4.2.1. Failure Chain
The experimental load-displacement curves of the deformed stagger-jointed segmental tunnel linings that are strengthened by the epoxy-bonded filament wound profiles is illustrated in
Figure 16, and the load levels of P1, which are associated with the progressive failure of the specimen, are listed in
Table 4.
When the loads that are acting on P1 reach 667.5 kN, the steel bars of the extrados at 270° and the steel bars of the intrados at 180° yield because of the tension that is occurring. When the loads that are acting on P1 reach 677.5 kN, the bolts at the 11.75°and 168.75° joints yield. Therefore, four plastic hinges form in the middle full-width ring at P1 = 677.5 kN, i.e., the 180° section, the 270° section, the joint at 11.75° and the joint at 168.75°. The unstrengthened ring becomes a multi-hinge structure with displacements that are rapidly increasing in the range of small load fluctuations.
The failure process of the linings that are strengthened by the FWPs can be divided into two stages, i.e., the elastic stage and the plastic stage.
When the loads that are acting on the P1 points reach 800 kN, a bond failure occurs between the FWPs and linings from 326.25° to 45°. The superposition between the FWPs and the linings in the crown is ineffective, and internal force redistribution occurs in the structure. The load-displacement curve shows that the structure enters into a plastic state. Its stiffness decreases significantly, with the displacement increasing sharply. This load is defined as the ultimate bearing capacity of the strengthened stagger-jointed segmental tunnel lining.
When the loads that are acting on the P1 points reach 802.5 kN, the extrados concrete of the upper and lower half-width ring at 168.75° is crushed. When the loads that are acting on the P1 points reach 835 kN, the extrados concrete of upper half-ring is crushed by compression at 0°. At the same time, the bolts at the joint at 33.25° yield. Then the loads keep decreasing, and the displacement increases continuously. Then the test is finished.
4.2.2. Weak Sections
The full-scale experiment results indicate that the initial failure of the strengthened staggered-jointed segmental tunnel linings originates from the debonding in the crown, which is same as with the continuous-jointed segmental tunnel lining experiment [
13].
The bonding interface is mainly connected by glue, and it is resistant to pull-out and shear. In the sagging moment area of the lining, i.e., the crown and the bottom of the lining, the bonding interface, is in the unfavorable state of shear and tension which makes it easier for interface damage to occur.
According to the research [
7], the peel stress of the bonding interface could be calculated. For example, when P1 = 800 KN, the average tensile stress of the FWPs at 0° is 73.59 MPa, and the inner diameter of the segment and the height of the FWPs are 2.75 m and 0.04 m, respectively. The calculation result of the peel stress of the bonding interface is 4.28 MPa, which is greater than the tensile bearing capacity of the bonding interface, which is 1.5 MPa. Therefore, the bonding interface at 0° is peeled off. Hence, the bonding interface at the crown and the bottom of linings are the weak sections of the FWP-strengthened linings.
The steel bars of the intrados at the crown and bottom of the lining and the steel bars of extrados at the waist are prone to yield failure. The internal force distribution of the unstrengthened lining is shown in
Figure 20. For the calculation method and the assumption of the internal force, refer to
Section 4.1.1. As shown in
Figure 20, the peak values of the sagging moment are at the crown and the bottom of the lining, while the peak value of the hogging moment is at the waist. At this load level, the steel bars of the intrados at the crown and the bottom bear the tensile stress, and the steel bars of the extrados at the waist bear the tensile stress.
The internal force of the strengthened lining with the incremental loads are shown in
Table 5. The sagging moment at the crown and the bottom of the lining and the hogging moment at the waist continue to increase. Therefore, the tensile strain of the steel bars at the intrados of the crown and the bottom and the steel bars at the extrados of the waist continue to increase, which are areas that are prone to yield failure. As shown in
Figure 12, the steel bars at the 0° zone of the intrados in the middle full-width ring and the steel bars at the 270° zone if the intrados in the middle full-width ring yielded, while the strengthening point and the tensile strain of the steel bars at the 180° zone of the intrados continued to increase after the FWPs construction. Therefore, the steel bars at the intrados of the crown and the bottom of the linings and the steel bars at the extrados of the waist are the weak sections of the FWP-strengthened lining.
After strengthening, the extrados concrete of the joints near the crown and the bottom of the lining is crushed. As shown in
Figure 7a,b, the sagging moment is large enough to make the bolts yield under tension and the extrados concrete of the joint crush. After the bonding between the segments and the FWP is intensified, the initial failure of the strengthened lining changed from the bonding failure at the crown to the crush of the 352° zone of the extrados surface joint concrete [
17]. Therefore, the extrados concrete of the joints near the crown and the bottom of the tunnel are the weak sections of the FWP-reinforced lining.
In conclusion, the bonding interface at the crown and the bottom of the linings, the steel bars at intrados of the crown and the bottom and extrados of the waist, and the concrete on the extrados of the crown and the bottom joints are the weak sections of the FWP-strengthened linings.