# Test Granular Materials Failure Using Bi-Directional Simple Shear Apparatus: A Review
## 1 Introduction  
Soil that mainly includes sand and clay is a widely available natural granular material, and it has been extensively used and studied as a construction material in engineering [1; 2; 3; 4; 5; 6; 7; 8; 9; 10]. However, because of the complexity of soils, the behaviour, especially the shear behaviour, is not well understood yet, in terms of research and construction. As a result, it is necessary to better understand the shear behaviour of soils through laboratory testing. The stress conditions in laboratory testing are usually different from the in situ conditions. This is due to the limitations of the apparatuses and methods. An appropriate apparatus can better replicate the in situ conditions. For example, an embankment under an earthquake loading can be better replicated using a bi-directional simple shear apparatus. Therefore, when choosing the equipment, factors such as the loading conditions, boundary conditions, and sample configurations should be considered. In addition, an appropriate sample preparation method to form the needed soil fabric, as well as a testing procedure that reflects the in situ loading, should be selected correspondingly. However, not all of these conditions can be met in most geotechnical experiments. For example, a saturated sample should have a constant volume, a constant vertical load, and a constant height, without lateral strain. However, a constant volume and constant load cannot be simultaneously and perfectly maintained in simple shear tests. Several element level tests are traditionally used in geotechnical testing, which are triaxial testing, simple shear testing, and torsional testing. Each has its merits and drawbacks. The following compares the apparatuses that are used in these tests, with emphasis on different stress paths and loading conditions. Triaxial apparatuses, which are widely available and easy to operate, have been widely used in previous studies, including research on liquefaction [11; 12; 13] and stress history [14; 15; 16; 17]. Triaxial testing uses a tall cylinder sample with a latex membrane, usually with a diameter to height ratio (D/H) of 0.5 [14; 15; 16]. The cell pressure is exerted by the water (or other medium) inside the cell. The water pressure is controlled via a cell pressure controller that is connected to the cell. The axial load is appliedby a motor at the base of the specimen, and is measured by the load cell above the specimen's cap. Because there is no physical constraint in the radial direction, the samples may develop radial (lateral) deformations, which are not representative of some in situ conditions. Although the cell pressure confines the tested specimen through the latex membrane to some degree, a constant volume can only be maintained globally. In addition, the radial strain is difficult to determine, as it is usually larger in the middle of a sample than on the top and bottom. The main problem with triaxial testing is the rotation of the principal axes of stresses. In the triaxial tests, the principal axes of the stresses instantaneously rotate 90 degrees during some cyclic tests. However, in the field and during simple shear testing and hollow cylinder testing, the principal axes of the stresses rotate smoothly during shearing. Hollow cylinder apparatuses are widely used for the controlled stress path testing of reconstituted soils in both drained and undrained conditions, with monotonic and cyclic loadings. Axial load, torque, outer cell pressure, and inner cell pressure can be independently exerted on a hollow cylindrical specimen, which enables the independent control of the rotation of the major-minor principal stress axes and the magnitudes of the three principal stresses. Therefore, some complex stress paths can be studied using a hollow cylinder apparatus. The hollow cylinder apparatuses use a hollow cylindrical sample with two latex membranes (inner and outer), usually with an outer diameter to height ratio of 0.5-1 [18; 19; 20; 21].