The booming city of Zhengzhou, in Henan province in east central China, was always a strategic crossroads for transport. Economic growth in the country has led to increased development and pressure on transportation systems in the city, which has had no metro but is now embarking on a major capital construction programme.

Located on the Yellow River, Zhengzhou presents an extensive challenge for tunnellers of relatively shallow, soft ground excavation to build a mostly underground, twin bore metro system in the built-up urban area.

A number of TBMs are being deployed on construction of the first phase of Line One of the metro, most of the shields having been manufactured locally. US manufacturer Robbins has supplied two EPBMS, assembled in China, for the west end section of Line One where the shields were launched late last year by the 11th Bureau of the China Railway Construction Corporation (CRCC).

Growing need in Zhengzhou
As a city of more than seven million people and set to become an even bigger national railway hub, Zhengzhou is a prime candidate for metro construction to help serve the transport needs of the growing population and expanding economy which is already operating with a road network heavily under pressure.

Initial planning for the metro system began a decade ago and the proposals that were drawn up led to a revision of the city master plan. Then, by 2003, the city had a pre-feasibility study for Line One underway.

In parallel, Zhengzhou has been preparing for the national rail transport plans that call for main North-South and East-West routes to run through the city, the former between Beijing and Guangzhou, and the latter between Xuzhou and Lanzhou. To tie-up to these intersecting lines, Zhengzhou will see four main rail links constructed separately to the metro building investment.

Subway development and operations for the city are undertaken by Zhengzhou Metro Company, which plans to construct a network of six lines by 2030. The company was established by the city in 2008. Its transport goal is to have most areas of the city covered and the centre to be accessible within 30-40 minutes of travel.

Zhengzhou metro
The metro network of six lines being planned for Zhengzhou will be developed in three main phases, the first of which is expected to be completed by 2015. However, the services on Line One, which forms a major part of the Phase One development works, are due to commence by around late 2013.

A number of local contractors are working in the different packages of the Phase One works – in addition to 11th Bureau of CRCC there are also the First and 16th, and Beijing Urban Construction Group, Shanghai Tunnel Engineering Company (STEC) and China Railway Tunnel Group, part of China Railway Engineering Corporation (CREC).

There are eight shields presently on Phase One of Line One, mostly manufactured by local firms, such as STEC and also China Railway Tunnel Equipment (via China Railway Tunnel Group it is part of China CREC), and there is the pair of EPBMs from Robbins. Eventually, there are to be a total of 12 TBMs on the first phase of Line One.

The machines are being used on the current, Phase One-only west end of Line One, between Kiaxuan Road and Tongpai Road; the line will be extended later on a route that turns north.

Line One
The route of Line One will run east-west through the heart of the city as part of a network that will have an overall shape that could be approximated to the radiating lines of a star upon which a circle has been imposed – Line Five, eventually.

Line One extends from a depot in the east of the city and runs parallel to a main railway for a short distance then continues west through the city, intersecting the rail line again at a few points as well as future metro lines. At the far west side of the route, Line One turns sharply north. The Phase One works do not include the northward leg.

In Phase One development, approximately 26km section of Line One will be constructed underground in twin bore tubes with a total of 52km of excavated route, including 22 cut and cover stations. The average distance between stations is slightly less than 1.3km. With this phase of the line’s construction to be completed in 2013, the client – Zhengzhou Metro – noted late last year that the underground works were approaching the halfway point.

When fully completed, though, Line One is to have a total length of approximately 35km and 28 stations, including a number of interchanges with all five of the other lines to be developed. The interchange stations will include those at Tongpai Road, Zijingshan, Convention Centre and Yellow River Road.

Other Lines
To run mostly on a north-south route through Zhengzhou, Line Two will have a total length of 27.3km with 22 stations, six of which will be interchanges. The average distance between the stations is, again, approximately 1.3km.

Line Three will run, broadly, between the northwest and southeast sections of the city over a total distance of 40.8km. The line will have 31 sites, including eight interchanges. The average distance between the stations is to be 1.36km.

Another north-south route will be Line Four, though it will have more of a “Z” shape on the metro network map, the extended legs at each end going west and east, respectively. The line will be 34.7km long with 25 sites, seven of which will be interchanges. The distance between stations on this line, at an average of 1.45km, will be the longest on the network.

A circle line in the centre of the metro network will be Line Five. The loop will be 40.6km long with 32 sites and 10 interchanges. The stations will be 1.3km apart, on average.

The last line of the metro network – Line Six – will have a zig-zag route between the south and east quarters of the city. The total length of the line is to be 24.3km with 19 sites, including six interchanges and stations on average 1.35km apart.

To develop this extensive network, Zhengzhou Metro plans for Phase One of construction to focus on the majority of Line One and Two, bringing a total of 45.4km of route into service by 2015.

By 2020, the company plans for Phase Two to complete the first lines and undertake the initial stages of Lines Three and Four. This phase is to add 50.2km to the network, taking the total length to almost 96km.

The third and final phase of the scheme is to add 106.7km of new construction between 2020-30, bringing the total length of the metro network to 202.5km.

The EPBMs
The 11th Bureau of the CRCC is building a 3.6km long stretch of twin bore between Kaixuan Road station in the west end and Tongpai Road in the east, and which a future interchange with Line Five (circle line). The stretch straddles the shallow Xiliu Lake and includes two further, intermediate stations – Xisanhuan (near Third West Ring Road) and Qinling Road.

Two 6.15m diameter EPBMs are being used to drive the tunnels between the stations on the section. The distances between the stations are:

– Kaixuan Road to Xisanhuan: 1,549m – Xisanhuan to Qinling Road: 1,316m – Qinling Road to Tongpai Road: 742m

Geology along the alignment of the Kiaxuan Road to Tongpai Road section is a mix of sedimentary strata – clay, fine sand, loess, pebbles and some groundwater.

Among the key anticipated challenges were the relatively shallow cover over the tunnels and the consequent delicate excavation work for the shields in the dense urban environment. The cover goes down to approximately 8m for about half of the route, and for the short distance below Xiliu Lake (which is only 1-2m deep) the distance from the tunnel to the bed is only 7m. In sections, the alignment passes below building foundations and a major road bridge.

The machines have a maximum thrust of 36,000kN, maximum torque of 4,785kNm, cutterhead speed of up to 2rpm and maximum stroke of 1.95m. The cutterheads have 750kW electric, variable speed motors, and are fitted with tungsten carbide knife-edge tools. Spoil removal is via a 800mm diameter shaft screw conveyor. The shields have two-chamber type manlocks.

The shields were assembled in Chengdu, at the facilities of Chengdu CSR Tunnelling Equipment Company. Components were brought from Japan (main bearing, main seal), US (electrical and hydraulic systems), Germany (transducers) and, locally (machine shield, cutterhead, backup gantries and additional equipment).

To reduce settlement, foam and bentonite are being injected for soil conditioning during the drives, and there is tight monitoring of both advance rates and volume of spoil being generated by the advancing drives.

The reinforced concrete lining is formed in 1.5m long rings (five plus one) built with 300mm thick segments. To prevent segment deformation, active articulation is employed in curvatures down to 200m radii. Standard cement mortar backfill is used to grout behind the rings.

Shield progress
The TBMs are being used by the Chinese contractor parallel bores that began in October 2010 and, following some waiting for station completion as well as relaunches plus dismantling, transport and reassembly, the drives were expected to be finished as T&TI went to press.

The first shield that was launched –EPB345 – set off from Xisanhuan, an intermediate station, on its eastward drive late last year and was followed three weeks later by its sister machine, EPB346. The contractor focused on getting the stretch between Xisanhuan and Tongpai Road constructed and then would return to relaunch the machines in the other direction to Kiaxuan Road station, which had not been excavated at the outset.

Kiaxuan Road was unable to accommodate the start of tunneling eastwards as a main water supply pipeline feeding residential areas in the west of the city had to be re-routed. Those works are only now being completed, which should enable the excavation of the station to be finished. The last leg of the tunnelling will then be able to be finished.

To commence tunnelling, though, at the outset the TBMs advanced from Xisanhuan and reached Qinling Road station in late December 2010 and early January this year, respectively. At each breakthrough the machines met the project’s standard rebar strips, arranged vertically in the walls at the portals and had to be cut away.

Following some brief maintenance, EPB345 was relaunched from Qinling Road a week after the arrival of the other shield which was itself subsequently sent boring again a month later. The machines reached Tongpai Road in early March and early April, respectively.

“The breakthroughs went perfectly,” said CRCC 11th Bureau’s general mechanical and electrical engineer on the project, Zhou Shuqing. “Machine downtime during tunnelling has been very minimal, and the organisation of this project has been very good.”

The crews on each machine work in two 10-hour shifts per day. The best advance rates on completing the first two sections to Tongpai Road were:

EPBM 345:
Best day = 31.5m (21 rings);
Best week = 201m (134 rings); closest 176m and then many in 140m-150m range.
Best month = 601.5m (401 rings).

EPBM 346:
Best day = 34.5m (23 rings);
Best week = 189m (126 rings); closest 183m and quite a few in two bands, the 162m-182m and the 100m-125m ranges, respectively.
Best month = 720m (480 rings); a project record by then, and also for a 6m-7m EPBM on Chinese projects, says Robbins

At Tongpai Road the TBMs were extracted and transported back to Xisanhuan station for reassembly and relaunch, this time westward, towards Kiaxuan Road. Most recently, EPB345 has gone as far as it can at present and, with 645m left to bore, is holding for Kiaxuan Road to be fully excavated before it can proceed. Its sister shield is a short distance behind.

On this last leg, though, these EPBMs have passed below Xiliu Lake, below which the ground was found to consist of very permeable soils, and the groundwater is connected to and fed by the lake due to vertical leakage. Before proceeding through this section of the drives the systems of the TBMs were re-checked and the tailgate brushes changed. During excavation, each shield kept tightly to restricted parameters – maintaining earth pressure at 1.1bar-1.3bar, the cutterhead speed to 1rpm and the advance rate to only 30mm/min-40mm/min.

With the final station in the section almost ready and the drives set to resume shortly, it is expected that the EPBMs will hole through by late October as T&TI went to press. Afterwards there is the potential for further use of the machines on the metro as they are reported to have performed well and the contractor is looking at procurement underway for other packages on the scheme.


Figure 1, Zhengzhou metro, China, highlighting EPBM-bored section Shallow tunnelling is called for on Line One of Zhengzhou metro One of the EPBMs on Line One of Zhengzhou metro The TBMs were assembled in Chengdu, China Zhengzhou is developing a major underground metro network