Natural soils are usually heterogeneous and characterized with complex microstructures. Sand–clay mixtures are often used as simplified soils to investigate the mechanical properties of soils with various compositions (from clayey to sandy soils) in the laboratory. Performing laboratory tests on a sand–clay mixture with definite clay fraction can provide information to understand the simplified soils’ mechanical behavior and better predict natural soils’ behavior at the engineering scale. This paper reviews previous investigations on sand–clay mixture and soil–structure interface direct shear test. It finds that even though there are many investigations on sand–clay mixtures and soil–structure interfaces that consider pure sand or pure clay, limited data on the mechanical behavior of the interface between sand–clay mixture and structure materials are available. Knowledge is missing on how the clay content influences the mechanical behavior of interface and how the soil particles’ arrangement changes as the clay content increases. Further study should be performed to investigate the interface in terms of a reconstituted sand–clay mixture and structure by interface direct shear test, to highlight the influence of clay fraction on the interface response, under various loading conditions.

A large number of experimental studies on sand–clay mixtures are well documented in the literature; however, the preparation protocol is rarely clearly detailed or varies a lot according to the authors. Variations in the preparation technique obviously increases the challenge of comparing different test results. As a consequence, sample preparation for sand–clay mixtures should be kept as constant as possible to ensure homogeneity and uniformity of samples and limit result variability. This paper develops a detailed procedure on how to prepare sand–clay samples for interface direct shear tests. Sand–clay mixtures are prepared with Fontainebleau sand, kaolinite clay and distilled water by the S1 (sand–water–clay) protocol. The uniformity of the reconstituted specimens is assessed by measuring the water content and density on three slices from the top to the bottom across the specimens. The repeatability of the samples is checked with oedometer and interface direct shear tests. This sample preparation procedure can be used for preparing sand–clay mixture for interface direct shear tests to investigate the influence of clay content or other effects (e.g., temperature) on the mechanical behavior of soil–structure interface. It has demonstrated great performance in preparing samples with good homogeneity and shape, compared to other traditional reconstitution techniques. With the sample preparation procedure, we can obtain repeatable test results as well.