GRS Fortrac® Natur

Installation Guidelines

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The mats shall be properly embedded between the reinforce-ment layers. Specification of a woven grid (e.g. HaTe 23.142/GR) is recommended where subsequent hydroseeding is planned (Fig. 2). An alternative planting solution involves the use of pre-seeded coir mats. A nonwoven fabric (e.g. type BS 12) is a suitable option for unvegetated retaining structures.

Contents

1. General 3

2. Summary 3

3. System components 3 3.1 Reinforcement 3.2 Erosion control 4. Delivery and storage 3-4

5. Installation 5 5.1 Safety 5.2 Construction plant, formwork and equipment 5.3 Subgrade 5.4 Geogrids 5.5 Fill 5.6 Borders 5.7 Drainage 5.8 Planting

6. Literature/References 5

7. Installation steps 7.1 Version with a temporary partial surface of a formwork 6-7 7.2 Version with a temporary large surface of a formwork 8-9 7.3 Version without formwork for slopes ≤ 45° 10-11

1. General

These installation guidelines explain the design of the GRS Fortrac Natur retaining wall system together with the steps needed for its successful construction. The descrip-tion provided here is of a general nature and cannot fully cover the wide-ranging applications and forms of geosyn-thetic reinforced retaining structures that can be built with Fortrac geogrids. Varying project parameters, local condi-tions and other factors may necessitate adaptation of the work sequence. This document is therefore no substitute for detail design and construction information. The individual opera-tions are described and illustrated. However, the values and sizes specified here serve only as recommendations and may require project-specific adjustment. Please contact us, if you need further details or have any queries. HUESKER will be glad to provide you with expert advice from design through to installation.

2. Summary

GRS Fortrac Natur is a plantable system solution for the installa-tion of geosynthetic reinforced retaining structures. The package includes Fortrac geogrids and erosion control mats; if necessary, formworks have to be provided on-site.

The stability of the retaining structure is guaranted by the Fortrac geogrids and the compacted fill. The geosynthetic reinforcement is incorporated layer by layer with the fill and wrapped back at the front face. In case of relatively low indications up to 45°, a fold-in need not to be obtained.

The erosion control mats are inserted in layers in the wrapped back areas to prevent the washing-out of fill. In the case of the version without back-up, the erosion control is ensured by a Fortrac 3D geogrid which is to be laid over the entire slope. The GRS Fortrac Natur system can be used for slopes of var-ious heights and with front batters between 30° and 90°. It is normally possible to plant permanent, maintenance-free veg-etation on the external slope face for batters of up to 70°.

3. System components

3.1 ReinforcementFortrac is a flexible, high-modulus geogrid manufactured from a variety of durable synthetic yarns and available in different mesh sizes and strengths. The product types suit-able for the particular application, the intervals between lay-ers and the embedment lengths are specified as part of the geotechnical design to Eurocode 7 [1], and on the basis of BS 8006 [2]. The Fortrac geogrids are supplied, as standard, in the form of 5 m wide sheets. It is important to cut and lay the geogrid sheets in the right direction. The geogrid's main di-rection of tension is generally the direction of unrolling (Fig. 1).

The names of the different Fortrac types indicate their short-term strength in longitudinal direction, e.g. Fortrac 110 T: Short-term strengh in long. dir. 110 kN/m.

3.2 Erosion controlThe erosion protection or grass seed mat to be provided in the front area can basically consist of different materials.

Fig. 2: HaTe 23.142/GR woven grid for erosion control

Fig. 1: Main and transverse direction of tension

Installation GuidelinesGeosynthetic Reinforced Soil (GRS) with Fortrac Geogrids

4. Delivery and storage

Fortrac geogrids are supplied in rolls wrapped in protectivesheeting. In compliance with DIN EN ISO 10320 [5], each ge-ogrid roll is clearly identifiable and is thus readily traceable through the quality assurance system (Fig. 3). Attention shall nonetheless be paid to ensuring the conformity of the sup-plied materials with the project specification. The Fortrac geogrids supplied by HUESKER are CE-marked and have ivg. (German Geosynthetics Industry Association) product certi-fication. The latter eliminates the need for on-site incoming goods inspections under ZTV E-StB [6]. The products may be unloaded by forklift (with stinger), self-unloader or other suitable site equipment, e.g. lifting/ laying gear (Fig. 4). All necessary precautions shall be taken

Fig 3: Example of Fortrac 110 T label

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tion can be facilitated through the use of a dispenser frame (Fig.6). In some cases, site logistics may be improved through provision for a central cutting point where the sheets can be cut to the required lengths, folded up and neatly stored

shall not be directly driven over. A minimum soil cover of 20 cm shall first be provided. The fill material shall be placed by the "end tipping" method and compacted layer by layer. Prior to any interruption of the works, all installed geogrids shall first be covered over with fill.

5.5 FillThe fill material shall be easily compactable and largely re-sistant to water action. It shall, at the very least, exhibit the properties prescribed by the project specifications and struc-tural design (e.g. shear strength, density, soil gradation, etc.). Water-sensitive and cohesive soils can also be used subject to the implementation of suitable additional measures. Suit-ability shall, in each case, be certified by the tests specified in ZTV E-StB [6]. Under BS 8006 [2], the soil types specified in DIN 18196 [8] are classed as generally usable, subject to verification of their suitability for the particular application or subject to allowance for the particular soil properties in the application.

The specified soils are as follows: ¡ Coarse-grained soil types of groups SW, SI, SW, GW, GI, GE ¡ Mixed-grained soil types of groups SU, ST, GU, GT, SU*,GT*, GU*, ST*

¡ Fine-grained soil types of groups UL, UM, TL, TM

The fill shall be placed and compacted in layers. The maxi-mum particle size shall not exceed 2/3 of the fill layer thick-ness. The instructions and special requirements for rein-forced retaining structures under ZTV E-StB [6] shall also be observed. In the absence of particular specifications, a mini-mum compaction level of DPr = 97 % is recommended. Heavily loaded structures may require higher compaction levels of DPr = 100 - 103 %.

5.6 Borders Project-specific measures may prove necessary to adapt the GRS structure to existing works or adjoining slopes. These shall be factored into the detailed design/ production infor-mation.

5.7 Drainage Privision shall be made in the detailed design/ production in-formation for an adequately sized drainage system. Surface run-off shall be collected and removed via drains and should not be shed over the slope of the retaining structure.

5.8 Planting A mixture of fill material and planting soil shall be placed to a depth of approx. 20 - 30 cm at the front of the structure (Fig. 8 + Fig. 9). The appointment of a garden or landscape architect is recommended to ensure the achievement of a fully vegetated surface appropriate to the location that ade-quately protects the exposed face against UV radiation. The seed mix and/ or cuttings or creepers shall be selected so as to be compatible with local conditions. We generally recom-mend extensive planting with indigenous species, which, once established, require little maintenance. It is recommended that a specialist contractor be appointed to plan and main-tain the vegetated surface during and after the initial growth period.

to prevent damage to the rolls. The geogrid rolls shall be stored well away from site traffic routes on a dry, clean and even base. The rolls may be stacked and shall be pro tected against slipping and unrolling. Unpacked goods must not be freely exposed to UV radiation for more than one month. The application guidelines in M Geok E (Guidance Paper on the Use of Geosynthetics in Earthworks for Roadbuilding Projects) [7] should be observed. The erosion control mats should be stored at a safe and dry location. To prevent seed germination prior to installation, any pre-seeded mats to be incorporated shall be additionally protected against light.

6. Literature/References

[1] DIN EN 1997-1: 2009-09, Eurocode 7: Geotechnical design – Part 1: General rules.

[2] BS 8006-1: 2010, Code of practice for strengthened/ reinforced soils and other fills, 2010 edition. British Standards Institute.

[3] Recommendations for design and calculation of earth bodies with geosynthetics reinforcements (EBGEO), 2010 edition, German Society for Geotechnics e.V.

[4] DIN EN ISO 10320: 1999, Geotextiles and geotextile-relat-ed products - Identification on site.

[5] ZTV E-StB (Special Technical Conditions and Guidelines for Earthworks in Highway Engineering), 2009 edition, FGSV (German Road and Transportation Research As-sociation), Earthworks and Foundations Working Group, Cologne, FGSV issue no. 599.

[6] M Geok E (Guidance Paper on the Use of Geosynthetics in Earthworks and Foundations for Roadbuilding Projects), 2005 edition, FGSV, Earthworks and Foundations Working Group, Cologne, FGSV issue no. 535.

[7] DIN 18196: 2011-05, Earthworks and Foundations - Soil classification for civil engineering purposes.

[8] DIN EN 14475: 2006, Execution of special geotechnical works, reinforced fill

Fig. 9: Vegetated GRS Fortrac Natur

Fig. 8: Permanently vegetated surface created by placing planting soil at front of structure

Fig. 6: Dispenser frame facilities cutting to length of sheets

Fig. 7: Efficient storage of cut-to-length geogrids

Fig. 5: Total Fortrac geogrid length

5. Installation

5.1 Safety The construction of geosynthetic-reinforced structures is subject to all statutory regulations and other provision governing on-site safety.

5.2 Construction plant, formwork and equipment Standard earth-moving plant can be used to install geo-synthetic-reinforced retaining structures. For the optimal construction, the use of temporary auxiliary formworks is recommended in order to enable sufficient front compaction. Climbing, partial area or large area formwork are custom-ary. The formwork elements are selected according to pro-ject-specific boundary conditons, but particularly flat slopes (e.g. < 45°) are often built without auxiliary formworks. For further information, please refer to the description of the in-stallation steps. Other useful equipment for the on-site works include utility/ battery-operated knives, measuring tape, colour spray, tying wire and pincers.

5.3 Subgrade The subgrade shall be prepared in accordance with ZTV E-StB [6]. Any existing slopes shall be adequately protected against erosion and collapse. Unless otherwise specified, a minimum foundation modulus EV2 [to DIN 18134] of 45 MN/m² shall be guaranteed. The compaction requirements under ZTV E-StB [6] for the prevailing soil type shall also be observed. Failure to meet the minimum bearing capacity requirements shall neces-sitate special measures (e.g. ground improvement or a geosyn-thetic reinforced foundation bed).

5.4 Geogrids The Fortrac geogrids shall be cut to the structurally re-quired length with additional allowance for the upstand and wrap-back lengths (Fig. 5).The sheets can be cut to length with standard utility or battery-operated knives. This opera-

(Fig. 7). The main direction of tension of the geogrids, which is generally the direction of unrolling, shall be indicated by a colour marking. As a general rule, the cut-to-length reinforcement sheets shall be laid horizontally with their main direction of tension perpendicular to the slope or wall face. Particular attention shall be paid to ensuring the correct orientation of the geogrid sheets during installation.

Unless verified by the structural calculation, no overlapping of sheets is permitted in the main direction of tension. A min-imum side oberlap of 10 cm is recommended (though not structurally necessary). The geogrids shall be laid tautly, without any folds or creases, so as to allow immediate load take-up. No special tensioning of the sheets is, however, necessary. To prevent the formation of folds or creases, the placement of fill on the geogrid shall commence at the slope side and progress towards the geogrid ends. The geogrids

Fig. 4: On-site storage of rolls and use of lifting/ laying gear

Lges = Total geogrid lengthLRs = Wrap-back length (min. 1.50 m)LHf = Upstand lengthLstat = Structurally required length

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adjusting theslope angle

min. 0.20

wrap around

layer thickness

depending onslope inclination

length accordingto design

min. 0.20

layerthickness

12 layer thickness

0.50

Wrap back the geogrid and secure with steel nails or fill material such that it is rigidly tautened at the rear step at a distance above the soil. Also wrap back the erosion control mat.

Place and compact the rest of the second layer of fill.

The following formwork units can be placed in the horizontal offset to achieve the desired slope batter.

Repeat steps 1 to 7.

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7. Installation steps 7. Installation steps

Following preparation of the subgrade, position and align of the first row of the temporary partial surface of a formwork.

7.1 Version with a temporary partial surface of a formwork

Installation

Lay the cut-to-length Fortrac geogrid horizontally, without folds or creases, with its main direction of tension perpendicular to the slope face. Unless the structural design specifies a greater value, a minimum wrap-back length of 1.50 m is recommended. Provide an approx. 10 cm overlap between adjoining sheets. It is essential to ensure that the geogrids are not directly driven over. Moreover, unless verified by the structural calculation, no overlapping of sheets is permitted in the main direction of tension.

Insert and fix the erosion control mat in the folded areas alongside the formwork. Turn back the mat by approx. 20 cm at the top and bottom.

Place a first layer of front planting soil and fill material, by end tipping, to a thickness of 20 - 30 cm and compact. The compaction operation shall commence at the slope face and progress towards the rear. Use a light compactor at the slope face. Rollers (≤ 12,5 t) may be used at a distance of over 1.50 m from the slope face.

Proceed by placing a 50 cm wide strip of fill along the front and compact until the specified layer thickness (normally between 40 an 60 cm) is achieved.

1 2

3 4

adjusting theslope angle

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length accordingto design

2.

min. 0.20

layerthickness

12 layer thickness

0.50

wrap around

layer thickness

Wrap back the geogrid and secure with steel nails or fill material such that it is rigidly tautened at the rear step at a distance above the soil. Also wrap back the erosion control mat.

Place and compact the rest of the second layer of fill.

Repeat steps 1 to 6.

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7. Installation steps 7. Installation steps

Following preparation of the subgrade, competently position, support an align the temporary large surface of a formwork.

Installation

Lay the cut-to-length Fortrac geogrid horizontally, without folds or creases, with its main direction of tension perpendicular to the slope face. Unless the structural design specifies a greater value, a mini- mum wrap-back length of 1.50 m is recommended. These can be temporarily fixed to the inside of the large-area formwork before the fill material is installed. Provide an approx. 10 cm overlap between adjoining sheets. It is essential to ensure that the geogrids are not directly driven over. Moreover, unless verified by the structural calculation, no overlapping of sheets is permitted in the main direction of tension.

Insert and fix the erosion control mat in the folded areas alongside the formwork. Turn back the mat by approx. 20 cm at the top and bottom.

Place a first layer of front planting soil and fill material, by end tipping, to a thickness of 20 - 30 cm and compact. The compaction operation shall commence at the slope face and progress towards the rear. Use a light compactor at the slope face. Rollers (≤ 12,5 t) may be used at a distance of over 1.50 m from the slope face.

Proceed by placing a 50 cm wide strip of fill along the front and compact until the specified layer thickness (normally between 40 and 60 cm) is achieved.

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7.2 Version with a temporary large surface of a formwork

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0.10

min. 0.25 m

length accordingto design

layerthickness

0.25-0.50

0.25-0.50

<45°

7. Installation steps 7. Installation steps

7.3 Version without formwork for slopes ≤ 45°

A structured erosion control mat is applied to the prepared embank-ment surface by earth anchors (approx. 1 piece/ m²) and covered with a vegetation-capable soil layer of approx. 10 to 15 cm thickness. The necessity for a drainage mat or a static proof of the erosion protection mat (antiglide reinforcement) must be checked in individual cases.

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Following preparation of the subgrade, the cut-to-length Fortrac geogrids shall be laid horizontally and without any folds or creases, with their main direction of tension perpendicular to the slope face. Provide an approx. 10 cm overlap between adjoining sheets. It is essential to ensure that the geogrids are not directly driven over. Moreover, unless verified by the structural calculation, no overlapping of sheets is permitted in the main direction of tension.

Installation

The filling material shall be placed and compacted in layers up to the layer thickness provided. The compaction is to be continued from the embankment front to the rear.

Repeat steps 1 and 2 up to complete construction height. After reaching the full construction height, the embankment could be smoothed with a slope spoon and then compacted with a special slope compressor (excavator attachment unit).

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min. 0.10 m

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Fortrac® und HaTe® are registered trademarks of HUESKER Synthetic GmbH.

HUESKER Synthetic is certified to ISO 9001 and ISO 50001.

HUESKER Synthetic GmbH

Fabrikstrasse 13–1548712 Gescher, GermanyPhone: + 49 (0) 25 42 / 701 - 0Fax: + 49 (0) 25 42 / 701 - 499Mail: info@HUESKER.deWeb: www.HUESKER.com