On-site stress tests of lining segments for Barcelona's underground tunnel

Construction of the new L9 underground line, currently being built in Barcelona, that comprises a single tunnel with two transport levels demanded load tests on the lining segments. The decision to undertake on-site testing was because of technical and economic considerations.

Introduction

Construction of the new L9 underground line, currently being built in Barcelona, that comprises a single tunnel with two transport levels demanded load tests on the lining segments. The decision to undertake on-site testing was because of technical and economic considerations. These tests were undertaken in December 2005 using a 1:1 scale test section.

The lining segments were manufactured with SFRC (Steel Fiber Reinforced Concrete) without reinforcement bars. The main aim of the load test, specifically designed for this investigation, was to obtain experimental proof of the behavior of these tunnel lining elements, made of SFRC and without any conventional reinforcement, under concentrated earth pressure. The load was measured using HBM pressure transducers.

The L9 project comprises a tunnel, approx. 40 km long, dug with a tunnel boring machine with a diameter of 12 m. The tubes consist of 7 + 1 segments each 1.8 m wide and an overall thickness of 0.35 m (Fig. 1).

The use of SFRC in ready-mixed parts is not usual, but it increases the toughness of the segments reducing the possibility of damage during removal from the mold, and during subsequent transport and installation. In the Barcelona tunnel the reinforcement cage was completely replaced with 60 kg/m3 steel fibers. Experiments on lining segments in a 1:1 scale under laboratory conditions had previously been carried out in The Netherlands and in Germany but load tests under actual site conditions have never been carried out before.

Test concept and measurement setup

The aim of the load test was to improve understanding of the behavior of the SFRC tunnel lining segments. On-site testing is sensible because it is possible to evaluate the contribution of the neighboring rings to the behavior of the investigated segments. It is assumed that this is mainly dependent on the remaining compressive stress in the longitudinal direction between the rings due to the axial pressure of the tunnel boring machine.

Fig. 1: Cross-section of the lining of the underground line L9 (Section 4a)

In addition, the results are very valuable for checking the assumed loads and methods of calculation used in the static calculations for lining segments. They are also used to evaluate the suitability of SFRC as a construction material for segments.

The test concept applied the load using three hydraulic flat-bed presses embedded in the outer curved surface of two ring segments. These presses were arranged as shown in Figure 2, so that they covered a sector of 48° in a vertical symmetrical arrangement at the upper end of the curve.

Fig. 2: Position of segment K in the ring, proposed loading direction of the hydraulic flat-bed presses

The positions of the recesses and the installation of the flat-bed presses are shown in Figure 3.

Fig. 3: Position and installation of the hydraulic flat-bed presses

On-site testing advantages and disadvantages

Technical advantages:

1. The reaction of the concrete construction is determined under actual on-site conditions.
2. The impact of all the elements is considered meaning that the tangential components in contact between the lining and the ground, along with the radial components are taken into account.
3. The pre-stressing in the longitudinal direction between the rings and the de-stressing is implemented under real conditions.
4. The stress/strain state before the test corresponds with the “normal state” in the tunnel after the lining has been fitted.

On-site testing disadvantages:

1. There are difficulties in the interpretation of the results due to natural uncertainties in the mechanical behavior of the site and the injection mortar.
2. As the measurements are implemented with predefined load application areas as it is not possible to change the position of the prestressing jack retrospectively.
3. The loads must lie below the breaking load to avoid damage to the lining.
4. The test sequence must be coordinated with the production program.

Economic advantages:

1. No special rooms or extra place and expensive laboratory equipment were required.
2. The measuring instruments used On-site can be used elsewhere as the measurements can be taken during or after the tests.

The section in the experiment comprises 16 SFRC rings (without reinforcement bars), of which five were equipped with embedded strain gages (SGs), pressure transducers and displacement transducers in the joints. A total of 150 SGs and 18 pressure transducers were embedded in these rings for monitoring. In addition, 7 transducers for vertical movements (Figure 4) and 52 further displacement transducers were applied to measure the tangential and vertical movements in the joints between the segments. This corresponds to four 2-D displacement transducers in the joints between the rings (Figure 5) and 44 displacement transducers for measuring the tangential movements in the joints between the segments of a ring. All sensors were connected with external measuring instruments before tests began.

Fig. 4: Measurement of deformation by displacement transducer

Fig. 5: 2D displacement transducer (radial and longitudinal) at vertex of curve between two rings

Hydraulic system and pressure transducers

The three embedded flat-bed presses were individually controlled via three manual valves with the sole purpose of controlling the hydraulic oil supply. The pressure in the pipe system was generated and controlled by an electric oil pump set up for 900 bar, the pressures were measured and recorded by a data acquisition unit using HBM’s P2VA1/200 pressure transducers, which have a very high sensitivity (Figure 6); 1 bar in the press corresponded to a force of 10 kN. The technical data of the pressure transducers are summarized in Table 1.

Tab. 1: Technical data of the pressure transducers used

A data acquisition system with high sampling rates was not required because the load was purely static and the probability of failure very low. Instead, a data acquisition system working with a multiplex technology was used (data logger) at a resolution of 6 1/2 digits and a maximum sampling rate of 60 channels/second. The data acquisition took a measured value every two seconds. The results showed that the pressure transducers were very reliable. Figure 6 shows the oil pumps, valves and the pressure transducer, Figure 7 shows the layout of the hydraulic circuit.

Fig. 6: Oil pump, valves and pressure transduce

Fig. 7: Schematic representation of hydraulic circuit

Loading

Loading comprised three phases and is shown in Figure 8. In phase 0 a load of 100 kN per press was applied and maintained for nearly 18 hours. This preparation phase provided information about the relaxation of the press pressure and the status of the overall system. In phase 1, a load of 500 kN per press was applied. Press No. 3 failed at the start of this phase. In phase 2, a load of 1500 kN was applied by the Presses 1 and 2, both simultaneously and separately.

Fig. 8: Force-time curve during test

Measurement results

Table 2 shows the vertical deflection at the vertex of the curve for various loading combinations. The maximum vertical deflection of 3.1 mm was reached in phase 2 with a load of 1500 kN using Press 2. Following the complete loading process, the remaining vertical deflection was 1.2 mm (38 % of the deflection under load).

Tab. 2: Maximum and remaining vertical deflection at vertex

Further results are shown in the following diagrams, where pressure and deformation are depicted along the y axis and time along the x axis.

Figure 9 shows the pressure trends and the radial displacement between the rings at the vertex of the curve in phase 2 where both presses continued to load the lining until 1500 kN was reached for both. This was the maximum loading applied during the tests. The radial displacements were not symmetrical on both sides of the loaded ring (1838). As can also be seen in Figure 9, they are practically negligible between the other rings.

Figure 10 shows the pressure and the relative vertical displacement between the rings at the vertex, also during phase 2. Both presses were also applying the maximum loading here. The circumferential displacements on the longitudinal joints between segments A2 and A3 are also not symmetrical in this case. This lack in symmetry is due to the eccentric position of the presses during the construction of the segments.

Fig. 9: Pressure and displacement in radial direction between rings at vertex of tunnel (loading phase 2)

Fig. 10: Pressure and displacement in circumferential direction at the longitudinal joint between segments A2 and A3

Fig. 11: Crack pattern on the uncovered interior tunnel surface after completion of phase 2

The first crack appeared in the vicinity of the curve vertex in segment A2 at a load between 40 and 500 kN, which was applied simultaneously by both presses 1 and 2 during phase 1. The crack pattern at 1500 kN per press is shown in Figure 11. It can be seen that the main cracks run parallel to the longitudinal axis of the tunnel. This, and the fact that the neighboring rings hardly moved, can be interpreted to mean that the loaded ring absorbed the majority of the loading. Figure 12 shows the cracks on the inside of the curve after phase 2. Water leaked through these cracks. This leaking was expected as the groundwater level lies 10 meters above the top of the tunnel.

Fig. 12: Cracks after phase 2

Concluding remarks

An on-site loading test was implemented on the lining of the L9 underground line in Barcelona which is under construction. The main aim of the test was to obtain experimental evidence of the bearing capacity of the SFRC segments under operating conditions. The loads applied were large enough to generate movement and significant cracks in the segments. Six different loading variants were achieved on the lining through the combination of various pressures with the flat-bed presses used. The maximum loading on the outer surface of the ring was 3000 kN.

The following conclusions can be made from the experimental investigations:

• Neighboring rings were only negligibly affected by the overall reaction of the loaded ring. Displacements were clearly noted between the loaded segments and the neighboring rings.
• The area affected by the loading was restricted to the upper half of the loaded ring. The transducers installed on the lower section of the loaded ring (both internal and external) showed hardly any changes during loading.
• The deformation of the loaded rings is mainly due to a rotation of the longitudinal joints between the segments.
• The non-linear 2D analyses carried out during the planning phase revealed deformations for individual rings that are comparable with the measured values. Additional analyses will be implemented to better interpret the results.

Acknowledgements

The authors thank the public company responsible for the planning and construction of the underground line L9 in Barcelona, Gestió d’Infraestructures, S.A. (GISA), for the financing of these research studies, which were carried out in the department of civil engineering at the Technical University of Catalonia UPC. The authors also thank all participating personnel from UTE L9, Payma Cotas and the UPC. Special thanks go to the personnel in the structural engineering laboratory, Mr. Carlos Hurtado and Mr. Jordi Lafuente as well as Ms. Roser Valls, Ms. Marta de la Torre and Ms. Gemma Viladomat.

References

[1] Molins, C.: Investigación teórica y experimental del revestimiento de túneles a base de anillos de dovelas prefabricadas: diseño de un ensayo in situ del revestimiento del túnel de la L9 del Metro de Barcelona. Sèrie Investigació: 706-I01-05. Departamento de Ingeniería de la Construcción, Universitat Politècnica de Catalunya. Barcelona 2005

[2] Molins, C., Marí, A. R., Aguado, A., P: Proyecto de prueba de carga del revestimiento del túnel a base de dovelas prefabricadas de la L9 del Metro de Barcelona. Proceedings of the III ACHE - Congreso de Puentes y Estructuras de Edificación de la Asociación, 14 -17 November 2005, 893-905, Saragossa, Spain