(1) Selection principles

The selection of the shield machine is mainly determined, after technical and economic comparison, based on engineering geological and hydrogeological conditions, tunnel cross-section shape, tunnel overall dimensions, tunnel burial depth, underground obstacles, underground structures, ground buildings, surface settlement and heave requirements, and so on. The main selection principles for the shield machine are: applicability, technical advancement, and economic rationality.

1) Principle of applicability

The cross-section shape and overall dimensions of the shield machine are applicable to the tunnel cross-section shape and overall dimensions, and its type and performance are applicable to the service conditions such as engineering geological and hydrogeological conditions, tunnel burial depth, safety requirements for underground obstacles, underground structures and ground buildings, and surface settlement and heave requirements. If the selected shield machine cannot fully meet the above service conditions, applicable auxiliary construction methods, such as compressed-air method and grouting method, should be added to ensure excavation face stability.

Since a shield machine has a relatively long reasonable service life and can be used for multiple construction projects, the shield machine should be selected according to the commonly expected service conditions during its service life, or the most unfavorable service conditions, so as to have relatively broad applicability.

2) Principle of technical advancement

Technical advancement has two meanings: first, different types of shield machines have different levels of technical advancement; second, the same type of shield machine has different levels of technical advancement due to differences in equipment configuration and functions.

Selecting a technically advanced shield machine, on the one hand, better adapts to the current and future engineering construction requirements of the construction unit and improves the market competitiveness of the construction unit; on the other hand, it maintains technical advancement during the reasonable service life.

Technical advancement must be premised on reliability, and advanced technologies that have been verified through engineering practice and have high reliability should be selected.

At present, the most technically advanced shield machines are the earth pressure balance shield machine and the slurry balance shield machine. With the continuous improvement and enhancement of design and manufacturing technology, their scope of application has become increasingly broad, and they have become the most widely used shield machines in shield tunnel construction.

3) Principle of economic rationality

Economic rationality means that when the selected shield machine and its auxiliary construction methods are used for engineering project construction, their comprehensive construction cost is reasonable on the premise of meeting construction safety, quality standards, environmental protection requirements, and schedule requirements.

(2) Selection procedure

Whether the shield machine is selected correctly or not affects whether normal tunneling construction can be carried out, and especially affects construction safety. Therefore, a scientific method must be adopted, and the selection must be made scientifically and reasonably according to a feasible procedure through steps such as planning, investigation, feasibility study, and comprehensive comparison and evaluation.