Corrosion problems in sanitary sewer conveyance systems are a worldwide issue and can reduce 100-year-design-life systems to less than 10 years and lead to sewer collapses. Moreover, effluent characteristics can vary leading to various corrosion rates over the life of the structure (EPA, 1991). Even CSOs with dry weather flows in northern climates may have corrosion problems (Chapman, 2002). Thus, robust higher initial cost liners are more cost effective when considering reduced maintenance, repairs, and longevity. External corrosion issues are reviewed in the original paper published at the Rapid Excavation and Tunneling Conference (San Francisco, 2011). The full version of the paper is available from It should be noted that this T&TNA article corrects errors published in the original paper.

One-pass versus two-pass liner
For small diameter conveyance lines (drive lengths between shafts are 200 to 450m):

• Corrosion-resistant pipe can be installed in one-pass if microtunneling is feasible (see Figure 1).

• Corrosion-resistant carrier pipes (e.g., fiberglass reinforced plastic, clay, polymer, PVC, HDPE pipe) can be installed within a jacked casing (Figure 1) or the pre-cast concrete tunnel liner (see Figure 2). The annular space between the conduits is backfilled with cellular concrete, sand or grout.

For tunnels that may be larger diameter and with longer drive lengths (>450m):

• A cast-in-place concrete lining is placed within the initial tunnel lining with a protective plastic liner (typically PVC or HDPE) mechanically anchored to the cast-in-place (CIP) concrete using a variety of anchorage systems (Figure 2). In lieu of CIP, a plastic liner can be attached to the initial liner using a thin layer of mastic, however, some problems with adhesion to the pre-cast concrete liner segments have been experienced on the Big Walnut Augmentation/Rickenbacker Interceptor (BAWRI) and Big Walnut Outfall Augmentation Sewer (BWOAS) CSO in Columbus, Ohio (Rush, 2010).

• A corrosion resistant precast segmented liner can be fabricated by placing an anchored plastic liner within the segment molds and subsequently the plastic membrane is welded in the field or polymer segments may be used, eliminating the thin plastic membrane (Figure 2).

One-pass liner
One-pass segmented tunnel liner systems, whereby the initial or primary support also functions as the permanent or final liner, have been employed for nearly 70 years. The first application of precast reinforced concrete segments as a permanent tunnel liner was for construction of the London Underground (Groves, 1943). The use of rubber gaskets for waterproofing segment joints goes back 45 years. In 1965, a sewer tunnel employed neoprene gaskets in New York (Anon 1965).

For a gravity sewer tunnel in warm climates, however, because of corrosion issues and high potential for hydrogen sulfide generation, the initial liner would have to be made of special corrosion-proof materials, have added thickness for the life of the structure or have an embedded liner. The first known usage of an embedded liner in a segmented liner is for a section of a Russian subway station; the segment molds were lined with stainless steel and then welded within the erected liner (Saumon, 2000). Recently, embedded plastic liners were used for the Upper Northwest Interceptor in Sacramento, California, the Panama City gravity sewer tunnel (T-Lock, TBM, 2009), and in a sewer tunnel in Moscow (Combisegments, Lang, 2010, 2011). Other corrosion-proof prototype one-pass liner systems have been proposed in small demonstration projects as discussed later in this article.

With the advances and development in corrosion materials in segmented tunnel liners, the cost advantages of one-pass systems, the shorter construction schedule, and the logistic of installing the second pass have made one-pass liner systems more competitive in recent projects as shown in Table 1.

One-pass options
On many projects, the variable or weak nature of the ground requires immediate support of ground using segmented and gasketed liners. The segments are typically pre-cast concrete with various accessories and a variety of different fastening devices. The use of rods and dowels (e.g., Sofrasar, TTC, Buclock) made of inert materials and no bolt pockets when gasket sealing can be accommodated without bolts have significant advantages in long term corrosion issues.

Thin membranes are inert materials attached to the surface of concrete and protect it from deterioration. When properly designed and installed, they offer dependable and durable long-term protection. From construction and cost viewpoints, most of the materials that fall in this category have some drawbacks. If used in conjunction with precast segmental tunnel lining system (i.e., embedded during the casting process), the numerous vertical and horizontal joints in the segment lining must be manually welded after completion of tunneling. Joint welding inspection is required and is included in project pricing. Field experience indicates joint welding and inspection is accomplished under budget.

T-Lock is a proprietary PVC liner manufactured by Ameron, which uses T ribs to anchor to concrete (see Figure 3, far right). It has been used in the wastewater conveyance industry for more than 50 years in pipelines such as RCP, RCCP and PCCP, as well as lined in cast-in-place vaults and junction structures. PVC was also specified as cast-in-place liners within tunnels for the Singapore Deep Tunnel Sewerage System (DTSS) but all PVC liners were effectively written out of the project specifications, as seen by many, by the requirement for unnecessary excessive tensile strengths and other material requirements.

Contractors had little choice but to use HDPE and its anchorage system, which may have resulted in higher project costs. Marshall et al. (Marshall, 2007) reported that HDPE is the method of choice for future projects similar to the DTSS because of a higher temperature resistance for fires that might occur during construction. However, HDPE is still combustible and will burn completely unless fire retardants are put into the material at manufacture. Even with fire retardants HDPE is flammable. By comparison PVC is flame-resistant and self extinguishing when the flame or fire is removed. For more information about HDPE flammability, see the Executive Summary, Metro Red Line Fire, the City of Los Angeles (7/13/1990).

It should be noted that standard specifications for PVC anchored liners require that pipe joints be tested before welding and that the joints remain open three days after external dewatering has been discontinued to see if there is any infiltration. Nevertheless, some owners, e.g., Metropolitan St. Louis Sewer District, do not prefer PVC in pipes because of the inability to test the joints after welding and the potential for seepage and bulges if there are leaks at the joints, even if the joints are relatively few (in comparison to a segmented liner). It should also be noted, however, there have been no failures of PVC liners due to corrosion; the issue is proper installation, as with any system.

The T-Lock PVC and HDPE embedded into segment system and polymer segmented liner system were specified for the 3.7m i.d. Upper Northwest Interceptor project in Sacramento (TBM, 2009), where the external groundwater pressure at the crown is a nominal 0.6 bars. T-Lock was chosen as the low cost material over polymer concrete and HDPE. The standard T anchors are designed for 30m of hydrostatic head. The PVC membrane is cut to size and laid in the precast concrete segment mold. The T anchors were installed parallel to the tunnel and for 360o around the perimeter. A small hole was placed at the invert; every 1.2m at the circle joint to relieve pressure on the membrane should leaks occur. After the segments were erected within the excavated bore, a crew welded overlapping strips with a heat gun at all joints and subsequently spark tested the liner for holidays [flaw or gaps]. The amount of heat welding for a 1.2m ring is over three times the amount of welding for a 3.6m section of RCP. The work was performed under the agency inspection and with training and periodic quality assurance of an Ameron field service representative.

The Combisegment (Lang, 2010, 2011) by Herrenknecht Tunnelling Systems incorporates GRP (glass reinforced plastic) and, like T-Lock, is placed in molds, but it is bounded with anchors and aggregate. Also, the GRP is molded at the corners to the gaskets, eliminating the need for welding at the joints. The Yuzhny Sewage Pipeline, a 500m-long, 2.75m i.d. tunnel, incorporated five trapezoidal segments with this embedded GRP. The project was complete in 2009. The tunnel was 18m deep with 11.5m of groundwater and is located in Tzarisyno, Moscow. The Russian contractor will continue to use the Combisegment technology on the next sewer jobs. Also, Combisegments are specified for a test section at the OARS (Olentangy-Scioto Intercepting Sewer Augmentation Relief Sewer) tunnel project in Columbus, Ohio, (6m i.d.), where tunneling operations are to start early 2012.

Embedded liner
The trend for embedded membrane liners will continue in order to save on installation costs and schedule and to provide the same robust system of a two-pass system (EPA, 2006). For example, the Steuler embedded liner system is under development and involves an HDPE membrane that is anchored to a cast-in-situ concrete lining using special molds. Also, a prototype plastic liner system by Construction Polymers, using non-woven geomembrane bonded to the back of Seaman Corporation’s XR-5 polymeric membrane, has been developed that is bonded to precast concrete segmental liner using microfibers without the use of a traditional T, X, +, or other macro anchors, or mastics. The bonding capacity has been tested extensively for high water pressures, bond pull off, and other corrosion tests. The advantages of such a bonding system would be the elimination of possible bulges except at joints, unless welding could be eliminated by wrapping or protecting the joint surfaces in other ways like the Combisegment system.

Thin membrane liners have been in the wastewater conveyance industry for more than 60 years. The emergence of the application of membranes integrated and embedded in precast concrete segment appears to be an economical choice at this stage of the evolution. Further developments are evolving. It is important to note that the standard capacity of anchors to withstand pullout from external hydrostatic pressure is 30.5m of hydrostatic head, suitable for many deep tunnel applications. Deeper applications would require engineering of anchors and their spacing as appropriate.

Precast polymer segments
Alternative non-cementitious materials for segment manufacturing such as thermosetting polymers (epoxy, vinyl ester) products as a substitute for the cementitious binder matrix have been an emerging technology and were approved as an option for tunnel work on the Upper Northwest Interceptor.

Cast polymer products (polymer composites) have been used in the mining and chemical industries for vaults, vats and conveyance pipes and channels for more than 25 years. Polymer concrete has been used in smaller diameter microtunneling pipe (Polycrete or Meyerpipe). Polymer conveyance products have not been widely used in the US in the wastewater industry, probably primarily due to the higher costs and concern by some owners regarding creep. Small diameter three-piece segmented liners made of polymer concrete less than about 1.2m diameter have been used in Europe. Prototype polymer cast concrete has been made for 2.9m i.d. segmented liners by Interpipe around 2006. SolidCast polymer has also been promoting its application to the segmented liner industry. It made a 2.9m diapeter prototype, and is proposing to make a larger diameter prototype.

There are several different types of corrosion-resistant polymer concrete that have been used in pre-cast applications. ‘Polymer concrete’ has been used very broadly to describe various products, only some of which are considered to be structural materials (Karbahri, 2006).

Polymer concrete (PC) uses a system that integrates a resin binder and aggregate together with no water or cement paste to form the composite material. PC commonly incorporates polyester, vinyl esters or epoxy resins. The polyester resin PCs use polyester with a styrene monomer; the vinyl resin PCs use vinyl esters with styrene monomers and oligomeric vinyl esters or epoxy resin PCs.

PCs that incorporate polyester resins with styrene or vinyl resins with styrene monomer release volatile organic compounds (VOC) and are a hazardous pollutant (HAP). During curing, styrene will react emitting fumes and also causing the polymer concrete matrix to shrink, which can be difficult if incorporating steel reinforcing elements. Flexiblizers are added to allow incorporation of steel reinforcement. Vendors who supply such technology include Interpipe (Osborn and Espeland, 2007).

PCs that incorporate oligomeric vinyl esters resins do not release VOC or HAP. Shrinkage or adding steel reinforcing does not require the addition of flexibilizers. Vendors who supply such technology include SolidCast (Cubeta, 1987 and Cubeta, 2008). PCs that incorporate epoxy resins minimize most of the environmental, corrosion, physical property and reinforcing issues. They are, however, slow to cure and generally require heat curing, additional molds and tooling to meet production goals. Removal of styrene reduces polymer concrete shrinkage.

PC is, by common definition, a material that uses a resin binder with aggregate to form a composite material. The resins are impregnated with a catalyst that induces chemical reaction(s) within the resin, which begins its set or hardening. Like cement, the reaction is exothermic. The resins and aggregate can be off-the-shelf items, but for certain applications the amount of catalyst in the resin and aggregate mix and how it is all mixed together is a closely guarded secret. Too much catalyst or mix time, and the resin sets up before it even gets into the forms. Too little, and the mix is still lumpy pudding two days later. The mixing and distribution of aggregate in a resin matrix is not something easily done (sometimes sands may be added to the matrix as well), and the process is highly confidential to each industry formulator. The blending of all the components (aggregates and resins) also determines the quality of the product. Once cure is complete (typically between 30 minutes to as much as four hours) the reaction(s) occurring in the resin are nearly complete, and most of the strength in the resin has been obtained. The amount of strength achieved versus time for the resin is logarithmic in nature, with a large majority of the cure occurring in a relatively short amount of time.

These products are considered structural and are generally used in precast concrete members, such as 24in (0.61m) storm water conduits, precast manhole risers and beams. The strength of this system comes from the interlock of the aggregates and the tensile, shear, and compressive strength of the resin. Often people question how a polymer concrete can have strength without cement. Quite simply, the ‘cement’ used in normal concrete is replaced by the ‘resin’ which has the required material properties to hold the aggregate interlock and utilize the strength of the aggregate. The limitation to this system is that the aggregate has to be clean and dry, so often washers and ovens are used to reduce the water content down to one to two per cent (since aggregates absorb eight to 13 per cent water content just sitting around in the open). The resins used are generally high-quality vinyl esters or polyesters, and set is quick and hot. Shrinkage is controlled in the mold, and stripping can occur within two hours. New advancements in resin technology have significantly reduced shrinkage by the removal of VOCs (styrene) from the base resin formula.

The advantages are the end material has high tensile strength, which can allow for lighter and thinner segments. The unit weight is similar to normal weight concrete. This helps control cracking and since the modulus of rupture is higher, allows for some conservative design using the tensile strength. The other big advantage is that the resins used are often highly resistant to corrosion. Use of this material in a sewer for example means that not only is the surface protected, but since the resin is throughout the matrix, even in the event of spalled materials (as for a coated or membrane system), the underlying material has the same corrosion resistance.

The disadvantages are mainly the industry impression. This formed a major misconception in the industry as to the application of polymer concrete and the appropriate and inappropriate applications and mixes. But aside from the materials, emissions can also be difficult to deal with. The EPA has really cracked down in recent years to enforce the emissions limits on industries.

Issue of PC creep
In terms of creep and deformation, there are two scenarios: deformation and creep during production, and in service. During casting and or application, the resins obtain a high level of strength quickly, but if loaded too quickly, the polymer chains can mobilize and deformation occur. As long as the product is protected until a sufficient amount of strength has been obtained, there should be no creep. In considering PC in tunnel segments, one must wait until the strength has been achieved to strip the segment, and then the segments must be appropriately supported in the storage yard to prevent deformation. Once placed into service, small instantaneous deformations under load are to be expected as in any structural application. Increases in deformation may occur if the in-service temperature is high enough to affect the polymer chains. For certain resins, this temperature can be close enough to ambient temperatures to affect its performance. For properly designed resins, the in-service temperatures will not cause relaxation of the polymer concrete matrix, and deformation due to heat is not an issue.

Further, since PC is considered an appropriate material for corrosive environments, there is concern about what will happen when exposed to corrosive environments over time, and if any deformation will occur under such elements. If the chemicals or molecules do not react with the polymer chains, there will be no reduction in cross section, and therefore no deformation will occur.

PC has been specified on a major one-pass tunnel segmented liner before, but was not chosen because of price. All long-term corrosion and material strength tests have been performed to the satisfaction of the engineers for that project. It is nevertheless an emerging technology that is worthy of consideration. It can be used in a structural application if it is properly designed with the correct resin, if the aggregate is uniformly distributed through the matrix and if the catalyst is added such that cure throughout the section is uniform. PC and its components are inert and are resistant to corrosion and should be considered for use in sanitary sewers and other such applications where long-term corrosion resistance is of importance. There is no need for a second pass to weld thin membrane liners to protect the joints, and finally it should be noted that ethylene propylene diene monomer gaskets are inert to sewer gases and salt water.

Thin membranes with adhesives
This category of adhesive linings could be considered a two-pass liner in the sense that after the precast concrete segmented tunnel is constructed a plastic thin layer is adhered to the segmented liner. This process consists of a broad spectrum of coating and lining systems, ranging from coal tar epoxy paints to high-performance newer products such as Linabond, graphite and carbon fiber composite linings. These products have historically been used for repairs. Regardless of the quality of these products, all of them rely heavily on proper surface preparation and dryness of the concrete surface during installation and curing. Humidity is a problem, and leaks at joints make installation problematic.

For this reason, most of these products have relatively poor performance records in sewer tunnel applications, because of the inherent difficulties in consistently meeting the ideal surface preparation requirements for the application of these products and are not recommended.

Table 1. Two-pass versus one-pass liner system (modified after Kaneshiro, et. al., 1996) Figure 1, Pipe jacked or micro tunnel lining alternatives Figure 2, Tunnel lining options Figure 3, T-Lock PVC, cut and laid in form under the rebar bage