GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE

Transcription

1

2 GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE Prof. J. N. Mandal Department of civil engineering, IIT Bombay, Powai, Mumbai , India. Tel

3 Module - 3 LECTURE- 12 Geosynthetic properties and test methods

4 RECAP of previous lecture.. Puncture resistance test Penetration resistance test (drop test)/ tear resistance Tensile behavior of geogrid Geogrid rib tensile strength Geogrid junction (node) strength Junction strength of geocell Tensile strength of gabions Direct shear test on geosynthetic

5 Pullout or anchorage resistance It is very important to compute the pullout capacity of reinforcement to ensure stability of any reinforced structure like reinforced soil retaining wall, reinforced slopes etc.

6 Two basic mechanisms are involved to mobilize or transfer pullout resistance between soil and geosynthetic 1) Interface friction, and 2) Passive resistance Only interface friction is associated with geotextile Both interface friction and passive resistance are associated with geogrid. Pullout resistance or anchorage capacity is expressed as the ratio of pullout force to the width of the sample (kn/m)

7 Pictorial view of pull-out test Schematic view Interaction coefficient of geotextile (C i ) P r = F/W = 2. L. n. C i. tan C i Pr 2 L ( h q ) tan

8 FEM analysis of pull-out test on cellular reinforcement Cellular reinforcement Stress distribution in cellular reinforcement

9 Ultimate pullout load was found increasing with increasing height of the reinforcement up to 30 mm, Further increase in height shows the decrease in ultimate pullout resistance. The optimization analysis shows that the spacing to height ratio of 3.3 gives the maximum pullout resistance for cellular reinforcements.

10 Example: Determine interaction coefficient. The following data is given. P = 65kN/m; L e = 1m; = 30 ; q = 60kpa. Solution: σ n = γ x h + q = 20x = 66 kpa P = 2 C i L e σ n tan 66 = 2xC i x1x66xtan30 C i = (66)/ (2x66x0587) = Interaction coefficient = C i = 0.849

11 Tensile behavior of geomembrane Smooth high density polyethylene (HDPE) and textured high density polyethylene (HDPE) geomembrane are used for conducting dumbbell shaped tests. Test specimens are dying cut from large sheets ASTM D 638, D 882, D 6693 (Dumbbell shape) Dumbbell shaped test specimen

12 Tensile behavior of dumbbell shaped geomembrane

13 Tensile behavior of wide width shaped Geomembrane is suitable in plain strain condition and much more design oriented compared to dumbbell shaped geomembrane Specimen is 200 wide with 100 mm gauge length Strain rate = 1 mm/ minute Wide width geomembrane (ASTM D4885)

14 Greater width of the specimen minimizes the contraction edge effect (necking) and provides closer results to actual material behavior (ASTM D4885). Tensile behavior of wide width shaped geomembrane (Smooth and textured HDPE)

15 Tensile behavior of smooth and textured 1.5 mm thick HDPE geomembrane Tensile property Dumbbell shape Narrow width (25 mm) Wide width (200 mm) ASTM D638 ASTM D882 ASTM D4885 Smooth Textured Smooth Textured Smooth Textured Strength at yield (kn/m) Elongation at yield (%) Strength at break (kn/m) Elongation at break (%) > 500 > 500 > 500 > 500

16 Tear resistance of Geomembrane (ASTM D 1004, D2263, D5884, D751, D1424, D1938, and ISO 34) The specimen has a 90 degree angle. Tearing resistance of geomembrane (a) schematic view and (b) pictorial view

17 Geomembranes can be joined for seam in shear and seam in peel test. Equipments for joining geomembrane Some typical seams of geomembrane (After Giroud, 1994)

18 Hydraulic properties Porosity Apparent opening size Percent open area Permittivity or cross plane permeability Transmissivity or In- plane permeability

19 Porosity Porosity (n) = (Volume of void / Total volume) = V v / V Total volume (V) = V s + V v V s = volume of solid = ( m. A) /, m = mass per unit area (g/m 2 ), A = Area (m 2 ), = density (g/m 3 ), V v = volume of void, V = total volume = A. t g t g = thickness of geosynthetics. n V V v V V V s 1 V V s n 1 m.a A.t g 1 m. t g

20 Apparent Opening Size (A.O.S.) or Equivalent Opening Size (E.O.S) [ASTM D4751] Apparent opening size can be measured in four ways: 1. By sieving glass beads 2. By image analyzers (Gours et al. 1982), and 3. By mercury intrusion (Holtz, 1988) 4. By bubble point method (Bhatia et al., 1996) Pictorial view of the glass beads of different sizes

21 The size of the beads which passes by less than or equal to 5 % is represented as Apparent opening size (A.O.S.) or O 95 expressed in millimeters. Determination of apparent opening size by dry glass sieving method The O 95 value is specifically used for design of any hydraulic structure.

22 Apparent opening size of different geotextile filters Apparent opening size of geotextile decreases with increase in the weight of geotextile.

23 Percent open area (POA) Percent open area can be defined as the ratio of total open area or total voids area of the geotextile to the total area of geotextile. It is expressed in percentage (%). POA Total area of the openings of geotextile Total area of geotextile The open area is measured by passing a light through the geotextile to a poster sized cardboard which is in the form of a graph sheet. From the graph sheet, number of squares can be counted. Otherwise, the voids can be mapped by a planimeter. Total area is measured by same magnification.

24 POA is applicable only for monofilament woven geotextile. The percent open area (POA) for monofilament and slit film wovens should be greater than or equal to four percentage. As the filaments of non woven geotextiles are closely tightened and very random, light cannot pass through it properly and as a consequence, the light passing method is not suitable for it.

25 Please let us hear from you Any question?

26 Prof. J. N. Mandal Department of civil engineering, IIT Bombay, Powai, Mumbai , India. Tel