As the provincial capital of Sichuan and home to more than 10,000,000 inhabitants, Chengdu is an important economic center in southern China. In an effort to keep pace with increasing transportation needs, Chengdu – like many cities in China – has begun building subways. Chengdu’s first metro line opened in 2010, with system expansion ongoing. Recently, Chengdu Metro Corporation saw the completion of the Fengxi Station–Nanxun Road Station section, part of the Line 4 Phase II West Extension.
The project, completed by contractor Sinohydro Bureau 7 Co. Ltd., involved a constructing tunnel totaling 862.3 m.
The overall Line 4 Phase II program comprises 17.34 km of underground alignment with 14 stations, with the West Extension totaling 10.67 km with 8 stations. The Fengxi–Nanxun section has been completed and is expected to open in 2017.
Chengdu is composed of the quaternary alluvial plain. Within the depth of metro tunnel, the ground is mostly sand-pebble, the nearer to the west, the larger the particle size of cobbles. Since the time metro line construction started in Chengdu in 2005, there has been an ongoing challenge in these ground conditions, namely equipping the machine to resist the abrasion of dense sandy-pebble, how to control the earth pressure during tunneling, and how to control the ground subsidence.
Based on the construction experience of Chengdu Metro Line 4 West Extension Line (Yangliu River Station – Fengxi Station – Nanxun Road Station) with an EPB TBM, this article elaborates on EPB tunneling in ground with high water content and large particle size cobbles and boulders, aimed at providing references and suggestions of selection of shield machine in similar ground conditions.
Chengdu Metro Line 4 Phase II section between Fengxi Station and Nanxun Road Station is an underground alignment. The project passes through medium dense cobbly soil and dense cobbly soil with pebbles and boulders. The boulders range up to 900 mm in size with compressive strengths ranging up to 300 MPa. Much of the ground in the area is water bearing.
As a whole, the ground condition of Chengdu Metro Line 4 Phase II is in a ground layer with high water content, large particle size, high strength cobbles and boulder, which makes tunneling in this kind of ground challenging.
Project Difficulty and Construction Risk
Based on the tunneling experience of Chengdu Metro, when a TBM passes through dense cobbles and boulders, the cuttinghead and cutters are under severe wear, so, during the whole project, to reinforce the cuttinghead, frequent replacement of cutters is unavoidable.
The largest particle size of boulders in this section is 900 mm, which is larger than the allowable particle size (300 x 500 mm) of cuttinghead and screw conveyor, so the large particle size of boulders must be cracked before going into the cuttinghead. If the cuttinghead cannot crack this large particle size of boulders without delay, multiple boulders will be stacked before the cuttinghead, which will increase the starting torque, slow down tunneling severely, and will lead to the cuttinghead getting stuck or the machine shutting down.
Because of the poor cementation or no cementation of boulders in this section, when a TBM passes through, soil above the cuttinghead will fall down after agitation, causing surface subsidence.
The excavation face is lower than the underground water line in this section, so this area has abundant underground water and high rock permeability. In this case, water content of muck is high, and water is easy to flow from the front of belt conveyor to the segment erection area, requiring manhours for cleanup and negatively impacting advance efficiency.
TBM Model Selection and Configuration
In comprehensive consideration of the power match, cuttinghead torque, RPM, margin of driving power, type of screw conveyor, and abrasion resistance of each component to achieve efficient advance rates in this ground condition, the project team chose an Earth Pressure Balance TBM manufactured by Liaoning Censcience Industry Co. Ltd. (LNSS), which in 2014 bought the assets of Caterpillar Tunneling (formerly known as LOVAT), including the entire intellectual properties of LOVAT/Caterpillar Tunneling. LNSS established Lovsuns Tunneling Canada Ltd. to serve North American and European markets. Lovsuns maintains the manufacturing facilities previously occupied by Lovat and Caterpillar, and has retained many of the employees as well.
The technical parameters of this shield machine are as follows:
The opening rate of cuttinghead is 34% (center opening rate is 50%), the basic configuration of cutters is 33 each 17-in. single disc cutters, 4 each 17-in. center twin disc cutters (could be changed by 18-in. cutter ring); No. 80 face scraper, and No. 24 gauge scraper. The opening rate of the cuttinghead is inversely proportional to the maximum quantity of cutters, 34% is suitable for this kind of ground condition. In addition, running radius of the center cutters on the cuttinghead is smaller than the others, so, these cutters will be under larger yawing force, this will affect cutting rate and operating life of disc cutters. Therefore, it is important to ensure the larger center opening rate of cuttinghead so that muck from the cuttinghead center can flow into chamber to avoid muck being stacked in front of cuttinghead and to reduce workload of center cutters.
The face of cuttinghead is welded with CAT wear-resistance plate, and the gauge of cuttinghead is welded with inlaid wear-resistance plate. There are seven injection holes on the cuttinghead. Wear resistance of the cuttinghead is controlled in two ways: one is selection of wear-resistant material, the other is reducing wear through effective ground conditioning.
There are 6 each 200-kW water-cooled and variable frequency motors on the cuttinghead. The nominal torque of the cuttinghead is 6,300 kNm, nominal rpm range is 0-1.8, maximum rpm is 3.2; and breakout torque is 8,300 kNm. In this kind of ground condition, the cuttinghead has more power reserve, high ultimate torque and big spindle. When suffering formation discontinuity and high variability of torque, the cuttinghead will not have deformation. At the same time, driving with variable frequency motors has the following advantages: high rigid torque, high ultimate torque, low power consumption, high efficiency, better operating environment (low noise and low temperature), less downtime, less maintenance, easy to maintain and less material consumption.
The inner diameter of screw conveyor is 850 mm, the allowable particle size is 300 mm (diameter) x 500 mm (length).
The maximum advance rate is 200 mm/min. The higher the advance rate, the greater the ability to go through serious ground condition, reducing the disturbance of soil, could reduce the chances of caving of the above soil, and decrease ground subsidence.
Challenges during Mining
This is the first LNSS to be used for Chengdu Metro construction. Based on the past tunneling experience in Chengdu, the project team adopted the excavation method of using half-full chamber and higher front pressure. According to the former tunneling experience of Chengdu Metro, rpm of cuttinghead should be controlled around 1 rpm to avoid strong disturbance of soil and ground subsidence.
The chamber of the cuttinghead is always kept half-full with bentonite injected to the cuttinghead to lower the torque in order to avoid the cuttinghead getting stuck. However, in the beginning of tunneling, there are also some problems: the cuttinghead getting locked, difficult to advance and ground subsidence.
Problem Analysis and Solution
With regards to the above problems, the project team and LNSS worked to find a solution to these problems.
In this section of ground, particle size of boulders is larger than the allowable size of cuttinghead and screw conveyor, so, boulders should be cracked, then be transported to back of the machine. If cuttinghead rotates at 1 rpm, it is difficult for disc cutters to crack the rock, and then there will be more and more rock in front of cuttinghead, leading to a stuck cuttinghead. LNSS’s cuttinghead has higher driving power, so there is torque rpm ratio in surplus. At 1.76 rpm, the torque of the cuttinghead is 6,300 kNm. In this case, the rpm of cuttinghead should be increased to 2 rpm and torque kept between 4,500 kNm and 5,500 kNm. With the high rotating speed, the cuttinghead can crack the big rock in front of it.
Also in this section, the ground is not self-supporting. If keeping the chamber half-full, the soil above the chamber can easily fall, causing ground subsidence. Therefore, because LNSS’s machine has torque in surplus, when tunneling, always keep the chamber full, and strictly control the volume of muck in order to control ground subsidence. After adopting this measurement, LNSS’s machine advanced smoothly, and the cuttinghead never stuck. At the same time, as the cuttinghead rpm is increased, cobbles are cracked quickly, and the advance rate is also increased. In these ground conditions it is possible to achieve advance rates of 150 mm/min and 370 rings in one month.
The successful experience of Chengdu Metro Line 4 Phase II Fengnan Section has great significance to selection of machines and tunneling parameters in the same ground condition. From the above analysis, the following points need to be considered when choosing machines in ground with high water content and large cobbles and boulders.
Driving power of cuttinghead. Based on the success experience in this section, the key point is the surplus power of the cuttinghead. In this type of ground, cracking of cobbles larger than the allowable size of the cuttinghead and screw conveyor is important to keep rock from piling up in front of the cuttinghead and stalling the machine.
Optimization of cutters. In unconsolidated soils, it is difficult to provide enough starting torque to the disc cutters, which will cause ineffective rolling of cutters. So, it is important to consider the starting torque of the disc cutters based on the ground conditions.
Cuttinghead structure and configuration. When tunneling in ground with large boulders, the torque of cuttinghead ranges between 4,500 kNm and 5,500 kNm, and instant maximum torque could be 7, 000 kNm, so, the overall structure strength of cuttinghead must be high. In addition, uniaxial compressive strength of cobbles and boulders is high, which adds wear to the cutters, so, the requirement of the wear resistance of face and gauge of cutters will be higher in order to avoid wear of cuttinghead before tunneling.
Advance rate. Under similar ground condition, keeping the higher advance rate (~80-150 mm/min) will bring great positive influence on other key parameters. The faster the advance rate, the shorter the advance time of each ring, the less the usage amount of grease and other comsumables. In this section, one barrel of HBW grease could maintain the machine tunneling more than 40 rings. In addition, the increase of advance rate could also prolong the life of cutters. In this section, the longest distance between replacement of cutters was 670 m.
The Chengdu Metro Line 4 Phase II Fengxi Station–Nanxun Road Station section showed that EPB tunneling in water-bearing ground with large cobbles and boulders can be accomplished successfully given the right set up.