In this paper, an extensive series of direct shear box tests (99 tests) were conducted to explore and compare the effects of raw and treated natural fibers, specifically Doum fibers on the mechanical behavior of three categories of sandy soils with distinct mean particle sizes (D50 = 0.63, 1, and 2 mm). Specimens from every soil category, containing 0 to 0.8% raw Doum fibers and 0 to 1% treated Doum fibers in incremental step of 0.2%, were reconstituted at an initial relative density of (Dr = 87 +/- 3%) and subjected to three different initial normal stresses (100, 200, and 400 kPa). The obtained results indicate that incorporating raw or treated Doum fibers improve the mechanical and rheological properties (internal friction angle, ductility, and maximum dilatancy angle) of the tested mixtures up to specific thresholds Doum fiber content (FD = 0.6% and FTD = 0.8% for raw and treated Doum fibers respectively). Beyond these limiting values, the mechanical and rheological properties decreased with further increases in Doum fiber content. Additionally, specimens reinforced with treated Doum fibers exhibit higher shear strength than that of the raw Doum fibers for all tested parameters. Based on the experimental results, it has been found to suggest a reliable correlation between Particle Size Distribution (PSD) characteristics and mechanical properties for all reconstituted specimens. The recorded soil trend is especially pronounced for the mean grain size (D50) ranging between 1 and 2 mm, where a notable increase in shear resistance is noticed. The analysis of the obtained outcome suggests the introduction of new enhancement factors (EF tau peak and EF phi degrees) as useful parameters for predicting the mechanical behavior of sand-fibers mixtures. Furthermore, new relationships have been developed to forecast changes in mechanical properties (peak shear strength, internal friction angle, and maximum dilatancy angle) of the tested mixtures under the impact of the selected parameters (FD/TD, D50, and sigma n).

This study explores the impact of non-plastic fines content, initial confining pressure, and grading characteristics on the undrained shear strength and excess pore pressure of sand-silt mixtures; a series of undrained compression triaxial tests were carried out on reconstituted Chlef sand (Algeria) samples with different percentages of silt content (Fc = 0, 5, 10, 15, and 20%), at an initial relative density (RD = 50%) subjected under three different confining pressures (p ' c = 20, 50, and 100 kPa). Observations from these tests unveiled intriguing insights. Notably, it was discovered that soil specimens with lower fines content and higher initial confining pressures showed increased resistance to liquefaction. Conversely, liquefaction resistance diminished under conditions of higher fines content and lower initial confining pressures. Moreover, the analysis of test results underscored the substantial influence of gradation on the peak shear strength and maximum excess pore pressure of sand-silt mixtures. This suggests that the distribution of particle sizes within the mixture plays a pivotal role in its mechanical behavior and susceptibility to liquefaction. Furthermore, the study's findings revealed the presence of straightforward correlations between various parameters. These correlations include those between peak shear strength (qpeak), maximum excess pore pressure (Delta umax), fines content (Fc), initial confining pressure (p ' c), and specific grading characteristics such as D10, D30, D50, D60, Cu, D10R, D50R, and CuR. These correlations offer valuable insights into the interplay of factors affecting the mechanical properties of sand-silt mixtures, aiding in the development of predictive models and engineering solutions for infrastructure projects.

A set of direct shear tests conducted to investigate the effects of sand mean grain size and geomembrane surface roughness on the shear behavior of the sand-geomembrane interface. Four types of sands with different mean grain sizes were used and the interfaces included a smooth geomembrane and four textured ones with different asperity characteristics. The samples were prepared in a direct shear box at five sedimentation angles. The experimental results showed that the peak friction angle of the soil-textured geomembrane interface was dependent on the mean grain size, inherent anisotropy, and surface roughness of the geomembrane. The shear strength of sand-textured geomembranes was two to three times larger than that of the sand-smooth geomembrane interface. The peak and residual friction angles of the sand-smooth geomembrane interface occurred at an orthogonal angle of sedimentation while it happened at higher angles for the sand-textured geomembrane interface. An increase in mean grain size increased the shear strength of the sand-textured geomembrane interface; while a reverse condition occurred for the sand-smooth geomembrane interface. The shear strength occurred at larger horizontal displacement as mean particle size increased and the effect of relative roughness on peak friction angle increased with increase in mean grain size.