Design and application of a new type of hierarchical control device

To realize coupling between the yielding and partition failure processes, a new hierarchical control device was designed. The new stage control device comprised an internal and external two-stage structure similar to “piston + sleeve.” Under the action of the surrounding rock pressure, primary yielding occurred first, and the cone slid along the inner wall of the yielding sleeve. With an increase in the surrounding rock pressure, secondary yielding occurred, and the tray slid along the outer wall of the yielding sleeve. The device had a simple mechanical structure and exhibited reliable mechanical properties. It had a convenient and rapid installation process, and the load-displacement curve was obtained by laboratory tensile tests. Mechanical test results showed that the device exhibited a clear function of graded pressure yielding, which was consistent with the designed sectional control curve. The results show that the new hierarchical control device exhibited a good control effect on the surrounding rock and is helpful for TBM in realizing fast driving.


Introduction
A TBM is an effective means to achieve rapid tunneling.However, if the ground stress is large, the surrounding rock strength is low and the water richness is strong, the surrounding rock is broken and large deformation occurs.This affects the TBM construction.In particular, when an open TBM is used, there is an insufficient booting force and other serious problems.Yielding support is currently an effective control method to solve large deformation of soft rock [1][2][3].This concept transforms traditional rigid support [4] into "rigid-flexi-rigid" support, and achieves the purpose of reducing working resistance by properly allowing surrounding rock to have a certain range of deformation.Several scholars have conducted research on yielding structure [5,6], including yielding anchor bolts and yielding arch frames.Jager [7] proposed a Cone Bolt that used a flat end and an anchoring agent to squeeze the sliding.The bolt was composed of a smooth sliding rod body and an end with a conical horn, which is used for grouting.Cantieni and Anagnostou [8] designed deformable tunnel supports compatible with the convergence of large tunnels.Li [9] designed a multi-node anchoring D-bolt based on the yield strength of steel.He et al. [10,11] prepared NPR anchors by introducing negative Poisson's ratio materials in the biomedical field because conventional anchor bolts are easily broken owing to tension.Wu et al. [12,13] proposed a large-deformation bolt with tension and compression coupling to make the stress of the bolt more uniform and improve its deformation characteristics.The reinforcement effect of the energy-absorbing rock bolt under different conditions was quantitatively estimated, and its mechanical work transfer ability was presented.Shucai et al. [14] considered the instability characteristics of the initial support structure and the radial load mechanism of underground caverns.Additionally, based on the support concept of first allowing, followed by resisting and stiffing, they proposed a new support system of steel grated concrete steel tube core tube.With the large-rigidity concrete core tube as the structural penetration barrier, further development of the surrounding rock deformation was restricted, and the problem of continuous large deformation of the surrounding rock was solved.Aiming at the problem wherein the initial support of a soft rock tunnel was damaged by excessive force, Qiu et al. [15] developed a limiting support resistance damper and applied it in engineering practice, which increased structural safety and reduced engineering costs.Barla et al. [16][17][18] successfully solved the problem of large extrusion deformation of the Lyon-Torino base tunnel using lining-embedded foamed concrete devices.Li et al. [19] established numerical models of energyabsorbing bolts based on the modification of cable elements in FLAC3D, and introduced the implementation process and specific capabilities of the model.Jinfeng et al. [20] designed a new adaptive compressible element that significantly improved the deformability of the support lining in squeezing rock, and the field application results showed that the surrounding ground pressure was reduced and more evenly distributed.
Although these yielding support devices achieve the effect of large displacement and small pressure corresponding to the intersection point of surrounding rock pressure-displacement curve and support resistance-displacement curve [21,22], they exhibit small yielding range, complex processes, and high price.Moreover, because these yielding support devices are mainly realized using the extension performance of the supporting component itself or optimizing the structure, they are not combined with the deformation characteristics of the surrounding rock.inconsequently, the process of yielding is not coupled with the deformation process, and it is difficult to provide different pressure for different deformation stages of the surrounding rock.
Based on the above studies, this study further analyzed the failure mechanism of weakly consolidated sandstone under the condition of rich water, proposed the concept of zoning deformation and failure of the surrounding rock at different depths, and designed a new stage control device accordingly.The device employs simple and straightforward mechanical principle, is easy to install, makes full use of conventional supporting materials to reduce the input and labor intensity of workers, and can achieve a large yield distance in a limited space.After testing the mechanical properties in the laboratory, it was successfully applied to the inclined shaft of the Kekegai coal mine, which is helpful for realizing the fast driving of an open TBM.

Failure mechanism of surrounding rock section of your paper
The essence of the surrounding rock deformation and failure during tunnel excavation is the redistribution of stress in the surrounding rock, which causes various changes in the surrounding rock.When the stress after redistribution does not exceed the strength of the surrounding rock of the tunnel, the surrounding rock of the tunnel will still be in the elastic stage, and the surrounding rock will have a small deformation but will still be in the stable stage.However, when the stress after redistribution exceeds the strength of the surrounding rock of the tunnel, a plastic zone begins to appear inside the surrounding rock; nevertheless, the overall stability is maintained.The plastic zone of the surrounding rock is fully developed, such that the rock mass slips along the fracture plane as a whole, yielding the rock mass in this area.If a support reaction does not occur, the surrounding rock in the area becomes unstable and collapses.Therefore, the plastic zone of the tunnel surrounding rock includes, but is not equivalent to, the zone of collapse of the surrounding rock; that is, the loose zone of the tunnel surrounding rock is within the range of the plastic zone after the tunnel excavation, as shown in figure 1.The traditional yielding support bears less surrounding rock pressure than the still support, as shown in figure 2. However, yielding occurs relatively late, by which time the surrounding rock has undergone significant deformation, and the stress from the shallow rock is transmitted to the deeper rock.Consequently, the loosened zone is enlarged.The stage-yielding support reduces the effect of reducing support force and minimizes the loosened zone by applying primary yielding to release the stress in the shallow rock, thereby protecting the deeper rock.As the tunnel excavation progresses, the surrounding rock pressure continues to increase, and by promptly implementing secondary yielding, the stress in the deeper rock can be released.This achieves the desired outcome of reducing the support force and minimizing the loosened zone.The new stage control device comprised an internal and external two-stage structure similar to "piston + sleeve".Under the action of the surrounding rock pressure, primary yielding occurred first, and the cone slid along the inner wall of the yielding sleeve.With an increase in the surrounding rock pressure, secondary yielding occurred, and the tray slid along the outer wall of the yielding sleeve.The structure and working process are illustrated in figures 3 and 4, respectively.(1) The primary yielding: After tunnel excavation, the surrounding rock deformation was small in the initial stage, and the bolt was in the elastic deformation stage; therefore, the stage control device did not work.When the deformation of the surrounding rock reached a certain degree and the pressure of the surrounding rock exceeded the designed first yielding resistance, the cone began to slide along the inner wall of the yielding sleeve, and the stage control device was in the first-level pressure-letting stage.
(2) The secondary yielding: The first yield released part of the accumulated variable performance.When the first yielding was completed, the surrounding rock pressure acting on the supporting system became increasingly large as surrounding rock zone failure occurred.When the surrounding rock pressure exceeded the set working resistance of the second yield, the tray began to slide along the outer wall of the yielding sleeve until the surrounding rock pressure and supporting resistance reached a balance.

Static tensile test of the new stage control device
Static tensile tests were performed to evaluate the mechanical properties of the new stage-control device.In the test, the displacement control method was adopted to analyze the mechanical and extension properties of the device installed on the bolt body, as shown in figure 5.The static tensile test process is shown in figure 6; the loading rate was maintained at 20 mm/min.Because the maximum width of the clamping device was 41 mm, the new stage control device contained a pallet, and the widest position was approximately 150 mm.Consequently, a special chuck was made before the test, as shown in figure 6 (a).The special chuck was composed of two steel plates with a thickness of 25 mm and four steel bars with a diameter of 30 mm.One side of the steel plate was connected to a 35 mm thick steel bar placed in the clamping device.The other side was a steel plate with a hollow middle, which allowed the steel bar to pass through and hold the tray simultaneously.Thus, the stage-control device can move under tension.Figure 6 (b) shows the clamping device, where the contact position between the clamps was padded with a layer of sandpaper to increase the friction force.Figure 6 (c) shows the installed staged control bolt, which undergoes tensile movement under the action of the hydraulic loading system and is connected to the hydraulic servo system, which loads at the set speed and uniform speed during the test.During the test, the stress change in the bolt was measured in real-time.The MG-400 bolt used in the field was selected to be closer to actual production, and the bolt dimensions were 22 mm × 2200 mm.Three groups of tests were conducted.The test results are presented in figure 7.   7, the static tensile curve is similar to that shown in figure 2. The changing process includes seven stages: the stretch bolt cone enters the sleeve cone sliding along the sleeve (primary yielding), the stretch bolt pallet is stretched out by the sleeve pallet sliding along the sleeve (secondary yielding), and bolt failure, indicating that the new stage control device has achieved the designed effect.

Conclusions
This study proposed a new stage-control device to address the challenges of insufficient thrust force and slow excavation speed encountered when using an open TBM in soft rock formations.This device coupled a support system to the surrounding rock deformation process.
1) Based on the deformation and failure mechanisms of the surrounding rock, the concept of zone failure of the surrounding rock was proposed.A new stage control device with a straightforward mechanistic principle and stable mechanical properties was designed, which can realize the effect of the coupling support with the deformation of the surrounding rock.
2) The new stage control device is composed of an internal and external two-stage structure similar to "piston + sleeve".Under the action of the surrounding rock pressure, primary yielding occurs first, and the cone slides along the inner wall of the yielding sleeve.With an increase in the surrounding rock pressure, secondary yielding occurs, and the tray slides along the outer wall of the yielding sleeve.
3) The mechanical test results showed that the device had an obvious function of graded pressure yielding, which was consistent with the designed sectional control curve.

3 .
New type of hierarchical control deviceOpen TBM projects require rapid support installation, minimal exposed anchor bolt length, and traditional support techniques that either take too long or result in excessive bolt exposure.This renders them unsuitable for open TBM excavation.Considering the zone failure phenomenon of the surrounding rock, a new stage control concept was proposed to realize the coupling of the support system and surrounding rock in terms of strength and stiffness at different deformation stages and improve the stability of the surrounding rock.The relationship between the support curve and ground reaction curve is shown in figure2.

Figure 2 .
Figure 2. Relationship between support curve and ground reaction curve.The traditional yielding support bears less surrounding rock pressure than the still support, as shown in figure2.However, yielding occurs relatively late, by which time the surrounding rock has undergone significant deformation, and the stress from the shallow rock is transmitted to the deeper rock.Consequently, the loosened zone is enlarged.The stage-yielding support reduces the effect of reducing support force and minimizes the loosened zone by applying primary yielding to release the stress in the shallow rock, thereby protecting the deeper rock.As the tunnel excavation progresses, the surrounding rock pressure continues to increase, and by promptly implementing secondary yielding, the stress in the deeper rock can be released.This achieves the desired outcome of reducing the support force and minimizing the loosened zone.

Figure 4 .
Figure 4. Working diagram of a new stage control device.

Figure 5 .
Figure 5. Static tensile test system.The static tensile test process is shown in figure6; the loading rate was maintained at 20 mm/min.Because the maximum width of the clamping device was 41 mm, the new stage control device contained a pallet, and the widest position was approximately 150 mm.Consequently, a special chuck was made before the test, as shown in figure6(a).The special chuck was composed of two steel plates with a thickness of 25 mm and four steel bars with a diameter of 30 mm.One side of the steel plate was connected to a 35 mm thick steel bar placed in the clamping device.The other side was a steel plate with a hollow middle, which allowed the steel bar to pass through and hold the tray simultaneously.Thus, the stage-control device can move under tension.Figure6(b) shows the clamping device, where the contact position between the clamps was padded with a layer of sandpaper to increase the friction force.Figure6(c) shows the installed staged control bolt, which undergoes tensile movement under the action of the hydraulic loading system and is connected to the hydraulic servo system, which loads at the set speed and uniform speed during the test.During the test, the stress change in the bolt was measured in real-time.The MG-400 bolt used in the field was selected to be closer to actual production, and the bolt dimensions were 22 mm × 2200 mm.Three groups of tests were conducted.The test results are presented in figure7.

Figure 7 .
Figure 7. Static tensile test curve.As shown in figure7, the static tensile curve is similar to that shown in figure2.The changing process includes seven stages: the stretch bolt cone enters the sleeve cone sliding along the sleeve (primary yielding), the stretch bolt pallet is stretched out by the sleeve pallet sliding along the sleeve (secondary yielding), and bolt failure, indicating that the new stage control device has achieved the designed effect.