Next month, the Pinglu water transfer tunnel – which was a greater tunnelling challenge than expected due to difficult ground over 25km – is due to go into operation on the Yellow River Diversion Project in China.

Taking more than four years to complete, the excavation took longer than initially envisaged. Overcoming the tough conditions, the Austrian-Chinese joint venture contractor finally holed through with the 4.82m diameter TBM before the end of 2010 to successfully complete the gargantuan undertaking. Since then the final insitu lining works have been underway to complete the project.

Yet, while the excavation of Pinglu tunnel was a major feat, the tube is but a fraction of the colossal 200km-long scheme that has been under construction for more than a decade.

Grand Diversion Scheme
Conceived as a long-term, strategic scheme to share a portion of national water resources, the Yellow River Diversion Project will transfer significant flows to chronically dry regions of Shanxi province. The province has rainfall of only about 400mm per year.

The client for the regional water transfer scheme is Shanxi Province Wanjiazhai Yellow River Project General Corporation.

Early development work on the scheme led to a decade of extensive initial construction work that came to a close at the end of the 1990s. More than 100km of conveyance tunnels were constructed to form the General Main and South Main sections of the scheme. Much of the tunnelling and construction work was undertaken by combinations of Italian and local contractors, and employed a number of TBMs, including Robbins machines and an NFM shield.

More recently, the client has focused its development efforts on the North Main Line section, which includes the Pinglu Tunnel connection and also about 15km of tunnels, excavated by drill and blast. The North Main Line will transfer water to the town of Pinglu and cities of Shuozhou and Datong.

Pinglu Tunnel
Austria’s Alpine (part of FCC group) undertook the tunnelling work at Pinglu in joint venture with a local partner, together operating as Sino-Austria Hydraulic Engineering Company (SAHEC).

They knew the geology along the alignment would be of unusually mixed strata, from soil and often sticky, medium hard rock, to saturated clay zones and highly abrasive sandstone as well as traces of coal; the project area in Shanxi province has significant coal deposits. To complicate the task further there are also fault zones. In the event, the tunnelling challenge proved to be even greater.

Alpine had a team on site since mid-2005. The JV had decided to excavate the long tunnel through challenging strata using only one TBM, a Robbins double shield which had been used earlier on the scheme. The TBM would only have one intermediate stop on the drive, at a shaft near Daliang, past the half-way point, where refurbishment could be performed before the machine would be relaunched.

The TBM would build a primary lining of precast concrete rings formed of hexagonal, honeycomb segments. The 4.32m i.d rings would be constructed of four segments in a longitudinally staggered formation. The design was developed to enable rapid, continuous boring with no downtime for segment erection. The secondary lining would be insitu cast concrete to complete the water conveyance tunnel to a finished diameter of 4.14m.

It was anticipated that the construction period would last until early 2009 under the EUR 47M (USD 66.6M) contract. The geological challenges, however, would see the main excavation continue until late 2010.

Alpine Meyreder had worked previously on Lot One of the Yellow River Diversion Project. The works had involved excavation of an intake as well as branch and access tunnels plus shafts and pumping station caverns (165m long by 18m wide by 39m high). The various pumping stations are used to lift the water from Wanjiazhai reservoir on the Yellow River and help overcome head losses from water flowing over long distances.

Excavation
The double-shield was used previously, from 2000, to bore 12km of tunnel on the earlier, Lot Five section of the Yellow River Diversion Project, during which, Robbins reports, it set two world records for machines in the 4-5m diameter class: a best month and monthly average of 1,855m and 1,352m, respectively.

Little adaptation was required to driving Pinglu tunnel as the geology was expected to be of an order that was broadly similar to that of the initial drive. Only refurbishment and maintenance were undertaken on the machine while the back-up system needed to be modified. Having also been used on the previous drive on the scheme, the back-up frame was extended from one to two strokes and so would allow the TBM to maintain good advance rates as journey times lengthened for supplies and spoil removal as the face advanced.

Segment production was undertaken by Alpine at a dedicated yard in Shuozhou, some 15km from the site, and manned by nearly 400 workers. In total, approximately 83,450 hexagonal honeycomb segments were produced to line Pinglu tunnel.

The TBM was launched on Pinglu tunnel in the third quarter of 2006. After two tough years, with often more difficult ground than expected, the machine completed 13km of the drive and holed through into the shaft near Daliang in October 2008. Despite the hard going, the TBM had achieved an average monthly advance rate of 750m.

Alpine’s managing director in charge, Siegfried Muller, then described the going as working through ‘extremely difficult’ ground. In the first section, the TBM had met a number of fault zones and there were also coal seams up to 12m thick.

The opportunity of the breakthrough allowed for a planned stoppage to refurbish the machine, and it was sent on its way again before the close of 2008. It was anticipated that the remaining section, of slightly more than 12km, could be excavated within about 16 months to enable final breakthrough and completion of the primary lining by May 2010.

Some six months into the relaunched drive, the TBM had excavated three-quarters of Pinglu tunnel and the 15,000th ring had been erected. Alpine Bau’s head of machinery dept, Paul Bargmann, commented that the TBM had achieved high output “in some very challenging ground conditions”.

By then, due to the highly varied geology, it was expected the final leg of the boring would not be over until near the end of 2010, a few months later than previously expected. The geological conditions encountered varied between thick coal seams and clay zones to extremely abrasive rock – sandstone with up to 70 per cent quartzite content.

Complicating the tunnelling task further were the demands on support to the face as the TBM advanced. In the final few kilometres of the drive the travel time to the face had extended to almost 90 minutes and supply trains were travelling hundreds of kilometres each day. There was also a complicated challenge for the ventilation systems. Yet, over the bore from the Daliang shaft the advance rates were able to reach almost 50 rings (70m) per day.

Bargmann said, “The key to this project’s success was the crew. We had the right mix of experienced people and young people hungry to learn.”

As per the revised programme, the TBM holed through before the end of the year and since then construction has focused on the cast insitu lining work. The tunnel will be ready shortly for operational use, carrying first diverted flows of the Yellow River to Pinglu and other cities.


Stacked hexagonal, honeycomb segment for tunnel lining Pinglu was a tough tunneling challenge in difficult geology Double shield completing bore of Pinglu tunnel Figure 1, location map of Pinglu tunnel on Yellow River Diversion Project, China