Experimental Evaluation of Shear Strength of Soil–Concrete Interface in Carbonate Sands from Northeastern Brazil

Analysis of Cement Addition to Shear Strength of Beach Sand

Liquefaction is a critical geotechnical hazard in sandy soils, particularly in earthquake-prone coastal regions, where loss of effective stress can compromise the stability of foundations and infrastructure. This study investigates the effect of cement addition on the shear strength of Palabuhanratu beach sand through direct shear testing. Sand samples were prepared with cement concentrations of 0%, 15%, and 25% by volume, with a fixed water content of 15%. Physical property tests confirmed poor gradation of the sand (Cu = 2.47, Cc = 0.84), with a dry density of 1.78 g/cm³ and specific gravity of 2.64. Direct shear tests conducted under varying normal loads revealed that the addition of cement significantly increased the shear strength parameters. At 15% cement concentration, the internal friction angle and cohesion improved by 9.11% and 1.88%, respectively, while at 25% concentration, improvements reached 21.26% and 5.00%. These findings demonstrate that cement stabilization enhances the shear resistance of beach sand, offering a simple and effective method to mitigate liquefaction risks in coastal construction projects.

Mechanical Behavior of Sandy Soils Under Different Moisture Contents Using UU Triaxial Tests

The objective of this study was to determine the influence of moisture variation on the shear strength and stress–strain response of sandy soils subjected to different confinement levels. The applied methods followed a quantitative, applied, and quasi-experimental approach using reconstituted cylindrical specimens measuring 7.26 cm in diameter and 14.52 cm in height. Moisture contents of 0%, 2.5%, 5%, 7.5%, and 10% were evaluated under confining pressures ranging from 50 to 200 kPa. The findings demonstrated that the mechanical response strongly depends on both moisture content and confinement conditions. Under dry conditions, shear strength increased considerably with higher confinement levels due to greater interparticle friction and granular interlocking. At 2.5% moisture content, strength improved because capillary suction effects generated apparent cohesion. The maximum shear strength value, 870.77 kPa, was obtained at 7.5% moisture content, where capillary suction and granular densification reached an optimal balance. In contrast, at 10% moisture content, strength decreased due to excess pore water, which reduced effective stresses and interparticle friction. The novelty of this study lies in identifying the optimal moisture content that maximizes the shear strength of sandy soils under UU triaxial testing conditions.

Measurements and DEM modelling of soil shear behavior under varying normal stresses for different soils

Experimental investigation on the shear strength, consolidation and permeability characteristics of cross-linked biopolymer-stabilized soils

This study investigates the influence of guar gum (G), xanthan gum (X), and their cross-linked blend on the shear strength and consolidation characteristics of soils with contrasting plasticity and mineralogical characteristics. Direct shear, one-dimensional consolidation, constant head permeability tests, SEM and XRD analyses were conducted on high-plastic clay, medium-plastic clay, and sand-dominant soil treated with standalone (2G, 2X) and combined (1G+1X) biopolymer dosages under curing periods up to 28 days. The cross-linked treatment produced the most consistent overall improvement. In the high-plasticity soil, cohesion increased from 56.87 to 81.39 kPa and the friction angle increased from 7.8° to 10.5°. In the medium-plasticity soil, cohesion increased to 77.47 kPa and the friction angle reached 12.2°. In the sand dominant soil, the friction angle increased from 39.1° to 42.9°, while cohesion increased from 12.74 to 23.53 kPa. At 800 kPa, the cross-linked blend reduced the void ratio to 0.18 and 0.14 in the high- and medium-plasticity soils, respectively. Hydraulic conductivity decreased to 8.45 × 10⁻⁹, 8.21 × 10⁻⁹, and 9.10 × 10⁻5 cm/s in the three soils. These improvements are attributed to polymer bridging, hydrogen bonding, pore filling, hydrogel formation, and mineralogy-dependent soil-polymer interactions. The findings demonstrate the potential of cross-linked biopolymers as sustainable stabilizers for geotechnical applications.

Shear strength, CBR, and interface interaction of pumice-modified clay reinforced with Geostrip: A large-scale laboratory study

Clayey soils are challenging in geotechnical engineering because of their high compressibility, moisture sensitivity, and low shear strength. This study investigates lightweight and sustainable pumice, locally termed Ponza, for stabilizing clayey soil collected from Al-Amiriya, Baghdad, Iraq. Pumice was incorporated at replacement ratios of 10%, 20%, 30%, 40%, and 50% by dry mass. For each mixture, the dry constituents were blended, mixed with water at the corresponding optimum moisture content, and compacted before testing; CBR specimens were then soaked for 72 h. The experimental program included modified Proctor compaction, soaked California Bearing Ratio tests, and large-scale direct shear tests under normal stresses of 25, 50, and 75 kPa for unreinforced and Geostrip-reinforced specimens. A 200-mm-wide shear box was selected to accommodate the 4.75–9.5 mm pumice particles and minimize particle-size and boundary effects. Increasing pumice content reduced the maximum dry density and produced an overall increase in optimum moisture content because of the lightweight and porous nature of pumice. CBR generally improved at higher pumice contents owing to greater particle interlocking and the development of a frictional load-bearing skeleton. The shear response gradually changed from cohesion to friction-controlled behavior. For unreinforced samples, the friction angle increased from 16.3° for natural clay to 31.0° for 50% pumice, and cohesion decreased from 19.69 kPa to 10.50 kPa. Average interface interaction coefficients of 0.94–0.99 are shown to be efficient at stress transfer and compatible with the clay-pumice mixtures and Geostrip reinforcement.

Engineering Performance and Interface Shear Behaviour of Crumb Rubber-Stabilized Clay Subgrade Reinforced with Geogrid

Clayey soils are generally characterized with low strength and high plasticity which may affect the stability of the subgrade in road infrastructure and hence encourage research into sustainable stabilization techniques. The objective of this study was to investigate the possibility of using recycled crumb rubber (CR) mixed with biaxial geogrid reinforcement to enhance the engineering performance and interface shear behaviour of problematic clayey soil. The experiments were performed on biaxial geogrid BX1100, waste crumb rubber, clay subgrade soil, type B subbase granular material and other materials. The subgrade soil of clay has been collected from the airport area of Al-Muthanna region, Baghdad. An extensive programme of laboratory tests was conducted on soil mixtures with 5%, 10% and 15% of crumb rubber (CR) and untreated soil to determine the effect of stabilization with crumb rubber. The protocol consisted of Atterberg limits, modified Proctor compaction, California Bearing Ratio (CBR) and large-scale direct shear testing. The results showed that the engineering properties of the clay soil were improved by using CR. Maximum improvement was observed at 15% CR content where CBR increased by 56.6% and plasticity index decreased by 44% over the untreated soil. In addition, the large-scale direct shear tests showed that the interface shear strength increased with increasing CR content under geogrid reinforcement. The calculated interaction coefficients were greater than unity for all the tested mixtures indicating effective bonding and interlocking between the reinforced soil layers. The results indicate that the synergistic effect of CR and geogrid reinforcement could improve the interface behaviour of the weak clay subgrade soils with sustainable reuse of waste tyre rubber.