Influence of soil confinement on the creep behavior of geotextiles

Geotextiles and Geomembranes 43 (2015) 351e358

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Technical note

Influence of soil confinement on the creep behavior of geotextiles Leonardo De Bona Becker a, *, Anna Laura Lopes da Silva Nunes b a b

ria, Rio de Janeiro, CEP 21941-909, Brazil Federal University of Rio de Janeiro, Centro de Tecnologia, Bloco D, Sala 207, Cidade Universita ria, Rio de Janeiro, CEP 21941-972, Brazil Federal University of Rio de Janeiro, Centro de Tecnologia, Bloco B, Sala 101, Cidade Universita

a r t i c l e i n f o

a b s t r a c t

Article history: Received 17 November 2014 Received in revised form 20 February 2015 Accepted 13 April 2015 Available online 15 May 2015

This work addresses the influence of soil confinement on the creep behavior of geotextiles by presenting the results of a full scale field test. Two samples of nonwoven polypropylene geotextile were inserted at different depths in a 3 m high compacted sand fill. The samples were loaded with a constant tensile load during a 1000 h period. To maintain a constant load during the test, a system of weights, pulleys and load cells was used. The sand fill and the samples were instrumented with several types of transducers in order to measure strains, displacements, applied forces, soil stresses and temperature. Direct shear and inclined plane tests were conducted to measure the mechanical properties of the interfaces. An interpretive model is proposed to analyze the field results. The confined creep behavior in the field is compared with results obtained by other authors and with in-isolation creep results obtained from laboratory tests. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Creep Confinement Geotextiles: nonwoven Geosynthetics

1. Introduction Creep behavior and stress relaxation have to be taken into account when using geosynthetics in the design of reinforced soil structures. Creep is common to many materials, including all polymers. Creep behavior is one of the most important properties to be evaluated when determining the allowable tensile strength of a geosynthetic in soil reinforcing applications. Long-term design strength can be determined using isochronous curves or by applying conservative reduction factors to the wide-width tensile strength (Tf). Bueno (2010) states that reduction factors typically range from 2 to 7. For example, the reduction factor of nonwoven polypropylene geotextiles can be as high as 5 (Task Force #27, 1991; Holtz et al., 1998). Such reductions are based on results of in-isolation creep tests. However, some authors have found that in-isolation creep tests tend to over-predict creep strain in some geosynthetics (McGown et al., 1982; Den Hoedt, 1986; Holtz et al., 1998). Koerner (2012) considers that, while expensive and time-consuming to perform, confined creep tests are important for setting realistic creep reduction factors.

* Corresponding author. Tel./fax: þ55 21 3938 7437. E-mail address: [email protected] (L.D.B. Becker). http://dx.doi.org/10.1016/j.geotexmem.2015.04.009 0266-1144/© 2015 Elsevier Ltd. All rights reserved.

In spite of the research conducted until now, the mechanism of confined creep is not yet fully understood, and its effect is usually disregarded in reinforced soil design. Several apparatuses have been developed for confined creep testing, but their results are difficult to compare because their boundary conditions are very different (McGown et al., 1982, Costa, 1999; Wu and Helwany, 1996; Boyle and Holtz, 1996; França and Bueno, 2010; 2011). Nonwoven geotextiles are currently seldom used to reinforce soil walls and slopes, but they were widely used approximately two decades ago. There are countless nonwoven reinforced structures around the world still in service. Some of these structures could be reassessed to allow load increases if their creep reduction factors are proven to be excessive. Nonwoven geotextiles used in other geotechnical applications can be under tensile stresses for long periods also. 1.1. Importance of soil confinement on geotextile behavior Soil confinement can affect the stress-strain behavior of some geotextiles, especially nonwoven ones (Yuan et al., 1998; Koerner, 2012). For geogrids and woven geotextiles, the effect of confinement may be negligible. The fibers of the nonwoven geotextiles are not aligned in any preferential direction but rather are sinuous and random. The strain of a nonwoven geotextile can be divided into two components. One is the strain of the fibers and the other is the strain due to their

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rearrangement, known as structure deformation. Den Hoedt (1986) speculated that structural deformation could represent up to 50% of the total strain in needle punched nonwovens. Under soil confinement, geotextiles may exhibit increases in short-term tensile strength (McGown et al., 1982; Ling et al., 1991; Leshchinsky and Field, 1987; Elias et al., 1998) and modulus, especially at lower strains (McGown et al., 1982; Boyle et al., 1996; Siel et al., 1987; Ling et al., 1991; Leshchinsky and Field, 1987; Yuan et al., 1998; Elias et al., 1998). Some authors have concluded that confinement does not influence rupture elongation (Leshchinsky and Field, 1987; Ling et al., 1991), whereas others have stated that rupture elongation is reduced (Elias et al., 1998). 1.2. Unconfined creep of geotextiles Three stages may be identified in typical unconfined creep results (Fig. 1). After an almost instantaneous deformation due to load application, the primary stage of creep begins. There is a fast increase in deformation, but the creep rate decreases. After a transition, secondary creep begins and the creep rate declines slightly. In some tests, after a critical strain, a tertiary stage begins and the creep rate increases leading to creep rupture (Cazzuffi et al., 1997). According to Mitchell and Villet (1987) and Allen (1991) creep behavior in the primary stage is strongly affected by structural deformation, whereas in the secondary and tertiary stages, the type of polymer controls the behavior. Cazzuffi et al. (1997) concluded that the shape of the time vs. strain curve depends on the dominant creep stage (Fig. 1c). 1.3. Confined creep of geotextiles The reduction factors used for creep at present are usually based on the interpretation of unconfined creep test results. However, the monitoring of several reinforced soil structures has shown equal or smaller creep strains or rates than those predicted by unconfined creep tests (Barrett, 1985; Mitchell and Villet, 1987; Delmas, 1988; Allen and Bathurst, 2002). Those results are difficult to compare because the real tensile load is usually estimated instead of measured. Koerner et al. (1993) state that creep tests of geotextiles must be executed under confinement to produce reliable results, especially in the case of nonwovens. In addition, to obtain reliable results about the influence of soil confinement on the creep of nonwoven geotextiles, it is important to conduct field experiments in order to minimize size effects.

Sample trimming to reduce samples sizes for laboratory tests implies cutting fibers and therefore reduces restraints on fiber realignment thus affecting the geotextile's behavior. McGown et al. (1982) conducted laboratory creep tests under sand confinement with normal stresses of 100 kPa. The equipment's boundary conditions could be described as similar to a confined wide-width test. Several types of geotextiles were used. The confinement reduced the primary creep by 40%e60% in the case of a nonwoven needle-punched geotextile. The secondary creep rates were also noticeably reduced. Since then, several authors have conducted confined creep tests on nonwoven geotextiles in different apparatuses. Wu and Helwany (1996) concluded that soil confinement increased or decreased geotextile creep depending on soil type. Normal stress and tensile loads were estimated. Boyle (1995) developed a device similar to Wu & Helwany's with a capacity to measure reinforcement tensile loads. The stresses were kept constant; however, the tensile load declined during the test. Elias et al. (1998) used an apparatus similar to the one described by McGown et al. (1982) and concluded that creep strain is significantly reduced in nonwoven geotextiles and that the creep of a geotextile may be affected by time-dependent sliding between fibers called shear creep. The authors believe that the normal stresses were high enough (69 kPa and 138 kPa) to increase friction between fibers and prevent their sliding, thereby improving the creep behavior of nonwovens. França and Bueno (2010) and França and Bueno (2011) presented results of confined and confined-accelerated creep tests on a polyester non-woven geotextile conducted in laboratory. The boundary conditions were similar to a confined tensile test, and the load was measured at both ends of the sample, but the load level in the center of the sample was not known. The normal stress was 30 kPa. The authors concluded that the confinement considerably reduced both the initial strain and creep rate. Wu and Hong (1994) conducted confined creep tests on nonwoven needle-punched geotextiles. The confining stresses were 0 kPa, 50 kPa, 100 kPa and 200 kPa. The authors found a significant reduction in creep only for 200 kPa. Comparison of those results is difficult because of their different boundary conditions, mainly tensile load level, tensile load distribution, normal soil stress, soil type, test duration and method of measuring (or estimating) tensile load. Furthermore, an ideal test should replicate the strain and stress conditions of reinforced soil structures, which was not the case for many tests.

Fig. 1. Primary, secondary and tertiary stages of geotextile creep: (a) time vs. strain, (b) time vs. strain rate and (c) creep behavior according to the dominant creep stage (Cazzuffi et al., 1997).


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2. Test set-up 2.1. Experimental fill and instrumentation Koerner et al. (1993) state that the creep tests of geotextiles should be executed under confinement to produce reliable results, especially in the case of nonwovens. In addition, full-scale experiments are desirable to reduce size effects. Trimming samples is undesirable because it implies cutting many fibers and it is hard to predict its effect on the transverse strain of nonwoven geotextiles. However, most creep and strength tests use trimmed samples because full-width samples are too large for laboratory devices. A full-scale field experiment was conceived to examine the effects of soil confinement on the creep behavior of a nonwoven geotextile (Becker, 2001). Two samples of nonwoven needlepunched polypropylene geotextiles were inserted to different depths in a 3 m high and 16 m wide compacted sand fill. The samples were full roll width (no trimming). The sand was compacted by vibratory plate. A drawback of some laboratory confined creep tests is unreal boundary conditions. By contrast, this full-scale experiment was designed to replicate the conditions of reinforcement inside the resistant zone of a reinforced soil mass. The samples were long enough to prevent pullout. Several types of transducers were used in order to measure strain, displacement, applied force, soil stress and temperature. The geotextiles were instrumented by strain gauges and telltales. Temperature sensors and earth pressure cells were also embedded in the fill. A section of the fill is shown in Fig. 2. Samples 1 and 2 were inserted at depths of 0.5 m and 2.5 m from the top, respectively. The vertical stress was 10 kPa and 48 kPa. The tensile load was kept constant during 1000 h by a system of cables, pulleys and dead weights. The applied load corresponds to 62% of the tensile resistance of the geotextile in wide-width tensile tests (Tf). Load cells were used to verify daily the tensile force on the cables. During the test, the geotextiles were pulled outward by a set of metallic clamps as shown in Fig. 3. The front part of every geotextile was reinforced with epoxy resin to reduce strain. To keep the unreinforced part under constant vertical stress, the reinforced part was enclosed in a lubricated HDPE geomembrane and extended beyond the vertical projection of the crest. Graphitic grease was used. The tensile stress was not constant along the sample length because the geotextile is extensible and transfers stress to the

Fig. 3. Longitudinal section of typical sample.

surrounding sand by friction. The distribution of telltales and strain gauges along the samples is shown in Fig. 4. Each geotextile was instrumented with 8 telltales and 7 strain gauges. Sample displacement was monitored by telltales consisting of stainless steel wires encased in PVC tubes to avoid soil friction. Strain gauges were installed between the telltales in an attempt to assure redundancy. Special high elongation type EP-08-250BF350 strain gauges by Micro Measurements Group were used. There is no unique well-established procedure for attaching strain gauges directly to geosynthetics because the environment in which such gauges need to perform is unusually harsh, relative to other applications of strain gauges (Leshchinsky et al., 2010). Sluimer and Risseeuw (1982) and Boyle and Holtz (1998) reported problems of under-registration of strain because of a difference in modulus between the adhesive of the strain gauges and the geotextile. In spite of careful bonding and waterproofing precautions, some strain gauges malfunctioned because of moisture, and the remaining under-measured strain. Therefore, the strain gauge results are considered unreliable and will not be considered.

2.2. Material properties The fill was entirely constructed with clean medium to coarse sand. Direct shear tests were conducted to determine the friction angle of the sand, with the same void ratio found in the fill. The peak friction and constant volume angles were 49 and 36 , respectively. A nonwoven needle-punched polypropylene geotextile was used. Its nominal physical and mechanical characteristics are shown in Table 1. Bueno (2000) conducted unconfined creep tests on this geotextile under four different load levels: 0.1Tf, 0.2Tf, 0.4Tf and 0.6Tf.

Fig. 2. Section of the experimental fill (Becker, 2001).

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The results shown refer to the upper geotextile. Unfortunately, it was not possible to perform the test on the lower geotextile due to successive failures of its reinforced part. Those failures were caused by unnoticed imperfect application of epoxy resin. The test occurred during the southern hemisphere spring from September 29th to November 10th. The ambient temperature ranged from 9.8 C to 33.4 C with an average of 20.4 C. The average minimum was 16 C and the average maximum was 24.8 C. The standard deviations were 3.1 C and 3.3 C, respectively, according to measurements by the National Institute of Meteorology e 8th District (INMET, 2011). The temperature within the fill was measured by several thermistors placed at different depths. Readings were taken in midmorning and midafternoon. The temperature at 0.5 m of depth varied between 18 C and 23 C, and the average was 20.5 C. Records show that the ambient temperature variations were attenuated inside the soil. The variation ranges of both ambient temperature and temperature inside the soil were narrower than those observed by Hsuan et al. (2002). 3.1. Strains obtained by telltale readings The average strain of the region between two telltales is as follows:

εi ¼ ðDxi Dxiþ1 Þ÷Li

(1)

where

Fig. 4. Distribution of telltales and strain gauges. All dimensions in cm (Becker, 2001).

The unconfined results (unfilled markers in Figs. 5 and 6) are presented along with the confined results. To assess the transfer of stress between sand and the geotextile, shear tests using the fixed shear box setup (Ingold, 1984) were conducted. The test results yield a sand-geotextile friction angle of 43 . The peak is reached for displacements of less than 1 mm, and there is negligible loss of shear resistance after the peak with a constant volume friction angle of 42 . The friction interaction coefficient between soil and geotextile was fs/GTX ¼ tan 43 /tan 49 ¼ 0.83. In the fill, the reinforced part of the geotextile was sheathed in lubricated membranes to reduce friction. Graphite grease was used for lubrication. An inclined plane device was developed to evaluate the loss of tensile load along the reinforced sample length. After testing two types of membranes, a 1.5 mm thick high density polyethylene geomembrane was selected. The friction angle obtained from inclined plane tests was slightly less than 1. 3. Results and analysis In this section, the results are shown along with a theoretical model developed for interpretation. Table 1 Nominal characteristics of the geotextile. Characteristic

Reference Value

Tensile strength from wide-width tensile tests (Tf) Elongation at rupture (εf) Mass per unit area Roll width Thickness

22 kN/m 60% 300 g/m2 2.15 m 2.8 mm

ε i is the average strain of a geotextile in region i (between telltales i and i þ 1), Dx i, iþ1 are the differences between the present reading and the first reading for telltales i and i þ 1 and L i is the distance between telltales i and i þ 1. The estimated strains are shown in Fig. 5 (filled markers). The strain is not constant along the geotextile length. Only regions 1, 2, 3 and 4 showed displacement. 3.2. Theoretical model of stress transfer during creep testing The tensile force is not constant along the sample because the geotextile is not rigid and transfers stress to the surrounding sand by friction. The maximum strain is at the beginning of the sample and decreases towards the free end. The strain in Fig. 5 can be directly related to tensile force neither by a stiffness modulus based on regular wide-width tensile tests nor by in-isolation creep tests because both confinement and creep may affect the modulus. However, the tensile force can be estimated by friction transfer. A constant rate of force transfer between geotextile and sand can be assumed because the difference between peak and post-peak interface friction angles is small and because the peak resistance of the sand-geotextile interface is reached for displacements less than 1 mm. A similar postulate was proposed by Sobhi and Wu (1996) based on numerical simulations of pullout tests. The post-peak friction angle is assumed to account for the displacement along the sample. The friction rate is as follows:

Tfriction ¼ 2$B$s0v $tanðdÞ ¼ 36:9kN=m

(2)

where Tfriction is the tensile force transferred to the sand by friction along a unitary length (friction rate),


355

Fig. 5. Confined and unconfined creep curves for the upper geotextile under different tensile loads.

Fig. 6. Confined and unconfined Sherby-Dorn curves for the upper geotextile under different tensile loads.

s0v is the vertical effective stress, B is the width of the geotextile and d is the mean interface friction angle. The length necessary to transfer the entire tensile force from the geotextile to the sand is as follows:

Ltransf ¼ To Tfriction ¼ 0:80m

(3)

where Ltransf is the geotextile length necessary to transfer the entire tensile force to the sand To is the tensile force at the beginning of the sample (To ¼ 0.624,Tf ,B ¼ 29.5 kN).

Fig. 7 shows the strain distribution along the geotextile estimated by telltale readings for different times. The strained length is in good agreement with the calculated Ltransf. In this model, the tensile force decreases linearly along the sample because the friction rate is constant. Therefore, the average force in a particular region depends only on the friction rate and the distance to the sample's beginning. In other words, the average force is not time dependent. Therefore, the tensile force in the four mobilized regions is approximately constant during the test and equal to the average value:

Tend ¼ Tbeg Tfriction $Lregion

(4)

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and Wu (1996) for pullout tests, the model proposed herein neglects that effect because this is not a pullout test and the additional length is small compared to the original length of the regions. The proposed model of interpretation implies that one single sample provides several different curves of %Tf. After mobilization, those percentages remain approximately constant during the test. 3.3. Confined creep analysis

Fig. 7. Strain distribution along the geotextile for different elapsed times.

Fig. 8. Assumed model of force distribution in a geotextile during creep testing.

Tavg ¼ Tbeg þ Tend 2

(5)

where Tend is the tensile force at the end of a region, Tbeg is the tensile force at the beginning of the region, Lregion is the length of the region and Tavg is the average tensile force within the region. Fig. 8 depicts the assumed model, and Table 2 shows the values of the average tensile forces. The shapes of the force distribution and strain distribution are approximately constant over time. However, only the force values are time-independent. The strain values increase over time because of creep processes. It takes some hours to mobilize Ltransf. After that, the elongation of a geotextile is caused by creep and will affect neither Tfriction nor Tavg. In reality, the strain on the geotextile increases the length of regions during the test. This results in more force transferred to the sand in every region. However, unlike the model proposed by Sobhi

Table 2 Force distribution in the geotextile according to the proposed model. Region

1

2

3

4

L region (m) T beg (kN) T end (kN) %Tf at the beginning of region %Tf at the end of region Avg %Tf

0.2 29.5 22.2 62.4 46.9 z55

0.2 22.2 14.8 46.9 31.4 z40

0.2 14.8 7.5 31.4 15.8 z25

0.3 7.5 0 15.8 0 z5

Fig. 5 shows a comparison between confined creep behavior for regions 1, 2 and 3 of the upper geotextile and the results of unconfined laboratory creep tests. The corresponding unconfined creep curves for 0.25,Tf and 0.55,Tf, were obtained by interpolation using 0.1Tf, 0.2Tf, 0.4Tf and 0.6Tf curves. Initial strains are much higher in unconfined tests, and logarithm of time vs. strain curves are approximately straight up to 1000 h. The confined test shows smaller strains than the unconfined tests during the 1000 h period. Fig. 6 shows the Sherby-Dorn confined and unconfined results. In spite of the scatter in the confined Sherby-Dorn curves, it can be seen that confined creep rates decrease for all loads except for the 0.55,Tf curve in which strain rate seems to stabilize after 20% strain. That behavior could be related to the beginning of a tertiary creep stage. The unconfined creep rates show a much more significant decrease. Soil confinement clearly alters the creep behavior of the nonwoven geotextile. The initial deformation, very high in the unconfined test, was negligible in the confined test, and the primary creep was noticeably reduced. That conclusion is in accordance with Mitchell and Villet (1987) and Allen (1991). They stated that the main influence of confinement is to reduce primary creep, whereas secondary and tertiary creep depend more on the type of polymer. This reduction in primary creep was also observed by McGown et al. (1982). However, a reduction in secondary creep rate was also observed. Logarithmic curves were fit to the experimental data of Fig. 5. Table 3 shows the parameters found. Parameter a is related to the initial strain while parameter b is related to the creep rate. The rows entitled “confined e all data” show the best fit for all points of every confined curve. Parameter a shows reductions in excess of 84% due to the elimination of the initial strain by confinement. Parameter b has a different trend. The reductions for 0.40,Tf and 0.25,Tf are much smaller, and there is an increase for 0.55,Tf. França and Bueno (2010) observed variations of 52% and 77% in parameters a and b, respectively. However, as stated before, some time is necessary for soil particle rearrangement and complete stress transfer between the reinforcement and the sand. After 7 he53 h, that process seems to have concluded and the confined creep curves in Fig. 5 become approximately straight. To expunge the effect of that delay, a new set of logarithmic curves was fit to the points after 7 h, 25 h and 53 h for 0.55,Tf, 0.40,Tf and 0.25,Tf, respectively. The parameter b obtained that way represents the slope of the straight portion of the logarithmic curves after mobilization. Unlike the results of other authors, the noticeable increases in parameter b for all curves show that the confined creep rates were higher than the unconfined ones in the present test. That behavior certainly will not last indefinitely because there is not any reason to assume that sand would increase the creep tendency of the geotextile. The confined creep curves may match asymptotically unconfined ones for longer times. The explanation for the higher creep rates in the confined test may be related to normal stress acting on the nonwoven geotextile. Some authors have used normal stresses ranging from 50 kPa to 200 kPa (McGown et al., 1982; Wu and Hong, 1994; Elias et al., 1998;


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Table 3 Effect of confinement on creep strain and creep rate. Tensile load

Creep test type

aa

Variation (%)

ba

Variation (%)

0.55,Tf

Unconfined Confined e all data Confined e straight portion Unconfined Confined e all data Confined e straight portion Unconfined Confined e all data Confined e straight portion

44.03 7.25 e 36.69 2.92 e 28.66 0.61 e

e 84% e e 92% e e 98% e

1.84 2.13 4.67 1.27 1.15 3.51 0.95 0.44 3.02

e þ16% þ154% e 9% þ176% e 54% þ218%

0.40,Tf

0.25,Tf

a

Note: ε ¼ a þ b , ln (t), where ε is the creep strain; a and b are adjusted parameters and t is time.

Costa, 1999; França and Bueno, 2010). Wu and Hong (1994) used four levels of normal stress (0, 50, 100 and 200 kPa) and concluded that reduction in confined creep was only important for 200 kPa. Unlike those cases, the normal stress in the present research was 10 kPa. Such a low stress may not have been enough to increase friction between fibers and thus could neither prevent their sliding nor reduce shear creep. 4. Conclusions A sample of nonwoven polypropylene geotextile was inserted into a compacted sand fill and loaded for 1000 h. Loads, displacements and temperature were monitored by several transducers. The boundary conditions resemble the portion of a reinforcement that extends beyond a potential failure surface in a reinforced soil mass. An interpretive model was proposed to analyze the test results. Interface friction angles were obtained by direct shear tests. The tensile forces along the sample were estimated assuming a constant rate of force transfer between the geotextile and the sand (friction rate). The load was assumed to be constant over time after the mobilization of every region because the increase in length due to elongation and creep is negligible when compared to the initial length. The deformed length estimated by friction rate agrees well with the test results. Comparison between the unconfined and confined results demonstrated that soil confinement reduced the initial strain and total creep strain up to 1000 h. Nevertheless, unlike other studies, creep rates were higher in the confined test than in the unconfined tests, showing that the benefits of confinement in this particular test may have been temporary. This likely was because the normal stress affects confined creep behavior (Wu and Hong, 1994) and the low normal stress of 10 kPa was not enough to prevent structural creep. Therefore, it seems advisable to disregard the benefits of confinement on the creep behavior of nonwoven geotextiles under low confining stresses. For example, to rely on perceived “conservatism” of creep factors when reassessing old structures to increase their height may be unsafe. Acknowledgments The authors express their gratitude to Prof. Eduardo Azambuja, CNPq e National Council for Scientific and Technological Development, Brazilian Chapter of IGS, INMET e National Institute of Meteorology and OBER S.A. References Allen, T.M., 1991. Determination of long term tensile strength of geosynthetics e a state of the art review. In: Proceedings of the Geosynthetics'91 Conference. IFI, Atlanta, St. Paul, pp. 351e380.

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Influence of soil confinement on the creep behavior of geotextiles

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