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Technical Paper: Slope stabilisation and erosion protection in a single operation

figure 4

Eberhard Gröner and Armin Roduner, Geobrugg

Arup Tech Paper logo (NB)

Arup Tech Paper logo (NB)

For a successful and long-term stabilisation of natural embankments and new cuttings in loose rock, their effective revegetation is very important. In combination with hydro or dry seeding, so-called 3D erosion control mats made of polypropylene are very effective.

On the one hand, the 3D structure reduces the impact energy of raindrops, and on the other, the drag force of the draining water is reduced. In addition, soil particles as well as organic substances such as small roots, branches and leaves can be retained in the loop structure, whic improves the natural nutrient supply.

In order to reduce the negative effect of heating by solar radiation which occurs when black variants are used, systems colour-matched to the soils result in less heating and better vegetation growth.

Geotextiles, which are used as planting aids, usually have only a low strength and are therefore only effective for low slope angles, or in combination with a stabilisation measure such as wire mesh.

In the last 15 years, meshes made of high-tensile steel wire for slope stabilisation in combination with soil nailing have become established. They can transfer larger forces and thanks to a very high puncturing resistance are better to introduce the load into the nailing. Their dimensioning for the stabilisation of superficial instabilities can be carried out with a design software, such as Geobrugg’s Ruvolum concept.

In order to increase the range of application of the erosion control mats described above, they were combined during production with a light mesh made of high-tensile steel wire to increase their load bearing capacity to 53kN/m. They are mainly used as roll-off protection and/or greening aids without nailing to the ground. The experience gained has now been used to produce a product that combines the advantages of high-tensile steel wire mesh and the erosion control mat. Loads of 150kN/m and puncturing resistances of 180kN can be introduced into the nailing when using a system spike plate.

This paper shows the development and application of both systems. Thanks to the systems described above, it is now possible to efficiently secure steep soil embankments in combination with a soil nailing system and to revegetate depending on the location.

1.0 Introduction

The creation and widening of infrastructure such as roads, railway lines and buildings, requires many new cuttings and the steepening of embankments in loose ground and rocks (Figure 1).

figure 1

figure 1

Figure 1 - Ad hoc slumping on a slope in the grounds of a sports facility

Ideally, the slopes should be at a low angle of inclination to avoid compromising the stability. If this is possible, simple greening aids such as erosion control mats are usually sufficient to protect against washouts. However, these only have a low strength and quickly begin to strain when steeper embankments have to be secured or when there are superficial or even global stability problems (Figure 2).

figure 2

figure 2

Figure 2 - Slumping on an embankment with a slope at approx. 25°, secured with a geotextile/erosion control mat for greening without nailing.

1.1 Support of greening with erosion control mats

An effective and functional layer of vegetation must be encouraged and/or produced to ensure the long-term stabilisation of embankments, (Rüegger, 2004 and 2006). Newly cut loose rock slopes are, for the most part, poor in nutrients due to the removal of the topsoil (Bosshard, 2013). Since the layer of vegetation is removed there is a lack of roots and natural seed deposits.

In principle, external erosion must be reduced or controlled. In doing so, the primary erosion caused by the so-called drop effect is distinguished from the secondary erosion caused by the flow effect (Figure 3). An erosion protection mat should minimize the impact energy of the droplets and retain soil particles in the draining water (Rüegger, 2006).

figure 3

figure 3

Figure 3 - Erosion channels caused by the flow effect of the draining water, which remove the soil and reduce and/or prevent the growth of plants.

There are a variety of erosion control mats and geotextiles available to support the greening of loose rock slopes, (SN standard 640 550, Geotextilhandbuch, 2003). In principle, there are two different starting materials: synthetic geotextiles – made of polypropylene – and natural geotextiles made of organic materials, such as jute or coconut fibers. Organic natural products can be used for flatter slopes. The advantage is that they store some of the water and release nutrients as they decompose, thereby promoting plant growth. In steeper slopes, synthetic products offer advantages thanks to their low weight, which only changes when permeated by water. The synthetic options also remain resistant for longer in comparison with the organic alternatives.

1.2 Flexible slope stabilisation systems using steel wire mesh

Flexible slope stabilisation systems made of steel wire meshes in combination with a soil nailing system are now widely used to secure loose ground and rock slopes. These systems have proven to be very effective if applied correctly (Figure 4). However, due to the lack of basic principles such as standards and directives, often no or insufficient requirements are demanded for the securing systems and verification of the stability is often neglected.

figure 4

figure 4

Figure 4 - Slope stabilisation with high-tensile steel wire mesh and erosion control mat

As a result, suppliers offer and/or install systems that are not adapted to the local static conditions or with components that are not matched to one another. This means that the systems may fail with potentially fatal consequences resulting from the material breaking, as well as drastic levels of instability. The load-bearing resistance of a system as a whole as well as of its individual elements (nets, meshes and attachments) must be known so that suitable dimensional models can be used for the necessary verifications of the stability (Rüegger, 2002 and Flum, 2014).

1.3 Dimensioning to prevent surface-related instabilities

Superficial instabilities can be measured according to the Ruvolum dimensioning concept. It is universal and can be used for dimensioning the securing systems of superficial instabilities in loose ground as well as for heavily loosened rock slopes. The bearing resistances form the basis for the dimensioning; these are determined in realistic tests that can be repeated. Rüegger described the Ruvolum dimensioning concept in detail in 2002 and 2004.

This concept includes the study of superficial slope-parallel instabilities (Figure 5), as well as the investigation of local instabilities between the individual nails (Figure 6).

The influence of excess hydrostatic pressure, flow pressure and seismic forces can also be taken into account.

figure 5 web

figure 5 web

figure 6 web

figure 6 web

2.0 Evaluation of an erosion control mat

Initial tests were carried out in 2000 to evaluate a suitable erosion protection mat for steeper slopes in combination with high-tensile steel wire mesh. These were expanded in 2003 with an erosion control mat that offered a wide range of applications.

The following requirements were defined in cooperation with Rüegger Flum:

  • Good permeability through spraying for wet and dry seed to get as many seeds as possible onto the substrate
  • Good adaptability to the ground
  • Low surface weight, even with the ingress of water
  • Good adhesion of the mat to the substrate/low risk of slipping
  • Good retention properties for soil particles, organic matter and seeds
  • Color-matched for a natural look/less heating

During the trials in 2000, various erosion protection mats were erected in a test area in Valais, Switzerland.

The following variants were used (Figure 7):

  • Area 1: Three-dimensional, very dense, three-layer anti-erosion mat made of polypropylene black.
  • Area 2: Three-dimensional corrugated erosion protection mat made of polypropylene black.
  • Area 3: Two-dimensional, flat geogrid made of polypropylene black.

figure 7

figure 7

Figure 7 - Test areas for the evaluation of an erosion protection mat.

The test area faced south-west, in a sparse location with extreme changes in dry and wet conditions. The greening was performed using a dry seeding process. The most successful greening was achieved on the three-dimensionally corrugated mat on area two. It is assumed that the retention properties of a two-dimensional, flat erosion protection mat, as used in area three, are too low for soil particles and seeds. For area one, it is assumed that the lower albedo of black surfaces causes a greater heating of the slope, which has led to the seeds drying up quickly, or not germinating at all.

Working off the basis of the previous tests, field tests were carried out in 2003 with a three-dimensional geomat. The aim was to study the mat’s retention properties, its adaptability to the substrate and the extent to which it could be permeated when sprayed. The shortlist included an 18mm-thick erosion control mat made of extruded monofilaments and a surface weight of 600g/m2. Its proportion of cavities is > 95%. Figure 8 shows the test areas in Bischofszell, Switzerland.

figure 8

figure 8

Figure 8 – Test areas and test frames at the time of sowing

The erosion control mats (geomats) as described above were inserted on the far right. The other surfaces are reference surfaces without erosion protection mats and/or similar products made from polypropylene. In order to be able to make claims about the permeability, wooden frames with an area of 1m2 were used. These can be seen at the bottom edge of the photo.

Verdyol germinated the seeds in the test area through hydroseeding. In this process, water is mixed with the seeds, a mulch and an algae-based adhesive. The mulch is a so-called export mulch made from hollow fibres <4mm in length. Roughly four weeks after the seeds have been sown there was a heavy rainfall over the test area, which caused superficial landslides on the slope. A further six weeks later, a study of the area revealed that the seeds in the reference area were reduced to about 10%. Correspondingly low plant growth was visibly noted. More than 90% of the surface in the area with the geomat, which had an incline of 30 – 45°, was covered with a layer of vegetation (Figure 9).

figure 9

figure 9

Figure 9 – Test area approx. 10 weeks after the hydroseeding process and after heavy rainfall. On the left the reference surface without an erosion protection mat with less than 10% greening success. On the right with a geomat and a greening success of more than 90%.

Working off these results, an erosion control mat was introduced to the market in 2004 under the trade name of Tectmat with the following characteristics (Figure 10):

  • Extruded monofilaments made of polypropylene with an irregular loop structure
  • Thickness: 18 mm
  • Surface weight: 600g/m2
  • Proportion of cavities: >95%
  • Color: curry green

figure 10

figure 10

Figure 10 – Erosion control mat in combination with a high-tensile steel wire steel wire mesh in the area of soil nailing.

3.0 Examples of use

3.1 Miraflores, Panama

Slope stabilisation using the Tecco G65/3 system in a heavily irregular rock embankment. The erosion control mat Tecmat could be adapted well to the soil. The fall of the folds shows that some areas do not fit (Figure 11). Due to the good conditions for growth, the neighboring areas are lush green. The initial signs of vegetation also appeared in the securing area.

figure 11

figure 11

Figure 11 – Secured rock embankment in Miraflores, Panama. Erosion protection mat and mesh could be adapted well to the irregular slope

3.2 Remscheid, Germany

Securing measure on weathered rock with the Tecco G65/3 system after a landslide (Figure 12). The growth conditions are good, as can be seen on the slope. The embankment is relatively even. It is difficult to secure the mat on a “solid” base, which is why there are slight shifts in the Tecmat.

figure 12

figure 12

Figure 12 – Securing a cut slope after a landslide in the weathered rock.

3.3 Dorndorf, Germany

Evenly profiled embankment of loose rock. Subsections were dug in the area of the nail heads in order to be able to harness the high-tensile steel wire mesh Tecco G65/3. The erosion control mat could be adapted well to the embankment - close fitting with only a small proportion of cavities under the erosion protection mat (Figure 13).

figure 13

figure 13

Figure 13 – Evenly profiled embankment in loose rock with close-fitting erosion protection mat

The three examples show that it is extremely feasible to apply a combination of erosion protection mats and high-tensile steel wire wire mesh. On slopes with irregular profiles it has been found that the erosion control mat does not always lie evenly on the embankment. It is therefore important when hydroseeding that the mat can be injected with an export mulch (length of fibres <4mm). Due to the lower water absorption capacity of synthetic mats, they are less prone to slipping under load. Depending on the conditions of the substrate, it is difficult to attach the mat. The example shown in 3.3 can be adapted well to the substrate. The mat is not anticipated to shift even when under load. Nevertheless, the mat must be suitable for spraying with a seeding process.

4.0 Structural reinforcement of geo-textiles

It was evident that the system could be optimised through corresponding combinations of erosion control mats and high-tensile steel wire wire meshes. Firstly, the erosion protection mat is structurally reinforced. This makes it possible to attach the securing measure with greater efficacy against slipping. A further advantage is a much simpler assembly: the erosion control mat and high-tensile steel wire mesh can be applied to the slope in a single operation.

The initial tests for a standard product were carried out in 2012 with the high-tensile chain link mesh Deltax G80/2. The internal diameter of the mesh is 80mm and the wire is 2mm thick. It has a load-bearing capacity of 53kN/m. It is often used without nailing as a drapery, even on steep slopes where there are likely to be just smaller stones. The reinforcement of the erosion control mat provides sufficient support to keep the small stones in place. Some systems were also applied with soil nailing. As described earlier, the erosion control mat cannot bear the loads in all applications without suffering some damage. The individual channels under the mesh are expected to shift in the fall-lines, especially if the mat is difficult to attach. In order to be able to adequately secure and green slopes subject to superficial landslides or with global stability problems, it was necessary to try and reinforce the erosion control mat with a high-tensile steel wire mesh Tecco G65/3 (Figure 14). The internal diameter of the mesh is 65mm and the wire is 3mm thick. It has a load-bearing capacity of 150kN/m with a puncturing resistance of 180kN against the system spike plate with edge lengths of 330 by 200mm. Figure 14 shows that the choice of mulch fibres is important. Long fibres are not able to sufficiently penetrate the erosion protection mat.

figure 14

figure 14

Figure 14 – After hydroseeding

5.0 Conclusions

In principle, naturally biodegradable, organic erosion protection mats are preferable. However, they reach their limits of usefulness when applied to steeper slopes. Three-dimensional geomats, such as those made of polypropylene, offer advantages owing to the fact that they weigh less and have good retention properties. Colour-matched, lighter erosion contraol mats heat up less and render superior greening results in addition to being less conspicuous.

The structural strengthening of geotextiles with high-tensile steel wire meshes, which are used as erosion control mats, increase their scope of use through their increased load-bearing capacity.

Superficial instabilities can be secured up to a depth of 2m with the high-tensile steel wire meshes that have a minimum puncturing resistance of 180kN. They can be reliably designed using Ruvolum. Global instabilities can also be secured if they are correspondingly dimensioned.

Furthermore, the extent to which the properties of synthetic geomats can be combined with those of organic ones was also studied. A variant is to attach the mats directly and reinforce them with the fibres of coconut, jute or wood wool. Further developments of natural geomats based on paper offer another possibility. The greening of embankments is certainly a very complex subject and depends on many factors.

References

Bosshard, A, Mayer, P, Mosimann, A, (2013). Leitfaden für Naturgemässe Begrünungen in der Schweiz. Ö+L Ökologie und Landschaft

Flum, D, Strolz, M, Roduner, A, (2014). Grossfeldversuche mit flexiblen Böschungsstabilisierungssystemen. Technische Akademie Esslingen, Beitrag für 9. Kolloquium Bauen in Boden und Fels

Rüegger, R, (1986). Die Hauptaufgaben der Geotextilien: theoretische Ansätze und Dimensionierungskriterien. Schweizer Ingenieur und Architekt, Band 104, Heft 40.

Rüegger, R, Flum, D, Haller, B, (2002). Hochfeste Geflechte aus Stahldraht für die Oberflächensicherung in Kombination mit Vernagelungen und Verankerungen (Ausführliche Bemessungshinweise). Technische Akademie Esslingen, Beitrag für 2. Kolloquium Bauen in Boden und Fels.

Rüegger, R, Weingart, K, Bickel, M, (2004). Flexible Oberflächensicherungssysteme aus hochfesten Drahtgeflechten in Kombination mit Boden- und Felsnägeln, 3 Fallbeispiele. Technische Akademie Esslingen, Beitrag für 3. Kolloquium Bauen in Boden und Fels

Rüegger, R, Eberle, T, (2006). Stützkonstruktionen aus bewehrter Erde: Richtlinie für Planung, Bemessung, Ausschreibung, Ausführung, Überwachung und Unterhalt.- Aarau: Departement Bau, Verkehr Umwelt, Abt. Tiefbau.

Rüegger, R and Flum, D, (2006). Anforderungen an flexible Böschungsstabilisierungssysteme bei der Anwendung in Boden und Fels. Österreichische Geologische Gesellschaft, Salzburg.

SN-Norm 640 550. (2003). Geotextilien und die Prüfvorschriften

Readers' comments (1)

  • Richard  Edwards

    I've been involved in a number of soil nailed slope projects where the 3D matting has inhibited growth due to being suspended off the surface by tree stumps/roots, rock outcrops etc. In my experience if you have an undulating irregular slope profile, a more effective option is to soil nail with rock netting then spray a high performance erosion control mulch with tackifiers through the open aperture of the netting. The mulch is far more effective than standard hydroseeding and if applied correctly can control surface erosion and speed up the rate of germination. I've done a number of rail and civils projects proving this method can be a viable option. It's more cost effective and sustainable in the right application. It's very common in the USA where I first saw this method used.

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