# Test Granular Materials Failure Using Bi-Directional Simple Shear Apparatus: A Review
## 2 The Development of Bi-Directional Simple Shear Apparatus  
The idea of simple shear was first developed by [PERSON] and [PERSON] [27] at the Norwegian Geotechnical Institute (NGI), in order to better duplicate the observed in situ conditions of the slipping plane of a landslide. In some cases, the soil is subjected to complex stresses and requires a simple shear apparatus that is capable of applying multidirectional shear stress in order to investigate such conditions.  
Over the last few decades, many efforts have been focused on laboratory testing using bi-directional simple shear apparatuses [16; 29; 30; 42; 43; 44; 45]. Unfortunately, few apparatuses can successfully control the boundary condition. This is mainly due to the difficulties in designing an apparatus that can perform multi-directional shearing. There are many technical issues with the existing bi-directional simple shear apparatuses, such as rocking and pinching problems [29]. In addition, these apparatuses that have been developed by researchers are not in widespread use.  
In recent years, a limited number of NGI-type bi-directional simple shear apparatuses have been developed in order to investigate the effect of static driving shear stresses on undrained shear behaviour [30; 37; 45; 46; 47; 48; 49]. In these studies, static driving shear stress is introduced so as to duplicate the stress state of the soil under a slope or foundation, and its magnitude depends on the inclination of its respective slope or structural weight. Static (monotonic) undrained shear stress represents horizontal forces, such as ice loading on an offshore arctic gravity structure, and cyclic undrained shear stress represents seismic loadings, such as transverse earthquake strikes. [PERSON] et al. [45], [PERSON] [46], and [PERSON] [47] studied the effect of the consolidation shear stress on clay in monotonic bi-directionalsimple shear tests. [PERSON] et al. (2018); [PERSON] and [PERSON] (2018), and [PERSON] (2018) studied the effect of consolidation shear stress on sand in cyclic bi-directional simple shear tests. In these studies, the specimens are first consolidated under a vertical stress and shear stresses along different directions, and are then sheared in undrained conditions along a fixed direction. The results of these studies show that the angle between the consolidation shear stress and the secondary shear stress has a significant effect on the stress-strain responses. Tests show the lowest strength occurs in tests with angles around \(90^{\circ}\), while the highest strength occurs at \(0^{\circ}\). In addition, [PERSON] (2018) concluded that the rotation of the principal stress and stress reversal have a profound influence on the excess pore water generation and the development of shear strain. However, such bi-directional simple shear tests do not consider the non-coaxiality. This is due to the difficulties in interpreting the stress state in the NGI-type simple shear tests.  
Generally, previous bi-directional simple shear tests can be divided into two categories, tests with level ground conditions and tests with sloping ground conditions. In the previous bi-directional simple shear tests with level ground conditions, some complex loading types were tested. These tests did not consider the effect of the consolidation of shear stress. In previous bi-directional simple shear tests with sloping ground conditions, the effect of the initial static driving stress was studied. The initial static driving stress is a type of consolidation shear stress. Schematic of a bi-directional simple shear testing apparatus is shown in Figure 1, and sectional details of a specimen in a mould are shown in Figure 2.  
Figure 1: Schematic of a bi-directional simple shear testing apparatus (**a**) YZ plane (**b**) XZ plane ([PERSON] and [PERSON], 2018).  
Figure 2: Sectional details of a specimen in a mould ([PERSON] and [PERSON], 2018).