Unsaturated soil shear strength of agricultural soils influenced by reclamation sequences in coastal China

Abstract Agricultural activities have raised increasing concern regarding their effects on soil properties and shear strength, which are related to root growth, crop yield and engineering stability. We collected undisturbed soil samples (0‐15 cm) from a No‐Reclaimed (NR) system, a Short‐term Reclaimed (SR) system for less than 7 years, and a Long‐term Reclaimed (LR) system for more than 30 years, in coastal reclamation areas. Direct shear tests were performed and soil water characteristic curves (SWCC) were developed for undisturbed soil samples at 96 soil sampling sites across five transects. Principal component analysis (PCA) was used to quantify tillage effects on soil properties and unsaturated shear strength including effective cohesion (c'), effective friction angle ( φ '), and friction angle related with matric suction ( φ b ). The LR system had significantly increased percentage clay and silt, organic matter (OM), microaggregates, gravity mean diameter (GMD) and effective cohesion (c'), but significantly decreased percentage sand, cation content and electricity conductivity (EC 1:5 ). The SR system had increased OM and GMD compared to the NR system. c' and φ b were significantly positively correlated with bulk density (ρ b ), clay, silt, OM and GMD. The correlations between the internal friction angle and these properties were opposite to those of cohesion. Two principal components (PC1 and PC2) explained the majority of the soil variables. PC1 had a strong relationship with clay, silt, OM, c' and sand, while PC2 had a strong relationship with EC 1:5 , SAR, GMD and ρ b . Thus, PC1 can be regarded as an indicator of soil texture; similarly, PC2 was able to represent soil salinity properties and structure. Confidence ellipses significantly separated the differences in soil between different reclaimed systems. The use of principal components (PCs) in the linear regression reduced the variables and estimated c' and φ ' accurately. This study increases the current understanding of the relationship between soil properties and shear strength under different tillage systems, and assesses the efficacy of soil property evaluation after reclamation in coastal areas. Highlights Effect of reclamation on soil properties and unsaturated shear strength was investigated. Long‐term reclamation increased unsaturated shear strength significantly. Soil properties are well explained by principal components (PC1 and PC2). Principal component regression is recommended for predicting unsaturated soil shear strength.