How can clogging and clay balling be prevented during excavation with an EPB tunnel boring machine?

The clogging and the clay balling During excavation with an EPB tunnel boring machine, these issues are prevented by evaluating the soil’s plasticity, mineralogy, moisture content, grain size distribution, and adhesiveness prior to construction, and by adjusting—during excavation—the soil conditioning, foam, water, polymers, chamber pressure, shear torque, advance rate, and auger extraction. The goal is to prevent clays and fine particles from adhering to the cutterhead, chamber, cutting tools, or auger, forming sticky masses that reduce performance or block the system.

In projects of pipe ramming, microtunneling y infrastructure crossings, this risk must be analyzed particularly when the soil contains plastic clays, clayey silts, loams, wet cohesive soils, or materials with sticky behavior. The EPB tunnel boring machine It requires that the excavated material have a consistency suitable for maintaining face pressure and being removed in a controlled manner.

Difference between clogging and clay balling

These two phenomena are related, but they are not identical. Clogging refers to the obstruction or adhesion of material within the system; clay balling refers to the formation of clay balls that hinder excavation and extraction.

Which types of land pose the greatest risk?

The risk increases when the route passes through:

• Modeling clays.
• Clay loams.
• Marls or cemented cohesive soils.
• Soils with a high proportion of fine particles.
• Soils containing expansive or highly adhesive minerals.
• Materials with moisture content close to their plastic limit.
• Mixed surfaces consisting of clay and gravel.
• Soils whose consistency changes rapidly.
• Areas with enough water to activate adhesion, but not so much that it makes the material runny.

In these cases, the selection of the cutting head for clay, sand, gravel, rock, or mixed soil It must be coordinated with the site preparation and the EPB extraction system.

How to Assess Risk Before Excavating

Before construction begins, test results and geotechnical data are reviewed to predict the soil's adhesion behavior:

When there is uncertainty, it may be advisable to conduct laboratory tests using the soil itself and various soil conditioners.

Preventive Measures in Design

Prevention begins before pile driving. The design must take the following into account:

Proper cutting head:
Configuration of tools, openings, scrapers, and geometry to reduce buildup and facilitate the flow of material into the chamber.

Site preparation:
Selection of foams, polymers, water, or additives to modify the consistency of the excavated material, reduce adhesion, improve workability, and facilitate its removal.

Compatible extraction system:
The screw conveyor must be able to transport the material without compacting, jamming, or losing its ability to regulate pressure.

Front Pressure Control:
The chamber pressure must be maintained within the specified range without causing excessive compaction of the material.

Maintenance and Repair Plan:
On sites where clogging is a risk, provisions must be made for shutdowns, cleaning, spare parts, and procedures for responding to blockages.

Managing changes in terrain:
If the soil profile consists of alternating layers of clay, sand, gravel, or a mixture of these, the conditioning process must be adjustable as work progresses.

Operational Control During EPB Excavation

During excavation, certain parameters are monitored to help detect clogging or clay balling:

• Increase in cutting torque.
• Increase in energy consumption.
• Reduction in feed rate.
• Pressure fluctuations in the chamber.
• Irregular discharge through the auger.
• Blockages or pulsations during extraction.
• Changes in the consistency of the excavated material.
• Increased thrust.
• Vibrations or abnormal behavior of the head.
• Difference between the excavated volume and the expected volume.
• Consumption of foam, water, or polymer.
• The need for frequent cleaning or shutdowns.

If signs of blockage appear, the conditioning dosage, feed rate, rotation speed, chamber pressure, and extraction are adjusted in a controlled manner.

Risks of Not Controlling Clogging

Failure to properly manage clogging or clay balling can result in:

• Loss of performance.
• Unplanned stops.
• Tools or openings are jammed.
• Increase in cutting torque.
• Worm screw overload.
• Loss of front-end pressure control.
• Over-excavation or lack of uniform excavation.
• Alignment deviations.
• Increased tool wear.
• Increase in thrust.
• Risk of subsidence.
• Cost overruns and delays.

In construction projects beneath roads, railways, existing utility lines, or in urban areas, these effects can also impact the monitoring of deformations and the safety of nearby infrastructure.

Information Required for Design or RFQ

To evaluate and prevent clogging or clay balling in an EPB excavation, it is advisable to provide the following information: geotechnical profile, percentage of fines, Atterberg limits, natural moisture content, clay mineralogy, particle size distribution, plasticity, consistency, presence of loam or expansive clays, water table, permeability, excavation diameter, driving length, depth, overburden, type of cutting head, openings, tools, available torque, auger configuration, anticipated chamber pressure, conditioning system, proposed foams or polymers, laboratory tests, environmental sensitivity, and production control criteria.

Request a Technical Review of the Risk of Clogging and Clay Balling in Excavation Using an EPB Tunnel Boring Machine before finalizing geotechnical work, cutting head, site preparation, or RFQ documentation.