Soil reinforcement employing geosynthetics has become an engineering solution for solving problems associated with soil structures such as retaining walls and embankments. It is an environmentally friendly and economically viable method without the need for skilled workers or specialized equipment that can be executed in all weather conditions. Jewell et al., 1984; Lee and Manjunath, 2000; Anas et al., 2016; Abdi and Mirzaeifar, 2017 have stated that for safe and economic design of reinforced soil structures knowledge of interaction at interface is crucial which is significantly influenced by reinforcement type and characteristics, soil properties, boundary and loading conditions, etc. Soil – geosynthetic interaction is usually considered under pull out or direct shear mode and the effects of influential factors such as soil density and type, particle shape and size, moisture content, geosynthetic tensile strength and geometry have been explored by Lopes & Ladeira, 1996; Athanasopoulos, 2002; Varuso et al., 2005; Latha & Murthy, 2006; Liu et al., 2009; Dash, 2010; Lopes & Silvano, 2010; Abdi & Arjomand, 2011; Tuna & Altun, 2012; Abdi & Zandieh, 2014; Chen et al., 2014; Ferreira et al., 2015; Thuo et al., 2015; Ferreira et al., 2016; Prashanth et al., 2016; Infante et al., 2016; Wang et al., 2016; Abdi & Mirzaeifar, 2017; Balakrishnan & Viswanadham, 2017. Xu et al. 2020, using experimental and numerical methods investigated the reinforcing mechanism of geosynthetic – reinforced granular soils under plane strain conditions. Wang et al., 2020 studied the creep behavior of sand – geogrid under different loading levels and reported that creep mainly occurs near the pullout point and deformations is reduced from tensile end to the fixed end of the geogrid. Effect of backfill compaction density on reinforced soil walls has been investigated using Centrifuge by Xu et al., 2020. Results showed that displacement of the model with inadequate compaction (Dr = 65%) was 30% greater than the model with Dr = 95%.
Land shortages and increased prices along with regulatory constraints, have necessitated the need for steeper and higher reinforced soil structures, cost of which mainly depend on the volume of earthworks and the amount of reinforcement required that are influenced by soil-reinforcement interactions. Because enhancing interactions has attractions from economical and constructional perspective which are crucial factors in the design of reinforced earth structures many researchers including Chenggang, 2004; Liu et al., 2014; Kargar & Mir Mohammad Hosseini, 2016; Moghaddas Tafreshi et al., 2016; Tavakoli Mehrjardi & Amjadi Sardehaei, 2017, Abdi and Safdari Seh Gonbad, 2018 have investigated different methods of improving interactions. Soil-geogrid interactions are due to friction at interfaces, soil-soil shear strength within apertures and passive resistance in front of transverse ribs. Goodhue et al. ,2001; Lopez, 2002; Palmeira, 2004; Abdi et al., 2009 and Xu et al., 2018 have shown that in direct shear mode the first two and under pull out condition the third mechanism is the main contributor to stability. As under direct shear mode, reinforcement tensile strength and passive resistance do not contribute significantly to stability of reinforced earth structures, this necessitates longer embedment lengths and larger volumes of soil, leading to greater costs. Thus, to alleviate this problem researchers have attempted to improve soil – geosynthetic interaction by different methods some of which are schematically shown in Table 1.
In current study and to complete the research conducted by Abdi and Safdari Seh Gonbad (2018), the effects of soil grading, attached element size and arrangements together with normal pressures on interactions in direct shear mode have been assessed. Poorly and well graded sands, a geogrid type, three different element sizes, arrangements and normal pressures were adopted for the investigation.