Why should double-hole tunnels with different geological conditions be excavated first?

Geological sketch work is divided into two parts: field work and indoor work. Now I mainly summarize the subway project you mentioned. The first step is to collect the geological conditions of the mine, including lithology, occurrence, structure and positional relationship. The second step is to sort out the data collected by field work and draw the geological conditions of one or two groups of tunnels according to a certain proportion. Need to attach a text description. If conditions permit, some special geological areas can be used.