Mixed-Face TBM Carbide Grade Selection — Soft vs Hard Ground Guide

Q: How do I choose the right carbide grade for mixed-face TBM conditions?

For mixed-face TBM conditions where soft ground and hard rock occur simultaneously across the cutterhead, start with SR8C as the baseline grade. SR8C at HRA 89.0 with 2.0–3.0 µm grain and 8% cobalt provides balanced toughness and wear resistance. If hard rock zones exceed UCS 120 MPa or fracture events exceed 15% of removed tools, upgrade those face zones to SR10C. For dominant soft-ground sections with low impact, SR8C remains the primary grade.

Q: What is the difference between SR7X and SR8C for mixed-face TBM tunneling?

SR7X runs at HRA 91.0 with 1.0–1.2 µm grain size and approximately 6% cobalt, optimized for pure abrasion resistance in uniform rock. SR8C runs at HRA 89.0 with 2.0–3.0 µm grain and approximately 8% cobalt, with flexural strength ≥ 2,200 MPa. In mixed-face TBM conditions, SR8C's coarser grain and higher cobalt allow it to absorb impact from hard rock zones without fracture, while SR7X would chip and shatter at the soft-hard boundary.

Q: Which carbide grade performs best under high-impact conditions in mixed-face TBM tunneling?

SR10C performs best when impact loading dominates. At HRA 88.0 with approximately 10% cobalt and flexural strength ≥ 2,200 MPa, SR10C maximizes fracture resistance at the cost of some wear performance. In mixed-face TBM conditions where hard rock inclusions or boulder zones generate repeated impact events that would shatter SR7X, SR10C survives the shock loading. The trade-off is accelerated wear in the soft-ground zones.

Q: How does cobalt content affect carbide performance in mixed-face TBM applications?

Cobalt content determines the toughness-hardness balance in mixed-face TBM conditions. Higher cobalt — 8% in SR8C and 10% in SR10C — increases flexural strength and impact resistance, allowing the carbide to survive the shock loads generated at the boundary between soft and hard ground. Lower cobalt around 6% as in SR7X produces higher HRA but creates brittleness. In mixed-face conditions where both abrasion and impact occur in the same cutterhead revolution, a cobalt range of 8–10% is the practical operating window.

Q: What causes premature carbide tip failure in mixed-face TBM tunneling?

Premature failure in mixed-face TBM conditions is most often caused by using a single grade across the entire cutterhead when the face contains zones of different hardness. Tools passing through soft ground survive while those crossing hard rock boundaries fracture, creating uneven loading that damages adjacent tool holders. Ruixin recommends face-zoning the cutterhead — specifying SR8C for soft zones and SR10C for hard rock zones — and tracking tip failure mode per radial position to validate the zone mapping.

Why Mixed-Face TBM Conditions Destroy Carbide, and Why a Single-Grade Solution Fails

A metro tunneling project in eastern China was driving through alluvial soils with a planned alignment 30 meters below the water table. The geotechnical survey showed uniform soft ground for the first 1,800 meters. At ring 912, the cutterhead encountered a buried granite boulder horizon — not a formation change, but scattered boulders embedded in soft clay across the lower half of the face. Tools on the left side of the cutterhead were cutting clay. Tools on the right side were hitting granite. The same carbide grade was installed on every tool.

Within two shifts, picks on the right side had lost tips to fracture. The unbalanced load then overloaded the left-side tools, which were designed for soft clay wear patterns. The entire cutterhead was retooled at ring 920 — an unplanned intervention that cost the project three days and ¥180,000 in replacement tooling.

The grade was not defective. The grade selection logic was: it assumed uniform face conditions.

Mixed-face TBM tunneling (where the cutterhead simultaneously encounters soft ground and hard rock across the same face) is not a geological transition problem. It is a load-distribution problem that destroys carbide in a pattern no uniform grade selection can solve. The tools in the hard rock zone fracture from impact. The tools in the soft ground zone overload because the fractured tools stop cutting. The failure cascades.

For a system-level diagnosis before changing carbide, continue with the TBM carbide cutting tools.

For projects where mixed-face conditions are confirmed or suspected, mixed-face TBM carbide grade selection must account for face zoning, not just formation type.

Mixed-face TBM cutterhead encountering both soft soil and hard rock simultaneously during tunnel excavation

Why Mixed-Face Conditions Create a Different Failure Mode

Sequential formation transitions are gradual. The entire cutterhead moves from soft to hard ground over several meters, and all tools experience the same transition at the same time. Mixed-face conditions do not work that way.

In a mixed-face TBM condition, tools on the left side of the cutterhead cut soft clay while tools on the right side cut granite — in the same revolution.

This creates three failure mechanisms that do not exist in uniform ground:

1. Differential tool loading. The tools in the hard rock zone experience 3–5× the cutting force of tools in the soft ground zone. The harder-ground tools wear or fracture faster, reducing their gauge height relative to the soft-ground tools.

2. Load redistribution. When hard-zone tools lose height or fracture, the cutting load shifts to the remaining taller tools, which are concentrated in the soft ground zone. These tools are now cutting material they were not designed for, at forces they were not designed to handle.

3. Cutterhead imbalance. Uneven tool wear creates eccentric loading on the cutterhead bearings. The resulting vibration accelerates wear on every tool regardless of face position.

The failure isn’t random. It is the predictable result of specifying a single carbide grade for a non-uniform face.

The Technical Variables That Control Mixed-Face TBM Performance

For mixed-face TBM carbide grade selection, three material variables determine whether the tools survive the simultaneous soft-hard contact cycle.

Hardness (HRA) and Abrasion Resistance

Higher HRA provides better resistance to abrasive wear in the soft ground zone. In clay, silt, and sandy soils with low quartz content, a harder grade extends tip life by resisting the fine abrasion of ground particles.

The constraint: high-HRA grades (HRA 91+) are brittle. When the same tool encounters a hard inclusion or rock boundary, the tip fractures rather than deforming. In mixed-face conditions, the tool cannot choose which ground it cuts — it must survive both.

Cobalt Content and Fracture Toughness

Cobalt is the binder that holds the WC grains together. Higher cobalt content increases flexural strength and impact resistance. In mixed-face TBM conditions, the cobalt content determines whether the tip survives the shock of transitioning from soft clay to hard rock within a single pass.

The relationship is inverse: increasing cobalt from 6% to 10% drops HRA from approximately 91 to 88, but flexural strength rises from approximately 2,000 to 2,200 MPa or higher. For mixed-face TBM carbide grade selection, the toughness gain from higher cobalt is worth the hardness loss, because fracture failure in mixed-face conditions is more expensive than accelerated wear.

Grain Size and Microstructural Stability

Grain size controls how stress distributes through the carbide matrix under impact loading. Coarser grains (2.0–3.0 µm) create more cobalt at grain boundaries, which absorbs impact energy through micro-deformation rather than cracking.

In mixed-face conditions, grain size is the least-discussed but most influential variable. Ruixin SR8C at 2.0–3.0 µm survives the repeated micro-impacts of mixed-face cutting because the coarser grain structure allows controlled stress distribution. Finer grains at the same cobalt content would concentrate stress at grain boundaries and initiate cracks.

Grade Options and Performance Trade-offs for Mixed-Face TBM

Application Scenario	Recommended Grade	Key Parameters	Why This Grade
Soft ground dominant (clay, silt, sand) with occasional hard inclusions < 80 MPa UCS	SR8C	HRA 89.0 ± 0.5, grain 2.0–3.0 µm, flexural strength ≥ 2,200 MPa	Balanced hardness handles abrasion in soft zones; coarse grain absorbs impact from occasional hard inclusions without fracture
True mixed-face (soft ground and hard rock > 80 MPa UCS simultaneously across face)	SR8C baseline + SR10C in hard zones	SR8C: HRA 89.0, cobalt ~8%; SR10C: HRA 88.0, cobalt ~10%, flexural strength ≥ 2,200 MPa	Face-zoning — SR8C for soft zones, SR10C for hard zones — prevents the failure cascade from differential tool wear
Hard rock dominant (UCS > 120 MPa) with soft infill or inclusions	SR10C	HRA 88.0 ± 0.5, grain 2.0–3.0 µm, flexural strength ≥ 2,200 MPa	Maximum cobalt content prevents impact fracture; wear rate in soft zones is an acceptable trade-off for fracture prevention
Boulder-rich ground, high-impact, unpredictable face	SR10C	HRA 88.0, cobalt ~10%, density 14.45 g/cm³	Highest impact toughness for unpredictable loading; highest resistance to catastrophic tip loss events

The right choice depends on the ratio of hard to soft ground across the face and the UCS of the harder fraction. For most projects with confirmed mixed-face conditions, SR8C is the baseline grade and SR10C is the hard-zone reinforcement.

Face-Zoning the Cutterhead: A Practical Mixed-Face Grade Strategy

In a mixed-face TBM condition where the geological survey or probe drilling confirms that different ground types occupy different sectors of the cutterhead, the most cost-effective approach is face-zoning: specifying different grades for different radial or angular positions on the cutterhead.

Radial Zoning

If the mixed-face condition is horizontal (water table interface, weathered rock surface), the harder ground sits below the softer ground. Tools on the lower half of the cutterhead require a tougher grade (SR10C) while tools in the upper half can run SR8C. This requires the tooling supplier to deliver two grades in identical geometries so that the pick body and mounting system do not change.

Sector Zoning

If the mixed-face condition includes localized hard inclusions (boulder zones, buried rock pinnacles), the hard zones occupy specific angular sectors of the face. Tools in those sectors run SR10C; the remainder runs SR8C. Sector zoning requires advance knowledge of the hard zone location from probe drilling or seismic face mapping.

Transition Buffer Zone

Regardless of zoning strategy, tools at the boundary between the soft and hard face zones experience the most severe loading: they cut intermittently through both materials. Zone a 15–20° buffer area on either side of the boundary with SR10C to prevent boundary-line fracture.

What Happens When You Choose the Wrong Mixed-Face Grade

1. Single high-hardness grade across a mixed face (SR7X).

Specifying SR7X at HRA 91.0 across a mixed face guarantees tip fracture in the hard rock zones within 50–100 meters. Tip life in the hard zones drops by 60–80% compared to SR10C at HRA 88.0. Fractured tips then damage adjacent tool holders, and the replacement cost per tool-holder repair adds 30–50% to consumable costs. The imbalance from differential wear doubles replacement frequency across the entire cutterhead.

2. Single high-toughness grade across a mixed face (SR10C).

Running SR10C on a predominantly soft-ground mixed face with only intermittent hard zones increases wear rate in the soft zones by 20–35% compared to SR8C. The cost per meter rises because tools in the soft zones wear faster than necessary, driving up total tool consumption across the drive.

3. Uniform grade selection without face zoning.

Even SR8C, the best single-grade compromise, underperforms when the hard rock zone exceeds 30% of the face area. Tools in the soft zone wear prematurely because the hard-zone tools fail and redistribute load. The optimum strategy is always face-zoned: SR8C in the soft zone, SR10C in the hard zone.

4. Ignoring grain size in grade selection.

Selecting by HRA and cobalt content alone without considering grain size produces acceptable short-term results but inconsistent long-term performance. At identical cobalt content, fine-grain grades (1.0 µm) fracture at impact loads that coarse-grain grades (2.5 µm) survive. In mixed-face TBM carbide grade selection, grain size is the parameter that separates surviving from failing at the soft-hard boundary.

How to Implement Mixed-Face Grade Strategy in Your TBM Operation

Pre-Project Steps

1. Map the face by zone. From the geotechnical report and any pre-drilling data, determine the angular and radial positions of the hard ground fraction. A probe hole or face camera inspection during the first intervention provides ground truth.

2. Specify grade per zone. Order SR8C for the soft sectors and SR10C for the hard sectors. Both grades must share the same tip geometry and mounting dimensions to allow installation on the same pick body. Ruixin manufactures SR8C and SR10C in identical geometries for this purpose — see our shield machine carbide tips for available geometries and batch specifications.

3. Mark and track zones. Cutterhead tools should be physically marked by zone during installation. Use a color code or alphanumeric marking system linked to the cutterhead position map.

During the Drive

4. Inspect at first intervention. Schedule the first cutterhead inspection earlier than normal, within 100 meters of entering mixed-face ground. Inspect tools from each face zone separately. If fracture events in the hard zone exceed 15% of installed tools, the grade is too brittle. If wear in the soft zone exceeds 50% of expected life, the grade is too soft.

5. Validate and adjust. Mixed-face TBM carbide grade selection is not a one-time decision. If the face changes (new probe data, harder or softer zones than expected), adjust the zone mapping and re-specify grades for the affected zones.

For additional context on how rock abrasivity interacts with carbide grade selection in TBM applications, see our TBM carbide cutting tools guide, which covers formation-based grade matching. For a deeper understanding of how cobalt content and grain size trade off against each other, our cemented carbide education article explains the material science behind every grade decision.

Ruixin SR8C and SR10C cemented carbide tips for TBM shield machine cutterhead face-zoning

Frequently Asked Questions

How do I choose the right carbide grade for mixed-face TBM conditions?

Start with the proportion of hard ground across the face and its UCS value. If hard ground occupies less than 30% of the face and UCS stays below 100 MPa, SR8C at HRA 89.0 with 2.0–3.0 µm grain is the correct single-grade choice. If hard ground exceeds 30% or UCS exceeds 100 MPa, face-zone the cutterhead: SR8C for soft zones, SR10C for hard zones. Send us your geotechnical survey data and probe hole results, and Ruixin will confirm the zone-by-grade recommendation within 24 hours.

What is the difference between SR7X and SR8C for mixed-face TBM tunneling?

SR7X runs at HRA 91.0 with 1.0–1.2 µm grain and approximately 6% cobalt, optimized for pure wear resistance in uniform abrasive ground. SR8C runs at HRA 89.0 with 2.0–3.0 µm grain, approximately 8% cobalt, and flexural strength ≥ 2,200 MPa. It is engineered for mixed loading where impact and abrasion coexist. In mixed-face TBM conditions, SR7X fails by fracture at the soft-hard boundary within a shift. SR8C survives the same zone for hundreds of meters.

Which carbide grade performs best under high-impact conditions in mixed-face TBM tunneling?

SR10C. At HRA 88.0 with approximately 10% cobalt and flexural strength ≥ 2,200 MPa, SR10C provides the highest fracture toughness in the Ruixin grade range. In mixed-face conditions where the cutterhead encounters boulders, rock pinnacles, or hard rock zones above 120 MPa UCS, SR10C absorbs the impact events that would destroy SR7X or stress-crack SR8C. The wear rate penalty in soft zones is real — approximately 15–25% faster than SR8C — but acceptable when fracture prevention is the priority.

How does cobalt content affect carbide performance in mixed-face TBM applications?

Cobalt content directly controls impact resistance. Ruixin SR8C at approximately 8% cobalt provides the baseline toughness for mixed-face survival. Ruixin SR10C at approximately 10% cobalt raises flexural strength further to handle extreme impact. Below 6% cobalt (SR7X range), the grade lacks the toughness to survive the shock loading at the boundary between soft and hard ground. The practical operating range for mixed-face TBM carbide grade selection is 8–10% cobalt.

What causes premature carbide tip failure in mixed-face TBM tunneling?

Two root causes cover most premature failures. First is using a single grade across the entire cutterhead when the face contains both soft and hard ground: tools in the hard zone fracture, tools in the soft zone overload. Second is specifying grade by HRA only without considering grain size or cobalt content. A tip failure analysis should begin with the face zone the tool was cutting: if failed tools cluster in one angular position, the mixed-face condition is the cause, not the grade quality. Ruixin provides batch-level CoA with every shipment so that grade quality can be ruled in or out as a variable.

Get a Custom Grade Recommendation for Your Mixed-Face TBM Project

Send us your geological survey data — formation type per face zone, UCS range, Cerchar Abrasivity Index where available, machine type (EPB, slurry, open-face hard rock TBM), and cutterhead geometry — and our engineers will produce a zone-by-zone grade recommendation with matching geometries within 24 hours.

Ruixin manufactures SR7X, SR8C, and SR10C in identical tip geometries for face-zoned cutterhead installations. OEM drawings accepted. Custom grade formulation available for non-standard formation profiles.

Email: info@ruixintungstencarbide.com
WhatsApp: +86-15253178777

Mixed-Face TBM Grade Selection — Soft/Hard Guide | Ruixin