EFFECT OF INCLUSION OF GLASS GRID ON FLEXURAL BEHAVIOR OF COVER SOIL

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1 IGC 2009, Guntur, INDIA EFFECT OF INCLUSION OF GLASS GRID ON FLEXURAL BEHAVIOR OF COVER SOIL A. Rawat Post-graduate Student of Deptt. of Civil Engineering, Indian Institute of Technology, Bombay, Powai, Mumbai , India. R.R. Rakesh Scientist, Bhaba Atomic Research Centre, Trombay, Mumbai , India. J.N. Mandal Professor, Department of Civil Engineering, Indian Institute of Technology Bombay, Powai, Mumbai , India. ABSTRACT: Cover and capping system is an essential component of any landfill project. Cover system must be insured for its performance in the differential settlement and crack formation under flexural loading. A testing program is carried out to evaluate the effect of inclusion of glass grid on the flexural strength of cover soil. Size of beams was selected as mm and all the beams were casted at optimum moisture content (O.M.C.). The three point loading arrangement for flexural loading was used for the test with strain rate of 1mm/min. The results were obtained in form of load v/s deflection curve. The tests were conducted with three different locations of reinforcement 30 mm, 20 mm and 15 mm from bottom edge of beam, and results are optimized by comparing the load-deflection curve for reinforced and unreinforced case. The length of crack was recorded in each test on transparent sheet. The results shows that the brittleness of soil beam is reduce due to the inclusion of glass grid as planer reinforcement. Improvement in the flexural rigidity is also observed. The crack length also reduced due to inclusion of glass grid in cover soil. Finally flexural loading and beam tests were modeled with the Finite Element Analysis software PLAXIS V8 version. The values of maximum central deflection of beam for different conditions of test obtained in PLAXIS V8 version were matched reasonably with tests results. 1. INTRODUCTION Nuclear technologies which are used for generation of electricity and a wide range of products for agriculture, medicine and industry generate the waste. These wastes are both non-radioactive and radioactive in nature. The basic approaches used in the disposal of radioactive waste are Delay and Decay Dilute and Disperse Concentration and Contain The selection of approach depends upon the type of waste. The delay and decay approach can be used only if the radionuclide involved have reasonably short half-lives and permit their decay to arrive at permissible discharge limits with in a reasonably short period. In this approach the waste is dispose-off either near the surface or in the greater depths in rock cavities or boreholes again depends upon the activity of the waste. There is an international consensus that surface disposal and Near Surface Disposal (NSD) are suitable options for shortlived low- and intermediate-level waste (L/ILW-SL) (IAEA- Safety Standard Series, 1995 and 1999). Low and intermediate level waste is placed in engineered facilities which are closed off by engineered caps and landscaping. The cover soil of engineered caps may fail to fulfill its objectives due to Differential settlement and subsidence, Erosion (Erosion of cover soil), Intrusion (Rainfall ingress, human, animal and plant). The entire above problems may lead to failure of cover system. The main objective of this study is to find out the solution of above problems related with the cover soil only. Beam tests were modeled with the Finite Element Analysis software PLAXIS V8 version. Previously considerable research has been carried out to assess the flexural and tensile strength of soil, clay liners reinforced with different types of geosynthetics by Mandal 1988, Indraratna et al. 1996, Kaniraj & Gayathri 2006, Wang The local soil which was collected from the disposal site from B.A.R.C. near surface disposal facility Trombay, 333

2 Mumbai was used for casting of beam. The characterization and the properties of soil were determined. The size of beam is selected as width = 100 mm, depth = 100 mm and length = 300 mm, so width to length ratio of beam used for flexural strength test is 1:3. All beams for different tests conditions were casted on Optimum Moisture Content (O.M.C.). Three point loading arrangement was used during test with the strain rate as 1 mm/min, and total 12 beams were tested in which three beams were tested for unreinforced case, and total 9 beams for three different locations of reinforcement were tested in reinforced case. The test is carried out on the universal tensile strength machine. used for the flexure loading with the strain rate of 1mm/min. The complete arrangement of loading system with the Universal Tensile Strength Machine is shown in Figure 1. C.P.U. Computer Monitor Load cell Soil Beam Loading Pad Loading arm Display Board 2. LABORATORY INVESTIGATIONS The soil samples were collected from the near surface disposal facility located at radioactive waste storage and management site (RSMS), B.A.R.C Trombay, Mumbai, from three different locations at a depth of 1 meter from ground surface. The properties of local soil are documented in Table 1. Table 1: Properties of Local Soil Collected from Waste Disposal Site from BARC, Trombay, Mumbai, India S. Property Value No. 1. Natural Water Content (%) Specific Gravity Atterburg Limits (a) Liquid Limit (%) (b) Plastic Limit (%) (c) Shrinkage Limit (%) 24.6 (d) Plasticity Index (%) Optimum moisture content of 20.5 soil (%) 5. Maximum dry density of soil (g/cc) 6. Permeability of soil (cm/sec) Shear strength parameters of soil 8. Undrained cohesion (kpa) 14.5 Angle of internal friction (degree) Beam Testing with Local Soil (Unreinforced Case) Initially soil samples were oven dried for 24 hours and sieved through 4.75 mm sieve and beams were casted on the optimum moisture content. Standard proctor compaction method was used for the compaction. The no. of blows was re calculated as per the energy imparted by the standard proctor test (594 kj/m 3 ). Beams were compacted in three layers, so the number of blows for the mould size of mm and volume of 30,00,000 mm 3 is obtained as 78 per layer. During the test, three point loading system was Fig. 1: Loading System Arrangement Used in Test Total three beams were tested for unreinforced case and testing data load and deflection were then averaged. Figure 2 shows soil beam testing for unreinforced case. During the test the load at which the beam ultimately failed and cracked were noted with their corresponding deflection, shown in Table 2. The load-deflection curve of unreinforced beam is shown in Figure 3. (a) Before test (b) After test, insight shows the crack pattern Fig. 2: Beam Test of Soil Beam for Unreinforced Case 334

3 Load in Newtons Effect of Inclusion of Glass Grid on Flexural Behavior of Cover Soil During the test the load-deflection data were noted. The loaddeflection behaviour of glass grid reinforced beam is shown in Figure 5. The test results and factors related with the flexure testing are shown in Table 2 and Sudden loss in strength (Brittle failure) Deflection in mm Fig. 3: Load v/s Deflection Curve for Soil Beam for Unreinforced Case The parameters related with the flexural loading as moment of resistance (M), radius of curvature (R) and modulus of flexure (E) related with the unreinforced beams are shown in Table Beam Reinforced with Glass Grid as Planer Reinforcement Figure 4 shows the testing of soil beam reinforced with glass grid for a typical location of 15 mm from bottom edge of beam. (a) Glass grid placed at 15 mm from bottom, before test b) After the test, glass grid placed at 15 mm from bottom Fig. 4: Beam Test for Soil Beam Reinforced with Glass Grid Placed at 15 mm Load in Newton Ductile-plastic (post-peak behavior) 15 mm 20 mm 30 mm Deflection in mm Fig. 5: Load v/s Deflection Curve of Soil Beam Reinforced with Glass Grid Placed Different Locations from Bottom Edge Table 2: Result of the Soil Beam Test Unreinforced and Reinforced Case Test condition P crack δ crack P u L c Soil beam unreinforced case Soil beam reinforced case 30 mm mm mm Where, P crack = Load at which crack forms in Newton s, δ crack = Max. Central deflection in mm, P u = Load at which beam fails in Newton s, and = Crack length from bottom edge in mm. L c Table 3: Parameters Related with Beam Test Reinforced with Glass Grid Test condition Moment of resistance (kn-m) Radius of curvature R, (m -1 ) Modulus of flexure E, (kn/m 2 ) Soil beam unreinforced case Soil beam reinforced case 30 mm mm mm

4 3. FINITE ELEMENT ANALYSIS OF BEAM TEST RESULTS (PLAXIS V8) The reinforcement (glass grid) was modeled with the geogrid chain line as shown in Figure 6. The geogrid (GG) was selected as elastic material type. The properties of geogrid were determined and assigned. Figure 6 shows the geometric modeling of soil beam reinforced with glass grid for reinforcement location 15 mm. Fig. 7: Mesh Formations and Deform Mesh Shape for Soil Beam Reinforced with GG Placed at 15 mm Fig. 6: Geometric Model Simulate in PLAXIS V8 of Soil Beam Reinforced with Glass Grid at 15 mm The properties of geogrid modeled as glass grid are shown in Table 4 and Table 5 shows the values of applied load in PLAXIS V8. Table 4: Properties of glass grid Properties of thermally Value bonded GT Material type Tensile strength of GT Elastic Interaction coefficient R inter kn/m Table 5: Values of applied load in FEM modeling Location of reinforcement Applied load system A 15 mm from bottom edge mm from bottom edge mm from bottom edge Mesh Generation and Deform Mesh The deformed mesh generated in PLAXIS V8 FEM analysis for soil beam reinforced with glass grid is shown as Figure 7 for reinforcement location 15 mm. The vertical displacement of soil reinforced with GG due to flexure loading is shown in Figure 8 for 15 mm locations. Fig. 8: Vertical Displacement (Extreme Value; U y 3.10 mm) Location of Reinforcement 15 mm from Bottom Table 6 shows the comparison in between experimental and finite element study. Table 6: Comparison in Between Experimental and Finite Element Study Description of test Applied central load Extreme vertical displacement in experiment study Extreme vertical displacement in FEM analysis GG 15 mm GG 20 mm GG 30 mm RESULTS AND CONCLUSION The flexural testing of local soil beam using three point loading indicates that a small deflection is required to cause cracking of soil beam. The unreinforced beam shows the brittle failure in the post peak region. From the test data (P crack = N and δ crack = 2.1 mm for location 30 mm, 336

5 P crack = N and δ crack = 2.85 mm for location 20 mm and P crack = N and δ crack = 3.05 mm for location 15 mm, it can be concluded that due to inclusion of glass grid the soil load-deflection behavior improves. Initially the load is increased with deflection in all the three cases, after getting a particular load i.e. P crack the crack formation is takes place, as the glass grid give more tensile strength at lower strain value so after the crack formation the load value is reduces gradually, which shows some short of ductile-plastic failure (post-peak behavior) of soil beam. During this reduction in load value the crack propagates and the load now transfers to glass grid. Now at this load the crack formation stopped for some fraction of time and the elongation of glass grid took place. The location of reinforcement nearer to bottom edge i.e. 15 mm gives us the optimum test results. The crack length gets reduced as compare to unreinforced case. As the crack formation in cover soil is a low strain occurring problem, and geogrid give its full tensile strength at lower strain values, so it is suitable to use glass grid in spite of geotextile as planer reinforcement. FEM analysis with PLAXIS V8 shows well agreement with experimental test results. REFERENCES Indraratna, B. and Lasek, G. (1996). Laboratory evaluation of the load-deflection behaviour of clay beams reinforced with galvanized wire netting, Geotextiles and Geomembranes, 14, International Atomic Energy Agency Document. (1995). Principles of radioactive waste management, Safety series, No. 111-F, 3 9. International Atomic Energy Agency (1999). Technical considerations in the design of near surface disposal facilities for radioactive waste, IAEA-TECDOC-1256, Kaniraj, S.R. and Gayathri, V. (2006). Behavior of fibrereinforced cement stabilized fly ashes, Journal of Testing and Evaluation, 34(4), 1 8. Mandal, J.N. (1988). Reinforced soil beam subjected to bending, The Indian Geotechnical Journal, 18 (3), Wang, J.J., Zhu, J.G. and Zhang, H. (2007). Experimental study on fracture toughness and tensile strength of clay, Engineering Geology, 94,