Post-Tensioning is commonly used in the construction of bridges, large buildings, nuclear power stations, and water tanks but its utilization in tunnels is still in its infancy.

The precast pre-stressed concrete segmental lining can be used for shield tunnels. It provides a reinforced lining ring by the following construction stages and as shown in Figure 1:

  • Segments to be assembled by TBM to complete a full ring
  • Steel strand to be inserted through embedded duct (sheath) within the segments
  • The strand to be tensioned by jack after anchorage of the other end
  • The strand to be fastened after reaching required tension load
  • The horizontal strands (if necessary) to be installed, tensioned and fastened
  • The recesses and gap between strand and duct to be filled by grouting



  • Due to the enforced compression stress on the segments, cracks can be reduced and the water-tightness of the lining will be improved.
  • The intrados of the lining will be smoother because of the eliminated bolt connection pockets.
  • Generally, the durability of the lining will be improved due to the increased quality.


  • Flexibility and resistance against seismic loading are improved due to pre-stressing of the segmental lining in both longitudinal and or circumferential directions by unbonded strands.
  • Resistance against squeezing rock will be improved
  • Extra resistance against breakage can be achieved in the joints segments.
  • As a result of induced fully- compressed rings and increased bending moment capacity, the resistance of the lining against internal water/effluent pressures as well as external soil or water pressures will be improved.


  • Bolt connections between segments can be eliminated since the segments are connected by the pre-stressing strands.
  • Thickness of the segment and tunnel outer diameter can be reduced due to improved moment strength capacity and subsequently excavation volume would be reduced.
  • Reinforcement can be reduced due to imposed pre-stress and increased strength in the segments.
  • Inserting the strand from one point, tensioning and fastening it is easier and faster in most cases than installing bolts between segments and fastening them individually.
  • Secondary concrete lining can be omitted due to the improved quality, durability, and resistance.
  • Due to the eliminated bolt connections, an automatic lining operation for inserting, tensioning, and fastening of the strands can be implemented.

Theoretical evaluation of increased resistance:

ASTM C76, C506, C507 and C497 have been considered as loading references to simulate the three-edge bearing application on the precast post-tensioned segmental lining modelling as shown at Figure 2. STAAD PRO software has been used for FE analysis for a common size sewer tunnel as an example with the following assumptions:

  • Lining OD= 2.80m
  • Thickness of segments= 0.20m
  • Breadth of segments= 1.0m
  • Divisions: 5 segments + 1 key
  • Strands: Unbonded single strand, dia.=20mm; 2 no. in circumferential direction and 4 nos. in longitudinal direction of the tunnel
  • Applied vertical Load: Equal to ASTM C76, Class II for D-Load (.01) = 125KN/m
  • Strand Tension load= 250 KN

Stresses are compared between segmental linings with and without PT strands. In Figures 3 and 4, positive and negative stresses address tension and compression stresses in the elements respectively in most critical cases. Also, top and bottom stresses represent stresses at outer and inner surfaces of the lining respectively. Results show that by post-tensioning the ring, tension stresses will be reduced around 18 per cent and 12 per cent at outer and inner surfaces of the ring, respectively, which means that tension resistance of the ring will increase up to 12 per cent at least.

Also, compression would be increased around 18 per cent and 18.5 per cent at outer and inner surfaces of the ring, respectively.

However, since concrete is much stronger in compression than tension, the increased amount of the compression can be easily tolerated by the ring. Further development of this system is done by using a spiral strand in a 6m-long helical tunnel as shown in the Figure 5. The following models share the same parameters as the previous models. In this configuration the spiral strands are parallel to the helical segmental lining. The most critical tension stresses would occur in the inner surface of the helical lining, which has been shown in Figures 6 and 7.

The outcome – by post-tensioning the spiral strands in the helical tunnel – is that the maximum tension stresses in the concrete elements will be reduced from 10.1 MPa to 8.44 MPa which means that tension resistance would increase up to around 16.5 per cent.

Using a post-tensioning strand in a tunnel’s segmental lining has various benefits. The strands can be used in diverse configurations including full ring, longitudinal, and spiral which would improve and increase lining quality and resistance as well as reduce the costs. It is predictable that this system would be executed widely in future projects.

The author was familiar with the post-tensioning (PT) systems employed in the construction of concrete beams and slabs, and started developing a PT system for the precast segmental lining of tunnels and manholes.

McNally Construction has implemented the PT system in a full-scale 2.744m ID segmental concrete ring. The full-scale testing results were satisfactory and verified the FEA results.

The segment moulds that were employed had been fabricated for previous projects and were being prepared for use in casting segments for a current project in Mississauga, Ontario. The precast rings are of the trapezoidal/ parallelogram style consisting of five segments and a key. The precast concrete manufacturer was instructed to embed conduit supplied by McNally in each of the segments of a single ring as shown in Figure 8.

The conduit was embedded in two rows aligned around the circumference of the ring. The segments were allowed to cure until they had achieved the required compressive strength and then the ring was assembled with the steel PT strand inserted through a pocket into the embedded conduit. With one end anchored in a pocket, the strand was tensioned by hydraulic jack until the required PT load was attained and then the other end was anchored in a second pocket.

The standard D-Load Method was used for load testing the assembled post-tensioned ring as shown in Figure 9 and the observed results successfully verified the theoretical FEA results.

In developing this PT tunnel lining system further, a universal segment has been conceptually designed where the PT strand is inserted into the leading circumferential edge of the segments, a sample of which is shown in the 3D model in Figure 10. The proposed helical tunnel lining method allows for segment erection and strand insertion to be completed continuously. The segments have short projections on the two trailing edges (circumferential and radial) and similar recesses in the opposite two leading edges. This forms a tongue-and-groove joint at both circumferential and radial joints. The PT strand is fitted into a continuous groove located at the base of the leading circumferential edge recess as shown in Figure 11.

Similar to conventional segmental linings, the TBM thrust cylinders will temporarily support each segment until the PT strand is inserted and the next loop of segments are erected. The TBM thrust cylinders would be operating at different extension lengths to push uniformly against the helical leading edge of the segments.

There are practical limits on the length of tunnel that can be constructed using a single length of PT strand. These include supply and tensioning length limits on the strands, project scheduling, and other constructability concerns. A special socket segment can be employed for terminating one strand and beginning another. Such a segment would include two pockets with conduits that cross over each other before emerging into the PT strand groove. The leading pocket is used to terminate the previous PT strand while the trailing pocket is used to begin the next PT strand (Figure 12).

An alternative method for anchoring a PT strand that does not require a special socket segment is anchoring and tensioning the leading end of the strand using a temporary steel frame then grouting the PT strand groove. Once the grout has cured, the temporary frame may be unloaded and removed as the PT strand tension will be locked in to the segmental lining structure via the grout. An additional recess within the PT strand groove located near the center of the segment would provide clearance for a coupler connection between the previous and new strands if required.

Two options have been considered for introducing alignment curves to the helical segmental lining. The first requires the use of tapered packers placed within the circumferential edge recess as shown in Figures 13 and 14. The second requires the use of length-modified segments, not unlike the conventional tapered segments currently in use; however, this option contradicts the universal concept of the helical tunnel segment.

The tapered packers could be manufactured from several materials including, but not limited to vulcanized rubber, GFRP, and steel. Tapered packer thicknesses would be expected to range from 5mm to 25mm or more. The segment recess depth will limit the maximum allowable packer thickness whereas the minimum thickness is expected to be approximately 5mm due to practical constructability.

These packers would be applied to one side of the tunnel, inserted into the circumferential recess once the PT strand has been inserted. By installing specifically chosen thicknesses in consecutive circumferential joints, the tunnel construction may follow an alignment through any curve: vertical or horizontal, constant or compound, or any combination of the these.

Different thicknesses of tapered packers can be stocked on a single project to allow a TBM to achieve different curve radii while maintaining packer placement within consecutive joints (Figure 15). This aids in the elimination of ‘missed’ packers.

However, a single thickness packer may also be employed at certain intervals to achieve different radii like how tapered rings are employed in certain orientation patterns to achieve different curve radii.

The segments’ trailing edge projections will allow for one or two rows of gaskets. The gasket would be expected to function similarly to that of a bell-and-spigot connection between precast pipes. Alternatively, the tapered packers may be constructed in such a manner as to function as both a gasket and a joint filler for completing alignment curves.

The helical PT system provides another advantage as the load induced by the stressed PT strand will be applied in both the circumferential and longitudinal directions effectively pulling the tunnel structure together.

It is expected that a purpose-built helical tunnel lining TBM could be automated such that it automatically inserts the PT strand and any tapered packer as the tunnel advances. It is further expected that a TBM may be able to automatically tension the strand and grout the groove after a predetermined length of tunnel construction.

This conceptual post-tensioned helical tunnel lining system and its proposed advancements have been patented.