Under critical infrastructures, microtunneling (typically in pipe ramming) is limited mainly by three “red lines”: admissible movements (seating/lifting), front and water control, y holder's requirements (permits + auscultation + method).
1) Settlement and deformation limit (dominant criterion)
In rail/metro/highway, movement thresholds are often very strict and require the crossing to be designed for minimize volume losses and maintain a steady progress (no “bad” stops). This conditions the method (usually closed shield tunnel boring machines if water or granular soils are present), the excavation plan and the contingency plan.
2) Licensee's requirements: permits, methodology and specific studies
Additional documentation (construction method, risk assessment, auscultation) and specific studies are usually required for high-capacity railways/metro lines or high-capacity roads. settling/vibration depending on the manager (e.g., road or rail administrations). In practice, this is managed as part of the management of subway infrastructure crossings.
3) Front and groundwater pressure monitoring
If there is high water table or high pressures, the constraint becomes keeping the face balanced to avoid seepage, fines entrainment and settling. In these scenarios, the need for pressurized shields (EPB/hydro-shield) and a pressure control strategy is reinforced; see management of pressures under high groundwater with hydroshields y closed shield hydroshield.
4) Geometric tolerances and “as-built” traceability”
At critical crossings, the execution is limited by shaft/quota tolerances and by the need for robust geometric control (laser/gyro, topographic network, etc.). Eurohinca details the approach in alignment, curvature and dimension control y real-time alignment control; The following table shows typical tolerances for control and monitoring during execution in crossroads management.
5) Implementation: where do the wells fit (and what size can they be)?
The typical “land” limitation is that you cannot always locate wells where you would like to (exclusion zones, easements, traffic, access). That conditions lengths, logistics and structural design of the well (reaction wall, hoisting, mud plant). See how to dimension the attack and reception wells y requirements for civil works and pile-driving wells.
6) Requirement for on-site instrumentation and control (not “optional”)
Under critical infrastructures, the operational constraint is that you have to execute with continuous monitoring (machine parameters + external auscultation) and with defined action thresholds. See microtunneling production, quality and safety metrics.
If you want to “get this down to spec” (thresholds, monitoring plan, method, pressure/feed rates and permit documentation), the most efficient way to do it is from Technical assistance and engineering and prepare the bid with the minimum info in what an RFQ should include to value a crossover.
ES 
EN 
FR 