Testing of Geosynthetics used in Reinforced Earth

PPT briefs about Geosynthetics

1. TESTING OF GEOSYNTHETICS Need for Testing- Identification of product. Selection of suitable materials as per the design specifications or regulations. Quality control – during production stage. Quality assurance – during the construction stage.

2. Collection of Test Samples ASTM D4354 “Standard Practice for Sampling of Geosynthetics for Testing” ISO 554 “Standard Atmospheres for conditioning and/or testing – specifications” ISO 9862 – “Geotextiles – Sampling and Preparation of Test Specimen” IS 14706 – “Geotextile- Sampling and Preparation of Test Specimen” During production stage, test samples are collected at uniform intervals for quality control purposes. During the construction stage also, test samples are collected at specified intervals. Number of specimen to be collected is given in respective standards.

3. Identification of the test samples Brand/Producer/supplier Description of type (grade etc.) Roll Number Date of sampling The sample shall be kept in a dry, dark place protected against chemical and physical damage at ambient temperature. Samples should be packed properly during shipping to the testing laboratories.

4. GEOTEXTILE PROPERTIES 1. Physical properties 2. Mechanical properties 3. Hydraulic properties 4. Endurance properties 5. Degradation properties

5. PHYSICAL PROPERTIES Specific Gravity Mass per Unit Area Thickness Stiffness

6. PHYSICAL PROPERTIES Specific Gravity – ASTM D792 It is the ratio of its unit volume weight to that of distilled, de-aired water at 27°C – as per IS. Determined by Pycnometer or density bottle method. The specific gravity of a base polymer is an important property as it can assist in identifying the base polymer of the geosynthetics. When the additives are added, the specific gravity of the resulting polymer may be higher or lower than that of the base polymer depending on the specific gravity and proportion of additive used.

7. Note that the specific gravity of some of the polymers (e.g., PP and PE) is smaller than 1.0, which is a drawback when working with geosynthetics in underwater applications as some of them may float. In such cases, Sinkers are used. Typical Values Steel – 7.87 Soil/Rock - 2.4-2.7 PVC – 1.69 Polyester – 1.22 to 1.38 Nylon – 1.05 to 1.14 Polyethylene – 0.90 to 0.96 Polypropylene – 0.91

8. 2. Mass per unit area – ASTM D5261 It is determined by weighing square or circular test specimens(5-10 specimen) of known dimensions (each specimen generally of area not less than 10000 mm 2 and combined total area of not less than 100000 mm 2 ), cut from locations distributed over the full width and length of the laboratory sample. Dimensions are measured without applying any tension. Measure the mass of the samples accurate to 0.01g. Mass per unit area = Total Mass/ Total area – reported in g/m 2 . For commonly used geosynthetics, the mass per unit area varies in order of magnitude from typically 100 g/m 2 to 1000 g/m 2 . In comparison to the geotextiles, the geomembranes may have substantially larger values of mass per unit area, even up to several thousands of grams per square metre.

9. 3. Thickness – ASTM D5199 The thickness of a geosynthetic is the distance between its upper and lower surfaces at a specified normal compressive stress (generally 2.0 kPa for geotextiles, and 20 kPa for geogrids and geomembranes) applied for 5 s. It should be measured by using a thickness-testing instrument to an accuracy of 1 mil ( = 0.001 inch = 0.025 mm). Expressed in mm. Woven geotextile- 0.25mm to 1mm. Non woven geotextile 1mm to more than 10mm

10. Thickness is not normally quoted for geotextiles, except for thicker nonwovens, but thickness is invariably quoted for geomembranes. The thickness values are required in the calculation of some geosynthetic parameters such as the permittivity and transmissivity. Since many geosynthetics, particularly geotextiles and some drainage geocomposites, are highly compressible, the thickness measure will greatly depend upon the applied normal compressive stress.

11. The mass per unit area of a geosynthetic can be related to its mass density (simply called the density) ρ or weight density (also called the unit weight) γ as where M is the mass, A is the surface area, V is the total volume, Δ x is the thickness of the geosynthetic specimen, and g ( = 9.81 m/s 2 ) is the acceleration due to gravity. If it is assumed that the density of the geosynthetic ρ is equal to the density of the polymer solid ρ s , then above Equation reduces to

12. where G is the specific gravity of the polymer solid, ρ w ( = 1000 kg/m 3 ) is the density of water, and γ w ( = 9.81kN/m 3 ) is the unit weight of water. Note that γ = ρg and γ w = ρ w g . Note that for the geomembranes, ρ ≈ ρ s . Q- For a geomembrane specimen, consider the following: Thickness, Δ x = 3 mm, and Mass per unit area, m = 2826 g/m 2 . Determine the specific gravity of the polymeric material of the geomembrane. Ans- 0.942

13. 4. Stiffness The stiffness (also known as the flexural rigidity ) of a geosynthetic is its ability to resist flexure/bending under its own weight. It is measured by its capacity to form a cantilever beam without exceeding a certain amount of downward bending under its own weight. Slide a 25mm wide strip of geotextile on an inclined plane at an angle of 41.5° and measure the length of the overhang(L) when tip bends under its own weight. Stiffness = (L/2) 3 × mass per unit area (mg-cm) Typical relation with Subgrade CBR value- Subgrade CBR Stiffness Requirements (mg-cm) <0.5 15000-25000 1-2 5000-10000 >2 ~1000

14. MECHANICAL PROPERTIES Compressibility Tensile Strength Seam Strength Fatigue Strength Burst Strength Tear Strength Puncture Tests

15. Compressibility Compressibility is the variation of the thickness of geotextile at different normal pressures. Thickness of woven geotextile is nearly constant at all normal pressures. Thick non woven (especially needle punched) exhibit marked reduction in thickness at higher pressure. Permeability properties are dependent on normal pressures. This decrease in thickness may result in the partial closing or opening of the voids of geotextile, depending on its initial structure and the boundary conditions.

16. KOERNER (1986)

17. Trapezoidal Tear Test- ASTM D4533 One of the test to ascertain construction survivability. Initially a small cut is given in the sample and the force required to tear the sample is measured. Force is applied on the sample in such a way that the initial tear is opened up. Result in reported in force units.

18. Wedge type grips used for trapezoidal tear test

19. Grab Tensile Strength (ASTM D4632) Construction survivability test Especially for separator applications in pavements. 25mm wide narrow grips used to perform the test. Test result is force in Newtons and rupture strain Loading at 300mm/min.

20. Tensile Strength Tests on Geotextiles (ASTM D4595) Different type of tests- Wide width tests 200mm wide and 100mm length. Narrow strip tests – 50mm wide strips and 200mm long. Samples usually gripped in roller grips – to provide smooth support to hold these geotextiles. Load applied at 10-20% strain per minute. Result is reported in units of force/unit width and the strain at peak load.

21. Geotextiles in retaining walls- use stiffer geotextile- higher strength at low strains – WOVEN Geotextile in coast line stabilization – geotextile tubes- undergo large strains due to deformations, ground movements, tides – Force developed in fabric is very low as they are not filled completely - NONWOVEN

22. Punching Strength – ASTM D4833 8mm diameter probe punched into a stretched geotextile Container diameter is 45mm Peak load developed is reported in Newtons

23. CBR Puncture Test ASTM D6241 using the same device Probe/Plunger is 50mm diameter Container is 150mm diameter Push it at rate of 300mm/min. 10 specimen are tested and average value is reported. Strength and Deformation are monitored.

24. Tensile strength from CBR Test Data Wide width tensile strength, T f = F p /(2  π r) T f = Force in kN/m F p = Punching Force, kN r = radius of CBR plunger Strain at failure, ε f = * 100 Where x= diagonal distance at failure a= horizontal distance between the outer edge of plunger and inner edge of mould  

25. Seam Strength Test Normal size of roll width is 3-5m and length ~ 100m. Larger size areas are covered by seaming geotextiles. Preferably the thread for seaming should be same type as the geotextile – polyester, polypropylene etc. Single stich, double stich, J-seam, Butterfly seam, etc. Seam strength is important as it can control the designs. Tensile strength tests are performed on the seams with same procedure. Efficiency = Strength of seam/ Strength of parent material

26. Burst Strength – ASTM D3786 Mullen burst strength – stone puncturing into a separation layer. Inflatable rubber membrane used to distort the geotextile into a hemisphere of 30mm diameter. When geotextile cannot deform any further, it will burst.

27. Fatigue Strength

28. Hydraulic Properties Apparent Opening Size Cross- plane permeability In plane permeability Gradient ratio

29. Apparent Opening Size- ASTM D4751 The ASTM method also called as dry method uses glass beads of uniform size. The test method involves in sieving uniform sized glass beads through the geotextile. Finer particles passing through is collected and analyzed. Main advantage of this method is that it is relatively faster compared to other methods.

30. Procedure of Dry Sieving Test Take 50gm mass of smallest size beads (75 µ) and sieve them for 10min utes and determine the percentage retained on geotextile. Repeat with next higher size glass beads until the percentage of glass beads passing through is x% or less. A graph is drawn between glass bead size on X-axis(log scale) and the %passing on Y-axis. If x% of a certain particle size is retained on a geotextile, the O x of the geotextile is the size of the particle in mm (usually 90% and 95% are used in the literature). ASTM designated AoS is O 95 corresponding to 5% particle passing through geotextile.

31. Limitations and Precautions Thick nonwoven geotextiles(especially needle punched type) may entrap the glass beads. Yarns in some geotextiles may move during the test thus affecting the AoS value. Glass beads may simply float instead of going through the geotextile because of their low mass. Electro-static forces may develop thus affecting the results – anti-static spray is used.

32. Hydrodynamic Test Method for AoS Also called as wet sieving method. Uniform sized particles are used in the test. Geosynthetic with sand particles are repeatedly dipped in water and taken out. Percent of sand particles passing through the geotextile is determined after each test. Procedure of test is similar to that of glass beads. Time taking process.

33. Cross plane permeability test – ASTM D4491 Constant head test 50mm head difference between the upper and lower surfaces of geotextile. Water allowed to flow through an opening of 25mm diameter. Volume of flow (>1 litre) in a given time (>30 seconds) is measured.

34. Q- Data from a test on cross-plane permeability is given below. Estimate the permeability coefficient and the permittivity. 1200 ml of water collected in 180 seconds under 50mm head of water. Thickness of the geotextile is 0.65mm. Diameter of the opening in the permeability device is 25mm.

35. Answer- Permeability = 1.77*10 -4 m/s Permittivity = 0.27/s

36. Falling head permeability test

37. In Plane Permeability Tests – ASTM D4716 Test is performed at different gradients of 0.25, 0.5 and 1.0. Normal pressure is applied on the sample. Minimum size of sample is 300mm*300mm. Geotextile should be sandwiched between two thick rubber sheets to prevent any leakage.

38. Constant head test θ = transmissivity (m 2 /s or m 3 /s-m)

39. Q- Data from an in plane transmissivity flow test on a jute geotextile is given below. Calculate the transmissivity and in plane permeability coefficient. 1 litre of water is collected in 60 seconds. Thickness of the geotextile is 2mm. Width and length of samples are 300mm. Head difference = 300mm.

40. Ans- i =300/300=1 Q = 1/60 = 0.0167 l/s = 1.67*10^-5 m^3/s Kp = q/( i *w*t) = 0.027 m/s θ = kp *t = 0.027*2/1000 = 5.55*10^-5 m^2/s.

41. Gradient Ratio Test – ASTM D5101 Flow through a soil column and geotextile layer to analyse the compatibility

42. Flow through a soil underlain by a geotextile filter layer. Compatibility between the two is established. Difference heads of water are measured and Gradient ratio is given as GR= For good compatibility between the geotextile and soil, steady state GR value should be less than 3.  

43. Gradient ratio tests on two different types of geotextiles and beach sand. After a long time steady state of flow is established.

44. Other Properties Of Geotextiles Abrasion resistance Durability properties

45. Abrasion Resistance – ASTM D1175 Abrasion is the “wearing away of a part of the material due to rubbing against another surface”. Degradation in strength and material loss may happen due to continuous rubbing with rough and hard surfaces during service life of a geotextile. Abrasion is generally important in below railway track applications. Rotary platform with a double head fitted with 1000g vitrified abrasion wheel is used in this test. Geotextile is a disk shaped sample with 90mm outer diameter and 60mm inner diameter. After 1000cycles of rotation, tensile strength test. Abrasion resistance is expressed as the ratio of tensile strength of the abraded sample to that of original sample. Typical loss of tensile strength is about 40% after 1000 abrasion cycle.

46. Abrasion by sand paper (ASTM D4866) The abrasion in this test is induced by rubbing a linear geotextile sample(50mm*300mm) by a sand paper(moves horizontally). A load of 6kg vertically is applied. Loss of weight and tensile strength after 750 abrasion cycle is reported as the abrasion loss.

47. Durability Test of Geotextile Accelerated Durability studies on geotextile – Mostly for coastal structures . Wetting of samples in sea water at night and Drying of samples in sun light during day time

48. Result of alternate wetting and drying