.17 m per day by using TBM, compared to an estimated value of 10 m per day before the construction. The applied maximum face pressure was obtained as 0.85 bar while its average value was measured as 0.3 bar. Along the tunnel, the measured and calculated values of face pressures are given in Figure 12. As can be seen from the figure, the applied pressure remained under the recommended pressure. In addition to face pressure, rotation velocity, torque, and thrust were recorded (Table 8) and the graphs depending on the segment number are given in Figure 13.

           

           

           

As shown in Figure 13, in the minimum overburden region (km: 376+500), the torque and the face pressure increased to 11 500 kN · m and 0.45 bar, while the penetration rate decreased to 24 mm/min. In the maximum slope region (km: 376+650), the penetration rate decreased to 17 mm/min while the face pressure and the thrust increased to 0.65 bar and 30 000 kN, respectively. Depending on the increase in overburden thickness, the penetration rate increased to 36 mm/min while the face pressure and torque decreased to 0.18 bar and 3800 kN · m. Initially, the excavation duration and ring construction time were notably longer, gradually decreasing as progress advanced. Particularly, the low overburden region significantly impacted the excavations, despite the implementation of significant engineering measures.

Using the data recorded during the excavation phase, a set of correlation analyses were conducted. Initially, simple correlations among the parameters were carried out and the results are summarized in Table 9. The graphs of the simple correlations between advance velocity and rotation velocity, face pressure, torque, and thrust are given in Figure 14. The simple regression analyses indicated that strong correlations among the parameters were not observed, and there were some meaningful regressions between the relations. To obtain a more meaningful correlation for penetration rate, multivariable correlation analyses were performed.

           

In the first stage of the multivariate correlation analyses, the automatic linear modeling module of the SPSS statistics package was used. As can be seen in Figure 15, the most important TBM parameters for predicting penetration rates were face pressure and rotation velocity. For this reason, the independent variables were selected as face pressure and rotation velocity for predicting the penetration rate. With the successful introduction of mechanized tunneling methods in geotechnical projects, estimation of TBM performance has become an essential step for determining the construction period and establishing project timelines (Lee et al., 2022).

           

           

6 RESULTS AND CONCLUSIONS

The T1 tunnel of the Ankara–Izmir High-speed Railway Construction Project was successfully completed with TBM instead of NATM as the initial plan. The main advantage of TBM is the reduced tunnel construction time. The detailed work on the construction time showed that this time for NATM was at least 1270 days, while that for TBM was 188 days. Consequently, the average advance rate was estimated as 10 m per day before the construction, and the measured advance rate was 16.17 m per day for TBM. This performance could be regarded as the best for tunnels with a diameter of 13–14 m.

One of the key findings from this study highlighted that the foremost parameter influencing TBM performance was face pressure. An inverse relationship was obtained between face pressure and rotation velocity. Therefore, the correct determination of the face pressure and selection of appropriate TBMs directly affected the successful construction of tunnels.

It is also important to correctly identify potentially problematic sections on the tunnel route and take necessary engineering measures. In this study, such sections were determined beforehand and necessary engineering measures were taken before excavations to prevent a possible TBM jamming.

The main differences between TBM and NATM were clarified in this study, which is an important case in terms of demonstrating that TBMs can be used successfully in large and weak ground conditions.

AUTHOR CONTRIBUTIONS

Servet Karahan: Conceptualization (equal); formal analysis (equal); writing—review and editing (equal). Candan Gokceoglu: Conceptualization (equal); formal analysis (equal); writing—review and editing (equal).

CONFLICT OF INTEREST STATEMENT

The authors declare no conflict of interest.