How is the risk of surface seating managed in sensitive urban environments?

Q: How is the risk of surface seating managed in sensitive urban environments?

The risk of settlement in urban environments is managed by a combination of: (1) a geotechnical and hydrogeological investigation aimed at identifying sensitive soils, strata changes and water pressure; (2) selection of the method and equipment that best controls face stability (usually EPB closed shields or hydro-shield when water or granular soils are present); (3) real-time monitoring of excavation parameters to maintain volume balance and face pressure within safe ranges (advance, torque, thrust, extraction/return and conditioning); (4) auscultation with warning/alarm/stop thresholds (surface and building topography, utility control, piezometry if applicable) and associated actions; (5) geometric control of alignment and elevation to avoid abrupt corrections; and (6) a contingency plan to act on deviations, adjusting parameters and applying corrective measures compatible with the design.

In urban environments (buildings, services, traffic and existing networks), the risk of seating is managed by a combination of geotechnical design, method/equipment selection, real-time parameter control y auscultation with alarm thresholds. In projects of pipe jacking (microtunneling), The objective is to avoid volume losses in the field and maintain a stable front.

1) Reducing uncertainty before work (design and planning)

Geotechnical and hydrogeological campaign aimed at identifying sensitive soils, abrupt changes and the presence of water; it results in a risk model and preventive measures (see geotechnical risks and mitigation).
Selection of the excavation system that best controls the stability of the face in urban areas: typically closed shield tunnel boring machines (EPB or hydroshield) when there is a water table, granular soils or high sensitivity to settling; see also choice of tunnel boring machine.
Design of wells and civil works to avoid movements induced by auxiliary excavations: how to dimension attack and reception wells y civil works and well requirements.

2) Control of the face and volume balance during excavation (which has the greatest impact on seating).

Maintain a adequate chamber pressure to balance the soil and water:
- In fines/cohesives it is usually applied to EPB closed shield.
- In saturated granulars or at high pressures, it is usually preferable to closed shield hydroshield and a strategy of pressure management under high groundwater.
Coordinated adjustment of advance, torque, thrust, pull-out/return (especially in slurry) to avoid over-excavation, loss of fines or “voids” that result in settling.
Lubrication and friction control to avoid blockages and forced stops (poorly managed stops increase the risk of pressure losses and deformations), within the approach of geotechnical risks and mitigation.

3) Auscultation and “observational method” (measure-compare-act).

A monitoring plan is defined with thresholds (alert/alarm/stop) and associated actions.
Real-time monitoring of digging and quality parameters, see metrics to be monitored in a microtunnel.
Typical external survey (according to criticality): topographic control of surface and buildings, control of sensitive services, piezometry if applicable. At intersections with administrative requirements (roads/railways), the approach is integrated in the management of subway infrastructure crossings.

4) Geometric accuracy for minimizing aggressive corrections
Maintaining tolerances and avoiding abrupt corrections helps to reduce lateral loads and ground disturbance: alignment, curvature and dimension control y real-time alignment control.

5) Contingency plan (what to do if entries appear)
In sensitive urban areas, in addition to prevention, a “plan B” is defined: immediate adjustment of parameters (pressure/advance/extraction), reinforcement of auscultation, and corrective measures compatible with the method (e.g., compensation/injection actions according to design), coordinated by Technical assistance and engineering.

In typical urban networks (sewers, water supply, crossings), these practices are systematically applied in the following areas urban sewage systems, urban supply systems y infrastructure crossings.