MECHANICAL BEHAVIOUR OF DAMAGED HDPE GEOMEMBRANES

Transcription

1 MECHANICAL BEHAVIOUR OF DAMAGED HDPE GEOMEMBRANES Guangming Xu, Mingchang Yang, Xianghai Zong Geotechnical Engineering Department, Nanjing Hydraulic Research Institute, China Abstract HDPE geomembranes is one of main leak-roof barriers used in modern waste landfill. Some minor damages caused by careless placement of them and filling on them are often neglected, but under certain conditions they may trigger an unexpected tragedy in construction engineering of modern sanitary landfill. In order to investigate the breakage causes of a large area of HDPE geomembranes on a landfill slope triggered by a series of small boreholes accidentally produced on its surface, extension tests are performed of membranes with and without damage. It is found that the measured value of elongation at break for HDPE geomembranes with damage is a great deal less than that of the HDPE geomembranes without damage, accompanying a premature failure. A qualitative geotechnical centrifuge model test was also conducted. A similar failure mode on the model membrane with damage holes was observed under the working conditions of large municipal solid waste (MSW) deformation created by centrifuge modeling. It is shown that the failure mechanism of membranes with damage is in that there is much high concentrated strain formed in its damaged section. Key words: municipal solid waste (MSW), damaged HDPE, tensile test, centrifuge test, mechanical behaviour Introduction Since HDPE geomembranes offer very high tensile strength and they are resistant and impermeable to most hazardous materials, they have been well developed to provide long-term bottom lining and capping barriers in waste landfills. Moreover, high friction properties can also be achieved by the newly developed HDPE geomembranes with one or two surface textured, which is suitable for use in lining steep slopes. Unfortunately, their resistance to damage boreholes is not very high especially when the membranes are under high tension loads. Recently, a breakage of a large area of HDPE geomembranes on a landfill slope is due to a series of small boreholes happened in a newly designed modern waste landfill in the South China. In the case, 2 mm thick HDPE geomembranes with two surfaces textured were adopted as primary impermeable containment in the landfill from bottoms to steep slopes. Hardly had MSW been compacted into it in layers when a sheet of HDPE geomembrane on a slope in a unit was found to break severely near the crest of the slope. After a careful inspection, the cause for the failure is identified to be a series of damage boreholes accidentally produced by rolling stones down the HDPE geomembrane during construction. As effects of damage on mechanical behaviour of HDPE geomembranes has rarely been studied so far, an investigation is done. Both HDPE geomembranes and their model membranes are 29

2 tested in extension tests and geotechnical centrifuge model tests, respectively. The damage on HDPE geomembranes is prepared by folding the sample, or drilling single hole or multi holes through the samples. The purpose of the extension tests is to identify the difference of deformation behaviour of the membranes with and without damage. At the same time, a model membrane is chosen according to the similarity law of geotechnical centrifuge model. The model membranes with and without damage holes are tested in the model under the working conditions of large MSW deformation, the same conditions as that of prototype, so that the similar failure may be duplicated and verified. Prior to test After test Prior to test After test Prior to test After test Folding Drilling φ2.5 mm Drilling φ1.0 mm Defect D4-folding D5-single hole D7-single hole plus defect Prior to test After test Prior to test Approaching breakage Drilling φ1.0 mm Drilling φ1.0 mm End D8-twin holes D10-eight holes Figure 1 Illustration of damage forms on HDPE membranes samples Effects of damage on mechanical behaviour of HDPE Geomembrane The samples in the extension tests are the prototype HDPE geomembranes. The nominal physical and mechanical indexes of HDPE geomembranes are shown in Table 1, which was supplied by the manufacturer. For intact samples (marked by I), there are three dimensions: 50 mm (length) 50 mm (width), 100 mm 50 mm, and 50 mm 200 mm. For samples with damage (marked by D), there are four dimensions: 50 mm 50 mm, 100 mm 50 mm, 50 mm 200 mm, and 100 mm 200 mm. Figure 1 illustrates four forms of damage, created by folding the sample, drilling single damage hole, twin and multi damage holes through the samples, and combining the original defect and one damage hole. All damage holes are drilled in the centerline and their spacing is 25 mm both for twin holes on the 50 mm wide samples and for multi holes on 200 mm wide samples. Figure 2 is their typical curves of tensile load and tension strain. Table 2 presents the mechanical properties for HDPE geomembranes with and without damage obtained in the present extension tests. 30

3 Table 1 Indexes of HDPE geomembranes by the manufacturer Physical properties Unit Test method Value Thickness mm ASTM D Density g/mm 3 ASTM D Ultimate tensile strength N/mm ASTM D638 > = 50 Tensile strength at yield N/mm ASTM D638 > = 36 Elongation at break % ASTM D638 >= 700 Elongation at yield % ASTM D638 >= 13 Elasticity modulus MPa ASTM D638 >= 600 Permeability coefficient cm/s ASTM E96 <= Tensile behaviour of HDPE geomembranes without damage It is seen in Figure 2 that the relationship curves for three dimensions of sample without damage are very similar. That is, the tension force increases with tensile strain, reaches its peak value, and drops to a steady value and then almost stays at this constant tension level until its elongation at break is reached. It is found that the measured indexes (Table 2) are very close to the ones supplied by the manufacturer (Tables 1). Therefore, the prototype HDPE geomembranes without damage have very good mechanical properties: high tensile strength (T ult ) of 40 N/mm and great elongation at break (ε ult ) of %. No Table 2 Comparison of extension test results of HDPE geomembrane with and without damage T L (mm) ult ε Damage conditions (N/mm y ε ult W (mm) (%) (%) ) E (MPa) I Without damage > I Without damage > I Without damage D Folding D Single hole of φ2.5 mm D Single hole of φ1.0 mm D Single hole of φ1.0 mm with defect at one edge D Twin holes of φ1.0 mm D Twin holes of φ1.0 mm D Multi-holes of φ1.0 mm D Multi-holes of φ1.0 mm

4 Unit tension/n/mm D4 D5 D6 D7 D8 D9 D10 D Axial strain/% Figure 2 Tension load against tensile strain I1 I3 I2 Tensile behaviour of HDPE geomembranes with damage It can be found from Figure 2 that there are likeness and differences in mechanical behaviour between HDPE samples with and without damage when they are subject to tension load. Initially, the relationship curves of two kinds of samples nearly coincide until they are elongated up to the strain at yield and reach the peak strength. Afterward, their curves begin to separate: the curves representative of HDPE samples with damage drop more abruptly than the ones representative of samples without damage. At the same time, the damage on samples changes their failure mode from the normal failure to a premature failure. The lack of a steady phase during extension test is the most distinct feature of deformation behaviour of all the samples with damage. Consequently, the measured value of elongation at break (ε ult ) of membranes with damage is about 44-90%, which is much smaller that 700% typical of membranes without damage. Other indexes such as elongation at yield (ε y ) and elasticity modulus (E) of membranes with and without damage are very approximate, but the measured value of ultimate tensile strength (T ult ) of membranes with damage is ranged from N/mm, a little lower than that of the membranes without damage. The damage of the tested membranes apparently becomes severe with tension load. The round holes of 1 mm in diameter were deformed prior to break into oval shape with about mm long and mm wide. For multi-hole damage on samples of 200 mm wide, eight small round holes became large oval openings before break, with two central ones being the largest (Figure 3). In order to get an insight on the detailed aspects of their deformation behaviour, an array of lines are marked in internal of equal spacing on some HDPE samples with damage (Figure 3). It is found that much greater concentrated strain takes place in the local damage section than in the other section without damage, indicating that the development of tensile strain is not uniform over the whole length of damaged membranes but concentrated in the small damaged area. This is the real cause of the premature failure with the lack of a steady phase during extension test. 32

5 The non-uniform deformation behaviour and premature failure mode can be a good interpretation to the failure in the above-mentioned case. (a) Prior to test (b) At break Figure 3 Non-uniform deformation behaviour of damaged HDPE membranes Modelling for deformation behaviour of HDPE geomembranes in geotechnical centrifuge In order to create the prototype working conditions of large MSW deformation to test the model membranes of HDPE geomembranes, geotechnical centrifuge modeling was employed. Two qualitative model tests were conducted in a 50-g ton centrifuge at Geotechnical Centrifuge Facility of Nanjing Hydraulic Research Institute of China. Some key specifications for the centrifuge are listed in Table 3. Table 3 Technical specifications for the centrifuge Key item Specification Payload capacity 50 g-tons Arm radius 2.25 m to the center of the swinging platform Maximum acceleration 250 g Strongbox s inside m m m sizes Similarity law of centrifuge model The principle of centrifuge modeling is based upon the requirement of similarity between the model and the prototype [1]. If a model of the prototype structure is built with dimensions reduced by a factor 1/N, then an acceleration field of N times the acceleration of gravity, g, will generate stress by self weight in the model that are the same as those in the prototype structure. The other aspects of similarity law can be established by a dimensional analysis proposed by Butterfield [2]. The modeling of deformation behaviour of membranes on an inclined slope of landfill is formulated in Figure 4 and the strain on the damaged section of membranes can be expressed in the dimensionless form 2 2 ( α, E /( ρ a H ), T /( ρ a H ), E t /( ρ a H ), µ, e H ) ε = f (1) MSW ult / 33

6 where ρ and H are density and thickness of MSW, respectively; a is inertial acceleration (which is 1 g = 9.81 m/s 2 in prototype and N g in centrifuge model, N = H p / H m, subscripts m and p denote the model and prototype, respectively); α is slope angle; T ult is ultimate tensile strength; E t, product of elasticity modulus (E) and thickness (t) of membrane, represents rigidity of membrane unit width; µ is friction between membrane and MSW; e is a measure characteristic of damage conditions on membranes in terms of diameter of holes. From equation (1), the three important requirements can be founded according to the principle of modeling similarity so that ε m = ε p a m T E = N g (2), m = Tult, p N (3) ult / t = E t N (4) m m p p / A model membrane of HDPE geomembranes is selected from commercial membrane products according to equations (1)-(4). The MSW is modeled by a mixed soil of small density and great compressibility. The controlled parameters are listed in Table 4 for two models: one for membrane without damage (M3) and the other for membrane with a series of boreholes of 3 mm in diameter and 25 mm in spacing (M4). Figure 4 also shows the model layout, by which a prototype with 20 m thick MSW is simulated by a 250 mm thick at 80 g of centrifuge acceleration. Settlement marks are installed during placing MSW in layers (Figure 5). Figure 5 illustrates the deformed MSW with large settlement happened. Table 4 Controlled parameters of two models α m H m ρ m am Tult, m Em tm ( ) (mm) (kg/m 3 ) a p Tult, p E p t p /70 1/80 One surface is textured by silicone glue. m µ m H m e 0 (M3), (M4) Deformation behaviour of membranes with and without damage in the models The model is brought to the desired acceleration (80 g) gradually in steps of 10 g. Upon stoppage of centrifuge spinning, a measurement is made through settlement marks to obtain unrecoverable deformation in layers for MSW. A careful observation is also made to the model membrane placed on the lined slope. It is seen in Figure 5 that the MSW of two models has undergone a great deal of settlement, with 10% of unrecoverable compression. It is indicated that large deformation conditions of MSW are successfully created in these centrifuge models. Detailed detection on two model membranes shows that there is not any traits of breakage occurred on model membrane without damage (M3) whereas the model membrane with a series of boreholes (M4) has approached failure. It is observed that there is a substantial downward 34

7 movement of 5 mm for the centerline of damaged part along inclined slope and an enlargement of damage holes of 3 mm to 5 mm in diameter, exhibiting large strain happened at the damaged section. It is also seen that the silicone network, which was glued on one surface to increase its roughness, was separated from the membrane surface around holes, registering that the damage section has elongated a lot than the other section and the silicone network. The deformation behaviour and failure mode of the membrane model membrane with damage are in agreement with those observed in the extension tests. The explanation to the failure mechanism under large deformation conditions in prototype has been verified by centrifuge modeling. Hm Settlement marks Anchoring block MSW Damage holes Membrane simultant α H y /mm Initial MSW Deformed M3 Deformed M4 #5 #4 #3 #2 #1 Figure 4 Model setup x /mm Figure 5 Large deformation conditions of MSW created (M3 & M4) Conclusions The effects of damage have been studied on deformation behaviour and failure mode of HDPE geomembranes in extension tests and centrifuge model tests. The main conclusions, which can shed light on the mechanism of a prototype failure of HDPE geomembranes under large deformation conditions of MSW, are drawn. (1) It is found that the measured value of elongation at break of the HDPE geomembranes with damage is about one tenth of that of the ones without damage, companying a premature failure. (2) The real cause of the premature failure with the lack of a steady phase during extension test is in that concentrated strain is developed greatly at the damage section. (3) The similar failure to the prototype one is also observed for the model membrane with damage holes under the large deformation conditions of MSW, which are successfully produced by centrifuge modeling. References [1] SharmaJ.S. and Bolton, M.D., Centrifuge modeling of an embankment on soft clay reinforced with a geogrid, Geotexiles and Geomembranes, Vol.14, 1996, pp [2] Butterfield, R., Dimensional analysis for geotechnical engineers, Geotechnique, Vol.49, No.3, 1999, pp

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