application of geogrid reinforced constructions: history

Procedia Engineering 172 ( 2017 ) 42 – 51

1877-7058 © 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of MBMST 2016

doi: 10.1016/j.proeng.2017.02.015

ScienceDirect

Available online at www.sciencedirect.com

Modern Building Materials, Structures and Techniques, MBMST 2016

Application of geogrid reinforced constructions: history, recent and

future developments

Martin Ziegler*

Chair of Geotechnical Engineering, RWTH Aachen University, Mies-van-der-Rohe-Straße 1, 52074 Aachen, Germany

Abstract

Geosynthetics are widely used for separation, protection, drainage, filtration and sealing. In addition, high-strength geogrids have

been more and more successfully used in recent times for the construction of steep slopes and geogrid reinforced bridge abutments,

the crossing of areas with soft soil by using geosynthetic encased sand columns and the bridging of areas susceptible to sinkholes.

Particularly for the last case geosynthetics with additional features have been developed allowing a permanent monitoring of the

deformations. The development of such intelligent geosynthetics with integrated chips and sensors for measuring strains,

temperature or environmental conditions are by no means at its end. Geosynthetics of the future will be equipped with such

additional functions enabling a permanent and non-destructive monitoring of structures built with geosynthetics. Anyhow, geogrid

reinforcement constructions show significant advantages in terms of economic and ecological aspects against classical concrete

structures. It is also well known from large-scale experiments and field tests that geosynthetic-reinforced constructions have a much

higher bearing capacity than calculated and that the deformations are much lower than expected. Therefore, it is quite evident that

the compound behavior of geosynthetic and soil is not yet completely understood. Recent research on this topic using large-scale

triaxial- and biaxial- tests combined with a modern method of visualization of the movement of the soil particles tries to fill this

gap.

2016 The Authors. Published by Elsevier Ltd.

Peer-review under responsibility of the organizing committee of MBMST 2016.

Keywords: Geogrid reinforcement; interlocking effect; triaxial and biaxial tests; particle image velocemetry.

* Corresponding author. Tel.: +49-241-80-25247; fax: +49-241-80-22384.

E-mail address: [email protected]

© 2017 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of MBMST 2016

43 Martin Ziegler / Procedia Engineering 172 ( 2017 ) 42 – 51

1. Introduction

The reinforcement of soil has a long tradition. Already 3500 years ago the Sumerians under King Kurigalzu I

erected the temple of Aqar Quf in Mesopotamia near Bagdad. They used reed mats to stabilize the foundations and

the brick walls (Fig. 1) as they had already understood that both brickwork and soil had nearly no tensile strength and

reinforcement elements were needed to induce tensile forces into their constructions for stabilization. It is remarkable

that it took nearly 1500 years until the Romans invented the so called Opus Caementitium, the ancestor of the concrete

used today with which they were able to construct such impressive monuments like the Pantheon in Rome.

In the last decades, beginning in the seventies, nonwoven and woven fabrics were and are still often used in road

and rail construction working as separators opposite weak, fine-grained subsoils. They prevent coarse granular

material, placed on the formation to construct the base course, being punched into and mixed with the finer-grained

subsoil over time. At the same time they also fulfil a reinforcement function, since, like a tie rod, they reduce deflection

and thus subsidence of the overlying soil package. The stiffness of the geosynthetic plays a crucial role here. The

higher the tensile stiffness of the geosynthetic, the greater the effect described. Therefore, the development of

composite materials made of a nonwoven fabric and a genuine reinforcement product such as a geogrid lead to

significant reductions in settlements. Hight-strength geogrids allow nowadays the constructions of steep slopes of

some decameter in height (Fig. 2).

Fig. 1. Stabilisation of foundations and brickworks with reet mats at Ziggurat temple at Aqar Quf (1500 B.C.) 0.


From many similar applications, we know that the ultimate bearing resistance of those geogrid-reinforced

constructions is much higher than calculated and the observed deformations are much lower than expected 0. This

suggests the assumption that our way of design of geogrid-reinforced constructions is too conservative as the

compound behavior of geogrids and soil is not yet completely understood. In the following sections, some typical

applications of geogrid-reinforced applications are illustrated. In addition, results of recent research on this topic using

large-scale triaxial- and biaxial- tests combined with a modern method of visualization of the movement of the soil

particles are presented. Those results help on a better understanding of the functioning of geogrids embedded in soil.

This paper is based on two other papers of the authors (0, 0).

2. Geogrids and geosynthetics applications

One of the most widely spread applications is the stabilization of a base course above soft ground. The effect of

the use of a geogrids combined with a nonwoven can clearly been seen in Fig. 3, where deep ruts only occur in the

unreinforced case. It is obvious that the cost for maintaining an unreinforced side road over a long period can easily

exceeds the relative small investment of an additional geogrid layer.

If a road or a railway line must cross an area of very soft soil, the base course must be supported on pile-like

elements. Here, depending on the stiffness required, use is typically made of grouted gravel columns or vibro

replacement stone columns. The latter have the advantage that, on account of their permeability, they accelerate the

consolidation in the same way as wick drains. In both cases, however, the soft layer must be covered with a sufficiently

Fig. 2. Geogrid reinforced steep slopes (Huesker).

Fig. 3. Stabilization of a site road a) without and b) with geogrids in combination with a nonwoven fabric .


thick base course, reinforced by one or even several layers of high-strength geosynthetic near its base, to bridge

between the pile-like elements and to transfer the loads into these by arching (Fig. 4).

If the shear strength of the subgrade is too low (cu < 10-15 kPa), the stone columns will not be adequately stabilised.

In such cases, geosynthetic-encased sand columns should be used, in which the hoop stress generated in the

geosynthetic supports the fill material. A steel tube with a hinged cover on the bottom is vibrated into the underground.

After reaching the end depth, the geosynthetic encasement is placed into the tube and filled with sand. After that the

cover at the bottom is opened and the tube is pulled out (Fig. 5a). Geosynthetic encased sand columns can also be

used for the foundation of ring dykes for land reclamation measures (Fig. 5b). One of the first and largest applications

in Germany was the land reclamation project for the production of the new Airbus 380 in Hamburg 0.

A similar task like the bridging between pile-like elements in soft ground is the bridging of sinkholes. Sinkholes

can develop in karst areas, or areas of former mining activities. If such areas need to be crossed by traffic routes, the

major task here is to prevent collapse of the base course if a sinkhole develops (Fig. 6). In conjunction with an early-

warning system, this allows the serious accidents to be prevented. Usually, the road or railway will no longer be

serviceable after a collapse, and repairs will have to be carried out before it is again fully usable.

Fig. 4. Embankment on reinforced concrete piles in soft soil (Huesker): a) cross-section, b) application.

Fig. 5. Geosynthetic- : a) production; b) foundation of a ring dyke.


Very close to the bridging of sinkholes is the coverage of tailings ponds with geogrids and coarse material in order

to make them accessible and to prevent further emissions as they often contain contaminated or radioactive materials

(Fig. 7).

However, the classical applications of geogrids are reinforced walls and steep slopes. Thanks to the development

of high-strength reinforcement elements with high tensile modulus, deep cuttings or high walls with steep slopes can

be built (Fig. 2). The geogrid-reinforced walls can be connected to a wide variety of facing elements and they can be

used for the construction of bridge abutments. Fig. 8, for example, shows such a bridge abutment with gabions as

facing element.

Fig. 8. Bridge abutment with gabion facing (Huesker).

Fig. 6. Sinkholes: a) opening; b) bridging with geogrids (Huesker).

Fig. 7. Bridge abutment with gabion facing (Huesker): a) cross-section, b) application.


3. Ecological benefit of geosynthetics

Summarizing, geogrid reinforced constructions are increasingly used as safe, ecological and economical solutions.

Compared to conventional solutions with concrete they often have the benefit of lower costs and less environmental

impact. This is impressively documented in a broadly based study of the European Association of Geosynthetic

product Manufacturers (EAGM), regarding a total of four case studies comparing a conventional construction method

with one or more alternatives using geosynthetics. The study was conducted by the ETH (Swiss Federal Institute of

Technology) in Zurich and ESU-services Ltd.. Evaluation criteria in this context included the cumulative energy

consumption, the global-warming potential, the photochemical formation of ozone (better known as summer smog),

the fine-dust potential, the acidification with nitrogen and Sulphur oxides, eutrophication, i.e. nutrient enrichment of

watercourses with fertilizer, and the land and water requirements. The study took account of the entire life cycle from

the extraction of raw materials, the manufacture of the product, the transport to and installation on site as well as

dismantling and disposal. For more details, refer to 0.

4. Intelligent Geosynthetics

In the garment industry, textiles incorporating chips based on RFID technology have been reality for a considerable

time now. They allow the wearer to be positively identified and linked with other stored data. While data-privacy

groups register legitimate concerns here, the transmission of data from structures built using geosynthetics is of great

interest for many applications, e.g. for a landfill sealing systems with geomembranes. Thus, electro-resistive sensors

which can detect possible leakage in the vicinity of the geomembrane have already been in use in landfill construction

for more than two decades. Only with these is remedial work at a reasonable cost possible in many cases, since

excavation work can be targeted on the area around the location of damage. While in this set-up the geosynthetic and

the leakage-locating system are still completely separated from each other, new developments seek to integrate the

sensor cable directly into the geosynthetic in order to detect the deformations of geogrid-reinforced structures. Those

are of great importance, particularly for slopes prone to slippage or for the bridging of sinkholes. Large and accelerated

deformations can be observed in real time so that hopefully effective counter-measures can be taken.

In the last 20 years, developments have taken place in this area which have proved their effectiveness in

corresponding research projects and initial applications, but are still not widely known on the market. Bragg grating

sensors based on fibre optics were already used for some years, but these had the disadvantage that the range of strain

measurement was limited and they only provided information from a few selected points. The integration of polymer

optical fibres (POF) into the reinforcement structure of geogrids in the last years was a further step in the direction of

full-area acquisition. POF work on the basis of the so-called Brillouin scattering. The installation of POF e.g. in a dam

enables continuous observation of the deformations so that a beginning collapse is indicated in advance and people

can be evacuated timely.

And, last but not least, information on the correct installation of geosynthetics and the required cover thickness are

of interest. For more details, see 0.

5. Research and theoretical background

In order to understand the functioning of geogrid-reinforced constructions it is helpful to have a look on the

principal behavior of soil under different stress conditions. The left diagram in Fig. 9 shows the range of possible

states of stress in a frictional soil. Only stress states within the club-shaped area are possible. The envelope outline of

this area represents the possible limits. This means that a soil element (brown) must always be subjected to

compressive stresses on three sides, whereas a metallic element (blue) may also be uniaxially loaded or even be

subjected to tensile stresses. In radially symmetric cases such as those pertaining in a triaxial test, all stress states can

be represented on a plane through the vertical 1 axis and the horizontal axis at 3 2. The boundary interface is then

represented by two curves that separate the possible from the not-possible stress states. If the commonly accepted

Mohr-Coulomb boundary condition is assumed to be valid, then the two curves degenerate into a straight line.


If a stress path is considered starting from an isotropic state of stress ( 1 = 2 = 3) and passing along the space

diagonal (green line), then the soil will exhibit only minimal deformations that result simply from the elastic

deformability of the mineral particles themselves, and it can theoretically reach infinitely large stresses. A physical

boundary only exists at the point at which failure of the mineral particles themselves eventually occurs. By contrast,

a stress path in which only the vertical stress is increased while the confining pressure is kept constant leads to the

development of a deviator stress, and relatively quickly to the limit condition (red line). Characteristic of this path is

that the deformations increase greatly at ever-lower increases in stress. A stress path, which runs more or less parallel

to the space diagonal ( 1 = 2 = 3), is therefore favourable with respect to a limitation of deformation and sufficient

distance to the limit boundary conditions (blue line). A stress path like this means that the limit boundary condition is

not reached and the three principal stresses increasingly converge, so that only minimal deformations are to be

expected further along the path. eosynthetic reinforcement altering the stress state in exactly this

direction, as can be shown for various applications (0, 0).

reinforced constructions let us first consider the large triaxial test shown

in Fig. 10. This is 110 cm high and 50 cm in diameter, and the tests were conducted unreinforced, and reinforced with

up to five layers of geogrids. The confining pressure was applied using a vacuum, so that only low stress levels could

be investigated, but these covered the range usually prevailing in sub-base layers. Angular gravel with a particle size

0 - 45 mm was used 0.

Fig. 9. Possible stress states in the principal stress space: 3D-stress-space (left) and 2D-stress-plane (right).


Fig. 11 impressively illustrates the effect of the geogrid reinforcement. With each geogrid layer, the ultimate load

increases and with 5 layers of grid reaches around 2.5 times that of the unreinforced sample. As the number of

reinforcement layers increases, the vertical compression, and in particular the lateral bulging, both decrease.

This constriction effect is obviously caused by the geogrid reinforcement, which induces a supporting effect in two

ways. First, if the material used contains relatively coarse particles compared to the grid opening size, these particles

brace themselves against the grid mesh, significantly reducing lateral spreading (Fig. 12). In the case of loading applied

over larger geogrid-covered areas, this so-called interlocking effect leads to a situation that soil particles can hardly

move laterally, since the individual soil regions in the grid openings buttress against one another. Although the grids

themselves then undergo virtually no deformation, the supporting effect of the geogrid in inhibiting deformation is

still present. This also explains the seemingly paradoxical observation in practice that a geogrid reinforcement also

works when it itself has undergone practically no strain, as can be seen repeatedly in on-site measurements.

Fig. 10. Large triaxial test with geogrid reinforced soil [8].

Fig. 11. Effect of geogrid layers in a triaxial test [8].


If the load is applied in a very concentrated manner, the soil can indeed move laterally, but such movement is

restricted by the geogrid, which is strained by the friction forces and reacts by transferring the activated load as a

reaction force back into the soil. In a triaxial test this means that the geogrids activated in the interior of the triaxial

sample have an effect similar to that of an additionally applied external confining stress. As Fig. 13 shows, the actual

effective stress path in the triaxial test under constant confining pressure does not lead vertically upwards, but is

deflected to the right by this additional confining pressure, so that it reaches the limiting boundary condition at a much

higher stress value. The stress path described thus largely corresponds to the favourable stress path as described in

Fig. 9. For more details see 0, 0.

This beneficial effect of activated reinforcement layers is therefore mainly due to the fact that the three-dimensional

stress state is positively influenced by the forces transferred by the activated reinforcement element. That is done in

such a way that the usually very low stress in the direction of the reinforcement layer is increased, so that the principal

stresses tend to equalise and produce a stress state which is more isotropic than strongly deviatory. This effect occurs

in many geotechnical applications with geosynthetic reinforcement (0, 0), e.g. a base course subjected to loading by a

running wheel, geosynthetic encased sand columns, reinforced bearing layer under a strip footing or the earth pressure

distribution on the facing of a geogrid reinforced wall (Fig. 14). The latter is impressively presented in 0.

Fig. 12. Interlocking of soil particles in the meshes of the reinforced grid.

Fig. 13. Real effective stresses and stress path in a reinforced triaxial test: a) cross-section, b) stress-path.


6. Summary and outlook

Reinforced constructions show in general an increase in bearing capacity and a remarkable decrease in

displacements if we compare the reinforced case with the unreinforced one at the same load. It was shown, that these

effects can be explained by the interlocking mechanism and the subsequent confining effect of the geogrid

reinforcement which leads to a more favorable stress path in the reinforced case by bending the more or less straight

deviatoric stress path in the unreinforced case to a more isotropic one. This means that we move away from the limit

state or we at least approximate the limit state line at a higher stress level resulting in higher bearing capacity and

fewer deformations. Compared to conventional solutions, geogrid reinforced constructions offers remarkable

advantages concerning economic and ecologic aspects. With the development of intelligent geosynthetics an area

covering monitoring of sensitive constructions in real time will become possible.

Acknowledgements

The author wants to express his gratitude to the companies of Huesker Synthetic GmbH and Naue GmbH & Co.

KG for their support and the providing of photos.

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Fig. 14. Large-scale testing device (left) and horizontal displacements obtained by PIV analysis in an earth pressure test (right) [8].

application of geogrid reinforced constructions: history

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