ap-3-11.dvi


Acta Polytechnica Vol. 51 No. 3/2011

3D Modelling of a Tunnel Re-excavation in Soft Ground

M. Hilar

Abstract

The construction of the shallow tunnel at Brezno started using the Pre-Vault Method. The tunnel excavation, in
complicated geological conditions, led to many difficulties which finally resulted in a collapse, when a significant part
of the temporary tunnel lining collapsed. Various options for re-excavating the tunnel were evaluated prior to further
construction. Finally a decision was made to separate the collapsed area into sections 9 m in length using 16 m-wide,
transversally oriented pile walls, to improve the stability of the collapsed ground. The walls were constructed from the
surface prior to excavation. It was also decided to re-excavate a collapsed area using the SprayedConcrete Lining (SCL)
method. Due to problematic soft ground conditions, which had been made even worse by the collapse, some additional
support measures had to be considered prior to re-excavation (ground improvement, micropile umbrellas embedded into
the pile walls, etc.)
This paper describes numerical modelling of the tunnel re-excavation through the collapsed area. Initial calculations

of the tunnel re-excavation were made using a 2D finite element method. Subsequently, further calculations to evaluate
the rock mass behaviour in the collapsed area were provided in 3D. The 2D calculations were used to provide sensitivity
studies, while 3D modelling was mainly used for evaluating the tunnel face stability (impact of the pile walls, impact
of ground improvement) together with other factors (length of advances, moment of the temporary invert closure, etc.)
The results of the modelling were compared with the monitoring results.
The paper also briefly describes the construction experience (technical problems, performance of various support

measures, etc.) The excavation and the primary lining construction were completed in 2006, and the tunnel was opened
for traffic in April 2007.

Keywords: tunnel, clay, soft ground tunnelling, sprayed concrete, New Austrian Tunnelling Method, NATM, Sprayed
Concrete Lining, SCL, numerical modelling.

History

The construction of theBrezno tunnel, with overbur-
den up to 30 m, started in 2000 using the Pre-Vault
Method (Perforex) [1]. The Pre-Vault Method prin-
ciple is such that a 20 cm-wide slit (for the Brezno
tunnel) is cut along the upper and side parts of a pre-
defined tunnel circumference. Step by step, simulta-
neously with cutting the prescribed number of seg-
ments in a prescribed sequence, the slit is filled with
sprayed concrete. The hardened concrete forms a
protective pre-vault reaching ahead of the rock exca-
vation. The pre-vault has a truncated conical shape.
This shape allows overlapping of the individual pre-
vaults. The length of the chain saw determines the
length of the pre-vault, which was 5 m in the case
of the Brezno tunnel. The pre-vault can overlap the
preceding prevault from 2.5 to 0.5m (the excavation
advance length is selected according to the geological
conditions that are encountered, from 2.5 to 4.5 m).
Full-face excavation then takes place under protec-
tion from the pre-prepared primary lining.
The tunnel excavationwaspredominantly inplas-

tic clays and claystones, and the maximum thickness
of the quaternary deposits (gravels and sands) was
about 6 m. The area was also affected by previous
undocumented mining activities. The very compli-

cated geological conditions caused many difficulties,
resulting in a significant collapse in 2003. The col-
lapse occurredwhen about 860m of the primary lin-
ing of the tunnel had been completed. About 77 m
of primary lining was destroyed (chain effect of pre-
vaults) and a further 44 m of the tunnel was filled
with collapsed material. Excavation ceased for sev-
eral months directly after the collapse.

Proposed tunnel recovery

A decision was made to separate the collapsed area
into 9m-long sections using 16m-wide tranverse pile
walls constructed from the surface. The walls were
formed from piles 1.18 m in diameter, and the walls
reached 3 m below the tunnel profile. The collapsed
tunnel was separated into 7 sections in the longitu-
dinal direction. A shaft had to be constructed in the
area of a buried Perforex machine (Figure 1).
The Sprayed Concrete Lining (SCL) method was

used for re-excavating the collapsed area. The pri-
mary lining was designed as sprayed concrete, rein-
forced by lattice girders and amesh. The tunnel face
had to be excavated in several stages. The proposed
excavationmethodhad tobeproperly statically eval-
uated prior to application, and all support measures
had to be optimised.

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Acta Polytechnica Vol. 51 No. 3/2011

Fig. 1: Longitudinal cross-section, including separation of the collapsed area using pile walls

Table 1: Input geotechnical parameters (the abbreviations of the layers correspond with Figure 2)

Geotechnical unit
Input parameters

γ (kN/m3) c (kPa) φ (◦) EDEF (MPa) ν

Quaternary deposits QD 19.2 11.5 18 17 0.30

Strongly weathered claystone SWC 19.2 11.0 10 19 0.40

Collapsed material CM 19.2 11.0 8 19 0.40

Weathered claystone WC 19.5 17.0 19 19 0.40

Claystone A CA 19.5 36.0 19 32 0.40

Claystone B CB 19.5 40.0 20 35 0.38

Claystone C CC 19.5 45.0 25 50 0.38

Coal seam CS 19.5 30.0 25 60 0.30

Fig. 2: 2D model finite element mesh (the abbreviations
of the layers correspond with Table 1)

The calculations were generated using the finite
element method (FEM). Due to the complexity of

the problem, ordinary 2D calculations were supple-
mented by 3D calculations to verify some 3D effects
(e.g. impact of tunnel separation by pile walls).

Original calculations

The initial static calculations for designing the pri-
mary lining and the excavation sequence were gener-
ated using 2DFEM(plane strainmodel) [2], and the
rock mass was modelled using a linear elasto-plastic
Mohr-Coulomb model (Figure 2). RIB software was
used for the calculations [4]. The primary tunnel lin-
ing was evaluated using interaction curves produced
by FINE software [5]. The input parameters were
derived from a supplementary site investigation con-
ducted after the collapse. The initial input param-
eters for each geotechnical unit are summarised in
Table 1. The value used for the coefficient of the
lateral pressure at rest was 0.8. The input parame-

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Acta Polytechnica Vol. 51 No. 3/2011

ters were derived from a supplementary site investi-
gation carried out after the collapse. The collapsed
ground was quite heterogeneous (a mixture of cohe-
sive and non-cohesive soils), and the selected mean
values were rather conservative.
The static calculations that were generatedmod-

elled the excavation and support installation in sev-
eral stages (top heading excavation, top heading lin-
ing, benchexcavation,bench lining, invert excavation
and invert lining closure), and the model included
two types of sprayed concrete — three-day-old green
sprayed concrete, and sprayed concrete with its final
parameters. The lining thicknesswas35 cm. The top
heading liningwas expected to be regularly closed by
a temporary invert, which is a crucial measure for
achieving equilibrium in geological conditions of this
type. Geometry also plays a very important role in
minimizing the bending moments (smaller eccentric-
ity) in the lining. The lining geometrywas optimised
in this way.
The calculated maximal axial forces were in the

range 1500 kN to 2450 kN, depending on the stage
of excavation. The calculations confirmed that the
maximum deformations of the primary lining should
not exceed 50 mm, and monitoring during construc-
tion generally confirmed these expectations. The
shape of the temporary top heading invert was de-
signed as a compromise between the optimum static
profile and the space requirement for the machinery.
The shape of the permanent invert was more appro-
priate from the static view, as no compromises were
required. The tunnel lining evaluation confirmed
that its capacity was sufficient.

Verification calculations
3D calculationswere generated using Plaxis 3DTun-
nel software [6]. The major aim of this modelling
was to evaluate the impact of the pile walls on the
excavation and the lining. The model was prepared
for analysing the conditions input into the 2D calcu-
lations (location of geotechnical units, input param-
eters, tunnel lining, etc.)
The3Dmodelwas127m inheight, 90m inwidth,

and 97m in length (Figure 3). The model comprised
half of the tunnel, and used symmetry. First, the top
heading constructionwasmodelled (Figure 4) in sev-
eral steps to simulate top heading excavation. Each
advancewasmodelled in two phases (excavation and
tunnel lining application).
Second, the bench and invert construction was

modelled in several steps (Figure 5). Again, each ad-
vance was modelled in two phases (excavation and
tunnel lining application). One model was generated
with pile walls (Figure 6); the second was generated
without them.

Fig. 3: 3D model finite element mesh (the geotechnical
layers and parameters correspond with the 2D model)

Fig. 4: Modelling of the top heading construction

Fig. 5: Modelling of the bench and invert construction

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Acta Polytechnica Vol. 51 No. 3/2011

Fig. 6: Details of the 3D model with pile walls

Fig. 7: Top heading tunnel lining deformations (impact of pile walls)

Impact of pile walls: The model included pile
walls spaced at 9 m, with a thickness of 1 m (Fi-
gure 6). The pile walls were modelled as a linear-
elastic material, and they were separated into two
parts to simulate the real structure (see Figure 6):
a. Lower part (in the tunnel area) filled with con-
crete with the properties: E =25 GPa, ν =0.2

b. Upper part (above the tunnel) filled by suspen-
sion with the properties: E =10 GPa, ν =0.2

Two calculations were generated: with walls and
without walls. The results showed the stiffening ef-
fect of pile walls. The construction of the pile walls

means a reduction in deformations (Figure 7) and
bendingmoments of about 50%. The differences be-
tween the 2D results and the 3D results (seeTable 2)
were caused by the original estimation of the relax-
ation. The choice of low relaxation (i.e. a fast ring
closure assumption) in the 2D calculations was de-
termined mainly by taking a conservative approach
to the primary lining analysis (to get higher axial
forces).
Impact of bench and invert excavation: A

further purpose of the 3D calculations was to evalu-
ate the effect of bench and invert excavation on the

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Acta Polytechnica Vol. 51 No. 3/2011

top heading lining performance (i.e. when the tunnel
invert should be closed). The invert was modelled to
be closedafter 2m(Figure8), 4m, and8madvances.
The calculations showed that the values of the inter-
nal forces in the top heading lining are not a signifi-
cant problem. A more significant problem would be
the deformations, which would double in the case of
8 m advances. The next problem was the forces in
the longitudinal direction and the shear forces in the
lining close to the walls (Figure 9). Thus, a max-
imum advance of 4 m was recommended for bench
and invert excavation.

Fig. 8: Bench and invert constructionmodellingwith 2m
advances

Fig. 9: Concentration of shear forces in the tunnel lining
on the interface of the pile walls and the ground

Topheading face stability: Calculationsof the
top heading face stability were also generated. The
bench and invert excavationwas expected to be sepa-
ratedat leastbyonepilewall tohaveaminimal effect
on the stability of the top heading face. Again the
calculation was performed in several stages to sim-
ulate the tunnel construction procedure (installation
of pilewalls, consequently several excavationsand in-
stallations of lining). The tunnel face stability was
calculated when the tunnel face was 2 m behind the
pilewall and1mof the excavationwasnot supported

by the tunnel lining. The safety factorwas calculated
using the phi-c reduction procedure (option available
in Plaxis for computing safety factors). In the phi-c
reduction approach, the strength parameters tan φ
and c of the ground were successively reduced until
failure occurs. The resulting safety factor is the ratio
of the initial and final shear parameters.
The calculations showed a safety factor of 1.1,

which indicated problems in the top heading face
stability. However, the calculation that was gener-
ated did not include the designed support measures
(supportwedge,micropile umbrellas and jet grouting
columns, further sequencing of the face, etc.) The
results showed a favourable effect of pile walls in lim-
iting the propagation of the deformations that were
generated (Figure 10).

Fig. 10: Propagation of the topheading face deformations

Construction experience

There was significant anxiety about the ground be-
haviour prior to the start of excavation, as the area
had been significantly disrupted by the previous col-
lapse (the area in and above the tunnel profile).
Coreswere thereforedrilled fromthe tunnel faceprior
to excavation of each section between the pile walls,
and a decision was made on ground improvement
and supportmeasuresbasedon the results of drilling.
In chamber 1 (see Figure 1), horizontal jet grouting
columns were installed into the face to increase the
face stability. This measure was also used in cham-
ber 3 (see Figure 1).
The tunnel profile was regularly protected bymi-

cropile umbrellas (Figure 11); the micropiles were
embedded into the pile walls on both ends. Some
attempts were made to embed the micropiles into

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Acta Polytechnica Vol. 51 No. 3/2011

Fig. 11: Tunnel construction under micropile umbrellas

the horizontal jet grouting columns (to increase their
stiffness), but like the jet grouting columns drilled
into the face, this approach was finished after the
third section.
All excavations were done with 1 m advances.

The excavatedprofilewas supported bywiremeshes,
lattice girders and sprayed concrete. The face sta-
bility was regularly increased by a support wedge
(ground left in the centre of the excavated profile).
In addition, a flash coat of sprayed concrete (sev-
eral centimetres)was immediately appliedonthe face
and tunnel perimeter after the excavation. The top
heading facewas sometimes excavatedand sprayed in
several steps (in cases of local instability). All these
measures helped significantly to increase the tunnel
face stability, and the calculated low tunnel face sta-
bility was confirmed during excavation by several lo-
cal failures. In addition, the temporary top heading
invert was closed regularly. Originally it was closed
in 2 m or 3 m steps, but later these sections were
extended to 4 m. Bench and invert excavation was
carried out more than 9 m behind the top heading
face (the length of one chamber). The excavation
started at the end of February 2006 and was com-
pleted without major problems at the beginning of
August 2006.

Results of monitoring

The ground deformations were recorded by ordinary
geotechnical monitoring. The sprayed concrete lin-
ing was monitored by convergence monitoring, with
threepoints on the topheadingand twopoints on the
bottom. The convergence cross-sectionswere located
in the centre of all pile walls and also between the
pile walls (generally one or two intermediate moni-
toring profiles between two pile walls). The surface
settlement wasmonitored on the top of all pile walls;
some intermediatepoints at ground level between the
walls were also monitored.

The maximum surface settlement monitored
above the tunnel was 28 mm (area above the cham-
ber 2). 2D modelling predicted surface settlement of
40 mm, and 3D modelling with pile walls predicted
20 mm. All convergences generally remained below
40mm, but themonitoring results showed significant
scatter of the tunnel lining deformations (Table 2),
mainlydue to theverydifficult heterogeneousground
conditions. In chamber 2, vault settlement of 93mm
was monitored. This high deformation was caused
by local problems (tunnel lining cracking), which did
not affect the overall stability of the tunnel. 2Dmod-
elling predicted vault settlement of 50mm, while 3D
modelling with pile walls predicted vault settlement
of 25 mm. The results are compared with the moni-
toring results in Table 2.
Naturally, even 3Dmodelling could not reflect all

aspects of the excavation. When the modelling re-
sults are compared with the actually measured de-
formation values, some differences become obvious,
but they are not too great in this particular case.
The differences are mainly caused by factors which
could not be properly included in the models (het-
erogeneousground, timing andquality of the support
measures, quality of the grouting, etc.)

Conclusion
The Brezno tunnel had to be excavated in very com-
plicated geological conditions. These ground condi-
tions were significantly worsened by the collapse of a
long section of the tunnel lining. The design of the
excavation procedure and appropriate support mea-
sures for re-excavation of the collapsed tunnel was
not a straightforward task. Static calculations of the
tunnel re-excavation were provided using the 2D fi-
nite element method (RIB software). Further calcu-
lations for evaluating the rockmass behaviour in the
collapsed area were provided using Plaxis FEM soft-
ware. 2Dcalculationswereused toprovidesensitivity
studies [3], and 3D modelling assisted the evaluation
of the tunnel face stability (impact of the pile walls,
ground improvement, etc.) The results of the mod-
elling were compared with the monitoring results.
The paper also briefly describes the construction ex-
perience (technical problems, performance of various
support measures, etc.) 2D and 3D modelling were
used to evaluate the ground and tunnel behaviour
prior to re-excavation.
The modelling provided very useful information

prior to the start of construction. It led to optimisa-
tion of the tunnel shapeand the excavation sequence.
It indicated tunnel face stability problems,whichhad
to be improved by various measures. It also showed
quite well some anticipated problems which subse-
quently appeared during the excavation (low stabil-
ity of the excavation face, concentration of stress be-

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Acta Polytechnica Vol. 51 No. 3/2011

Table 2: Comparison of monitored and calculated tunnel lining crown settlement

Tunnel chainage (m) Monitoring (mm) 2D Model (mm) 3D Model (mm)

2004 5 50 25

2007 19 50 25

2012 21 50 25

2019 20 50 22

2025 20 50 25

2027 27 50 25

2031 33 50 22

2034 37 50 25

2036 50 50 25

2040 93 50 22

2043 38 50 25

2048 39 50 22

2052 40 50 25

2057 40 50 22

2061 40 50 25

2066 31 50 22

2070 24 50 25

2075 26 50 22

2079 11 50 25

2081 17 50 24

tween the unclosed and closed linings, the positive
influence of the temporary invert and dividing pile
walls, etc.)
Regarding the excavation itself, a flexible ap-

proach to the construction work on the part of the
contractor was essential. In some cases, it was nec-
essary to respond to the properties and behaviour of
the ground in a very flexible manner; it was impos-
sible to optimise all aspects of the excavation in the
planning phase. The excavation procedure was rea-
sonably well optimised during construction, so the
collapse recoverywas completed without any further
significant difficulties. The Brezno tunnel construc-
tion was successfully completed, and the tunnel was
opened for traffic in April 2007.

Acknowledgement

Financial support from research grant TACR
TA01011816 is gratefully acknowledged.

References

[1] Hilar, M., Heřt, J., Smida, M.: Soft Ground Ex-
cavationof theBřeznoTunnel.Proceedings of the
World Tunnelling Congress, Agra, 2008.

[2] Hilar, M., John, V.: Recovery of a collapsed sec-
tion of theBrezno tunnel.Tunel, Vol.16, 3/2007.

[3] Barták,J.,Hilar,M., Pruška,J.: NumericalMod-
elling of the Underground Structures. Acta Poly-
technica. Vol. 42, No. 1/2002.

[4] RIB — software for civil engineers.
http://www.rib.cz

[5] FINE — civil engineering software.
http://www.fine.cz

[6] Plaxis — software for geotechnical engineers.
http://www.plaxis.nl

doc. Ing. MatoušHilar, MSc., PhD., CEng.,MICE
Phone: +420 604 862 686
E-mail: hilar@d2-consult.cz
D2 Consult Prague, s.r.o.
Zelený pruh 95/97 (KUTA)
140 00 Praha 4, Czech Republic

Department of Geotechnics
Faculty of Civil Engineering
Czech Technical University in Prague
Thákurova 7, 166 29 Praha 6, Czech Republic

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