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Technical Manual for Design and Construction of Road Tunnels - Civil Elements

Chapter 16 - Tunnel Rehabilitation

16.1 Introduction

This chapter focuses on the identification, characterization and repair of typical structural defects in a road tunnel system. The most significant problem in constructed tunnels is groundwater intrusion. The presence of water in a tunnel, especially if uncontrolled and excessive, accelerates corrosion and deterioration of the tunnel liner. This chapter identifies the methods for measuring the flow of water from a leak; describes proper methods for identifying the types of remedial action to be taken including sealing of the liner with either chemical or cementations grout; and describes the procedures to install the various types of grout. A comparison of types of grout available at the time of writing and a chart indicating which type of grout is best suited for each condition is provided. Typical details are included to illustrate the proper methods for grouting.

This chapter presents various structural repair methods to reinstate the structural capacity of a deteriorated tunnel liner including methods for demolition of unsound concrete, brick or steel and methods for the restoration of the tunnel liner to its original condition and function. Details for the repair of concrete, steel reinforcement, and embedded elements of the tunnel liner system are provided. Most of the repair methods presented are designed to be used in active tunnels which permit minimum daily shutdowns. Repairs can be performed in a limited time frame allowing the tunnel to be returned to service on a daily basis.

This Chapter also addresses the structural bonding of cracked concrete. Details are presented to illustrate methods for demolition, surface preparation and placement of concrete to complete repairs. Current state-of-the-art materials available for repair of cast-in-place and precast concrete, steel and cast iron linings are discussed. Special procedures required for the repair of each lining material are presented.

This Chapter also addresses the various methods for the repair of components of segmental liners, including gaskets, attachments and fasteners. Guidelines for the repair of each type of segmental lining are presented. Design details of tunnel segmental lining are discussed in Chapter 10. The repair of hangers for suspended ceilings is discussed.

Repairs of steel/cast iron components addressed hereafter include roof beams columns, knee braces etc. which are often subject to severe corrosion and often need to be upgraded, replaced or rehabilitated. This Chapter covers typical details required for the restoration of riveted sections, rolled steel beams and other specially fabricated steel and cast iron elements of a tunnel system, and includes details on surface preparation, coatings for corrosion protection and proper methods for fire protection of the steel /cast iron elements of a tunnel.

This Chapter also address repairs of brick, dimension (Ashlar) stone and concrete masonry elements that exist in many tunnel systems. Methods of evaluating the condition of the masonry elements and methods for the restoration of masonry elements include removal and replacement, repair of mortar joints and methods for repointing joints. Procedures for the support of masonry structures during rehabilitation are discussed.

Lastly, structural repairs of unlined rock tunnels are briefly discussed in this Chapter.

16.2 Tunnel Inspection and Identification

Tunnel inspection requires multi-disciplinary personnel familiar with various functional aspects of a tunnel including civil/structural, mechanical, electrical, drainage, and ventilation components, as well as some operational aspects such as signals, communication, fire-life safety and security components.

Recognizing that tunnel owners are not mandated to routinely inspect tunnels and that inspection methods vary among entities that inspect tunnels, the FHWA and the Federal Transit Administration developed guidelines for the inspection of tunnels in 2003 and updated them in 2005 known as "Highway and Rail Transit Tunnel Inspection Manual" available at www.fhwa.dot.gov/bridge/tunnel/inspectman00.cfm (FHWA, 2005a). Note that at the time of preparing this manual, the FHWA is proposing to create a regulation establishing National Tunnel Inspection Standards (NTIS) which would set minimum tunnel inspection standards that apply to all Federal-aid highway tunnels on public roads.

This Manual and Chapter focus on the civil/structural aspect and assumes tunnel inspection to be performed by experienced personnel who are familiar with the types of materials found in tunnels, have a basic understanding of the behavior of tunnel structural systems, have had experience in the inspection of transportation structures and are familiar with the FHWA Bridge Inspection Training Manual (FHWA-FD-91-015), and Highway Rail and Transit Tunnel Maintenance Rehabilitation Manual (FHWA-IF-05-017) (FHWA, 2005b). In addition to the information identified in the Bridge Inspection Training Manual, protocols are described herein that are applicable to the inspection of road tunnels. The following sub-sections discuss the standard parameters for inspection and documentation.

16.2.1 Inspection Parameter Selection

Inspection parameters are chosen based upon the preliminary inspection of the tunnel and the scope of work. Particular emphasis should be placed on determining the presence of special or unique structures that require the addition of special inspection parameters for inclusion in the project database.

16.2.2 Inspection Parameters

Standardized inspection parameters are necessary to speed the processing and evaluation of the observed data. The use of standardized coding of information, necessary for consistency of reporting, also helps to assure quality control by providing guidelines for inspection personnel and standardizing visual observations. The Deficiency and References Legends in Appendix H provides a recommended standard coding for cataloguing tunnel defects.

16.2.3 General Notes in Field Books

All general field inspection/repair notes, consisting of a chronology of events, must be kept in a bound field book. Each member of the field team must carry a bound field book at all times when on site. The information contained in the field book should include notes on safety issues and on discussions with contractors, operations personnel and other interested parties. Entries into the field book must be chronological by date and time, and consist of clear, concise and factual notification of events and appropriate sketches. Field records, notes and the inspection database shall be maintained in one location. Field books should be copied on a weekly basis to prevent loss of data.

Nowadays electronic notebooks and/or special laptop computers are often used to record field data and sketches digitally which can also include digital photographs and/or videos with date, time, and GPS location information embedded.

16.2.4 Field Notes

The three types of field notes required for effective inspection of roadway tunnels are:

  • General notes in field books
  • Documentation of defects on field data forms
  • Documentation of defects by photographs/video.
16.2.5 Field Data Forms

Field data forms document the information required for a particular project. In general, these forms are developed for the project and are project specific. The forms provide a project standard for the tabulation of the data obtained from the inspection. This information is transmitted to the data management personnel for input into the project database.

16.2.6 Photographic Documentation

The documentation of tunnel defects is best supplemented by the use of a digital camera. Photographs should be taken of typical and atypical conditions. Additionally, the photographs should also be used as documentation for special or unique conditions.

Photographs must:

  • Exhibit the project number, date, time, location, photographer and a general description of the item
  • Be catalogued and stored in a systematic manner for future recall. Note: It is helpful to name all photo logs in a consistent manner that is outlined in a directory; i.e., using the structure number as a pre-fix to each individual photo file name.

It is essential to follow the photographic method of documentation referenced above. Instituting this method at the beginning of the project will prevent mislabeled or unlabeled data from being distributed or misinterpreted.

16.2.7 Survey Control

All condition surveys require a definitive baseline for location (survey) purposes. Generally most highway systems have an established survey baseline. The post construction baseline survey of the highway system is usually performed for the maintenance of the roadway and tunnel structure. Such stationing systems are usually well defined with permanent markers located on the tunnel walls. Some tunnels may already have a baseline condition established by laser scanning techniques (Chapter 3).

The tunnel inspection documentation must be linked to the existing baseline stationing system for the following reasons:

  • Allow inspection data to be used for long-term monitoring of the tunnel structure by the owner's engineering/maintenance staff
  • Allow defects to be readily located for future inspection or repairs
  • Facilitate rapid start-up of inspection teams
  • Reduce project costs and confusion

In addition to locating the tunnel defects along the alignment, it is necessary to locate them in relation to their position within the structure. To locate defects within the tunnel, the limits of the walls, roof, and invert must be delineated for conformity (Figure 16-1). Circular tunnels are divided up into 30-degree segments clockwise from the highpoint of the tunnel crown as shown in Figure 16-2. This delineation is always performed looking upstation on the established baseline survey.

Typical Cut and Cover Inspection Surfaces and Limits




LC&RC: Left& Right Ceiling ; LW&RW: Left & Right Wall; and LR&RR: Left & Right Roadway Wearing Course

Figure 16-1 Typical Cut and Cover Inspection Surfaces and Limits (Russell, 1992)

Delineation of Typical Circular Tunnel


Figure 16-2 Delineation of Typical Circular Tunnel

The development of standard inspection parameters and the associated calibration of inspection crews, prevent many of the errors and omissions that can occur when the work is performed by numerous separate teams. In addition, timely reviews by the project advisory committee allow for program modifications and speedy implementation of supplemental procedures as required.

The documentation for each tunnel, boat section, ventilation building, cross passage, utility room, low point sump, pump station, air duct or other element is made looking up-station. The element being inspected is divided about the centerline. Each component of the element having deficiencies/ observations to the left of the centerline will have a prefix of (L), whereas those to the right of the centerline will have a prefix of (R).

Standardized codes are developed for deficiencies that correspond to each component of the tunnel structure. These deficiencies can be tracked easily in the field and conformed to by the inspection crew. Existing codes for deficiencies are depicted by symbols and identification for both concrete: spalls, delaminations, cracks and joints and steel: reinforcing and framing. Also identified are bolt connections, and tunnel moisture.

Spalls and delaminations may occur in concert and are almost always found in association with structural cracks. There are documented instances where spalls are the result of impact (cars, etc.), insufficient concrete cover over the reinforcing steel or poor quality control of workmanship or materials. Standardized symbols for concrete spalls can be referenced in Deficiency and References Legends, Appendix H and in Table 16-1.

An example of typical structural defects documented using standard inspection parameters is shown below. In this example, a concrete spall located at a construction joint on the right wall panel at station 250+55 is 2-square feet in surface area, 4-inches deep, with exposed reinforcing steel (rebar) ( R) which has a section loss of approximately 20% and has a glistening surface of water (GS) is documented as follows:

StationLocation TypeArea (depth)Re-RodMoisture Comments
250+55RWS-22 S.F. (4")R-2, 20%GSAt construction joint

Note: Typical Spall Classifications: S-1 Concrete spall less than 2", S-2 Concrete spall to reinforcing steel, S-3 Concrete spall behind reinforcing steel, S-4 Special concrete spall.

Lists of standardized identification codes for deficiencies are included in Appendix H.

16.3 Groundwater Intrusion

16.3.1 General

Groundwater intrusion can be mitigated either by treating the ground outside the tunnel or by sealing the inside of the tunnel. This section will deal with the sealing of an existing lining rather than formation grouting outside of the tunnel.

The selection of the proper repair product for the conditions found on the project is key to the success of a leak containment program. Each site has its own particular environmental and physical properties. The pH, hardness, chemical composition, turbidity of the groundwater entering the tunnel all contribute to the ability of the chemical or particle grouts to effectively seal the leaking defect. The physical conditions that created the defect, movement of the crack or joint, the potential for freezing and the amount of water inflow all are site specific constraints for the selection of the repair material and all of these parameters must be assessed. Ideally, if any movement of the crack or joint is suspected it is best to monitor the defect for a period of time sufficient to allow for an estimation of actual movement.

The selection of the proper grout to seal a tunnel liner is dependent on the degree of leakage into the tunnel from the defect. Typically the tunnel defects that cause leakage are construction joints liner gaskets, and cracks that are the full depth of the liner. Standardized terms have been developed to describe the inflow of water. Standardized terms are useful in the selection of the grout because they allow all personnel including individuals who have not visited the tunnel to be familiar with the degree of water inflow. This familiarity of all personnel including the grout manufacturer facilitates the selection of the proper product and procedure for sealing the leak lists common terms used for the identification of leakage in the United States .

Table 16-1 Common U.S. Descriptions of Tunnel Leakage (Russell, 1992)
MoistMDiscoloration of the surface of the lining, moist to touch
Past MoisturePMArea showing indications of previous wetness, calcification etc
Glistening SurfaceGSVisible movement of a film of water across a surface
FlowingFContinuous flow of water from a defect; requires volume measurement
DryDStructural defect illustrates no signs of moisture
16.3.2 Repair MaterialsM

The selection of the proper repair product for the site-specific condition is key to the successful repair of a tunnel or underground structure leak. The most common way to seal a tunnel liner is to inject a chemical or cementitious grout. The grout can be applied to the outside of the tunnel to create a "blister" type repair that seals off the leak by covering the affected area with grout. The selection of the grout is dependent on the groundwater inflow and chemical properties from the soil and water.

The most common method of sealing cracks and joints that are leaking is to inject a chemical or particle grout directly into the crack or joint. This is accomplished by drilling holes at a 45 degree angle through the defect. The holes are spaced alternately on either side of the defect at a distance equal to 1/2 the thickness of the structural element. The drill holes intersect the defect and become the path for the injection of the grout into the defect. All holes must be flushed with water to clean any debris from the hole and to clean the sides of the crack or joint prior to injection to ensure proper bonding of the grout to the concrete. Typical injection ports are shown in Figure 16-3 . Figure 16-4 shows field injection of grout. Figure 16-5 illustrates the typical location of injection ports and leaking crack repair detail (FHWA, 2005b).

Typical Injection Ports for Chemical Grout

Figure 16-3 Typical Injection Ports for Chemical Grout

Leak Injection, Tuscarora Tunnel PA Turnpike

Figure 16-4 Leak Injection, Tuscarora Tunnel PA Turnpike

The selection of the grout is dependent on the width, moisture content, and potential for movement within the crack or joint. For joints that move, only chemical grout is appropriate. The movement of the joint or crack will fracture any particle grout and will cause the leak to reappear. Single component water reactive polyurethane chemical grout is the most effective grout for the full depth sealing of cracks and joints that have moisture present within the defect. If the defect is subject to seasonal wetness and is dry at the time of repair a hydrophilic grout should be used. When utilizing a hydrophilic grout, water must be introduced into the defect to catalyze the grout. Hydrophobic grouts have a catalyzing agent injected with the chemical grout or premixed into the grout prior to injection. In both cases water or a catalyst is used to gel the grout. Alternatively, hydrophobic chemical grout may be utilized. Hydrophobic chemical grouts rely upon a chemical reaction to cure whereas hydrophilic chemical grout require water to catalyze. Common hydrophobic grouts are acrylates and closed cell polyurethane. The installation of both types of grout is similar to that described here.

Typical Location of Injection Ports and Leaking Crack Repair Detail

Figure 16-5 Typical Location of Injection Ports and Leaking Crack Repair Detail (FHWA, 2005b)

In situations where the defect is not subject to movement and is dry at the time of repair an epoxy grout can be injected into the defect in the same manner that concrete is structurally rebonded. The grouts shown in Table 16-2 are typical grouts for the injection cracks and joints in a tunnel liner. The particle grouts are often used for formation grouting outside of the tunnel liner or in very large dry cracks and joints. The most commonly used grouts for the sealing of cracks in tunnel liners are the polyurethanes and acrylates.

Table 16-2 Typical Grouts for Leak Sealing (Russell, 1992)
Particle Grout
Flyash Type F;CMed (50cps-2:1)LowHighNon flexible
Type I CementMed(50Cps- 2-1)LowHighNon flexible
Type III CementMed (15cps- 2:1)LowHighNon flexible
Microfine CementLow ( 8cps- 2:1)LowHighNon flexible
BentoniteMed ( 50 cps 2:1)LowLowSemi flexible
Chemical Grout
AcrylamidesLow (10 cps 2:1)HighLowFlexible
AcrylatesLow (10 cps)LowHighSemi flexible -No shrinkage: Good success record
SilicatesLow ( 6cps)LowHighNon flexible- High Shrinkage
LignosulfatesLow (8 cps)HighLowFlexible not widely used
Polyurethane (MDI)High (400 cps)Med.LowFlexible: Good success record (Hydrophilic)
Polyurethane (TDI)High (400 cps)Med.LowFlexible : Good success Record(Hydrophobic)

Porous concrete can be sealed from the interior (negative side) of the tunnel to provide for a waterproof seal within the tunnel. Crystalline cementitious grouts that are applied to the interior of the tunnel and kept moist for 72 hours after application form a chemical bond with the free lime in the concrete and reduce the pore size of the concrete such that the free water vapor in the concrete cannot pass through. The success of these materials is varied and is to be used when no other alternative is available.

Interior side waterproofing is also performed by covering the interior surface of the wall with a cementitious coating consisting of two 1/8-inch thick coats applied to a moist concrete surface. Figure 16-6 illustrates the success of this type of coating in a tunnel in Pennsylvania with an external hydrostatic pressure of approximately 400 feet of water.

16.4 Structural Repair - Concrete

16.4.1 Introduction

The repair of concrete delaminations and spalls in tunnels has traditionally been performed by the form-and-pour method for the placement of concrete, or by the hand application of cementitious mortars that have been modified by the addition of polymers. Both of these methods are not well suited for highway tunnels that are in continuous daily operation. This daily operation usually permits the tunnel to be out of service for very short periods of time. Therefore, the repair process must be rapid, not infringe on the operating envelope of the daily traffic and be a durable long-term monolithic repair.

Negative Side Cementitious Coating, Tuscarora Tunnel PA Turnpike

Figure 16-6 Negative Side Cementitious Coating, Tuscarora Tunnel PA Turnpike

Today, the repair of concrete structural elements is performed typically by two methods: the use of hand applied mortars for small repairs and the use of shotcrete for larger structural repairs. In either case the preparation of the substrate is the same, only the type of material differs.

Shotcrete (also discussed in Chapters 9 and 10), is the pneumatic application of cementitious products which can be applied to restore concrete structures. This process has been in use for over decades in the US for the construction and repair of concrete structures both above and below ground. Shotcrete is defined by the American Concrete Institute as a "Mortar or concrete pneumatically projected at a high velocity onto a surface."Since the 1970's the use of low-pressure application of cementitious mortar has been commonplace in Europe and is known as Plastering. Over the years, developments in materials and methods of application have made the use of polymer cementitious shotcrete products for the repair of defects in tunnel liners in active highway tunnels cost effective. The selection of the process type, and the material to be applied is dependent on the specific conditions for tunnel access and available time for the installation of the repair. Shotcrete is preferred to other repair methods since the repair is monolithic and becomes part of the structure. The use of shotcrete is a process that allows for rapid setup, application and ease of transport into and out of the tunnel on a daily basis.

This section only provides the procedures utilized to delineate the extent of the repairs to the liner, and the work required to implement the shotcrete repairs. Refer to Chapter 10 for a more general discussion regarding shotcrete. Table 16-3 lists the most commonly used materials for the repair of tunnel liners.

Table 16-3 Comparison of Repair Materials (Russell, 2007)
ApplicationTwo-Component Self Leveling MortarPolymer Shotcrete Wet ProcessTwo Component MortarPolymer Shotcrete Dry ProcessPolymer Masonry Mortar
On Grade; above, belowyesyesyesyesyes
On horizontalyesyesyesyesyes
On verticalnoyesyesyesyes
Overlay systemyesNoyesnoyes
Structural repairyesyesyesyesyes
Leveling materialyesyesnoyesyes
Filler: voidsnoyesyesyesyes
Maximum depth3 inchesunlimited1 inch/liftunlimited1 inch/lift
Minimum depth1/2 inch1/4 inch1/4 inch1/4 inch1/8 inch
Extended w/ aggregateyesNoyesnoyes
High abrasionyesyesyesyesyes
Good bond Strengthyesyesyesyesyes
Compatible coefficient of expansion w/concreteyesyesyesyesyes
Resistant to saltsyesyesyesyesyes
High early strengthyesyesyesyesyes
High FlexuralyesyesYesyesyes
Good freeze- thawyesyesyesyesyes
Vapor BarrieryesNononono
Ok Potable wateryesyesyesyesyes
Open to traffic 1-2 hoursyesyesyesyesyes
Low rebound dustyesyesyesnoyes
16.4.2 Surface Preparation

The surface preparation for concrete repair requires removal of all unsound concrete by either the use of chipping hammers or the use of hydro-demolition. Unsound concrete is removed to the full depth of the unsound concrete. In cases where chipping hammers are used it has been found that limiting the size of the hammers by weight is the best way to control over excavation. Limiting the weight of the chipping hammers with bit, to less than 30 lbs. (13.6Kg) reduces the risk of over excavation of concrete. These hammers are too weak to excavate concrete in excess of 4,000 psi. (27,580 Kpa). The use of Hydro-demolition requires testing on site, at the beginning of the project to determine what pressures are required to excavate the unsound concrete without removing the sound substrate (Figure 16-7).

Hydro-demolition should not be used in areas that house electrical equipment, cables, or other mechanical equipment that may be effected by the excavation process. The area to be repaired must not have feather edges, and must have a vertical edge of at least 1/8 inch in height. This vertical shoulder is necessary to prevent spalling at the edge of the new repair.

Substrate after Hydro-demolition, Shawmut Jct. Boston

Figure 16-7 Substrate after Hydro-demolition, Shawmut Jct. Boston

After the unsound concrete is removed, any leaking cracks or construction joints must be sealed prior to the application of the reinforcing steel coatings and the shotcrete. This sealing should be performed using a chemical grout suitable for the type and magnitude of the leakage. In general single component polyurethane grouts are the most successful in effectively sealing most tunnel leaks. Refer to Section 16.3.2 for more information on sealing leaks.

16.4.3 Reinforcing Steel

Once the unsound concrete has been removed, reinforcing steel must be cleaned and if a loss of section is evident the damaged reinforcing steel must be removed and replaced. All rust and scale must be removed from the reinforcing steel and any exposed steel liner sections or other structural steel elements. The cleaning is generally to a white metal commercial grade cleaning. Once cleaned the reinforcing steel is to be evaluated for loss of section and if the loss of section is greater than 30% an analysis must be performed. If the results of the analysis indicate that the lining does not have adequate strength with the remaining reinforcing steel, then the damaged steel must be replaced. Mechanical couplers are used when splicing new reinforcing steel to existing. Mechanical couplers eliminate the need for lap splices in the reinforcing steel and thereby reduce the amount of lining removal required to replace the reinforcing steel. (Figure 16-8 )

Typical Mechanical Coupler for Reinforcing Steel

Figure 16-8 Typical Mechanical Coupler for Reinforcing Steel

After the steel has been cleaned a coating must be placed on the steel to protect the steel from accelerated corrosion due to the formation of an electrolytic cell. Numerous products exist for this purpose, including epoxy and zinc rich coatings. Zinc rich coatings are better suited for this application due to the fact that they do not form a bond-breaker as do many epoxies. This is important since these materials are applied by the use of a paint brush and it is difficult to prevent the concrete surface from being accidentally coated. The application of the zinc rich coating is to be performed within 48 hours of the cleaning and not more than 30 days prior the application of the shotcrete.

16.4.4 Repairs

Small shallow spalls are repaired by the use of a polymer modified hand patch mortar as shown in Figure 16-9. Hand patch mortar is a prepackaged polymer modified mortar that is applied in lifts of 1 to 2 inches. The patch areas are generally less than 2 square feet in area and require keying into the substrate by the use of "j" hooks and welded wire mesh or rebar. Unsound concrete is removed by either a hydro-demolition hand wand or by a chipping hammer with a weight of less than 30 lbs, including bit. The limiting of the hammer size provides for the removal of concrete of less than 4,000 psi compressive strength and limits over excavation since the hammer energy is not sufficiently strong to remove higher strength concrete.

Other than small repairs which utilize the repair mortars, the most commonly used material is shotcrete (or specifically prepackaged polymer modified fibrous shotcrete). Figure 16-10 illustrates the details of typical concrete repairs for deeper spalls. Discussions of the deeper spall repairs are included in Section 16.4.5 Shotcrete Repair.

16.4.5 Shotcrete Repairs

As discussed in Chapter 10, there are two processes for the application of shotcrete; Dry Process and Wet Process. Both processes have been in use for many years and are equally applicable for use in tunnel rehabilitations. The wet process creates little dust and is applicable for use in tunnels when partial tunnel closures allow traffic inside the tunnel during the repair work. The dry process creates extensive dust and is not suitable for partial tunnel closures due to limited visibility created by the dust.

The successful application of shotcrete regardless of the process chosen relies on the skill of the nozzleman (Figure 6-11) (In the case of the wet process both the nozzleman and the laborer mixing the mortar). A successful repair program requires the nozzleman and the other members of the shotcrete crew to be skilled and tested on site using mock-ups of the types of areas to be repaired. These mock-ups should closely duplicate the shape and surfaces to be repaired. This testing program is often used to certify the skill of the shotcreting crew and provides for better quality control during the progress of the work. The testing program develops an understanding between the Engineer, Owner and Contractor that defines an acceptable product for the work.

Once the reinforcing and structural steel elements have been cleaned and coated, welded wire mesh is to be placed over the area to be shotcreted (Figure 16-12). The mesh is placed to within 2 inches of the edge of the repair. The wire mesh is attached to the existing reinforcing and to the substrate by the use of "J" hooks.

Shallow Spall Repair

Figure 16-9 Shallow Spall Repair (FHWA, 2005b

Typical Sections at Concrete Repair

Figure 16-10 Typical Sections at Concrete Repair (FHWA, 2005b)

The purpose of the wire mesh is to assist in the buildup of the shotcrete and to provide for a monolithic repair that becomes part of the host structure. The wire mesh should be hot dipped galvanized after fabrication and is best if delivered to the site in sheets rather than on a roll. If epoxy coated mesh is used it must be in sheets in order to eliminate field touch-up of the cut ends of the mesh. The mesh size for dry process is a 2 X 2 inch mesh and for wet process 4 X 4 inch mesh. The larger mesh is required for the wet process to prevent clogging of the mesh by the shotcrete and therefore creating voids behind the mesh surface.

Nozzleman Applying Wet Process Shotcrete, USPS Tunnel Chicago

Figure 16-11 Nozzleman Applying Wet Process Shotcrete, USPS Tunnel Chicago

Reinforcing Steel for Repair, Sumner Tunnel Boston

Figure 16-12 Reinforcing Steel for Repair, Sumner Tunnel Boston

After the entire area to be patched is filled with shotcrete the material is allowed to cure for 20-30 minutes, at which time the mix is screeded and troweled to the desired finish (Figure 16-13 ). Trying to work the shotcrete prior to this time will result in tearing of the surface and make finishing very difficult. Caution must be exercised to monitor the drying rate of the shotcrete since the times stated here will vary depending on wind conditions and relative humidity. After the repair has been troweled to the desired finish a curing compound must be sprayed on the surface of the new shotcrete to prevent rapid drying. The manufacturer of the premixed shotcrete will recommend a curing compound best suited for the job site conditions.

Shotcrete Finishing, Shawmut Jct. Boston

Figure 16-13 Shotcrete Finishing, Shawmut Jct. Boston

16.5 Structural Injection of Cracks

Cracking is the most common defect found in concrete tunnel liners. While most of the cracks are a result of thermal activity, there are cracks that are a result of structural stresses that were not accounted for in the design. It is important to note that cracks also occur as a result of shrinkage and thermal stresses in the tunnel structure. Cracks that exhibit thermal stresses should not be structurally rebonded since they will only move and re-crack. However, structural cracks that occur as a result of structural movement, such as settlement and are no longer moving should be structurally rebonded. Any crack being considered for structural rebonding must be monitored to assess if any movement is occurring. A structural analysis of the tunnel lining should be performed to ascertain if the subject crack requires rebonding.

There are three types of resin typically available for injection of structural cracks in tunnels. They are:

  • Vinyl Ester Resin
  • Amine Resin
  • Polyester Resin

Vinyl ester resin is the common type of resin used for bridge repair work and is usually not suited for tunnel work since most cracks in tunnels are damp or wet. The vinyl ester resin will not bond to surface saturated concrete and will not structurally rebond a damp or moist crack. However, if the crack is totally dry during the injection process this epoxy will provide a suitable rebonding of the concrete.

Amine and polyester resins are best suited for the structural rebonding of cracks in tunnels. Both resins are unaffected by moisture during installation and will bond surface saturated concrete. Cracks with flowing water must be carefully injected and the manufacturer's advise must be obtained to ensure proper installation of the resin.

In all cases the manufacturer's recommendations must be followed for the injection of epoxy resins, particularly in the case of overhead installation. Figure 16-14 illustrates a typical installation of epoxy resin for the structural rebonding of cracks in concrete. The procedure for rebonding masonry and precast concrete elements is similar.

Typical Structural Crack Injection

Figure 16-14 Typical Structural Crack Injection (FHWA, 2005b)

16.6 Segmental Linings Repair

As discussed in Chapter 10, segmental lining can be made up of either, precast concrete, steel or cast iron. A segmental liner is usually the primary liner of a tunnel. The segments are either bolted together or keyed. The only segmental liners that are keyed are the precast liners. The most common problems with segmental liners is deformation of the flanges in the case of steel and cast iron liners and corner spalling of precast concrete segments. The spalling of precast segments and deformation of the flanges of steel/cast iron segments usually occurs at installation or as a result of impact damage from vehicles. In addition the rusting through of the liner plate of steel/cast iron segments occasionally occurs.

16.6.1 Precast Concrete Segmental Liner

The repair of spalls in precast concrete liner segments is performed by the use of a high performance polymer modified repair mortar which is formed to recreate the original lines of the segment. In the event the segment gasket is damaged the gasket's waterproofing function is restored by the injection of a polyurethane chemical grout as described above. Damaged bolt connections in precast concrete liner segments are repaired by carefully removing the bolt and installing a new bolt, washer, waterproof gasket and nut. The bolts are to be torqued to the original specification and checked with a torque wrench.

16.6.2 Steel/Cast Iron Liner

The repair of steel/cast iron liners varies according to the type of liner material. Steel, if made after 1923, is weldable while cast iron is not. Common defects in these types of liners are deformed flanges and penetration of the liner segment due to rusting. Deformed flanges can be repaired by reshaping the flanges with hammers or heat. Holes in steel liner segments can be repaired by welding on a new plate. Bolted connections often have galvanic corrosion which is caused by dissimilar metal contact and often require the entire bolted connection to be replaced. When the bolted connection is replaced a nylon isolation gasket is used to prevent contact between the high strength bolt and the liner plate. Figure 16-15 shows the repair of a rusted through steel segment and a repaired bolted connection.

Steel Segmental Liner Repair

Figure 16-15 Steel Segmental Liner Repair (Russell, 2000)

Repairs to cast iron liner segments is similar to those for steel. However, since cast iron cannot be welded the repair plate for the segment is installed by brazing the repair plate to the cast iron or drilling and tapping the liner segment and bolting the repair plate to the original liner segment. In some instances it is easier to fill the area between the flanges with shotcrete. Figure 16-16 illustrates a test panel for filling a liner plate with shotcrete.

Cast Iron Segmental Segment Mock-up of Filling with Shotcrete, MBTA Boston

Figure 16-16 Cast Iron Segmental Segment Mock-up of Filling with Shotcrete, MBTA Boston

16.7 Steel Repairs

16.7.1 General

Structural steel is commonly used at the portals of tunnels, support of internal ceilings, columns, segmental liners and as standoffs for tunnel finishes. The repairs to steel elements is to be site specific and to be performed in accordance with the appropriate standard (Figure 16-17 ). The American Welding Society's Standard Structural Steel Welding Code AWS D1.1/D1.1 Structural Welding Guide most recent version should be utilized for the construction of all welded steel connections. Repairs to Rivets and bolting must comply with AASHTO Specification.

Typical Framing Steel Repair at Temporary Incline

Figure 16-17 Typical Framing Steel Repair at Temporary Incline

16.8 Masonary Repair

The restoration of masonry linings composed of clay brick or Ashlar (dimension) stone consists of the repointing of deficient mortar. As shown in Figure 16-18 , the repointing of masonry joints consists of raking out the joint to a depth of approximately one inch (2.54cm). Once the joint has been raked clean and all old mortar removed, the joints are repointed with a cemtitious mortar, or a cementitious mortar that has been fortified with an acrylic bonding agent.

Typical Masonry Repair

Figure 16-18 Typical Masonry Repair

Replacement of broken, slaked or crushed clay brick requires a detailed analysis to determine the causes and extent of the problem. Once the problem is properly identified a repair technique can be designed for the particular structure. Caution must be exercised in the removal of broken or damaged brick. The removal of numerous bricks from any one section may cause the wall or arch to fail. Therefore it is imperative that any repair work on masonry be performed by competent personnel having experience in the restoration of brick and stone masonry.

16.9 Unlined Rock Tunnels

Unlined rock lined tunnels do not required a permanent concrete, brick or steel lining since the rock was competent and illustrated sufficient strength with minimal reinforcement to remain standing. These roadway tunnels are also usually very short in length. Most have support consisting of various types of rock reinforcement; including rock dowels, rock bolts, cable bolts and other reinforcement which were placed at various angles to cross discontinuities in the rock mass. These rock reinforcement elements typically range in length for 5 to 20 feet in length and are installed and grouted with resin or cementitious grout. Please refer to Chapter 6 for more detailed discussions about various types of rock reinforcement elements.

Rock reinforcement elements, may deteriorate and loose strength due to the corrosive environment and exposure typical in tunnels, and require replacement and installation of new rock reinforcement elements. Replacement of rock reinforcement elements requires a detailed investigation of the structural geology of the tunnel which is performed by an engineering geologist or geotechnical engineer having experience in geologic mapping and the rock stability analysis as discussed in Chapter 6.

Another more frequent cause for the need to repair unlined rock tunnels is the falling of rock fragments which over time become loose and drop onto the roadway. There are many ways to prevent this from occurring, the most common of which is to scale (remove) all loose rock on a periodic basis from the tunnel roof and walls by the use of a backhoe or hoe ram. Other methods include the placement of steel liner roof as a shelter, additional rock bolts and wire mesh to contain the falling rock fragments, and shotcrete on the areas of concern as shown in Figure 16-19 and Figure 16-20.

Rock Tunnel with Shotcrete Wall Repair and Arch Liner

Figure 16-19 Rock Tunnel with Shotcrete Wall Repair and Arch Liner (I-75 Lima Ohio)

Rock Bolts (Dowels) Supporting Liner, I-75 Lima Ohio Underpass

Figure 16-20 Rock Bolts (Dowels) Supporting Liner, I-75 Lima Ohio Underpass

16.10 Special Considerations for Supported Ceilings/Hangers

Numerous highway tunnels in the United States have suspended ceilings for ventilation purposes and in some cases aesthetics. These ceilings are generally supported by keyways in the tunnel walls and by hanger rods that are attached to the tunnel liner either by means of cast-in-place inserts or post-installed mechanical or adhesive (chemical) anchors. FHWA issued a Technical Advisory in 2008 strongly discouraging the use of adhesive anchors for permanent sustained tension or overhead applications (see Appendix I). Any use of adhesive anchors in road tunnels must conform to current FHWA directives and other applicable codes and regulations.

The inspection of these hangers is important to tunnel safety and a rigorous and regular inspection program that considers importance and redundancy is strongly recommended to maintain an appropriate level of confidence in their long-term performance.

During inspection one method used to verify hangers are in tension is by "ringing" each hanger. Ringing a hanger is done by striking it with a masons hammer. A hanger in tension will vibrate or ring like a bell after being struck while a hanger that is not in tension because of a connection or other defect will not ring. Hangers that exhibit a defect or lack of tension should be closely inspected and checked for structural suitability. Examples of typical hangers and their components are shown in Figure 16-21.

Typical Hanger Supports for Suspended Ceiling
Figure 16-21 Typical Hanger Supports for Suspended Ceiling

The repair of ceiling hangers depends on the particular type of defect. If the hanger rod, clevis, turnbuckle or connection pins are broken or damaged they can be simply replaced with similar components which are readily available from many sources, including most large hardware supply retailers (Figure 16-22).

Typical Replacement Hanger Hardware
Figure 16-22 Typical Replacement Hanger Hardware

The repair of loose connections at the tunnel arch is of primary concern. The recommended repair for failed adhesive anchors is to replace them with undercut mechanical anchors typical examples of which are shown in Figure 16-23.

Typical Undercut Mechanical Anchors
Figure 16-23 Typical Undercut Mechanical Anchors

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Updated: 06/19/2013
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