SPIDER® Rock protection system - Geobrugg AG

SPIDER ® Rock protection system
Use of fl exible spiral rope nets for rock protection – information
gained from large scale fi eld tests – conclusions for practice
Technical documentation / March 2010

SPIDER ® Rock protection system / Technical documentation / March 2010
Use of fl exible spiral rope nets for rock protection
– information gained from large scale fi eld tests,
conclusions for practice
2
Daniel Flum Rüegger+Flum AG, St. Gallen, Switzerland
Armin Roduner Geobrugg AG, Protection systems,
Romanshorn, Switzerland
Daniela Engl AlpS – GmbH, Innsbruck, Austria
ABSTRACT
Flexible rope nets have been used in alpine regions for some decades for
protecting rock slopes against unstable boulders or critical rock masses. The
confi guration of the protection measure was often based on the years‘ of
experience of individual specialists. There was a lack of adequate dimensioning
concepts or extremely simplifi ed models were used. To better understand
the supporting behavior of fl exible rope nets and their interaction with nails
or extruded piles, to thoroughly analyze the force distribution and investigate
the infl uence of dynamic infl uences, comprehensive large scale fi eld tests
were carried out in Felsberg near Chur, Switzerland. This paper summarizes
the information gained and conclusions drawn for the practical application
of fl exible rope nets anchored by nails or extruded piles.
INTRODUCTION
Flexible rope net systems frequently open up interesting and cost effective
application possibilities. Often used in the past or still in use are square or
rectangular nets of stranded ropes with a diameter of normally 8 – 10 mm,
joined at the crossing points with cross clamps or wire coils. Sometimes a
thicker rope running along the periphery is used to strengthen the boundary
zone. A representative example of this system is the PENTIFIX ® system. The
nail pattern is determined by the size and geometry of the panels. This renders
project specifi c adaptation and optimizing of the nailing diffi cult. Originally
such systems were used for protecting creeping slopes. Deformation of the
protection system was taken for granted. In addition to large area applications
fl exible rope nets are often used to protect individual critical boulders which
can endanger persons or infrastructure objects.

Based on material technology innovations it was possible to replace stranded
ropes with their 0.8 –1.0 mm thick individual wires by statically equivalent
spiral ropes with a wire diameter of 3.0 – 4.0 mm. Moreover the manufacturing
process was optimized to the degree that nets can now be produced on
machines in rolls. This has decisively infl uenced the application: Previously the
panels were separately installed and fi xed to nails distributed in the corner
zone of the net. Now 3.5 m wide nets can be placed rationally and fl atly in
rolls up to a length of 20 m and with a weight of 190 kg. These are force-locked
together at the sides by shackles, with minimum slip. The nets can be optimally
stretched over the rock surface using simple spike plates.
Along with corrosion protection which has likewise undergone improvement,
probably the most important new feature is the freedom of choice in the
positions of the anchors. This permits optimal adaptation of the nailing to
the project specifi c circumstances which has a positive infl uence of the supporting
behavior. This step was an essential milestone in the history of rope
nets.
Due to the freedom of choice in the arrangement of the nailing, e.g. surrounding
a critical block, this can be optimally secured via a back anchored net.
Here, optimal means using the least number of anchoring points, securing
the block so that it can only move in its position to the degree permitted by
the project requirements. Assuming the block cannot be directly stabilized
via a dowel or an anchor.
Representative of this new kind of rock protection system satisfying the re-
quirement for a free choice of nail arrangement is the SPIDER ® rock protection
system. The spiral rope used for this application consists of three twisted
together high-tensile steel wires, each 4 mm in diameter, with a yield stress
of at least 1,770 N/mm2 . This spiral rope is fi rst crisscrossed to form the spiral
shape and then twisted together to form a net. The ends of the spiral cables
are tied to one an-other to permit the full transmission of force to the adjoining
panels.
The following photos are intended to provide a brief overview of the application
possibilities of the SPIDER ® rock protection system.
There follows a detailed description of the test concept of the large scale fi eld
test, the performed tests and the determining infl uencing variables. Finally
the knowledge thus gained and the fi nal conclusions are summarized. The
large scale fi eld tests were carried out in the framework of a joint research
project of Geobrugg AG, Rüegger+Flum AG, alpS-Zentrum für Naturgefahren
und Risikomanagement and the University of Innsbruck.
Fig. 1: Taubenlochschlucht near Biel,
Switzerland
Fig. 2: Gondo, Switzerland
3

SPIDER ® Rock protection system / Technical documentation / March 2010
Fig. 3: Blow-out niche as test terrain
Fig. 4: Anchorage arrangement
Fig. 5: General view of the Calanda
massif, west of Chur, Switzerland with
the test location
4
TEST CONCEPT
The objectives of the large scale fi eld tests were to investigate under the
most realistic conditions the supporting behavior of the rock protection
system, the interaction between the system components and the direction,
also the level of the forces transferred from the unstable block via the net
into the individual anchorage points in a 1:1 scale under different boundary
conditions.
For this purpose an ideal site was found at the foot of the Calanda massif
in the district of Felsberg west of Chur, Grisons, Switzerland, which was accessible
to transport vehicles and satisfi ed the work safety requirements.
Thanks to the support of the local community and a local contractor, the
test installation was speedily erected and the necessary auxiliary equipment
made available without problems.
This comprised a U-shaped blow-out niche, open at the bottom. The sliding
surface slopes forward at an angle of approx. 55°, therefore away from the
Rheintal. In this area the banking thickness of the solid limestone is approximately
0.40 m and corresponds to the thickness of the blown-out slab.
The carbonate formation belongs to the Helvetian blanket and was steeply
inclined through the mountain folding process.
The width of the niche is approximately 2.5 m at the top widening to approximately
4.0 m towards the bottom. The length viewed in the line of dip
measures approximately 3.5 m. The lower area of the test site is formed
more or less vertical over a height of approximately 2.5 m.
Directly above the test site a light-weight fence was erected as a work
protection from dislodged small stones, for example by rock goats or by
weathering.
Three nails type GEWI with D = 28 mm were installed above, below and at
both sides of the niche for the point anchorage of the net. In addition two
anchors were installed at the side in the niche itself. This arrangement permitted
tests, taking into account different anchor confi gurations.
Test location

It was also possible to mount a stranded rope with a diameter D = 14 mm
laterally both top and bottom against spiral rope anchors with D = 14.5 mm.
The anchorage length of the nails was 1.0 – 1.5 m and that of the spiral rope
anchor 1.5 m. The nails in each group were approximately 1.0 m apart. Figure
6 shows the anchorage arrangement.
Fig. 8: Positioning the test block
Fig. 6: Arrangement of the anchorages
Fig. 10: Setting up the test installation Fig. 11: Overall view of the test system
without the inclusion of a boundary
rope
5

SPIDER ® Rock protection system / Technical documentation / March 2010
Fig. 10: Test to investigate the infl uence
of the boundary rope on the supporting
behavior and the forces
Fig.11: Hemp rope for releasing the
block
Fig. 12: Measuring the direction and
inclination of the force measuring cells
using a geologist’s compass and an
inclinometer
6
To permit the performance of the tests under the most natural conditions, a
more or less cubical test block from the neighboring stone quarry was used,
whose roughness corresponded to the rock in the area of the test site. Its
weight was 1160 kg. Two lengths of reinforcing bar bent to form loops were
cemented in to the top edge of the test block. These enabled the block to be
suspended and brought into position using a crane rope (Habegger).
The block displacement was measured via a cable pull potentiometer with a
maximum measuring length of 1500 mm.
Specially developed aluminum force measuring plates were used to measure
the forces transferred via the net to the various anchorage points. In this way
it was possible to adapt these optimally to the test conditions.
With large scale fi eld tests the main problem is to record all relevant infl u-
ences but avoid as far as possible unavoidable constraints which are not quan-
tifi able using measuring techniques. Therefore on the basis of many years’
test experience, the anchorages, including the geometry of the niche were
selected so that these could be defl ected and loaded more or less freely and
without constraints. In addition to the electronic recording of the forces, the
direction vectors of the force measuring cells were measured before and after
the test. This enabled the reaction of the net and the boundary ropes on the
anchorages to be determined as realistically as possible. Another test comprised
suddenly causing the block to slip. This was achieved using a hemp rope
which thanks to the friction through twice encircling the suspension, was able
to be released manually.
As a basis for the back calculation the anchor points were calibrated with an
accuracy of +/- 5 cm. The following fi gure shows the spatial position of the
anchorages. A marks the suspension point. Points 13 – 16 represent the positions
of the spiral anchors.

FRICTION TESTS
Before starting with the actual tests using a rope net, friction tests were carried
out as a basis for the subsequent analysis of the forces. The friction angle between
the test block and the sliding surface was able to be determined by making a
distinction between static and sliding friction. The force necessary to hold the
block in place was measured in order to determine the static friction. To measure
the sliding friction the block was lowered in a linear fashion by means of a cable
crane (Habegger) over a length of approximately 0.20 - 0.25 m and the force was
measured dependent upon time and displacement (cf. fi g. 19). The force fl uctuations
are due to the lever movement necessary for actuating the Habegger.
The dark staircase line describes the displacement of the block over time and the
grey line shows the force in the cable over time. The curves up to the start at 17
sec. result from the relieving of load via the chain hoist. The results from the fric-
tion tests were as follows: for the static friction, a friction angle of ϕ = 28 – 29°
H
and for the sliding friction, a friction angle of ϕ = 24 – 25°.
G
Fig. 13: Central perspective side view
showing the spatial position of the
anchor
Fig. 14: Friction tests
Fig. 15: Result of a friction test for
measuring the sliding friction. The dark
staircase line describes the displacement
of the block over time and the
grey line shows the force in the cable
over time. The curves up to the start at
17 sec. result from the relieving of load
via the chain hoist.
7

SPIDER ® Rock protection system / Technical documentation / March 2010
Fig. 16: Overview and confi guration I
Fig. 17: Confi guration II (left) and III
(right)
Fig. 18: Confi guration IV (left) and V
(right)
Fig. 19: Confi guration VI (left) and VII
(right)
8
LARGE SCALE FIELD TESTS
A total of 29 tests were performed. 7 different anchorage confi gurations were
taken into account.

Since a discussion of the test results for all confi gurations would be outside
the scope of this paper, the description and analysis is restricted to two examples.
The following were covered: test 7 with consideration for confi guration
I and test 20 with consideration for confi guration V.
Test 7
The following representation shows the spatial orientation of the force vectors
in the anchorages, both before and after the test. The direction of these
vectors is determining for the equilibrium analysis taking into account their
values.
The progress of the forces over time was able to be recorded with adequate
accuracy using a measuring frequency of 25 Hz. A distinction is made between
the load immediately upon impact (peak) and the subsequent forces (residual
value) when the block is once again at rest.
In test 7 for example the maximum force in anchorage 11 (force in the middle
at the bottom, see fi gure 16) is 11.2 kN which then falls to 9.1 kN. The acceleration
of the block, also the rigidity of the system with the braking process
has a direct infl uence. The more the block can accelerate, the greater the
dynamic effects.
The measuring device permitted the measurement of a maximum of 8 channels
in parallel. So it was possible to measure another 7 forces in addition to
the position pick-up. As a result of this, connection of the force measuring
cell at anchorage 1 was not possible with test 7. In Table 1 those values were
interpolated as appropriate around the block, taking into account the force
distribution.
Fig. 20: Test 6 confi guration I
Fig. 21: Spatial orientation of the force
vectors
Fig. 22: Measured forces in the
anchorages
9

SPIDER ® Rock protection system / Technical documentation / March 2010
Fig. 23: Max. and residual forces
in test 7
Tab. 1: Dynamic effects, residual forces
and relationships in test 7
10
top 1 2 3 1+2+3
dynamic [kN] 5.5
5.8
5.2
16.6
residual [kN] 4.6
4.9
4.1
13.6
bottom 10 11 12 10+11+12
dynamic [kN] 2.7
11.2
0.6
14.5
residual [kN] 2.2
9.1
0.3
11.6
side 5 8 Ø
dynamic [kN]
5.1
4.2
4.7
residual [kN]
4.2
3.6
3.9
force relationship: bottom - top η
dynamic
0.87
residual
0.85
force relationship: side - top ζ
dynamic
0.28
residual
0.29
dynamic - residual relations κDR top
1.22
bottom
1.25
side
1.20
The maximum forces under dynamic actions are essentially dependent on the
acceleration or deceleration during the braking process. The velocity or deceleration
was determined by diverting the displacement over time once or
twice. For example the following maximum values resulted from test 7:
maximum velocity = 1.12 m/s
maximum acceleration = +3.19 m/s 2
maximal delay = –6.58 m/s 2
A comparison of the forces from the dynamic action and the relevant maximum
residual forces with the forces determined purely statically on the basis
of the RUVOLUM ® ROCK concept [1], taking into account a three-dimensional
equilibrium consideration, yields the factors stated in the following
table.

forces from static back calculation kN
total force to be transferred upwards
9.2
total force to be transferred downwards
7.8
total force to be transferred sideways
2.8
dynamic – static relationships κDS top
1.8
bottom
1.85
side
1.69
residual – static relationships κRS top
1.48
bottom
1.48
side
1.41
This example shows that through the dynamic infl uence, the maximum
forces occurring in reality are signifi cantly greater than those estimated
purely statically. For instance the result obtained in test 7 is a factor of κ DS
= 1.69 – 1.85.
Also clear is that the remaining residual forces in the system are still approximately
higher by a factor of κ = 1.41 – 1.48 than those statically de-
RS
termined. This is explained by the fact that through sliding into the net the
boulder also becomes wedged against the subsoil. The system is not elastic
enough to enable the boulder to spring back suffi ciently and to relieve the
net once again. Through this constraint the boulder is protected from sliding
down further due to the mobilization of additional friction.
Test 20
Test 20 is based on confi guration V. 14 mm stranded ropes are braced laterally
at the top and bottom against the spiral anchors D = 14.5 mm (anchorage
points No. 13 – 16). At the top and bottom the net, together with the boundary
ropes is suspended at two anchors. The net is also held by two anchors at
the side. The side edges are not additionally reinforced via boundary ropes.
The following photos show the situation at the start and end of the test. The
rigidity of the protective measure and the associated load transfer are essentially
infl uenced through the arrangement of the supporting ropes.
In contrast to test 7, in test 20 certain forces are also transferred via the top
and bottom supporting rope. This renders an analysis of the force transfer
more complex.
Tab. 2: Statically determined forces and
relationships
11

SPIDER ® Rock protection system / Technical documentation / March 2010
Fig. 24: Test 20 at start of test
Fig. 25: At the end of test, defl ection of
the bottom boundary rope
Fig. 26: Spatial orientation of the force
vectors
Fig. 27: Measured forces in the anchorages
12

Represented in Table 3 are the forces from the maximum dynamic action
(peaks) together with the corresponding residual values. The forces S u and S o
mentioned in Figure 27 correspond to the forces in the top and bottom bound-
ary ropes. The force in anchorage 5 was set on the basis of the results from
the other tests at 50% of the force in anchorage 4. These tests with boundary
ropes exhibited a more or less symmetrical supporting behavior. For this
reason the forces in the side anchorages 7 and 8 were selected identical to
the values in anchorages 4 und 5.
top anchorages 1 3 1+3
dynamic [kN]
9.2
8.8
18.0
residual [kN]
3.3
3.6
6.9
bottom anchorages 10 12 10+12
dynamic [kN]
5.9
3.4
9.3
residual [kN]
2.7
1.7
4.4
side anchorages 4 / 7 5 / 8 4+5/7+8
dynamic [kN]
7.4
3.7
11.1
residual [kN]
2.4
1.2
3.6
boundary ropes top bottom
dynamic
6.5
15.0
residual
2.3
6.1
force relationships bottom : top η
dynamic
0.52
residual
0.64
force relationships side : top ζ
dynamic
0.61
residual
0.52
dynamic - residual relationships κ DR
top anchorages 2.63
bottom anchorages 2.13
side anchorages 3.09
top boundary rope 2.77
bottom boundary rope 2.45
Fig. 28: Max. and residual forces in
graph form
Tab. 3: Dynamic actions, residual forces
13

SPIDER ® Rock protection system / Technical documentation / March 2010
Tab. 4: Statically determined forces and
relationships
14
Compared with test 7 the acceleration path was approximately twice as long
at 1050 mm. This directly infl uenced the maximum velocity and the accel-
eration. The following maximum values were measured in test 20:
• maximum velocity = 2.30 m/s
• maximum acceleration =+6.42 m/s 2
• maximum delay = –15.67 m/s 2
The acceleration in test 20 was higher by a factor of 2.0 compared to test 7
and the delay was even greater by a factor of 2.4. Through the arrangement
of the boundary ropes the protection system had a more rigid action. The
fact that in test 20 the 2 side anchorages contributed certainly also had a
stiffening infl uence. The relationship of the upwardly transferred forces to
the total forces directed downwards falls to η = 0.52 - 0.64. On the other
hand the side anchorages gain infl uence with ζ = 0.52 – 0.61.
If the forces from the static back calculation are compared with the measured
dynamic maximum values the resulting relationship is κ = 2.3 – 2.7. An
DS
interesting point is that the κ relationships are almost 1.0. This means that
RS
through the arrangement of the supporting ropes the boulder is less clamped
and the system behavior is signifi cantly more elastic. In this case the residual
forces can be estimated with suffi cient accuracy with a simple equilibrium
consideration.
forces from static back calculation kN
total force to be transferred upwards
6.8
total force to be transferred downwards
4.1
total force to be transferred sideways
4.1
dynamic – static relationships κDS top
2.6
bottom
2.3
side
2.7
residual – static relationships κRS top
1.0
bottom
1.1
side
0.9

KNOWLEDGE GAINED AND CONCLUSIONS
FOR PRACTICE
Spiral rope nets, fabricated like mesh cover, provide new possibilities for securing
unstable boulders prone to come loose on steep slopes due to their
high longitudinal and transverse tensile strength and their high knot strength,
which is important if the anchorage is subjected to a point force.
The forces measured in before made model experiments scaled 1:3.5 in reallife
scenarios were congruent with the results derived from the simple twodimensional
theoretical model.
The large scale fi eld tests showed the practical suitability of rope nets such
as e.g. the SPIDER ® rock protection nets. Further, a range of tests enabled
the direction and amount of the force vectors to be determined dependent
on the arrangement of the anchorages. The acceleration distance of the
boulder played an important role here. The tests yielded the following information
and conclusions for practice:
If a critical boulder is calculated purely statically on the basis of an
equilibrium consideration the forces in the anchorages can sometimes
be massively underestimated. As shown from the tests, the forces
from the dynamic infl uence exceed the statically determined forces
by a factor of 1.5 – 2.5 or more. Consequently a dynamic factor κ is D
to be taken into account when dimensioning fl exible rock protection
systems.
In principle the forces are more likely to be transferred upwards. The
size of the relationship η of the upward forces to the downward
forces depends on the nature of the meshing of the boulder with the
rope net and whether boundary ropes are installed.
The large scale fi eld tests have shown that when using a large mesh
net for securing individual boulders, boundary ropes are to be fi tted
to the top and bottom and where possible also at the sides. This can
essentially improve the supporting behavior of the system.
The dimensioning of fl exible rock protection systems can be carried
out using a simple model based on the equilibrium consideration. It is
obligatory for the individual relationship factors and above all the
dynamic effects to be adapted to the local and project specifi c
circumstances.
LITERATURE
[1] Rüegger, R.; Flum, D.: Eine neue Generation von Spiralseilnetzen zur Sicherung von Felsbö-
schungen – Versuche, Bemessung, Anwendungsbeispiele. Technical Academy Esslingen, 6th
Colloquium: Bauen in Boden und Fels, January 2008, Ostfi ldern, Germany
15

Geobrugg protects people and infrastructures from the
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