## 4 Discussion  
Our results show that soil cohesion decreased with the increase in soil moisture content and increased with higher clay content and soil density, which is supported by [34], who found a strong positive correlation between soil cohesion and clay content. Similarly, [19] also demonstrated that soil cohesion increased with clay content. Higher soil wet bulk density resulted in smaller gaps between the soil particles, thereby leading to a larger internal friction angle as well as greater soil cohesion. Many published works suggested that the shear strength increased with water content but decreased when maximum shear strength was reached at a particular water content [18; 35]. Ref. [18] suggested a quadratic polynomial relationship between soil cohesion and soil water content (\(R^{2}>0.75\), \(p<0.05\)), with the maximum cohesion reached when the soil water content was between 14 and 21%. In the current study, the moisture contents of our paddy soil samples were within 22-40%, thus the soil cohesion decreased with increasing soil water content. The effect of moisture content on cohesion was mainly caused by the water film bonding force. With the increase in moisture content, the distance between soil particles and the thickness of the water film increased, and as a result, cohesion between soil particles decreased. Our finding was in order with [20], who indicated a negative impact of soil moisture content on cohesion.  
The internal friction angle mainly reflected the friction characteristics between the soil particles, including the sliding friction caused by the roughness of the particle surface, and the biting friction produced by the movement of the particles. Although [19] suggested that soil friction was poorly associated with soil water content and bulk density, our study indicated that soil moisture content played a vital role in the friction angle, as shown in Table 4 and Figure 2. The internal friction angle increased quickly when moisture content increased from 30% to 34%, but then rapidly dropped when it increased from 34% to 50% because lower soil moisture content led to a reduction in the thickness of the water film between soil particles, which means that the gaps between the particles were filled and the inner friction angle became larger. When the soil moisture content exceeded the liquid limit, the inner friction angle would be reduced to zero. As at this time paddy soil would be in a rheological state and the gaps between particles were filled with water, the particles could move freely without resistance [36]. Our results are supported by [20], who also showed experimental evidence that the internal friction angles decreased with increasing soil water content. Similarly, our study indicated that the internal friction angle both increased at first to the maximum value and then dropped when wet soil bulk density kept increasing. The internal friction angle in paddy soils increased with clay content when it was less than 30%, which is in order with [34], who found fine clays decreased frictional strength greatly in the calcareous soils of central Iran. However, the internal friction angle started to decrease when the clay content continued increasing, which is supported by [37], who reported lower internal friction angles for clayed soils than sandy soils.