Tunnel face deformation mechanism

In order to study deformation mechanisms of pipe jacking tunnels traversing frozen ground, the partial failure mechanisms are used as shown in Figure 2.12, corresponding to the blow-out of upper part of the tunnel and the collapse of the lower part of the tunnel. In the upper part blow-out mechanism, as shown in Figure 2.12(a), the top of failure area passes through the tunnel crown. In the lower part collapse mechanism as shown in Figure 2.12(b), the bottom of the failure area passes through the tunnel invert (Li, Emeriault, Kastner, & Zhang, 2009). These two partial failure mechanisms are the very common and dangerous failure mechanisms within the underground tunneling engineering. Therefore, four types of deformation mechanism without failure are taken into consideration based on the results of face deformation.

Figure 2.12 Typical partial failure mechanisms (Li et al., 2009).

Figure 2.13 shows the schematic diagram of deformation mechanisms of tunnel face of pipe jacking in frozen ground, which are divided into the following types: 1) type I:

bulges outward as a parabola along with tunnel centerline, always occurs in the condition with comparatively larger tunnel diameter or small tunnel diameter with low property parameters, such as low deformation modulus, cohesion and friction angle; 2) type II: the maximum tunnel face deformation comes up at the point of 1/3 diameter near tunnel invert, which always appears with low mechanical properties or insufficient slurry pressure and may cause partial collapse; 3) type III: bulges inward as a parabola along with tunneling direction, and just behaves oppositely with type I, which happens with large slurry pressure; 4) type IV: the maximum deformation performs towards the tunnel invert along with jacking direction, which also occurs with the situation of too large slurry pressure. When pipe jacking is used in frozen ground, much more attention should be paid to type IV because of the gradient temperature distribution causes the strength gradient of frozen ground along vertical direction from low to high temperature, which may cause uneven cutting of tunnel face and results in jacking misalignment or even get stuck of drivage machine.

Figure 2.13 Deformation mechanisms of tunnel face.

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Figure 2.14 Deformation mechanism of type I with friction angle (magnified 10 times). (a) 20°, (a) 30°, (a) 40° and (a) 50°. (T=-5℃, c=10kPa, C=4m, D=10m)

Figure 2.15 Face deformation profiles of type I with friction angle. (a) 20°, (b) 30°, (c) 40° and (d) 50°. (T=-5℃, c=10kPa, D=4m, C=10m)

Figure 2.14 and Figure 2.15 give the typical results of deformation mechanism of type I with the same ground temperature (T), cover depth (C), tunnel diameter (D), cohesion (c), and the friction angle (φ) ranges from 20 degrees to 50 degrees. Obviously, the tunnel face deformation decreases with increasing friction angle. However, when the pipe jacking tunnel locates in shallow ground with relative large tunnel diameter (e.g.

D=5m), the deformation mechanism will behave as face deformation mechanism type II as shown in Figure 2.13. It causes the face collapse and even penetrates the cover depth to ground surface and results in ground subsidence (see Figure 2.12(b)). Figure 2.15 illustrates the face deformation profiles and Figure 2.16 shows the contour and vector map of face deformation under the same condition with Figure 2.14. The contours refer to the deformation scales and vectors stands for the value and direction of displacement in this figure. It can be clearly seen that as the increasing of friction

angle, the deformation mechanism changes from type II to type I, because higher friction angle makes the heading face more stable. For your reading convenience, the legend in the graphs is defined in this same way: for example, the legend “10/20, 2/4”

means that cohesion of 10kPa, friction angle of 20 degrees, diameter of 2m and cover depth of 4m, or the legend “-5℃, 20 degrees, 2/4” means that temperature of -5℃, friction angle of 20 degrees, diameter of 2m and cover depth of 4m.

Figure 2.16 Contours of face deformation with friction angle. (a) 20°, (b) 30°, (c) 40°

and (d) 50°. (T=-5℃, c=10kPa, D=4m, C=10m)

Figure 2.17 illustrates the typical deformation mechanism of type II. These four simulation schemes have the same geometric parameters, cohesion and friction angle, only with the different ground temperature, from -5℃ to -20℃. The type II is a common phenomenon faced within the urban underground infrastructure construction, especially

in the shallow buried depth, such as shallow large pipe jacking tunnel and TBM tunnel construction. This is the same with large pipe jacking tunnel construction in frozen ground. Large diameter tunnel construction with shallow cover depth should take into consideration of type II deformation mechanism. Because this behavior leads to large ground subsidence and is very dangerous to the nearby structures and surface buildings.

Appropriate slurry pressure applied on tunnel face is an effective way to prevent the face collapse, which is also the main goal of this chapter to achieve. It can be concluded that the deformation of tunnel face reduces with decreasing ground temperature.

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Figure 2.17 Deformation mechanism of type IIwith ground temperature (magnified 10 times). (a) -5℃, (a) -10℃, (a) -15℃ and (a) -20℃. (c=10kPa, φ=20°, D=5m,

C=4m)

Figure 2.18 and Figure 2.19 show the typical deformation mechanism of type III, with effective face support pressure of 2.0 times of horizontal stress at tunnel centerline. It

can be seen from the figures that the face deformation decreases with increasing friction angle. Generally, if the type III continues developing or increasing the effective face support pressure, it will evolve into failure mechanism of type IV. The deformation mechanism of tunneling face is different with that of unfrozen ground and hardly no blow-out damage happens as conducting pipe jacking within frozen ground. What is more, the strength gradient of frozen ground forms along vertical direction from low to high temperature, which may cause uneven cutting of tunnel face and results in jacking misalignment or even get stuck. This is also the reason that the deformation develops towards tunnel invert rather than tunnel crown. In addition, Figure 2.19 shows the face deformation profiles of type III, and Figure 2.20 illustrates the contours and vector map of face deformation of this type. Inevitably, while jacking process, the thrust and slurry pressure are applied on the tunnel face, and therefore, avoiding the misalignment induced by the strength gradient should be paid much attentions.

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Figure 2.18 Deformation mechanism of type III with friction angle (magnified 10 times). (a) 20°, (a) 30°, (a) 40° and (a) 50°. (T=-5℃, c=10kPa, D=4m, C=10m)

Figure 2.19 Face deformation profiles of type III with friction angle. (a) 20°, (b) 30°, (c) 40° and (d) 50°. (T=-5℃, c=10kPa, D=4m, C=10m)

Figure 2.20 Contours for face deformation with friction angle, when σP/σH=2.0. (a) 20°, (b) 30°, (c) 40° and (d) 50°. (T=-5℃, c=10kPa, D=4m, C=10m)