Case Study: Silica Monitoring for a Major Infrastructure Tunnelling Project

Client: Major Infrastructure Authority and Principal Contractor

A metropolitan rail authority engaged a major construction consortium to deliver a 7.2-kilometre twin tunnel rail project in a major Australian capital city. The project involved:

Tunnel boring machine (TBM) operations: Two parallel TBMs excavating soft ground with rock (1.8 km drive per machine)
Breakout chambers and station excavation: Two major stations and two crossover chambers (conventional excavation methods including drilling and blasting)
Support and lining: Spray-applied concrete, cast segments, and temporary propping
Estimated duration: 36 months of primary excavation
Workforce: Peak workforce of 350–400 personnel underground

The project was classified as critical infrastructure with high regulatory scrutiny and high visibility. Any occupational health incident (particularly silica-related disease diagnosis in workers) would generate media attention and regulatory action. The authority and principal contractor were acutely aware that dust monitoring and health surveillance had to be exemplary.

The Challenge

Underground infrastructure tunnelling presents unique occupational hygiene challenges:

Confined underground environment: Limited natural ventilation; dust accumulation potential is high; visibility is reduced, making control verification difficult
Mixed excavation methods: TBM work (cleaner, semi-enclosed process) and conventional excavation (open-face drilling and blasting, high dust generation) coexist on the same project
Compressed air operations: Some work zones used compressed air; potential for high-risk exposures and health effects beyond silica (nitrogen narcosis, decompression sickness)
High-skill, mobile workforce: Crews rotated between work fronts; consistency of dust control practices was difficult to ensure
Regulatory uncertainty: Whilst underground work was WHS-regulated, specific guidance on silica monitoring protocols for tunnelling was limited; the project would set a precedent

The principal contractor engaged Anitech Group to design a comprehensive, real-time dust monitoring program that would protect workers, document compliance, and meet emerging industry best practice for infrastructure tunnelling.

Our Approach

1. Pre-Project Planning and Baseline Design

Months before excavation commenced, Anitech conducted a pre-project occupational hygiene assessment:

Reviewed geological predictions: Identified sections of high-silica material (fine sandstone, quartz-rich rock) vs. lower-silica sections (mudstone, clay)
Assessed excavation methods: TBM performance data from similar projects to estimate dust generation in enclosed TBM environment; conventional excavation planning to predict drilling and blasting dust
Identified high-risk work zones: Station breakouts (exposed face, higher dust) vs. TBM tunnel (enclosed, better containment)
Consulted with the TBM specialist: Reviewed manufacturer specifications for TBM dust suppression systems
Designed baseline monitoring protocol: Plan to sample intensively during the first 3 months (TBM commissioning and early operation) to characterise exposure before full-scale work

2. Baseline Monitoring (Months 1–3)

When excavation commenced, Anitech deployed a comprehensive baseline program:

Personal Air Sampling:
– TBM operators and supervisors (3–4 workers per shift)
– Tunnel workers at active face (conventional excavation areas)
– Support workers (spray application team, segment erectors, temporary propping crew)
– Shift engineer/supervisors overseeing work
– Sampling frequency: 2–3 times per week across different shifts to capture variability

Static/Area Monitoring:
– TBM tail shield zone (highest dust area of the TBM environment)
– Station breakout face area
– Tunnel boring machine gantry (assembly and maintenance area)
– Main tunnel access and egress (to assess general tunnel air quality)

Real-Time Dust Screening:
– Hand-held optical particle counters deployed daily to map dust distribution within the tunnel
– Real-time data used for immediate process feedback and control adjustment

Ventilation Assessment:
– Air velocity measurements in main tunnel
– Quantification of dust suppression system effectiveness (water/foam spray on TBM cutterhead)

Baseline Results (Key Findings):

Work Area/Task	Worker Role	TWA (mg/m³)	% of WES	Note
TBM tail shield	TBM operator	0.06	120%	Just above limit; elevated baseline
Station breakout face	Drill/blast crew	0.22	440%	High-risk; conventional excavation
Tunnel support (spray concrete)	Spray operator	0.08	160%	Respirable concrete dust + RCS
Segment erection	Erector	0.04	80%	Lower-risk; semi-enclosed TBM
Main tunnel (general)	Supervisors/walkthrough	0.012	24%	Background air; acceptable

Critical finding: The TBM tail shield (0.06 mg/m³) and especially the conventional excavation breakout (0.22 mg/m³) exceeded the WorkExposure Standard. The TBM itself was borderline—the built-in dust suppression (cutter-head water spray) was helping but not sufficient.

Immediate response:
– Breakout excavation protocols modified: reduced blasting charge sizes to generate less dust, extended post-blast ventilation time, temporary water spray systems added to the breakout face
– TBM dust suppression: upgraded the cutter-head spray system pressure and flow; added mist suppression in the tail shield backfill area
– Temporary local exhaust ventilation trial: experimental dust collection duct deployed at the TBM tail shield during a test period to assess feasibility

3. Rapid Control Upgrade Phase (Months 3–6)

Results from baseline monitoring triggered immediate engineering and procedural improvements:

TBM Modifications:
– Upgrade tail shield dust suppression: higher-flow water spray system, improved mist application geometry
– Installation of a portable dust collection unit (HEPA-filtered, powered) positioned at the tail shield transition zone; trials confirmed 40% reduction in dust levels
– Operator cabin air filtration: upgraded to HEPA-grade cabin filters on all TBM cabs

Conventional Excavation (Breakout areas):
– Reduced charge weights in blasting rounds (smaller charges = less dust from detonation)
– Post-blast ventilation protocol: all high-risk zones received 30 minutes of additional tunnel ventilation (two extra ventilation cycles) before re-entry
– Temporary water suppression: installed fog spray system at breakout face during excavation; pre-wetting of exposed rock to reduce airborne dust
– Portable dust collection at breakout: local exhaust ventilation unit with flexible ducting positioned to capture dust at the face

Spray-Applied Concrete:
– Upgraded spray rig water supply and mist suppression
– Addition of a temporary fume/dust hood at the spray nozzle area
– Enhanced tunnel ventilation during spray operations

Control Cost: Approximately AUD $2.1 million for upgraded ventilation, dust suppression equipment, and portable collection systems.

4. Post-Control Monitoring and Sustained Compliance (Months 6–36)

After controls were fully implemented, monitoring was established on an ongoing schedule:

Phase 2 Monitoring (Months 6–12): Post-control baseline establishment
– Repeat baseline sampling across all work areas to quantify control effectiveness
– Real-time monitoring continued daily

Results after controls:

Work Area/Task	Worker Role	New TWA (mg/m³)	% of WES	Reduction
TBM tail shield	TBM operator	0.032	64%	47% ↓
Station breakout face	Drill/blast crew	0.06	120%	73% ↓
Tunnel support (spray)	Spray operator	0.04	80%	50% ↓
Segment erection	Erector	0.022	44%	45% ↓

Outcome: TBM operations moved to acceptable compliance (0.032 mg/m³, below limit). Breakout excavation (0.06 mg/m³) remained above the limit; the consortium implemented further controls: compressed air alternative (ground stabilisation) for high-risk sections, further procedure refinement, and mandatory P3 respirator use during blasting and immediate post-blast period.

Phase 3 Monitoring (Months 12–36): Sustained programme
– Monthly personal sampling of high-risk workers (TBM operators, breakout crews, spray operators)
– Quarterly static monitoring and real-time screening
– Health monitoring (see below)

Compliance track record (Months 12–36):
– Compliance rate: 92% of personal samples ≤ 0.05 mg/m³
– Non-compliant samples: 8 of 100 monthly samples exceeded the limit
– All 8 occurred during expected high-risk events (major blasting rounds, drill crew shift changeovers, equipment failures)
– All non-compliant workers were notified, controls were reviewed/upgraded, and re-sampling confirmed return to compliance within 2–3 weeks
– Real-time screening: Detected early warning of rising dust 15 times; allowed preemptive control activation before dust exceeded limits

5. Occupational Health Monitoring Program

Parallel to air monitoring, Anitech designed a health surveillance program:

Baseline occupational health assessments:
– All underground workers (phase 1: first 180 workers recruited for early-shift teams)
– Assessments comprised: chest X-ray, spirometry, occupational health questionnaire, clinical review

Baseline findings:
– Abnormal X-rays: None (reassuring baseline)
– Reduced lung function: 12 workers (6.7%) showed FVC or FEV1 below 80% predicted
– Detailed assessment: 7 were smokers (smoking effects); 3 had prior silica exposure (previous construction or mining work); 2 remained unexplained
– These 12 workers were enrolled in close surveillance (6-monthly rather than 2-yearly follow-up)
– Respiratory symptoms: 31 workers (17.2%) reported cough, shortness of breath, or chest tightness
– Most symptoms resolved or improved within 4 weeks of commencement (acclimatisation to underground work, tunnel environment adaptation)

Surveillance protocol (Months 6–36):
– All underground workers: occupational health surveillance every 12–24 months
– 12 workers with baseline abnormalities: surveillance every 6 months
– New workers joining project: baseline assessment within first month

Surveillance results (Months 6–36):
– No new silicosis cases detected in 18 months of surveillance (170 workers screened)
– No significant decline in lung function in follow-up assessments
– Symptom improvement: Workers re-assessed at 12 months showed improvement in respiratory symptoms (down to 11% reporting symptoms), consistent with adaptation and improved air quality

Worker engagement: Health monitoring was voluntary but well-attended (>95% uptake). Workers valued the structured health surveillance and felt protected by the comprehensive monitoring program.

Outcomes

Regulatory Success

The project’s comprehensive dust monitoring program and health surveillance exceeded regulatory expectations. The regulator commented (in project communications) that the standard was exemplary and should become model practice for infrastructure tunnelling projects.

Worker Health and Retention

No silicosis cases were detected; no work-related respiratory disease claims were filed during the project. Worker satisfaction surveys indicated high confidence in the project’s commitment to health protection. Retention rates for underground crews exceeded project averages.

Knowledge Advancement

The project generated detailed baseline and compliance data on tunnel dust exposure for an Australian infrastructure context. This data was shared with industry (with confidentiality protections) and contributed to emerging best-practice guidance for tunnelling occupational hygiene.

Cost and Value

Monitoring cost over 36 months: Approximately AUD $1.2 million (personal and area sampling, health assessments, real-time equipment, reporting)
Control cost: Approximately AUD $2.1 million (ventilation upgrades, dust suppression, portable collection)
Value delivered: Zero occupational health incidents, regulatory compliance, competitive advantage in future projects, and protection of workforce health

Lessons and Takeaways

Underground work requires proactive baseline planning: Designing monitoring protocols before excavation commenced allowed early problem detection and rapid control response.
Real-time monitoring enables adaptive control: Daily hand-held dust screening allowed the work team to detect rising dust and activate controls before exceedances occurred.
Health monitoring is feasible even in remote underground settings: Bringing health assessments on-site (portable X-ray, spirometry) maximised participation and worker confidence.
Blended control strategies are necessary: TBM work needed equipment upgrade (spray system, dust collection); conventional excavation needed procedure change (reduced charges, extended ventilation) and temporary suppression; spray work needed operator-level controls. No single control solved all areas.
Precedent-setting projects benefit industry: The detailed data, transparent reporting, and demonstrated success became a template for future tunnelling projects and contributed to emerging occupational hygiene best practice in Australian infrastructure.

Supporting Major Tunnelling and Infrastructure Projects Nationally

Underground infrastructure projects present unique occupational hygiene challenges. Dust monitoring must begin before excavation and be sustained throughout project life. Anitech’s expertise in tunnelling and underground work is recognised across Australia.

Request a project monitoring program design for your tunnelling or major infrastructure project.

IMAGE_REQUIREMENTS

FEATURE: 1200x630px. Underground tunnel interior: tunnel boring machine visible in distance with dust suppression spray mist visible around the cutting head; lighting from TBM lamps and portable site lighting; tunnel walls visible; worker safety gear in foreground; shows the TBM dust control in action, professional underground infrastructure scene.
BODY_1: 800x450px. Close-up of TBM tail shield area: dust suppression mist spray system visible in operation around the tail area; HEPA dust collection unit with ducting visible on the side; demonstrates real-time dust control systems in the high-risk tail shield zone.