Introduction

Geotextiles used as filters must be dimensioned in such a way that the soil to be filtered is protected from erosion, the load-bearing grain structure is kept stable, the passage of fine grains is only permitted to a non-critical extent so that clogging – the accumulation of fine material on the filter – is avoided or remains within harmless limits, and groundwater can flow through the filter.

To this end, the mechanical filter effectiveness (soil retention), the hydraulic filter effectiveness (prevention of a rise in the seepage line) and the robustness must be verified.

For road construction, the "Additional Technical Contract Conditions and Guidelines for Earthworks in Road Construction" (ZTV E-StB) provide information on the design of geotextile filters with reference to the "Information Sheet on the Use of Geosynthetics in Earthworks for Road Construction" (M Geok E).

In August 2017, the revised version of the DWA-M 511 information sheet "Filtering with Geosynthetics" was published, replacing the information sheet "Application of Geotextiles in Hydraulic Engineering", DVWK 221/1992. This develops a new design procedure for mechanical filter stability based on a comparison of national and international recommendations, and also formulates requirements for hydraulic filter efficiency, filter thicknesses and robustness.

In the field of waterways – embankment and bed protection – the design of geotextile filters is dealt with in the "Information sheet on the use of geotextile filters on waterways" (MAG).

The MAG dates from 1993. According to information from the BAW Karlsruhe, revisions recommend replacing part of the filter effectiveness verification with the corresponding verification in accordance with DWA-M 511. This is discussed in this article where appropriate.

The technical delivery conditions for geotextiles and geotextile-related products on waterways (TLG) of the Federal Ministry of Transport and Digital Infrastructure, Waterways and Shipping Department, were reissued in 2018, replacing the TLG from 2008. In Annex 2 of the new TLG, the soil type of the building ground is divided into three classes, A, B and C, whereas in the TLG from 2008 there were classes 1 to 4. The MAG information sheet (1993) and the guideline for testing geotextiles in waterway construction (RPG, 1994), which were valid at the time this manual was written, contain soil types 1 to 4, analogous to the TLG 2008. Soil type A in the 2018 TLG is essentially a combination of soil types 1 and 2, soil type B corresponds to soil type 3 and soil type C to soil type 4. As the MAG information sheet and the RPG guideline are still valid, this handbook continues to use soil types 1 to 4.

This article presents the verification procedures of these information sheets and recommendations and attempts to clarify their application. For this purpose, flowcharts for the verification of geotextile filters in accordance with DWA-M 511 and MAG 1993 have been developed. In the following text, the relevant chapters of the application manual are indicated based on the flowcharts for the verifications to be carried out.

Examples of necessary partial verifications can be found in the "Application manual for the information sheets DWA-M 511 Filters with geosynthetics and BAW Application of geotextile filters on waterways (MAG)", which is provided by NAUE GmbH & Co. KG. The programme printouts for the verifications were created using the GGU Sieve and GGU Filter Stability programmes.

Planning principles for geotextile filters

The following planning principles must be known in order to be able to dimension the geotechnical filters for their respective application situation with regard to their filter properties and other strength properties.

1.1 Building ground:

• Layer structure,
• water levels,
• Grain size distribution curves of soils adjacent to the filter,
• plastic properties of soils adjacent to the filter, undrained shear strength, c_u, plasticity index I_p
• Possible internal friction angle (stability of slopes, friction coefficient of the geotextile)
• Water permeability coefficients,
• filter construction (MAG),
• Choice of cover layer (MAG)

1.2 Effects

• Hydraulic stresses
• mechanical stresses from construction work – puncture stresses, etc.,
• Mechanical stresses from overburden,
• chemical effects, sintering/ochre formation

1.3 Determination of the permeability coefficient k_f from the grain size distribution curve

Water permeability, which forms the basis for investigating hydraulic filter stability, can be estimated using the grain size distribution curves according to the methods described in the literature. The information sheet "Application of grain filters on federal waterways (MAK)" summarises methods and their areas of application for different soils. Input variables are the undrained shear strength c_u and the effective grain diameters at 10, 20 and 25 mass %, d₁₀, d₂₀ and d₂₅.

2. Flow chart for filter design according to the information sheet "Filtering with geosynthetics", DWA-M 511

The procedure for filter design according to the information sheet "Filtering with Geosynthetics", DWA-M 511, is shown in the following flow chart for ease of use. For the verifications to be carried out, the diagram indicates the relevant chapters of DWA-M 511 and the application manual for the information sheets DWA-M 511 Filtering with Geosynthetics and BAW Application of Geotextile Filters on Waterways (MAG).

Fig. 1: Flow chart for the design of a geotextile filter according to DWA-M 511

After a basic distinction between cohesive – at least medium-plastic fine-grained and mixed-grain – and non-cohesive – coarse-grained and slightly plastic mixed-grain and fine-grained soils – (chapter 3.2), the need for filtration is determined (chapters 3.3.1 and 3.3.2). The verification methods used according to Cistin/Ziems and MMB 2013 are well known and have proven themselves for this purpose.

For the verification of suffosion safety (Chapter 3.4.2), reference is also made to MMB 2013, where criteria from Ziems, Kenney/Lau and Burenkova are applied.

The verification of mechanical filter effectiveness for soils not at risk of suffosion and static hydraulic loads (Chapter 3.4.1.1) is presented with a new approach in DWA-M 511. The required opening width of the geotextile filter O₉₀ is determined using the coefficient of uniformity C_U and the effective grain diameter d₅₀ of the soil to be filtered. The equation for the lognormal distribution is:

This approach attempts to take into account the commonalities of the well-known approaches of the Canadian Geotechnical Society (CFEM, 2006), Giroud, Luettich et al. (1982) and DVWK Merkblatt 221 (1992), but at the same time to achieve a continuous dependence on aperture width and coefficient of uniformity, thereby avoiding discontinuities. In order to take the storage density into account, as in the Giroud approach, a bandwidth of ±10% is introduced.

Fig. 2: Distance ratio O₉₀ /d₅₀ depending on the unevenness coefficient C_U of the soil

For dynamic hydraulic loads such as turbulent flows and wave action, verification in accordance with DWA-M 511 can be carried out in accordance with EAK 1993 for fine and medium sands with 0.1 mm < d₅₀ < 0.3 mm as follows (Chapter 3.4.1.2):

O₉₀ ≈ d₅₀

In the case of suffusive soils (Section 3.4.3), the filter should not be designed to retain all fine particles that can be displaced as a result of suffosion in order to prevent a colmation-induced increase in water pressure. The filter can be dimensioned according to Lafleur (1993) with

O₉₀ ≤ d₃₀

.

The proportion of fine particles that can be considered erodible must be determined on a case-by-case basis. To estimate the largest grain diameter of the suffosive fraction, the approach according to Ziems can be used in accordance with DWA-M 511.

For cohesive soils, the requirements for the opening widths of geotextile filters are formulated in DWA-M 511 (Chapter 3.4.4) depending on the plasticity properties and the undrained shear strength of a soil:

Table 1: Criteria for mechanical filter effectiveness, cohesive soils

To verify hydraulic filter stability (Section 3.5), the requirement specified in DWA-M 511 7.5 must be met

If the soil is highly permeable and the gradients are low, the permeability of the geotextile can be determined as:

According to DWA-M 511 7.7, a sufficient filtration length must be ensured for geotextile filters by guaranteeing a minimum thickness of

(Section 3.6).

According to DWA-M 511, the following requirements apply to geotextile filters with regard to the minimum weight per unit area depending on the backfill (Chapter 3.7):

Table 2: Required weight per unit area depending on the overburden

3. Flow chart for filter design according to the information sheet "Application of geotextile filters on waterways", MAG

The information sheet "Use of geotextile filters on waterways", MAG (1993) applies to geotextiles used as filters for slope and bed protection on waterways and associated structures such as dams and ditches.

The "Technical Delivery Conditions for Geotextiles and Geotextile-Related Products on Waterways", TLG, specify requirements for the properties and testing of the materials used.

3.1 Flow chart

The procedure for filter design according to the information sheet "Application of geotextile filters on waterways" MAG (1993) is shown in the following flow chart for ease of use. For the verifications to be carried out, the diagram lists both the relevant chapters of this manual and the chapters of the MAG for ease of use.

Fig. 3: Flow chart for the design of a geotextile filter according to DWA-M 511

Prior to further investigations, the soil to be filtered is classified as non-cohesive or cohesive soil in accordance with MAG 1993.

For non-cohesive soils with a failure grain size and for non-cohesive soils with a uniformity coefficient of C_u ≥ 8, suffosion safety must be verified in accordance with MAG 1993.

According to information from the BAW at the time of writing this manual, the MAG 1993 information sheet is currently being revised, meaning that the suffosion risk is already being assessed in accordance with the flow chart in MMB 2013 (Chapter 3.4.2).

According to Figure 4, the geotextile must be dimensioned in terms of its filter properties depending on the selected construction method for the adjacent materials – subgrade, top layer, levelling layer, grain filter (Chapter 4.3).

Fig. 4: Filter construction methods from MAG 1993

The necessity of a geotextile filter between the mineral grain layers used, depending on the filter construction method, is determined in accordance with MAG 1993 using the diagram according to Cistin/Ziems (Chapter 4.4).

For the dimensioning of the geotextile, the requirements specified in MAG 1993 with regard to filter effectiveness and material properties in accordance with TLG 2018 must be met. MAG 1993 recommends the BAW soil type method (Chapter 4.5.1.1), in which the requirements are specified by assigning the subsoil and selecting a filter and cover layer design.

The filter rule according to AK 14 no longer applies to the verification of dynamic and static hydraulic loads and is replaced by the verification of mechanical filter effectiveness according to DWA-M 511 (Chapter 4.5.1.2).

The technical delivery conditions for geotextiles and geotextile-related products on waterways were reissued in 2018, replacing the TLG from 2008. The soil type of the building ground is divided into three classes, A, B and C, whereas in the TLG from 2008 there were classes 1 to 4. Soil type A in the TLG 2018 is a combination of soil types 1 and 2, soil type B corresponds to soil type 3 and soil type C to soil type 4.

Fig. 5: Soil types 1 and 2 according to TLG 2008, soil type A according to TLG 2018

As the MAG information sheet and the RPG guideline are still valid, the manual continues to use soil types 1 to 4.

Once the soil type or, for grain size ranges, the soil types have been determined as shown above, the design according to the soil type method follows. The following table from TLG 2008 is used for this purpose (Chapter 4.5.1.1).

Table 3: Standard requirements for the filter effectiveness of a geotextile from TLG 2008

In future, the standard requirements from TLG 2018 will apply. Figure 10 shows the new requirements.

Table 4: Standard requirements for the filter effectiveness of a geotextile from TLG 2008

According to MAG 1993, a geotextile has sufficient filter effectiveness for all soils of a soil type if, in a basic test prescribed for each product

• for soil types 1 – 3 using the
flow-through method• for soil type 4 using the turbulence method

complies with the values specified in the following table for layer thickness, soil passage and permeability at a soil load of 25 cm.

For soils whose grain size range exceeds the limits of validity of a soil type, the geotextile filter must be designed for all soil types covered by the grain size range with grain sizes d₅ to d₆₀.

General material requirements such as tensile strength and abrasion resistance are specified in the TLG (Chapter 4.5.2). The required puncture resistance depends on the selected filter components and is specified in line 3. The thickness of the filter layer is generally specified by the soil type method.

Table 5: Material requirements for a geotextile from TLG 2008

The new requirements according to TLG 2018 are shown in Table 6.

Table 6: Material requirements for a geotextile from TLG 2018

The water permeability of the geotextile used must be selected in accordance with MAG 1993 as follows:

• silty soils: k_v ≥ 50 ⋅ kSoil
• slightly silty soils: k_v ≥ 10 ⋅ kSoil

However, according to information from the BAW, the value specified in the soil type procedure for the permeability of the soil-filled geotextile is completely sufficient to prevent soil deposits and does not need to be further checked under the above conditions.

Filter design according to M Geok E

For geosynthetics used in earthworks and drainage systems in road construction, the M Geok E information sheet provides details on possible applications and the necessary verification procedures.

In the course of verifying the mechanical filter effectiveness, M Geok E distinguishes between so-called hydraulic safety cases with regard to hydraulic effects. For three safety cases, information is provided on how to verify the mechanical filter effect (Chapter 5.1).

According to M Geok E, proof of hydraulic filter effectiveness is provided if the following condition is met (Chapter 5.2):

The following conditions must be met to rule out clogging. To protect the soil against erosion and thus keep the load-bearing grain structure stable, an opening width is selected with

gew. O₉₀ ≤ 1.0 ⋅ zul O₉₀

To ensure that clogging remains within harmless limits, an opening width must be selected with:

gew. O₉₀ ≥ 0.2 ⋅ zul O₉₀

For geotextile filters, geotextile robustness classes GRK 3 to GRK 5 are required (Chapter 5.3).

Table 7: Determination of the required geotextile robustness class according to TL Geok

Table 8: Geotextile robustness classes according to TL Geok

Literature

/1/ Application of geotextiles in hydraulic engineering, Merkblätter 221 / 1992, published by the German Association for Water Management and Cultural Engineering (DVWK), Bonn, 1992

/2/ Recommendations for the execution of coastal protection works, EAK 2002, corrected edition 2007, published by: Board of Trustees for Research in Coastal Engineering /3/ Foundation Engineering Handbook, 7th edition, Part 2, published by: Univ.-Prof. Dr.-Ing. Karl Josef Witt

/4/ BAW Information Sheet on the Use of Grain Filters on Federal Waterways (MAK), published by: Federal Waterways Engineering and Research Institute, Karlsruhe, 2013

/5/ Amendment to A1:2015 to BAW Information Sheet on the Use of Grain Filters on Federal Waterways (MAK), publisher: Federal Waterways Engineering and Research Institute, Karlsruhe, 2013

/6/ Information sheet on the application of standard construction methods for slope and bed protection on inland waterways (MAR), publisher: Federal Waterways Engineering and Research Institute, Karlsruhe, 2008

/7/ Information sheet DWA-M 511, Filtering with geosynthetics, published by: DWA German Association for Water, Wastewater and Waste, 2017

/8/ Information sheet on the use of geotextile filters on waterways (MAG), published by: BAW Federal Waterways Engineering and Research Institute, Karlsruhe, 1993

/9/ Information sheet on material transport in soil (MMB), publisher: Federal Waterways Engineering and Research Institute, Karlsruhe, 2013

/10/ Information sheet on the use of geosynthetics in earthworks for road construction (M Geok E), published by: Research Association for Road and Traffic Engineering, Cologne, 2016

/11/ Guideline for the approval of geotextiles for filtering and separating landfill liners, published by: Federal Institute for Materials Research and Testing, 2016

/12/ Technical delivery conditions for geosynthetics in earthworks for road construction, (TL Geok E-StB 05), published by: Road and Transportation Research Association, Earthworks and Foundation Engineering Working Group, 2005

/13/ Technical delivery conditions for geotextiles and geotextile-related products on waterways (TLG), published by: Federal Ministry of Transport, Building and Urban Development, Waterways and Shipping Department, 2008

/14/ Technical delivery conditions for geotextiles and geotextile-related products on waterways (TLG), publisher: Federal Ministry of Transport, Building and Urban Development, Waterways and Shipping Department, 2018

/15/ Technical delivery conditions for armour stone (TLW), 2003

/16/ Additional Technical Contract Conditions and Guidelines for Earthworks in Road Construction, ZTV E-StB, 2009