## 2 Materials and Testing Programs
### Sample Preparation
#### 2.2.2 Test of the Water Stability  
The water stability of aggregates has a significant impact on the erosion resistance of the slope topsoil. Aggregates with good water stability will not be washed off easily, thereby reducing the loss of soils and nutrients on the slope. Combined with vegetation growth promoted by the good structure between soil aggregates, the erosion resistance of the slope topsoil can be further improved under the joint action of self-stability of aggregates and reinforcement of soils by vegetation. First, the soils were oven dried and sieved through a 2 mm mesh to ensure homogeneity of the soil. The dried soils were then mixed well with water to the optimum moisture content (OMC), which was determined by a standard Proctor compaction test (GB/T 50123-1999). Thereafter, the humid soils were placed in sieves with mesh sizes of approximately 20, 10 and 5 mm. The sieves were shaken to divide the aggregates into two groups: the aggregates that remained on the 10 mm (the diameter of the aggregates was between 10 and 20 mm, which was noted as an average diameter of 15 mm) and 5 mm (the diameter of the aggregates was between 5 and 10 mm, which was noted as an average diameter of 7.5 mm) sieves.  
The preparation of the polymer solution was the same as that used to test the effective infiltration depth. The different polymer solutions were then sprayed onto the surfaces of the different groups of aggregates and kept at a constant temperature of 25 \({}^{\circ}\)C for three days. The results are collected in Table 3, where the notation of the test group included the average diameter of the aggregates and the applied solution (UNT indicates water without ADNB).  
The water stability test required immersing the aggregates in static water. Fifty aggregates were randomly placed on a sieve covered by filter paper. To avoid the sudden disintegration of aggregates, immersion was begun with capillarity; water was added to a level at which the filter paper was just immersed. Thereafter, water was slowly added until all aggregates were submerged. The immersion time was set as 10 min, and the cumulative number of collapsed aggregates was recorded every minute for 10 min. The water stability index K was calculated with the following formula:  
\[K=\frac{(a_{1}\times 5+a_{2}\times 15+a_{3}\times 25+\dots+a_{10}\times 95+a_{ \infty}\times 100)}{50}\]  
where \(a_{1},a_{2},a_{3}\dots a_{10}\) represent the number of collapses at an immersion time of 1 min, 2 min, 3 min,... 10 min and \(a_{\infty}\) is the number of aggregates that remained intact after 10 min. The coefficients of 5,  
Figure 2: Apparatus of the effective infiltration depth test.  
15, 25 and 100 represented the water stability of the soil aggregates during each minute. That means that if an aggregate collapsed within 1 min, its coefficient was defined as 5, which was calculated by \((0+10)/2\). If an aggregate collapsed between 1 and 2 min, its coefficient was defined as 15, which was calculated by \((10+20)/2\). If the aggregate never collapsed during the immersion time of 10 min, its coefficient was defined as 100. The \(K\)-value had a range of 5 to 100. A higher \(K\)-value represented higher water stability of the soil aggregate.