## 4 Application Example
### Case for Impact Loading  
#### 4.2.1 Laboratory Drop Weight Impact  
The multi-layer pavement slab was subjected to an 1181-kg drop weight impact. The cross-section of the multi-layer pavement is the same as that given in Figure 7. The basic mechanical properties of the, HSC, ECC and AC are determined according to ASTM standards, and the results are summarized in Table 9. The drop weight was a cylindrical projectile with a hemispheric head (100 mm in diameter and made with high strength steel), and the pavement slab was subjected to two times of impact from the same drop height of 1.5 m. During the test, the multi-layer pavement slab was placed on the top of compacted soil/sand in a steel strong box. Directly below the slab was the geocell (MiraCell MC-100) which was filled with compacted soil/sand. This was to enhance the strength of the soil/sand layer and to provide a solid sub-base as to simulate the practical condition. The setup for the multi-layer pavement slab is given in Figure 13. Various instruments were also installed to monitor the response of the pavement during the drop weight test. Figure 14 shows the positioning of the potentiometers. A photodiode system was used to trigger the data acquisition system during the test. It consists of two photodiodes and two laser sources placed 100 mm vertically apart. The data acquisition system would be triggered when the falling projectile crosses the top laser emitter. Impact velocity could be determined using the time interval that the projectile took to cross the second laser emitter. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Item** & **Field Trial Test** & **Numerical Result** & **Deviation from Field Trial Test** \\ \hline Max. vertical & & & \\ acceleration (m/s\({}^{2}\)) & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Vertical acceleration of the multi-layer pavement slab. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Item** & **Field Blast Test (kPa)** & **Numerical Result (kPa)** & **Deviation from Field Trial Test** \\ \hline TPC1 & Destroyed & 13,393 & Sensor destroyed as pressure \textgreater{} range \\ TPC2 & 273 & 267 & 2\% \\ TPC3 & 200 & 241 & 20\% \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Peak reading for the total pressure cell. In the numerical model, the AUTOMATIC_SURFACE_TO_SURFACE contact algorithm is employed to model the interaction between pavement slab and soil. The contact algorithm TIEBREAK is also used to simulate the interface behaviour between the AC and HSC layer, which is the same as that in the numerical model for blast loading as mentioned in Section 4.1.2. As the impactor might penetrate the AC layer, the erosion method is employed in the simulation, in which the maximum plastic failure strain of 0.2 is used to delete the distorted element once the actual strain exceeds this predefined failure strain. The multi-layer pavement slab, the drop weight head and soil mass are discretised in space with one point Gauss integration eight-node hexahedron elements. The geogrid is simulated with the four-node Belytschko-Tsay shell element that allows no bending resistance. In addition, the PLASTIC_KINEMATIC material model is employed to describe the bi-linear behaviour of the geogrid under tensile loading [23]. The parameters used for the geogrid are the same as given in Table 4.  
Figure 14. Positioning of the potentiometers. Figure 13. Setup for the impact test. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameters** & **AC** & **HSC** & **ECC** \\ \hline Young’s modulus \(E\) (MPa) & 598 & 40,000 & 18,000 \\ Compressive strength \(f_{c}\) (MPa) & 4.6 & 90 & 80 \\ Tensile strength \(f_{t}\) (MPa) & 0.7 & 4.35 & 5 \\ Poisson ratio \(\
u\) & 0.35 & 0.2 & 0.22 \\ \hline \hline \end{tabular}
\end{table} 
Table 9. Basic properties of the materials in the multi-layer pavement in the drop weight test. The Drucker-Prager model is used to simulate soil mass.