What contingency measures are implemented in the event of a loss of slurry flow?

Faced with a loss of slurry flow In hydro-shield microtunneling, contingency measures aim to stabilize the tunnel face, prevent ground loss, maintain hydraulic control, reduce slurry consumption, and prevent surface or environmental impacts. The response may include halting or slowing the advance, adjusting pressure and flow rate, modifying the slurry’s rheology, incorporating sealing materials, controlling backflow, verifying flow rates and pressures, enhancing monitoring, and, if the loss persists, applying ground stabilization treatments or reviewing the excavation method.

In works of microtunneling, pipe ramming, river and watercourse crossings o submarine emissaries, loss of circulation is particularly significant when working with hydro shield in permeable soils, gravel, loose sand, fractured rock, karst, cavities, backfill, or areas with hydraulic connectivity.

What does "loss of slurry circulation" mean?

Circulation loss occurs when some of the drilling mud escapes into the ground, fractures, voids, cavities, backfill, or preferential pathways, rather than returning through the return line to the separation plant.

Immediate Steps to Take in the Event of a Loss

The initial response should prioritize safety, stability at the front, and environmental control.

Common measurements:

• Slow down or temporarily stop moving forward.
• Maintain a face pressure consistent with the stability of the ground.
• Check the flow rates of the supply and return lines.
• Check the density, viscosity, and solids content of the slurry.
• Reduce the flow rate if the loss increases without improving the return.
• Avoid sudden movements that could widen the escape route.
• Check pumps, valves, sensors, and piping.
• Check whether the leak coincides with a geotechnical change.
• Perform auscultation on the surface or nearby structures.
• Check for any sludge discharge in wells, watercourses, manholes, or on the surface.
• Record the time, pressure, flow rate, volume lost, and position of the tunnel boring machine.

The decision to stop or continue should be based on a balance between the stability of the face, fluid loss, the risk of subsidence, and the sensitivity of the surrounding environment.

Slurry and Circuit Adjustments

If the loss is associated with permeable or fractured rock, the drilling fluid formulation can be modified to reduce filtrate and improve sealing.

The following can be evaluated:

These adjustments must be coordinated with the rheological properties of sludge and by calculating pressure drops in the slurry circuit.

Treatments if the loss persists

When the loss cannot be controlled through operational adjustments, additional measures may be necessary:

Surface treatment of the soil:
Injections, grouts, microcement, gels, jet grouting, or localized treatments to reduce permeability or seal voids.

Treatment from the well or the front line, if possible:
Localized injections, sealing, or specific interventions in entry/exit areas or accessible points.

Changing excavation parameters:
Adjustment of head pressure, flow rate, feed rate, pump speed, or slurry density.

Enhanced auscultation:
Topographic monitoring, piezometers, subsidence, deformations, flow rates, turbidity, and potential discharge points.

Review of the route or method:
If the leakage is associated with karst, cavities, highly fractured rock, or critical hydraulic connectivity, redesign, pretreatment, or a review of the construction strategy may be required.

In areas at risk of cavities, karst features, or holes, these contingencies must be taken into account as early as the design phase.

Risks That Must Be Managed

An unmanaged loss of circulation can lead to:

• Pressure drop across the orifice.
• Ground instability.
• Uncontrolled water ingress.
• Loss of fines.
• Over-excavation.
• Settlements.
• Appearance of mud on the surface.
• Damage to waterways, drainage systems, or utilities.
• Filling of voids or cavities.
• Higher consumption of bentonite, water, and additives.
• Blockages or malfunctions in the separation plant.
• Delays and cost overruns.
• Environmental Incidents.
• The need to stop the sinking.

At infrastructure crossings, rivers, urban areas, or environmentally sensitive areas, the action threshold should be more conservative.

How to Document an Emergency

Any loss of circulation must be documented to ensure technical and contractual traceability.

The following are recorded:

• The point on the route where it occurs.
• Date and time.
• Depth and coverage.
• Estimated land area along the front.
• Frontal pressure.
• Pump flow rate.
• Return flow.
• Estimated loss volume.
• Density and viscosity of the slurry.
• Additives added.
• Parameter changes.
• Progress before, during, and after the incident.
• Observations on the surface or in the riverbed.
• Auscultation readings.
• Measures Taken.
• Recovery results.
• Recommendations for the next section.

This documentation helps assess whether the loss is due to an anticipated geotechnical condition or a significant deviation from the project baseline.

Information Needed to Review the Project

To assess contingencies in the event of a loss of slurry circulation, the following information should be provided: excavation diameter, driving length, depth, cover, geotechnical data, permeability, grain size distribution, water table, water pressure, presence of gravel, fractured rock, karst, or cavities, expected slurry flow rate, face pressure, rheological properties, separation plant, pumping distance, allowable losses, environmental sensitivity, proximity to watercourses, utilities, or buildings, planned monitoring, availability of additives, sealing materials, grouting, ground treatment, shutdown criteria, and an emergency plan.

Request a Technical review of contingencies related to the loss of slurry circulation in a hydroshield Before finalizing geotechnical work, slurry formulation, the separation plant, or RFQ documentation.