TN-The Design of Green Terramesh Embankment against Rockfall

TECHNICAL NOTE
Rev: 02, Issue Date: April 2012
GREEN TERRAMESH® ROCKFALL PROTECTION EMBANKMENTS
1. Introduction
The “correct” solution to a rock fall problem must necessarily depend on the site conditions, the nature
of the problem and the finances available to pay for the solution over both long and short terms. The
protection strategy can involve the installation of systems (such as high strength meshes) which are
designed to retain the rocks “in situ” on the slope. Alternatively the most suitable strategy may involve
the installation of a system designed to prevent falling rocks from impacting vulnerable structures/areas.
In the latter scenario, the client or engineer is presented with two main options to intercept falling rocks
and prevent them from causing damage; to install either a dynamic rockfall fence or to install a rockfall
protection embankment.
Fig. 1: Comparison of successful rockfall interceptions of an embankment (>7500kJ) and a dynamic rockfall fence (2200kJ)
Dynamic rockfall fences are highly effective at intercepting individual falling blocks and falls composed
of numerous smaller rocks. Indeed modern fences (such as the Maccaferri CTR and RMC fences) are
developed and tested to be able to accept multiple impacts (up to 5000kJ) without failure. A
disadvantage of rockfall fences is that they undergo un-recoverable (plastic) deformation during the
process of interception. This dictates that a following a successful interception, maintenance of the
system is required, including replacement of any “spent” components, such as energy dissipaters. This
resets the system and prepares it for subsequent impacts.
In contrast to dynamic rockfall fences Green Terramesh reinforced soil embankments have a variety of
advantages, primarily their theoretically unlimited energy absorption (>5000kJ) and debris volume
capacities. Reinforced soil embankments offer the additional advantage of multi-functionality in that they
can be designed to offer effective protection from rock falls, debris flows and avalanches. Another
strength of the reinforced soil embankment is its capacity to accept rockfall (and other) impacts whilst
only requiring minimal (if any) maintenance aside from clearing the intercepted material.
Maccaferri Green Terramesh embankments for rockfall protection have been designed and constructed
worldwide and have proven to be successful, cost effective and reliable solutions.
1
Maccaferri reserves the right to amend product specifications without notice and specifiers
are requested to check as to the validity of the specifications they are using.
2. Green Terramesh Rockfall Embankment
The Green Terramesh® reinforced soil embankment system is an effective solution for protection against
high energy rockfall impacts. It is extremely cost effective with respect to both price and maintenance.
The embankment is constructed from high specification, factory prepared Green Terramesh® modular
units. The units are fabricated from double twist steel wire mesh and feature a standard face inclination
of 70 degrees. The steel wire mesh is galvanized with Galmac (Zn + 5%Al) and additionally PVC
coated.
Figure 2: Modular Green Terramesh® reinforced soil system unit
Figure 3: Completed Green Terramesh embankment (small
size) prior to establishment of vegetation.
Figure 4: Construction of 11.5m high Green Terramesh
rockfall embankment in European Alps.
For an embankment to work effectively, the design should take account of the following:
1. The embankment height shall be sufficient to intercept all necessary rock trajectories
2. The area upslope of the embankment must provide sufficient volume to accumulate fallen rocks
3. The embankment must have sufficient thickness and density in order to prevent the rocks from
penetrating through the embankment.
When compared to a dynamic rockfall barrier, a Maccaferri reinforced soil embankment will be able to
absorb a higher total energy of rockfall impacts and require little or no maintenance.
2
Maccaferri reserves the right to amend product specifications without notice and specifiers
are requested to check as to the validity of the specifications they are using.
Feature
Green Terramesh Embankment Rockfall Energy Fence
Energy absorption
capacity
•
•
Tests up to 5000kJ
Computational checks to >
5000kJ for higher capacity
systems
•
Current products tested up to
5000kJ
Resistance to multiple
maximum energy level
impacts
•
Yes
•
Yes (depending on specific fence
type)
Downslope deformation of •
structure from impact
•
Very low-no downslope
deformation
Falling bodies are retained
behind, or embedded within the
embankment
•
Yes (current fences are all
‘dynamic’ or semi-dynamic
systems)
Ability to intercept
•
maximum velocity impacts
Very high (no theoretical limit)
•
Variable (~35m/s maximum)
depending on fence type and
manufacturer
Installation in immediate
proximity to vulnerable
infrastructure
•
Yes (due to low/negligible
deformation of structure)
•
No. Minimum standoff distance
required due to required
elongation of the fence (varies
between fence types and
manufacturers)
Maintenance requirement
after low energy impact
•
None (under normal
circumstances)
•
Variable (depending on fence type
and manufacturer)
Installation tolerances
(geometric)
•
Few specific requirements
•
Small geometric installation
tolerances (to ensure correct
functioning)
Required slope
•
topography for installation
Suitable only for medium to low
gradient slopes/sites
•
Can be installed on any type of
slope and in any orientation
Obstruction to wildlife,
human and vehicle
passage on slope
•
Comparable to a fence
•
Comparable to an embankment
Cost of installation of the
structure
•
Most cost effective >3000kJ
•
Most Cost effective < 4000kJ
Certification and testing
•
•
No certification
•
Tested in accordance to standard
UNI 11167
•
Variable depending on
manufacturer
Commonly tested and certified
according to ETAG 027 (Europe)
Table 1: Feature comparisons between rockfall embankments and rockfall fences
Table 1 above contrasts the principle differences between reinforced soil embankments and dynamic
rockfall fences. Maccaferri reinforced soil embankments can provide many advantages over dynamic
rockfall fences, principle among which is their unlimited maximum energy and volumetric capacity.
3
Maccaferri reserves the right to amend product specifications without notice and specifiers
are requested to check as to the validity of the specifications they are using.
3. Design Methodology
The dynamic impact characteristics of falling rocks is usually determined by statistical evaluations that
are developed with numerical simulations of trajectories, evaluated on a case-by-case basis. This is
usually carried out by computer software packages. The kinetic energy of the impact is determined
considering the translational velocity and the mass of the falling block.
While the height of the embankment is based on the calculated impact trajectory height, the
embankment thickness is proportional to the energy (and therefore force) of the impacting rock. A
procedure, developed by Maccaferri in conjunction with the Polytechnico Di Torino (Italy), presents a
simple graphical approach which determines the greatest expected impact penetration into the Green
Terramesh embankment and thereby enable the dimensioning of the embankment. Using this
methodology, Maccaferri have designed and built embankments to accommodate up to 20,000kJ
energy, with heights above 15m using the Green Terramesh reinforced soil embankment system.
As part of the development process, Finite Element Method (FEM) analyses were performed to evaluate
the effects of block impacts on Green Terramesh embankments. Different impact and embankment
construction characteristics were evaluated as part of this FEM work and design charts were calculated.
These enable engineers to specify the correct embankment geometry for a given impact scenario.
Note: A full paper on the numerical modeling of the embankment and general aspect on the design
procedures is available.
Figure 5: FEM modelling of the impact of a cubic, rigid body on a
Green Terramesh reinforced soil embankment
4
Figure 6: Real case of “piercing impact on
Green
Terramesh
reinforced
soil
embankment (7500kJ impact)
Maccaferri reserves the right to amend product specifications without notice and specifiers
are requested to check as to the validity of the specifications they are using.
Figure 7: Derived maximum penetration of an impacting block in relation to impact energy
Figure 8: Indicative embankment layout and definition of the relevant embankment parameters
5
Maccaferri reserves the right to amend product specifications without notice and specifiers
are requested to check as to the validity of the specifications they are using.
Based on Figure 7 & 8 above, the maximum impact energy on Green Terramesh embankments and
their relationship with the bounce height with boulder size can be estimated. For a Green Terramesh
embankment with a minimum top crest width of 1.1m, with a condition that the minimum width of the
embankment at the point of impact is at least 2 times the penetration depth; an indicative embankment
height with the anticipated energy capacity can be produced. This is summarized in Table 2 below. A
Factor of Safety of 1.50 has been introduced to the bounce height for the estimation of minimum
embankment height.
Bounce Height (m)
1.0
1.5
2.0
2.5
Max Impact Max Impact
Boulder Size GTM Height, Fill Volume/m, GTM Base
Energy (SLS) Energy (ULS)
3
(m)
H (m)
(m )
Width, B (m)
(kJ)
(kJ)
0.90
2.40
4.74
2.85
700
N.A*
1.56
3.00
6.58
3.30
1600
3500
1.56
4.20
11.04
4.20
1600
3500
1.96
4.80
13.67
4.60
2000
9000
1.56
4.80
13.67
4.60
1600
3500
1.96
5.40
16.55
5.00
2000
9000
1.56
5.40
16.55
5.00
1600
3500
1.96
6.00
19.70
5.50
2000
10000
Table 2: Maximum Impact Energy on Green Terramesh embankment and indicative required height
*Note: Boulder size and bounce height is not able to produce high energy for ULS condition
SLS (Serviceability Limit State) and ULS (Ultimate limit state) are defined as follows:
SLS - The penetration depth at the up slope side following the impact is lower than 20% of the
embankment thickness at the impact height and not greater than 70cm deep.
•
ULS– The deformed shape of the Green Terramesh reinforced embankment after the creation of
the crater during the impact is no longer stable statically.
•
Thus, the SLS conditions have to permit an easy
maintenance of the structure, simple patch up
repair is possible and the embankment will be able
to absorb further multiple rockfall impacts. ULS
condition is the energy level that would cause the
reinforced embankment
to collapse and
reconstruction is required on the impacted section.
That is, the embankment is no longer stable to
take another impact.
Note that Table 2 above is produced based on a
minimum top crest width of 1.1m, the ULS energy
can be enhanced if necessary by increasing the
top width.
Figure 9: Definition of Green Terramesh embankment
geometry
6
Maccaferri reserves the right to amend product specifications without notice and specifiers
are requested to check as to the validity of the specifications they are using.
4. Performance
A residential dwelling in Sumner, Christchurch, located at
the foot of a weathered rock slope, has long been at high
risk from rock falls. In August 2006, loose rock fell from
the slope and hit one of the nearby building. This
prompted the property owner to lodge an application with
the EQC.
After analysis of solutions in the market, a Maccaferri
Green Terramesh embankment was selected as being
most appropriate for the conditions, anticipated loads and
minimal future maintenance requirements. Maccaferri
provided technical assistance to the project designer; the
final embankment dimensions were 3.0m total height with
1.5m embedment on the downslope side. A containment
area was excavated between the upslope face of the
embankment and the toe of the rock slope. Construction
began in mid-June 2010 and was completed by early
August 2010.
Completed GTM embankment with rockfall face
During the September 2010 earthquake of Mw 7.1 in
Darfield, a number of falling rocks (up to approximately
250mm in diameter) were successfully stopped and
contained by the Green Terramesh embankment. Due to
the relatively small size of the fallen rocks, no penetration
was observed into the embankment face.
Green Terramesh embankment after Sept. 2010 earthquake
Subsequent inspection of the embankment noted impact
penetration of up to 250mm on the upslope side of the
embankment while no damage or displacement was
visible on the downslope (house) side of the
embankment. This clearly demonstrated the capacity of
Green Terramesh embankments.
The Green Terramesh embankment
protected the property and its occupants.
undoubtedly
Green Terramesh embankments have been designed and
constructed successfully worldwide for rockfall protection
and are proven to be effective reliable solutions. They can
permit the absorption of multiple very high energy impacts
without the requirement for extensive, complicated or
expensive maintenance works.
Green Terramesh embankment after Feb. 2011 earthquake
Maccaferri NZ Ltd.
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Tel. (+64) 9 6346495 - Fax (+64) 9 6346492, FREEPHONE 0800 60 60 20
E-mail: sales@maccaferri.co.nz - Web site: www.maccaferri.co.nz
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The information presented herein is, to the best of our knowledge and belief,
correct and is subject to periodic review and revision. The validity of the
information relative to the subsoil, hydraulic and other engineering conditions
must be ascertained by a suitably qualified person. No warranty is either
expressed or implied. Unauthorised reproduction or distribution is prohibited.
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© 2010 Maccaferri. All rights reserved. Maccaferri will enforce Copyright.
The aftershock with Mw 6.3 which struck Christchurch in
February 2011, caused multiple rock falls from the slope.
This site location is approximately 5km from the epicenter
of the earthquake. All rockfalls were successfully stopped
and contained by the same embankment. The largest
individual rocks to fall were up to 2.5m in diameter, the
total volume of fallen material was estimated to be 200
m3. The energy levels of individual block impacts were
estimated at between 700kJ and 2,600kJ.