Journal of Natural Disaster Science, Volume 30, Number 1, 2008, pp25-33

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New Protection Wall Against Rockfall Using a Ductile Cast Iron Panel
Diop AMINATA1, Atsushi YASHIMA1, Kazuhide SAWADA1, Eunsu SUNG1,
Kensaku SUGIMORI1, and Shoichi INOUE2
1
Department of Civil Engineering, Gifu University
2
Protec Engineering Co. Ltd(Received for 30 May, 2007 and in revised from 1 Oct., 2007)

ABSTRACTIn Japan, slope disasters occur due to earthquakes, abnormal weather and inappropriate land development. It is
important to identify safe and economical countermeasures against rockfall disasters. We developed a simple,
long-lasting and low-cost structure with maximum impact dissipative action when stopping rockfall. In order tosimulate rockfall impact, various energies of falling rocks were made to collide with real-scale protection structures. The new type of protection wall against rockfall using a ductile cast iron panel is an efficient barrier and has
an effective dissipative function. It also has very good permeability. The structure is flexible and the design andconstruction can take into account of the natural environment and topographic features.

1. INTRODUCTION
In Japan, the risk of slope disaster is very high because of thegeology, weather conditions and the patterns of land use. Rockfall
is a continuous threat not only to inhabitants of mountainous
regions, but also to roads and railway tracks where areas for natural
escarpment or excavation exist. Despite usually involving limited
volumes (Rochet, 1987), rockfalls are characterized by high energy
and mobility, making them a major cause of landslide fatality.
Rockfalls can be triggered by a variety of factors including earthquakes (Kobayashi et al., 1990), rainfall, freeze-and-thaw cycles(Matsuoka and Sakai, 1999) or the progressive chemical weathering of rocks and their discontinuities.
Rockfall, the free movement of stones on steep slopes, occurs
when rocks on a slope becomes dislodged and fall down the slope.
The falling mass often detaches from a steep cliff along geologicalplanes, cracks and joints. A rockfall may be a single rock or amass of rocks, and the falling rocks can dislodge other rocks as
they collide with the cliff. Boulders fall, roll or bounce down theslope according to the slope angle and surface roughness. In someinstances, boulders may also slide down the slope (Matsuo et al.
2002; Ritchie, 1963).
The results of the road disaster prevention survey carried out
in 1996 across the whole country revealed a huge number of dangerous slopes in Japan. As a result, 145,500 slopes were identified
as needing monitoring, and 56,700 slopes as needing urgent disaster prevention projects. In the case of Gifu Prefecture, which is thetarget area in this research, 1,882 slopes were identified as needing
disaster prevention among 5,000 monitored slopes along 4,200 kmof road maintained by the prefectural government. Furthermore,
slope disasters have been increasing every year due to earthquakes,
which have occurred continuously in recent years, and also theabnormal weather and land development. Efficient countermea-

sures against dangerous slopes are required to protect human lifeand property from the danger of rockfall or soil flow at many dangerous slopes in Gifu Prefecture.
The scale of rockfalls is predicted by field surveys, and thenthe energy can be roughly calculated by using the fall height estimated values. From the rockfall energy levels mentioned in thefalling-rocks manual standards, appropriate rockfall protection
structures are selected as shown in Fig.1 (Japan Road Association:
Rockfall Countermeasures Manual, 2000). Although it is important that countermeasure works are determined by energy levels, it
is also necessary to identify construction methods that respond tovarious demands, such as lower environment impact, inexpensive
construction cost and maintenance.

Fig. 1 Choice of protection structures according to the rock fall energy
levelsKEY WORDS: Rockfall, Disaster, Protection wall, Ductile cast iron panel, Shock absorber

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D. AMINATA, A. YASHIMA, K. SAWADA, E. SUNG, K. SUGIMORI AND S. INOUE

There are many studies on the stability evaluations of slope
and soil flow (Sumi and Yashima, 2000). However, with a limited
budget framework and with various environmental problems, disaster prevention programs are not progressing sufficiently.
The most commonly used protection structures against rockfalls are rock sheds, fills, retaining walls, ditches, prevention fences
and prevention nets. These structures are commonly placed close
to the elements at risk and their role is to intercept and stop the
rockfall by transforming kinetic energy to elastic and plastic deformation but also to retain satisfactory performance over time while
not requiring much maintenance. In 1990, the distribution of theprotection structure types was as follows: 46% for fences, 38% fornets, 14% for walls, 1.3% for rock sheds and 0.7% for other methods (Matsuo et al. 2002). New construction methods against rockfalls have been developed based on the locally prevailing circumstances, the degree of danger and the expected event. These barriers are capable of absorbing energy of up to 3000 kJ within a single
rockfall event. Their main idea is not to use a rigid barrier but tostop the rock gradually over a relatively long distance in order toreduce peak load in all barrier s components (Heierli et al., 1981).
These flexible barriers need several components such as steel nets
supported by cables, posts and braking elements acting together.
In this study, we examined new countermeasures against rockfalls and soil flow using ductile cast iron panels. The objective wasto develop a simple, long-lasting and low-cost structure with maximum impact dissipative action when stopping the rockfall. In
order to evaluate the performance of different countermeasures, toenhance the comprehension of their mechanical behavior during
impact and improve their design and repairing methods, we carriedout real-scale field tests.
A ductile cast iron panel of 50 cm in height and 100 cm inlength was used. This type of mesh panel has so far been used forriverbed, slope works, etc (Sawada et al, 2005 and 2006).
A wall using ductile cast iron panels has the following advantages:
1) The design and construction take the environment into consideration by using local ground material, and enable vegetation
growth. Since local, natural stone can be used as the fillingmaterial, a porous structure can be formed and inside water pressure is not accumulated.

Fig. 2 Outline of the first-step field test2) Compared with steel, ductile cast iron has good durability
against salt and acid, and has almost no water retention andenvironmental pollution. It is also recyclable.
3) Since a wall using a ductile cast iron panel is made from a combination of standardized panels, it can be assembled swiftly.
The protection wall using a ductile cast iron panel can be erected
in two days; compared to the two weeks for a geogrid-reinforced
wall and the one month for a concrete wall.
4) Less care is required for compaction and density when the panel
is filled up with boulders compared to a geogrid-reinforced wall.
5) A design, which follows any topography, is possible, and thedissipative process of the impact load takes place easily.
6) A wall using a ductile cast iron panel can be constructed even on
soft ground without using a rigid foundation, whereas an excavation and construction of a rigid foundation are necessary in thecase of a concrete wall.
2. EXPERIMENTAL METHODOLOGIESIn order to simulate rockfall impact, various energies of falling
rocks and imitation rocks were assumed and made to collide withreal-scale protection structures using ductile cast iron panels. In
this study, three experimental investigations were carried out in
2004, 2005 and 2007 as follows.
2.1 First-step field test
Experiments were carried out in Ibigawacho Quarry in
October 2004. Three kinds of loads were allowed to roll and fall
down a 17-m high slope at a 45-degree inclination as shown in Fig.
2. The loads were made to collide with the rockfall protection
walls, which have a ductile cast iron panel frame shock absorbing
material. The cast iron spherical falling mass was of 150, 300, 600
mm in diameter, and 0.36, 1.67, and 5.00 kN in weight respectively.
The impact load test was conducted several times with orwithout panel frame shock absorbing material. Photo 1 shows the
rockfall protection wall, which has panel frame shock absorbing
material and Photo 2 shows a backhoe dropping a rock at the test
site.
After assembling the load receptacle stand and rockfall protection wall made of H-type steel, panel frame shock absorbing material was arranged for the test. One unit of the panel frame was (1.0

Photo 1 Protection wall using ductile cast iron panel

NEW PROTECTION WALL AGAINST ROCKFALL USING A DUCTILE CAST IRON PANEL

Photo 2 Backhoe dropping a rockm x 2.0 m x 0.5 m) assembled, then extended to 6.0 m in length,
1.0 m in width and 3.0 m in height (six steps) was installed in frontof the load receptacle stand. The framework of the panel was filled
up with boulders. The kinetic quantity of the impact load and the
involved deformation were measured. Eight sets of load cells were
fixed to the load receptacle stand and response loads were measured using a digital dynamic strain meter.
The sampling interval was set to 0.2 ms (5000 Hz), and themeasurement time was set to about 3 seconds. For the measurement, several tests were performed under the same conditions.
Visual observation and measurements were carried out on the characteristics of the load collision place at the panel frame shock
absorbing material, the horizontal movement of the protection wall,and the damage of the panel frame shock absorbing material.
Moreover, the falling path of the rock was recorded with a highspeed camera. After that test, the cast iron panel frame wasremoved, and then a direct impact load test was performed on theprotection wall (H-type steel) without panel frame shock absorbing
material. Similar dynamic measurements and visual observation
were carried out.
2.2 Second-step field test
During the first-step field test, a spherical mass was released
to fall down the sloping ground. However, it is difficult to control
the impact forces due to friction between the mass and rough sloping ground when falling. For this reason, free fall collision tests
were carried out in November 2005 in Kakamigahara (Fig. 3).
Two cranes were used for the tests. The two cranes lifted a mass of
5.00 kN to a fall height of 11 m, and then dropped the mass asshown in Photo 3.
A 3-dimensional accelerometer was connected to the falling
load and then the impact force and the energy were computed fromthe acceleration record, and the displacement (interpenetration condition) of the falling weight at the time of collision.
Each physical quantity is computed using the time series data
(sampling frequency 5000 Hz), and the results were compared.
The calculation method of each physical quantity is shown first.
From the time series data of measured falling load acceleration a,velocity v, displacement u, impact force f, protection wall absorbed
energy Ea were calculated. Velocity v is obtained by the time integration of acceleration a through the following formula.

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Fig. 3 Outline of the second-step field test

Photo 3 Experimental condition of the second-step field testv = Ú adt …………………………………………………(1)Falling weight displacement u is obtained by the time integrationof falling weight velocity v, through the following formula.
u = Ú vdt …………………………………………………(2)The falling weight collision direction is considered as positive.
The impact force is obtained by the product of acceleration a andweight m as shown in the following formula.
f = ma ……………………………………………………(3)Protection wall absorbed energy Ea is obtained by the time integration of the product of impact force f and displacement increment Δu:
Ea = Ú fDudt ………………………………………………(4)However, since displacement increment Δu is used, when thevalue of velocity v changes to negative, energy Ea absorbed by theprotection wall is considered constant. Variations in the calculated
values of various physical quantities over time are shown below.
In the second-step field test, in order to examine the perfor-

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D. AMINATA, A. YASHIMA, K. SAWADA, E. SUNG, K. SUGIMORI AND S. INOUE

mance of the wall using a ductile cast iron panel, experiments wereconducted on both a ductile cast iron panel and a geogrid-reinforced wall (Photo 4).
The geogrid-reinforced wall consists of impact catchment
material, which directly catches rockfall and structurally distributes
the impact force to an impact resistant structure, which responds tothe transmitted power. Large bags made of high elastic textiles
filled with similarly sized broken stones were used as the impact
catchment material (soilbags). Although the filling material is not
the most convenient, the bags constitute a flexible impact-absorbing layer. The resistant structure consists of horizontal layers ofgeogrid-reinforced soil of 500 mm in height. The geogrid-confined
geomaterial resists the force distributed by the flexible impact
catchment bag.
2.3 Third-step field test
In February 2007, real-scale field tests were carried out again
in Kakamigahara. Experiments were performed at different impact
points: 50 cm and 75 cm on a ductile cast iron panel wall and 50
cm and 125 cm on a new hybrid design wall combining wire meshpanel and ductile cast iron panel (Photo 5). The front face of the

Photo 4 Geogrid-reinforced wall

Photo 5 Hybrid wall combining wire mesh panel and ductile cast ironpanel

hybrid wall is made with a square cross-section wire mesh and theother faces with a ductile cast iron panel. After assembling theframe of the hybrid protection wall, it was filled up with the boulders in a similar way as the wall using ductile cast iron panel.
In this step of the field test, experiments were conducted in asimilar way as in the second-step field test. A falling load energyof 100 kJ was used in this case as shown in Fig. 4. A 3-dimensional accelerometer was connected to the falling load and the calculation methods of velocity v, displacement u, impact force f, protection wall absorbed energy Ea are the same as those used in the second-step field test.
3. EXPERIMENTAL RESULTS
Figs. 5 and 6 obtained from the first-step field test show the
waveform of the impact force on the protection walls with andwithout panel frame shock absorbing material respectively. The
maximum impact force shown in Figs. 5 and 6 are obtained byadding the measurement of eight load cells installed in the back.
The results obtained from several tests under the same condition
are different because it was difficult to control the impact force due
to friction between the mass and rough sloping ground (Fig. 5).
The analysis of Fig. 6 where there is no panel frame shock
absorbing material shows that the impact force is 10 times larger
than that of Fig. 5 where there is a panel frame shock absorbing
material.
The impact waveform in the case where there is no panel
frame shock absorbing material is very sharp and shows a large
force at the time of collision, and the impact force dissipates in ashort time. However, the dissipation time of the impact force islonger in the case with panel frame shock-absorbing material.
The relation between falling load weights and maximum values of the impact force are illustrated in Fig. 7. If the panel frame
shock absorbing material is used, the impact force is very small,compared to that of the case where there is no panel frame shock
absorbing material. After the impact test with the 0.36 kN load,
collision marks were noticed on the panel frame as shown in Photo
6, however neither fracture nor remarkable damage was observedon the panel. Concerning the 1.67 kN load, the panel frame fractured after the first collision. Even after a second collisionoccurred at almost the same part, the damage range increased but

Fig. 4 Outline of the third-step field test

NEW PROTECTION WALL AGAINST ROCKFALL USING A DUCTILE CAST IRON PANEL

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Fig. 5 (a) 0.36 kN, (b) 1.67 kN, (c) 5.00 kN Waveform of the impact force(with ductile cast iron panel)Fig. 6 (a) 0.36 kN, (b) 1.67kN, (c) 5.00 kN Waveform of the impact force(without ductile cast iron panel)Fig. 7 Relation between load weight and maximum impact force

Photo 6 Panel after collision with 0.36 kN

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D. AMINATA, A. YASHIMA, K. SAWADA, E. SUNG, K. SUGIMORI AND S. INOUE

the filling material remained intact and did not flow out (Photo 7).
For the 5.00 kN load, although no fracture was noticed on thepanel, the collided part curved at a depth of about 25 mm (Photo8). In the case of the 1.67 kN load, the damage is smaller, because
the kinetic energy was absorbed by the slope by bounding and sliding, when the load was slipping down.
Accordingly, the protection wall using cast iron panel andboulders is locally damaged by the rockfall. If the repairing
method and the method of selecting the filling material are established, the protection wall using ductile cast iron panel can be used
for low rockfall energy.
In the second-step field test, the results obtained from the protection wall using a ductile cast iron panel are compared with thoseof a geogrid-reinforced wall. The measured time of all the results
shown below is 0.1 second after the collision time.
Fig. 8 and Fig. 9 show respectively the variations per time of
the acceleration and velocity of the falling loads used in the second-step field test. The variations in time of the barrier-absorbed
energy and the impact force are given in Fig. 10 and Fig. 11. The
relation between the displacement and time after collision is shownin Fig. 12. The figures illustrate the results in the case of thegeogrid-reinforced wall and the protection wall using a ductile castiron panel.
The falling load acceleration shows a negative value after collision, and the velocity is decreasing accordingly (Fig. 8 and Fig.

Photo 7 Panel after collision with 1.67 kN

Photo 8 Panel after collision with 5.00 kN

9). However, in the case of the geogrid-reinforced wall, the falling
load acceleration has a higher value than the wall using the ductile
cast iron panel, and the dissipation time of the acceleration is also
shorter. Consequently, the velocity of the falling mass becomes
negative in the early stage (Fig. 9), and the increase rate of theabsorbed energy is also large (Fig. 10). This means that at thisFig. 8 Acceleration time history (tests in 2005)

Fig. 9 Velocity time history (tests in 2005)

Fig. 10 Absorbed energy time history (tests in 2005)

NEW PROTECTION WALL AGAINST ROCKFALL USING A DUCTILE CAST IRON PANEL

Fig. 11 Impact force time history (tests in 2005)

Fig. 13 Acceleration time history (tests in 2007)

Fig. 12 Displacement time history (tests in 2005)

Fig. 14 Velocity time history (tests in 2007)

energy level, the geogrid-reinforced wall is stiffer than the wallusing ductile cast iron panel.
The displacement-time relationship is illustrated in Fig. 12;
the peak value expresses the maximum magnitude of penetration of
the falling load but in the case of the geogrid-reinforced wall the
peak value is smaller compared with the wall using ductile castiron panel. This result also confirms the stiffness of the geogridreinforced wall. On the other hand, as the impact force is smaller
and the deformation relatively larger for the ductile cast iron panel,the deformation of the whole structure at this energy level showsthat the shock was effectively absorbed.
In the third-step field test, experiments were conducted with afalling weight of 9.70 kN and a fall height of 10.5 m. Fig. 13 andFig. 14 show respectively the variations per time of the acceleration and velocity of the falling mass used in the third-step field test.
The hybrid wall has the higher falling load acceleration than that ofthe ductile cast iron panel (Fig. 13). Consequently, the velocity of
the falling mass changed in the early stage (Fig. 14), and theincrease rate of the absorbed energy is also large (Fig. 15). Thismeans that, the hybrid wall combining a wire mesh panel and ductile cast iron panel is stiffer than the wall using a ductile cast ironpanel. The variation of the impact force is given in Fig. 16. Fig.
17 shows the displacement-time relationship. The peak value,31

Fig. 15 Absorbed energy time history (tests in 2007)

which expresses the maximum magnitude of penetration of thefalling load, is smaller in the case of the hybrid wall compared withthe wall using a ductile cast iron panel. This result also confirms
the stiffness of the hybrid wall.
Photos 9 and 10 show respectively the condition of the hybrid
wall and the wall using ductile cast iron panel after collision. The

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D. AMINATA, A. YASHIMA, K. SAWADA, E. SUNG, K. SUGIMORI AND S. INOUE

Photo 9 Hybrid wall after collision

Fig. 16 Impact force time history (tests in 2007)

Photo 10 Ductile cast iron panel after collision history (tests in 2007)

Fig. 17 Displacement time history (tests in 2007)

filling material of the hybrid wall remained intact and did not flowout (Photo 9).
The results obtained from the third-step field test show that thewall using ductile cast iron panel and the hybrid protection wall
combining a wire mesh panel and cast iron panel are locally damaged by the falling load energy of 100 kJ. With a proper repairing
method and material selection, the use of these types of protection
wall is recommended.
4. CONCLUSION
This study shows the results of real-scale tests of the protection wall against rockfalls. Although there are few actual examples
of application, the features of material configurations and their
excellent performance as shock absorbers are demonstrated.
With the recent reductions in public investment, development
and maintenance of the existing transportation network are essential. Therefore, it is important to identify economic and safe methods and, moreover, to develop a life cycle cost method for rockfall
disaster prevention.
The main conclusions obtained from the tests are:
(1) Local material or surplus soil from other construction sites canbe used. Recycling is possible.
(2) The barrier performs well and it has an efficient dissipative
function.
(3) The proposed protection wall can be adjusted to the site geometry and the design and construction can take topographic features into account.
(4) Construction materials are mainly stone, so vegetation andplanting are possible on the wall face.
(5) The type using ductile cast iron panels has very good permeability (its development can be expected). Considering theseepage effect of ductile cast iron panels, a hybrid design withother methods of construction is possible.
(6) A wall using ductile cast iron panels can be assembled swiftly(in two days) and less attention is required for the compaction
and density when the panel is filled up with boulders. It canalso be constructed even on soft ground without using rigid
foundations.
Since the numbers of field tests are limited, numerical simulations with larger impacts will be carried out in the next step of thisstudy series, in order to investigate the characteristic related-impact
conditions such as stress and strain inside the structure. We will
also propose methods of repairing damaged parts of the panels
after collision with the falling load.
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