The influence of geogrid’s geometry and structure on interface behavior

European experience in pullout tests

By Daniele Cazzuffi, Lidia Sarah Calvarano, Giuseppe Cardile, Nicola Moraci, and Piergiorgio Recalcati

1. Introduction

The use of geosynthetics to improve the soil mechanical response has become increasingly common practice in geotechnical engineering applications.

Geosynthetic materials are commonly used in soil reinforced structures, in embankment toe reinforcements on compressible ground, and in paved and unpaved roads.

The knowledge of equivalent friction parameter is important in the soil structures design (Cazzuffi et al., 1995). It can be done by direct shear tests and pullout tests carried out to simulate, as closely as possible, the on-site conditions.

The geogrid pullout resistance is a function of skin friction, generated between the solid portion of the reinforcement and the soil, and the passive resistance mobilized in correspondence of the bearing members placed transversely to the pullout force direction.

In particular, the main soil parameters that influence the pullout behavior are the same that typically affect the dilatancy, such as the relative density, the grain size distribution (in relation to the geogrid mesh size), and the vertical effective confining stress (Moraci and Montanelli, 2000).

About the reinforcement, the geometry and thickness of the transverse elements, the mesh size (in terms of spacing between elements), and the longitudinal and transversal members’ stiffness play an important role on the mechanical behavior of geogrids.

Therefore, for the extensible polymeric inclusions, the apparent coefficient of friction mobilized at the interface is a function of both the soil and the reinforcement characteristics.

This article deals with some results of a wide experimental program developed in Europe aimed to study the behavior in pullout conditions at constant rates of displacement of different geosynthetics embedded in a granular compacted soil. The pullout tests were performed on two high-density polyethylene (HDPE) extruded mono-oriented geogrids, four polypropylene (PP) extruded bio-oriented geogrids, one polyester (PET) woven geogrid, and one PET welded geogrid.

In particular, the main factors that influence the mechanical pullout response (in terms of peak pullout resistance and peak apparent coefficient of friction) of different geosynthetic reinforcement (different geometry and structure), installed in compacted granular soils, are reviewed.

2. Test apparatus

To develop the test program, large-scale pullout equipment from the geotechnical laboratory of Mediterranea University of Reggio Calabria was used.

The test apparatus was developed in previous research (Moraci and Montanelli, 2000; Moraci et al., 2003; Moraci and Recalcati, 2006; Gioffrè and Moraci, 2006; Moraci and Cardile, 2009). The test apparatus is essentially composed of a rigid steel large pullout box (1700mm × 680mm × 600mm), a vertical load application system, a horizontal force application device, two special clamps with sleeve system, and all required control and data acquisition instruments.

Figure 1 shows the test apparatus and Figure 2 is a sketch illustrating its components.

The test procedure was developed on the basis of the results from previous researches on the soil-geosynthetic interaction (Moraci and Montanelli, 2000; Moraci and Recalcati, 2006), and helps to ensure a good reproducibility of results and to minimize the scale effects.

All pullout tests described herein have been performed at controlled rates of displacement (CRD) equal to 1.0mm/min, until geogrid tensile rupture or until a total horizontal displacement of 100mm was achieved.

For the application of the pullout force, an electric double-acting hydraulic jack connected to a load cell and to a special clamping system was used.

To apply the tensile load to the different types of reinforcement specimens, two internal clamps were specifically designed and used. The internal clamping system has several advantages compared with the external ones most commonly used. The first is to ensure constant anchorage length during the pullout test. The second is the possibility to measure displacements and strains only in confined conditions.

In contrast, the clamping system inside the pullout box needs a preliminary pullout test, carried out under the same test conditions, only on the clamp (without the geogrid), to evaluate the skin friction.

In particular, for each type of clamp, three different calibration tests were performed at the vertical effective stress values ​​equal to σ’_v=10, 25, 50 kPa. Therefore, the pullout force due to the clamp friction (measured at each displacement level) needs to be subtracted from the pullout force measured in the test with the geosynthetic at the same displacement

Finally, this internal clamping system permits the study of the reinforcement confined failure. The internal displacements along the geogrid specimen were measured and digitally recorded during the pullout test.

3. Test materials

The test materials used in this study have been classified and mechanically characterized by standard laboratory tests allowed to evaluate the main strength and deformability parameters necessary to develop the next phase of pullout test analysis and discussions.

3.1 Soil characterization

The results of the classification test (Figure 3) indicate that the soil 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 Standard Proctor compaction test performed indicates a maximum dry unit weight γ_dmax=16.24 kN/m³ and an “optimum” water content w_opt=13.5%.

The direct shear tests and the triaxial tests have been carried out at an initial unit weight equal to 95% of γ_dmax, obtained at a water content of 9.3% (corresponding to D_R=76%). The peak shear strength angle φ’_p yield, in range between 48° and 42°, where the higher and the lower values refer respectively to the lower (σ’_v =10 kPa) and the higher (σ’_v=100 kPa) confining pressure.

The shear strength angle at constant volume φ’_cv results equal to 34°.

3.2 Geometrical and mechanical characteristics of tested geogrids

The pullout tests have been performed on eight different geogrids with different geometry, structure, and tensile stiffness characteristics.

The pullout tests have been carried out on: two HDPE extruded mono-oriented geogrids (called GGEM1 and GGEM2), four PP extruded bi-oriented geogrids (named GGEB1, GGEB2, GGEB3, and GGEB4), one PET woven geogrid (GGK), and one PET welded geogrid (GGW).

Figure 4 shows a schematic cross section of a geogrid bearing members (spacing between transversal bars in the pullout direction equal to S) that is placed transversely to the direction of pullout force.Table 1 shows the geometrical characterization of the geogrids, where W_r and B_r are the node width and thickness, respectively; W_t and B_t are the width and thickness of the bar portion between two nodes, respectively (Figure 4); and A_b is the area of each rib element (including the node embossment and the bar portion between two nodes A_t + A_r) where the bearing resistance can be mobilized.

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

4. Analysis of pullout response

Figure 5 shows the classical pullout curves that report the pullout force (P) vs. the displacement measured at the edge attached to the clamp, for the investigated anchorage length (L_r=1.15m), varying the applied normal effective confining pressures [10 kPa (a), 25 kPa (b), 50 kPa (c)].

The pullout curves show, for GGEB2 and GGEB3 geogrids at vertical effective confining stresses lower than 25 kPa, for GGEM1, GGEM2, and GGK geogrids at effective normal confining tensions lower than 50 kPa, and for GGW geogrid under all applied confining stress levels, a softening mechanical response. In this case, there is a decrement of the pullout resistance after a peak value​​ ​​at large displacements.

On the contrary, for GGEB1 and GGEB4 under all applied confining stress levels, and for the remaining geogrids at higher confining stress, a strain-hardening behavior, with a progressive increase of the pullout resistance with the increase of the displacement of the first geogrid confining section, until its maximum value more or less constant ​at large displacements, was observed.

It is possible to say that the pullout interaction mechanism is progressively developed along the reinforcement specimen length, in the latter behavior, while it is developed almost at the same time along the geogrid in the former one.

In the test carried out on all bi-oriented geogrid specimens and in the range of confining stress used, pullout tensile failure occurred. Particularly, it was achieved for GGEB2 and GGEB3 geogrids at normal effective confining stress equal to 25 kPa, and for GGEM1 and GGEM2 geogrids for confining tension equal to 50 kPa.

4.1 Influence of the geogrid geometry on pullout response

To study the influence geometry of the reinforcement on the mechanical pullout behavior, the results of the tests conducted on geogrid of the same structure and comparable strength tensile and stiffness were compared.

The extruded geogrids were characterized by measuring the spacing between transversal bars in the pullout direction, the surfaces on which it is possible to mobilize the friction and the passive interaction mechanisms after defining the number of transversal and longitudinal bars (Table 3).

Figures 6 and 7 show the peak pullout resistance envelopes varying the normal effective confining stresses, for extruded mono and bi-oriented geogrids (the filled symbol indicates the confined tensile failure).

The GGEM2 geogrid, for all applied confining tensions, exhibited greater peak pullout resistance than the GGEM1 geogrid. The peak pullout resistance percentage difference varied about 23.5%.

These experimental results may be due to the different surface values on which it can be mobilized, including the interaction mechanisms of passive and friction types.

In fact, for the GGEM2 the above-mentioned areas were more than 8% different if compared to the GGEM1.

This result is confirmed by applying the theoretical model developed by Moraci and Gioffrè (2006) (valid in absence of interference phenomena between the bearing members).

The extruded bi-oriented geogrids for all applied vertical effective stresses showed comparable values ​​of the peak pullout resistance when the tensile failure did not occur.

This experimental result is particularly interesting because it disagrees with the comparison between the values​​ of the overall frictional and bearing surfaces on which it is possible to mobilize the interaction mechanisms. In particular, the GGEB3 and GGEB4 geogrids have higher values ​​of frictional and bearing surfaces when compared with the GGEB1 and GGEB2 geogrids, but the former ones are those that exhibit the ​​lower spacing values between the bearing members than the GGEB1 and GGEB2 geogrids.

Therefore, the comparable values ​​of the peak pullout resistance, at the same test condition, are due to the interference phenomena between the passive failure surfaces that are generated in front of the bearing members and unloading stress areas at the back of these bars (Palmeira, 2004). In this case, the theoretical model (Moraci and Gioffrè, 2006) is not applicable.

Consequently, to study the interference phenomena that occur when the distance between the bearing members decreases, additional tests were performed by removing some transversal bars from the specimens of GGEB4 geogrid, 0.90m long, which originally were spacing bars equal to S = 39.5mm. The new specimen was characterized by spacing between the bearing bars equal to 86.7mm and 129.8mm, respectively.

Figure 8 shows the results of pullout tests carried out on GGEB4 geogrid specimens ( L_R = 0.90m) varying the spacing between the transverse elements. All tests were performed at confining pressure equal to 50 kPa.

In particular, Figure 8 shows the peak pullout resistance envelopes, in the same test conditions, for different geogrid mesh density.

The experimental results illustrated in Figure 8 show the optimum spacing, S_opt, which maximizes the peak pullout resistance. In fact, when the transverse bars spacing is lower than the “optimum” value, the pullout response appears to be affected by the interference between the bearing members.

When the transverse bars spacing is higher than the “optimum” value, the pullout resistance decreases due to the minor number of bearing members that provide passive resistance contribution to the overall pullout resistance.

These results demonstrate the significant effect of the density of the bearing elements of the geogrid mesh on the mechanical response of polymeric geogrids.

4.2 Influence of the geogrid structure on pullout response

To study the influence of the reinforcement structure on the geogrid pullout behavior, the results of pullout tests conducted on geogrids of different structure and similar strength tensile and stiffness, were compared.

In this case, the analysis relates to the extruded geogrid GGEM2, the woven geogrid GGK, and the welded geogrid GGW. The results were analyzed in terms of peak pullout resistance and of peak apparent coefficient of friction, the latter defined as:

where:

P_R= pullout resistance, corresponding to the maximum pullout force (per unit of width) measured during a pullout test;

L_e= confined effective length of the reinforcement equal to the sum of initial reinforcement length plus the geogrid specimen elongation Δl acquired at the value of P_R;

σ’_v= vertical effective confining stress acting at soil-reinforcement interface.

It is important to note that the determination of µ_S/GSY by using equation (1) can be performed without any assumption about the values of the soil shear strength angle mobilized at the interface; it can be easily determined from the pullout tests.

Figure 9 shows, for each geogrid specimen and for the same anchorage length, the peak pullout resistance envelopes vs. the applied vertical effective confining pressure.

It is possible to observe that, for all pullout tests performed, the maximum peak pullout resistance was measured for the extruded mono-oriented geogrid GGEM2 (although it is characterized by lower values ​​of tensile strength and stiffness).

In particular, the maximum percentage difference of peak pullout resistance equal to 19% is given with the comparison between the extruded geogrid GGEM2 and the woven geogrid GGK, while the greater percentage difference is equal to about 21% in the comparison between the extruded geogrid GGEM2 the welded geogrid GGW.

This result is due to interference effect and geometry effect, the latter associated to the different production processes, as evidenced by the data reported in Table 1.

In Figure 10 the pullout test results are interpreted in terms of peak apparent coefficient of friction mobilized at the interface.

For geogrids of different structures (with the same anchorage length Lr=1.15 m), the trend of the peak apparent coefficient of friction is mobilized at the interface vs. the effective vertical confining stress.

In all the analyzed cases (for different geogrid processing structure types and equal length) it is possible to observe a reduction in the mobilized peak pullout interface apparent friction coefficient with the increase of the applied vertical effective stress.

This result is connected to the soil dilatancy phenomena, whose entity decreases with the increases of the confining normal effective pressure σ’_v , that developed in correspondence with the three-dimensional passive failure surfaces that arise at the bearing members of the geogrid.

It is also important to note that geogrids GGK and GGW, for confining stresses higher than 25 kPa, mobilized a peak apparent coefficient of friction lower than the constant volume shear strength angle value refers only to the soil.

While the extruded geogrid GGEM2, for all applied confining condition, shows a µ_S/GSY higher than the constant volume shear strength angle refers only to the soil.

Comparing the experimental results refer to pullout tests carried out on geogrids varying structure types, at the same applied vertical effective stress, to be not influenced by soil dilatancy phenomena, it is possible to evaluate the effects of reinforcement different structures on the peak apparent coefficient of friction mobilized at the soil-geosynthetic interface.

In particular, higher µ_S/GSY values​​, for each investigated confining pressure, are associated with extruded mono-oriented geogrid GGEM2.

Particularly, the greater percentage difference of µ_S/GSY values (Table 4), equal to 20.71%, is given with the comparison between the extruded geogrid GGEM2 and the woven geogrid GGK, while the maximum percentage difference is equal to about 22.69% in the comparison between the extruded geogrid GGEM2 and the welded geogrid GGW.

5. Conclusions

The experimental results presented in this article show the influence of the different parameters studied (geogrid’s geometry and structure) on the mechanical response of different geosynthetic reinforcement in interaction with compacted granular soil, in pullout conditions.

Particularly with the increase of the surface where the passive and friction interaction mechanisms are mobilized, in the absence of interference phenomena, it is possible to note corresponding enhancement both of peak pullout resistance and mobilized peak pullout interface apparent friction coefficient.

In contrast, when the proximity of the bearing members produces interference phenomena, the increases in the bearing surface are not associated with increases of the mechanical response parameters.

In the latter case, the effect of interference between bearing members shows the existence of an optimum spacing bars, S_opt, below which the pullout response appears to be affected by the interference phenomena.

Acknowledgments

This paper was originally presented in Italian by Calvarano et al. (2011) at the XXIV Geotechnical National Conference held in June 2011 in Napoli, Italy.

It has been edited for Geosynthetics magazine style and format.

Daniele Cazzuffi—CESI SpA, Milano, Italy; IGS past-president and a member of the Editorial Advisory Committee for Geosynthetics magazine
Lidia Sarah Calvarano, Giuseppe Cardile, and Nicola Moraci—Mediterranea University of Reggio Calabria, Italy
Piergiorgio Recalcati—Tenax GTO, Milano, Italy

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