Tunnel boring machines are used under which conditions?

Tunnel boring machines are used under which conditions?

• A. when RQD is high and uniaxial compressive strength is moderate
• B. when RQD is low and UCS is high
• C. in soft clays with low groundwater
• D. in highly fractured rock with low strength

Answer: (A)

Tunnel boring machines work best when the rock mass is relatively intact and stable, with a balance of strength that allows efficient cutting without collapse or excessive cutter wear. A high RQD means the rock has few fractures and discontinuities, so the tunnel face remains coherent as the machine advances. Moderate uniaxial compressive strength indicates the rock isn’t too weak to hold the tunnel or too hard to cut quickly; it provides a stable face and predictable support needs. Under these conditions the TBM can bore smoothly, with continuous advance and consistent rock support requirements. In contrast, very fractured or weak ground (low RQD) or soft soils with groundwater present pose stability challenges that are better tackled with other methods or TBM types designed for soft ground. Soft clays with low groundwater, while potentially workable with certain TBM configurations, do not embody the stable, competent rock scenario where TBMs excel. Highly fractured rock with low strength leads to unstable faces and excessive ground control problems, making TBMs far less suitable.

Tunnel boring machines work best when the rock mass is relatively intact and stable, with a balance of strength that allows efficient cutting without collapse or excessive cutter wear. A high RQD means the rock has few fractures and discontinuities, so the tunnel face remains coherent as the machine advances. Moderate uniaxial compressive strength indicates the rock isn’t too weak to hold the tunnel or too hard to cut quickly; it provides a stable face and predictable support needs.

Under these conditions the TBM can bore smoothly, with continuous advance and consistent rock support requirements. In contrast, very fractured or weak ground (low RQD) or soft soils with groundwater present pose stability challenges that are better tackled with other methods or TBM types designed for soft ground. Soft clays with low groundwater, while potentially workable with certain TBM configurations, do not embody the stable, competent rock scenario where TBMs excel. Highly fractured rock with low strength leads to unstable faces and excessive ground control problems, making TBMs far less suitable.