Mega-TBM completes complex highway drive 13 Feb 2020

Final breakthrough of the bored Beiheng Expressway drive in Shanghai marked the conclusion of more than three years of work described as an encyclopaedic project of underground urban TBM excavation.

Alignment of the Beiheng Expressway

The Beiheng Expressway is one of the three main east-west highways of Shanghai, with a total length of 19.1km and a combination of at grade, underground and elevated sections. Lot II of the project – owned by the SMI Road Group; designed by the Shanghai Municipal Engineering Design Institute and the Shanghai Tunnel Engineering & Rail Transit Design and Research Institute; and constructed by STEC, the Shanghai Tunnel Engineering Co – is the most important and difficult part of the Beiheng Expressway project. With a full length of 7.8km, all underground, the main element of Lot II is the bored tunnel consisting of two drives of 2.76km and 3.66km long each either side of an intermediate access and ventilation shaft.

The bored tunnel was designed as a double-deck six-lane highway of 15m o.d. and an i.d. of 13.7m. A new 15.56m diameter Mixshield TBM, the largest of its kind ever used in Shanghai, was procured from Herrenknecht for the project. The whole tunnel alignment runs along a densely residential and commercial corridor; the route passes a large number of structures including buildings, utilities, flood retaining wall and subway system, which made the project incredibly challenging.

Double deck highway design

Unprecedented challenges

During excavation, the mega TBM faced unprecedented challenges in urban highway tunnel construction. First was the curved alignment that presented several tight S curves of as little as 500m radius. Second were the extremely highly sensitive environmental conditions. On its drive the 15.56m diameter x 120m long x 3,000 tonne TBM passed under 89 buildings and structures, including three live metro lines, four flood retaining walls of the Suzhou River, and a number of urban utilities. For these reasons the project is known as an encyclopaedia of urban excavation.

Tight S curves

Some 80% of the alignment is curved and 50% of this is through curves of 500m to 600m radius. Tight curved alignments require careful navigation. Attention is needed to avoid staggered segmental lining rings, tailskin wear by the segments, failure of the tail seal due to uneven tail skin clearance, and the scraping of the inner lining by the TBM backup gantries. These considerations challenged the selection of the TBM, the design of the segments, the water tightness of the tail seal and segmental sealing gaskets, and control of construction risks.

Compact TBM for navigating tight S curves

To ensure both safety of the TBM and adaptability to the alignment, much work was carried out during TBM selection. The first concern was whether the front sections of the TBM shield should be articulated. Comprehensive assessments on safety, cost and adaptability were carried out before a fixed tailskin structure was chosen. The shield design was governed by the load case of 36m overburden plus additional six-storey building overload. Under this load condition, the radial shield deformation at the articulation section would be more than 10cm, and no strengthening measures could be taken because of the articulation special design. Leakage of the articulation seal would therefore be inevitable. After detailed investigation and evaluation of three different shield design concepts STEC and Herrenknecht concluded that a shield with a fixed tailskin was the safest solution for the Beiheng project.

Secondly, the TBM shield and the gantries were designed to be compact and much shorter than usual and were designed to be conical to increase the flexibility of the TBM in the ground and decrease the friction between the TBM and the ground. For the 15.56m diameter machine, the diameters of the front shield, middle shield and tail shield gradually decreased by 3cm respectively. In addition, four over cutters were included on the cutterhead design to overcut the ground on the inner side of each curve. To compensate for the enlarged annulus created by the conical shape of the TBM and the over cutters on the curved alignment, the machine was equipped with a 10-point annular grouting system to ensure adequate simultaneous segmental lining back grouting.

Mega TBM assembly

Slurry separating system

Due to the curved alignment, two types of universal segmental lining were designed: one with a taper of 4cm, for straight sections of the alignments and where the curve radius was more than 1,000m; and the other of 8cm erected in sections where the turning radius was less than 600m.

During excavation, horizontal displacement of 2m wide segment rings was monitored continuously. From the monitored data, the attitude and tendency of the oncoming rings was predicted. This was then used to select the curve shape and calculate advancing quantities, which were modified in time according to updated data. In the first sharp curved section, with a turning radius of 500m, the TBM made a successful S curve turn within 13 days and at an advance rate of 5.9 rings or 11.8m/day working 24 hours a day.

Close encounters

Of the 89 buildings along the alignment, 30 were under passed and 59 were side passed. Among these buildings was the nearly 100-year-old Zhaofeng Villa buildings. As the TBM passed under the old buildings it was also negotiating a 500m radius sharp curve, was under shallow cover and in unfavourable ground. The buildings were subjected to careful structural analysis and a risk control model was prepared. Before passing by the ancient structure 40 rings were chosen as test section to verify the control parameters. During the passing, every ring erected was monitored by an independent company to check the settlement of buildings and of the ground within the area of influence, and the parameters were adjusted as necessary ahead of excavating and building the next ring. These procedures proved successful and protected the ground and buildings with a ground loss rate of only 2%.

Tight radius curves on the horizontal alignment

Even more challenging than passing by the buildings was the TBM crossing under operating metro lines with 6.6m diameter double tracks. The Line 11 transit tunnel has a minimum clearance of 7.06m. It was the first time that a 15m diameter TBM has crossed under a metro tunnel, and no references could be provided, which presented a high risk.

On the premise that the operation of Line 11 could not be disturbed by the 44m long underpass, it was decided that the speed of the trains would be limited and the TBM would progress without any stops. After 70 hours, the TBM completed the 44m distance with a progress rate of 7 rings/day. Deformation of the metro tunnel was under the specified level of maximum allowed settlement of 20mm, and the operation of rail transit was not influenced.

Route under buildings with shallow foundation

With the experience of passing under Line 11, the TBM also succeeded in passing under Metro Line 7. At this point and after advancing 4.8km of the total 6.42km long drive, the TBM suffered serious wear of the tailskin, which made this passing more risky. The TBM finished this underpass in 71 hours, crossing a distance of 38m and limiting deformation of the Line 7 metro tunnel to 8mm.

There were other hazards including passing through horizontal radius curves of 50m and passing under and close by buildings and municipal utilities.

Green construction and smart maintenance

In recognising the current trend for greener construction, the whole process of the Beiheng Expressway construction was based on full lifecycle construction management. As part of this effort the project was the first in Shanghai to use fair-faced concrete in its interior structure. The technique is based on single castings, without any additional finishing, and using the natural surface of the in-situ cast concrete as the final finish. The design was in line with international practices including those developed in Japan. It is a breakthrough for the design of interior structures and construction technology of long, large diameter tunnels in China.

During the project, efforts to lower noise, vibration and dust emission were also adopted. As examples, dry powder grout was used in annular grouting, an integrated and modularized slurry separation system was established, muck recycling was applied as far as possible, and many of the structural components were prefabricated off site for faster, more efficient installation.

Mega TBM breakthrough in October 2019

BIM was applied at the beginning of the Beiheng Expressway project. The model was made based on the various information from the construction project and by simulating the real information of the building by digital information. A full lifecycle co-ordination management platform was also developed for Lot II of the project. The platform integrated a 4D model, construction progress, safety, quality management, real time hazard identification and management, and data monitoring, making it a comprehensive information sharing platform.

The successful development and application of technologies such as a mega TBM design, continuous boring through tight radius curves, and passing under, over and by large scale sensitive buildings and structures, has advanced the design and construction practices of high risk tunnel excavation in China. As the urban highway networks are developing in multiple dimensions in Chinese cities, the experience gained in this project will be able to be used for many underground urban highway projects around the world.