Design & Planning of Ventilation Systems for Construction of Large Underground Caverns

Ventilation duct in a large underground cavern

As underground caverns grow in scale and excavation advances deeper, maintaining safe working conditions becomes an increasingly complex engineering task. This technical paper by Dr. R. K. Khali, Senior Vice President, and Naveen K. Bahuguna, Senior General Manager, GR Infraprojects Limited, examines the staged planning of a ventilation system for the Dibang Hydropower Project. Using computational fluid dynamics (CFD) simulations, the study evaluates airflow, duct configurations, pressure losses and fan power requirements to develop a ventilation strategy suited to the changing demands of cavern excavation.

Abstract

Ventilation is needed for underground caverns to provide breathing air to the personnel working inside, provide fresh air quality by removing atmospheric contaminants such as gases and dust, and provide oxygen and cooling for operating vehicles/construction equipment in the cavern.

Environmental conditions such as temperature, gases, and airflow concentration have to be controlled and kept within an acceptable range in underground structures where there is a fleet of construction equipment and working crews. The ventilation system therefore has to take into account all the ventilation parameters to deal with efficiency and health and safety issues at the same time.

The design of ventilation system for construction of large underground caverns is very crucial to deal with efficiency, health and safety issues. As underground cavern sizes increase, the airflow quantity that reaches the deepest part usually decreases due to an increase in airway resistance which then reduces the fan performance and air quality in this area. This issue requires a detailed CFD analysis.

The aim of this paper is to design an effective system for managing an underground environment for safe working during construction with the help of CFD simulation studies, and propose an optimum ventilation system for large underground caverns with respect to air quality and power consumption.

Introduction

Figure 1: Layout of Water Conductor System

The Dibang Multi-Purpose Project has been conceived to serve dual objectives of hydropower generation and flood moderation. It utilizes a net rated head of 222.5 m through the construction of a concrete gravity dam on the Dibang River, with an installed capacity of 2880 MW (12 × 240 MW units) and an annual energy generation of 11,223 MU. Water is conveyed from the reservoir through power intakes, headrace tunnels, and pressure shafts to the powerhouse for electricity generation.

The underground power complex comprises three large caverns: the Main Inlet Valve (MIV) cavern (17 m W × 29.1 m H × 250 m L), the Powerhouse cavern (23.5 m W × 54.3 m H × 427 m L), and the Transformer cum GIS cum Draft Tube Gate cavern (19 m W × 31.5 m H × 407.8 m L). Multiple adits and access tunnels have been planned to facilitate construction, including two D-shaped tunnels of 6.5 m diameter leading to the MIV cavern at EL 286.0 m and EL 264.8 m, respectively. For the powerhouse cavern, two 6.5 m D-shaped adits are provided, one at EL 262.4 m from the MIV floor and another at EL 301.3 m connected to the cable-cum-ventilation tunnel (8.0 m D-shaped). Two 9.0 m D-shaped main access tunnels (MAT-1 and MAT-2) will approach the powerhouse service bays at EL 283.7 m. Similarly, construction of the transformer cavern will be supported by a 9.0 m D-shaped access tunnel at EL 312.5 m and an 8.0 m cable-cum-ventilation tunnel at EL 334.5 m.

Figure 2: Underground Caverns Layout

Excavation of such large underground caverns generates significant amounts of dust and exhaust gases from blasting and heavy equipment operations. If not controlled, these pollutants can deteriorate air quality, affect machinery performance, and endanger worker health. Temporary forced ventilation is therefore essential to supply sufficient fresh air, dilute contaminants, and maintain acceptable working conditions. The requirement is particularly critical in major structures such as the MIV, powerhouse, and transformer caverns, where excavation progresses in stages, progressively increasing both the excavated volume and the corresponding ventilation demand. Hence, the ventilation system must be designed to adapt dynamically to each stage of excavation while meeting the minimum airflow and safety criteria.

Materials And Methods

Table 1. Deployment of equipment for the PH cavern construction

The maximum utilization of above deployed HP considering load factor as 50% and hence total horsepower requirement will come down to 1461 HP. Accordingly, temporary ventilation system is computed for 1500 HP requirement.

Table 2. Calculation for Minimum Flow required

The following table indicates the minimum threshold value to be maintained inside the caverns

Following duct diameter and fan flow have been considered for simulation purpose:

Pressure drops and Friction factors are computed using CFD analysis:

Flexible ducts, typically constructed from a combination of rigid circular reinforcement and flexible sheet lining, exhibit complex flow behavior at high velocities. Under such conditions, turbulence significantly alters the effective friction factor, necessitating detailed analysis rather than reliance on empirical values alone. To quantify this, Computational Fluid Dynamics (CFD) simulations were carried out for both circular and elliptical ducts.

Figure 3: Circular Duct

A representative 100 m length of flexible duct was modeled, with a design flow rate of 100 CMS for the circular duct and 150 CMS for the elliptical duct. Transient simulations were performed to capture flow instabilities and accurately determine pressure losses.

The key parameters for the 3D CFD simulations are summarized as follows:

● Mesh size: 0.05 m, fully unstructured polyhedral mesh

● Inlet velocity: 16.7 m/s and 25 m/s (depending on duct configuration)

● Air density: 1.15 kg/m³

● Turbulence model: k–ω SST

Figure 4: Elliptical Duct

For the circular duct (2.5 m diameter), the simulation indicated a pressure drop of 294 Pa with an effective friction factor of 0.031. In comparison, the elliptical duct (2.7 m hydraulic diameter) exhibited a lower pressure drop of 172 Pa, while maintaining the same friction factor of 0.031. Simulations for the ventilation system were carried out in different stages. The excavation of the powerhouse cavern is executed in a systematic, multi-stage sequence to ensure structural stability, safe working conditions, and effective ventilation throughout the process. Each stage incorporates a specific ventilation scheme designed to meet airflow requirements, remove dust and gases, and ensure worker safety, thereby aligning with best international tunnelling and underground construction practices.

The stages of excavation and their corresponding ventilation schemes are explained below:

Stage 1: Extension of Adit to powerhouse top and Adit to powerhouse bottom through the full length of powerhouse.

Figure 5: Stage 1 ventilation scheme

Stage 2: Sidewise Excavation of Adit to Powerhouse Top for full width of Powerhouse.

Figure 6: Stage 2 ventilation scheme

Stage 3: Total 6 no. of pilot shafts of diameter 2.1 m will be completed.

Figure 7: Stage 3 ventilation scheme

Stage 4.1: Partial bench down excavation above service bay level

Figure 8: Stage 4.1 ventilation scheme

Stage 4.2: Partial bench down excavation up to service bay level

Figure 9: Stage 4.2 ventilation scheme

Stage 5: Full bench down excavation up to service bay level

Figure 10: Stage 5 ventilation scheme

Stage 6: Partial bench down excavation below service bay level

Figure 11: Stage 6 ventilation scheme

Stage 7: Full bench down excavation of powerhouse cavern.

Figure 12: Stage 7 ventilation scheme

For the simulation of CO, NOx & opacity emissions, each simulation was runs for 20 minutes, and the peak values of the noxious gases were monitored and displayed in graphs.

Air speed and carbon monoxide level simulations

Figure 13: Air Speed / Figure 14: CO level

Carbon dioxide and nitrogen dioxide level simulations

Figure 15: CO2 level / Figure 16: NO2 level

Figure 17: Extinction Coefficient

Results and Discussion

Table 5. Powerhouse Cavern Stage Wise Pressure Drop & power

Access tunnels to service bays via MAT-1 and MAT-2

Figure 18: Access tunnel to Service bay-1 via MAT-1 / Figure 19: Access tunnel to Service bay-2 via MAT-2

Adit to powerhouse bottom and top ventilation layouts

Figure 20: Adit to PH Bottom via MAT-2 / Figure 21. Adit to PH Top via CVT

Table 6. Requirement of fans & duct size for the ventilation of powerhouse cavern

Conclusions

The design of duct systems for large underground caverns is constrained by the limited clearance available in the Main Access Tunnels (MATs) and the Cable-cum-Ventilation Tunnel (CVT). In addition, minimum airflow velocities must be maintained within the excavated sections to ensure adequate dilution of contaminants. Smaller duct diameters, while easier to handle, lead to higher air velocities and consequently excessive pressure drops. These conditions may induce duct instability, vibrations, and noise, posing safety risks to the workforce. Moreover, prolonged fan operation under stall conditions significantly increases the likelihood of equipment failure. For this reason, duct velocities should not exceed 25 m/s. To mitigate these risks, the use of ducts with at least 2.5 m circular diameter or 2.7 m hydraulic diameter (elliptical) is recommended.

CFD-based simulations were performed for multiple excavation stages of the powerhouse cavern. The main outcomes are summarized below:

A minimum of 153 m³/s of fresh air was ensured across all excavation stages, meeting the requirements of both the workforce and equipment, except in Stages 1 and 2 where the demand was proportionally reduced.
The target threshold velocity of 0.3 m/s was consistently achieved through combinations of 50 and 75 m³/s fans.
The minimum number of fans required for the powerhouse cavern is eight (six rated at 50 CMS and two rated at 75 CMS).
Concentrations of CO, NO 2, and particulates were maintained below the permissible threshold limits.
Given the progressive nature of excavation, Variable Frequency Drives (VFDs) are recommended for adjusting airflow in real time.
Following each blast cycle, fans should operate in extraction mode for at least 30 minutes to evacuate residual fumes.
Pressure losses increase significantly with smaller duct diameters; therefore, larger ducts are essential for operational stability.
The effective duct length for each stage should be limited to 15–20 m, with extensions added progressively as excavation advances.

These findings establish the ventilation system design parameters necessary to maintain safe and efficient working conditions during the staged excavation of large underground caverns.

References

GRIL, Amberg Engineering (2024). Simulation report of MIV, PH, TR & Associated Build Environment for Dibang Hydro Power Project).
Sharma, Ashok Kumar, (2020). Tunnel Ventilation System- During construction. IARJSET ASCE Journal, Vol-7, Issue 4, April 2020.
Fang Lin (2018). Construction Network Ventilation System for an Underground LPG Storage Cavern. Civil Engineering journal Vol-4, No. 7, July 2018.
Adrian Halin, Catrin Edelbro (2020). State-of-the Art Underground Ventilation. Rock Tech Centre, Lulea University of Technology.

Acknowledgement: This article has been reproduced from the proceedings of ITA WTC 2026 with the kind permission of the organisers and the author.

Published on 03 November 2026

Published in NBM&CW November 2026