When I evaluate an FB-39 Friction Bolt for a tunneling project, I begin with three questions: whether the drilled hole can remain within the 35–38 mm range, whether the required anchorage fits the bolt’s tested capacity, and whether the steel finish matches the underground environment. The 39mm rock bolt is generally used for underground mining, tunnel reinforcement, strata mesh pinning, and primary or secondary support where immediate friction anchorage is required.
| Use case | Required hole range | Selection caution |
|---|---|---|
| Mesh pinning and secondary support | 35–38 mm | Confirm plate and mesh compatibility |
| General tunnel reinforcement | 35–38 mm | Verify design anchorage and bolt length |
| Primary support with longer bolts | 35–38 mm | Confirm geology, capacity, and installation access |
| Wet or corrosive underground areas | 35–38 mm | Specify hot-dip galvanizing or another approved coating |
An FB-39 friction bolt is a 39 mm slotted steel tube used to reinforce rock around underground excavations. Its standard length range is 600–3000 mm, with longer lengths commonly selected when the bolt must penetrate beyond the loosened rock zone. Unlike resin or cement-grouted anchors, the FB-39 develops holding resistance through continuous contact between the expanded tube and the borehole wall.
The tube includes a longitudinal slot, a tapered installation end, and a ring or collar assembly used with a bearing plate. The supplied dimensions include a 30 mm taper-end diameter, a 65 mm taper length, a 17 mm tube slot width, and material thicknesses from 2.0–2.5 mm. These dimensions affect insertion behavior, contact pressure, steel deformation, and the bolt’s response to installation equipment.
During installation, a hydraulic rock-bolting machine drives the tube into a hole that is slightly smaller than the nominal bolt diameter. The tube compresses as it enters the hole, creating radial pressure and friction along the contact surface. This mechanism allows the bolt to begin supporting the surrounding rock immediately after installation, provided that the hole diameter, tube condition, drive energy, and plate seating are controlled.
The FB-39 does not depend on a resin cartridge curing around a threaded bar. Instead, its split tube is forced into a borehole with a recommended nominal bit size of 35–38 mm. The difference between the nominal tube diameter and the hole diameter produces interference, which presses the steel against the borehole wall.
The resulting anchorage depends on several variables rather than diameter alone. Rock roughness, fracture frequency, groundwater, hole cleanliness, drilling deviation, tube thickness, and the length of effective contact all influence the final holding behavior. For this reason, I treat the stated load figures as product and test references, not as a substitute for a project-specific ground-support design.
The FB-39 data lists a minimum yield strength of 345 MPa, equivalent to approximately 85 kN, and a typical yield strength of 445 MPa, or approximately 110 kN. The tube ultimate tensile strength is listed at a minimum of 470 MPa, approximately 115 kN, with a typical value of 530 MPa, approximately 130 kN. The listed typical breaking capacity is 124 kN, while the minimum breaking capacity is 89 kN.
I recommend that buyers review the entire specification sheet rather than selecting a bolt only by its nominal diameter. A 39 mm tube may be suitable for one tunnel heading and unsuitable for another if the hole-quality distribution, design load, water chemistry, or installation equipment differs. The following values are the key product data available for the TRM FB-39 product range.
| Specification | FB-39 value |
|---|---|
| Nominal bolt diameter | 39 mm |
| Standard bolt length | 600–3000 mm |
| Recommended bit size | 35–38 mm |
| Hole diameter range | 35–38 mm |
| Taper-end diameter | 30 mm |
| Taper length | 65 mm |
| Tube slot width | 17 mm |
| Taper slot width | 2 mm |
| Material thickness | 2.0–2.5 mm |
| Ring wire gauge | 6 mm |
| Ring open gap | 5.0–6.0 mm |
| Cross-sectional area | 245 mm² |
| Mass per meter | 1.92 kg |
| Minimum yield strength | 345 MPa / 85 kN |
| Typical yield strength | 445 MPa / 110 kN |
| Minimum tube tensile strength | 470 MPa / 115 kN |
| Typical tube tensile strength | 530 MPa / 130 kN |
| Typical breaking capacity | 124 kN |
| Minimum breaking capacity | 89 kN |
| Recommended initial anchorage | 3–6 tons / 27–53 kN |
| Typical ultimate axial strain | 21% for thickness below 16 mm |
The product is listed in untreated and hot-dip galvanized versions. The standard catalog includes 600, 900, 1200, 1800, and 2400 mm lengths, with packaging quantities of 150 pieces for the listed configurations. The catalog also gives approximate unit masses, ranging from 1.20 kg for an untreated 600 mm bolt to 4.50 kg for a galvanized 2400 mm bolt.
For procurement, I would request confirmation of the actual steel grade, thickness tolerance, length tolerance, coating specification, and batch test records before approving a purchase order. A supplier should also identify whether the quoted breaking capacity refers to tube rupture, weld failure, anchorage pull-out, or another test mode. Those failure modes are not interchangeable in a tunnel-support calculation.
The selected length should extend through the expected loosened or fractured zone and provide sufficient embedment in more competent rock. A 600 mm FB-39 may be useful for mesh pinning or shallow secondary support, while 1800–2400 mm lengths may be considered for more substantial tunnel reinforcement. I would not select length from tunnel diameter alone because joint orientation, block size, excavation damage, and deformation depth can change the required anchorage zone.
A practical selection process uses geological mapping, probe drilling, convergence observations, and the project’s ground-support classification. The engineer should define the bolt spacing, pattern, angle, and required load before the purchasing team chooses the final length. If the support design depends on a specific anchorage depth, the effective embedded length must be checked after drilling and installation.
The FB-39 is used in several layers of underground ground-support systems. Its main value is immediate friction anchorage without waiting for grout or resin to cure. That makes it relevant to advancing headings, temporary stabilization, mesh installation, and situations where support must follow excavation closely.
In underground mining, the FB-39 may be used in development drives, crosscuts, declines, shafts, stopes, and service excavations. The bolt can be paired with bearing plates and strata mesh to control loose rock fragments between adjacent bolts. It may also be used where the mining cycle requires drilling and support within a short operating window.
For mining contractors, the main operational factors are installation speed, compatibility with the bolter, available bolt lengths, and the ability to inspect installation records. I would also review whether the mine’s ground-control plan requires friction bolts as primary support, supplementary support, or mesh-retention hardware. A friction bolt should not be assigned a support role beyond the capacity and deformation behavior demonstrated by the project’s design and testing program.
In transportation, hydropower, utility, and access tunnels, FB-39 bolts may support rock around the crown, walls, portals, and local overbreak zones. The 39 mm diameter provides a smaller drilling requirement than a 47 mm friction bolt, which can be useful where the equipment fleet is configured for smaller holes or where drilling time and consumables are constrained.
For tunnel work, I would connect the bolt selection to the excavation sequence. A short FB-39 may provide temporary restraint after blasting, while longer bolts can be incorporated into a systematic pattern where the ground-support design requires deeper reinforcement. The final arrangement should be coordinated with shotcrete thickness, welded wire mesh, lattice girders, drainage, and any subsequent permanent lining.
One documented use of the 39 mm friction bolt is securing strata mesh, including installation into existing 47 mm friction-bolt support systems with a smaller plate. This application is different from using the FB-39 as the primary structural anchor for a large unstable block. Here, the bolt and plate transfer mesh loads into the rock surface and help reduce the risk of small pieces falling between larger support elements.
Plate compatibility must be checked before installation. The plate opening, plate shape, ring geometry, mesh aperture, and contact area should all be suitable for the FB-39. A plate that does not seat correctly can create local bending, mesh slippage, or incomplete load transfer even when the tube itself is properly installed.
I would normally consider the FB-39 when the project requires immediate support, the drilling system can consistently produce 35–38 mm holes, and the required load falls within the verified performance range. It is particularly practical for general tunnel support, mesh pinning, short-cycle underground construction, and temporary or supplementary reinforcement. The bolt is also suitable when the project team wants to avoid curing time associated with resin or cement systems.
The decision becomes less favorable when holes are routinely oversized, highly irregular, or contaminated with cuttings and water. An oversized hole reduces interference between the tube and rock, which can lower radial pressure and anchorage. If the measured hole diameter is regularly above 38 mm, I would stop treating the FB-39 as a direct replacement and reassess the drilling process, bolt size, or support design.
| Project condition | FB-39 decision |
|---|---|
| Consistent 35–38 mm holes | Generally suitable for evaluation |
| Immediate support required | Favorable because friction anchorage develops during installation |
| Mesh pinning | Suitable with compatible plate and ring |
| Dry or moderately wet ground | Select untreated or approved coated finish according to design |
| Persistent water or aggressive chemistry | Consider hot-dip galvanized or specified protective coating |
| Highly deformable ground | Require project testing before approval |
| Oversized holes above 38 mm | Correct drilling or consider another support solution |
| Grouted permanent anchorage required | Select a grouted system instead |
| Higher design load than verified capacity | Use an engineered higher-capacity alternative |
Diameter should be treated as a design variable, not a simple ranking system. A smaller bolt may reduce drilling and handling demands, while a larger bolt may require a larger hole and different installation energy. The correct choice depends on the interaction between tube geometry, hole size, steel thickness, anchorage demand, and the surrounding ground.
| Bolt type | Typical selection logic | Main caution |
|---|---|---|
| FB-33 | Smaller-diameter friction-bolt applications and restricted drilling conditions | Confirm whether its capacity meets the support design |
| FB-39 | General underground support, mesh pinning, and 35–38 mm hole applications | Requires consistent hole control and compatible accessories |
| FB-42 | Special-diameter applications where a project specifies approximately 40–42 mm | Obtain confirmed product data before substitution |
| FB-47 | Larger friction-bolt support and applications using larger drilling equipment | Larger holes increase drilling, energy, and consumable requirements |
The FB-39 may be preferable to FB-33 when the design requires more tube area or higher verified anchorage, but I would not infer the exact capacity difference without comparable test data. It may be preferable to FB-47 when the contractor wants to limit hole size while retaining a larger tube than the FB-33. FB-42 should be treated as a special-required diameter unless the supplier provides a complete technical datasheet and batch testing information.
Before drilling, I confirm the approved bolt spacing, length, angle, plate type, mesh arrangement, and installation sequence. The pattern must reflect the geological conditions observed at the face rather than relying only on a generic tunnel drawing. If the face mapping shows wider fractures, water inflow, blocky rock, or deformation, the responsible engineer may need to revise the support class.
The FB-39 should be installed as part of a complete system that may include mesh, bearing plates, shotcrete, straps, or other reinforcement. The bolt alone does not control every instability mechanism. Its function must be clearly defined as primary reinforcement, secondary support, mesh retention, temporary support, or a combination approved by the design team.
I use a drill bit intended to produce a 35–38 mm hole and monitor bit wear during the shift. A worn bit can change hole diameter, while excessive feed pressure or unstable drilling can create an irregular borehole. The hole should be drilled to a depth that accommodates the selected bolt length and leaves enough clearance for the tapered end and the required seating arrangement.
After drilling, the hole should be checked for blockage, collapse, excessive deviation, and loose cuttings. Flushing or air cleaning must follow the site’s safety and equipment procedures. I also record the hole diameter where the project requires quality verification, because a nominal bit size does not prove that the finished borehole remains within tolerance.
The plate or mesh plate should be placed so that it can bear against the rock surface or mesh without rocking. The bolt’s tapered end enters the borehole first, and the ring or collar must remain compatible with the selected plate. Misalignment at this stage can cause the plate to sit at an angle or prevent proper contact around the hole mouth.
The installation machine should be configured for the bolt length, rock condition, and approved drive procedure. The operator should avoid using excessive impact that damages the ring, plate, tube end, or bolting equipment. A controlled drive sequence is more useful than simply maximizing impact energy.
The tube is driven into the borehole until the plate is seated and the bolt reaches the specified installation position. I monitor drive time, impact response, alignment, and the final plate-to-rock contact. A sudden change in drive resistance may indicate a fractured zone, an obstruction, a collapsed hole, or a hole that does not match the required diameter.
Perpendicularity is important when the plate must distribute load over mesh or shotcrete. A severely inclined bolt can create eccentric bearing and reduce the effective contact area. The site record should capture the bolt location, length, installation time, operator, equipment, and any abnormal installation behavior.
A visual inspection should confirm that the plate is seated, the mesh is captured, the bolt is not visibly damaged, and the installation point matches the approved pattern. Pull testing may then be performed according to the project’s inspection plan. The supplier states that pull tests are used to assess welding quality and breaking strength, but an installed-bolt pull test must also be interpreted in relation to borehole condition and rock behavior.
The listed recommended initial anchorage is 3–6 tons, or approximately 27–53 kN. That value should not automatically be treated as the acceptance criterion for every project because the engineer may specify a different test load, proof load, displacement limit, or sampling frequency. A test that reaches the target load with excessive displacement may require a different response from a test that reaches the same load with stable movement.
I use the following checklist during installation audits because most field problems are linked to process variation rather than the nominal product name:
Confirm the drill bit is designed for a 35–38 mm finished hole.
Replace or inspect worn bits before diameter drift affects anchorage.
Check that the hole is clear of cuttings, loose fragments, and collapsed material.
Record holes that exceed 38 mm or cannot accept the bolt to design depth.
Maintain the planned bolt angle and avoid severe plate eccentricity.
Confirm that the plate opening matches the ring and tube geometry.
Record drive time and abnormal impact or refusal behavior.
Inspect the taper, slot, ring, welds, and coating before installation.
Separate accepted, damaged, and questionable bolts in the work area.
Interpret pull-test results using both load and displacement.
Investigate failed or low-load tests before continuing the same pattern.
Maintain batch traceability between installed bolts and supplier records.
Untreated FB-39 bolts may be appropriate in dry or short-duration applications when allowed by the project specification. Wet mines, groundwater-bearing tunnels, coastal infrastructure, and chemically aggressive environments require a separate corrosion assessment. The coating choice should consider humidity, water chemistry, chloride exposure, expected service life, ventilation, and whether the bolt is temporary or permanent.
The product range includes hot-dip galvanized versions, with a small mass increase compared with untreated bolts. For example, the listed 1200 mm untreated configuration is approximately 2.40 kg, while the galvanized version is approximately 2.50 kg. That difference affects handling and shipping calculations only slightly, but the coating specification, thickness, adhesion, and damaged-area repair procedure remain important.
I would not assume that galvanizing alone makes a friction bolt suitable for every permanent support system. If the design requires a grouted annulus, corrosion monitoring, sacrificial thickness, or a specified design life, the complete support system must be evaluated. In some tunnels, a galvanized friction bolt may be suitable as temporary or secondary support but not as the sole permanent reinforcement.
The FB-39 is not automatically suitable for every tunnel or mining project. I would reconsider it when the borehole is consistently oversized, when the ground is highly deformable and demands a specifically tested energy-absorbing system, or when the design load exceeds the verified capacity. It may also be unsuitable where the project requires resin or cement grouting to provide long-term corrosion protection and load transfer.
Weak or fractured ground requires careful interpretation. Fractured rock can provide useful surface contact, but open fractures, voids, crushed zones, and washed-out holes may reduce continuous friction. Before approving the bolt for such conditions, I would conduct representative pull tests and compare the results with the design anchorage requirement.
A 39 mm diameter should also not be treated as a direct substitute for FB-33, FB-42, or FB-47 without checking drilling requirements and test evidence. The correct replacement must match the hole range, material thickness, plate system, installation equipment, design load, and corrosion specification. If one of those factors changes, the support design may need formal review.
I recommend using a five-stage selection process for an FB-39 rock bolt:
Define the support duty: Identify whether the bolt is for primary reinforcement, secondary support, temporary support, or mesh pinning.
Confirm the geology: Review rock mass quality, jointing, water, deformation, excavation damage, and expected load transfer.
Check drilling compatibility: Verify that the equipment can produce and maintain 35–38 mm holes.
Match the specification: Select length, material thickness, finish, plate, ring, and packaging based on the project documents.
Validate field performance: Use installation records, pull tests, inspection data, and trial sections before large-scale deployment.
This process prevents diameter-led purchasing decisions. It also gives the contractor measurable acceptance criteria for both product compliance and installation quality. If the first trial section shows inconsistent drive times, excessive hole variability, or pull-test results below the design requirement, I would correct the process before expanding production.
The FB-39 Friction Bolt: When to Use the 39mm Rock Bolt in Tunneling Projects question is best answered through hole control, design load, geology, installation method, and service environment. I would choose the FB-39 when a project requires immediate friction anchorage, can maintain 35–38 mm boreholes, and needs support for tunnel reinforcement, underground mining, strata mesh, or temporary and secondary ground control.
The product data provides a 39 mm diameter, 600–3000 mm length range, 2.0–2.5 mm material thickness, 89 kN minimum breaking capacity, 124 kN typical breaking capacity, and recommended initial anchorage of 27–53 kN. Those figures are useful for preliminary selection, but they must be checked against project-specific testing and engineering requirements.
Before purchasing, I would request material certificates, dimensional records, mechanical tests, coating documentation, pull-test results, packaging details, and delivery commitments. I would then run a controlled trial installation using the selected bit, plate, equipment, and inspection procedure. The FB-39 is a practical choice for many tunneling applications, but its suitability depends on verified installation conditions rather than nominal diameter alone.
