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The influence of soil type on interface behavior under pullout conditions

June 1st, 2014 / By: / Feature, Geogrids, Testing & Codes

Part 3: European experience in pullout tests

1. Introduction

The rapid increasing application of geosynthetics as reinforcement elements for earth structures requires a careful examination of soil-reinforcement interaction mechanisms.

The soil-geosynthetic interaction can be complex because it is affected by structural, geometrical, and mechanical characteristics of the geosynthetic, as well as by boundaries and loading conditions (Moraci and Montanelli, 2000; Moraci and Recalcati, 2006; Moraci and Gioffrè, 2006; Calvarano et al., 2012 and 2013; Cazzuffi et al., 2011; Cazzuffi et al., 2014; Moraci et al., 2014) and the mechanical properties of soil.

The main objective of this article is to focus on the effects of soil type on the interface behavior under pullout loading conditions.

In this article, some results of a wide experimental research program developed by geotechnical research group of Mediterranea University of Reggio Calabria, are reported. The pullout tests were performed, at constant rate of displacement (1mm/min), on three extensible geogrids, embedded in two different compacted granular soils. The pullout test results—in terms of peak pullout resistance at soil-geosynthetic interface—are analyzed. Some considerations about deformation behaviour analysis are also reported.

2. Test apparatus

To develop the test program, aimed to study the influence of the soil type on interface behavior under pullout conditions, a large-scale piece of pullout equipment (1700 × 680 × 600mm3) was used. The pullout test apparatus, which is capable of applying a displacement controlled static pullout force, was previously described. A more detailed description of the test apparatus can be found in previous researches (Moraci and Recalcati, 2006; Moraci and Cardile, 2009 and 2012).

Image 1

3. Test materials

3.1 Soils

Two granular soils were used in the research. The first soil (following called Soil A, Figure 1) is a uniform medium sand (SP according to USCS classification system, A-3 according to CNR-UNI 10006 classification system), with grain shape from sub-rounded to rounded, uniformity coefficient, U, equal to 1.96, and average grain size, D50, equal to 0.32mm. The second soil (following called Soil B, Figure 1), is a well-graded sand with gravel (SW according to USCS classification system, A1-b according to CNR-UNI 10006 classification system), with grain shape from sub-rounded to rounded, uniformity coefficient, U, equal to 7.48, and average grain size, D50, equal to 1.47mm.

The Standard Proctor compaction tests performed on Soil A indicate a maximum dry unit weight, γdmax =16.24 kN/m3, at an “optimum” water content wopt =13.5%. For Soil B a maximum dry unit weight, γdmax =18.36 kN/m3, at an “optimum” water content wopt =9.8% was obtained.

The direct shear tests, as all the pullout tests, were carried out at an initial unit weight corresponding to the 95% of γdmax for each sand studied. The peak shear strength angle, φ’p, at confining pressure was equal to 100 kPa, is 42° for the Soil A, and 49° for the Soil B.

Table 1 Geometrical characteristics of the studied geogrids
Geogrid S [mm] Wr [mm] Wt [mm] Br [mm] Bt [mm] Beq [mm] Ab [mm2]
GGEM2 240.0 11.86 6.00 4.65 4.48 4.37 85.35
GGEB1 39.48 14.81 25.84 3.76 2.90 3.29 107.10
GGEB2 56.92 16.24 59.49 4.95 3.72 4.02 269.70

3.2 Geogrids

The pullout tests were performed on one HDPE extruded mono-oriented geogrid (called GGEM2) and on two PP extruded bi-oriented geogrids (named GGEB1, GGEB2).

Table 1 shows the geometrical characterization of the geogrids, where Wr and Br are the node width and thickness, respectively; S is the spacing between transversal bars in the pullout direction; Wt and Bt are the width and thickness of the bar portion between two nodes, respectively; and Ab is the area of each rib element (including the node embossment and the bar portion between two nodes) where the bearing resistance can be mobilized (Moraci and Gioffrè, 2006; Cazzuffi et al., 2011, Cazzuffi et al., 2014).

TABLE 2 Mechanical proprieties of the tested geogrids
Geogrid Polymer TF [kN/m] J2% [kN/m] J5% [kN/m]
GGEM2 HDPE 88.69 1749 1212
GGEB1 PP 55.90 825 618
GGEB2 PP 35.83 535 390

The mechanical properties of the different geogrids were evaluated by means of wide-width tensile tests according to EN ISO 10319 (see also Cazzuffi, 1996), but performed at the same rate of displacement used in the pullout tests (1mm/min). The tensile test results are reported in Table 2.

TABLE 3 Pullout test program
Test Geogrid Direction Soil LR [m] σ’v
[kPa]
T 01÷12 GGEM2 TD A 0.4 / 0.9 / 1.15 10 / 25 / 50 / 100
T 13÷24 GGEM2 TD B 0.4 / 0.9 / 1.15 10 / 25 / 50 / 100
T 25÷36 GGEB1 TD A 0.4 / 0.9 / 1.15 10 / 25 / 50 / 100
T 37÷48 GGEB1 TD B 0.4 / 0.9 / 1.15 10 / 25 / 50 / 100
T 49÷60 GGEB2 TD A 0.4 / 0.9 / 1.15 10 / 25 / 50 / 100
T 61÷72 GGEB2 TD B 0.4 / 0.9 / 1.15 10 / 25 / 50 / 100

4. Analysis of pullout test results

Pullout tests were carried out varying, for each geogrid, the specimen length (LR = 0.40, 0.90, 1.15 m), the applied vertical effective stress (σ’v = 10, 25, 50 and 100 kPa), and the type of the soil in contact (Soil A and Soil B). The pullout test program is summarized in Table 3.

All pullout tests were performed at constant rate of displacement equal to 1.0 mm/min, until geogrid tensile rupture or until a total horizontal displacement of 100mm was achieved.

The soil particle size, according to the geogrid mesh size, is one of the most important factors affecting the soil-geogrid interface behavior (Jewell et al., 1984; Palmeira and Milligan, 1989; Palmeira, 2009; Calvarano et al., 2012).

To study this effect, the results of pullout tests performed on one mono-oriented geogrid (GGEM2) and on two bi-oriented ones (GGEB1 and GGEB2), embedded in two different compacted granular soils (Soil A and Soil B) were analyzed.

The soils and geosynthetics properties were described in section 3. The influence of the type of soil can be showed comparing the pullout curves that refer to the tests carried out at the same test condition (relative density, normal effective confining stress and anchorage length, type of geogrid) embedded in two different granular soils (Soil A and Soil B).

It is clear that the scale effects, due to the average size of the soil particles (D50), the size of the geogrid openings (S), and the thickness of the transversal bars (Beq), affect the soil-geogrid interface shear resistance (Palmeira, 2009).

The influence of the relative sizes of soil particles in respect to the transversal ribs spacing on soil-reinforcement interaction in direct shear tests was studied by Jewell et al. (1984). The authors stated that the interaction coefficient of direct sliding increases with soil particles diameter and has its maximum value when the grain size is similar to that of the geogrid apertures.

In fact, when soil particle size is similar to grid aperture size, the rupture zone is forced away from the grid plane so the shear resistance would be equal to the full shear resistance of the soil. In the extreme case in which the soil has particle size too large to penetrate the grid apertures, these remain directly on the grid plane surface and the shear resistance would be due to contact between themselves and the plane grid surface.

To avoid the scale effect described by Jewell et al. (1984), the authors recommended that the ratio between the size of the openings of the geogrid (S) and the mean soil grain size D50 must be higher than 3.

TABLE 4 Ratio between the size of the geogrid openings and the average diameter of soil particles for the different soil-geogrid combinations
Geogrid S [mm] S/D50(SoilA) [-] S/D50(SoilB) [-]
GGEM2 240.00 750.00 163.27
GGEB1 39.48 123.39 26.86
GGEB2 56.92 177.86 38.72

The results reported in Table 4 show that for all investigated geogrids, embedded in the two different soils, the ratio S/D50 is always greater than 3, so there would not be this scale effect.

Image 2

Based on result of pullout tests carried out with metallic grids and geogrids, Palmeira (2009) and Palmeira and Milligan (1989) showed the influence on the bearing stress ratio of the relative size of the soil particle with respect to the grid transverse member thickness. This influence can be seen in Figure 2, where the normalized bearing stress is plotted against the transverse member thickness normalized by the average soil particle diameter. The authors carried out pullout tests on isolated transverse members with different shape embedded in dense sands.

When the size ratio, B/D50, is less than 10, the mobilized bearing stress can be improved by more than two times, depending on the bearing members’ shape. Normalized bearing strength starts to be independent on the soil particle size only when the ratio between member thickness and average particle diameter is equal to 10.

TABLE 5 Ratio between the equivalent thickness of the transverse bars and the average diameter of soil particles for the different soil type-geogrid combinations
Geogrid Beq [mm] Beq/D50(SoilA) [-] Beq/D50(SoilB) [-]
GGEM2 4.37 13.65 2.98
GGEB1 3.29 10.29 2.24
GGEB2 4.02 12.56 2.73

Table 5 reports, for the investigated geogrids embedded in Soil A and in Soil B, the ratio Beq/D50. In particular, Soil A is characterized by D50 equal to 0.32mm, while Soil B is characterized by D50 equal to 1.47mm. Soil B has about 84% of the particles with larger diameter of 0.32mm and shows a maximum diameter equal to 4.75mm (compared to 0.85mm of Soil A) which is greater than the thickness of the transverse elements, Beq, of each investigated geogrid.

In particular, as shown in Table 5 for all the geogrids of this comparison, the transverse member thickness normalized by the average soil particle diameter is lower than 2.98 when the inclusions are in contact with Soil B. For Soil A, the Beq/D50 ratio is always higher than 10.29, therefore, when the geogrid specimen is in Soil B (sand with gravel), the scale effect, due to the average soil particles size (D50) relative to the thickness of the transverse geogrid bars, is relevant.

Image 3_1
Image 3_2
Image 3_3

FIGURE 3 Pullout curves obtained at the same test condition (relative density, normal effective confining stress and anchorage length, type of geogrid) for different geogrids embedded in the two different soils (Soil A and Soil B): a) GGEM2; b) GGEB1; c) GGEB2

Figure 3 shows pullout force vs. displacement for the GGEM2, GGEB1, and GGEB2 geogrid specimens of length LR =0.90m embedded both in Soil A that in Soil B, either respectively compacted to 95% of γdmax, studied applying a confined pressure σ’v =25 kPa. Analyzing these curves, it is clear that higher pullout resistance was always obtained with Soil B. This result can be explained when considering that pullout forces are the integrals of the tangential stresses mobilized along the interface (along the active length that is the geogrid length that works in the anchorage zone, along which the elementary interaction mechanisms can be mobilized). These tensions, at equal length, structural characteristics, shape, tensile resistance, and stiffness of the reinforcing elements, depend on shear strength properties of the soil in contact with the reinforcement and on the soil particle size. Similar considerations apply to all tests, as summarized in Figure 4 and Table 6.

Image 4_1
Image 4_2
Image 4_3

FIGURE 4 Peak pullout resistance envelopes vs σ’v, for different LR varying the fill soil (Soil A and Soil B): a) GGEM2; b) GGEB1; c) GGEB2

TABLE 6 Peak pullout resistance for the GGEM2, GGEB1, and GGEB2, embedded in Soils A and B, for the studied applied confined stresses and anchorage lengths
(*) Confined failure
PR [kN/m]
10 kPa 25 kPa 50 kPa 100 kPa
Geogrid Length [m] Soil A Soil B Soil A Soil B Soil A Soil B Soil A Soil B
GGEM2 0.40 12.51 16.86 20.84 31.73 34.78 43.78 44.92 53.78
0.90 22.38 32.50 38.36 54.71 55.82 87.57 85.56 101.44*
1.15 25.84 39.40 44.88 67.04 70.64 98.17 96.96 107.56*
GGEB1 0.40 8.29 16.86 12.84 30.58 23.61 44.93 39.15 53.88*
0.90 19.03 36.88 35.09 52.36* 48.06* 54.91* 48.53* 56.67*
1.15 21.15 37.11 40.34 53.97* 52.18* 56.44* 53.36* 61.73*
GGEB2 0.40 8.64 15.97 15.07 32.26 21.87 43.47* 31.56 44.75*
0.90 19.61 31.11 33.55 37.34* 41.76* 44.18* 37.65* 44.18*
1.15 22.64 37.33 36.27 40.71* 42.46* 43.59* 42.89* 46.86*

In particular, Figure 4 shows, for each geogrid and for each anchorage length studied, the envelopes of peak pullout resistance vs. the vertical confined pressure, varying the fill soil (Soil A and Soil B). For ease of reading, the dotted lines and open symbols refer to Soil A and the solid lines and solid symbols refer to Soil B. The results show that the higher pullout resistance are mobilized for Soil B.

These results also confirm the importance of soil particle size, geogrid aperture size, and thickness of bearing members. A marked increase in soil-geogrid interface pullout resistance was observed when the soil contained a significant percentage of particles with size similar to the thickness size of the geogrid-bearing members, but smaller than the geogrid openings.

Therefore, the marked increase in pullout resistance at the soil-geosynthetic interface depends on the greater shear strength properties of the soil in contact and on the scale effect (as a result of lower ratio between the equivalent thickness of the transverse bars and the average diameter of soil particles).

The effect of the fill soil type on the deformation behavior under pullout loading conditions was also analyzed.

Image 5_1
Image 5_2

FIGURE 5 Displacements measured along the specimen for a) GGEM2 and b) GGEB1 geogrids

Figure 5 shows the distribution of the nodal displacements, along the reinforcement specimens, for GGEM2 (Figure 5a) and GGEB1 (Figure 5b) embedded in Soils A and B (for which the confined failure do not occur) measured in correspondence of the peak pullout resistance, PR, and of the trigger pullout resistance (that causes the movement of the last bar), Pin, at the same test condition (gsoil = 95% γdmax, LR =0.90 m, σ’v =25 kPa), that are obviously different for the two investigated soils.

This distribution provides information about the mobilization of the elementary interaction mechanisms (shear stress mobilized at the soil-geosynthetic interface) along the reinforcement, under pullout conditions, and therefore, the process of stress transfer from the soil to the reinforcement. The shear stresses mobilized for the different soil type-geogrid combinations, which are higher when the geogrids are embedded in Soil B, are reached for greater displacement value at the edge attached to the clamp, if the fill soil is Soil B.

Finally, to evaluate the influence of soil type on the deformation behavior, the response in terms of iso-displacement curves was analyzed.

These curves, which can be used in the design analysis performed by means of displacement methods, represent the mobilized pullout resistance referred to a given admissible displacement of the first geogrid confined section.

Image 6_1
Image 6_2

FIGURE 6 Comparison between the iso-displacement curves (δ=25mm) vs. applied confined pressure relative to the GGEM2 embedded in Soil A and in Soil B, for different geogrid specimen lengths: a) short (LR = 0.40m); b) long (LR = 1.15m)

Image 7_1
Image 7_2

FIGURE 7 Comparison between the iso-displacement curves (δ=25mm) vs. applied confined pressure relative to GGEB1 embedded in Soil A and Soil B, for different geogrid specimen lengths: a) short (LR = 0.40m); b) long (LR = 1.15m)

Considering a maximum allowable displacement equal to 25mm, Figure 6 and Figure 7 show, respectively for GGEM2 and GGEB1 embedded both in Soils A and B, the comparison between the iso-displacement curves vs. applied confine stress, referring to the “short” reinforcement (LR = 0.40m, Figure 6a and Figure 7a) and the “long” one (LR = 1.5 m, Figure 6b and Figure 7b).

By the result comparisons, it is clear that for a given admissible displacement measured at the edge attached to the clamp equal to 25mm, GGEM2 and GGEB1 embedded in Soil B (with larger variety of the grain size, higher D50, and therefore, better mechanical properties), show higher pullout resistance.

4. Conclusions

The experimental results presented in this article clearly show the influence of soil type on soil-reinforcement interaction under static pullout loading conditions.

On the basis of experimental results, the following conclusions could be drawn:

  • Pullout resistance depends on soil mechanical properties and on geosynthetics stiffness and geometry. In particular, pullout resistance increases with the increase of soil shear strength and of geogrid bearing area Ab and with the decrease of the ratio B/D50 (due to scale effects). Therefore, a marked increase in the pullout resistance was observed in Soil B.
  • The response in terms of iso-displacement curves, for a fixed displacement measured at the edge attached to the clamp, shows that the confined stiffness increases also with the increase of soil shear strength and of bearing area Ab and with the decrease of the ratio B/D50. Therefore, the higher confined tensile stiffness was observed when the geogrid specimens are embedded in Soil B.

Daniele Cazzuffi—CESI SpA, Milano, Italy; IGS past president; and a member of the Editorial Advisory Committee for Geosynthetics magazine.

Nicola Moraci, Lidia Sarah Calvarano, Giuseppe Cardile, and Domenico Gioffrè—Mediterranea University of Reggio Calabria, Department of Civil, Energy, Environment and Materials Engineering (DICEAM), Italy.

Piergiorgio Recalcati—Tenax SpA, Viganò (LC)

Acknowledgments

All authors have contributed in equal manner to this article.

The research was funded by: PON01_01869 TEMADITUTELA.

References

Calvarano L. S., Cardile G., Gioffrè D., Moraci N., 2013. “Experimental and theoretical study on interference phenomena between the bearing members of different geogrids in pullout loading conditions”, Geosynthetics 2013, Long Beach, paper n.78, pp. 496–502.

Calvarano L. S., Cardile G., Moraci N., Recalcati P., 2012. “The influence of reinforcement geometry and soil types on the interface behaviour in pullout conditions”, 5th European Geosynthetics Congress (EuroGeo 5), Valencia, Vol. 1, pp. 708–714.

Cazzuffi, D., 1996. “Evolution of European standardization on geosynthetics, with special reference to mechanical tests”, Proceedings Index 96 Nonwoven Congress–Construction session, Geneva, pp. 1–15.

Cazzuffi, D., Calvarano, L. S., Cardile, G., Moraci, N., Recalcati P., 2011. “European experience in pullout tests: The influence of geogrid’s geometry and structure on interface behavior”, Geosynthetics, Vol. 29 (5), pp. 42–51.

Cazzuffi, D., Moraci, N., Calvarano, L. S., Cardile, G., Gioffrè, D., Recalcati P., 2014. “European experience in pullout tests: Part 2—The influence of vertical effective stress and of geogrid length on interface behaviour under pullout conditions”, Geosynthetics, Vol. 32, (2).

EN ISO 10319, 2008. “Geosynthetics wide-width tensile test”, International Organization for Standardization, ISO, Geneva.

Jewell, R. A., Milligan, G. W. E., Sarsby, R. W., Dubois, D. D., 1984. “Interactions Between Soil and Geogrids”, Proceedings from the Symposium or Polymer Grid Reinforcement in Civil Engineering, Ed. Thomas Telford, London, pp.18–30.

Moraci N., Montanelli F., 2000. “Analysis of pullout tests on extruded geogrids embedded in a compacted granular soil”, Rivista Italiana di Geotecnica (Italian Geotechnical Journal), n. 4(16), pp. 5–21 (in Italian).

Moraci, N., Recalcati, P.G., 2006. “Factors affecting the pullout behaviour of extruded geogrids embedded in compacted granular soil”, Geotextiles and Geomembranes, Vol.24 (22), pp. 220–242.

Moraci, N., D., 2006. “A simple method to evaluate the pullout resistance of extruded geogrids embedded in granular soil”, Geotextiles and Geomembranes, Vol. 24 (12), pp. 116–128.

Moraci, N., Cardile, G., 2009. “Influence of cyclic tensile loading on pullout resistance of geogrids embedded in a compacted granular soil”, Geotextiles and Geomembranes, Vol. 27, pp.475–487.

Moraci, N., Cardile, G., 2012. “Deformative behaviour of different geogrids embedded in a granular soil under monotonic and cyclic pullout loads”, Geotextiles and Geomembranes, Vol. 32, pp.104–110.

Moraci, N., Cardile, G., Gioffrè, D., Mandaglio, M.C., Calvarano, L.S., and Carbone, L., 2014. “Soil geosynthetic interaction: design parameters from experimental and theoretical analysis,” Transportation Infrastructure Geotechnology, 1 (2), pp. 165-227.

Palmeira, E M., Milligan, G. W. E., 1989. “Scale and other factors affecting the results of pull-out tests of grid buried in sand”, Géotechnique, Vol. 11 (3), pp. 511–524.

Palmeira, E. M, 2009. “Soil–geosynthetic interaction: Modelling and analysis”, Geotextiles and Geomembranes, Vol. 27 (5), pp. 368–390.

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FIGURE 1 Grain size distribution curves of the two different soils.

FIGURE 2 Result of pullout test on isolated transverse members with different cross section (Palmeira, 2009).

FIGURE 3 Pullout curves obtained at the same test condition (relative density, normal effective confining stress and anchorage length, type of geogrid) for different geogrids embedded in the two different soils (Soil A and Soil B): a) GGEM2; b) GGEB1; c) GGEB2

a) GGEM2 geogrid

b) GGEB1 geogrid

c) GGEB2 geogrid

FIGURE 4 Peak pullout resistance envelopes vs σ’v, for different LR varying the fill soil (Soil A and Soil B): a) GGEM2; b) GGEB1; c) GGEB2

a) GGEM1 geogrid

b) GGEB1 geogrid

c) GGEB2 geogrid

FIGURE 5 Displacements measured along the specimen for a) GGEM2 and b) GGEB1 geogrids

a) GGEM2 geogrid

b) GGEB1 geogrid

FIGURE 6 Comparison between the iso-displacement curves (δ=25mm) vs. applied confined pressure relative to the GGEM2 embedded in Soil A and in Soil B, for different geogrid specimen lengths: a) short (LR = 0.40m); b) long (LR = 1.15m)

a) LR=0.40m

b) LR=1.15m

FIGURE 7 Comparison between the iso-displacement curves (δ=25mm) vs. applied confined pressure relative to GGEB1 embedded in Soil A and Soil B, for different geogrid specimen lengths: a) short (LR = 0.40m); b) long (LR = 1.15m)

a) LR=0.40m

b) LR=1.15m

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