By Vojtech Ernst Gall
Modern TBM segments have come a long way from their first use several decades ago. More and more, the tunneling industry is optimizing segment shapes, introducing new reinforcement methods, and improving segment components to make segments more robust and more economical. Of particular note is the improvement in segment accessories. Modern segment accessories such as segment dowels, bicones, or modern gaskets, not only improve segment construction, but they allow for segmental lining systems to be employed across a wider variety of geotechnical conditions and in a wider range of structural arrangements than ever before.
One such area in which segment accessories have improved segment performance is in segment cuts. Segmental linings must inevitably be cut to create openings for connections to adjacent structures. Cuts in segmental linings, however, greatly disturb the static equilibrium of the structural system and require a redistribution of loads to adjacent rings around a cut opening. This typically leads to high shear forces in ring joints, high tension forces in segments above and below the cut, and high thrust forces in the segments adjacent to the cut opining.
Traditionally, segment cuts are implemented by introducing an external steel frame from within the tunnel to support the cut rings until the connection is made between the main tunnel and the connecting structure. This approach is expensive and intrusive, especially in smaller diameter tunnels where the steel frame reduces the available space for the main operation of the tunnel. Furthermore, this situation potentially impacts the construction schedule as the connection cannot be made until the main tunnel construction is completed.
In order to avoid these drawbacks, Gall Zeidler Consultants (GZ) is developing innovative approaches for self-supporting segment cuts by incorporating high capacity bicones plus additional segment reinforcement into specially designed segments. These bicones and additional reinforcement work together to function as an internal frame that allows for loads to be transferred around a segment cut.
The Pawtucket Combined Sewer Overflow (CSO) Tunnel is one such project for which GZ has developed a special segment design. The Pawtucket tunnel is a segmentally lined TBM tunnel currently under construction in Pawtucket, RI. The 2.2 mi long rock tunnel is being built by the CB3A Construction Joint Venture, composed of CBNA and Barletta. The segmental lining, has an inner diameter of 30’-2”, is 14” in thick, and is primarily reinforced with steel fibers.
The main tunnel connects to several 8’ diameter mined adits along its alignment, for which the segmental lining must be cut. GZ’s engineers have therefore developed a special segment design for the segments immediately around the cut that eliminates the need for external bracing. Due to the combination of the high-capacity bicones and additional reinforcement cage within each segment, the special segments can support the existing earth and water loads, and can redistribute these loads around the temporary openings. Furthermore, as the segments are precast using the same molds as the regular segments, they are installed just as normal segments and have no negative impact on construction.
GZ is using a similar approach for the cross-passage design of the Silvertown tunnel. The Silvertown Tunnel is a new, 1.4 km long twin bore TBM road tunnel crossing beneath the River Thames in soft ground. The outer tunnel diameter is 37’-5” and the lining is 15.75” thick. The TBM tunnels will be connected by 7 15’-9” cross-passages, of which 5 will be constructed using ground freezing for temporary ground stabilization. Using high capacity bicones in addition to an internal reinforcement cage, GZ has developed a design that eliminates the need for external bracing around the segment cuts needed for the cross passage. In contrast to the Pawtucket segments, the Silvertown cross passages are constructed purely in soft ground. In addition, the special segments for the Silverton tunnel must carry additional freezing loads, demonstrating that the bicone plus internal reinforcement approach has a high degree of applicability for segment cuts made for various purposes in a wide array of ground types.
The above designs demonstrate that, through the use of new segment components, and with some clever engineering, new segment designs can not only improve the TBM tunnel structure itself but can support follow on works behind the TBM as well. We therefore believe that innovative designs such as the ones described above will become more and more commonplace in future segment designs as familiarity with new segment technologies grows.
By Vojtech Ernst Gall, Senior Tunnel Engineer, Gall Zeidler Consultants