Исследование механических свойств при растяжении геотекстильных полотен и георешеток, используемых в гражданском строительстве

В данной работе было проведено исследование механических свойств при растяжении геотекстильных полотен и георешеток. Были исследованы пять образцов различной структуры, включающие тканое геополотно, нетканое геополотно, основовязаную георешетку с нетканой подложкой и экструдированную георешетку. Определены основные показатели свойств геосинтетических материалов, такие как прочность при растяжении, удлинение при максимальной нагрузке, нагрузка при растяжении при определенном удлинении и секущий модуль. Показано, что геосинтетические материалы обладают анизотропией свойств, прочность при растяжении исследуемых образцов находится в диапазоне от 8,4 до 87,4 кН/м, удлинение при максимальной нагрузке от 2,4 % до 103,5 %. Значения секущего модуля лежат в диапазоне от 300 до 2115 кН/м. Рассматривается взаимосвязь структуры и свойств.

Ключевые слова: геотекстиль, георешетки, тканые полотна, нетканые полотна, механические свойства, жесткость, прочность при растяжении, секущий модуль, испытания, строительство

1. Literature review

The key to improving the quality of building structures, reducing construction time, and partially reducing the cost of construction lies in the application of advanced technologies and materials such as geosynthetics. Geosynthetic materials are widely used in civil engineering applications presenting a special class of modern building structural materials [1,2]. They have found a number of applications and have a great potential in various fields of civil engineering, including the construction of engineering structures, road construction, railway construction, hydraulic engineering, building of sports facilities, etc. [3–10]. Historically this type of materials used in the largest quantities in road construction. Nowadays, however, geosynthetic materials are widely used for road and railroad construction, drainage systems, landfills, building of sports facilities, etc. At present, geosynthetics are used almost in each project. Geosynthetics can improve pavement performance and reduce life cycle costs [11–12]. Most geosynthetics are man-made materials made from various types of polymers both natural and synthetics used in environmental, transportation and civil engineering projects. Geosynthetics are used in one or more of the following functions: separation; reinforcement; protection; filtration; drainage; erosion control; barrier. The common types of geosynthetics are the following: geotextiles; geogrids; geonets; geomembranes; geocells; geocomposites. In this work two major tyopes of geosynthetics: geotextiles and geogrids are considered.

Geotextiles have been used very early for various building and landscaping applications. According to [13], a geotextile is defined as a planar, permeable, polymeric (synthetic or natural) textile material, which may be nonwoven, knitted or woven, used in contact with soil and/or other materials in civil engineering applications The structure of the material in primarily determined its production technology and in accordance with the conventionally mainly divided into three classes include woven, knitted and nonwoven products [14,15]. Woven geotextile is a geosynthetic material produced by weaving technology. Knitted geotextiles is a geosynthetic material obtained by knitting technology. Nonwoven geotextile is a geosynthetic material obtained by technology for manufacturing of nonwoven materials.

Geogrids is defined as a planar, polymeric structure consisting of a regular open network of integrally connected, tensile elements, which may be linked by extrusion, bonding or interlacing, whose openings are larger than the constituents [13]. Geogrids elements called ribs may have different apertures (up to 20 cm) and different rib junctions (bonding or crossover joining). The generally include uniaxial, biaxial and triaxial grids. Geogrids are designed to be a support structure, and, therefore, must have both high strength and low elongation.

2. Problem definition

The testing of geosynthetic materials by engineers is always in terms of forecasting whether a design will hold or not. The two opposing principles to achieve this are the scientifically accurate experiment and the principle of abstraction. The modelling of a built-in geosynthetic material could be carried out in a manner similar to that of the fibre–matrix in a composite, i.e., by modelling the soil as the matrix material and the geotextile as the reinforcing fibre. Especially with simple geosynthetic structures such as geogrids, this is theoretically feasible. However, this method is not described in any of the literature or standards. In fact, the filling is always excluded from the equation and only the geosynthetic material is submitted to tests because the filling can have a wide range of properties (which are usually neither isotropic nor even homogeneous), depending on the materials available on-site. Thus, unlike steel, where a few, often one-dimensional tests will give values that can be used for very detailed and accurate construction, the possibilities for detailed construction using filling are limited. The tests therefore must be carried out as a specialized case and, in fact, many of them have to be done on-site. Test that are done in the laboratory are only concerned with the properties of the geosynthetic material itself and not with the properties of the construction as a whole. The degree of useful abstraction thus always depends on the site and the application.

The tensile properties of geosynthetics define their relationship to the action of forces applied to them, the action of which they are deformed. They are important in choosing the material especially in cases where geosynthetics performs the primary load-bearing function. The choice of geosynthetic material depends on the function, application and based on tensile properties such as strength, elongation, stiffness. Tensile properties is one of the most common ways of obtaining mechanical characteristics. The main focus of this work was to investigate and analyse the tensile properties of geotextile fabrics and geogrids for further suitable application.

3. Description of the research

For the experiment, five different types of geosynthetics including geotextile fabrics and geogrids have been investigated. Table 1 summarizes the designation and characteristic of samples investigated. They include two geotextile samples and three geogrid samples. Geotextile samples consist of woven fabric (denoted as 1-GTX-PET), which have a mass per area of 480 g/m² functioning as separation and reinforcement fabric and made of polyester and needle-punched nonwoven fabric (denoted as 2-GTX-PP) which have a mass per area of 160 g/m² and made of polypropylene functioning as filter and protection layer. Geogrid samples consist of warp-knitted geogrid (denoted as 1-GGR-PET) with layer of nonwoven fabric, the components are sewn together, which have a mass per area of 285 g/m² functioning as reinforcement and made of polyester; extruded plastic geogrid (denoted as 2-GGR-PP), which have a mass per area of 530 g/m² and made of polypropylene functioning as reinforcement; warp-knitted geogrid (denoted as 3-GGR-GL) made of glass fibers treated with bitumen polymer.

Table 1

Experimental data

One of the most common methods for determining the maximal tensile stress and the maximum tensile elongation of textile materials is the tensile test. The essence of this method is to apply a load to a unit sample until it ruptures.The easiest way is to obtain stress-strain diagram, which represents the relationship between the elongation and the load until failure of the sample in the deformation mode at a constant speed. The tensile test was performed on an Instron 5965 tensile machine. The determination of tensile behavior of geotextiles under uniaxial loading is performed by standard method named strip test. The stress-strain diagrams were obtained with a 100-mm sample base and clamp movement of 100 mm/min. Testing of geogrids was conducted on single ribs. No fever than five specimens in the machine direction (MD) and five samples in the cross machine direction (CMD) were tested. It should be noted that tensile strains were calculated from crosshead displacement. The tensile diagrams of samples investigated in MD and CMD are presented in figures 1 and 2 respectively. Using the test results, the relative values of the tensile strength normalized to the sample width; elongations at maximum load tensile strength and tensile load at specified strain (2 %, 5 % and 10 %) were determined from stress-strain curves and given in the table 2. As seen in the figures, the values of tensile strength of the geosynthetic samples are in a very wide range.

The tensile strength of a geosynthetic is expressed in kilonewtons per meter (kN/m) directly from the data obtained from the tensile testing machine as follows:

,(1)

where

F_max is the recorded maximum load, in kilonewtons;

c is the specimen width.

For woven and nonwoven c is determined as follow:

,(2)

where

B is the specimen nominal width, in meters.

(3)

where

N_m is the minimum number of tensile ribs within a 1 m width of the geogrid;

N_s is the number of tensile elements within the test specimen.

Fig. 1. Stress-strain curves of geosynthetic samples (MD)

Fig. 2. Stress-strain curves of geosynthetic samples (CMD)

Table 2

Tensile properties of investigated materials

The maximum strength of the samples exhibit samples 1-GTX-PET and 3-GGR-GL (more than 80 kN/m), that undoubtedly advantage while used in a reinforcement function. Sample 3-GGR-GL has the lowest elongation (less than 3 %) and may perform a reinforcing function in the asphalt concrete. Samples of 1-GGR-PET and 2-GGR-PP have the strength of 40 kN/m, which is sufficient for reinforcing of various soil structures. that is quite enough for reinforcement function. The minimum tensile strength and maximum elongation has sample 2-GTX-PP.

The stiffness of a geosynthetic at a given strain level is the slope of a load-strain curve from a tensile test. A secant slope at 2 %, 5 % and 10 % strain is used to define the geosynthetic stiffness in this work. The secant stiffness J_sec is expressed in kilonewtons per meter (kN/m) at a specified strain as follows:

,

where

F is the determined load at strain ε, in kilonewtons;

ε is the specified strain, in percent.

The calculated values of secant stiffness of samples investigated in machine direction (MD) and cross-machine direction (CMD) at 2 %, 5 % and 10 % strain are presented in figures 3 and 4 respectively. As seen in the figures, the values of secant modulus of the geosynthetic samples are in a very wide range. The minimum value of 30 kN/m has the sample of nonwoven fabric (2-GTX-PP) that is not important because the material performs the function of filtering and separating and not reinforcing. There are more important properties in the through-thickness direction of the fabric. Another thing is the materials with reinforcement function, for which stiffness is a necessary condition for efficient reinforcing. The values of secant stiffness are in the range of 300–2115 kN/m. Further, it can be seen that the highest value of secant stiffness has glass fiber geogrid (3-GGR-GL) about 2000 kN/m that due to the properties of the raw material source.

Fig. 4. Secant stiffness of geosynthetic samples (cross machine direction)

4. Conclusions

1. Five samples of geosynthetic materials with different structures: woven geotextile fabric, nonwoven geotextile fabric, warp-knitted geogrid with layer of nonwoven fabric extruded geogrid and warp-knitted geogrid were investigated.
2. Tensile tests on investigated samples were done and the values of tensile strength, elongation at maximum load, tensile load at specified strain, secant stiffness were calculated.
3. It was shown that geosynthetic materials are anisotropic materials and their tensile strength of investigated samples is in the range from 8,4 to 87,4 kN/m; elongation at maximum load is in the range from 2,4 % to 103,5 %. The values of secant stiffness are in the range of 300–2115 kN/m.

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