ISO 12957-1 Direct shear test apparatus machine of Geosynthetics — friction characteristics.
This machine is used to determine the shear resistance and the pullout resistance of a geosynthetic against soil, a geosynthetic against another geosynthetic, or a Geosynthetic clay liner against an adjacent material, by the directly shear test and by the pullout test separately. The test results obtained are used in the design of structures that employ geosynthetics, also used for quality control during construction of these structures. It is suitable for geotextiles, geomembranes, geonets and geogrids, etc.
ISO 12957-1 document specifies an index test method to determine the friction characteristics of geosynthetics in contact with a standard sand as described in EN 196-1, i.e. with a specified density and moisture content, under a normal stress and at a constant rate of displacement, using a direct shear apparatus.
The same testing procedure can be used with any type of soil with the density and moisture content that are required to evaluate the performance under specific conditions or with another geosynthetic under a normal stress and at a constant rate of displacement, using a direct shear apparatus.
The procedure can also be used for testing geosynthetic barriers.
The direct shear pull test for geosynthetic materials mainly measures the friction characteristics between materials like geotextiles and grids and soil, which is particularly important for the design of engineering projects such as road construction and retaining walls. For instance, when constructing a reinforced soil retaining wall, it is crucial to understand how tightly the materials and soil can grip each other; otherwise, there may be a risk of sliding or collapse. So how is it done? Let’s explain it step by step.
1, the equipment must be prepared. A displacement gauge is needed to measure movement distance, and a load cell is needed to measure tensile force; all these must be calibrated in advance to ensure data accuracy. A loading device is also needed to apply vertical pressure (simulating the weight of soil) and horizontal pull. A direct shear pull apparatus is used; this machine has two boxes, with the bottom one able to move horizontally while the top one presses down on the soil and materials. For example, the sensors need to be calibrated every six months using a standard weight to check if the displayed force matches the actual force.Next, the test samples are prepared. Geosynthetic materials are usually cut into strips 20 centimeters wide and 30 centimeters long, with specific dimensions depending on standards; for instance, the road industry may require wider strips. When making a clay layer, the moisture content should be close to the optimal moisture content. Layered compaction is done using a rammer, with each layer being 5 centimeters thick and compacted 30 times to ensure the density matches that of the field. The soil must also be selected based on actual project requirements, for example, either sandy or clayey soil, while controlling moisture content and density. Then, the material is laid flat on the soil, followed by a layer of soil on top, with edges aligned to prevent wrinkling of the material; otherwise, the measured force may be underestimated.
2, the sample is installed. Weight is added vertically or pressure is applied using a hydraulic device, typically at levels of 50kPa, 100kPa, 200kPa, and 300kPa to simulate soil pressure at different depths. The prepared soil and material are placed into the bottom box of the shear apparatus with the top box pressing down, and the position is adjusted so that the center of the material aligns with the pull direction. For instance, when constructing a 3-meter high retaining wall, the soil pressure at the bottom would be approximately 300kPa, so the friction value needs to be measured at this pressure.
3, The test begins. Horizontal pulling is controlled by displacement, slowly pulling the material at a rate typically of 1 millimeter per minute; it cannot be too fast, as the soil would not have time to react, leading to inflated measured forces. The material is pulled until it is completely extricated or breaks, observing the failure mode. During the pull, the displacement gauge and load cell will record data in real time; for example, when pulled 2 millimeters, the force may read 100 Newtons, at 5 millimeters the force peaks at 200 Newtons, and then may drop to a stable 150 Newtons, which is the residual strength. If the material breaks, it indicates that the material itself lacks strength; if it slips between the material and the soil, it signifies that there is insufficient interface friction.
