Physical and numerical modelling of strip footing on geogrid reinforced transparent sand
Introduction
Geosynthetic reinforcements have been widely used in engineering practice to improve the behavior of soil-foundation system, embankment and lining systems (Tafreshi and Dawson, 2010; Rajesh and Viswanadham, 2012; Saha Roy and Deb, 2017a; Ouria and Mahmoudi, 2018). The commonly used geosynthetic reinforcement products are geogrid, geotextile and geocell. Among them, geogrid is one of the most widely used to reinforce soil for shallow foundations (Latha and Somwanshi, 2009b). Over the last few decades, many researchers have investigated the behavior of geogrid-reinforced soil under footing loading using laboratory model tests (Das et al., 1994; Shin et al., 2002; Dash et al., 2004; Patra et al., 2005; Latha and Somwanshi, 2009a, Latha and Somwanshi, 2009b; Abu-Farsakh et al., 2013; Prasad et al., 2016; Wang et al., 2018; Useche-Infante et al., 2019; Xu et al., 2019), field tests (Abu-Farsakh et al., 2008; Demir et al., 2013; Venkateswarlu et al., 2018) and numerical modelling (Laman and Yildiz, 2007; Latha and Somwanshi, 2009a, Latha and Somwanshi, 2009b; Chakraborty and Kumar, 2014; Demir et al., 2014; Tran et al., 2015; Hussein and Meguid, 2016; Lai and Yang, 2017).
Most of existing literatures focused on the influence of the depth of the top reinforcement layer, the length of reinforcement, the number of reinforcement layers and the spacing between reinforcement layers on the bearing capacity and settlement of the foundations. Latha and Somwanshi, 2009a investigated the effect of different reinforcement parameters on the behavior of a square footing resting on reinforced sand foundations using model tests and numerical simulations. It was reported that the optimum spacing of reinforcement layers was about 0.4B; the effective depth of the reinforced zone below the footing was 2B; and the optimum length of reinforcement was about 4B, where B is the width of the footing. Abu-Farsakh et al. (2013) conducted a series of plate load tests on geosynthetic-reinforced sand foundation and reported that the optimum depth of the top layer was 0.33B, and the effective geosynthetic reinforcement length was 6B. They also found that geogrid reinforced sand foundations performed better than geotextile reinforced foundations.
Although many studies have been done to investigate the influence of different reinforcement parameters on the performance of reinforced soil foundations, limited research has been done to understand the failure behavior of reinforced soil foundation, especially the failure of geogrid. Yamamoto and Otani (2001), Michalowski and Shi (2003) and Zhou et al. (2008) performed a series of model loading tests to investigate the deformation pattern and progressive failure mechanism of reinforced soil foundations by analyzing soil displacement using a digital image correlation (DIC) technique. However, the relationship between the failure of foundations and the fracture of reinforcement layers has not been well investigated and understood.
From the above discussion, it can be noticed that it is essential to understand the nature of internal soil displacement, soil-geosynthetic relative displacements and the relationship between the failure of foundation and the rupture of reinforcements. Due to the opacity of soil materials, it was not possible to explore the factors using loading tests. Recently, transparent soils have increasingly been used to study the soil-structure interaction problems, such as, soil displacement during pile installation or pullout (Kong et al., 2015; Xiao et al., 2017), interaction between soil and reinforcement (Ezzein and Bathurst, 2014; Bathurst and Ezzein, 2016; Peng and Zornberg, 2019), tunneling-induced ground settlement (Ahmed and Iskander, 2011; Xiang et al., 2018), three-dimensional deformation in soils (De Cataldo et al., 2017; Yuan et al., 2019), etc. Although transparent soils have been widely used to investigate soil-structure interaction, there has been limited application on behavior of footings resting on reinforced soils. Further studies need to be done to reveal the failure mechanism of reinforcement in the foundation.
Chen et al., 2019a, Chen et al., 2019b performed several plate load tests on transparent soils reinforced with polyamide net. The authors observed progressive failure of reinforcement during the loading tests, but due to the white colored nature of the polyamide net, it was hard to distinguish the failure sequence of the reinforcement layers. Also, as the dimensions of model foundation (500 mm in length, 85 mm in width and 240 mm in height) were relatively small and the polyamide net had low stiffness, the test results could not capture the prototype behavior of geogrid reinforced foundations. Hence, there is a need to establish the failure pattern and load-settlement response of reinforced soil foundation on a relatively larger models using suitable reinforcement.
The main objective of the present study is to understand the failure mechanism of geogrid reinforced soil foundation using experimental and numerical tests. A number of plate load tests were performed on strip footing resting on transparent soil reinforced with different number of layers and lengths of geogrid spaced at 0.25B. A two-dimensional finite difference program was used to study the fracture of geogrid under strip loading considering the geometry of the model tests. The evolution of deformation field in the foundation, and forces in the reinforcement layers have been studied to reveal the failure mechanisms of the reinforced soil foundations.
Section snippets
Test apparatus
The test system includes a soil tank, a loading system, a digital camera and two laser transmitters. The internal dimensions of the tank are 800 mm × 200 mm × 590 mm (length × width × height). The test box is made of 19-mm-thick toughened glass framed with L shape steel beams. The deformation of soils and reinforcement layers during loading can be observed through the glass wall. The loading system includes an electronic universal loading machine (WDW-600KN), and a 80 mm wide and 198 mm long
Load-settlement response
The load-settlement behavior of the footings was monitored to investigate the effect of reinforcement layout on the performance of reinforced soil foundations. The improvement in the ultimate bearing capacity of footings due to the inclusion of geogrids is quantified using a non-dimensional parameter: bearing capacity ratio (BCR), which defines the ratio of the ultimate bearing capacity of a reinforced soil foundation (qult,reinf) to that of the unreinforced soil foundation (qult,unreinf). The
Numerical modelling
The two-dimensional finite difference program FLAC (Itasca, 2011) was used to investigate the performance of reinforced soil foundations and unreinforced soil foundation. Plane strain condition was used to simulate the experimental test conditions. The displacements and stresses in x and y directions at various normalized settlements were computed to assess the performance. The mobilized tensile load distribution in geogrid reinforcement were also determined. When the mobilized tensile load
Load-settlement behavior
Fig. 12 compares the measured and simulated load-settlement curves of the unreinforced foundation and the foundations with four and six layers of 3B long reinforcement. It shows that the numerical predictions in general agree well with the experimental results, especially for the reinforced foundations. The numerical model over predicted the bearing capacity of the unreinforced foundation. This may be due to the inclusion of soil cohesion in the numerical model. Fig. 12 also shows that, for
Conclusions
A number of plate load tests were performed on unreinforced and geogrid-reinforced transparent sand in a model tank with the inner size of 800 mm × 200 mm × 590 mm. A 198 mm long and 80 mm wide steel plate was used to apply strip loading. Fused silica sand particles were saturated in transparent pore fluid to prepare the transparent soil. The deformations of soil and reinforcement were observed using a digital camera with the aid of laser transmitters to illuminate the reinforcement layers. The
Acknowledgements
Funding received from the National Natural Science Foundation of China under grant No. 41772289, and the Shanghai International Science and Technology Cooperation Fund under grant No. 18230742700, is gratefully acknowledged. The fourth author Dr. Sathiyamoorthy Rajesh from Indian Institute of Technology Kanpur is thankful for the Talented Young Scientist Program supported by China Science and Technology Exchange Center.
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