laboratory tests for estimation of static and dynamic ...

LABORATORY TESTS FOR ESTIMATION OF STATIC AND DYNAMIC INTERFACE CHARACTERISTICS OF GEOSYNTHETIC F. CASTELLI*, A. CAVALLARO* AND M. MAUGERI* * University of Catania, Faculty of Engineering, Department of Civil and Environmental Engineering, Viale Andrea Doria n. 6, 95125 Catania, Italy

SUMMARY: In this paper direct shear tests and shaking table tests to measure the static and dynamic interface shear strength properties of different combinations between geotextile and sand, geotextile and clay, geotextile and geomembrane, geomembrane and sand, geomembrane and geogrid, geotextile and geogrid interfaces are reported. The influence of geosynthetic position during static tests, frequency of excitation during dynamic tests, vertical stress and relative density of the sand was studied. It was concluded that proper simulation of field conditions in laboratory experiments is important to obtain suitable friction angle values to be used in design.

1. INTRODUCTION The dynamic frictional properties of different geosynthetic interfaces play an important role in the stability analyses of various geotechnical structures that incorporate geosynthetics. The results of extensive static and dynamic test on the interface shear strength properties of various geosynthetic materials and soil have been reported by Myles (1982), Martin et al. (1984), Saxena and Wong (1984), Williams and Houlihan (1986), Miyamori et al. (1986), Negussey et al. (1989), O’Rourke et al. (1990), Mitchell et al. (1990), Yegian and Lahlaf (1992), De (1996) and De and Zimmie (1998). Many of the geosynthetic surfaces in composite liner system of modern landfills are characterized by relatively low friction angles. This may causes locations of slip movement at the interfaces between such elements. Moreover the use of geosynthetic as isolation can be proposed to mitigate earthquake hazard. Recent requirements to include considerations for seismic impact in modern landfill designs have led to need for understanding the dynamic frictional properties of these interfaces. This work presents and discusses laboratory test results on different combinations between geotextile and sand, geotextile and clay, geotextile and geomembrane, geomembrane and sand, geomembrane and geogrid, geotextile and geogrid interfaces. Moreover different geogrid orientations were used to evaluate the nature of the interface contact. The experimental shear strength evaluation of soil-geosynthetic system was carried out by the interpretation of interface direct shear testing results using varying shear displacement rates and normal stresses on compacted soil/geotextile interfaces. The interface direct shear tests were conducted using a direct shear device that consists of an upper shear box and a lower shear box. The results obtained are presented in terms of peak shear strength and of secant friction angle for 1

each test. The angles of internal friction correspond to a linear failure envelope that passes through the origin and the peak shear strength. A comparison is also reported between the shear strength results obtained for the cases of soil with and without geosynthetic reinforcement. The dynamic frictional properties were estimated using shaking table tests. The tests revealed various important characteristics regarding the dynamic friction properties of the geosynthetic interfaces, including a dependence of some of the interfaces on the level of normal stress and the excitation frequency.

2. STATIC LABORATORY TESTING PROGRAM For practical use of geotextile as a soil reinforcement material, it’s suitability should be checked by evaluating not only mechanical properties, but also soil-geotextile interaction properties. With the development of soil reinforcement technique, also soil-geotextile interaction testing have been much performed. In the design of reinforced soil the friction between the soil interface and the reinforcing element is usually investigated by direct shear tests or pull-out tests. In this case it is a common practice to employ the Mohr-Coulomb criterion to estimate the friction at the interface (Cazzuffi et al.,1994). A laboratory investigation carried out on soil reinforced samples with geotextile allowed to obtain some information about the interaction between these different materials. In particular, to evaluate the improvement of soil mechanical properties, the results of some direct shear tests on reinforced sandy soil samples were analysed in terms of overall strength increasing.

Reinforcing Element a)

Reinforcing Element b)

Figure 1. Installation of the reinforcing element on soil samples: a) scheme 1; b) scheme 2.

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300 Calcareous sand

σv =300 kPa

250 200

τ (kPa)

τ (kPa)

The direct shear tests were carried out on geotextile-reinforced soils, using three different cohesionless soil: calcareous sand, volcanic sand and silica Catania sand. Direct shear tests were performed on the portion of soil finer then 2 mm using the Casagrande shear box having a surface of 10x10 cm2 and a height of 2 cm. The experimental results have shown the modification of the mechanical behaviour up to failure, due to the presence of the reinforcing element, and the increase of the overall shear strength of the reinforced materials. To investigate the influence of the relative position of the geotextile against the soil layer on the test results (Nakamura et al., 1996), a series of direct shear tests have been carried out according two different positions (Figure 1). The employed three soil types have been reinforced with a geotextile characterised by an ultimate pull-out force equal to 21.5 kN/m, installed according to the scheme reported in Figure 1 a) and Figure 1 b). The results have been compared with those obtained on the unreinforced soil samples. All tests were run immediately after the placement and compaction of soil in the shear box with values of normal stress ranging from 100 kPa up to 300 kPa. The stress-strain curves obtained on both unreinforced and reinforced specimens are reported in Figure 2. In particular Figure 2 a), c) and e) regard the comparison between the experimental 300 σv =300 kPa

Calcareous sand 250

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Figure 2. Stress-strain curves obtained by direct shear tests on reinforced and unreinforced soil samples.

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results obtained for unreinforced soil and soil samples reinforced according to scheme 1 (Figure 1 a), while Figure 2 b), d) and f) regarding the comparison between unreinforced soil and soil samples reinforced according to scheme 2 (Figure 1 b). The results obtained generally show that the presence of the geotextile determines a valuable increase of the shear strength and the effectiveness of the reinforcement seems to increase for higher strains. In terms of conventional shear strength parameters (Figure 3), the reinforced samples are characterised by a certain apparent cohesion up to 15 kPa, while the cohesion was zero for the unreinforced samples. No increase of the friction angle was observed for the reinforced samples.

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Figure 3. Failure envelopes of reinforced and unreinforced samples in direct shear test: a) calcareous sand; b) volcanic sand; c) silica Catania sand.

3. DYNAMIC LABORATORY TESTING PROGRAM

3.1 Shaking table tests The shaking table available at the University of Catania was described by Cascone and Maugeri (1995). Figure 4 is a schematic layout of the shaking table facility used to evaluate the dynamic interface properties between different combinations of geosynthetic. The shaking table consisted of a vibration exciter connected to a rigid aluminium table mounted on frictionless linear bearing pillow blocks moving on two stainless steel guide rails. The dimensions of the table are 200 cm in the direction of the motion 100 cm in the transverse direction. The motion is provided to the table by a loading unit consisting of an electric threephase synchronous engine with a still disk mounted on the engine shaft. The position of the disk is adjustable allowing to produce different eccentricities in the range 1 – 10 mm. The motion is transferred from the engine to the table by means of a ball bearing placed on the edge of the table. The contact between the disk and the bearing is maintained by a spring fixed on a contrast beam and kept compressed throughout the dynamic testing. The amplitude

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Figure 4. Shaking table facility used to evaluate the dynamic interface properties between geosynthetic and geosynthetic. and the frequency of the table motion were controlled by a signal generator. In Figure 4 the geomembrane is fixed to the shaking table, while the geotextile is fixed to a superimposed concrete block with a base of 50 cm x 30 cm and a height of 10 cm. Static weight was added on the concrete block to vary normal stress, from 2.25 kPa to 8.62 kPa. To test the behaviour of the interface soil-geosynthetic the soil used in the dynamic tests is a silica uniform (D60/D10 = 1.60) sand from the southern coast of Catania (Sicily) which has small (D50 = 0.3 mm) sub-angular grains, respectively maximum and minimum unit weight γmax = 16.8 kN/m3 and γmin = 14.5 kN/m3 and an average peak value of the angle of shear resistance φ = 35°, obtained as a result of a certain number of direct shear tests at different values of relative density (Cascone et al., 2000). The backfills in the test box were prepared by dry pluviation of sand from a different height, ranging between 75 cm and 120 cm, in order to obtain a relative density (Dr) of 15 %, 30 % and 60 %, respectively. The acceleration of the table and that of the block were measured simultaneously by piezoelectric accelerometers at a rate of 100 measurements per second. In addition the displacement of the block relative to the table was recorded using a linear variable-displacement transducer (LVDT). All data recorded by the acquisition data unit were analysed by a personal computer and a commercially available software. Xb

ab BLOCK

W(ab/g) W = m.g Xt F = W.tan δd,y

at

TABLE

Figure 5. Scheme of the dynamic forces acting on the table and on the block.

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3.2 Dynamic interface behaviour evaluation A series of shaking-table tests were performed to evaluate the maximum shear stress (or acceleration) transmitted to the block, and its dependency upon the table acceleration, frequency of motion of the table and normal stress conditions. Figure 5 shows schematically the dynamic forces acting on the table and on the block As reported in Figure 5 when the table accelerates with an acceleration at it transmits a frictional force F to the block. This frictional force cannot exceed the interface shearing strength between the geomembrane and the geotextile. Assuming a Mohr-Coulomb type of failure mechanism, the value of F can be written as: F = W tanδd

(1)

where: - W = weight of the block and superimposed weight; - δd = geomembrane-geotextile interface dynamic friction angle. This limiting shear force induces a limiting block acceleration ab, given by: W tanφd = W (ab/g)

(2)

or ab = g tanδd

(3)

This implies that starting from the at-rest position, as the acceleration of the table is increased, the block and the table move together for as long as the table acceleration (at) is smaller than the limiting block acceleration (ab) given by (3). When the acceleration of the table (at) exceeds the limiting value (ab), relative movement will be induced between the block and the table. Thus, measurement of the acceleration of the block can provide the dynamic characteristics of the interface shear properties between the geomembrane and the geotextile as given by: δd = tan-1 (ab/g)

(4)

3.3 Results and discussion Different combination between geotextile and sand, geotextile and clay, geotextile and geomembrane, geomembrane and sand, geomembrane and geogrid, geotextile and geogrid interfaces were performed. Moreover different geogrid orientations were used to evaluate the nature of the interface contact. During shaking table tests when the block acceleration was almost identical to that of the table no relative displacement between the geotextile and the geomembrane was recorded by the LVDT. This indicate that there was a complete transfer of interface shear stress from the geomembrane to the geotextile. When the peak table acceleration (Figure 6) was larger than that of the block, a relative displacement along the geotextile-soil interface (between the block and the table) was recorded by the LVDT, as show in Figure 7. In Figure 8 a relative displacement along the geomembrane-soil interface recorded by the LVDT is shown. 6

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Figure 6: Recorded block acceleration for geotextile-soil interface.

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Figure 7: Relative displacement during shaking table tests of Figure 6 for geomembrane-soil interface. In Table 1 the values of dynamic friction angle obtained for different conditions of relative density and vertical stress are reported for the different type of interface examined. To evaluate if the slippage in the dynamic interface shear strength with the increasing table acceleration could be due to the effect of frequency upon the geomembrane-geotextile interface,

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Relative Displacement [cm]

1 0 -1 -2 -3 -4 -5 -6 0 2

4 6

8 10 12 14 16 18 20 22 24 26 28 30 Time [sec]

Figure 8. Relative displacement during shaking table tests for geomembrane-soil interface. the series of tests reported in Table 1 were repeated. In these tests, the frequency of the table motion was varied and the normal stress was maintained constant and equal to 8.62 kPa. The obtained data indicate that the frequency of excitation, within the range investigated (2 10 Hz), has little effect upon the peak block acceleration. Furthermore, for each frequency investigated, the general tendency of the block acceleration to increase with increasing table acceleration could still be observed (De and Zimmie, 1998; Yegian and Lahlaf, 1992). Additional tests were also performed to evaluate the influence of the normal stress on the dynamic interface friction angle. The tests were performed using different weights on the concrete block. To avoid the potential effect of the rocking inertia that might come into play as the weights are placed over the block, a maximum normal stress of 8.62 kPa was maintained. The general conclusion that can be made is that the increase in block acceleration with increasing table acceleration beyond the onset of sliding cannot be attributed to the potential effect of normal stress (Yegian and Lahlaf, 1992). Table 1 – Type of examined interface and dynamic tests results. N°. Type of Interface Dynamic Friction Angle I Geotextile Soil (Sand) 32° (a) 32.5° (b) 33° (c) II Geomembrane Soil (Sand) 19° (a) 19.5° (b) 20.5° (c) III Geotextile Geomembrane 10° (1) 10° (2) 10° (3) 10° (4) IV Geomembrane Geogrid (P) 17° (1) 17° (2) 17° (3) 17° (4) V Geomembrane Geogrid (N) 18° (1) 18° (2) 18° (3) 18° (4) VI Geotextile Geogrid (P) 16.5° (1) 16.5° (2) 16.5° (3) 16.5° (4) VII Geotextile Geogrid (N) 33.5° (1) 33.5° (2) 33.5° (3) 33.5° (4) Where: P = Parallel to the motion direction; N = Normal to the motion direction; (a): Dr=15%, (b): Dr=30%, (c) Dr=60%; (1): σv=2.25 kPa, (2): σv=4.19 kPa, (3): σv=6.09 kPa, (4): (σv=8.62 kPa.

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Moreover, as regard the geomembrane-soil interface, the effects of relative density on the dynamic friction angle for the silica Catania sand, was shown to be negligible. A modelling of the interface behaviour results by shaking table tests is proposed by Carrubba et al. (2001), in terms of time-history of displacement, velocity and acceleration.

4. CONCLUSIONS Direct shear tests and shaking table tests results on the static and dynamic interface shear properties of different geosynthetic combinations were presented. The following observations and conclusions are made from this research: • The static results obtained generally show that the influence of the geotextile determine a valuable increase of the shear strength and the effectiveness of the reinforcement seems to increase for higher strains; • The reinforced samples are characterised by a certain apparent cohesion, while the friction angle is practically the same for both the examined position of the geotextile; • For silica Catania sand, the value of static friction angle evaluated by direct shear tests is not substantial different from that of dynamic friction angle evaluated by shaking table; • The dynamic friction angle for geomembrane-geotextile interface is very low, thus stability evaluation must be carefully performed in the design; • During dynamic tests the frequency of excitation has little effect upon the peak block acceleration for each frequency investigated and the general trend of the block acceleration to increase with increasing table acceleration could still be observed; • Normal stress and relative density of the sand for the performed tests did not influence the behaviour of different combinations of geosynthetic.

ACKNOWLEDGEMENTS The Authors wish to thank Mr. Gioacchino Marzo and Mr. Alfio Strano for their valuable help in the direct shear tests and shaking table tests respectively.

REFERENCES Carrubba P., Massimino R., Maugeri M. (2001) Dynamic friction of geosyntethic interfaces by shaking table tests. Proceedings 8th International Waste Management and Landfill Symposium, Santa Margherita di Pula, Cagliari, Italy, 1-5 Ottobre. Cascone E. and Maugeri M. (1995) Shaking table test of gravity retaining walls. Proceeding of the 7th International Conference on Soil Dynamics and Earthquake Engineering, Chania, 1995. Cascone E., Lo Grasso A. S. and Maugeri M. (2000) Dynamic model tests on L-shaped gravity retaining walls. Proceeding of the 15th International Conference on Soil Dynamics and Geotechnical Engineering, Aucland, 2000. Cazzuffi D., Picarelli L., Ricciuti A. and Rimordi P. (1994) Behaviour of geogrid reinforced gravel in large scala triaxial tests. Proceedings 13th International Conference of Soil Mechanics and Foundation Engineering, New Delhi, Vol.1, 271-274, 1994. De A. (1996) Study of interfacial friction of landfill geosynthetics: Static and dynamic. Ph. D.

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Thesis, Rensselaer Polytechnic Institute, Troy, New York, USA, 245 p. De A. and Zimmie T. F. (1998) Estimation of Dynamic Interfacial Properties of Geosynthetics. Geosynthetics International, Vol. 5, N°. 1-2, pag. 17-39. Martin J. P., Koerner R. M. and Whitty J. E. (1984) Experimental friction evaluation of slippage between geomembranes, geotextiles and soils. Proceedings International Conference on Geomembranes, Industrial Fabrics Association International, Denver, pag. 191-196. Mitchell J. K., Seed R. B. and Seed H. B. (1990) Kettleman Hills waste landfill slope failure. I: Liner-system properties. Journal of Geotechnical Engineering, Vol. 116, N°. 4, pag. 760-779. Miyamori T., Iwai S. and Makiuchi K. (1986) Frictional Characteristics of Non-Woven Fabrics. Proceeding of the 3rd International Conference on Geotextiles, Vienna, Aprile 7-11, 1986, Vol. III, pag. 701-705. Myles B. (1982) Assessment of soil fabric friction by means of shear. Proceedings 2nd International Conference on Geotextiles, Las Vegas, pag. 787-791. Nakamura T., Ikeura I. and Mitachi T. (1996) Some factors affecting the results of soil-geogrid direct shear test. Proceeding of the International Symposium on Earth Reinforcement, Fukuoka, Japan, 1996, Vol.1,123-128. Negussey D. Wijewickreme W. K. D. and Vaid Y. P. (1989) Geomembrane interface friction. Canadian Geotechnical Journal, 26(Feb.), pag. 165-169. O’Rourke T. D., Druschel S. J. And Netravali A. N. (1990) Shear strength characteristics of sand-polymer interfaces. Journal of Geotechnical Engineering, Vol. 116, N°. 3, pag. 451-469. Saxena S. K. And Wong Y. T. (1984) Frictional characteristics of a geomembrane. Proceedings International Conference on Geomembranes, Industrial Fabrics Association International, Denver, pag. 187-190. William M. K. And Lahlaf A. M. (1992) Discussion of Kettleman Hills waste landfill slope failure. I: Liner –system properties. Journal of Geotechnical Engineering, ASCE, 118(4), pag. 643-645. Yegian M. K. and Lahlaf A. M. (1992) Dynamic Interface Shear Strength Properties of Geomembranes and Geotextiles. Journal of Geotechnical Engineering, Vol. 118, N°. 5, May 1992, pag. 760-779.

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