# A Procedure to Prepare Sand-Clay Mixture Samples for Soil-Structure Interface Direct Shear Tests
## 6 Conclusions  
This study presents a method to prepare sand-clay mixture samples for interface direct shear tests. The procedure for the presented method should be performed with three main phases as follows:  
* Mixing sand, clay, and distilled water into a sand-clay mixture slurry with the S1 (sand-water-clay at 1.5 \(w_{\rm L}\)) scheme;
* Performing a preconsolidation on the slurry sample with the designed oedometer cell;  
Figure 11: Repeatability of the interface direct shear test on S1 sand–clay mixture samples of 75% clay content: (**a**) shear stress as a function of horizontal displacement and (**b**) Mohr–Coulomb envelope, critical adhesion and interface friction angles of the 1 st and 2 nd tests.  
* Trimming the preconsolidated sample down to a square sand-clay mixture solid and move it into the shear box. Regarding the high number of experimental studies involving very variable preparations of soil samples, mixtures consisting of Fontainebleau sand, kaolinite clay, and water were prepared by the S1 protocol (sand-water-clay) to verify the uniformity and repeatability of the samples. This kind of sand-clay mixture specimens can be used to study the influence of clay content or other different effects (e.g., temperature) on the mechanical behavior of soil-structure interface. The conclusions are summarized as following: 1. The water content and density of different slices confirm that the S1 sand-clay mixture samples are uniform; 2. The water content, global void ratio, and density of samples with clay content from 0% to 100% indicate that the S1 samples are repeatable. Moreover, the oedometer and interface direct shear tests confirm this repeatability; 3. The sand-clay mixture samples are well suitable for the interface direct shear tests. The S1 sample preparation can avoid some inherent problems which exist in other traditional methods so that it is more suitable for preparing sand-clay mixture specimens for interface direct shear tests. For instance, the dry pluviation method cannot guarantee a homogeneous and saturated specimen of sand-clay mixture or well-graded sand; the wet pluviation cannot be used for well-graded soil or sands containing fine particles because of the particle segregation [52]; dry tamping sample preparation can successfully avoid the honeycomb structure [66; 67] induced by the capillary forces, but it enhances the heterogeneities and uncertainty of physical properties in the sand-clay mixture specimens due to the possible segregation effect; consolidation of the sand-clay mixture slurry on the interface direct shear device may result in soil leakage. Furthermore, as well as the advantages mentioned above, after a sand-clay mixture specimen has been obtained successfully by this method and the tools and preconsolidation cell are available, then another specimen can be prepared with the same procedure. Hence, it is more efficient than traditional specimen reconstitution procedures since the sample preparation can be preformed in parallel with the interface direct shear test. However, it should be noted that when using this method to prepare a sand-clay mixture with a low clay content (e.g., <15%), the prepared specimen may be not in a good shape. Future studies of repeatability assessments are suggested to examine the differences of the microstructure of the samples.  
Conceptualization, methodology, data curation, writing--original draft preparation, K.Y.; formal analysis, K.Y., J.L.; writing--review and editing, J.L., A.-R.V., E.D.F., K.O.; All authors have read and agreed to the published version of the manuscript. This research was funded by the China Scholarship Council.  
Not applicable.  
All data generated or used during the study appear in the submitted article. The authors would like to express their acknowledgment to [PERSON] for his important technical assistance. The authors declare that they have no conflict of interest.