During a hydro-shield microtunneling The rheological properties of the slurry must be controlled to stabilize the cutting face, transport the excavated material, limit seepage into the ground, and maintain a stable excavation cycle. The most important of these are density, viscosity, elastic limit, gel strength, filtrate, solids content, particle size distribution in suspension, pH, retained sand, and suspension stability.
In works of pipe ramming, microtunneling o crossings under rivers and waterways, the hydro shield It uses slurry—usually bentonite or conditioned mixtures—to maintain pressure at the face and transport the excavated material to the screening plant.
Critical rheological properties of slurry
| Property | What does this indicate? | Why It Matters at Hidroescudo |
|---|---|---|
| Density | Weight of sludge per unit volume | It affects head pressure, solids transport, and circuit stability |
| Viscosity | Fluid resistance to flow | It affects pumpability, pressure drops, and entrainment capacity |
| Elastic limit | Minimal effort required to get the flow going | Helps keep particles suspended and prevent sedimentation |
| Gel Strength | Ability to form a structure at rest | It reduces settling during shutdowns, but excessive use may make restarting more difficult |
| Filtered | Water loss into the ground | It affects cake formation, front stability, and fluid consumption |
| Solids Content | Percentage of particles incorporated into the sludge | It affects density, wear, separation, and circuit efficiency |
| Sand Content | Abrasive particles present in the fluid | Increases wear and tear on pumps, pipes, valves, and plant equipment |
| pH | Chemical condition of the fluid | It affects the stability of bentonite, additives, corrosion, and chemical compatibility |
| Suspension Stability | Ability to avoid segregation | Prevents sediment buildup, blockages, and pressure fluctuations |
| Temperature | Thermal condition of the fluid | It can modify viscosity, additive behavior, and pumpability |
Sludge Density
The density It affects the hydraulic pressure available at the cutting face and the slurry's ability to transport excavated solids. If the density is too low, it may not adequately contribute to the stability of the cutting face or to the transport of material. If it is too high, it can increase pressure drops, energy consumption, wear and tear, and the strain on pumps and pipelines.
During operation, the density of the feed slurry, return slurry, and recirculated slurry is monitored by comparing it after the sludge separation plant.
Viscosity and Pumpability
The viscosity It defines how easily the slurry flows through pipes, pumps, chambers, and return lines. Insufficient viscosity can reduce the ability to carry solids; excessive viscosity can lead to pressure drops, reduced flow rate, pump overload, and difficulties in the separation plant.
In a hydraulic shield, viscosity must balance three requirements:
- Keep the front stable.
- Transport solids to the surface.
- Enable efficient separation and recirculation.
Elastic limit and gel strength
The elastic limit and the gel strength They are important for keeping particles suspended, especially during flow reductions or temporary shutdowns.
If these values are too low, sand, silt, or heavy particles may settle in the circuit. If they are too high, pumping problems, increased pressure, difficulty restarting, or a loss of separation efficiency may occur.
These properties are particularly important in long sections, deep boreholes, or soils with variable grain sizes.
Filtration and Cake Formation
The filtered indicates the tendency of the mud to lose water into the ground. In a hydroshield, controlled seepage can help form a film or cake at the front, reducing permeability and contributing to stability. However, excessive seepage can lead to fluid loss, high consumption of bentonite or additives, local instability, and changes in working pressure.
Special attention should be paid to:
- Permeable sand.
- Gravel.
- Cracked land.
- Fractured rock.
- Areas with a high water table.
- Crossings under waterways or in sensitive areas.
Solids and Sand Content
The solids content It increases as the slurry carries excavated material. If the separation is insufficient, the recirculated fluid may return to the tunnel boring machine with too many solids, increasing its density, viscosity, abrasiveness, and wear.
The sand content is particularly critical because it increases wear on:
- Pumps.
- Pipes.
- Valves.
- Hydrocyclones.
- Excavation camera.
- Cutting head.
- Supply and return ducts.
For this reason, rheological control must be coordinated with the separation plant's performance and with waste management.
pH, additives, and chemical stability
The pH It affects the stability of bentonite, the effectiveness of polymers or additives, equipment corrosion, and compatibility with the soil or groundwater. Sudden changes in pH can alter viscosity, flocculation, clay dispersion, or separation performance.
In marine environments, saline soils, aggressive waters, or the presence of contaminants, it may be necessary to adjust the slurry formulation and monitor chemical compatibility during excavation.
How Sludge Is Managed During Construction
The check should be performed by comparing the prepared sludge, the feed sludge, the return sludge, and the sludge treated by the separation plant.
The following are commonly reported:
- Density.
- Viscosity.
- pH.
- Sand content.
- Total solids content.
- Filtered.
- Gel strength.
- Supply and return flow rates.
- Circuit pressure.
- Pressure drops.
- Temperature.
- Consumption of bentonite, water, and additives.
- Excavated volume.
- Volume of sludge generated.
- Performance of screens, hydrocyclones, and fine separation.
Monitoring this data allows us to adjust the formulation, detect changes in the ground, prevent circuit saturation, and maintain a stable face pressure.
Risks of Not Controlling Slurry Rheology
Poorly controlled rheology can lead to:
- Loss of stability at the front.
- Mud seepage into the ground.
- Sediment buildup in pipes.
- Circuit blockages.
- Increased pressure or pressure drops.
- Accelerated wear and tear on pumps and pipes.
- Difficulty in transporting solids.
- Overload of the separation plant.
- Excessive use of water, bentonite, or additives.
- Changes in frontal pressure.
- Lower feed rate.
- Unplanned stops.
- Increased waste volume.
- Environmental Risks Due to Poor Sludge Management.
At submarine emissaries o sea water catchments, these risks must also be coordinated with permits, turbidity control, discharges, stockpiles, and environmental management.
Information Required for Design or RFQ
To define the rheological control of slurry in hydroshield microtunneling, the following information should be provided: excavation diameter, drive length, shaft depth, anticipated circulation flow rate, face pressure, pumping distance, geotechnical conditions, particle size distribution, percentage of fines, permeability, water table, salinity, water pH, abrasiveness, slurry type, bentonite or planned additives, target density, target viscosity, allowable filtrate, maximum sand content, maximum solids content, separation plant capacity, recirculation system, discharge limits, waste management, environmental monitoring, and quality documentation requirements.
Request a Technical review of slurries, rheology, and separation plant for hydro-shield before finalizing the fluid formulation, the slurry circuit, or the RFQ documentation.
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