## 5 Design Considerations
### Practical Application: Seismic Design of ERSGI Foundation Systems  
\"Eco-rubber geotechnical seismic-isolation foundation systems\" or in short \"ERGSI foundation systems\" are integrated systems comprising two key elements (Figure 13):  
* a seismic energy-absorption shallow horizontal layer of GRMs; and
* a flexible fiber-reinforced rubberized concrete raft foundation.  
The geotechnical design of the GRM dissipative layer of a ERSGI foundation system requires considering not only the strength and compressibility criteria shown in Figure 12 (permeability is expected to be fulfilled given the fact that the GRMs are coarse granulated free-draining materials), but also the seismic performance of these synthetic materials.  
Typically, in New Zealand, 1- to 2-storey timber-framed residential buildings transfer a vertical effective stress (\(\sigma_{\text{v}}\)') from the structure to the foundation of 7-10 kPa and 10-15 kPa [47; 48], respectively. Considering only G-RL (due to its acceptable leaching characteristics, Figure 11) from Figure 7b, it is evident that mixtures with \(\mathit{VRC}\leq 95\%\) would have a friction angle much greater than 30\({}^{\circ}\), implying a good stability under static load conditions (\(\sigma_{\text{v}^{\prime}}\leq 10\)-15 kPa) for both 1-storey and 2-storey buildings.  
On the other hand, to guarantee the serviceability during and after the superstructure construction (i.e., to avoid damage to pipelines and other essential services that may  
Figure 12: Proposed geo-environmental acceptance criteria for use of GRMs in geotechnical applications.  
Figure 13: Schematic illustration of the “Eco-rubber geotechnical seismic-isolation” (ERGSI) foundation systems (adopted from [16]).  
be embedded in gravel-rubber foundation layer), and meet the compressibility criteria (\(\epsilon_{\
u}\leq 3\%\)), it appears that only the G-RL mixtures with \(VRC\leq 40\)-45% would be suitable for 1-storey buildings and those with \(VRC\leq 35\%\) would be suitable for 2-storey buildings (Figure 14).  
The seismic performance of ERGSI foundation systems was assessed by small-scale impact load tests conducted on GRMs [49]. In such pilot tests, a foundation prototype was placed on a 60 cm thick layer (made of mixtures with \(VRC=0\), 10, 25, and 40%). As shown in Figure 15, it was found that as more rubber aggregates were added into the mixtures, the natural frequency of the foundation system shifted toward a smaller value (base-isolation effect) while the amplitude of the output peak acceleration decreased (i.e., damping effect).  
It is also evident that even a small amount of rubber (i.e., \(VRC=10\%\)) contributes significantly in the dissipation of seismic energy. On the other hand, \(VRC\) of 25% and 40% seem to provide the same dissipative effects.  
Based on the above, it is recommended that the range of suitable GRMs for ERGSI foundation systems should be limited to \(VRC\leq 40\%\) and \(VRC\leq 35\%\) for 1-storey and 2-storey buildings, respectively. However, to achieve the best seismic performance in terms of seismic isolation and energy dissipation, it was eventually suggested to use \(25\%\leq VRC\leq 35\)-40%.  
Figure 14: Selection of suitable GRMs for foundation applications based on maximum allowable vertical strain (\(\epsilon_{\
u}=3\%\) contours are derived from the 1-D compression test results shown in Figure 5): (**a**) 1-storey building; and (**b**) 2-storey building.  
Figure 15: Dynamic properties of G-RL mixtures from impact load tests: (**a**) decay of natural frequency with increasing \(VRC\); and (**b**) reduction in output acceleration with increasing \(VRC\).