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 / 

Germany 

2 

INTRODUCTION 

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. 

OBJECTIVES 

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 

conditions. 

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) 

3


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

test in Goldach SG, Switzerland 

4 

ERFA SINKHOLE PROTECTION SYSTEM 

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 

problems. 

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 

tk 

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 

characteristics: 

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 

5


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 

6 


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

Connectors 

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. 

ERFA LIGHT: 


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 

tk 

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

Total fracture force per m R = 1060 kN/m 

tk 

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 

system 

Fig.7: Schematic representation of the 

ERFA Light system 

7


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 

8 

ERFA MED: 



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

tk 



tk 

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

ERFA PRO: 



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

tk 



tk 

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

APPLICATION 

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. 

FULL-SCALE FIELD TESTS 

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 

(asphalt) 

9


Fig. 12: Break-in model from EBGEO 

02/2009 

10 

DIMENSIONING 

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. 

CORROSION PROTECTION 

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 

here. 

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) 

11


Fig. 14: Results of salt spray tests carried 

out by Prof. Nünninghoff; interpolation 

through Geobrugg AG 

12 

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 

name of GEOBRUGG SUPERCOATING ® , GALFAN ® or BEZINAL ® . 

MAKING-UP 

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. 

PLACING 

General 

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 

system: 

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 

13


Abb. 17: Strand couplings 

Depth of the foundation under terrain 

Fig. 18: End anchorage possibility 

14 

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 

ERFA-System 

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. 

CLOSING REMARKS 

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. 

15

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 

www.geobrugg.com • info@geobrugg.com 

A company of the BRUGG Group ISO 9001 certifi ed 

1.402.24.EN.1003

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

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