Pullout resistance of inclined anchors embedded in geogrid reinforced sand
Introduction
Foundations of several civil engineering structures like transmission towers, buried pipelines, chimneys, retaining walls, etc., are more likely to experience uplift forces and overturning moments. In these structures, the compressive forces on the foundations from weight of the structure along with several other live loads are less as compared to the uplift forces and moments due to the wind, earthquake, and buoyant forces. In the past, many types of foundations were employed to resist uplift forces. Among these, plate anchors or the anchor foundations are commonly used as they are economical and easy to install (Robinson and Taylor, 1969). Although both horizontal and vertical anchors can be used to withstand pullout forces, inclined anchors are more suitable to resist overturning moments (Ghaly and Clemence, 1998).
It has been reported in the literature that with inclination, the pullout capacity of anchors improves compared to horizontal anchors. Robinson and Taylor (1969) conducted a series of field tests on different types of anchors to compare their pullout capacity and feasibility and concluded that the cast in place concrete anchor foundation meets the requirements at a reasonable cost. They also conducted tests on helical screw anchors with inclination angles () 10°, 23° and 35°, where is the inclination angle of the anchor rod with the vertical. It was reported that the pullout capacity increased with the inclination angle.
Vesic (1971) reported that for inclined steel circular plates embedded in fine sand, the axial pullout capacity perpendicular to the plate surface or the breakout force increased with the inclination angle.
A theoretical model was proposed by Meyerhof (1973) to predict the pullout capacity of inclined anchors, which is a function of uplift coefficients (for cohesionless soil), (for cohesive soil), inclination angle and other parameters. For a given friction angle of the soil, the coefficient increases with the inclination angle and results in higher pullout capacity with inclination.
Larnach and McMullan (1975) conducted experiments on anchors in dry sand at higher depth. They reported that an individual anchor's highest capacity is achieved at an inclination angle of 20° with horizontal.
Murray and Geddes (1989) conducted laboratory experiments on inclined anchors in dense sand and reported an increase in the ultimate passive resistance of the anchors when the inclination angle increased from 0° to 90° with the horizontal.
Geddes and Murray (1991) performed experiments on circular, rectangular, and square anchor plates in dry sand. They observed that, for all types of anchors, the pullout capacity increased with inclination angle from 0° to 90°. However, in circular plates, a local peak was observed within the interval . No such peaks were observed in rectangular and square plates.
Goel et al. (2006) presented an analytical expression for the breakout resistance () for inclined circular plate anchors in sands using limit equilibrium approach considering different parameters. The factor is a factor of lateral earth pressure and other parameters and increased with the inclination angle.
Bhattacharya and Kumar (2014) determined the pullout capacity of inclined strip anchor plate embedded in sand using the lower bound limit analysis and indicated an increase in the pullout capacity with an increase in the inclination angle.
A theoretical model for predicting the pullout capacity of strip anchors based on the cavity expansion theory for biaxial stress conditions can be found in Zhuang and Yu (2018).
Kumar and Rahaman (2019) studied the vertical uplift capacity of strip anchor plate placed in sand using lower bound limit analysis theorem and concluded that the vertical uplift decreases with an increase in the vertical inclination of the resultant pullout load and the eccentricity.
A few studies were reported on the uplift capacity of anchors in reinforced sand. Based on model tests conducted on horizontal anchors in reinforced sand, Krishnaswamy and Parashar (1994) concluded that the vertical uplift capacity could significantly increase using geosynthetics. Consoli et al. (2012) conducted model tests on horizontal anchors placed in fibre-reinforced cement stabilised backfill and reported that the fibre reinforcement significantly increased the uplift capacity of the anchor plate and also induced ductility in the soil-anchor system. Khatun and Chattopadhyay (2010) reported improvement in the ultimate breakout capacity of circular anchors in sand due to the presence of geotextile reinforcement. Based on model tests, Ghosh and Bera (2010) reported an appreciable improvement in the anchors’ uplift capacity by providing geotextile ties.
Recently, extensive studies have been reported on the improvement of pullout capacity of horizontal and vertical anchors in reinforced sand. Based on finite element analysis, Rahimi et al. (2018b) indicated that, by geocell inclusion, the uplift capacity of anchors could be increased by 5%–215% depending upon the depth of embedment. The uplift capacity of horizontal anchors was found to increase significantly in presence of a geocell layer (Rahimi et al., 2018; Choudhary et al., 2019b). Experiments on anchor groups also exhibited a substantial increase in the uplift capacity when placed in sand with geogrid reinforcement (Choudhary et al., 2019a). It has been reported that the load-carrying capacity of vertical plate anchor in geocell reinforced sand increased by four times as compared to the capacity of anchors in unreinforced sand (Dash and Choudhary, 2018, 2019). Most recently, Kumar and Ilamparuthi (2020) studied the effect of geocell reinforcement on uplift resistance of shallow anchors and reported that uplift resistance increased due to reinforcement. These studies demonstrate the beneficial use of reinforcement in improving the performance of both horizontal and vertical anchor plates in sand.
However, the studies reported related to inclined anchors are either based on theoretical models or model tests conducted in unreinforced soil. To the authors’ knowledge, no study has been reported to understand the pullout behaviour of inclined anchors in reinforced soil mass. Therefore, a detailed investigation is required to understand the behaviour of inclined anchors in reinforced soil.
In view of this, the main objective of the present study is to investigate the pullout behaviour of inclined anchors embedded in unreinforced and reinforced sand through laboratory experiments and numerical analysis. The effect of different parameters, such as the size of reinforcement, depth of embedment, relative density of soil, and inclination of the plate, on the overall behaviour of the anchor system has been analysed. Besides, the implications of the study are illustrated with reference to a typical transmission tower foundation design, and the advantage of using geosynthetics is brought out.
Section snippets
Experimental program
Laboratory experiments were carried out to analyse the performance of inclined anchors in a cohesionless medium. Model tests were performed in six different series, as detailed in Table 1. In the first series, the effect of geogrid size on the pullout capacity of anchor placed at 45° inclination was studied, and the optimum size of the geogrid was determined. In the following series, the effect of different parameters on the anchor performance was investigated. In series 2 and 3, the effect of
Influence of reinforcement
In the first series, tests were performed to determine the optimum size of the geogrid layer. For this purpose, four tests were conducted with different dimensions of the geogrid layer. Results obtained from these tests are shown in Fig. 3. It was observed from the experimental results that the pullout capacity increased significantly when the geogrid size was increased from to . However, further increasing the size to did not produce a significant improvement
Numerical study
A detailed parametric study is not feasible in the laboratory due to several constraints. Thus, numerical models can be used to study the behaviour of the structures thoroughly. Results obtained from numerical analyses can also be used to compare the experimental results. In the present study, numerical analyses were carried out in (Fast Lagrangian Analysis of Continua in 3D) to compare the experimental data. It is a three-dimensional explicit finite difference scheme which works with
Practical design example
In this section, the use of inclined anchors in transmission tower foundations is shown through a simple practical example. A typical transmission tower was designed in STAAD Pro software. Wind load was generated in the positive -direction as per ASCE 7–2010 guidelines. Details of the tower and wind velocity are given in Table 4. The orientation of the transmission tower is shown in Fig. 16. The forces developed at the foundations due to the self-weight of the structure and the wind load are
Limitations of the present study
Although extensive experiments were conducted, and a numerical model was developed to compare and validate the results in this paper, some drawbacks of the present study exist. This is due to the limitations associated with the small-scale experiments and certain assumptions made in the numerical modelling. An elastic-perfectly plastic Mohr-Colomb constitutive model was assigned to the soil in the numerical analysis. However, during the experiments, the sand was observed to flow like individual
Conclusions
This study investigated the pullout behaviour of inclined square anchor plate in reinforced sand. The effect of different parameters on the ultimate pullout capacity was studied. Numerical analysis was carried out, and the results were compared with the experimental data. Based on the findings of this study, the following conclusions are drawn:
- 1
The axial pullout capacity of the anchor increases significantly by the inclusion of geogrid reinforcement. The reinforced soil-anchor system can sustain
Declaration of competing interest
None.
Acknowledgement
This study is a part of the research project titled “Analysis of Performance of Inclined Plate Anchors Embedded in Geosynthetics Reinforced Soils for Transmission Tower Foundations” funded by CPRI, Bangalore (file number: CPRI0007) to the third and fourth authors at the Department of Civil Engineering, Indian Institute of Science, Bangalore. The authors gratefully acknowledge the financial support provided by CPRI, Bangalore, for this study. The authors are thankful to the anonymous reviewers
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