Precast Tunnel Segments Research Articles

Experimental and numerical investigation of flexural behavior of precast tunnel segments with hybrid reinforcement

Precast concrete segments reinforced with a combination of steel fibers and conventional rebar (RC-SFRC) have gained prominence in the mechanized construction of tunnels in recent years due to the advantages offered by this hybrid reinforcement solution in enhancing the mechanical behavior of these structural members. This research aims to understand better the flexural behavior of RC-SFRC tunnel segments proposed to replace conventional reinforcement in São Paulo Metro Line 5. An experimental program and numerical analyses using a multiscale model were conducted to predict the post-cracking parameters of SFRC through three-point bending tests according to EN14651 and assess the flexural behavior of full-scale tunnel segments. In both scenarios, 2D multiscale numerical models with discrete and explicit representations of the reinforcements were applied. The comparison of numerical and experimental results in terms of crack width, mean crack spacing, deflection, and the ultimate and service loads derived from design predictions demonstrate that the proposed RC-SFRC reinforcement represents an advantageous alternative, significantly enhancing the performance of the segments for both serviceability and ultimate limit states. Furthermore, the responses show that the adopted numerical strategy is promising and can be consolidated as a tool for optimizing the design process.

Statistical Process Control of Fiber-Reinforced Concrete Precast Tunnel Segments

Statistical Process Control of Fiber-Reinforced Concrete Precast Tunnel Segments

Full-scale testing of precast tunnel lining segments under thrust jack loading: Design limits and ultimate response

In modern practice, precast segmental tunnel linings are typically installed via a tunnel boring machine (TBM), which advances by thrusting against the previously installed segmental ring. The forces applied through the thrust jack pads can induce significant bursting and spalling tensile stresses and strains in the segment, and improperly designed segments can suffer from cracking as a result. An experimental study has been conducted to evaluate the progression of damage from initial cracking to ultimate capacity for full-scale precast tunnel liner segments under thrust jack loading. The baseline segment design is composed of steel fiber reinforced concrete (SFRC), and the impact of supplemental conventional steel bar reinforcement and load application eccentricity were also investigated. Six full-scale tests were performed with a thrust jack load per pad up to 22.2 MN (which is ∼ 3.8 times the maximum expected installation thrust force). At the maximum expected thrust jack load during installation (5.78 MN per pad), the segments were virtually undamaged, and hairline cracking initiated between the load pads on only one test. At the TBM’s ultimate jacking capacity (9.55 MN per pad) surface cracking was observed between and under the load pads; however, the crack width remained below 0.2 mm for all specimens. The formation of cracking limit states was accurately predicted by pre-test linear and nonlinear finite element (FE) models. At overload conditions, the baseline SFRC-only segment exhibited a radial bursting failure. The inclusion of supplemental conventional reinforcement does not reduce the level of cracking damage or strain development below the TBM’s ultimate jacking capacity; however, at overload conditions, the supplemental reinforcement mitigates cracking and prevents a radial bursting failure at 20.3 MN per pad. A load eccentricity of 38 mm towards the extrados surface increased the transverse strain and the formation of transverse cracking at a lower load level.

Mechanical response of precast tunnel segments with steel and synthetic macro-fibers

Precast tunnel lining segments manufactured with synthetic macro-fibers and steel fibers as the primary reinforcement were tested under flexural and edge compression loading. Since steel fibers are usually prescribed as the primary reinforcement of tunnel sections, the dosage of synthetic fibers was determined by matching their residual flexural strength. The performance of tunnel segments was verified using the extensive data collected from instrumented deflection, crack growth, quantitative DIC, and strain gauge results and correlated with the load vs. deformation response. Results show that synthetic fibers can meet the design load requirements specified for the steel fibers reinforcement. Flexural test results indicate a 16% increase in macro-fiber content from 0.80% to 0.93% by weight resulting in a 32% increase in the residual strength of standard beam samples and a 41% increase in the residual strength of full sections. Compared to the steel fiber samples, samples with polymeric macro-fiber had a 6.5% higher average toughness. The full-scale load-deflection results were within 7% of the average values in each category of samples which is almost half the variation in the flexural ASTM C1609 results. The edge compression tests indicated that the mode of failure under concentrated jack forces is governed and dominated by the tensile lateral bursting stresses resulting from inefficient distribution of the load to the supporting edge.

Fracture modelling of steel fibre reinforced concrete structures by the lumped damage mechanics: Application in precast tunnel segments

A lumped damage model for steel fibre reinforced concrete (SFRC) is proposed in this paper. In the proposed model, the inelastic phenomena are lumped at hinges, where the damage variable accounts for concrete cracking and the plastic rotation represents the permanent deformation. Such lumped damage model was obtained by a flexure experiment and then applied to two precast tunnel segments. The results show that the proposed model is quite accurate for the SFRC tunnel segments analysed in this paper.

Study on the effects of cooling phase and construction technology on the fire performance of R/C tunnels

Fire performance of tunnels can represent a critical issue in the design phase, even more than for other structures and infrastructures, due to some inherent features such as (a) the fire compartment geometry often leading to very high temperature (also making difficult the intervention of fire brigades), (b) the structural redundancy caused by soil restraint fostering the development of relevant indirect actions, and (c) the high compression state in the lining (all the more during the fire exposure) that increases the spalling propensity and severity. Within this context, the role played by key parameters such as lining thickness and stiffness are investigated by comparing the fire performance of two different technological solutions for the lining: (1) traditional cast-in-situ lining and (2) pre-cast segmental tunnel lining. The fire scenario also considers the cooling phase, in order to discuss the main critical points to be solved when facing the final stage of the fire. 3D finite element analyses have been performed, proving that (I) higher thickness and stiffness does not necessarily correspond to a higher safety factor due to the indirect actions, and (II) fire cooling phase (if any) can be even more critical than the heating phase.

A model for predicting the splitting bearing capacity of Fiber Reinforced Concrete elements under partially loaded areas

AbstractConcrete elements are frequently subjected to partially loaded areas; a typical example is represented by precast tunnel segments. In fact, during the excavation process, the hydraulic jacks of the boring machine exert, on the last assembled ring, high forces concentrated on small areas with respect to the size of ring joint. Similarly, when lining is embedded in ground, high hooping forces are transmitted through longitudinal joints, which present a contact zone smaller than the segment cross section. In both cases, spreading these loads into tunnel segments result in tensile transverse stresses which may cause splitting cracks. In the last decades, the growing desire to find more economic and sustainable solutions has driven enormous efforts in the tunnel industry to find new design solution such those based on Fiber Reinforced Concrete (FRC) or a combination of FRC and traditionally steel rebars. Research studies have already demonstrated the ability of fibers in controlling splitting crack phenomena. However, there is a lack of knowledge on reliable analytical approaches to quantify these benefits. The main aim of this paper is to develop a new model for determining the bearing capacity of FRC elements under partially loaded areas when splitting failure occurs. The analytical model herein reported allows a good prediction of the maximum loads exhibited by FRC prisms tested under high concentrated loads, whose data were selected from the literature.

Role of the tensile constitutive modeling on the structural response of fiber reinforced concrete flat slabs: A numerical study

AbstractFor decades, fiber reinforced concrete (FRC) has been primarily used in elements for which the consequences of potential failure are minor, like slabs on grade and precast tunnel segments, just to name a few examples. With reference to slabs supported on walls and/or columns, the use of fibers as partial or main reinforcement has increased over the last 15 years. Despite the existence of advanced design codes like the fib Model Code 2010, experimental data, and standing buildings that prove the technical feasibility of FRC, there are still barriers for its use at a wider scale. Technical aspects such as the uncertainties in the identification procedure of FRC uniaxial tensile laws and the influence of the FRC post‐cracking strength class on the serviceability and ultimate limit states response are among the factors that prevent a larger diffusion of FRC. In this paper, the role played by several tensile post‐cracking constitutive models—among those most commonly used both in research and in design practice—on the structural response of FRC flat slabs is investigated. The effect of numerical modeling choices such as (i) the influence of shell or brick elements and (ii) the influence of material homogeneity or heterogeneity along the slab thickness on the structural response of FRC flat slabs is also examined.

A parametric numerical study on the behavior of large precast tunnel segments during TBM thrust phase

During the last two decades, Fiber Reinforced Concrete (FRC) was adopted in several segmental tunnel linings. The benefits related to the inclusion of fiber reinforcement in cementitious composites are mainly related to the increase of post-cracking tensile residual properties. FRC allows to reduce or avoid conventional reinforcement leading to a more efficient segment production process. The application of Tunnel Boring Machine (TBM) thrust jack forces during the lining construction stage is one of the most demanding loading conditions, since high-concentrated forces are introduced in last installed segments, especially for large diameter lining. Hence, special attention should be devoted to this phase by considering parameters which may affect segment structural response. To this aim, the behavior of a lining having large diameter during TBM operations was investigated by means of a broad parametric numerical study concerning segment configuration, irregularities and lining slenderness. The following reinforcement solutions were also considered: traditional steel rebars only, steel fibers reinforcement only and a combination of them (hybrid solution). The study clearly proved that segment configurations as well as irregularities strongly affect the structural behavior of segments. The hybrid reinforcement or FRC only solution having high post-cracking performance may guarantee the same response of RC solution.

A hybrid solution proposal for precast tunnel segments

AbstractThe construction of underground structures is playing a relevant role in modern society. As a result of noticeable tunnel boring machine (TBM) evolution, mechanized excavation methods are gaining increased market shares. TBM driven tunnels are generally used in combination with precast concrete segments as the main support system for reducing or minimizing surface disturbances during construction and to fulfill the project requirements with regards to quality, construction times, budget and safety. Within this framework optimization approaches strongly focused on these process parameters, rather than the general structural performance of the segments. However, improving the performance by using conventional reinforcement, High Performance Fiber Reinforced Concretes (HPFRCs) or a combination of such enables a possible reduction of the lining thickness or the amount of reinforcement. To this aim, this paper presents a new hybrid solution for precast tunnel segments with a reduced thickness, which leads to new perspectives in reducing the total amount of concrete, in decreasing the volume of disposal materials from the boring process as well as to minimize the TBM size. A parametric numerical study is developed for evaluating the effectiveness of proposed hybrid solution. The latter is based on high strength reinforced concrete in combination with HPFRC at the longitudinal joints, whose geometric configuration is properly modified to maximize the segment's bearing capacity.

Tolerances for TBM thrust load based on crack opening performance of fiber-reinforced precast tunnel segments

In the past twenty years, several precast tunnels were constructed using Fiber-Reinforced Concrete (FRC). Also, it has been reported that concrete segments experience damage during TBM operations due to factors as load eccentricity and uneven supports. Simulating in a laboratory such conditions together with interactions between elements, as in a field, is challenging. So numerical models can provide essential guidance for the design. Therefore, the current research presents a broad investigation considering three usual tunnel diameters, different number of TBM shoes by segments, shoe dimensions, and configurations (German and French). Also, FRC classes, determined according to the fib Model Code, are tested. It was found that the French distributions of shoes presented the best performance for all studied cases when compared to the German one. Tunnels with an internal diameter of Di = 6 m were the only ones sensitive to the increase from two to three shoes by segment. In the case of four pads, all tunnel diameters increased the load capacity. The FRC class change from 1.5a to 3a significantly enhanced the load capacity relative to the crack opening of tunnels with Di of 6 m and 10 m. For changing class from 1.5a to 4a, all tunnels had enhanced performance. For Di = 10 m, the increases in load capacity were 2.23, 2.18, and 2.60 for segments with 2, 3, and 4 shoes, respectively. Also, the best aspect ratios found for the shoes’ geometry are reported in the paper.

Experimental study on hybrid precast tunnel segments reinforced by macro-synthetic fibres and glass fibre reinforced polymer bars

Increasing the popularity of using fibres in precast structures has brought questions about their usability in segmental tunnel linings as an alternative to the conventional reinforced lining. Most prior studies have already revealed that the replacement of conventional steel reinforcement is possible with steel fibres. However, the usability of macro-synthetic fibres (MSFs) as reinforcements in precast tunnel segments is still unclear and one of the controversial questions to be answered. As such, this study aims to evaluate the structural applicability of using polypropylene (PP) MSFs in precast tunnel segments by means of an experimental program on full-scale specimens. Within this framework, totally fourteen full-scale precast tunnel segments of Mecidiyekoy – Mahmutbey underground railway tunnel in Istanbul/Turkey characterized by four different reinforcement cases were analysed: i) typical conventional steel reinforcement; ii) the combination of MSFs and conventional reinforcement; iii) the combination of MSFs and glass fibre reinforced polymer (GFRP) rebars; and iv) MSFs only. Flexural tests were carried out to compare the flexural behaviour of specimens at the allowable crack opening width, while point load tests were conducted to observe the structural performance of precast tunnel segments under the effect of design thrust forces. The experimental results showed that the combination of MSFs and GFRP could be an innovative solution for precast tunnel segments in case of using a suitable quantity that satisfied the project requirements. Although PP fibres exhibited adequate spalling and splitting stress control, it is observed that they could not overcome high flexure forces without using reinforcement bars at a low volume of fractions. Thus, more comprehensive studies need to perform on GFRP + MSFs segments because of having the advantages of corrosion resistance in the presence of an aggressive surrounding environment.

Numerical investigation into the effect of stepping on the circumferential joint in the precast tunnel segments under TBM thrust jacks

The mechanized tunnelling process is mainly coupled with precast concrete segments, which include segmental rings that they are placed in their place simultaneously with the boring operation. Segments are mainly loaded by thrust jacks during the installation stage. It is essential to limit the cracks and damages of the segments during this stage to be sure of their structural integrity and sustainability. Stepping on the circumferential joint is a problem that can occur during the ring assembly. The main purpose of this study is to investigate the effect of the stepping under the load caused by thrust jacks. For this aim, the occurred stepping has been analyzed in the Tabriz metro line 2 project. In this research, the stepping has been investigated in four states, including non-stepping state, 1-cm stepping state, 2-cm stepping state, and 3-cm stepping state. Each state also has four positions, which include stepping on the right side of the ring, on the left side, on the floor, and on the roof. According to the results of the numerical modeling, increasing the amount of the stepping increases the damages. These damages have reached their highest level when the segment has stepping at the roof position.

New American Concrete Institute (ACI) Code for Design, Manufacturing and Construction of Tunnel Segmental Lining

American Concrete Institute (ACI) as one of the most powerful advocates for concrete construction in the world is aiming to publish its first guide (ACI 533.2R) led by the authors of this paper on general aspects of precast tunnel segments. This paper is intended to present salient features of the guide including the most recent developments on all aspects of design, manufacturing and construction. This document is drafted based on the knowledge and the experience gained on projects in Asia, Europe, and North America, and available national and international reports and recommendations. Procedures to perform structural design for Ultimate and Serviceability Limit States (ULS, SLS) during production, transportation, construction and final service stages are explained. Details of segmental ring geometry and systems, concrete strength, curing, and reinforcement detailing are discussed. Gasket design, segment connection devices, anchorage systems, tolerances, measurement and dimensional control, and repair of defects are among other topics that are presented. This document also addresses testing and performance evaluation, durability and degradation mechanisms of tunnel linings and their mitigation methods. While some design parts of this guide may only consider the procedures adopted by ACI, they can be extended to other national and international codes and used worldwide.

New ACI 533 guide on general design and construction aspects of precast concrete tunnel segments

AbstractAmerican Concrete Institute (ACI) is aiming to publish its first guide (ACI 533.2R) on general aspects of precast tunnel segments. This paper presents salient features of the guide including the most recent developments on major aspects of design, manufacturing and construction. This document is drafted based on the knowledge and the experience gained on projects in Asia, Europe, and North America, and available national and international recommendations. Procedures to perform structural design during production, transportation, construction and final service stages are explained. Details of segmental ring geometry and systems, concrete strength, curing, and reinforcement detailing are discussed. Gasket design, segment connection devices, anchorage systems, tolerances, measurement and dimensional control, and repair of defects are among other topics that are covered. This document also addresses durability and degradation mechanisms of tunnel linings and their mitigation methods. While some parts of this guide may only consider the procedures adopted by ACI, they can be extended to other national and international codes and used worldwide.

Precast tunnel segments for metro tunnel lining: A hybrid reinforcement solution using macro-synthetic fibers

The addition of fibers has been proven successful to simplify reinforcement in precast tunnel segments, allowing as a function of both segment typology and fiber reinforced concrete (FRC) toughness a total or partial replacement of the conventional reinforcement. Results from an experimental research aimed at comparing the structural behavior of segments made with conventional (only rebars, RC) or hybrid (rebars + fibers) reinforcement are presented. The experimental program consisted of flexural and point load tests (which reproduces the jack actions during TBM operations) carried out on four large-scale precast tunnel segments representative of a metro tunnel lining characterized by an internal diameter of 5.80 m and a thickness of 0.30 m. The main goal of the experimental program was to evaluate the possibility of using macro-synthetic polypropylene fibers (Polypropylene Fiber Reinforced Concrete, PFRC) in combination with a lower amount of conventional rebars (optimized reinforcement, RCO) for guaranteeing the required segment performance. Experimental results indicate that macro-synthetic fibers may be very effective in combination with conventional rebars to withstand the main stresses that arise in a segment both at initial and final phases, proving that the adoption of hybrid reinforcement solution using macro-synthetic fiber is possible for metro tunnel lining.

Hybrid precast tunnel segments in fiber reinforced concrete with glass fiber reinforced bars

The interest in using fiber reinforced concrete (FRC) for the production of precast segments in mechanical excavated tunnel lining is continuously growing, as witnessed by the studies available in literature and by the actual applications. The possibility of adopting a hybrid solution of FRC tunnel segments with Glass Fiber Reinforced Polymer (GFRP) reinforcement is investigated herein. The proposed solution can cover the situations in terms of requirements that cannot be satisfied with the FRC solution only. On the same way the use of GFRP bars can avoid the problems related to the use of traditional steel rebars. A typical metro tunnel geometry is considered, and full-scale segments are designed, cast and experimentally validated. In particular, one flexural and one point load full-scale tests are carried out, for the evaluation of the structural performances (both in terms of structural capacity and crack pattern evolution) of tunnel segments with hybrid solution, under bending, and under the TBM thrust. The experimental behaviors are compared with the ones obtained on fiber reinforced only segments, with the same geometry and concrete material. Finally, the obtained results are discussed, and the effectiveness of the proposed technical solution, in increasing the bending resistance and, mainly in reducing the crack width under flexural and TBM thrust, is highlighted.

Failure mechanism of joint waterproofing in precast segmental tunnel linings

Water leakage is a major issue for shield tunnels in both the construction and operational stages. The practical solution in industry with relation to the prevention of groundwater penetration is to implement ethylene-propylene-diene monomer (EPDM) gaskets at tunnel joints. Although a few past research has focused on joint waterproof performance, this paper proposes a sealant mechanism based on the finite element analysis. A conceptual model is first established to identify sealant behavior of gasketed joint subjected to lateral water pressurization. A nonlinear finite element model is developed to simulate the gasket-in-groove sealant behavior, and the performance of the model is validated with experimental data. Results of the nonlinear numerical analyses (incorporating a hyperelastic constitutive model of the gasket, complex contact properties, and large deformation) demonstrate the failure mechanism of joint waterproofing. Emphasis is placed on quantifying the effects of joint opening, joint offset and joint rotation on waterproof capacity. The computed results indicate that joint waterproofing capacity significantly decreases with increasing joint opening or joint rotation. The waterproofing capacity varies under different joint offset scenarios because of change of contact interfaces. For the prototype tunnel, it is suggested to control a 6.42 mm joint opening, a 0.0505 rad positive joint rotation or a 0.0141 rad negative joint rotation to maintain its waterproofness.

Void detection in two-component annulus grout behind a pre-cast segmental tunnel liner using Ground Penetrating Radar

Ground Penetrating Radar (GPR) is being considered as a potential non-destructive technique for ensuring complete annulus grout backfill behind pre-cast segmental tunnel liners. Current methods for ensuring grout backfill involve drilled verification holes through the waterproof liner. The Rondout bypass tunnel, in Newburgh, NY, where void formation behind the tunnel liner is possible due to high groundwater pressures, provided an example project to evaluate GPR capabilities. While the application of GPR for void detection in one component grouts behind tunnel liners has been well-documented, site specific calibration of the GPR equipment and the application to two-component grouts is necessary. Experiments were conducted on actual tunnel segments to determine the most appropriate antenna frequency that can penetrate through the steel-reinforced segments and produce the best resolution. Mock air and water voids were embedded within areas of complete grout behind the segment to demonstrate the GPR signatures. Steel reflectors buried within the grout provided indirect measurements of the electromagnetic (EM) properties of the two-component grout. All field results were compared against analytical evaluation and numerical models to validate void detection. The findings of this research allow for direct implementation of the GPR technique for detection of voids behind segmental tunnel liners with two-component grout backfill.

Practical design, testing & verification guidelines for pre-cast segmental tunnel linings subjected to fire loading

This paper presents a practical 5-stage fire design procedure for bored pre-cast segmental tunnel linings subjected to an accidental fire load. A case study is presented with each stage discussed together with references to design assumptions, structural analysis, concrete mix design, fire test specifications and guidance on the review and interpretation of fire test results. Finally, the effects of progressive local deep spalling are investigated with a non-linear transient thermal-mechanical coupled finite element analysis. The spalling behaviour is modelled with the removal of elements during the analysis to match those recorded during fire tests.