TECHNICAL NOTE

Impact of Overcut on Interaction Between Shield and Groundin the Tunneling with a Double-shield TBM

Rohola Hasanpour1 • Jamal Rostami2 • Yılmaz Ozcelik1

Received: 20 November 2013 / Accepted: 15 August 2015 / Published online: 23 August 2015

� Springer-Verlag Wien 2015

Keywords Double-shield TBM � Overcut � Shieldjamming � Squeezing ground � 3D numerical simulation �Thrust force

1 Introduction

Double-shield TBMs (DS-TBM) are among the most

technically sophisticated excavation machines in use by

tunneling industry. The use of shields around the TBM

allows the machine to pass through weak grounds and

adverse geological conditions. However, there are limita-

tions in applicability for DS-TBM in some ground condi-

tions where large deformations are anticipated. The

presence of the shield limits access to the tunnel walls for

observation of ground conditions. This means limited

possibilities of observing and analyzing ground conditions

to avoid certain problems. Similarly, the presence of the

shield does not allow the intrusion of the ground into the

tunnel envelope, which is the main objective of using a

shielded machine in the first place, yet it also creates the

possibility of ground pressing against the shield. In such

conditions, TBM may get stuck (including shield jamming

and cutterhead blocking) in complicated geological struc-

tures, especially under high ground cover or in weak rocks,

where large convergences are expected. This could cause

major delays and impose a heavy and expensive burden on

the tunneling operation. Some of the issues related to

application of DS-TBMs in squeezing ground have been

discussed in Hasanpour (2014) and Hasanpour et al.

(2014a, b) and some possible scenarios and concepts for

mitigating the related problems are offered.

There are several performance parameters that should be

considered with high accuracy at the design stage of a

TBM for preventing machine entrapments. Size of the

annular space or gap between ground and shields (created

by overcut), length and diameter of shields, thrust force and

torque, and machine advance rate are the most important

performance parameters in tunneling by a shielded TBM.

However, selecting the correct overcut, compared to other

performance parameters, has a significant impact on pre-

venting shield jamming. Selecting an appropriate or opti-

mum value for overcut at the design stage of DS-TBM

tunnel and implementing the predetermined overcut is the

easiest way to address machine jamming, with the possi-

bility of adjustment along the tunnel by using movable

gage cutters. The adjustments can be directly related to

ground properties and optimized to reduce the risk of

machine jamming, while minimizing both the amount of

material that is excavated and hauled out of the tunnel and

the amount of grout that is placed behind the segments.

For preventing the shield seizure, increasing the annular

gap between the rock and shield is often utilized at the

machine design stage. This feature can be included in the

design of the cutterhead to accommodate a given overcut as

a base design, and as needed, the excavated diameter of the

tunnel, and hence the gap above the shield can be increased

to react to bad ground where large convergences are

& Rohola Hasanpour

roha93@gmail.com

Jamal Rostami

rostami@psu.edu

Yılmaz Ozcelik

yilmaz@hacettepe.edu.tr

1 Department of Mining Engineering, Hacettepe University,

Beytepe, 06800 Ankara, Turkey

2 Department of Energy and Mineral Engineering,

Pennsylvania State University, University Park, PA, USA

123

Rock Mech Rock Eng (2016) 49:2015–2022

DOI 10.1007/s00603-015-0823-x

expected, or in cases when longer and normal machine

delays are planned. Furthermore, the stepwise increase of

the annular gap by decreasing the diameter of the rear

shield relative to the front shield is another solution that is

incorporated in machine design and can be observed in all

double-shield TBMs manufactured in recent years.

In order to choose a suitable value for the overcut, more

comprehensive and detailed three dimensional numerical

analysis of the ground is needed at the design stage of a

double-shield TBM. This is due to the fact that ground

convergence and hence the pressure imposed upon the

shield and thus the thrust force needed to propel the shield

is a function of the complex interaction between the rock

mass, the tunneling machine, its subsystems and compo-

nents, and the final tunnel support in tunneling by a

shielded TBM. Therefore, three dimensional models that

include all of these components are essential for modeling

the interactions correctly and avoid the errors created by

assumption of plane strain conditions or axisymmetric

modeling.

There are a number of studies in the literature that

related to the numerical analyses of mechanized tunnel-

ing in squeezing conditions (Lombardi and Panciera

1997; Einstein and Bobet 1997; Graziani et al. 2007;

Sterpi and Gioda 2007; Wittke et al. 2007; Ramoni and

Anagnostou 2006, 2007, 2008, 2010, 2011; Amberg

2009; Schmitt 2009; Zhao et al. 2012). The developed

model in this study enhances the computational model of

Zhao et al. (2012) and describes a comprehensive 3D

modeling used for simulation of the DS-TBMs for

excavation of long deep tunnels through rock masses that

exhibit squeezing behavior. The model has some prop-

erties that distinguish it from other 3D models that have

been developed for numerical simulation of shield TBMs

in the past.

Finite difference analysis with large strain assumption

was used in this paper for numerical computation. More-

over, the results of analysis were presented in simple 3D

and geometrically correct shapes that are practical for

engineering applications. The model estimates tunnel

convergence during excavation and predicts the loads on

the shields at various time steps, corresponding to short

term response of the ground (Hasanpour et al. 2014a, b). In

addition, to avoid errors in the analysis due to large dis-

placements in weak grounds, the method of displacement

control has been applied to represent contact surfaces

between the ground and the shield. For this purpose, a

FISH routine was developed in FLAC3D that controls all

displacements with respect to non-uniform overcut at each

solving step of numerical analysis. Increasing of gap due to

conical shape of the shield is also considered in the geo-

metric setting of the model and in the contact detection

code. These properties of model distinguish the simulations

used in this study from other 3D models that have been

developed for numerical simulation of shield TBMs in the

past (Hasanpour 2014).

A numerical study on the impact of changes in overcut

was subsequently carried out to allow for observation of

impact of overcut on the possibility of machine jamming in

squeezing ground. The results demonstrate that the contact

forces on shields are considerably lower when a larger

overcut is provided. But, in squeezing ground, it leads to a

bigger plastic zone and instabilities above shields and lin-

ing and creates higher dead weight loads that could cause

jamming of the backup system or failure of the concrete

segmental lining. On the other hand, the smaller overcut

will result in high contact forces, leading to a smaller

plastic zone; however, shields may be entrapped due to

high frictional forces. Design of overcut should be per-

formed by accounting for the above mentioned

phenomenon.

In this paper, the results of 3D numerical modeling of

rock mass and TBM components are discussed with ref-

erence to previous research work by Hasanpour et al.

(2014a, b) and Hasanpour (2014). The output of the

modeling includes longitudinal displacement and contact

forces and also sectional ground pressure on shields versus

different amount of overcut. Furthermore, the size of

plastic zones for four different values of overcut and the

required thrust force to overcome frictional forces when

shield is in contact with the rock mass are investigated.

2 Numerical Modeling

The study presented in this paper is based on 3D numerical

modeling and simulation of ground response when using a

double-shield TBM and large ground convergences are

anticipated. The current part of the study is an extension of

the previous work by the authors on this topic (Hasanpour

2014; Hasanpour et al. 2014a, b). The current study builds

upon the 3D models previously used by changing the

overcut values for evaluation of the impact of overcut on

contact forces between the shield and the ground. Rock

mass parameters, geometric dimensions, and mechanical

properties for DS-TBM components are given in Tables 1

and 2.

Various excavation stages are programmed as several

steps in the numerical model. These stages were imple-

mented based on the design and excavation diameter of the

cutterhead, diameters of the front and rear shields for a

given double-shield TBM, and incorporating the time

steps. In this study, a total of 41 excavation steps were

simulated including the initialization step 1 and 40 exca-

vation steps (each excavation step is modeled by advancing

of the face by 1 m).

2016 R. Hasanpour et al.

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The contact between the TBM main shield and the rock

mass was modeled by interface elements and considering

the gap between the ground and the shield, with a non-

uniform overcut in the shielded TBM. This allows for

accurate simulation of the overcut which is maximum at

the crown and zero at the invert, where the TBM shield

slides on the invert by its own weight. Normal and shear

stiffness values (kn, ks) were assigned to interface elements

for simulation of interaction phenomena. kn and ks values

of shields were selected to be ten times the equivalent

stiffness of the softer neighboring zone:

K þ 4=3ð ÞGDZmin

;

where K and G are the bulk and shear moduli of the

ground, respectively, and DZmin is the smallest width of an

adjacent zone in the normal direction, equal to 1 cm

(FLAC3D manual, 2006). kn and ks are calculated to be

2.7e13 Pa/m according to the above-noted relationship.

Advance rate of machine was taken into account by

controlling of the unbalance forces that are created after

each excavation step during face advance. For this purpose

and in order to correctly represent the continuous excava-

tion by a shielded TBM and considering the advance rate of

machine into step-by-step analyses, an 87 % relaxation of

the unbalance forces for each step of analysis was used

(Zhao et al. 2012).

3 Results of Numerical Modeling

The modeling results are illustrated in the following by

giving the displacements, forces, and pressure along the

lines at the tunnel crown and side-walls. The overcut is

defined as the gap between the ground and the front shield.

The overcut between the ground and the rear shield is

higher due to a stepwise reduction in diameter of the

shields. Shield thickness is considered to be 3 cm. The gap

between the rock mass and the cutterhead is slightly

smaller than that of the front shield when overcut in the

crown is 20 cm. For other overcut settings, this magnitude

is different and is adjusted with respect to the overcut in the

front shield. Figure 1 shows the different overcut settings

at the crown. DR is the gap between the front shield and the

ground, which is also shown as DRf.

Given that the overcut between the ground and shields is

non-uniform in the cross section of the tunnel, there are

different values of overcut at the tunnel sectional bound-

aries. This means that the overcut has the maximum value

at the crown and gradually decreases to its minimum value

equal to zero at the invert. The numerical analyses were

performed to capture the non-uniform overcut as intro-

duced in the models.

3.1 Shield–Ground Interaction

One of the important parameter in DS-TBM design is the

stepwise reduction of the shield diameter, thus defining the

variation DR of the radial gap along the shield based on the

initial excavation diameter defined by the cutterhead. The

positive effect of a stepwise construction is reducing the

contact forces (which govern the required thrust force)

acting upon the shields. Figure 2a, b illustrates the simu-

lation results in terms of the longitudinal contact force

profile (LFP), which is proportional to the longitudinal

displacement profile (LDP) at tunnel circumference along

the tunnel crown and side-walls, respectively. As expected,

the contact forces are considerably low (both for the front

and rear shields) when a larger overcut is provided. In the

case of a very large overcut DR = 20 cm, the gap between

the ground and shield closes at and the shield experiences

lower contact forces.

Table 1 Rock mass parameters

and geometric dimensions for

DS-TBM components

DS-TBM components Rock mass parameters

Cutterhead length (m) 0.75 Elastic modulus, E (GPa) 1.40

Front shield length (m) 5 Poisson’s ratio, m (–) 0.25

Rear shield length (m) 6 Cohesion, c (MPa) 0.60

Shield thickness (cm) 3 Friction angle, / (�) 28

Lining segment width (m) 2 Dilatancy angle, w (�) 8

Lining segment thickness (cm) 45

Tunnel inner diameter (m) 8.10

Table 2 Mechanical properties

of DS-TBM componentsMaterial properties Unit Shield Segmental lining Soft backfill Hard backfill

Elastic modulus GPa 200 36 0.5 1.0

Poisson’s ratio – 0.3 0.2 0.3 0.3

Unit weight kN/m3 76 30 24 24

Impact of Overcut on Interaction Between Shield and Ground in the Tunneling with a Double… 2017

123

The impacts of overcut and stepwise reduction of the

shield diameter on decreasing the total ground pressures

acting on the shields are summarized in Table 3. The

results show that a wide gap is more important for the rear

shield, because the convergence of the ground increases

with the distance behind the face. For example, the total

ground pressure acting upon the front shield decreases from

23.3 MPa where DR = 1 cm to 4.2 MPa at DR = 20 cm,

corresponding to a reduction of about 81.9 %. This per-

centage is about 86.3 % for the rear shield where the

ground pressure reduces from 19.6 to 2.7 MPa.

3.2 Comparison of the Plastic Zones for Different

Depths of Overcut

A comparison of the plastic zone around the tunnel for

different sizes of overcut has been performed to evaluate

the impact of depth of overcut on the size of plastic zone.

As shown in Fig. 3, the size of the plastic zone for different

overcut increases linearly with the size of the radial gap,

DR (DR = 1, 5, 10, and 20 cm). The larger overcut

requires more time to close the gap but leads to a bigger

plastic zone in the longitudinal direction. Therefore, if

DR = 20 cm of overcut gap remains open for longer time

and takes shield length L for initial contact between the

rock and shield, the extent of plastic zone will increase due

to additional room to expand and related increased volume

of rock involved in ground deformation.

Although a larger overcut creates a lower load on the

shield and subsequently a lower frictional force during

machine advance, however, a larger overcut leads to larger

deformations around the tunnel and consequently forms an

extended zone of overstressed ground. Thus, the ground is

loosened and softened due to large deformation. This cre-

ates higher loads above the shield at the crown of tunnel

and can cause backfilling problems (Ramoni and Anag-

nostou 2011). The problems related to loosening and

softening of ground in tunnels are particularly important

for the design of a yielding support, because both strength

loss and major loosening call for a higher yield pressure in

the support system (Anagnostou and Cantieni 2007).

When an additional dead load is placed against the

shield and ground support due to development of extended

plastic/weak zones, it may appear as overstressed segments

that can fail or develop cracks in various directions. This

could explain the ground behavior in the case of tunnels

where failed segments were observed after passing of the

machine. One example is the T26 Tunnel (Istanbul-Ankara

high speed rail project) in Turkey which has experienced

serious failure in the segments and backup jamming, and

sometimes resulted to the entrapment of the shields

(Hasanpour and Rostami 2013).

It should be noted that proper placement of backfill and

grouting is essential for uniform redistribution of pressures

around segmental lining. This issue has direct implication

on the design of the lining system and optimizing the

amount of backfill/grouting around segments. Applying

larger overcut to relieve the shield loading and reduction in

the required thrust force for propelling the TBM in bad

ground can be considered as a reasonable solution. How-

ever, the amount of overcut should be reduced in good

ground where it could result in the need for higher amounts

Fig. 1 Illustration of four

assumed different overcut sets

(1–4) used in numerical analysis

2018 R. Hasanpour et al.

123

Fig. 2 LDP and LFP for a 1-m

length of cutterhead, 5 m front

shield and 6 m rear shield at

different overcut sets, DR, of 1,5, 10 or 20 cm a at the crown

b at side- wall

Impact of Overcut on Interaction Between Shield and Ground in the Tunneling with a Double… 2019

123

of material for backfill/grouting that means increased cost

of grouting and more expensive excavation of tunnel on a

per foot basis.

3.3 Thrust Force Calculations

The thrust force required to overcome shield skin friction

can be calculated by integrating the contact pressure over

the shield surface and multiplying the results by the skin

friction coefficient. Sectional contact pressure profiles,

between the ground and front shield as well as rear shield,

are shown in Fig. 4a, b. Figure 4c, d also shows the

required thrust force to overcome the frictional forces on

the shield versus the overcut for the two operational

stages including ongoing excavation and restart after a

standstill.

The skin friction coefficient was assumed to be

l = 0.15–0.30 for ongoing excavation and l = 0.25–0.45

for restart after a standstill, where the lower friction coef-

ficient values aim to illustrate the positive effects of

lubrication of the shield extrados, e.g., by bentonite or

other lubricants (Ramoni and Anagnostou 2010).

Table 3 Ground pressure acting upon machine components

Overcut, DR (cm) Ground pressure acting on (MPa) Pressure reduction comparing to 1 cm overcut (%) on

Cutter head Front shield Rear shield Cutter head Front shield Rear shield

1 19.7 23.3 19.6 0 0 0

5 19.2 17.7 8.8 2.5 24 55.1

10 11.7 10.6 5.7 40.6 54.5 70.9

20 4.5 4.2 2.7 77.2 81.9 86.2

Fig. 3 Plastic zone for a 1-m cutter head, 5 m front shield and 6 m rear shield at variable DR a 1 cm, b 5 cm, c 10 cm, d 20 cm

2020 R. Hasanpour et al.

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4 Conclusion

A comprehensive 3D modeling study of mechanized

excavation by a DS-TBM was performed to allow for the

assessment of the ground shield interaction at various

points along the shield for various overcut size. Numerical

analysis was used in the simulations to evaluate the mag-

nitude of the ground loading on the shield and possible

TBM jamming. The effect of a stepwise shape of the shield

in reducing the ground pressure acting against it, for dif-

ferent values of the overcut was investigated. The results

show that a larger overcut decreases shield loading and

therefore would lead to a lower frictional resistance during

shield advance. The required thrust force to overcome

frictional forces was determined to be lower for increased

depth of overcuts. This was true for two operational cases

of ongoing excavation and restart after long delay or

standstill.

The results also show that a higher overcut can lead to a

larger plastic zone and increases the possibility of experi-

encing instabilities above the shields and increased loading

against lining by imposing higher values of dead weight

loads on the shield. This could cause failure of the seg-

mental rings. On the other hand, smaller overcut results in

high contact forces, leading to a smaller plastic zone.

However, shields may be entrapped due to high frictional

forces resulting in high contact forces.

The results indicate that a larger overcut is not by itself a

solution for coping with squeezing conditions and avoiding

shield entrapment. Increasing the overcut can be

Fig. 4 Sectional contact pressure profile between ground and a front

shield, and required thrust force for the ongoing excavation, l = 0.25

and b rear shield restart after standstill, l = 0.40. c Amount of propel

force needed to move the machine forward in ongoing excavation and

d in case of restart after long delay or stand still

Impact of Overcut on Interaction Between Shield and Ground in the Tunneling with a Double… 2021

123

considered for preventing machine jamming in squeezing

ground in special cases but should be carefully optimized.

This paper has described the type of a parametric study that

can be used for evaluating the impact of overcut on the

shield loading and the extension of plastic zone. This

procedure can be used to optimize the overcut size in

specific ground conditions and geometry of the shield.

For optimizing the amount of overcut, ground properties

along tunnel should be determined with high degree of

accuracy. This is essential for selecting the input parame-

ters for simulation of the shielded machine. A full 3D

numerical simulation of the shielded TBM tunneling

should be performed for evaluation of the different rates of

overcut on shield loading as well as on plastic zone around

tunnel. Appropriate value of overcut can be selected based

on a sensitivity analysis of the overcut and resulting ground

load on the shield, including the extent of the plastic zone

around the tunnel.

Acknowledgments The authors gratefully acknowledge the finan-

cial support of the Scientific and Technological Research Council of

Turkey (TUBITAK) under Project No. MAG-114M568.

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