The Final grouting of the annular space in a microtunnel It consists of filling, in a controlled manner, the void between the outside of the driven pipe and the excavated soil. Its purpose is to stabilize the area surrounding the pipeline, reduce residual voids, limit deferred settlement, improve soil-pipe contact, and leave the site in suitable condition for the pipeline’s service life.
During the pipe ramming or the Microtunneling in Above-Ground and Underground Applications, the annulus may be temporarily filled with bentonite or lubricating slurry to reduce friction and driving forces. The technical literature on microtunneling describes this use of bentonite in the annulus during tunneling and subsequent backfilling of the borehole as a measure to reduce the risk of collapse or soil loss.
Why Is There an Annular Space in Microtunneling?
The annular space occurs because the tunnel boring machine's excavation diameter is usually slightly larger than the pipe's outer diameter. This overcut allows the machine to advance, guides it, and reduces friction between the pipe and the ground.
During execution, that space can be used for:
- Inject bentonite or lubricating fluid.
- Reduce driving forces.
- Compensate for soil-pipe friction.
- Help stabilize the immediate environment.
- To facilitate movement over long distances or in areas with higher friction.
Lubrication of the annular space is an important practice in pipe jacking and microtunneling because it can reduce thrust forces and improve advance performance.
Objectives of the Final Injection
The final injection should not be confused with lubrication during driving. Its main purpose is fill and stabilize the annular space once a section or phase of advancement has been completed.
Its main functions are:
| Objective | What he brings to the project |
|---|---|
| Filling Residual Gaps | Reduces gaps between pipes and the ground |
| Limit deferred seats | Reduces the risk of subsequent ground settlement |
| Improve ground-to-pipe contact | It provides more consistent perimeter support |
| Replace or displace temporary fluids | Reduces reliance on bentonite or construction slurry |
| Seal priority waterways | It can help reduce seepage, depending on the mixture and soil type |
| Improve closure traceability | It allows you to record volumes, pressures, and injection zones |
At infrastructure crossings, urban areas, roads, railways, or critical infrastructure; this phase is particularly important because residual voids can contribute to delayed settlement or deformation.
How the Final Injection Is Designed
The design must define mixing, pressure, volume, sequence, injection points, and stop criteria. It’s not just a matter of “pumping grout”: the grouting process must be adapted to the soil conditions, the diameter, the undercut, the water table, and the sensitivity of the surrounding environment.
1. Theoretical volume of the annulus
It is calculated based on the excavation diameter, the outer diameter of the pipe, and the length of the section. This volume serves as an initial reference, but the actual volume may vary due to losses to the soil, irregularities, over-excavation, voids, permeability, or prior consumption of bentonite.
2. Type of mixture
The mixture may consist of a cement slurry, bentonite-cement slurry, fluid mortar, or another formulation compatible with the project. It must have sufficient workability to fill the annulus without generating pressures that could damage the pipe, displace the soil, or cause heaving.
3. Injection pressure
The pressure must be sufficient to distribute the mixture, but must remain below the permissible limits for the soil, the pipe, the joints, and the surrounding area. Excessive pressure can cause heaving, hydraulic fracturing, unwanted seepage, or damage to nearby utilities.
4. Injection Points
Injection can be performed through openings or ports in the pipeline, points in the well, or specific devices, depending on the system design. The location and spacing of the injection points must allow for gradual and traceable filling of the annulus.
5. Forward sequence
The sequence is defined to prevent unfilled pockets and control displacement. It can be carried out in phases, in sections, or starting from alternate points, depending on length, slope, diameter, accessibility, and observed behavior.
6. Completion Criteria
The grouting operation is considered complete when defined criteria regarding volume, pressure, return flow, controlled discharge, stabilized consumption, or confirmation through site records are met. These criteria must be established before the operation begins.
How It's Implemented on Site
Execution typically follows a controlled sequence:
- Inspection of the completed section, piping, joints, and injection points.
- Mix Preparation and Dosing Control.
- Inspection of equipment, pumps, hoses, pressure gauges, and valves.
- Gradual injection starting from the defined points.
- Continuous monitoring of pressure, volume, and flow rate.
- Monitoring for returns, leaks, or abnormal behavior.
- Change the injection point according to the sequence.
- Record of the actual volumes injected by section.
- Cleaning of pipes or injection ports, if necessary.
- Final validation and closing documentation.
In sensitive construction projects, grouting must be coordinated with monitoring, topographic surveys, and monitoring of existing utilities.
Which parameters are monitored?
During the final injection, the following are recorded:
- Theoretical volume of the annular space.
- Actual volume injected.
- Injection pressure.
- Pump flow rate.
- Injection site.
- Time and sequence.
- Type of mixture and dosage.
- Density, fluidity, or viscosity of the mixture.
- Observed returns.
- Abnormal consumption or leakage into the ground.
- Possible leaks in wells or joints.
- Sitting up, standing up, or moving if auscultation is being performed.
- Incidents and Corrective Actions.
These records are important for tracking the completion of the project and for verifying that the annular space has been treated in accordance with the procedure.
Risks of a Poorly Designed Injection
A poorly planned final injection can cause:
- Unfilled residual voids.
- Deferred seats.
- Excessive grout consumption.
- Losses to the ground.
- Excessive pressure on pipes or joints.
- Surface lifts.
- Leaks into utilities, drains, or wells.
- Hydraulic fracturing of the ground.
- Blockage of injection ports.
- Lack of traceability regarding the actual volume installed.
Therefore, the procedure must be coordinated with the choice of tunnel boring machine, pipe design, the lubrication system, thrust control, and the technical assistance and engineering for trenchless construction sites.
Information Required for Design or RFQ
When designing or reviewing the final grouting of the annular space, it is advisable to provide the following information: excavation diameter, outer pipe diameter, driving length, planned over-excavation, alignment geometry, slope, geotechnical conditions, permeability, water table, presence of voids or backfill, lubrication system used, pipe type, joint type, allowable pressure, available grouting points, environmental sensitivity, planned monitoring, rejection criteria, proposed mixture, and final documentation requirements.
Request a Technical review of the final grouting of the annular space in microtunnels before completing the pile driving process, the grout mixing, or the RFQ documentation.
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