Tunnelling and underground civil projects present some of the highest respirable crystalline silica (RCS) exposure risks in Australian construction. Workers in tunnel boring machine (TBM) environments, drill-and-blast teams, and shotcrete applicators face extended exposure to high dust concentrations in enclosed spaces with limited ventilation options. This creates a unique and challenging compliance environment for managing dust and airborne contaminants in Australian workplaces.
This guide explains the exposure profile in tunnelling, the regulatory requirements, and how to design a monitoring programme that protects workers and meets legal obligations. While this guide focuses on tunnelling, the underlying monitoring principles also apply to silica dust monitoring on above-ground construction sites that encounter high-silica geology.
Why Tunnelling Presents Extreme RCS Exposure Risk
Tunnelling combines several factors that create high silica exposure:
For workplaces with diesel-powered equipment, understanding workplace exposure standard for DPM is an essential part of WHS compliance.
Geology: Australian tunnelling projects encounter diverse rock types — sandstone (high silica content), granite (high quartz), and mixed-face geology. Each generates RCS dust when excavated.
Work intensity: Tunnel excavation requires continuous operation: drilling, blasting, mucking (removing broken rock), scaling (removing unstable rock), and shotcrete application. Workers spend 8–10 hours underground.
Enclosed space: Unlike surface work, ventilation is limited by space constraints. Air flows are restricted, and dust concentrations build up during work shifts.
Dust generation mechanisms:
– Percussion drilling produces high-velocity dust clouds (drill and blast method)
– TBM cutterhead rotation generates continuous dust (TBM method)
– Blasting creates intense dust clouds that linger in the enclosed space
– Shotcrete application generates silica-rich aerosol
– Mucking operations resuspend settled dust
Historical context: Australia’s major infrastructure projects (Melbourne Metro, Cross River Rail, Sydney’s Suburban Rail Loop, and numerous mining operations) have revealed that many workers in tunnelling environments are exposed to RCS concentrations of 0.1–0.5 mg/m³ or higher — 2–10 times the RCS workplace exposure standard (0.05 mg/m³).
This makes tunnelling a priority environment for RCS monitoring and control.
Exposure Profile in Tunnelling: Risk by Role
Different worker roles experience different exposure levels:
TBM Operators (highest risk): Spend 6–8 hours in the TBM cabin, exposed to dust from the cutterhead chamber. Concentrations often exceed 0.1 mg/m³.
Shotcrete Applicators (highest risk): Apply sprayed concrete in dust-laden air. Exposures often 0.15–0.5 mg/m³, especially if respiratory health effects of DPM protection is inadequate or fitting is poor.
Mucking Crew (high risk): Operate loaders and dumpers to remove broken rock. Dust is resuspended from floor and equipment surfaces. Exposures typically 0.08–0.2 mg/m³.
Tunnel Scalers (high risk): Remove unstable rock from tunnel walls and crown after blasting. Work in high dust concentrations from blasting residue. Exposures 0.1–0.3 mg/m³.
Drill Operators (high risk): Operate percussion drilling rigs for blastholes. Directly exposed to drill dust. Exposures 0.2–0.8 mg/m³ if no water suppression.
Shotcrete Labourers (medium-high risk): Support shotcrete operation; exposed to spray aerosol and rebound. Exposures 0.05–0.15 mg/m³.
Tunnel Supervisors (medium risk): Spend time in various zones but less in direct dust sources. Exposures 0.05–0.1 mg/m³.
Ventilation Plant Operators (low-medium risk): Work in auxiliary tunnel managing ventilation systems; exposed to dust drawn from main tunnel. Exposures 0.03–0.08 mg/m³.
Regulatory Framework for Underground Civil Work
Tunnelling is typically regulated under the Work Health and Safety Act 2011 (not mining regulation, which applies only to extractive mineral mining). Key obligations:
- Duty to manage RCS exposure — applies to principals (project owners), contractors, and subcontractors
- Workplace Exposure Standard (WES) — 0.05 mg/m³ (until December 2026)
- Risk assessment required — before work begins and after significant changes
- Exposure monitoring required — if you cannot confirm exposure is below WES
- Health surveillance required — baseline and periodic assessment for workers in RCS roles
- Controls must follow the hierarchy — engineering controls first; RPE as last resort
- 30-year record retention — for dust samples and health records
The Safe Work Australia Model Code of Practice for RCS applies in full. See Safe Work Australia’s 2025 Model Code of Practice for RCS for detailed obligations.
Types of Tunnelling Operations and Silica Risk Profiles
TBM (Tunnel Boring Machine) Tunnelling
Process: A large rotating cutterhead breaks rock at the tunnel face. Broken material is conveyed to the rear and loaded into trucks (or hoppers in slurry/EPB shields).
Silica risk profile:
– Dust generated continuously during TBM operation
– Concentration depends on rock type (sandstone and granite are high-risk)
– Dust sources: cutterhead chamber, scrapings from tunnel walls, conveyor belt operation
– Typical exposure: 0.05–0.15 mg/m³ in TBM cabin (high if ventilation is poor)
Monitoring challenge: Limited space in TBM cabin and muck car; difficult to place stationary monitors; personal DPM monitoring is essential.
Drill-and-Blast Tunnelling
Process: Percussion drills create blastholes; explosive charges break rock; mucking crew removes broken rock.
Silica risk profile:
– Drilling generates high dust concentrations (1–5 mg/m³ locally if no water suppression)
– Blasting creates intense dust clouds that persist for minutes to hours depending on ventilation
– Post-blast dust concentrations: 0.2–1 mg/m³ in the working zone
– Mucking resuspends settled dust
Monitoring challenge: Highly variable dust concentrations; real-time monitoring is essential to understand peak exposures and ventilation effectiveness.
NATM (New Austrian Tunnelling Method)
Process: Combination of drilling, blasting, and sprayed concrete lining; similar to drill-and-blast but with immediate shotcrete application.
Silica risk profile:
– High during drilling and blasting phase
– Very high during shotcrete application (aerosol and rebound; 0.1–0.5 mg/m³)
– Lower during support installation if ventilation is effective
Monitoring challenge: Multiple exposure peaks throughout the shift; different roles have vastly different exposures.
Cut-and-Cover Tunnelling
Process: Open excavation, construct tunnel structure, backfill. May involve breaking concrete decking and excavation in weathered silica-bearing rock.
Silica risk profile:
– More controlled than deep tunnelling (open-air for much of the work)
– Risk during concrete breaking and rock excavation
– Typical exposures: 0.03–0.1 mg/m³ if ventilation and water suppression are used
Designing a Tunnelling Monitoring Programme: Best Practice
1. Initial Exposure Assessment (Pre-Commencement)
Conduct before the tunnel closes or the TBM arrives:
- Site geology: obtain geological maps and boring logs to identify silica-bearing rock
- Task risk matrix: map all work tasks and identify highest-risk roles (TBM operator, shotcrete team, drill operator, etc.)
- Benchmark: consult similar projects. What exposures were recorded on comparable geology and equipment?
- Real-time screening: deploy a portable TEOM or pDR for 1–2 weeks in preliminary work (e.g., portal drivage, test excavation). Identify peak concentrations and worst-exposed roles.
Output: Baseline exposure profile; prioritised list of high-risk roles for gravimetric sampling.
2. Baseline Gravimetric Sampling (Weeks 1–4 of Tunnelling)
- Target highest-risk roles: TBM operator, shotcrete applicator, drill operator, scaler (if drill-and-blast)
- Collect 4–6 personal samples from each role over 2–4 weeks
- Full 8-hour shifts (or task-specific samples if roles are part-time or intermittent)
- Use NATA-accredited lab with FTIR analysis
Static monitoring (optional but recommended):
- Place stationary samplers in high-exposure zones (TBM cabin, shotcrete spray zone, drilling area)
- 4–8 hour samples; helps characterise exposure zones independent of individual work practices
Output: Baseline exposure data; identify roles or tasks that exceed WES.
3. Implement Initial Control Measures
Based on baseline results, implement engineering controls targeting the worst exposures:
For TBM cabins:
– Upgrade cabin ventilation (e.g., dedicated fresh air supply with filtration)
– Seal cutterhead chamber leaks
– Use air curtains to prevent dust infiltration
For drill-and-blast operations:
– Mandated water suppression on all percussion drills (achieves 70–90% dust reduction)
– Design optimal blast timing and ventilation procedures to minimise post-blast dust concentration
– Implement dust extraction behind the drill (if equipment permits)
For shotcrete application:
– Deploy local exhaust ventilation (LEV) at the spray zone (capture nozzle spray and rebound)
– Improve general ventilation (larger-diameter ventilation ducts, higher fan capacity)
– Enforce RPE protocols: P2 respirators, fit-tested, replaced when clogged
For mucking operations:
– Water suppression on loader buckets (reduces resuspension)
– Improve ventilation timing: allow settling period before muck operations begin
– Dust suppressant on tunnel floor (polymer-based; reduces resuspension)
4. Real-Time Monitoring for Process Optimisation
Deploy real-time instruments (TEOM, pDR, or optical counters) alongside gravimetric sampling:
Purpose:
– Identify peak dust periods throughout the shift (e.g., immediately post-blast, during shotcrete spray, during drill operation)
– Evaluate ventilation effectiveness in real time (does ventilation fan ramp-up reduce dust as expected?)
– Assess control improvements (compare dust concentrations before and after implementing water suppression, LEV, etc.)
– Provide real-time feedback to workers (powerful behavioural tool)
Method:
– Deploy portable monitor with data logger for 1–2 hour periods in high-risk areas or on specific workers
– Download data; analyse trends; share findings with the project team
– Use to inform scheduling decisions (e.g., “drill and blast should be scheduled in the morning to allow 6-hour settlement before mucking”)
Frequency: Continuous during the project; intensified after control changes.
5. Follow-Up Gravimetric Sampling (Weeks 6–10)
After implementing initial controls, conduct follow-up gravimetric sampling:
- Same roles and workers as baseline (to track individual exposure reduction)
- If baseline exceeded WES, target the improved controls to assess control effectiveness
- Document all control changes in the monitoring report
Expected outcome: Exposure should be ≤0.05 mg/m³ or show a significant reduction (e.g., 50% lower) if controls are effective.
6. Ongoing Monitoring (Monthly to Quarterly)
Once initial assessment and control optimisation are complete, establish ongoing monitoring:
- High-risk roles: monthly personal sampling
- Medium-risk roles: quarterly sampling
- After any process change: re-sample within 2 weeks
Low-risk roles or areas: Annual sampling (if exposure is consistently below half the WES and controls are stable).
Ventilation as the Primary Control in Tunnelling
Ventilation is critical in underground work — it is the only ambient control available. Design ventilation systems carefully:
Key design parameters:
- Air velocity in tunnel: minimum 0.5 m/s to dilute dust and carry it toward the extraction
- Fresh air supply rate: sufficient to replace the air consumed by equipment (e.g., diesel engines) and dilute dust to below WES
- Dedicated dust extraction: separate exhaust ductwork (not mixing fresh and exhaust air)
- Dust settling zone: if possible, create a section downstream of work areas where dust can settle before muck loading (increases settling time from minutes to hours)
Monitoring ventilation effectiveness:
- Real-time dust monitors in the spray zone and work area show ventilation impact (if dust drops when fan increases speed, ventilation is working)
- Periodic static sampling confirms that ambient dust levels are acceptable
Major tunnelling projects increasingly employ sophisticated ventilation modelling (Computational Fluid Dynamics — CFD) to optimise air distribution before construction begins. This is a best-practice investment.
Respiratory Protection in Tunnelling
RPE is necessary in tunnelling despite control implementation:
Requirements:
- Fit-tested P2 respirators — minimum standard for general tunnel exposure above 0.05 mg/m³
- Supplied air respirators — for high-exposure roles (e.g., shotcrete application, drill operation) where concentrations are anticipated to exceed 0.15 mg/m³
- Fit testing — mandatory before first use and annually thereafter (more frequent if multiple respirators are issued)
- Training — workers must understand when and how to use respirators, how to check fit, how to maintain them
Compliance note: RPE cannot be the sole control. It must be supported by engineering controls (ventilation, water suppression) and health monitoring.
Health Monitoring for Tunnelling Workers
All workers in RCS roles on major tunnelling projects must receive health monitoring:
Baseline assessment (before first exposure):
– Medical questionnaire (smoking history, previous silica exposure, respiratory symptoms)
– Spirometry (baseline lung function)
– Chest X-ray (baseline to detect pre-existing abnormalities)
Frequency:
– Annual if exposure is likely to exceed 0.1 mg/m³ (high-risk roles)
– Every 2 years if exposure is 0.05–0.1 mg/m³ (medium-risk roles)
– Every 3 years if exposure is likely below 0.05 mg/m³ (low-risk roles)
Administration:
– Partner with occupational physician or AIOH-accredited clinic
– Provide results to workers with explanation
– If decline in lung function is detected, escalate: increase monitoring frequency, review exposure controls, consider redeployment
Working with an Occupational Hygienist on a Major Tunnelling Project
For large infrastructure projects, engaging an experienced occupational hygienist is essential:
Their role:
– Design the monitoring programme (baseline, follow-up, ongoing)
– Specify sampling and lab protocols (NATA-accredited analysis)
– Interpret results and advise on control improvements
– Liaise with the health surveillance provider
– Audit compliance (training, RPE fit testing, ventilation maintenance)
– Prepare compliance reports for regulatory review
What to look for:
– AIOH membership (FAIOH or MAIOH preferred)
– Experience with tunnelling or underground construction
– Familiarity with NATA-accredited labs and FTIR analysis
– Track record on major infrastructure projects
At Anitech, we provide end-to-end support for tunnelling projects: initial risk assessment, baseline and follow-up monitoring, real-time process optimisation, health surveillance coordination, and regulatory compliance documentation.
Frequently Asked Questions
Q: What exposure levels are typically found in tunnelling?
A: Highly variable. TBM operations: 0.05–0.15 mg/m³ in cabin if ventilation is poor; can be below 0.05 mg/m³ with good ventilation. Drill-and-blast: 0.2–0.8 mg/m³ during drilling; 0.2–1 mg/m³ immediately post-blast. Shotcrete: 0.1–0.5 mg/m³ in spray zone. Mucking: 0.08–0.2 mg/m³ if dust suppression is used. Baseline monitoring is essential to establish your project’s specific profile.
Q: How long do results take to come back from the lab?
A: Gravimetric analysis (FTIR): typically 7–10 days from receipt at the lab. Real-time data: immediate (displayed on screen). Plan your monitoring timeline accordingly; allow 2 weeks between sampling and results before making control decisions.
Q: Can I use only real-time monitoring to prove compliance?
A: No. Real-time monitoring is excellent for process control and optimisation but is not NATA-accredited and cannot serve as sole evidence of WES compliance. Use gravimetric (NATA) as your compliance method; use real-time to support control design.
Q: Do contractors working on a tunnelling project have the same monitoring obligations as the principal contractor?
A: Yes. All contractors with workers exposed to RCS have a duty to manage exposure. The principal contractor should coordinate a unified monitoring programme and share results with all contractors. Avoid duplication and ensure all workers are covered.
Q: What is the WEL transition and how does it affect my tunnelling project?
A: The December 2026 transition from WES (0.05 mg/m³) to WEL will introduce lower exposure limits. Exact WEL values are pending, but prepare now by increasing control investments. Projects running into late 2026 should plan control upgrades before 1 December 2026. See Silica WEL 2026 for guidance.
Q: How do I sample in a TBM cabin if space is limited?
A: Personal sampling (pump and filter on the worker’s belt) is the standard approach. Static sampling in the cabin is difficult but can be done with permission from TBM operators. Consider a short sampling period (4 hours instead of 8) if the cabin is very cramped, but ensure you capture the representative exposure period.
Q: Should I conduct health monitoring for workers on short-term contracts?
A: Yes. Any worker with RCS exposure requires baseline assessment before first exposure. If contract duration is less than 3 months, a baseline is sufficient (follow-up periodic assessment is not required if exposure ends). If the worker may return, retain baseline results for their file.
Q: How do I ensure fit testing is compliant in an underground setting?
A: Fit testing should occur above ground (in a clean space) before workers descend into the tunnel. Use a fit testing service provider; specify the respirator model to be used in the tunnel. Test annually or after significant worker changes. Document all fits in your training register.
Summary
Tunnelling and underground civil projects demand rigorous RCS monitoring. The combination of high dust generation, enclosed spaces, and limited ventilation options creates a uniquely challenging exposure environment. Best practice requires:
- Baseline assessment — characterise exposure before full project commencement
- Targeted controls — implement engineering controls (water suppression, ventilation, LEV) for highest-risk roles
- Real-time optimisation — use portable monitors to refine controls and identify trends
- Gravimetric verification — NATA-accredited sampling to confirm WES compliance
- Health surveillance — baseline and periodic assessment for all RCS-exposed workers
- Ongoing monitoring — monthly to quarterly gravimetric sampling throughout the project
Australia’s major infrastructure projects (Metro, Cross River Rail, Suburban Rail Loop) are setting new standards in RCS management. By implementing a comprehensive monitoring programme early, you protect workers, meet regulatory obligations, and build a defensible compliance record.
Supporting Australia’s infrastructure: Anitech has guided RCS compliance on major tunnelling projects nationally. Contact us to design a monitoring programme for your project. Get in touch.
IMAGE_REQUIREMENTS
-
FEATURE (1200×630px): Cross-section diagram of a tunnel under construction showing multiple work zones (TBM cabin at front, shotcrete spray zone, muck removal area, ventilation ductwork). RCS dust clouds visible in high-risk zones. Depth and scale evident.
-
BODY_1 (800×450px): Close-up of a tunnelling worker wearing a P2 respirator in a dusty work environment, with ventilation ducting visible. Caption: “Health protection in tunnelling: engineering controls + respiratory protection + health monitoring.”
-
BODY_2 (800×450px): Real-time dust monitor screen displaying time-series dust concentration data with spikes corresponding to drilling and blasting phases. Caption: “Real-time monitoring shows exposure peaks during critical work phases, guiding ventilation and scheduling decisions.”
