# Research on the Impact Force of Rockfall Impacting Sand Cushions with Different Shapes
## 3 Experimental Results  
The impact force results of five kinds of falling stones under five falling heights were analysed. The acceleration curves of different shaped drop hammers collected by the dynamic signal acquisition instrument were compared and analysed, as shown in Figure 3. Taking the curve with a drop height of 1.2 m as an example, it can be seen that the acceleration at the initial position of the curve was zero. This was because, at this time, the drop hammer was in a free fall state after the power was cut off by the electromagnet. The acceleration of the drop hammer remained unchanged before contacting the cushion, which was always equal to the gravity acceleration. The acceleration sensor used in this experiment could not collect the constant acceleration but only the variation in the acceleration. Regardless of the shape of the rockfall, the form of the hammer acceleration curve was consistent. After contacting the cushion, the hammer reached the peak acceleration in a very short time except for the D1 hammer, and then the acceleration decreased to zero. D4 and D5 almost reached the peak acceleration at the same time, and the time for D4 and D5 to reach the peak acceleration was earlier than that for D3, D2, and D1. Compared with other shapes \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**No.** & \(m\) **(kg)** & \(h\) **(mm)** & \(d\) **(mm)** & \(b\) **(mm)** & \(\xi\) \\ \hline D1 & 1.629 & 96 & 72 & 144 & 4 \\ D2 & 1.612 & 60 & 90 & 90 & 2 \\ D3 & 1.696 & 38 & 114 & 57 & 1 \\ D4 & 1.633 & 24 & 144 & 36 & 0.5 \\ D5 & 1.677 & 15 & 180 & 22.5 & 0.25 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Drop hammer mass and size. of falling stones, D5 and D4 had a large contact area with the cushion. It was observed in the experiment that D5 and D4 moved in the cushion with a short distance; therefore, they could reach the peak acceleration quickly. It can be seen that the duration of the impact also had the following order: D5 > D4 > D3 > D2 > D1. The test results showed that the rockfall shape had an impact on the order of acceleration reaching the peak and the duration of impact, and the impact of rockfall shape could not be ignored when considering the impact time of rockfall. The impact force results of five shapes of rockfall impacting the sand cushion are shown in Figure 4. The figure shows that the impact force increased with falling height. Under the same falling height, the impact force results always had the law of D1 < D2 < D3 < D4 < D5. The impact force results of D5 were greater than those of sphere D3 commonly used in the impact force model and were much greater than the impact force results corresponding to D1. When the falling height was 1.2 m, the impact force corresponding to D5 was 2.926 kN, the impact force corresponding to D3 was 1.243 kN, and the impact force corresponding to D1 was 0.468 kN. D5 reached six times D1, which fully shows that it is very dangerous to ignore the rockfall shape when considering the rockfall impact problem. When the five falling rocks with the same mass fell at the same falling height, the corresponding initial kinetic energy was the same. Observing the deformation of the cushion behind the impact cushion of the five falling rocks, it can be found that the depression depth corresponding to D5 was much smaller than that corresponding to D1. Combined with the time history curve in Figure 3, taking D5 and D1 as examples, D5 reached the peak acceleration in a very short time, and the impact duration was short with a large impact force. The peak acceleration time of D1 was relatively backwards, and the impact duration was long. The results of the impact force and impact time in the experiment could well match the functional principle. It can also be seen from Figure 4 that the impact force of rockfall varied nonlinearly with the change in falling height but varied exponentially with a number less than 1.