TBM Performance During Excavation: Let's Check It Out!

Hello, APT Friends! In our previous article, we discussed Brittle Failure During Tunnel Excavation. Now, let's continue with the next topic: TBM Performance During Excavation. So, the performance of the Tunnel Boring Machine (TBM) during excavation needs to be monitored through several parameters, such as face pressure and advance rate. We maintain TBM face pressure at around 1.69 MPa, though sometimes the load drops due to the cutting head losing contact with the tunnel face. The average advance rate of the TBM is 49.98 mm/hour, with a total excavation time of about 2.80 hours to move forward 146 mm.

The tunnel we're digging has an average diameter of 51 mm, so its radius is around 25.5 mm. Interestingly, the tunnel wall isn't perfectly round but rather wavy. The cutting performance is also influenced by the rock mass properties, especially its brittle response. The synthetic sandstone used in this experiment shows brittle behavior, which allows the TBM to advance faster than when synthetic shale was used previously.

Fluctuations in the advance rate can occur due to the cutting head losing contact with the rock specimen. During mechanical excavation, the TBM operates under quasi-static load, and the rock mass properties greatly determine the feasibility of excavation.
 

Figure 9: Monitoring records of miniature TBM parameters during the excavation phase; (a) face thrust pressure; and (b) advance speed.

During excavation, the tunnel remains under load, and analysis shows several cracks parallel to the tunnel axis. Thin rock fragments also detach, indicating spalling failure around the tunnel boundary. The surface is not smooth but wavy, which worsens the damage.

After epoxy resin was injected into the tunnel, the rock microstructure remained well-preserved. Macro photography of the half-axial and longitudinal cross-sections shows clear damage characteristics. However, the Excavation Influence Zone (EIZ) is hard to identify because the elastic deformation is not visible.

The damage in the tunnel wall is divided into four zones:

1. High-Damage Zone (HDZ), marked by significant brown discoloration and numerous cracks;
2. Excavation Damage Zone (EDZ), with slight color changes due to plastic deformation;
3. Construction Damage Zone (CDZ), appearing denser with a light gray color;
4. Spalling Zone, marked by thin material separation along the walls.

Spalling is associated with micro-cracks parallel to the wall, occurring due to extensional failure. Interestingly, the measured spalling depth is about 2 mm, or about 8% of the tunnel's radius. So, if we imagine a tunnel with a 10 m diameter, the spalling depth could reach 0.4 m!

Figure 11: Half cross-section of a rock tunnel lining along a 50 mm advance from the tunnel portal. HA stands for Half-Axial, while the following codes correspond to plate numbers and shot locations (upper, middle, and lower). Different types and colors of demarcation lines indicate the induced damage zones around the tunnel boundary.

This figure shows a half-section of the tunnel after a 50 mm advance, complete with close-up macro photos from three locations. You can see four separate rock fragments caused by spalling on the tunnel wall. These separated materials are long and super thin, detaching in parallel from the wall, clearly indicating spalling as the cause rather than random debris from the drilling process.

Figure 12: Half axial cross-section of a rock tunnel lining along a 100 mm advance from the tunnel portal. HA represents Half-Axial, followed by slab numbers related to distance from the portal and shot location (upper, middle, lower, and invert).

This figure shows four thin materials separating from the tunnel wall, with the Construction Damage Zone (CDZ) clearly visible around the tunnel boundary, about 0.2 mm thick. Significant color changes up to 5 mm indicate a High-Damage Zone (HDZ) surrounded by the Excavation Damage Zone (EDZ).

Figure 13: Longitudinal cross-section of the lined rock tunnel. LU and LL represent upper and lower longitudinal sections, with the following numbers indicating shot locations.

This figure shows the tunnel's longitudinal cross-section. You can see spalling of thin rock segments and the almost continuous Construction Damage Zone (CDZ) surrounded by the Excavation Damage Zone (EDZ). The boundary of these zones follows the wavy, irregular excavated tunnel wall.

Conclusion:

This study focuses on investigating damage in a model tunnel excavated in brittle synthetic rock using a true triaxial cell. A miniature TBM was used to excavate the scaled-down tunnel model under realistic in-situ stress conditions. Lab tests were conducted to understand the mechanical behavior of the rock, especially its brittle response and cracking in synthetic rock specimens.

After the experiment, macro photography of the failed tunnel boundary confirmed the damage and failure mechanisms. Spalling, resulting from increased stress and tangential stress around the tunnel boundary during stepwise loading, is a common failure mechanism. The findings are crucial for improving techniques in real-world tunnel construction!

Dari hasil eksperimen ini, kita bisa menarik beberapa kesimpulan yang penting:

Gambar 14. Hasil pelacakan yang didigitalkan pada perpindahan terowongan dari penampang longitudinal. (a) Diagram sebar dengan pemilihan titik dari invert dan mahkota terowongan. (b) Deformasi terowongan dan kegagalan spalling sepanjang profil longitudinal terowongan yang diberi beban dengan perbesaran 5x untuk tanah yang terdeformasi. Perlu dicatat bahwa penggalian awal dengan TBM miniatur menghasilkan bentuk terowongan yang tidak teratur.

Hasil penelitian ini memberikan beberapa kesimpulan penting, Sobat APT!

1. Terdapat tiga zona kerusakan yang teridentifikasi: Zona Kerusakan Konstruksi (CDZ), Zona Kerusakan Tinggi (HDZ), dan Zona Kerusakan Penggalian (EDZ), serta pemisahan material akibat terkelupas. Zona-zona ini punya kaitan erat dengan mekanisme kegagalan yang berbeda, mirip dengan kurva stres-regangan sebelum, saat, dan setelah puncak.
2. Model terowongan menunjukkan kegagalan terkelupas dengan ketebalan sekitar 2 mm, setara dengan 8% dari radius terowongan. Temuan ini penting banget untuk memahami potensi bahaya dalam konstruksi terowongan di kondisi in-situ yang sebenarnya.
3. Kegagalan ekstensional di dekat batas terowongan, yang terlihat dari pola kegagalan, sejalan dengan perilaku dinding terowongan dan sampel uji UCT, keduanya menunjukkan retakan yang sejajar dengan tegangan utama.
4. Investigasi pasca-eksperimen di penampang longitudinal juga mengungkap potensi lokalisasi geser dan mikro-retakan akibat profil terowongan "as-built" yang bergelombang, dengan HDZ terlokalisasi erat mengikuti batas akhir terowongan.

Ini semua penting untuk pengembangan dan perbaikan teknik dalam proyek terowongan, Sobat APT!