With this paper on the Testing of Geotextiles we are trying to address some of the major obstacles in the satisfactory use of geotextiles. Geotextiles are made up of synthetic materials such as polythelene, polyester and nylon which are used for stabilizing and improving the strength of soil. The particular function of geotextile include separation, reinforcement, filtration, drainage and moisture barrier. The rapid growth in the development and use of geotextiles has led to problems of design and specification for this diverse range of materials and their numerous applications. Although a number of countries are actively engaged in the preparation of national design codes and model specifications, a major obstacle has been a lack of internationally recognised standard test methods.
GENERAL ASPECTS OF TESTING GEOTEXTILE:
Like any other material that is use for industrial or construction purpose, geotextile are submitted to testing for the determination of the material properties. According to different level of interest, four main categories of testing level can be distinguish
- DEVELOPMENT TESTS
- FUNDAMENTAL TESTS
- SUITABILITY TESTS
- QUALITY CONTROL TEST
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| NonoWoven Geotextile Needled |
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| NonWoven Geotextile ,Heat Bonded |
Tests during the stage of development of new product are performed for the determination of basic properties, with respect to the producibility and applicability of the new product. They are carried by chemist and physicist in the research laboratories of geosynthetics producers. Their result are kept secret, unless they are useful for marketing and at the same time unless for the competitor in the market.
Fundamental Tests:
Fundamental Tests on geosynthetics aim at a better understanding of their properties and performers. They are subjected to scientific research carried out at institutes that are independent of the industrial producers. Since geosynthetics are usually forming composite systems with soils ,the great variety of soil on one hand and the great variety of geosynthetics on the other hand, provide a wide field of unsolved problems and ample room for scientific research and development .
Suitability Tests: -
Suitability Tests of the geosynthetics are carried out to evaluate their suitability for certain technical assignment. These investigations can comprise chemical, mechanical and hydraulic test on the geosynthetics or on systems composed of geosynthetics and soil. Suitability test will always comprised the application of established and standardized testing method to evaluate the suitability of a new product for design purpose. These test also cover some aspect of quality control because they provide basic data fro specifications which have to be met continuously in production and construction.
Quality control test:
Quality control test on geosynthetics are required for quality assurance. Chemical, mechanical and hydraulic test that render information about the properties, relevant to the technical application on carried out on construction projects by specialized institute. It must be possible to carry out quality control test quickly, efficiently and regularly during production or construction. These test, therefore, should be of short duration and simple to set up and have an obvious relationship to the qualification. Quality control tests are an essential part of construction supervision but since many civil engineers in practice have no knowledge at all or only insufficient knowledge about geosynthetics, expert are usually needed for the quality control of geosynthetics on all major construction project where geosynthetics are installed.
TESTS DETERMINING OF MECHANICAL INDEX PROPERTIES ARE : -
- Tensile test
- Plunger test
- Cone penetration test
- Creep test
Tensile test:
There are many different ways to perform a tensile test. In Germany, the strip tensile test has become a standard .The free testing length of the samples is cut 100mm wide and folded to 50mm width for testing. The tensile test is performed with a constant strain rate of 200mm/min. At least 5 samples have to be tested in the longitudinal and in the transverse direction each. The tensile forces are recorded in the unit kN/m and strain in %. Mean values have to be reported of the maximum tensile force and strain at maximum tensile force.
Plunger test:
When a geotextile is used for the separation of two soil layers, in particular above soft ground, it is stressed biaxially and the puncture strength and biaxially deformability criteria for design. Such a loading condition is approximated by the plunger test according to DIN 54307. The test is carried out in a modified CBR testing machine. The sample is fitted between the two circular tension rings with an inner diameter of 150mm and the outer diameter of 230mm. The hollow cylinder has an inside diameter of 150mm and the must be sufficiently high for the plunger piston to move downwards over the distance of 150mm. The plunger is the circular cylinder with a diameter D = 50mm and a height of 75mm. In the test it moves downwards at a constant speed of 60 + 10 mm/min. The force acts on the geotextile sample and the deformations are recorded. The force-displacement curve is plotted continuously or by stepwise method for 100, 500,1000 N etc. The test has to be carried out on ten samples of the geotextile and the mean values of the puncher force, standard deviation and variation coefficient have to be reported.
Cone penetration test:
The cone penetration test was developed in the Norway. It is some times carried out in the Germany. The geotextile specimen is clamped into a hollow cylinder like in the plunger test. A cone of 1 kg weight, 50mm diameter and an angle of 450 at the point is dropped from the height of 500mm and makes a hole into the geotextile specimen.
Rammer Test :
A test for investigating the resistance of a geotextile to puncturing by falling stone that is carried out with a special 30kg rammer is called test. Depending on size of stone that will be dropped on geotextile at particular site, the height that the rammer is dropped is chosen as1.0 or 2.0.The test is positive if no puncher is recorded on three consecutive samples of soil, tested with variation of the direction of the rammer blade with respect to the geotextile:-parallel, perpendicular and inclined under 450 to the longitudinal of the geotextile.
Creep test:
The time dependent of geotextiles can be studied by simple creep test. Samples of 200mm free length are subjected to constant tensile loads the longitudinal are measured after 1,24,96 & 500 hours. This test is very simple and may not be sufficient for the design of reinforcement structures, where the long term stress-strain behavior of the geotextile is of significance. There is no standard method of testing for such situations at the present time that make it necessary to perform specific tests for individual projects.
TESTS FOR HYDRAULC PROPERTIES OF GEOTEXTILES:
Application of geotextiles to the construction of hydraulic structures such as dams, shore and bank protection or drainage facilities obviously require the knowledge of the hydraulic properties of the geotextiles. For this purpose simple standard test are carried out for the determination of
- Permeability
- Transmissibility
- Effective opening size
The permeability for the flow direction perpendicular to the plane of the geotextile is determined under conditions of a small constant head for laminar flow. Circular samples of geotextiles of 145 mm diameter are laid in layers to a total thickness of above 20mm and kept submerged for 24 hours. The test sample is then de-aerated by applying a vacuum. The permeability test is carried out under a hydraulic gradient i < 2 with de-aerated water. During the test the normal pressure on the sample is increased step-wise from 2 kN/m2 to 200 kN/m2.
Transmissivity test:
In situations where geotextile acts as a drain ,the permeability in the plane of the geotextile in which the water s transmitted –or the transmissivity in the plane should bee known .Since most of the geotextiles are anisotopic, usually the permeability n the plane direction is determine on the rectangular samples in two perpendicular directions. Samples 100mm x 165mm are inserted in the apparatus to a thickness of > 20mm when loaded by the surcharge in steps from 2kN/m2 to 200 kN/m2.Before the application of surcharge load, the sample is submerged for 24 hours and de-aerated by vacuum .
Effective Opening size:
Filter criteria for geotextile filters are directly related to the effective opening size dw .The effective opening size in geotextile dw is determined by wet sieving quartz sand of known composition using modified equipment. In the test the geotextile functions as a sieve .The geotextile sample of 180 mm diameter s clamped in to a sieve frame. The sand, which has a dry weight of 100grams form wovens and a dry weight of 300 grams for non wovens, sprinkled on the sample. The sand is then sprayed continuously with water and sieved for 15 minutes in modified sieve machine with a vertical amplitude of 3mm at a frequency of 50 Hz. The sand which passes through the geotextile sample is caught on a paper filter and then analysed with the following sieves 2500 microns, 700 microns, 500 microns, 400 microns, 200 microns, 100 microns, 63 microns and 45 microns. The sand retained on and in the geotextile is analysed in the same way separately. The effective opening size dw is found from the coarsest sand fraction which has passed through the test sample and in which still a defined quantity in relation to the original fraction.
CONCLUSION:
Since the composite system of soils and geotextiles exhibit rather complex behaviour, there are still many unsolved questions. Testing programs such as those discussed above will contribute to the understanding of the performance of the geotextiles. They will also make us aware of the limitations and will help us in avoiding the failures. These will ultimately play a great role in promoting the use of geotextiles in civil engineering.
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