Rock Engineering in Difficult Ground Conditions – Soft Rocks and Karst – Vrkljan (ed)
© 2010 Taylor & Francis Group, London, ISBN 978-0-415-80481-3
Optimization of rock mass support systems during deep excavations
Ž. Arbanas
Department of Civil Engineering, University of Rijeka, Croatia
Geotechnical Design Department, Sector IGH Project, Institute IGH, Rijeka, Croatia
M. Grošić
Geotech Ltd., Rijeka, Croatia
D. Udovič
Werkos Ltd., Osijek, Croatia
V. Jagodnik
Faculty of Civil Engineering, University of Rijeka, Rijeka, Croatia
ABSTRACT: During excavations in the rock mass for constructions of roads and buildings in urban
areas, the only parameter that can be predicted is the excavation slope. In urban areas excavations sometimes have to be made vertically with demanding support systems, which require strict technical construction. The deformability and strength properties of the rock mass are determined from geotechnical
investigation and the use of the rock mass classification system. It is then possible to make the geotechnical model and perform the stability and stress-strain analyses. It is first assumed that the rock mass slope
is unsupported during excavation. If a low initial factor of safety is obtained, the stability analysis should
include a support system. The interactive design is applied throughout construction of the support system
and includes extensive monitoring. A re-design of the excavation and additional support measures are
required when monitoring imply on significant deviations of defined conditions.
1
INTRODUCTION
During excavations in the rock mass for the construction in urban areas, the only parameter that
can be predicted is the excavation slope. The excavation slope depends on the rock conditions and
the chosen support system for the excavation stability. In urban areas excavations sometimes have
to be made vertically with safe and demanding
support systems, which require strict technical construction of the support system.
The deformability and strength properties of the
rock mass are determined from geotechnical investigation and the use of the rock mass classification
system. It is then possible to make the geotechnical
model for the rock mass and perform the stability and stress-strain analyses. It is first assumed
that the rock mass slope is unsupported during
excavation, which results in the initial value of the
factor of safety. If a low initial factor of safety
is obtained, the stability analysis should include
a support system. The design support system
will thus result from the stability analysis, which
provides a satisfactory factor of safety during all
phases of excavation.
Due to the uncertainties regarding the rock
mass and the supporting system characteristics
that are included in the described design process, it
is necessary, especially for demanding geotechnical
structures, to use additional methods in order to
secure the structure stability. The interactive design
for deep excavations is applied throughout excavation and construction of the support system and
includes extensive monitoring. According to the
suggested procedures, a new design of the excavation and/or the application of additional support
measures are required when instability of the excavation, or insufficient stability, is detected during
construction. If additional support measures are
necessary, they usually consist of modifications or
optimization of the design support system.
In this paper we are describing the whole process
of interactive design of deep excavations, start with
field investigation, and proceeds with the classification of rock mass and laboratory determination
of rock characteristics, supported by additional
rock strength determination during excavation and
optimization of applied support system. It then
includes extensive monitoring in order to get sufficient knowledge on the actual behavior of the rock
427
mass with the support system during excavation
and construction. The contingency measures consist of design modifications and optimization of
support elements in case when monitoring results
and back-analyses show real behavior of supported
rock mass during excavation.
2
DESIGN OF CUTS IN ROCK MASS
As a rule, the cutting of rock slopes occurs in civil
engineering during the excavation of a building
foundation, or in mining when extracting minerals and raw material, for which the only design
parameter one can generally influence is the slope
of the cut, resulting in the total height of the cut
slope. For civil engineering applications, the time
required to cut the slope is significantly shorter
than the duration the cut must remain stable as
part of the engineering construction. In mining,
economical advantages can be gained in varying
the geometry (slope angle) of the cut, whereas in
civil engineering, the possible consequences of a
slope failure necessitate a more conservative design.
For example, in urban areas cuts are designed with
very high safety factors, often by applying excessive
support; in mining, slopes are sometimes designed
with slope angles that often bring the risk of possible instability.
The cut slope inclination can be vertical or
inclined, depending on conditions in the slope and
conditions of the support construction that ensures
the stability of the cut angle. This often depends
on the relationship of the cost incurred if failure
occurs and the construction cost of the retaining
works. It is clear that in urban environments the
price associated with potential damage claims significantly exceeds the price of ensuring the stability of the cut, which requires strict adherence to
geometry with adequate reinforcement systems,
compared to less conservative design geometries
with minimal support. Therefore, design conditions
in urban environments are frequently on the limit
of practical realization and they demand extremely
safe support systems that need to adhere to technical conditions of the designed works (Arbanas
2002; Arbanas et al. 2003), Figures 1, 2.
Designing rock mass cuts comes down to selecting an adequate geometry and/or necessary support
systems. Stacey (2003) summarizes the design principles in rock engineering as defined by Bieniawski
(1991, 1992). The principles are as follows:
• Design principle 1: Clarity of design objectives
and functional requirements,
• Design principle 2: Minimum uncertainty of
geological conditions,
• Design principle 3: Simplicity of design
components,
Figure 1. Deep excavation of open pit Zagrad in Rijeka,
a view of the open pit during excavation (Arbanas et al.
2006).
Figure 2. Deep excavation of open pit Kantrida in
Rijeka during excavation.
• Design principle 5: Optimization,
• Design principle 6: Contractibility.
Based on design principles the results of the
geotechnical investigation and rock mass classification carried out, strength parameters and rock
mass deformability parameters can be defined
and a geotechnical model developed to perform
a stability analysis. Stability analyses are carried
out depending on the possible failure mechanism,
using one of a number of established methods,
whereby, as a result of the conducted analysis, the
factor of safety is calculated. Most often stability analyses are performed that adopt limit state
(limit equilibrium) methods using one of the
much commercial software intended for this purpose. Stability analyses are carried out with the
initial hypothesis that the rock mass, for a given
cut geometry, stands unsupported in all excavation
428
phases, for which initial safety factor values are
derived. If the safety factor values calculated are
deemed inadequate, the effects of using selected
support systems are then included into the geotechnical model to ensure that the required safety
factor is maintained throughout all phases of the
construction works (Arbanas 2002).
While selecting support structure systems one
needs to take into consideration numerous conditions, including the behavior of individual elements
of the support structure (Windsor 1992, 1996).
Rock mass reinforcement by rockbolts, cable bolts
or pre-stressed geotechnical anchors often form a
constituent part of the support system. The contribution of a single rock mass reinforcement element
is defined, as a rule, by its bearing capacity, and in
order to calculate its impact on the factor of safety
in a stability analysis, its element length and element grid are defined in the geotechnical model.
By including reinforcement elements into the geotechnical model, and depending on possible failure mechanisms on the slope or cut, the position
of a critical failure surface or even critical failure
mechanism can be varied. By conducting stability analyses, a support structure is adopted, and
it satisfies the wanted stability of the geotechnical
model expressed by the wanted factor of safety.
Rockbolts, as rock mass reinforcement elements
in the support structure, are frequently used today
due to several advantages, as outlined by Stillborg
(1994). Rockbolts may take various forms, from
reinforcing bars of standard diameters used in civil
engineering, to specially designed profiles manufactured according to their specific purpose. When
using rockbolts, the main issue is how many rockbolts are required to properly reinforce the rock mass
during cutting of the excavation. In the stability
analysis, the benefits of the rockbolts, with respect
to the cut’s factor of safety, is calculated based on
the bearing capacity of the rockbolts (assuming
that the calculated force value is achieved).
This approach raises several questions as to its
validity:
1. The rockbolt capacity, as a rule, is based on
data from the manufacturer’s booklet for a
given rockbolt or the steel used to manufacture
it and largely based on pullout tests.
2. Rockbolts at the moment of installation do
not represent a rock mass reinforcement element. To serve as a rock mass reinforcing element, a natural interaction must occur between
the forces and displacements in the rock mass
and those then induced in the support structure
at the observed location (Windsor 1996). The
activation of the rockbolts is a consequence of
rock mass displacements that are a consequence
of rock mass relaxation after further deformation that happened during next stages of
excavation. The amount of the forces activated
in the rockbolts directly depends on the amount
of deformation, which is not analysed as a rule
in limit equilibrium stability analysis methods
(Arbanas 2002).
3. By inserting the rockbolt calculated forces into
the geotechnical models at the location of the outside connections of the reinforcement elements
and the rock mass (Windsor and Thompson
1996), the state of stress in the rock mass is significantly changed. Taking into consideration
that the shear strength directly depends on normal stress on the failure surface, it is obvious that
normal stress on the possible slip surface unjustifiably increases by introducing rockbolt calculated forces, and as a consequence, rock material
strength increases as well.
The above points suggest that the stability analysis approach adopted may result in safety factors
that in reality may significantly differ because of
an inaccurate hypothesis of the influence of rockbolts on the rock mass behavior. Determining the
real value of safety factors is achievable only by
conducting stress-strain analyses during the design
phase that give us information on possible rock
mass strain during cutting, from which it is possible to estimate also what the contribution of rockbolts is to the overall rock cut stability. Stress-strain
analyses provide displacement calculations along
points where the rockbolts are installed, which enable calculated values of the activated forces in the
rockbolts to be defined through estimated rockbolt
stiffness values. Only based on the calculated strain
and achieved forces in the rockbolts is it possible to
conduct a cut stability analysis for a rock mass that
can show a correct estimation of the safety factor.
This points to the need of using stress-strain analyses
and slope stability analyses in parallel, especially for
deep excavations where it may be necessary to analyze the different stages of the excavation (together
with the influence of the installation of rockbolts).
Strain results determined through stress-strain analyses, according to excavation phases, will define the
needed support and rock mass reinforcement systems, both with respect to installation geometry and
bearing capacity. Rock mass reinforcing elements
that meet the required safety factors will be used in
a subsequent iteration of stress-strain analysis until
the final required stability is achieved in the rockbolt reinforced rock mass.
3
INTERACTIVE DESIGN OF DEEP
EXCAVATION
An interactive rock mass excavation cut design,
based on the observation method (Terzaghi and
Peck 1967; Peck 1969; Powderham 1998; Nicholson
429
et al. 1999; Szavits-Nossan 2006), was introduced
in the phase of the construction. Rock mass cut
design methodologies were shown by Hoek & Bray
(1977, see also Wyllie and Mah 2004), Hoek &
Brown (1980) and Bieniawski (1991, 1992); and
have been amended to include the selecting of support structures (Windsor and Thompson 1992;
Arbanas 2002, 2004) and the appropriate rockbolts (Stillborg 1994). With the proposed procedure, the need to re-design the cut and/or using
additional support measures could be enacted if
during the excavation process a possible instability was detected or foreseen. If adequate support
measures are proposed, then it still may be necessary to think about the possibility of modifications
that would improve the stability state or optimize
the design solution even further (Windsor and
Thompson 1992; Arbanas 2002, 2004). Stillborg
(1994) indicates the need to supervise all parts of
the project as a unique a design process. It should
be emphasized that the most important thing here
is that the overall control of all activities, from
investigation works, design, execution and monitoring, etc., is in the hands of one engineer, acting
like designer and supervising engineer.
The interactive design phase, or the second
design phase, comes down to the supervising of
the constructing works during excavation and supporting the cut, as well as monitoring all the necessary activities that need to ensure safe execution of
works within the required cut stability limits. Overall activity includes the carrying out of the supervising and monitoring as outlined in the design
(Kovačević 2003), as well as engineering-geological
mapping of the slope including rock mass classification and rock mass strength testing, during the
excavation (Arbanas 2002). Based on the above
information, the following can be asserted said
(Arbanas et al. 2007):
• Based on engineering-geological mapping data,
the in-situ state of the rock mass can be established in the open cut much more precisely than
during the geotechnical investigation works and
the possible failure mechanism in the cut can be
confirmed, as well as rock mass classification
elements Figure 3.
• Data for carrying out the rock mass classification is completed based on rock mass strength
estimations and characteristics of the discontinuity sets present (Bieniawski 1989). Methods to
define the unconfined rock strength include the
Point Load Test (ISRM 1985), by which intact
rock strength values can be defined quickly
and through more samples. Based on the rock
mass classification carried out, the rock mass
properties may be calculated using established
empirical relationships (Hoek and Brown 1997;
Hoek et al. 2002).
• Acquisition of data that enables for the rock mass
deformability modules to be established may
be based on geodetic surveys or displacements
measured by inclinometers and extensometersdeformeters. This is possible when interactive
design (observational method) is used (Arbanas
et al. 2005, 2006; Kovačević and Szavits-Nossan
2006). One of the main elements of interactive
design is monitoring during excavation and
the construction of the support system, which
includes surveying a net of benchmarks and
measurement of rock mass displacements by
inclinometers and extensometers-deformeters,
Figure 4. These measurements make it possible
Figure 3. Engineering-geological cut mapping during
deep excavation of open pit Zagrad in Rijeka with different rock mass classification zones (Arbanas 2002).
Figure 4. The support system and measuring devices
on open pit Zagrad in Rijeka (Arbanas et al. 2004).
430
to determine with some accuracy the values
of the deformation modulus of the rock mass
through back analysis of its stress-strain behavior in which measured and calculated displacements are matched to the level of engineering
accuracy deemed acceptable (Arbanas 2002,
2003, 2004; Arbanas et al. 2003, 2004, 2005,
2006, 2007; Kovačević and Szavits-Nossan
2006). Rock mass deformability modules, as it
has already been described, can be estimated
in correlation with rock mass classifications
(Bieniawski 1989; Serafim and Pereira 1983;
Hoek et al. 2002; Arbanas et al. 2005; Hoek and
Diederichs 2006). However, these estimations in
some cases may be too generalized and cannot
compete with measured values taken from the
monitored location.
• Based on pull-out testing results (ISRM 1981),
stiffness of the installed rockbolts is defined
in the area of the design rockbolt work force.
In combination with measured displacements,
it is possible to define an acting force in the
rockbolt for certain stages of excavation of the
rock mass cut. A more reliable method would
involve installing load cells with the rockbolt
or using extensometer data from one installed
immediately next to the rockbolt (Thompson
et al. 1995).
These estimations enable a back analysis of the
rock mass behavior. The stress-strain back analysis, together with adopted rock mass strength parameters based on the rock mass classification and
measured displacements, are then used to establish
the rock mass deformability modules. Based on
measured displacements and established stiffness
of the installed rockbolts, the values of activated
forces in the installed rockbolts are defined. The
defined force values in the rockbolts are then used
in the cut stability analyses, carried out analysis,
using one of the limit state analyses methods. The
result of analysis provides a closer representation
of the true stability state of the rock cut, reinforced
by rockbolts, expressed in terms of the factor of
safety. This procedure has to follow the cut excavation stages in the rock mass (Arbanas 2002;
Arbanas et al. 2007) and enabled optimization of
the rock support construction.
Geotechnical design, following the described
methodology, better establishes the stability state
of the cut as a function of the calculated values of
the forces in rockbolts used in the stability analysis. Stress-strain analyses are used in defining limit
values of the allowed rock mass deformations in a
rock cut, in the first design phase so as during construction, when deformations also play important
role in construction optimization. Based on measured deformation values during initially stages of
construction and determinations of deformation
Figure 5. Optimization design scheme during deep
excavation.
modules of rock mass it is possible to estimate
finale deformation values and in comparison with
limit values optimize rock support construction,
Figure 5.
Using described methodology numerous deep
excavations in limestone rock mass are successfully
constructed with significant effects of optimization
on rock support structures (Arbanas 2002, 2004;
Arbanas et al. 2003, 2004, 2005, 2006, 2007).
4
CONCLUSIONS
During excavations in the rock mass for constructions of roads and buildings in urban areas, the only
parameter that can be predicted is the excavation
431
slope. In urban areas excavations sometimes have
to be made vertically with demanding support systems, which require strict technical construction.
The deformability and strength properties of the
rock mass are determined from geotechnical investigation and the use of the rock mass classification
system. It is then possible to make the geotechnical
model and perform the stability and stress-strain
analyses. It is first assumed that the rock mass
slope is unsupported during excavation. If a low
initial factor of safety is obtained, the stability
analysis should include a support system. Based
on design principles the results of the geotechnical
investigation and rock mass classification carried
out, strength parameters and rock mass deformability parameters can be defined and a geotechnical model developed to perform a stability analysis.
Stability analyses are carried out depending on the
possible failure mechanism, using one of a number
of established methods, whereby, as a result of the
conducted analysis, the factor of safety is calculated. An active design procedure was established,
which made possible changes required in the rock
mass reinforcement system during excavation of
the cuts. Stress-strain back analysis, based on the
measured deformations, and performed pull-out
tests of rockbolts, enabled observation and the
prediction of rock mass behavior in the cuts to be
applied to future phases of excavation. Measured
strain on measuring equipment enabled analysis to
provide a closer representation of the true stability state of the rock cut, reinforced by rockbolts,
expressed in terms of the factor of safety and
needed optimization of the support structure.
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