The 2 x 2-lane Xiaotang-Gantang Expressway (XGE) covers a length of 109km from the Guangdong/Hunan provincial border to a point on the outskirts of Shaoguan. The XGE is being constructed by local contractors under ten FIDIC contracts with a total value of $210m. The contracts were awarded on November 11 1998, some having construction periods of 30 months, those including tunnels, 36 months. The project is 30% funded by the World Bank.
The existing, mostly 2-lane, Pingru Highway took the most advantageous route through the area and, consequently, the XGE has been forced to adopt a much more challenging alignment. In many places, the Pingru Highway imposes severe constraints on construction of the XGE, particularly where the routes cross. The twin tunnels have a route length of 7km (i.e. 14km in total). Many of the cuttings are more than 30m deep and have steep faces. Several cuttings and tunnel portals are being re-designed as a consequence of the actual ground conditions being experienced, which are less favourable than expected.
The topography and geology are complex and the XGE is probably the first expressway in China facing such difficult conditions. In many locations there are significant karst features, which have led to major risks and difficulties in constructing the tunnels and bridge foundations. The project comprises a succession of bridges (11.8% of the total length), tunnels (6.3%), and earthworks through terrain having very steep cross slopes.
The tunnels consist of seven pairs of 2-lane tubes as listed in the table above.
Pingshi (500m long) is, for the most part, relatively straightforward, being excavated in sound pelitic sandstone. Excavation of the the Meiziao Tunnel (760m) in karstic limestone was completed in April 1999. The first difficult tunnel is that at Pishuang’ao. At the Beijing end, progress in the left tube is slow. In the right tube, there was a big setback in May 1999. A major karst feature was encountered and this funnelled 30m to the surface. Fortunately, no-one was hurt. Although the feature has been made safe and the debris removed, tunnelling has yet to be resumed. At the Zhuhai end, a 30m long pipe arch is being constructed in the left tube following a deterioration in tunnelling conditions. The adjoining right tube is being excavated by a twin pilot method through a sandy, clayey material.
The next tunnel on the freeway route is that at Yangpeng. At just over 2km long, this is the longest tunnel on the project. It also has the most complex geology. Limestone is the predominant medium along the route, followed by sandstone, with the remainder comprising fault zones and Quaternary deposits. At the Beijing end, progress in both tubes has been slow owing to difficult ground. The left tube is being advanced under the protection of a 30m long pipe arch, while, in the right tube, a six cell multi-drift (CRD) excavation method is being employed.
At the Zhuhai end, steady progress in both tubes was being achieved through sandstone until recently. This all changed dramatically in the right tube on the morning of October 21 1999, when there was a massive face collapse, disgorging 4000 cu.m of rock and mud with an initial inrush of water, said to be of the order of 2 cu.m/s. Fortunately, the drive is upgrade and is thus self draining. No-one was hurt. The water inflow has subsided to some 12 litres/s and work to recover the face continues.
The third troublesome tunnel is at Wukengba and is 1.2km long. Here, excavation is being carried out through a series of conformable sandstones. At the Beijing end, severe transverse ground slope conditions have delayed the start of tunnelling at the portal of the left tube. A substantial retaining wall has been built to provide the necessary lateral support to the portal. To mitigate delays on this account, two-way development of the left tube has begun via the right tube and a pedestrian cross passage some 200m in from the portal. Progress in the right tube itself has been relatively good, with more than 600m excavated to date.
Portal problems
At the Zhuhai end of the left tube, there are portal problems of a different kind. The portal position lies under side cast material from the construction of the Pingru Highway, leading to design and construction difficulties which have yet to be resolved. In the interim, two-way development is proceeding slowly by conventional heading and bench, working via an adit driven immediately adjacent to the portal of the right tube. Steady but slower progress was being achieved in the right tube until October 22 1999, when a collapse close to the face occurred, which resulted in a hole funnelling nearly 30m through to the surface. Again, no-one was hurt. The problem was overcome within a month and excavation work has resumed.
The Beijing end of the Wequan Tunnel (380m) is adjacent to and high above the Pingru Highway, making portal operations very difficult. Accordingly, most of the tunnel excavation (through sandstone) has been performed from the Zhuhai end.
The Shimen’ao Tunnel (1.2km) has been driven through karstic limestone. The left tube holed through in September 1999 and only 14m remained at the time of writing to be excavated in the right tube. This is, however, at the location of a major karst feature and progress is very slow.
An in-situ concrete lining is being provided in addition to the primary lining of rockbolts and shotcrete. Emphasis is being placed on the need to construct the secondary lining as soon as possible after shotcreting and rockbolting has been carried out.
Notwithstanding the difficulties and the fact that the project’s first anniversary has now passed, it is still hoped to complete the tunnelling work within 36 months. However, the 2km long Yangpeng Tunnel is now seriously behind schedule, which does give cause for concern. At the time of writing (early Dec 1999), excavation was only 25% complete.
Related Files
Figure 1: Location of the road tunnels
