Durability Design of CSO Tunnels

By Irwan Halim and Robert Vail

Nowadays, 100-year service is typically demanded for many combined sewer overflow (CSO) tunnel systems. This article describes the measures required to protect such systems against degradation and corrosion for achieving the intended design life.

CSO tunnel programs are designed to reduce CSOs from the surrounding communities. The projects typically include a main conveyance and storage tunnel constructed using tunnel boring machine (TBM); TBM launch and receiving shafts which are then transformed into grit screening and ventilation facilities; tunnel dewatering pump station (TPS); and flow drops and vent shafts with connecting adits to bring the excess wet-weather flow into the tunnel. The launch/grit screening shaft is connected to the TPS shaft with a suction header tunnel as shown in Figure 1, and the typical components of vortex style flow drop system in rock are shown in Figure 2 below.

Figure 1. Pump Station and Launch Shafts.

Figure 2. Typical Vortex Chamber, Deaeration Chamber and Adit.

Exposure Threats to Durability and Design Mitigation

The service life for conventional steel reinforced concrete is usually defined as the total duration to the initiation of steel corrosion plus five (5) years of corrosion propagation. Figure 3 below shows a simplified illustration of the service life of conventionally reinforced concrete.

Figure 3. Service Life of Conventionally Reinforced Concrete

The primary exposure threats to the durability of the tunnel structures include:

• Chloride ion diffusion
• Carbonation
• Sulfate attack
• Acid attack from hydrogen sulfide
• Freeze/thaw degradation
• Abrasion/Erosion

Reinforced concrete sewer tunnels and structures are typically required to be designed in accordance with ACI 350. According to ACI 350, structures for which liquid-tightness, gas-tightness, or enhanced durability are essential, design considerations should also conform to the requirements of environmental engineering concrete structures. Reinforced concrete subject to attack by chemical solutions or corrosive gases should be protected by using the appropriate type of cement, aggregates, and other ingredients; proportioning the concrete mix with the correct amount of each ingredient; and proper batching, mixing, placing, consolidating, finishing, and curing operations. The measures recommended in ACI 533.5R for mitigation of acid attack resulting from H₂S exposure include: dense/high-quality concrete; coatings; sacrificial layers; and use of calcareous aggregates. The evaluation of these measures is discussed in the paragraphs below.

Chloride Ion Diffusion

The threat from diffusion of chloride ions into conventionally reinforced concrete can be modeled utilizing the Life-365^TM software. The surface chloride concentration value in the model is set to be representative of the exposure expected for the project structures. Chemical analysis data from groundwater surrounding the structures as well as the typical CSO fluid that will be transported by the tunnel can be utilized in the model to assess the corrosion threats to the interior and exterior surfaces of the reinforced concrete structures and exposed hardware. The model calculates the time required for chloride ions to diffuse to the depth of reinforcement at a concentration above the threshold for initiation of corrosion. The modeling also accounts for seasonal deicing chemical (containing chloride ions) application, wet/dry cycles, and seasonal temperature fluctuations.

Mitigation of chloride ion diffusion in conventionally reinforced concrete is addressed by decreasing the permeability of the concrete and providing adequate cover for the reinforcement. The permeability can be decreased by decreasing the water-to-cementitious materials (w/cm) ratio and adding supplementary cementitious materials to replace a portion of the cement binder. ACI 350 provides limitations to the supplementary cementitious materials for cast-in-place concrete exposed to deicing chemicals, a common source of chloride ions. The standard allows up to 25% cement replacement with fly ash or 40% slag and an addition of up to 10% silica fume. The total combination of fly ash and silica fume cannot exceed 35% and the total combination of slag and silica fume cannot exceed 50%.

In ACI 350, the requirements for concrete exposed to deicing chemicals and for mitigation of erosion include a minimum compressive strength at 28 days of 5,000 psi; maximum w/cm ratio of 0.40; maximum entrained air content of 6%; minimum cementitious materials content of 610 lb./yd.³; and the use of hard, dense, clean aggregates. The commentary suggests the replacement of a portion of the cement with silica fume improves erosion resistance when hard aggregate is not utilized. The requirements for increased compressive strength and cementitious materials content typically reduce the permeability of the concrete and also improves resistance to chloride ion diffusion.

For example, these parameters were utilized to evaluate six different concrete mixes with 5,000 psi and 6,000 psi strength in the Life-365 modeling software. The model parameters were set for achieving a minimum 100-year design life with a minimum clear cover of 1.15 in. over conventional steel reinforcement for mitigation of chloride ion diffusion for each of the concrete mixes. Out of the six mixes evaluated for each w/cm ratio, the mixes that achieved a minimum required 100-year design life for either the 5,000 psi or 6,000 psi concrete were:

• 25% Fly Ash, 0% Silica Fume, 2 gallons/yd³ 30% calcium nitrite inhibitor.
• 40% Slag, 0% Silica Fume, 2 gallons/yd³ 30% calcium nitrite inhibitor.
• 25% Fly Ash, 8% Silica Fume, no corrosion inhibitor.
• 40% Slag, 7% Silica Fume, no corrosion inhibitor.

The model shows the slag concrete performs slightly better than the fly ash concrete and an increase in silica fume decreases the permeability of the concrete to chloride ion diffusion. Silica fume also increases the compressive strength of concrete. Calcium nitrite inhibitor is primarily utilized to mitigate reinforcing steel corrosion due to chloride ion diffusion. Calcium nitrite has also shown some benefits to mitigation of carbonation and sulfuric acid attack. The modeling shows a 100-year design life is anticipated to be achieved for the minimum required clear cover for chloride ion mitigation when no silica fume is included, and calcium nitrite is added at a rate of two gallons per cubic yard of concrete. An alternate to the calcium nitrite inhibitor is an addition of silica fume to the concrete mixes. The typical serviceability requirement that limits the maximum allowable crack width and depth in CSO tunnel structures also contributes to the water tightness of the tunnel liner and thereby help ensure long-term protection from durability threats due to chloride corrosion.

Carbonation

The threat of deterioration due to carbonation of the concrete in the tunnel structures is anticipated to be low. Carbonation of an interior surface of a tunnel structure installed below the groundwater level is rare due to the saturation of the concrete. Carbonation can also occur in groundwater exposure when the groundwater is high in bicarbonate content. Mitigation of carbonation in conventionally reinforced concrete is addressed in a similar manner as mitigation of chloride ion diffusion.  Mitigation is achieved by decreasing the permeability of the concrete, sufficient reinforcement cover, and limiting cracking. Limiting the maximum crack width to 0.25 mm and depth to 5 mm for the purpose of mitigating chloride ion diffusion will also mitigate the low threat of carbonation.

Sulfate Attack

The threat of sulfate damage to the exterior surfaces from groundwater and the interior surfaces from the CSO fluid of reinforced concrete structures depends on the sulfate ion concentration in the fluid. This threat is normally addressed in accordance with of ACI 350 based on the threat levels. Sulfate damage to concrete can be mitigated with sulfate-resistant cement (C₃A <8%) such as ASTM C150 Type II or C595 Portland-limestone cement, a low w/cm ratio, high compressive strength, high cement content and the use of supplementary cementitious materials. ACI 350 requires a maximum w/cm of 0.42 and a minimum compressive strength of 4,500 psi for exposure to a moderate sulfate ion threat.

Acid Attack from Hydrogen Sulfide

An approach adopted by the EPA and ASCE can be used to assess corrosion of the structure internal surfaces from exposure to hydrogen sulfide. This approach estimates the loss of material as a function of time and CSO fluid characteristics.

Corrosion rate determination due to exposure to hydrogen sulfide was addressed by the Environmental Protection Agency (EPA) in 1974. ASCE Manual of Practice No. 69 also adopted the EPA procedure for determining the concrete corrosion rate. The concrete corrosion rate is a function of the hydrogen sulfide gas flux to the tunnel (pipe) crown, the concrete alkalinity, and the fraction of produced sulfuric acid which reacts with the concrete. This relationship is described by Expression (1).

in which

C     = maximum rate of concrete corrosion by sulfuric acid at the tunnel (pipe) crown, in/yr

Ø_sw    = hydrogen sulfide gas flux to the tunnel (pipe) wall at 20^oC (68^oF), g/m²-hr

A     = concrete alkalinity; expressed as equivalent grams of calcium carbonate per gram of concrete

k      = acid reaction factor

CCF =          crown corrosion factor, ranges from 1.5 to 2.0

TCF =          turbulence corrosion factor, equal to 1.0 for uniform flow conditions well removed from turbulence points.

The acid reaction factor k is the sulfuric acid fraction which reacts with cement at the tunnel wall. This fraction is a function of the amount of H₂S released to the sewer atmosphere, the acid runoff, and failure of all the acid to react. When acid production is rapid or if a significant condensate amount is formed, k may be as low as 0.30 because the acid runs down the lining and back into the wastewater. For slow, uniform acid production, the value of k will approach 1.0. Unless specific system information is available, a value of k = 0.95 is typically used to estimate the maximum potential corrosion rate in tunnels larger than 60 inches in diameter.

The alkalinity, A, or acid neutralizing capacity of the tunnel material can be determined by analysis or can be estimated from the composition of the concrete mix. Anhydrous cement has an alkalinity, expressed as calcium carbonate equivalent, equal to about 1.18 times its weight. Thus, the cement proportion used, expressed as cured concrete weight percentage, multiplied by 1.18, results in the alkalinity contributed by the cement. For concrete made with granitic aggregate, the alkalinity is usually between 16% and 24%.

The H₂S flux, ᶲ_sw, to the tunnel lining exposed above the water surface is expressed as:

in which

s       =          the slope of the flow energy grade line (i.e., tunnel slope for uniform steady flow)

v       =         stream velocity, ft/s

[DS] =         dissolved sulfide concentration in waste stream, mg/l

J       =          fraction of dissolved sulfide presents as H₂S

b      = surface width of waste stream, ft

P’     =          lining perimeter exposed above waste stream surface, ft

The [DS] is typically taken from the CSO fluid test data. The total sulfide concentration includes dissolved and precipitated metal sulfides. Assuming the [DS] comprises only H₂S and HS-, the J parameter can be calculated for a given pH at 20°C (68°F) by deriving it from the equilibrium expression as:

The ᶲ_sw can be estimated by Expression (2) or using nomographs provided in ASCE Manual of Practice No. 69.

The CCF in Expression (1) accounts for the increased corrosion rate at the tunnel (pipe) crown. In tunnels with uniform hydraulic conditions well removed from points of turbulence, the crown corrosion is calculated to be about 1.5 times the average perimeter corrosion. The ASCE recommended design value for CCF is 2.0.  The TCF is introduced to account for the sulfide corrosion effects being accelerated at turbulence points. The design of the drop shafts, deaeration chambers and adit connections to the main tunnel are to minimize turbulent conditions. A TCF value of 1.0 can be utilized in the calculations to account for the turbulence mitigation design. Over the range of temperatures normally encountered in CSO tunnels, the temperature effect on transfer rate may be neglected.

In the EPA procedure the sacrificial concrete thickness to mitigate hydrogen sulfide damage can be approximated utilizing the corrosion rate calculated above and the service life requirement for the tunnel liner. The sacrificial concrete thickness is approximated using:

z = LC                                                                                         (4)

in which

L =     service life, yr

z =     allowable thickness loss, in.

C =    the maximum expected corrosion rate, in./yr.

Combining Expressions (1) and (4) yields the life factor Expression (5):

Az = 0.45 k Ø_sw L (CCF) (TCF)                                                   (5)

Life factor Az is a combined factor which accounts for material thickness and acid neutralizing capacity. This suggests there is some design flexibility in how the life factor requirement can be met (EPA 1974).

The calculation of concrete lining corrosion rate should be based on estimated CSO fluid depth inside the tunnel and total duration over a period of one year. The corrosion rate associated with each flow depth can be calculated and the weighted average of the calculated corrosion rates used to estimate the expected loss of concrete lining over the lifetime of the tunnel.

Dry pump station shaft is not anticipated to be exposed to the CSO fluid and can be designed in accordance with ACI 350 to achieve its design life. The interior surface of the pump station shaft will be exposed to air and the external surface will be exposed to soil, rock and groundwater.

The internal surface of the TBM launch and grit screening shaft will be exposed to varying levels of the CSO fluid. The minimum CSO elevation is expected to be at the approximate invert elevation of the tunnel at the connection to the launch shaft. It is expected that most of the degradation due to hydrogen sulfide that requires mitigation will occur within the 5-foot vertical zone around the minimum CSO fluid elevation. Protective coatings for mitigation of hydrogen sulfide degradation can be applied over this zone but will require maintenance over time. Otherwise, sacrificial concrete thickness can be provided to mitigate the threat of damage due to hydrogen sulfide generation in the fluid and microbiologically influenced corrosion. The receiving and ventilation shaft is not anticipated to maintain a level of CSO fluid and is therefore not expected to be exposed to degradation from the generation of hydrogen sulfide. In this case the minimum clear cover for the conventional reinforcement can be provided for mitigation of corrosion due to chloride diffusion.

Freeze/Thaw Degradation

The threat of damage due to freeze/thaw is typically very low for CSO main tunnel due to the temperature of earth at the depth of burial.  However, if precast tunnel liner is used, protection against freeze/thaw damage should be considered to prevent damage that may occur during storage of the tunnel liner segments prior to installation. There is also threat of damage due to freeze/thaw for the portion of the other structures from grade down to the depth of frost line. Air entrainment in concrete mixes mitigates the threat of freeze/thaw damage but can lessen resistance to abrasion. ACI 350 recommends a maximum air entrainment of 6% for mitigation of possible abrasion.

Abrasion/Erosion

The threat of cavitation in the vortex chambers is anticipated to be low based on the anticipated maximum fluid velocity and smooth surface. In general, cavitation typically does not occur on a smooth surface at fluid velocities less than approximately 40 ft/second. The threat of abrasion/erosion in the vortex chambers is anticipated to be low to moderate because of the upstream structures typically utilized to minimize the size and number of entrained solids in the CSO stream. The threat of abrasion/erosion due to cavitation is anticipated to be moderate for the deaeration chambers due to the turbulent conditions that can exist during high flow rates. The air entrained in the CSO fluid in the drop shaft will help to mitigate the risk of cavitation damage to the concrete in the deaeration chamber. The threat of abrasion/erosion due to cavitation in the deaeration chamber and adit tunnel is low based on the maximum anticipated fluid velocity and smooth surface in the chamber.

The risk of abrasion/erosion in CSO tunnel is low due to the infrequent high flow rates within the tunnel. For concrete structures subjected to abrasion, ACI 350 imposes the following additional requirements:

• Minimum f’c = 5,000 psi at 28 days
• Maximum w/cm ratio = 0.40
• Maximum air content = 6 percent, or 3 percent if not subject to freezing and thawing
• Minimum 610 lbs/yd³ of cementitious materials
• Hard, dense, clean aggregates

Increased f’c and reduced w/cm ratio from those permitted by the ACI standard will further improve the abrasion/erosion resistance.

Material Selection

Many recent CSO tunnels utilize steel fiber reinforced concrete (SFRC) as final lining. Research has shown that SFRC does not respond to chloride ion diffusion in the same manner as conventionally reinforced concrete. The chloride ion concentration threshold for corrosion of SFRC has been shown to be approximately ten times higher than for conventionally reinforced concrete (ACI 533.5R). Damage due to corrosion of steel fibers is generally limited to the outer 1 to 5 mm in aggressive environments and is considered aesthetic (The International Federation for Structural Concrete (fib) Bulletin 83). Cracking is not a threat due to environmental exposure but rather a serviceability consideration. Cracking is controlled in SFRC much better than conventionally reinforced concrete and this feature contributes to the increased resistance of SFRC to the environmental exposure conditions such as chloride ion diffusion. There appears to be a consensus that corrosion of the steel fibers will occur where cracks with a width exceeding 0.5 mm exist (fib Bulletin 83). The existing research data is somewhat inconclusive for smaller crack widths but suggests corrosion is minimized with crack widths of 0.25 to 0.30 mm or less. The diffusion of chloride ions into the concrete modeled utilizing the Life-365^TM software or fib model codes is not applicable to SFRC. SFRC does not have a consistent depth of cover over the reinforcement. Therefore, the rate of diffusion of chloride ions is not directly relevant to the durability of structures constructed with SFRC. The durability threat to SFRC from Hydrogen Sulfide exposure can be mitigated using sacrificial concrete in the design.

The suction header is typically a pressurized steel-lined pipe connecting the grit screening and TPS shafts, consisting of welded steel pipe with outside backfill concrete. Mitigation of corrosion on the exterior surface of the suction header pipe should be provided by the concrete backfill. Steel in concrete with a pH above approximately 9.5 is in a passive state with minimal corrosion. An external protective coating, typically consisting of hydrogen sulfide resistant polyurethane compliant with AWWA C222, can be applied to the pipe surface to supplement the protection provided by the concrete. This coating should have a total dry film thickness of 35-45 mils.

The interior surface of the steel suction header pipeline will be continuously submerged in the CSO fluid. There is a potential for release of hydrogen sulfide gas from the CSO fluid as discussed above for the reinforced concrete elements. However, there is not expected to be an open headspace in the suction header to provide water/moisture on the pipe wall necessary to create corrosive sulfuric acid. Cement mortar linings compliant with AWWA C205 are intended for steel pipelines that are continuously filled and is a suitable material for mitigation of internal corrosion of the suction header.

Fiber reinforced polymer mortar (FRPM) material can be used for smaller shafts construction such as the drop and vent shafts and is highly resistant to degradation due to abrasion, sulfates, chlorides, hydrogen sulfide and carbonation and does not require additional corrosion mitigation.

Conclusions

This article describes a systematic and integrated durability design approach for CSO Tunnel and ancillary structures to achieve its required design life as summarized in the Figure 4 below. However, the durability threats and mitigations are project specific and must be identified based on project specific conditions.

Figure 4. Systematic Approach to Durability Design

REFERENCES

ACI-201.2R, Guide to Durable Concrete, American Concrete Institute, 2016.

ACI-224R, Control of Cracking in Concrete Structures, American Concrete Institute, 2001.

ACI-350, Code Requirements for Environmental Engineering Concrete Structures and Commentary, American Concrete Institute, 2020.

ACI-533.5R, Guide for Precast Concrete Tunnel Segments, American Concrete Institute, 2020.

ASCE, Manuals and Reports on Engineering Practice – No. 69: Sulfide in Wastewater Collection and Treatment Systems, American Society of Civil Engineers, 1989.

ASTM C150, Standard Specification for Portland Cement, ASTM International, 2020

ASTM C595, Standard Specification for Blended Hydraulic Cements, ASTM International, 2023

AWWA C205, Standard Cement-Mortar Protective Lining and Coating for Steel Water Pipe, 2018

AWWA C222, Standard Polyurethane Coatings for Interior and Exterior of Steel Water Pipe and Fittings, 2018

EPA, Process Design Manual for Sulfide Control in Sanitary Sewerage Systems, U.S. Environmental Protection Agency, October 1974.

Federation international du beton (fib) Bulletin 83, Precast tunnel segments in fibre-reinforced concrete, 2017.

Abot the Authors

Dr. Halim is AECOM Chief Engineer for water tunnels with 34 years of experience in geotechnical and underground projects. He has provided engineering and design services for major water/wastewater as well as transit agencies throughout North America. Irwan’s expertise includes design and construction of mined and cut-and-cover tunnels in both soft ground and hard rock; large TBM-driven tunnels, drill-and-blast, major SEM excavations, and tunneling through complex and mixed-face ground conditions.

Robert Vail is the AECOM Discipline Leader for Stray Current and Corrosion Control with 39 years of experience in the industry.  He is certified by the Association for Materials Protection and Performance as a Corrosion Specialist.  He provides engineering and design services for a very wide variety of structures both domestic and internationally.