Geotextiles and Geomembranes 12 (1993) 441-450

An Expert System for the Design of Geotextiles

Gernot Mannsbart & Siegfried Resl

Polyfelt Ges. m.b.H., St. Peterstr. 21, A-4021 Linz, Austria

ABSTRACT

This paper describes the design programs that have been evaluated for the design of Polyfelt geotextiles. The basis for the design is the design charts in the technical manual 'Polyfelt TS Design and Practice'.

In addition to simply putting in figures and printing the result of the computation, this program also provides useful background information on important topics related to the use of geotextiles, such as geotextile functions, geotextile properties and specifications, useful construction hints, etc.

Furthermore, even without a precise knowledge of the input parameters, a design engineer may approach the final design step by step, by providing background information for an estimation of unknown input parameters.

The program was written with an expert-system language called KnowledgePro, issued by Knowledge Garden Inc., USA.

Design programs are available for the following applications: road construction, hydraulic construction, drainage systems, retaining walls, and geomembrane protection.

l INTRODUCTION

During the last few years, many different kinds of geotextile-design methods have been established, mainly by the manufacturers, but also by independent institutes and by people editing national standards or guidelines. Since these design suggestions were all based upon design manuals and design charts, it was often a hard, time-consuming job to design a geotextile for a certain application, and the engineers therefore often hesitated to use these materials.

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442 Gernot Mannsbart, Siegfried Resl

With the development of an easy-to-handle adequate computer program, which can be used like an expert system, this disadvantage could be eliminated.

1.1 KnowledgePro: hypertext and expert system

The Polyfelt design-disk offers not only solutions for all application areas of geotextiles but also the possibility to ~communicate" with the computer by using hypertext and an expert system (Thompson & Thompson, 1988). Hypertext is a feature that optionally offers background information, which is provided by the program itself and can, but need not necessarily, be viewed by the user of the program.

Expert systems allow the computer to communicate ideas and inform- ation where the program is able to ask 'intelligent" questions depending on available information.

The combinat ion of Hypertext with an expert system gives the possibility of interactive design, taking into account the user's state of knowledge.

The Polyfelt design disk is based on the renowned 'Polyfelt Design and Practice Manual" (1986), supplemented by the results of the latest geotechnical research.

2 GEOTEXTILE DESIGN OF UNPAVED ROADS

In the design disk, the design of a geotextile for the construction of unpaved roads is divided into two situations, as follows.

(a) Geotextiledesign." This part of the program suggests the choice of the correct Polyfelt grade as a function of subgrade strength and condition, traffic load, type of fill material, and thickness of the first fill layer.

(b) Unpaved-road design: Here the required fill depth for an unpaved road over a weak subgrade is a function of traffic load, and the allowable rut depth. The required rut depth is determined and the correct Polyfelt grade chosen.

The design of the fill depth is done in two steps:

• design of the fill depth to avoid shear failure for static loading; • introduction of traffic stress.

The static required thickness Do is a function of the subgrade CBR, the axle load, and the friction angle of the fill material. This method is summarized in a design chart, which can be found in the Polyfelt literature. The chart has been derived from plate-loading tests carried out with different subsoils and fill materials.

The influence of traffic is introduced by the traffic-influence factor.fv,

Expert ,2vstem ,for design of geotextiles 443

which is a function of the allowable rut depth and the total number of load repetitions during the design life. This again is summarized in a design chart, which has been derived from practical experience, including some aspects of the design recommendations in the Swiss Geotextile Manual (published by SVG - the Swiss Association of Geotextile Experts (1988)).

The final depth is then calculated by multiplying Do andfv with an adequate factor of safety, which considers uncertainties in the assumption of the design parameters.

Experience has shown that a certain min imum thickness is required to guarantee trafficability of the road. For CBR values < 1, this is 40 cm; for CBR values > 3, this is 20 cm; for values between these limits, it can be interpolated. Furthermore, the fill thickness should also be a min imum of 1.5 times the maximum grain size.

2.1 GeotextUe-grade selection

For the design of the required geotextile, three criteria have to be taken into account:

• the puncture criterion: • the hydraulic criterion; • the installation criterion.

A certain min imum grade of geotextile is required to allow easy installation without damage. The relevant parameters are the subgrade strength, subgrade condition, and type of fill material. The following categories have been established:

subgrade strength: • ~high" (CBR > 3) • "medium' (CBR = 1-3) • ~low' (CBR -- 0.5-0.75) • ~very low' (CBR -- 0.25) subgrade condition: • 'cleared" • 'partially cleared' • 'uncleared'

according to the recommendat ions in the FHWA manual (USA) (FHWA, 1989);

fill material: • ~typical' (stone size < 100 mm) • ' intermediate' (stone size 100-250 mm) • ~unsorted blasted' (stone size > 250 mm).

444 Gernot Mannsbart, Siegfried Resl

For the lowest stress level ('high', 'cleared', 'typical'), the required min imum grade is Polyfelt TS420. Each single increment of the stress level corresponds to the next higher Polyfelt standard grade.

2.2 Puncture criterion

Sharp-edged aggregate results in puncture stress to the geotextile due to ground pressure caused by axle loads.

Relevant parameters are:

• axle load • fill thickness • mean grain diameter

(i.e. dm,x/4 for non-uniform material (Cu > 5), and din,x/2 for uniform material (CL, < 5)).

Based on laboratory puncture tests with sharp-edged pyramid pistons. a design chart has been established, which can be found in the Polyfelt design literature (Polyfelt TS - - Design and Practice or other publication).

When rounded aggregate is used. the lowest grade of Polyfelt (TS420) is generally sufficient, as the puncture stress is nil.

2.3 Hydraulic criterion

The hydraulic characteristics of the geotextile must guarantee the free exit of the rising pore water and simultaneous retention of the fines. The requirements are mainly based on practical experience and laboratory tests.

The most critical situation occurs when coarse-grained fill material is used (i.e. ds0 > 50 mm and Cu < 5) and medium/heavy daily traffic is applied (i.e. more than ten trucks per day): in this case, the required effective opening size, Dw, must be smaller or equal to the ds5 value of the subgrade soil; for light traffic (i.e. fewer than ten trucks per day)~ a maximum Dw of0.11 mm is required. For all other types of fill material, a Dw of less than or equal to 0.12 mm is sufficient.

3 GEOTEXTILE DESIGN FOR HYDRAULIC CONSTRUCTION

For hydraulic constructions, two basic categories of works are covered:

(a) river-bank protection and (b) coatal protection

Expert system for design of geotextiles 445

River-bank protection covers all erosion-protection work where the soil has to be protected from flowing water (rivers, streams, canals, etc.).

Coastal protection deals with all erosion-protection work where the soil has to be protected from wave attack (sea coasts, banks of lakes, etc.).

Proceeding with the program, the user is asked to enter the following information as input parameters:

gradation of the subsoil, stiffness of the subsoil, type of revetment, stone size, and type of installation,

for river-bank protection, and

gradation of the subsoil, stiffness of the subsoil, wave height, slope geometry, block size, and type of installation

for coastal protection. In order to fulfil the filtration function, the geotextile has to meet the

following requirements:

• hydraulic requirement: an adequate opening size to retain the fine soil particles, and high water-permeability to allow the water to pass through: and

• mechanical requirement: it must not be damaged during the placement of the revetment blocks.

3.1 Hydraulic requirements

The filter criteria differentiate between cohesive and non-cohesive soils. Since cohesion prevents the separation of individual soil particles by water, the filter criteria for cohesive soils can be less strict. It is recommended that, for such soils, the effective opening size (Dw) of the geotextile should not exceed O. 11 mm. The filter criteria for non-cohesive soils have been evaluated by a large number of laboratory tests.

The decisive parameters are:

• the ds0 value of the soil to be filtered, • the uniformity coefficient, Cu (the more uniform the soil, the higher

is the danger of erosion), and • the ratio d~5/dso.

446 Gernot Mannsbart. Sieg[ried Res'l

The required effective opening size D,,. is a maximum o f A * dso. where A can be read from Table 1.

3.2 Mechanical requirements

During the installation of the revetment, geotextiles are subject to great mechanical stresses. When riprap stones or blocks are dropped from a certain height, the geotextile will be subject to an impact stress. The drop energy is calculated by multiplying the block weight by the drop height and an adequate factor of safety.

When a block is dropped onto a cushion layer of smaller stones, which is placed on top of the geotextile, the drop energy of the block is distributed over several stones. To take this into account, the drop energy is reduced by the factor (d~tonJdbl,,~.k) j3~, where

d~o,~c = diameter of the cushion-layer stones: and dNock = diameter of the revetment blocks

On the basis of this design, the drop energy, and the CBR value of the subgrade, the required Polyfelt grade can be selected. The relevant diagram can be found in the Polyfelt design literature. The weaker the subgrade, the lower is the danger of puncturing the geotextile, since the needlepunched nonwoven can more easily adapt to the deformations caused by the stones or blocks.

4 GEOTEXTILE DESIGN FOR DRAINAGE SYSTEMS

In drainage systems, geotextiles are used to avoid the infiltration of fine soil particles into the drainage system, which could otherwise be clogged up. In order to fulfil its filtration function, the geotextile must meet two basic requirements:

• hydraulic requirement: an adequate opening size to retain the soil particles, and high-water permeability to allow the water to pass through; and

Table 1

Cu 1-2 Dss/Ds0 > 4 2-4

1-3 1.0 1.5 1.5 3-6 1.2 1.8 1.8 > 6 !.0 1.6 2.0

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• mechanical requirement: the geotextile must not be damaged during installation.

4.1 Hydraulic requirements

The filter criteria differentiate between cohesive and non-cohesive soils. Cohesion prevents the separation of individual soil particles by water, so the filter criteria for cohesive soils can be less strict. It is recommended that, for such soils, the effective opening size (Dw, 0~) of the geotextile should be less than 0.15 mm. The filter criteria for non-cohesive soils have been evaluated by a large number of laboratory tests.

The decisive soil parameters are:

• the ds0 value of the soil to be filtered; and • the uniformity coefficient, CL. (the more uniform the soil, the higher

is the danger of erosion). The criterion has been summarized in a design diagram, which is taken from the Polyfelt design literature.

4.2 Mechanical requirements

On the basis of the grain shape, the grain size of the drainage gravel, and the drop height during installation, a certain minimum grade is recommended to avoid puncturing the geotextile. For rounded drainage gravel, the dange of puncturing the geotextile is nil, so Polyfelt TS21 is sufficient.

For angular drainage gravel, the recommended minimum Polyfelt grade can be read from Table 2.

5 GEOTEXTILE DESIGN FOR RETAINING WALLS

The internal-slope stability of an earth-retaining wall/steep slope can be increased by introducing several horizontal layers of geotextiles. The

Table 2

Drop height Grain Size." Diameter (ram)

< 100 100-200 200--300 > 300

< 1 TS21 TS21 TS22 TS420 1-2 m TS21 TS22 TS420 TS500 2-3 m TS22 TS420 TS500 TS550 3-4 m TS420 TS500 TS550 TS600

448 Gernot Mannsbart, Siegfried Resl

external stability is not covered by this calculation, and this has to be checked by some other conventional stability analysis.

5.1 Design method

A reinforced-earth wall is designed by calculating the total active earth pressure, which then has to be taken up by the tensile forces in the geotextile layers.

E = A . * ( p + y * h ) * h / 2

where E = total active earth pressure; ,,1. = earth-pressure coefficient: p = surcharge load: ~" = unit weight of fill material; h = wall height

In the case of cohesive fill material, the earth pressure is reduced by:

2*C*AA0.5

The active-earth-pressure coefficient, is calculated by a formula taking into account the internal friction and the angle between the wall surface and the vertical.

For the calculation, the earth-pressure distribution is not trapezoidal as in the theory but simplified to an even distribution over the whole wall height. The total earth pressure, E, is divided by the number ofgeotextile layers, resulting in the required tensile force R. This is now compared with the tensile strength of the Polyfelt grades, reduced by an adequate safety factor.

6 GEOTEXTILE DESIGN FOR G E O M E M B R A N E PROTECTION

In the construction of tunnels and landfills, the waterproofing liners often consist of a geomembrane. In order to protect this membrane against puncture and abrasion, both during installation and after completion, a geotextile will be placed adjacent to the geomembrane. These calculations will lead to the required Polyfelt grade to fulfil the protection function. Other important aspects of the design, such as slope stability, drainage capacity, etc., are not covered by these calculations.

6.1 Design method

The design is based on the pyramid puncture test. which has been developed by Polyfelt Ges.m.b.H. in order to quantify the protection

Expert system for design of geotextiles

Table 3

Unit w e i g h t Membrane thickness (g m 2)

0.5ram I.OOmm 1.5ram 2.00mm

208 0.37 0.76 1.22 1.68 409 0.68 1.16 1.64 2.31 608 0.88 1.51 2.03 2.54 810 1.17 1.91 2.49 3.11

1004 1.58 2.41 3.00 3.61 1216 1.93 3.00 3.51 4.19

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capability of geotextiles. In this test, a pyramid piston simulating an angular stone is pressed onto the geotextile/geomembrane composite layer, which is placed on a metal plate, simulating a rigid sub-base (the worst condition that can occur in practice). Electrical equipment indicates the limit force when the geomembrane is punctured. After a long series of tests, the basic values for different geomembrane thicknesses and geotextile unit weights have been derived. Some typical test results (in kN) are shown in Table 3 (Puehringer, 1990).

In doing the design, the maximum overburden pressure on the lining system is derived (either from surcharge load or from the ground pressure of the construction equipment), and the load on a single stone is calculated (on the basis of the geometrical conditions). This value is then multiplied by a factor of safety and compared with the laboratory test results.

In choosing the safety factor, the following parameters must be taken into account: the allowable deformation of the membrane, the long-term creep deformation, the influence of temperature, stresses applied during construction, the consequences of a possible failure, etc. In the construction of a municipal landfill, a safety factor between 3 and 10 is recommended. The higher value of the surcharge load caused by the overburden pressure of the solid waste or the normal pressure caused by the construction equipment is relevant for the design.

REFERENCES

Anon. (1986). Polyfelt TS Design and Practice Manual. Polyfelt G.m.b.H., Linz, Austria.

FHWA. (1989). FHWA Manual, Research Development and Technology Center, McLean, VA, USA.

450 Gernot Mannsbart. Siegfried Resl

Puehringer, G. (1990). Pyramid puncture test. Paper presented at ASTM Symposium on Geosynthetics Testing for Waste Containment Applications, Las Vegas, Nevada, USA.

SVG. (1988). Schweizer Geotextilhandbuch, SVG, St. Gallen, Switzerland. Thompson, B. & Thompson, B. (1988). KnowledgePro expert system shell.

Knowledge Garden, Inc., Nassau, NY, USA.