As excavation proceeds, temporary support must react to the condition of the ground as it becomes exposed. Installation time can be limited, and geological conditions may evolve from round to round.
Examples:
The anticipated ground conditions should be reviewed continually as tunnel excavation progresses. The support solution implemented for one tunnel reach should not be assumed to be appropriate for another zone with a different geological condition.
Wanfeng provides reinforcement and surface-retention components that can be part of a system designed as initial support for a project.
Rock bolts, threaded bars and friction bolts are used to reinforce or restrain rock surrounding an excavation. Bearing plates and steel straps are used to help transfer and distribute applied loads on the surface. Welded wire mesh is used to help retain smaller loose fragments between bolts or other points of restraint.
Components may be used in conjunction with:
Related systems may only be referred to as Wanfeng products when confirmed available for supply by Wanfeng.
Functions of a Temporary Support System
An engineered system of temporary support components can be designed to:
TEMPORARY DOES NOT EQUAL UNCONTROLLED
Just because a support component is defined as temporary does not mean it will be replaced quickly. Depending on the construction sequencing, a temporary support can stay in place for weeks or months. It should meet the specified mechanical, dimensional, installation and durability requirements for its actual lifespan.
Component specifications should not be compromised just because they are part of what could be defined as a temporary support system.
Engineering Judgment Disclaimer
The final support type, timing of installation, allowable unsupported advance, bolt pattern and length, anchorage, mesh configuration, shotcrete thickness, excavation sequence and instrumentation should be designed or reviewed by qualified geotechnical, tunnelling, mining and structural engineers. Wanfeng will only supply components specified on approved drawings and is not responsible for determining when an excavation is safe to enter or if temporary support is enough for the tunnel being constructed.
Note that each of these products can have proprietary options or have additional trade names assigned by different manufacturers. Each product category includes examples of key specification fields and common additional fields that should be considered during product selection.
1.Rock Bolts
Steel bolts that can be installed around the tunnel crown, shoulders and sidewalls throughout excavation.
Key specification fields
Additional fields
2.Friction bolts / Split Sets
Friction type bolts that are designed to be quickly installed where the borehole and ground condition are appropriate for the system installed.
Key specification fields
Additional fields
Application note
Verification of friction-bolt performance should be accomplished through controlled installation and test loading as specified on the project drawings.
3.Fully Grouted Threaded Rebar Bolts
Steel bars with threads along the full length or defined lengths that can be grouted with cementitious material or resin.
Key specification fields
Additional fields
4.Cable bolts
Longlength steel elements designed for reinforcement where potential instability may exceed the capacity of conventional bolts.
Key specification fields
Additional fields
Application note
Longlengeth cables are not necessarily required for temporary ground support applications. Their use should be fully justified by means of an approved ground-support design.
5.Bearing plates
Steel plates that are generally used to help transfer load from compatible bolts to the rock surface.
Key specification fields
Additional fields
6.Welded wire mesh
Panel type product used to restrain smaller loose fragments between active reinforcement locations.
Key specification fields
Additional fields
Application note
Mesh strength, fastener capacity, panel overlap and distance from rock surface should be confirmed in the approved support drawing and installation sequence.
7.Steel straps
Flat or profile steel elements used to help support fractured rock masses or to distribute support reaction between bolt locations.
Key specification fields
Additional fields
8.Nuts, Couplers and accessories
Steel elements used with compatible bolts and installed using approved equipment and procedures.
Key specification fields
Available configurations may include some or all of the following:
| Typical excavation or ground condition | Typical support requirement | Product categories or systems that may be relevant |
| Competent rock with isolated loose blocks | Localized reinforcement | Rock bolts with bearing plates |
| Jointed crown or sidewalls | Reinforcement across discontinuities | Rock bolts or threaded bolts |
| Fractured surface rock | Reinforcement and surface retention | Bolts with welded wire mesh |
| Small fragments between bolt positions | Continuous surface retention | Mesh with suitable plates and fasteners |
| Blocky ground or intersecting joints | Load distribution between bolts | Bolts, mesh and steel straps |
| Rapid drill-and-blast cycle | Timely installation after scaling | Approved rapid-installation bolts with mesh where required |
| Variable but stable boreholes | Installation system compatible with measured hole variation | Only bolt systems verified for the specified borehole range |
| Collapsing or water-bearing holes | Anchorage suitable for poor hole conditions | Project-designed grouted, cased or alternative system |
| Fault or shear zone | More continuous initial support | Fully grouted bolts, mesh, straps and shotcrete as designed |
| Weak ground with short stand-up time | Support installed close to the excavation face | Project-designed pre-support, shotcrete, bolts, ribs or closed support ring |
| Local deep instability | Reinforcement beyond the shallow loosened zone | Longer threaded bolts or cable bolts where verified |
| Temporary wet environment | Construction-period corrosion resistance | Galvanized, coated or otherwise protected components |
| Squeezing or deforming ground | Controlled deformation capacity | Only verified yielding or deformable products in an engineered system |
| Dynamic or seismic demand | Verified energy absorption | Only products supported by relevant dynamic test data |
| Initial support intended to contribute to final support | Documented structural and durability performance | Products specifically assessed for permanent design requirements |
Selection Disclaimer
The following matrix should be used as a preliminary guide only. The following products or systems may be appropriate for certain conditions and are not to be considered a recommendation for all applications.
Support type, unsupported advance, installation distance from face, bolt spacing, anchorage, shotcrete and testing should be determined by site-specific engineering.
Temporary support and permanent support should be clearly distinguished in project documents.
| Design consideration | Temporary or initial support | Permanent support |
| Primary purpose | Stabilize the excavation during construction | Meet the specified long-term structural and service requirements |
| Exposure period | Construction period or defined temporary duration | Full design life |
| Design basis | Construction-stage ground loads and sequence | Long-term loads, durability and serviceability |
| Corrosion protection | Based on actual temporary exposure | Based on groundwater, environment and design life |
| Inspection | Frequent construction-stage inspection | Planned long-term inspection and maintenance |
| Structural contribution | Not automatically included in final design | Included only where explicitly designed and verified |
| Removal | Only under an approved sequence | Normally remains as part of the completed works |
| Documentation | Installation and construction records | Permanent works records and compliance documents |
Design Note
Permanent status should not be assumed for initial support.
Prior to designing temporary bolts, mesh, shotcrete or other means of support as part of the permanent load-resisting system, the project engineer should confirm that:
Short-term performance criteria do not ensure permanent acceptability.
Wanfeng can evaluate product schedules and approved drawings for custom manufacturing capability.
These attributes are candidates for customization:
All customization requirements should be confirmed on an approved drawing, product schedule or technical agreement prior to manufacturing.
Inspection criteria should be commensurate with material type, project specification and mutually agreed inspection protocol.
1.Raw Material Inspection
2.Dimensional and Thread Inspection
3.Mechanical Property Testing and Mechanical Testing
4.Weld Inspections
5. Surface Coating Controls
6. Marking, Identification and Final Inspection
7. Certificates
List standards such as ASTM, EN, ISO, AS/NZS or others only if conformity can be assured for the product being supplied and the project requirements.
Performance of temporary support is reliant on the entire installation and inspection regimen being completed.
The approved project plan may include:
Replacement Warning
Damaged or misplaced support should be identified and corrected in a timely manner following the approved ground-control procedure. Sequence of replacement should be considered and should never leave workers unprotected and exposed to unsupported ground.
Wanfeng does not define the safe removal sequence of installed tunnel support.
1.What is temporary tunnel support?
Answer:Temporary support is installed during excavation as needed to stabilize exposed ground and/or provide safe construction access prior to installation of subsequent or permanent support. Examples include bolts, mesh, straps, shotcrete, ribs, etc.
2. Common products used for initial tunnel support?
Answer:Rock bolts, friction bolts, threaded bars, bearing plates, welded wire mesh, steel straps and compatible accessories are commonly used, depending on ground and excavation conditions. The complete system should be defined by the project engineer.
3. How soon should temporary support be installed?
Answer:This will vary based on ground stand-up time, sequence of excavation and requirements of the approved ground-support plan. Wanfeng cannot provide a general recommendation for safe distance or unsupported period that will apply in all situations.
4. Are friction bolts ever used as temporary support?
Answer: Friction bolts can be appropriate where borehole diameter, rock condition, applied load and expected construction duration are suitable for the product being considered. Installation procedure and performance should be confirmed for project conditions.
5. How do bolts and mesh work together?
Answer: Bolts are used to reinforce/restrain the rock mass and provide attachment points at the surface of the excavation. Mesh is used to retain smaller fragments between bolts, and plates/straps may help distribute loads and secure the mesh to the rock.
6. Can temporary support be part of the permanent design?
Answer: Only if its contribution to overall structure, installation quality, durability, corrosion protection and interaction with the final lining have been evaluated and accepted by the project engineer.
7. Does temporary support need corrosion protection?
Answer:Yes, depending on exposure to water, chemicals, anticipated lifetime and whether the materials will remain in place. “Temporary” does not automatically mean ” unprotected”.
8. What should be done if temporary support is damaged?
Answer:Support that is damaged or displaced during construction should be inspected and repaired/replaced as part of the approved ground-control procedure. Load-carrying support should not be removed by personnel unless an engineered sequence and appropriate protection have been designed.
