Effects of initial moisture and bulk density on the soil compression characteristics of black soil

Soil structure can deteriorate under agricultural field vehicle compaction. Physical soil quality has posed a serious threat to agriculture production in Northeast China's farmland. Typically, the widespread and heavy use of agricultural machinery can be responsible for this instance. Soil compression characteristics can greatly contribute to the quantitative analysis of the soil compaction process. But it is still unclear on the variation of black soil compression characteristics with different initial moisture and initial bulk density. This study aims to investigate the influence of initial moisture and initial bulk density on the repacked black soil. The soil compaction risk was also quantified and predicted after evaluation. Six initial moisture levels were set at 0.15, 0.20, 0.25, 0.30, 0.35, and 0.40 g/g, and six initial bulk density levels were at 1.00, 1.10, 1.20, 1.30, 1.45, and 1.60 g/cm³. Uniaxial confined compression tests were conducted using a consolidator to measure the soil’s ratio under different applied stress. Soil compression curves were collected using the Gompertz equation. Three important characteristics of soil compression were calculated from curves, such as the pre-compression stress, the compression index , and the swelling index. The results showed that the initial moisture, bulk density, and their interaction all shared a significant influence on the compression characteristics of repacked black soil (P<0.001). A series of soil pedo-transfer functions were established to predict the compression characteristics. The σ_PC of black soil ranged from 10.42 to 1 106.17 kPa, which was positively correlated with the initial moisture content, and negatively correlated with the initial bulk density in a linear relationship (P<0.05). The compression index ranged from 0.311 to 0.852, indicating a bivariate polynomial equation relationship with the initial moisture and bulk density. There was a decrease as the initial bulk density increased and reached the maximum at medium moisture. The swelling index ranged from 0.007 to 0.321, which was positively correlated with the initial water content, and negatively correlated with the initial bulk density. Therefore, the black soil presented pre-compression stress lower than 200 kPa and high compression stress greater than 0.4, when the initial moisture exceeded 70% of the field capacity or the initial bulk density was lower than 1.2 g/cm³. Such soil hydraulic and structure conditions indicated a high risk of soil compaction under field traffic. It was recommended to fully consider the risk for the cultivation operations without delaying farming. In summary, soil compaction has been caused by agricultural field traffic, although agricultural mechanization has been beneficial for the production in the black soil region. This finding can provide a strong reference to understanding the effects of the initial moisture and bulk density on compressive characteristics. A set of pedo-transfer functions was built using the initial soil moisture and initial bulk density. An effective prediction was offered for the black soil pre-compression stress, compression index, and swelling index. These predictive pedo-transfer functions presented the potential to quantify and predict the level of soil compaction risk induced by heavy machines during wheeling and field operations. The implementation of pedo-transfer functions can also provide crucial data for the input parameters of soil compaction models. Therefore, an impactful basis can be obtained for the soil workability assessment and field operating conditions.