Designing Tunnels for Effective Odor and Corrosion Control

The construction of large diameter, deeply buried sewer interceptors and in-line storage tunnels has increased dramatically over the past 15 years. Tunnels have proved to be a cost-effective way to eliminate numerous pump stations, prevent accidental overflows and efficiently store and move wastewater in collection systems. However, there have been incidences of odor complaints due to unusual and forceful ventilation phenomenon associated with tunnels. In addition, tunnels are more prone to corrosion resulting from hydrogen sulfide gas conversion to sulfuric acid because there is less exchange of air with the atmosphere.Over the past 15 years we have come to recognize the classic “deep tunnel” ventilation dynamics but the engineering community has had little formal research or hard data upon which to base designs for ventilation correction. The issue of deep tunnel ventilation has received more attention in the past 10 years and researchers have been developing relationships between flow volume, friction factors, dropstructure configuration and airflow/pressure in a sewer.Recent work has yielded new data that suggest a relationship between pipe diameter, slope, water velocity and dropstructure design which can reasonably predict unusual ventilation events from deep tunnels and allow the design of corrective measures before startup. Relationships between dropstructure design, size, flowrate and air ventilation have been developed. In addition, refinement of the friction drag coefficients for normally flowing gravity sewers and tunnels has led to more accurate prediction of the full-scale ventilation potential. Proactive incorporation of sewer ventilation and odor control designs into major interceptor and deep tunnel sewer designs prevents public relations problems following major public works projects.Corrosion protection for deep tunnels requires a new way of thinking since typical sewer corrosion methods do not apply or are uneconomical. HDPE or FRP pipes are not an option with single-pass tunneling methods and coatings and liners on pre-cast tunnel segments have met with mixed success. New construction methods which allow a wider range of corrosion protection alternatives for single-pass tunnels have recently been developed.This paper provides a detailed discussion of tunnel ventilation, odor and corrosion control, including calculation guidelines for predictive ventilation of deep tunnels from friction drag dynamics, tunnel storage conditions and dropshaft air eduction.