Influence of Transverse and Longitudinal Members of Coated Polyester-Yarn Geogrid on Pullout Response Under Low Normal Stress

Abstract

This paper presents an experimental investigation of the effectiveness of longitudinal and transverse members of bitumen-coated polyester-yarn geogrids under low normal stress using large-scale pullout apparatus. The tests first compare the pullout behavior of a geosynthetic sheet with that of the geogrid. It then compares the pullout behavior of geogrids with different spacing of longitudinal and transverse members. The results show that under low normal stresses, the behavior of the sheet and the geogrid is comparable. The spacing between transverse ribs displays very little influence on peak pullout resistance of the geogrids under low normal stress. Increase in spacing between longitudinal ribs reduces the interaction between them. As a result, after the critical spacing of 70 mm determined for present test conditions, the total pullout resistance of the geogrid specimens is the product of the pullout resistance per rib with the number of longitudinal ribs in the respective specimens.

Appendix

1.1 Appendix 1A: Calculation of Pullout Resistance for Sheet

Since the top soil moves with the geosynthetic reinforcement in both cases at the peak, the frictional resistance provided by the top surface of the reinforcements is not considered. However, the movement of the top soil block will impart side resistance to the pullout in the initial stages as shown in Figs.

Fig. 20

Conceptualized pullout interaction mechanism for sheet under low normal stress—side view

20

Fig. 21

Idealized pullout interaction mechanism for sheet under low normal stress—perspective view

and 21. Thus, the peak pullout resistance (P_rs) of the sheet [10] will be the sum of interface shear resistance at the bottom (P_r-bottom) and frictional resistance on the back and sides (P_r-sides) as given below:

$$P_{{{\text{rs}}}} = \, P_{{\text{r - bottom}}} + \, P_{{\text{r - sides}}} ,$$

(1)

$$P_{{\text{r - bottom}}} = L_{{\text{r}}} \sigma_{{\text{n}}} f_{{\text{b}}} B\tan \varphi ,$$

(2)

$$P_{{\text{r - sides}}} = \left( {2 \times 0.5 \times K_{0} \gamma \, \left( {h^{2} /4} \right) \, L_{1} \tan \varphi } \right) + \left( {2 \times 0.5 \times K_{0} \gamma \, h^{2} L_{2} \tan \varphi } \right) + \left( {0.5 \times K_{0} \gamma \, h^{2} B\tan \varphi } \right),$$

(3)

where L_r is the length of the geosynthetic (= L₁ + L₂ = 0.4 m + 0.4 m = 0.8 m), σ_n is the normal stress on the reinforcement (= 5 kPa), f_b is the bond coefficient (= 1, assuming soil–reinforcement resistance equal to soil–soil resistance), ϕ is the angle of shearing resistance of the soil (= 43°), γ is the unit weight of soil (= 16.5 kN/m³), K₀ is coefficient of earth pressure at rest (= 1 − sin ϕ), h is the height of the overburden soil above geosynthetic, and B is the width of the geosynthetic reinforcement (= 0.3 m).

1.2 Appendix 1B: Calculation of Pullout Resistance for Geogrid

The peak pullout resistance of geogrids (P_rg) is assumed to be the sum of the interface shear resistance at bottom (P_rg-bottom), bearing resistance (P_rgb), and frictional/shear resistance from the sides of the top soil block (P_r-sides) as given below:

$$P_{{{\text{rg}}}} = P_{{\text{rg - bottom}}} + P_{{{\text{rgb}}}} + P_{{\text{r - sides}}} ,$$

(4)

$$P_{{\text{rg - bottom}}} = \alpha_{{\text{s}}} L_{{\text{r}}} \sigma_{{\text{n}}} f_{{\text{b}}} \;\tan \;\varphi ,$$

(5)

$$P_{{{\text{rgb}}}} = \alpha_{{\text{b}}} \left( {L_{{\text{r}}} /S} \right)\sigma_{{\text{b}}} t,$$

(6)

where α_s is the fraction of geogrid solid area, α_b is the total frontal area available for bearing resistance, S is the spacing between T-ribs (= 0.002 m), t is the thickness of the T-rib (= 0.001 m), and σ_b is the bearing stress mobilized on T-ribs. For the present case, α_s was estimated as 0.34 and α_b as 0.80. Evaluation of σ_b can be done using different failure mechanisms. Jewell [10] used the punching failure mechanism (GG-PS-P in Fig. 11) which gives the lower bound value of the maximum pullout resistance. The upper bound value is obtained when the general shear failure mechanism (GG-GS-P in Fig. 11) is used [29]. Bergado et al. [30] used the modified punching failure mechanism (GG-mPS-P) and Matsui et al. [31] used the Prandtl local shear failure mechanism (GG-LS-P).