Keywords

1 Introduction

Shield machine is a large high-tech construction equipment dedicated to underground tunnel excavation. Modern shield machine integrates optical, mechanical, electrical, hydraulic, sensor and information technology, involving geology, civil, mechanical, mechanics, hydraulic, electrical, control and measurement and other disciplines [1].

In the large water content, large pore ratio, high compressibility, deep soft land foundation highway, railway, airport and other facilities, the shield machine often encounters the plastic vertical drainage(PVD) board left by the soft land foundation treatment [2,3,4]. The middle of the PVD board is wavy or harmonica shape extruded plastic core board shown in Fig. 1, which is mixed with polypropylene (PP) and polyethylene (PE). Plastic core board surrounded by non-woven filter layer which is the skeleton and channel of the PVD board. PVD board is a commonly used geotechnical material for soft foundation treatment. It can increase the drainage way of soil layer, shorten the drainage distance. Under the action of the upper load, additional stress is generated to make the pore water through the drainage board, reduce the pore ratio and water content, increase the soil compaction. It can accelerate the consolidation and settlement of the foundation for achieving a higher consolidation degree in a short time and improving the bearing capacity of the foundation soil [5,6,7].

Fig. 1
A photo of a sheet of P V D board. It highlights its translucent and ribbed texture.

Plastic core board of PVD board

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PVD board has good toughness, corrosion resistance, so it is difficult to degrade. PVD board can remain underground for decades. When the shield machine cross through this section, it would block the cutterhead, screw conveyor and reduce the tunneling efficiency [1]. According to the literature, the current research on PVD board cutting is mostly in the qualitative simulation test study in the laboratory, but the theoretical research on the damage mechanism of the composite matrix is not deep enough [8].

The finite element model based on the macroscopic anisotropic composite, predict the plastic drainage plate and the method, and reveal the matrix damage, the evolution of matrix damage and the surface damage mechanism [9, 10].

2 3D Model Establishment

2.1 Material Setting

LS-DYNA is both explicit and implicit algorithm, the main advantage is to deal with material failure and large deformation problems. LS-DYNA calculation process based on ANSYS Workbench environment.

As shown in the Fig. 2, Singapore T316 project may have PVD board from Changi Bound CH99+950 to CH100+170 and Woodlands Bound CH99+970 to CH100+120 about 220 m zone with a spacing of 1.2 m. The stratum in this area is soft soil which is mainly fine sand and clay strata through PVD board during tunneling.

Fig. 2
An aerial view of an airport layout highlighting the P V D zones with labeled areas including the Southeast Apron and Northeast Apron.

PVD zone

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The cutterhead is a composite cutterhead, and the front disc cutter spacing is 115 mm. Disc cutter can be replaced with PVD special single-edged tear cutter according to the construction needs. In this project, the 6.7 m shield machine is analyzed by numerical simulation. During the single cutter cutting process, the blade material mold steel is T13. According to the characteristics of single-cutter cutting, the cutter is assumed to be rigid. Mechanical property parameters of the PVD board materials are shown in Table 1. The PVD board is assumed to be an anisotropic elastic oplastic ribbon material with a sectional size of 100 mm × 2 mm in order to simplify the calculation model. It's worth noting that segmented linear elastoplastic material is very commonly used because the data we measure experimentally is often of this type.

Table 1 Mechanical property parameters of the PVD board materials
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Unidirectional reinforced composites are orthogonal anisotropic materials. As shown in the Fig. 3, the load along the main direction of the material is called in the main direction load, and the corresponding stress is called in the main direction stress. The strength of the following characteristics:

Fig. 3
A schematic of the basic strength of single-layer composite material featuring a rectangular single-layer composite material with axes labeled X and Y, and stress vectors indicated.

Basic strength of single-layer composite materials

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  1. (1)

    For anisotropic materials, because the maximum acting stress does not necessarily correspond to the dangerous state of the material, the maximum main stress unrelated to the material direction is meaningless, while the stress in the main direction of the material is important.

  2. (2)

    If the material has the same strength during stretching and compression, there are three basic strengths of the orthogonal anisotropic monolayer material:

X—axial or longitudinal strength (along the main material direction 1);

Y—transverse strength (along the main direction of the material 2);

S—Shear Strength (along the 1–2 plane).

In this strength theory, the stress in the main direction of the material must be less than the strength of the respective direction, otherwise the damage occurs. For tensile stress is:

$$\left. {\begin{array}{*{20}c} {\sigma_{1} < X_{{\text{t}}} } \\ {\sigma_{2} < Y_{{\text{t}}} } \\ {\left| {\tau_{12} } \right| < S} \\ \end{array} } \right\}$$
(1)

The model was imported into ANSYS-WorkBench, and the surface grid was refined when dividing the grid. If the cutterhead speed is 0.85 r/min, the simulated cutter speed is 150 mm/s. And the cutting depth is 5 mm, 10 mm, 15 mm, 20 mm, 25 mm and 30 mm respectively [11].

3 Cutting Force Experiment

3.1 Cutter Selection

The selection of cutting force experimental cutter based on Singapore T316 project engineering demand. The main design purpose is to improve the cutting effect of the PVD board, because the ground is soft soil layer. So the cutter edge with sharp Angle design improve the cutting ability of the tool. The wider cutting area on cutter could improve the impact resistance and durability of the cutter. At the same time, the cutter which is easy to meet the positive and reverse running requirements of the cutterhead is designed as a double direction blade type. The slice blade design is shown in Fig. 4, referred to as PVD cutter.

Fig. 4
Two lateral and frontal view schematics of a P V D cutter, featuring its detailed design and dimensions.

PVD cutter

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3.2 Cutter Wear Calculation

The relationship between the wear height t (mm) and operating distance λ (km) can be based by the accumulated experience of Komatsu in the following [12]:

$${\text{t}} = {\text{K}}\lambda \times {10}^{{ - 3}}$$
(2)

In the formula:

K—Wear coefficient of PVD cutter. According to the similar strata and similar cutterl use experience in China, the wear coefficient of the steel wear resistant cutter of the mold K is about 25–45.

The tunnel distance that can be excavated when reaching PVD cutter wear limit

$$L = 10{,}000 \times P{\text{e}} \times {\text{t/}}\left( {{2}\pi {\text{R}} \times K} \right)$$
(3)

In the formula:

L—The tunnel distance that can be excavated when reaching PVD cutter wear limit, m.

K—Wear coefficient of PVD cutter. Combined with the similar project—Zhoushan Lujiazhi project and the formation factors, the wear coefficient is precalculated according to the highest value K = 45.

t—Allow wear height of PVD cutter, mm.

R—Installation radius of the outermost peripheral cutter, m.

Pe—The cutting depth of PVD cutter, cm r–1.

$$P{\text{e}} = V/N$$
(4)

V—The excavation speed of the shield machine, cm min–1.

N—Cutterhead rotation speed, r m–1.

According to the analysis, when the wear height of the cutter head is calculated at 25 mm, the tear cutter is expected to reach 526 mm.

3.3 Cutting Test Equipment

The instrument performs single cutting test using the linear cutting device shown in Fig. 2. The data acquisition system uses FC3D three-axial force sensor fixed between the cutter pedestal and the planer roof-plate to test the force values of X, Y and Z. The X axis side force direction is perpendicular to the side of the cutterl; the Y-axis normal force direction is perpendicular to the surface of the cutting material; the Z axis cutting force direction is parallel to the feed direction. The sensor range is 10 kN shown in Fig. 4. The auxiliary load measuring device uses the S-type tension sensor installed on the fixed edge of the drainage plate. The sensor range is 7 kN shown in Fig. 5 [13].

Fig. 5
A photo of a cutting equipment in an industrial setting, with exploded views of the three-axial force sensor and S type tension sensor.

Cutting test equipment

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According to the experience of cutting the PVD board, the main failure forms are expected to present three main states shown in Fig. 6: (1) complete cutting off; (2) complete pulling failure on the fixed side; (3) cutting and pulling failure. For the cutting test of the PVD board, the placement state of the test device has the following two types. According to the cutterhead rotation speed of 0.85 r/min, the simulated cutting speed is 150 mm/s. The different placement PVD boards are tested mainly by changing the cutting depth, and the cutting depth is kept unchanged during the single cutting process [14].

Fig. 6
A schematic of two different placement states of a P V D board during cutting, with arrows indicating the cutting direction.

Two types placement state

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A comparison of the different placement PVD boards cutting peak forces are made in Table 2. By observing the cutting state under horizontal placement shown in Figs. 7 and 8, the pulling failure of the PVD board accounts for a large part of the fracture of the drainage plate. The common damage process is: (1) cutting produces a cut; (2) the proportion of the pulling fracture increases; (3) pulling failure.

Table 2 Cutting working condition data
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Fig. 7
A photo of a machine tool positioned above a metallic piece marked A 2 placed on a workbench, ready for cutting.

Before cutting state under H placement

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Fig. 8
A photo of a machine tool positioned above a metallic piece marked A 2 placed on a workbench.

After cutting state under H placement

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By observing the cutting state under vertical placement shown in Figs. 9 and 10, the cutting failure of the PVD board accounts for a large part of the fracture of the drainage plate. The common damage process is: (1) cutting produces a cut; (2) the proportion of the cutting fracture increases; (3) pulling failure.

Fig. 9
A photo of a machine with a drill positioned above a block of material marked B 1, ready for cutting.

Before cutting state under V placement

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Fig. 10
A photo of a human hand holding a piece of cut metal with B 6 written on it.

After cutting state under V placement

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It can be seen that the cutting failure process of PVD board is from the incision to the expansion of the incision and then to the final pull damage.

4 Conclusions

The validity and rationality of the simulation model are verified by comparing the simulated cutting force value with the experimental value. The results show that the cutting process of the plastic vertical drainage board is from the notch generation to the notch expansion and then to the final breaking process. So it can be concluded that the sharp-edge cutter is more suitable for the cutting of the plastic vertical drainage board. In the actual construction, ensuring the wear resistance and impact resistance of the sharp-edge cutter is the key to improve the service life.