The ERFA system prevents a collapse and protects ... - Geobrugg AG


The ERFA system prevents a collapse and protects ... - Geobrugg AG

ERFA sinkhole

protection system

Sinkhole hazard in the roads and highways sector?

The ERFA system prevents a collapse and protects road users

System description / March 2010

ERFA sinkhole protection system / System description / March 2010

Sinkhole hazard in the roads and highways

sector? The ERFA system prevents a collapse

and protects road usersd

Photo on front page: State offi ce for

geology and mining of Saxony-Anhalt /




Sinkholes and depressions occur in connection with the leaching of soluble

rock such as limestone, gypsum or salt or are the consequences of the collapse

of artifi cially created subterranean cavities for instance by the underground

mining of brown coal or lignite. They occur sporadically and are to date, diffi

cult or impossible to predict.

As existing road and highway networks must be constantly adapted and extended

due to demographic development it is often unavoidable to cross

areas with a high risk of sinkholes. Depending on specifi c project requirements

and boundary conditions a highway routes run on dams, on more or less fl at

terrain or in cuttings.

With respect to actions for sinkhole prevention, a distinction is made between

those providing full and partial protection.

Protection methods already established on the market require either a correspondingly

thick covering with spoil or rubble or the insertion of a massively

reinforced concrete slab. Where a road is to be created on a proposed

dam, the principal economic method is certainly the use of high tensile

strength geoplastics. In this case there is a requirement for an adequate covering

in order to activate the arching effect.

Where – with the aim of full protection – very high requirements are to be

satisfi ed with respect to maximum permissible deformations, for example for

German Federal Railways high-speed train routes, then often only rigid protection

measures are applicable, such as the installation of massively reinforced

concrete slabs. Structure changing measures such as the use of explosives,

dynamic intensive compression or the removal and replacement of the

subsoil are outside the scope of this documentation.

Numerous projects are now in existence which - for reasons of economy - favor

the smallest possible covering with rubble for bridging areas with a risk

of sinkholes (in cuttings for instance). Also the concept of partial protection

is often preferred instead of cost-intensive full protection.

The aim of Geobrugg AG, based upon its many years’ experience with protection

measures in the sphere of natural hazards, such as rockfall, avalanches,

earth slips or shallow landslides is to offer an economic system for protecting

roads and highways endangered by sinkholes. In the fi rst place, the required

covering with spoil or rubble should be kept to a bare minimum, and secondly

the rigidity of the system should be selected so as to be able to minimize

the anticipated deformations as far as possible.

This technical documentation describes the newly developed system with its

components, summarizes the results of the performed full-scale fi eld tests

and informs on the installation and dimensioning.


The ERFA system pursues the notion of partial protection with the aim of reducing

the risk of sinkholes and the associated costs to an acceptable level

through cost-effective and simple action. Here, in the case of an event the

system may deform to the extent that a sinkhole is visually perceptible while

protecting the safety of road users. Compared to a rigid structure, it can then

react promptly to prevent a progressive breaking away of the subsoil.

Installation should be able to take place simply and quickly and the required

insertion depth kept to a minimum.

Great emphasis is placed on the high degree of strength of the system elements,

thereby enabling insertion damage to be prevented or kept to a bare minimum.

The system is to be conceived and structured on a modular basis to permit

simple adaptation to different project requirements and changing boundary


It must be determined whether existing dimensioning concepts are compatible

with the effective load bearing behavior of the new protection system. Where

necessary an adequate dimensioning concept is to be elaborated.

Special attention should be paid to corrosion protection and thus adapted to

project-specifi c requirements.

Fig. 1: Examples of sinkholes: Pizza Hut

in Bearver County, Pennsylvania, USA

(above) and Munich, Germany (left)


ERFA sinkhole protection system / System description / March 2010

Fig.2: Photos from the full-scale fi eld

test in Goldach SG, Switzerland



The ERFA sinkhole protection system is characterized by linear, rigid bearing

elements combined with a high-strength diamond-shaped steel mesh acting

as a fl at force spreading element.

Serving for load transfer in the road longitudinal axis are standard reinforce-

ment steel strands of high-tensile steel wire. As a result of the high deformation

rigidity, the linear load bearing elements can transfer considerable forces with

very slight defl ection. A force transfer of 2 000 – 2 500 kN/m is possible without


The system rigidity plus the load bearing capacity is directly infl uenced by the

choice of the number of strands per running meter and their cross-sectional

area. By this means, the system can be optimally adapted to the project-specifi

c requirements and boundary conditions.

The linear load bearing elements are held together by means of the hightensile

steel wire mesh which is placed over them, exerting a spreading effect.

This prevents the breakage of individual strands through the pavement.

Due to optimal interaction with the rubble the anchoring section of the combi

product can be considerably reduced longitudinally.

Through the choice of diameter of the reinforcement steel strands and their

mutual spacing, the load bearing capacity and the maximum defl ection in the

case of an event is adaptable to the project-specifi c requirements.

Linear load bearing elements

According to the supplier data the linear main load bearing elements used in

the standard ERFA sinkhole protection system possess the following characteristics:

Parameter Value

Nominal diameter in inches D = 0.6’’

Nominal diameter in mm D = 15.7 mm

Nominal cross-section A = 150 mm P 2

Weight G = 1180 g/m

Yield stress f = 1590 N/ mm y 2

Minimum tensile strength f = 1770 N/ mm tk 2

Fracture force P = 266 kN


Number of wires 6 + 1

Diameter of the outer wires 5.20 mm

Diameter of the inner wire 5.35 mm

Elongation at maximum load 3.5 %

Contraction Ψ = 30 %

E-modulus (mean value) E = 195’000 N/ mm P 2

Relaxation over 1 000 h, 20°C, 0.70 ftk max. 2.5 %

The following illustration shows a schematic tension-elongation diagram for

a reinforcement steel strand according to the specifi cation in Table 1.

Flat load bearing element

The high-tensile steel wire mesh used with the ERFA system as a force spreading

load bearing element, for instance TECCO ® G65/4, was specially developed

by Geobrugg AG for applications with high demands. It possesses the following


Tab. 1: Characteristics of a reinforcement

steel strand as a linear main load

bearing element

Fig. 3: Tension-elongation diagram for a

reinforcement steel strand with

nominal cross-section A p = 150 mm 2


ERFA sinkhole protection system / System description / March 2010

Tab. 2.: Characteristics of the TECCO ®

G65/4 steel wire mesh

Fig. 4: High-tensile TECCO ® G65/4 steel

wire mesh with a tensile strength of

250 kN/m

Fig. 5: Tensile test of a high-tensile

TECCO ® G65/4 steel wire mesh, net size:

13 x 7 meshes, width of the tested mesh

sample: 1.08 m, fracture load: 280.8 kN,

test carried out on 25.01.2008


Parameter Value

Diagonal x · y = 83 · 138 mm (+/- 3%)

Mesh width D I = 63 mm (+/- 3%)

Number of longitudinal meshes per m n l = 7.2 Stk/m

Number of transverse meshes per m n q = 12.0 Stk/m

Material high-tensile steel wire

Minimum tensile strength f tk = 1770 N/ mm 2

Wire diameter d = 4.0 mm

Tensile strength of a single wire Z w = 22 kN

Mesh tensile strength z l = 250 kN/m

Weight g = 3.3 kg/m 2


In the full-scale fi eld tests in Goldach SG, Switzerland the strands were nonpositively

connected using wire rope clips, topped by the TECCO ® G65/4 steel

wire mesh. This type of connection was expedient for test purposes. For projectrelated

applications however, the wire rope clips are to be replaced by other

elements permitting more rational mounting.

One possibility in place of wire rope clips is the use of aluminum press sleeves,

which can be mounted at the factory. Here it is recommended to join the mesh

over the knots with the strands.

A further possibility is the use of intermeshing metal rails, which surround the

strands and the mesh and are non-positively connected together.

It is recommended to non-positively connect the strands with the mesh at intervals

of approx. 2.5 – 3.0 m.

ERFA systems

The tensile strength of the ERFA system can be optimally adapted to the project-

specifi c requirements by the choice of main load bearing elements and their

number per running meter. Three standard ERFA systems are available.


Parameter Value

Nominal diameter of linear load bearing element D = 15.7 mm

Nominal cross-section A = 150 mm P 2

Fracture strength per linear load bearing element P = 266 kN


Number of linear load bearing elements per m n = 4 Stk/m

Total fracture force per m R = 1060 kN/m


Total cross-sectional area per m A = 600 mm tot 2 /m

Fig. 6: Connecting the mesh with the

strands in the full-scale fi eld tests in

Goldach SG, Switzerland

Tab. 3: Characteristics of the ERFA Light


Fig.7: Schematic representation of the

ERFA Light system


ERFA sinkhole protection system / System description / March 2010

Tab. 4: Characteristics of the ERFA Med


Fig. 8: Schematic representation ERFA

Med system

Tab. 5: Characteristics of the ERFA Pro


Fig. 9: Schematic representation ERFA

Pro system



Parameter Value

Nominal diameter of linear load bearing element D = 15.7 mm

Nominal cross-section Ap = 150 mm2 Fracture strength per linear load bearing element P = 266 kN


Number of linear load bearing elements per m n = 6 Stk/m

Total fracture force per m R = 1590 kN/m


Total cross-sectional area per m A = 900 mm tot 2 /m


Parameter Value

Nominal diameter of linear load bearing element D = 15.7 mm

Nominal cross-section Ap = 150 mm2 Fracture strength per linear load bearing element P = 266 kN


Number of linear load bearing elements per m n = 8 Stk/m

Total fracture force per m R = 2120 kN/m


Total cross-sectional area per m A = 1200 mm tot 2 /m


The ERFA sinkhole protection system is usable for the protection of highways

and roads with asphalt and concrete coverings. To permit subsequent restoration

and problem-free surface removal, it is recommended that the reinforcement

is placed in the frost protection layer and not in the asphalt load

bearing layer.

In reference to the guidelines for road surface superstructure standardization

RStO 01 (edition 2001), this means in construction class II for instance, with

a dimension-relevant traffi c load B of 3 – 10 mill. equivalent 10-t axle transits,

that the reinforcement is installed in the 54 cm thick frost protection layer

with an 80 cm frost-proof superstructure.


To verify the functional suitability of the ERFA system, full-scale 1:1 fi eld tests

were carried out in Goldach SG, Switzerland in October 2008 and February

2009. Installed fi rst was an asphalt load bearing layer and secondly a concrete

slab with thicknesses of 20 cm and widths of 2.5 m. The concrete slab was

not reinforced. The modeled sinkhole exhibited a rectangular hollow with a

free span width of 3.0 m. A total of 17 reinforcement steel strands with the

characteristics detailed in Tab. 1 were installed.

After the form removal, both constructions were loaded with weights. The

deformations of the slab were then measured with reference to the loading.

A total loading of 30.4 tons resulted in a depression in the slab center with

a length of approx. 20 cm. This load corresponds to traffi c regulation loading

for heavy goods trucks with a total load of 600 kN in accordance with DIN

1072. The loading was not able to be increased due to the unavailability of

further weights. The ERFA system showed considerable reserves to be available.

Fig. 10: Placing the ERFA system and

placing the concrete slabs

Fig. 11: Full-scale fi eld test in Goldach

SG, Switzerland, in October 2008

(concrete) and in February 2009



ERFA sinkhole protection system / System description / March 2010

Fig. 12: Break-in model from EBGEO




In the latest draft of the EBGEO (Recommendations on Soil Reinforcement

with Geosynthetics), edition 02/2009, a distinction is made between the break-

in model and the arch model.

According to Figures 1a and 1b, break-in models are more likely to develop

where non-cohesive rubble material exhibits a relatively low layering density,

or where the diameter of the sinkhole is relatively large compared to

the thickness of the rubble layer.

Where the non-cohesive rubble material exhibits a high layering density and

good serration, then with adequate thickness in the bridging zone, an arch

or coupling structure will develop (Figs. 2a and 2b). The broken up material

below the pressure zone increasingly loads the reinforcement with its own

weight and any applied load. Over the course of time the arch can collapse,

in particular with dynamic effects, so that then the reinforcement is fully

loaded by the applied load weight (as in Fig. 1a).

When using the ERFA system, the thickness of the spoil plus the combined

binder and top layers is relatively small compared with the sinkhole diameter

(H/D < 1). As a result no load bearing arch can form. Corresponding to the

break-in model, the total applied load is to be carried through the membrane.

The resulting forces are to be transferred laterally in the road longitudinal axis.

The ERFA system possesses pronounced anisotropic characteristics and hence

can be dimensioned for the level case according to the membrane theory. As

a simplifi cation, the membrane can be regarded as a single rope. This represents

an easily bent load bearing element which can absorb tensile forces only.

In order to estimate the anticipated deformations as a function of the rigidities

of the load bearing elements and the adjacent subsoil, fi nite element analyses

can be performed as a supplement to this or for plausibility checking.


Because the elements of the ERFA system are steel products, appropriate attention

must be paid to corrosion protection. Geobrugg’s many years’ experience

in the fi eld of protection measures against natural hazards is a bonus


The aggressiveness of the environment decisively infl uences the corrosion process.

The rate of wear is generally assumed to be low in rural districts, providing

that road salt is not used locally. In comparison, a signifi cantly greater

corrosion effect is to be anticipated in an industrial or coastal zone.

This is also confi rmed by the results of investigations carried out by Prof. Nünninghoff

on zinc-aluminum coated wire samples which were exposed to corrosion

for up to 21 years. The anticipated wear over a period of 100 years is

approx. 32 μm in industrial zones in comparison with only approx. 16 μm in

rural districts. Moreover the rate of corrosion is essentially dependent on the

microclimate in the immediate surrounds of the steel wire products.

The usual Zn/Al coating of the linear and fl at load bearing elements is 150 g/m 2 ,

corresponding to a coating of 21 μm. According to project requirements, the

Fig. 13: Rope under a given load q(x)


ERFA sinkhole protection system / System description / March 2010

Fig. 14: Results of salt spray tests carried

out by Prof. Nünninghoff; interpolation

through Geobrugg AG


cladding can be increased to 200 g/m 2 (= 28 μm) or 250 g/m 2 (= 35 μm). Stainless

steels can be used where very high demands are placed on the corrosion protection.

If a pure Zn coating is used, a 3 – 4 times shorter service life is to be

expected in comparison to a Zn/Al coating of the same cladding quantity. This

is shown in the following diagram from Prof. Nünninghoff’s study, which represents

the results of salt spray tests on wires with a Zn, respectively a Zn/Al

cladding of 300 g/m2 . The red and blue curves were interpolated through

Geobrugg AG. The 95% Zn / 5% aluminum cladding is known under the trade



The ERFA system is delivered coiled on reels. The linear load bearing elements

are non-positively attached to the fl at load bearing element. The standard width

is 2.0 m. On request the panel width can be adapted specifi cally to a project. The

maximum possible panel width is 3.5 m. The standard roll length is 100 m. On

request the length of a roll can be specially adapted to the project.



The ERFA system is placed using a reel unwinding device. Care is to be taken that

the system is placed as tautly as possible. Normally the system is not actively

tensioned. The ERFA system is to be placed to produce the best possible bonding

with the rubble material. Cavities in the region of the load bearing elements are

to be avoided. The required deformation moduli are to be guaranteed.

The maximum grain size of the preferably broken rubble for the frost protection

layer should be adapted to the opening width of the high-tensile mesh. If the

maximum grain size is signifi cantly smaller than the width of the mesh openings,

steps are to be taken to prevent rubble material falling though on the occurrence

of a sinkhole, for instance by placing a non-woven or fl eece covering.

Longitudinal butt joints

Two basic possibilities exist for the longitudinal butt jointing of the ERFA


1. Overlapping with point connections

2. Use of strand couplings

If the ERFA system is joined by overlapping, the end of the fi rst roll with the

projecting mesh is to be placed over the projecting exposed strands of the

second roll. The strands must mutually overlap suffi ciently so that depending

on the type of connection, the fully load bearing capacity can be transferred

from the one strand of the fi rst roll to the other strand of the second roll.

Fig. 15: End of the fi rst roll with

projecting mesh

Fig. 16: Beginning of the second roll with

projecting strands


ERFA sinkhole protection system / System description / March 2010

Abb. 17: Strand couplings

Depth of the foundation under terrain

Fig. 18: End anchorage possibility


Foundation height

Depth of the anchorage

Foundation width

Foundation of the anchorage

of the ERFA system

A second possible variant is the use of strand couplings according to the manufacturer’s

data. Normally these consist of a coupling head, a coupling sleeve, a

wedge set and a spring.

Transverse butt joints

Due to the pronounced anisotropic characteristics, the lateral interconnection

of the panels is not critical. The edge meshes of the high-tensile steel wire mesh

can, for instance, be interconnected by appropriate clips or shackles.

End anchorage

The execution of the end anchorage is basically dependent on the project-specifi

c boundary conditions. The fi gure below shows one possibility as to how at

least part of the load bearing capacity of the protection system can be anchored

via an end beam.

Length of the anchorage zone

Asphalt wearing course

Asphalt binder course

Asphalt base course


Frost protection layer

The position and dimensions of the foundation are to be adapted to the project

and its boundary conditions. Where necessary this should be additionally backanchored

via anchors and extruded posts.

The withdrawal strength of the protection system in the frost protection layer,

which is directly dependent on the applied load may be taken into account in

the area outside the zone directly infl uenced by the mobilization of the passive

earth resistance. The anchorage of the ERFA system is to be located suffi ciently

far outside the area endangered by the sinkhole.

Reinforcing of the boundary zone

In the event that the boundary zone requires reinforcement, an additional

strand or a spiral rope of comparable rigidity can be installed at the factory or

on-site and fi xed non-positively.

Bends, roundabout traffi c, recesses

By placing additional linear load bearing elements such as reinforcement steel

strands or spiral ropes of comparable rigidity, the load bearing behavior and

capacity can be adapted to special boundary conditions such as in bends, with

roundabout traffi c and in the area of recesses.


The ERFA protection system is adaptable economically and according to proj-

ect-specifi c requirements. It has proved its functional suitability in 1:1 full-scale

fi eld tests.

Due to the high load bearing capacity of the reinforcement steel strands

combined with rigid load bearing behavior, there are only slight deformations

with considerable span widths and infl uences.

Compared with geoplastic solutions, creep deformations are negligible. Steel

products also provide a high load bearing capacity to being driven over during

installation or under transverse stresses.

The ERFA sinkhole protection system is installed in the loose stone layer below

the fi rst bituminously or hydraulically bonded layer or the concrete supporting

layer. Expenditure on earthwork in particular with restorations or in cuttings

is kept to a minimum. The dimensioning concept is adapted accordingly.

The ERFA protection system could also be installed directly in the concrete

slab. This essentially changes the load bearing behavior. Also in this case,

subsequent restoration work will be very costly. Due to the reinforcement,

simple surface removal is no longer possible.

Direct reinforcing of the asphalt slab is not recommended. High loading can

result in the detachment of parts of the asphalt layers which become arched,

endangering road users.


Geobrugg protects people and infrastructures from the

forces of nature

It is the task of our engineers and partners to analyze the problem

together with you in detail and then, together with local consultants,

to present solutions. Painstaking planning is not the only thing

you can expect from us, however; since we have our own production

plants on three continents, we can offer not only short delivery paths

and times, but also optimal local customer service. With a view towards

a trouble-free execution, we deliver preassembled and clearly

identifi ed system components right to the construction site. There

we provide support, if desired, including technical support – from

installation right on up until acceptance of the structure.

Rockfall barriers

Rockfall drapes

Slope stabilization systems

Debris fl ow barriers

Avalanche prevention structures

Open pit rockfall barriers

Special applications

Geobrugg AG

Geohazard Solutions

Aachstrasse 11 • CH-8590 Romanshorn

Phone +41 71 466 81 55 • Fax +41 71 466 81 50 •

A company of the BRUGG Group ISO 9001 certifi ed


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