Clogging of slurry-shield tunnel-boring machine drives in sedimentary soft rock: A case study

Abstract

This paper presents a case study of the clogging of a slurry-shield tunnel-boring machine (TBM) experienced during tunnel operations in clay-rich argillaceous siltstones under the Ganjiang River, China. The clogging experienced during tunneling was due to special geological conditions, which had a considerably negative impact on the slurry-shield TBM tunneling performance. In this case study, the effect of clogging on the slurry-shield TBM tunneling performance (e.g., advance speed, thrust, torque, and penetration per revolution) was fully investigated. The potential for clogging during tunnel operations in argillaceous siltstone was estimated using an existing empirical classification chart. Many improvement measures have been proposed to mitigate the clogging potential of two slurry-shield TBMs during tunneling, such as the use of an optimum cutting wheel, a replacement cutting tool, improvements to the circulation flushing system and slurry properties, mixed support integrating slurry, and compressed air to support the excavation face. The mechanisms and potential causes of clogging are explained in detail, and the contributions of these mitigation measures to tunneling performance are discussed. By investigating the actual operational parameters of the slurry-shield TBMs, these mitigation measures were proven to be effective in mitigating the clogging potential of slurry-shield TBMs. This case study provides valuable information for slurry-shield TBMs involving tunneling in clay-rich sedimentary rocks.

Graphical abstract

Keywords

slurry-shield TBM / geological investigation / clogging / argillaceous siltstone / TBM performance / mitigation measures

1 Introduction

Over the last few years, shield tunneling has played a pivotal role in various tunnel construction activities [1–5] owing to its advantages, such as high tunneling efficiency [6–8], excellent construction safety [9–11], and fewer negative impacts on the surroundings [12–16]. Slurry-shield tunnel-boring machines (TBMs) have been widely employed for tunneling in China, such as in the Shanghai Yangtze River Tunnel [17], Hangzhou Qingchun Road Cross-River Tunnel [18], Shiziyang Tunnel in Guangzhou–Shenzhen–HongKong High Speed Railway [19,20], Nanjing Yangtze River Tunnel [21,22], and Sutong GIL Yangtze River-Crossing Cable Tunnel [23]. However, numerous case studies have shown that earth pressure-balanced (EPB) and slurry pressure-balanced shields often encounter clogging problems during construction while tunneling in clayey soils or sedimentary rocks containing clay minerals [24–28]. Clogging is a common phenomenon that occurs during shield tunneling in clayey soils or clay-rich rocks owing to cohesive materials adhering to the disk cutter housings in the mixing chamber, cutterhead faceplate, and slurry line during pipeline transport [29,30]. In these cases, it has a considerable negative impact on shield tunneling, including excavation difficulties, damage to cutting tools, project schedule delays, and inevitable budget overruns.

The potential impacts on shield-tunneling performance have prompted increased attention to the clogging phenomenon over the past few decades. For example, Ryu et al. [31] developed a slurry clogging criterion that can be used to assess slurry penetration characteristics or types using slurry-shield TBMs. The grain-size distribution of the ground, slurry characteristics, and injection pressures were considered when laboratory-scale–slurry-penetration tests were performed. Zhai et al. [32] proposed a real-time self-updating–machine-learning system for clogging early warnings using minimal tunneling data for slurry-shield tunneling in mixed mudstone–gravel ground. Li et al. [33] established the criteria for cutting-head clogging occurrence based on machine-driving parameters (e.g., total thrust, cutting-head torque, advance rate, and cutting-head rotation speed). Moreover, many factors can contribute to the clogging potential during EPB or slurry-shield TBM tunneling, such as the type of soil, opening of the cutterhead, clay content, mineralogical composition, and adhesion of clayey muck to the metal surfaces, which include the cutters, cutterhead, and chamber bulkhead [34–37].

Several studies have investigated the clogging mechanisms of TBMs and proposed different methods to evaluate the clogging potential during slurry or EPB-shield TBM tunneling. Currently, all available methods are divided into two primary categories: semi-empirical methods and physical modeling [38–42]. In addition, considering that clogging is caused by the adhesion of soil to metal surfaces, numerous studies have attempted to assess it using various laboratory tests, such as direct-shear, ring-shear, static lateral-adhesion, and vane-shear tests [35,36,41–44]. Peila [35] presented different types of laboratory tests employed on conditioned soils for EPB-shield tunneling. Peila et al. [36] proposed several different laboratory test procedures to evaluate the effect of clay conditioning on the adhesion between clayey soil and metal during EPB-shield tunneling. Kang et al. [41] conducted vane-shear tests of mixtures that were composed of two different clays (bentonite and kaolin) to evaluate the clogging potential of conditioned mixtures. The results showed that mixtures with bentonite had a higher clogging potential than pure kaolin. Kooistra et al. [43] investigated numerous soil parameters (e.g., Atterberg limits, water content, percentage of clay, and clay mineralogy) that affect the adhesion and adhesive friction of clayey soils in contact with metal surfaces by performing shear-box tests. Using direct-shear tests of normally consolidated clay, Zimnik et al. [44] suggested that this could cause a higher adherence potential of clay when the water content decreased. Sass and Burbaum [45] proposed a test to assess the adhesive effect of clayey soils by simulating the actual situation in which clay adheres to the cutting wheel of the shield machine. Feinendegen et al. [46] analyzed the clogging behavior of different soils by means of cone pull-out tests, which showed several new manipulation methods that could reduce adhesion or clogging, such as modified electrolyte concentrations or the application of an electric charge to the steel parts. Zumsteg and Puzrin [47] developed a new shear-plate apparatus that provides the possibility of accurately measuring the tangential adhesion and sliding resistance between metal surfaces and conditioned soil. Zumsteg et al. [48] quantified the limited efficiency of the existing commercially available conditioning chemicals (e.g., foam or polymer) and proposed polyamine chemicals for reducing the clogging potential of clays. The working mechanisms of these new chemicals were investigated using microscopy and adsorption measurements. Zumsteg et al. [49] investigated the effect of both clay mineralogy and the composition of the supporting slurry on the stickiness of the excavated clay using novel stickiness tests, including the mixing and model TBM cutterhead tests. Ye et al. [50] proposed a new scheme of soil conditioning during an EPB-shield tunneling of argillaceous siltstones that considered both the slump value and liquidity index of the soil that was conditioned by foam. Wang et al. [51] investigated the tangential adhesion strength between clay and metal considering different influence factors, such as the total normal stresses, consistency indices of clay, and dispersant contents, using rotary-shear tests. Cui et al. [52] conducted numerous piston pull-out tests that focused on the adhesion mechanisms and considered many factors, such as water content, compression pressure, compression time, metal-surface roughness, hydrophobic material, and metal-surface structure. In addition, the clogging risk of an EPB shield was analyzed by combining the classical prediction chart of the clogging potential with the adhesion results of the pull-out tests. In summary, many researchers have investigated the clogging problems encountered during EPB-shield TBM tunneling in clayey soils; however, research on the clogging of slurry-shield TBM driving in sedimentary rock remains limited. Moreover, these laboratory tests have not been applied in tunneling practice because the relationship between adherence tests and hindrance by clogging in practice remains unclear.

In this context, this paper presents a case study related to the clogging of slurry-shield TBMs during tunneling in clay-rich argillaceous siltstones under the Ganjiang River, China. The objectives of this study are as follows: (i) to present the geology and hydrogeology of the study site as well as the geotechnical characteristics of weathered argillaceous siltstone; (ii) to investigate the effect of clogging on slurry-shield TBM tunneling performance; (iii) to address mitigation measures against the clogging of slurry-shield TBM; and (iv) to summarize the mechanism and potential causes of clogging in detail. Finally, the contribution of mitigation measures to shield-tunneling performance was investigated.

2 Project description

The twin-track metro tunnel extends from Qiushui Square Station to Tengwang Pavilion (formerly called Zhongshan West Road) station as part of Nanchang Metro Line 1, China, and runs underneath Ganjiang Middle Avenue, Qiushui Square, Ganjiang River, and Yanjiang Middle Avenue. This is the first cross-river tunnel under the Ganjiang River in Nanchang, China. Two slurry-shield TBMs were used for the excavation of the twin running tunnels, tunneling from the northwest to the southeast. The lengths of the left and right tubes are 1880.574 and 1889.518 m, respectively, including a length of 1242.5 m undercrossing over the Ganjiang River in each tube. Both twin tubes had overburden depths ranging from 5.4 to 21.5 m. The elevation along each tunnel alignment has a V-shaped characteristic, showing low values in the middle and high values at the ends of the tubes. Fig.1 shows a typical cross section of the shield-tunnel segment lining. The outer and inner diameters of the tunnel lining are 6.0 and 5.4 m, respectively. The tunnel liner is composed of three standard segments, two adjoining segments, and one key segment. The ring width and thickness of the segment lining are 1.2 and 0.3 m, respectively.

Slurry-shield TBM S-367 and NFM-07 were respectively employed for the construction of left and right tubes. The two TBMs were manufactured by two different equipment-manufacturing companies. One (S-367) was manufactured by Herrenknecht AG (Germany), and the other (NFM-07) was manufactured by NFM Technologies, France. Fig.2 shows the cutterhead structures of the two slurry-shield TBMs. The cutterhead of the slurry-shield machine NFM-07 is a faceplate disk with an opening ratio of 32%. It was composed of six spokes and equipped with 153 cutters, including six dual-disk cutters, 19 double-edged disk cutters, three single-edged disk cutters, nine replaceable rippers, 34 immovable rippers, 46 front scrapers, 12 bucket lips, and 24 peripheral protection bits. The cutting wheel of the slurry-shield machine S-367 had an opening ratio of 35% and was equipped with 129 cutters, including six dual-disk cutters, 13 double-edged disk cutters, 30 rippers, 64 front scrapers, and 16 overcutters. Tab.1 lists the primary characteristics of the two slurry-shield TBMs.

3 Site geology and hydrogeology

3.1 Engineering geology

The construction site of the twin cross-river tunnels on Nanchang Metro Line 1, China, which is located in the Pingxiang–Yueping depression of the South Yangtze River anticline, is influenced by the Ganjiang River fault. The geological structure is rather complex in the study area, where fractures and rift basins are well developed. In general, detailed site investigations are critical for shield tunneling, but site investigation for underwater tunnels is usually more difficult and costly than for mountainous tunnels, as there are difficulties in the geological investigation of these underwater tunnels.

Prior to slurry-shield TBM tunneling, geological investigations at the actual site were conducted using the core drilling method. Results from borehole drilling showed that geological units were predominantly composed of artificial backfills, silts, fine sands, coarse sands, gravelly sands, and pebbles during the Quaternary Period, as well as highly, moderately, and slightly weathered (SW) argillaceous siltstones of the Tertiary-aged Xinyu Formation. Fig.3 shows the geological cross section along the tunnel route.

A mixed-face ground condition occurs at the actual tunneling site when the slurry shields the TBMs’ launch and arrival. Mixed-face ground is the ground where two or more geological materials with significant differences in material properties simultaneously appear on the tunneling face. Soft soils were present in the upper part of the excavation face at the real site (e.g., coarse sands, gravelly sands, and pebbles), but the lower part was hard (i.e., argillaceous siltstones). There was a total length of approximately 647 m at both ends of the twin tunnels for the mixed-face ground, accounting for 34.2% of the entire tunnel length. The remainder along the tunnel route is fully faced weathered argillaceous siltstone. Tab.2 lists the physical and mechanical properties of the soils and rocks at the study site.

3.2 Geotechnical characteristics of weathered argillaceous siltstone

Argillaceous siltstones soften easily when exposed to water, while they crack and disintegrate when the water inside the rock mass is lost. The weathering speed of the rock mass is comparatively fast after excavation and exposure, mainly because clay minerals in the rock mass expand when exposed to water but shrink as they lose water. Argillaceous siltstones generally exhibit characteristics such as easy cracking after drying and complete spallation after water absorption. Argillaceous siltstone has a purplish-red color and a medium-thick layered structure composed of silty sand, argillaceous, and calcareous cements. According to the laboratory test results of 98 sets of rock samples, the natural uniaxial compressive strength (UCS) of rock mass varies within the range of 6.0–20.4 MPa, classified as soft rock.

The weathering grades of argillaceous siltstones from top to bottom at the shield tunneling face mainly include highly weathered (HW), moderately weathered (MW), and SW rocks. Completely weathered rocks are not distributed at the construction site. Tab.3 summarizes the geological descriptions and indices for the grading of weathered argillaceous siltstones. Tab.4 presents the quantitative indices of weathered argillaceous siltstones. As presented in Tab.3 and Tab.4, most of the HW argillaceous siltstone structures have been destroyed. The rock mass was fractured, and the quality grade was ranked as V. The MW argillaceous siltstone maintains the rock structure of its parent rock and its integrity as rock; however, joints and fractures are comparatively developed. The UCS of MW rock masses varies within the range of 6.0–14.2 MPa. The quality grade of the rock mass was ranked IV. The fresh rock and SW argillaceous siltstone maintained the structure and properties of the parent rocks with few weathering cracks. The UCS of SW rock ranges from 6.0 to 14.2 MPa, and the UCS of fresh rock varies from 9.2 to 20.4 MPa. The quality grades of both fresh rock and SW rock masses were ranked as IV. Furthermore, weathered argillaceous siltstone is relatively impermeable, owing to its high clay mineral content.

3.3 Hydrogeology

The case study site was located in Nanchang City, Jiangxi Province, which has a subtropical monsoon climate. The annual rainfall in this area has uneven distribution characteristics. The annual rainfall from April to June is concentrated, but there is little rainfall from November to February each year. There is approximately 1610.08 mm of annual rainfall on average. Nanchang City is located on the alluvial plain of the Ganjiang River. The surface water system is well developed in this area. The Ganjiang River is the largest river in Jiangxi Province and runs through the city. The surface water flow changes with the season.

The groundwater at the construction site consists of three types of water: perched water, pore water, and confined fissured water supplemented by surface water (the Ganjiang River). A small amount of perched water was stored in an artificial backfill. The perched water is recharged by atmospheric rainfall, and the water level varies greatly with the climate. Pore groundwater is mainly stored in Quaternary and Holocene sediments, such as silt, fine–coarse sand, gravelly sand, and pebbles. Groundwater in these Quaternary sediments is abundant at the study site and is closely connected to the Ganjiang River. The water table of the Ganjiang River varied within the range of 15.5–19.6 m during the period of slurry-shield TBM tunneling. Moreover, fissured water mainly occurs in argillaceous siltstones of the Tertiary-aged Xinyu Formation, connecting the pore water above through fracture zones. Fissured water was confined and contained a small amount of water. The head of the confined fissured water was equal to the water table of the Ganjiang River.

4 Effect of clogging on slurry-shield tunnel-boring machine tunneling performance

4.1 Characterization of slurry-shield tunnel-boring machine tunneling performance

Fig.4(a)–Fig.4(d) display the time-history curves of several main operational parameters (e.g., advance speed, total thrust, torque, and penetration per revolution) during slurry-shield TBM NFM-07 tunneling in the right tube. The following can be observed:

1) The advance speed varied between 12 and 48 mm/min with strong fluctuations when the slurry-shield TBM NFM-07 tunneled at the first 80 rings in the right tube. Simultaneously, the total thrust of the cutter wheel ranged from 4000 to 18000 kN, and the cutterhead torque varied in the range of 800 to 2400 kN·m. The penetration per revolution of the shield machine varied between 15 and 55 mm/r. These performance data show that there is a high tunneling efficiency when slurry-shield TBM tunneling occurs at the first 80 rings in the right tube, but there are strong fluctuations in the operational parameters. These fluctuations can be attributed to the inherent soil properties of both gravelly sand and pebbles. In addition, it may cause variations in the operational parameters owing to the inhomogeneity and looseness of these mixed soils.

2) The advance speed and penetration per revolution decreased, but the total thrust and cutterhead torque increased as the slurry-shield NFM-07 excavated from 80 to 170 rings in the right tube. The main reason for this was that the slurry-shield tunneling encountered mixed-face ground. The upper layer is soft (gravelly sands and pebbles), but the lower layer is hard (weathered argillaceous siltstones). With the increasing proportion of rock masses at the excavation face, the advance speed and penetration per revolution decreased. Most of the advance speeds ranged between 6 and 12 mm/min, and the penetration per revolution was lower than 10 mm/r while tunneling from 100 to 150 rings in the right tube. However, the total thrust and cutterhead torque increase as the slurry-shield machine moves forward. The total thrust of the cutter wheel ranged from 14000 to 18000 kN, and most of the cutterhead torque varied between 2000 and 2600 kN·m.

3) Subsequently, the slurry-shield TBM started tunneling in full-face argillaceous siltstones after the 170-th ring in the right tube. The advance speed and penetration per revolution were always low, but the total thrust and cutterhead torque were high. Most of the advance speeds ranged between 5 and 8 mm/min, and the penetration per revolution was lower than 8 mm/r. The total thrust of the cutter wheel varied from 14000 to 20000 kN, and most of the cutterhead torque ranged between 2000 and 2800 kN·m when tunneling from 170 to 400 rings in the right tube. These operational parameter data indicate that the slurry-shield TBM NFM-07 encountered soil clogging.

4.2 Estimation of clogging risk during slurry-shield tunnel-boring machine tunneling

To evaluate the clogging potential during EPB or slurry-shield TBM tunneling, some empirical charts have been proposed by previous researchers. For example, Thewes [24] initially developed a diagram of clogging potential evaluation for cohesive soils using two critical indices (i.e., plasticity index (I_P) and consistency index (I_c)) through empirical investigations in order that it could estimate the clogging risk for hydro-shield tunneling.

The clogging risk of a TBM during tunneling can be estimated using a combination of the plasticity index (I_P) and consistency index (I_c), where I_P and I_c are defined by the following equations:

(1)I_{\mathrm{P}}=W_{\mathrm{L}}-W_{\mathrm{P}},

(2)I_{\mathrm{c}}=\frac{W_{\mathrm{L}}-W_{\mathrm{n}}}{W_{\mathrm{L}}-W_{\mathrm{P}}},

where W_n is the natural water content, W_L is the liquid limit, and W_P is the plastic limit.

Subsequently, Hollmann and Thewes [34] developed a new universal classification diagram to estimate the clogging potential of various types of shield machines while tunneling in clayey soils (for example, EPB shields, open-face shields, and slurry-supported shields). The abscissa of the diagram represents the difference between the plastic limit and the water content (W_P – W_n), while the ordinate represents the difference between the liquid limit and the actual water content (W_L – W_n). Therefore, these empirical classification charts can be employed to estimate the clogging risk of slurry-shield TBM in this case study.

According to the geological investigation, the natural water content (W_n) of the argillaceous siltstone in the field was 11.4%. The liquid limit (W_L) and plastic limit (W_P) of excavated soil in the shield chamber were 28.4% and 18.2%, respectively. Thus, the plasticity index (I_P) was calculated to be slightly greater than 10% and the consistency index (I_c) was calculated to be approximately 1.67 based on Eqs. (1) and (2), respectively. Thus, the soil is classified as silty clay (CL) in accordance with Casagrande’s plasticity chart.

With reference to the empirical classification chart of clogging suggested by Thewes [24], there may be a medium potential for clogging, whereas slurry TBM tunneling occurs in argillaceous siltstone. In addition, according to the general classification chart for the evaluation of clogging that is proposed by Hollmann and Thewes [34], Fig.5 shows the clogging potentials of the natural soil sample, which is plotted on this universal diagram. Thus, the muck is maybe in the form of “lumps” in the excavation chamber while tunneling in natural argillaceous siltstone, but water content may increase significantly due to the use of a slurry-support fluid during tunneling. The water content of the soil has a significant effect on the clogging of the shield cutterhead. The soil in the excavation chamber may be moved to a lower critical consistency as the water content increases.

4.3 Actual clogging in the excavation chamber

The construction units organized engineers and technicians to carry out inspection operations on both the cutting wheel and cutters by opening the excavation chamber under compressed air pressure conditions. After accessing the excavation chamber, well-trained working personnel discovered that soil clogging appeared on the cutterhead panel and cutter housing. In addition, massive clay lumps adhered to the corbels, supporting the cutterhead of the slurry-shield machine. Fig.6(a)–Fig.6(d) show the clogging of the slurry-shield TBM and clusters of soils from the separation system.

5 Mitigation and countermeasures

5.1 Optimization and control of slurry properties

Because a high slurry density increases the likelihood of clogging, low-density bentonite slurry is employed while slurry-shield TBM tunneling in full-face argillaceous siltstones. The physical properties of the slurries are listed in Tab.5. An additional centrifuge was added to the slurry separation and treatment system so that the slurry properties in the circulation line could be improved to meet the construction requirements. Moreover, an optimum scenario for slurry separation was applied, which included four phases: screening, hydrocyclone separation, precipitation, and centrifugation. Under these conditions, the properties of the slurry materials were ensured, and the density of the bentonite slurry was either excessive or too low.

5.2 A mixed support system used for slurry-shield tunnel-boring machine tunneling

Most of the geology at the excavation face along the tunnel route is full-face argillaceous siltstones when the slurry-shield TBM encounters the problem of clogging. Considering that argillaceous siltstones have good stability owing to their inherent rock properties, a mixed support system is employed to support the excavation face, consisting of compressed air in the upper half of the excavation chamber and slurries in the lower half (Fig.7). Compared to the full-face slurry-support system, the clogging potential of the cutterhead was significantly reduced when a mixed support system was used for tunneling in full-face argillaceous siltstones. The main reason for this is that fine-grained cuttings or rock chips fall off easily from the cutterhead when it turns around during slurry-shield TBM tunneling because compressed air is used in the upper half of the excavation chamber. Compared with the full-face slurry-support system, the probability of the accumulation of fine-grained cuttings on the cutterhead faceplate was significantly reduced when the mixed support system was used. In addition, the “flushing” of the cutterhead faceplate has been dramatically improved when compressed air is employed in the upper half of the cutting chamber because the resistance of high-pressure air is far less than that of slurry materials.

5.3 Improvement of cutting wheel and cutters

The cutting wheel and cutters had to be improved so that the efficiency of shield tunneling could be enhanced because the slurry-shield TBM NFM-07 tunneling encountered clogging in the right tube. Therefore, the cutting wheel and cutters were transformed prior to slurry-shield TBM S-367 tunneling in the left tube to reduce the clogging potential and decrease the inspection and maintenance frequency of the cutters. Several crucial aspects of the improvement of the slurry-shield TBM are as follows.

a) The dual-disk cutters at the center of the cutterhead were changed to removable tearing cutters so that the opening rate of the cutterhead could increase compared with the previous one. There was a 175–210 mm height beyond the surface of the cutterhead for the double-edged tearing cutters. The spacing between the cutting edges was 170 mm.

b) Four spherical-tooth disk cutters with high abrasion resistance were installed at the edge of the cutterhead. For these gauge disk cutters, a height of 175 mm was beyond the surface of the cutterhead, and the spacing between the cutting edges was 100 mm (Fig.8).

c) Multiple mixing wings were installed both at the back of the cutterhead and at the bulkheads of the excavation chamber such that the cut mucks could flow easily. Under this condition, clogging of the shield cutterhead could be prevented.

5.4 Improvements of flushing system of slurry tunnel-boring machines

A flushing system is crucial for a slurry-shield TBM to address the clogging problem. When comparing the two slurry-shield TBMs, it is discovered that the design of the flush pipes is basically the same in both. The major improvements in the flushing systems of two slurry-shield TBMs include two aspects: 1) an additional flushing pump that is added to each slurry-shield machine, and 2) simultaneously, a decrease in the caliber of the flushing pipelines. Under this condition, the flushing pressure from the slurry feed line can increase, preventing the appearance of clogging on the shield cutterhead. Fig.9 shows the location plan of the flushing pipelines. The improvements in the flushing pipelines are as follows.

1) An additional pump with a power of 55 kW was employed for each slurry-shield TBM to increase the slurry feed pressure. Under these conditions, it can improve the “flushing” of clayey material on the cutterhead faceplate. In addition, a connector with a nominal diameter of 150 mm was machined onto the previous slurry injection pipeline, and slurries were transported into the added pump through it. The pump was connected to the previous slurry feed line to flush the cutterhead faceplate.

2) The previous slurry injection hole (nominal diameter of 80 mm) located on the upper left of the slurry-shield TBM NFM-07 was improved into five small nozzles with a diameter of 16 mm. These small holes were distributed along the radius of the cutterhead; in this case, the clogging materials could be flushed out of the shield cutterhead by the supported slurries in various paths.

3) A bentonite pipeline with a nominal diameter of 80 mm located on the right side of the slurry-shield TBM S-367 was extended to both sides of the slurry gate inside the cutter chamber. The slurry gate was then flushed separately using the bentonite pipeline. In such cases, the muck could be discharged smoothly and prevent clogging at the slurry gate.

4) The previous flushing pipeline with a nominal diameter of 80 mm, located at the center of the cutterhead, was transformed into four flushing nozzles with diameters of 25 mm. A flushing nozzle was reconstructed at the center of the cutterhead, and the three remaining nozzles were located around it. These nozzles were mainly applied to flush the center cutters in order to prevent clogging at the center of the cutting wheel of the slurry TBM S-367. Fig.10 shows the improved nozzles of the two slurry-shield TBMs.

6 Discussions

6.1 Mechanism of clogging

Generally, the cutting wheel is pressed into the excavation face during slurry-shield TBM tunneling, and the plastically deformable argillaceous siltstones are then pushed to both sides of the cutting wheel where they are cut by the drag picks in the form of “lumps” [39]. Fine clay mineral grains contained in these excavated clayey lumps then disintegrate and accumulate in the inflowing groundwater or supported slurry liquid (Fig.11(a)). Clogging occurs in both the cutting chamber and cutting tool housing because of the inherent cohesion of clay mineral grains and the adhesion of clay to a metal component surface (Fig.11(b)). Between these two, adhesion seems to be the most important single mechanism, resulting in the stickiness of clay mineral grains.

In addition, the water around the clay mineral grains inside the excavation chamber may form a water film that exists at the interface between the clay mineral grains and metal owing to the effect of surface tension, as shown in Fig.11(c). As the water content increased, the clay mineral grains gathered to form soil blocks under the action of the water film. In general, the water content has a significant effect on the adhesion between clay mineral grains and the metal surface. The adhesion stress at the interface between clay mineral grains and the metal surface increases when the water content of the soil approaches the liquid limit [52]. Under such conditions, clogging is likely to occur. Nevertheless, clay mineral grains flow freely with water when the mixture of clay mineral grains and water is similar to a slurry state. In this case, the adhesion stress may be quite small or no longer exist at the interface between the clay mineral grains and the metal surface. The clogging also disappeared from the metal surface of the shield cutterhead. Thus, it is believed that clogging often occurs during TBM tunneling owing to the aforementioned interactive mechanism.

6.2 Potential causes of clogging

Through a detailed investigation of the case study, the reasons for the clogging of slurry-shield TBMs can be explained by the following two crucial aspects: 1) geological lithology and 2) the design and operation of slurry-shield TBMs.

6.2.1 Geological lithology

The mineralogical composition of the argillaceous siltstones was investigated using a fully automatic X-ray scanner. Tab.6 lists the mineralogical compositions and contents of the rock specimens at the construction sites. As presented in Tab.6, the argillaceous siltstone specimens contain six different mineral components. Three of these (chlorite, illite, and hydromica) are clay minerals, and the total clay mineral content in argillaceous siltstone reaches 48.6%–50%. Hollmann and Thewes [34] believed that the existence of clay minerals was the main potential cause of clogging, especially when the clay mineral composition exceeded 10%. Because clogging was to be expected above a clay mineral content of more than 10%, it caused a considerably high potential for soil clogging during slurry TBM tunneling under such geological conditions.

6.2.2 Design and operation of slurry-shield TBMs

1) The adaptability of cutter tools to the practical geology encountered is critical when a tunnel is excavated by shield machines, which plays a very important role in preventing the clogging of shield TBMs and improving working efficiency. In general, a detailed investigation of geological conditions must be conducted prior to selecting and assembling excavation cutters for a shield machine. The construction company lacked experience with slurry TBM tunneling in such geological conditions in the beginning, so the design of cutter tools and cutting wheels was not perfect. It includes the following aspects: (a) a small opening of the cutterhead structure, (b) unreasonable quantities and types of cutters, (c) an insufficient height difference between the surface of the cutterhead and the installed cutting tools, and (d) a lower pressure for the “flushing” of the cutterhead faceplate in the excavation chamber.

2) The improper operation or control of the slurry-shield TBM, including the advancing and slurry circulation systems, is another crucial cause of the clogging of the TBM. Sometimes, many TBM operational parameters are not updated in time by the operator in the control room, particularly when unexpected geological conditions or muck in the excavation chamber are discharged under abnormal conditions. In this case, the potential for clogging may increase during slurry-shield machine tunneling.

3) Moreover, the physical properties of slurries in circulation pipelines (e.g., viscosity, density, and sand content) did not improve over time because they may ignore the high-frequency test of the properties of slurries in separation plants at actual tunneling sites. Under such circumstances, the density of the slurry increases in the circulation lines. The cut lumps in the excavation chamber were barely discharged. To some extent, this causes clogging of the slurry-shield TBM over time. In summary, the aforementioned factors induced the clogging of the slurry-shield TBM in this case study, which mainly concentrated on the supporting bracket and cutters of the cutterhead structure.

6.3 Contribution of mitigation measures to shield tunneling performance

Fig.12(a)–Fig.12(d) present the time-history curves of the main operational parameters (e.g., advance speed, total thrust, torque, and penetration per revolution) of the two slurry-shield TBMs from 180 to 600 rings in the left and right tubes. The data of 400–600 rings in the chart are the tunneling data after the improved measures are employed.

As shown in Fig.12, both the advance speed and penetration increased, but the total thrust and torque decreased after mitigation measures against clogging were applied for slurry-shield TBM NFM-07 tunneling from 400 to 600 rings in the right tube. The advance speed varied between 8 and 18 mm/min, and the average advance speed was more than twice that before. The total thrust of the cutter wheel ranged between 8000 and 16000 kN, the average of which decreased by approximately one-third. The cutterhead torque varied between 1000 and 2300 kN·m, and the average torque decreased by as much as a quarter. The average penetration per revolution increased by more than twice over the previous value.

In addition, it could be seen that the tunneling performance of slurry-shield TBM S-367 in the left tube was obviously better than that of slurry-shield TBM NFM-07 in the right tube during tunneling from 180 to 400 rings. The advance speed of the slurry-shield TBM S-367 was faster than that of NFM-07, but both the total thrust and torque of S-367 were smaller than those of NFM-07 when tunneling from 180 to 400 rings in the left and right tubes, respectively. However, there was no obvious difference in the penetration per revolution between the two slurry-shield TBMs during tunneling from 180 to 400 rings because the rotation speed of the cutterhead increased during the slurry-shield TBM S-367 tunneling. Overall, through the implementation of the aforementioned mitigation measures, the tunneling performance of the two slurry-shield TBMs increased significantly.

Therefore, the mitigation measures proposed in this study make a significant contribution to the tunneling performance of slurry-shield TBMs, and they are very successful. The contributions of mitigation measures to shield tunneling performance include the following aspects.

a) Both the supported slurry properties and the slurry circulation flushing system were significantly improved, which increased the ability of the slurry fluid to transport muck.

b) Because the resistance of high-pressure air is actually far less than that of slurry materials, it significantly reduces the clogging potential of the cutterhead when compressed air is used in the upper half of the cutting chamber.

c) These optimum cutting tools ensure that the excavated mucks can flow easily owing to the high adaptability of the cutter tools to geology.

Finally, the construction units organized technicians to inspect the cutting tools under normal atmospheric pressure when full-face, MW argillaceous siltstones were encountered. The results showed that there was no obvious clogging phenomenon in the excavation chamber and disk cutter housing; however, slight and normal wear of the cutting tools occurred. The performance conditions of the cutting tools are shown in Fig.13.

7 Summary and conclusions

The twin-track metro tunnel connecting Qiushui Square Station and Tengwang Pavilion Station as part of Nanchang Metro Line 1, Jiangxi Province, China, is a very interesting case of slurry-shield TBM tunneling under the Ganjiang River in argillaceous siltstones containing rich clay minerals. The occurrence of clogging events resulted in major problems during project implementation, such as higher maintenance requirements, longer machine downtimes, and higher costs. Several lessons have been learned, and the conclusions drawn from this case study are as follows.

There is a high potential for soil clogging during slurry-shield TBM tunneling because of the clay mineral content of argillaceous siltstones. It is crucial to design and assemble various cutting tools for TBMs based on the actual physical properties of rock masses, and the mineralogical compositions of rocks must be investigated. An additional centrifuge is essential for improving the slurry properties of the slurry circulation system. Because the flushing system of the slurry-shield TBM did not improve prior to tunneling, the TBM performance was affected by clogging. By adopting a mixed support system that consisted of compressed air in the upper half and slurries in the lower half within the excavation chamber, success in preventing the clogging of the slurry-shield TBM was achieved during tunneling in stable sedimentary rocks. Consequently, the total thrust and cutterhead torque increased significantly, whereas the advance speed and penetration decreased.

Finally, based on the feedback of the main operational parameters, such as advance speed, total thrust, torque, and penetration per revolution of the slurry-shield TBM NFM-07, the average advance speed more than doubled. The average thrust and torque decreased by approximately one-third and one-quarter, respectively. The average penetration per revolution more than doubled. In addition, the tunneling performance of slurry TBM S-367 in the left tube was superior to that of slurry TBM NFM-07 in the right tube. Thus, the aforementioned comprehensive mitigation measures have been proven to proactively mitigate the clogging potential of slurry-shield TBMs during tunneling in argillaceous siltstone. This case study provides knowledge and references for similar slurry-shield TBMs when tunneling in clay-rich sedimentary rocks.

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