THE SIGNIFICANCE OF MODELING THE EXCAVATION SEQUENCE IN NUMERICAL
ANALYSIS OF UNDERGROUND OPENINGS
*Y. H. Hatzor, Y. Tal+, and G. Yagoda-Biran++
Dept. of Geological and Environmental Sciences
Ben-Gurion University of the Negev
Beer Sheva, Israel 84105
(*Corresponding author: hatzor@bgu.ac.il)
+
Present address: Dept. of Earth, Atmospheric and Planetary Sciences, MIT
++
Present address: Nevada Seismological Laboratory, University of Nevada, Reno
X. T. Feng
State Key Laboratory of Geomechanics and Geotechnical Engineering
Institute of Rock and Soil Mechanics
The Chinese Academy of Sciences
Xiaohongshan Wuchang
Wuhan 430071China
THE SIGNIFICANCE OF MODELING THE EXCAVATION SEQUENCE IN NUMERICAL
ANALYSIS OF UNDERGROUND OPENINGS
ABSTRACT
When studying jointed rock mass deformation around underground openings it is of great importance to
model correctly the excavation sequence during the numerical simulation. This is because for effective
arching mechanism to develop in the roof minute block displacements must take place and some of the
available discontinuities shear strength must be mobilized. If from the beginning of the simulation the
underground opening already exists in the numerical model, block displacements into the excavation space
will initiate and develop from time zero, before the initial stresses in the model have attained their actual
initial level in the field. These initial block displacements are therefore artificial and exist only in the
numerical model, not in the physical reality, and may lead to erroneous assessment of the stability of the
designed structure, typically on the conservative side.
We demonstrate the significance of this numerical effect on a 25 meter deep 2000 year old underground
limestone quarry below the old city of Jerusalem, Israel which is still standing today unsupported with a
free span of 40 m. We show that when the opening exists from the beginning of the simulation the required
friction angle for roof stability is 25 degrees, whereas when the opening is introduced after all stresses and
displacements in the model have been stabilized the required friction angle for stability is only 15 degrees.
We then use the ability to remove sections of the opening during the simulation to obtain the in-situ
stresses in the 2500 meter deep tunnels of the Jinping hydroelectric project in China. We utilize
displacement data obtained underground in the course of tunnel excavation using the sliding micrometer
technique to find the in situ stresses at the depth of the excavation by inversion. We model exactly the
sequence of excavation which consists of removal of the top heading followed by the bench, and compare
between the measured and computed displacements to find the in situ stresses which return minimum root
mean square error (Figure 1). Thus we are able to constrain the in situ stress field at these depths where
standard in situ stress measurement procedures are extremely difficult to execute properly.
Finally, we demonstrate the applicability of modeling sequential excavation by constraining the extent of
the loosened zone around deep underground excavations in densely jointed rock masses which are
comprised of columnar basalts. We show that empirical guidelines for rock bolt dimensioning in such
blocky rock masses are not always applicable and indeed may be un-conservative. Our results are validated
using in situ displacement data obtained in the roof and side walls of a tunnel using multiple point borehole
extensometer data.
We conclude that realistic modeling of jointed rock mass deformation around tunnels, shallow or deep,
must incorporate provisions to model the excavation sequence properly. Gravitational stresses must be
allowed to develop in the model before the planned opening is removed. When sequential excavation is
incorporated in the numerical simulations the results are more realistic and allow for more accurate
analysis, including stress inversion and dimensioning of safe reinforcement.
s1= -68.35 MPa
3.5
measured
numeric
3
z
U [mm]
2.5
South side
x
North side
2
q=-13.1º
s2= -32.14 MPa
1.5
1
0.5
0
4
6
8
10
12
14
16
18
20
Borehole depth (m)
Figure 1 – Stress inversion results for the 2500 meter deep tunnels of Jinping hydroelectric project, China
B = 20m
Figure 2 – Minimal reinforcement pattern required to ensure stability in columnar basalts
KEYWORDS
Tunneling, numerical analysis, in situ stress, loosening zone, DDA, NMM