Pore water pressure
Pore water pressure was observed by installing two pore water pressure transducers at 30 cm from the bottom of the sandbox, either for the loose sand or dense sand parts. Excess pore water pressure measured was converted to pore water pressure ratio (PWPR) by dividing excess pore water pressure with initial vertical effective stress (σv’). Pore water pressure ratio time histories are shown in Fig. 7.
Generally, the results obtained show an insignificant difference in all cases, both for P1 and P2. As can be seen in Fig. 7, for water pressure meter placed in the loose sand zone (P1), although the maximum PWPR obtained is around 1 for Case 2, but the maximum value in Cases 1 and 3 is also immensely close to 1, around 0.97, which indicate that liquefaction occurred. In Case 4, the maximum PWPR is only slightly lower and showed a faster water pressure dissipation, compared to other cases. Correspondingly, for the dense sand state (P2), the maximum PWPR acquired is almost similar for all cases of about 0.4, even though the highest PWPR in Case 4 is little higher compared to other cases. These results signify that no liquefaction occurred in this zone.
According to the results, it can be said that the effect of the use of gravel and geosynthetics on pore water pressure is insignificant in these experiments. Since the main purpose of pore water pressure measurement is to determine the occurrence of liquefaction in the sand layer, therefore the influence of the use of gravel and geosynthetics on pore water pressure is not a major concern.
Acceleration
Figure 8 shows the acceleration time histories of all cases at the ground surface of the loose sand (A1), the ground surface of the dense sand (A2), and input acceleration (A3). As can be seen from this figure, although the disparity of acceleration is not significant, still can be observed that the presence of gravel and geosynthetics, could decrease the average amplitude of the acceleration obtained. In Case 3, which ground strengthened with gravel and geosynthetics type I, and the ground in the dense state resulted in the lower average amplitude among others. These results are in line with the results acquired in the ground amplification experiments described previously, ground density and the use of reinforcing materials have a positive effect on ground acceleration.
Vertical ground displacement
The vertical ground displacement occurred through ten different points at the ground surface was measured. To simplify understanding, the displacement values are averaged, and the results can be seen in Fig. 9. It can be observed that based on the averaged vertical ground displacement measured, the presence of the proposed mitigation could reduce vertical displacement in various amounts, for example, by use gravel only (Case 2), in the loose sand condition, the settlement was decreased around 4 mm, from 20.9 mm to 16.9 mm, and reach approximately 1.9 mm for the dense condition, from 5.6 mm to 3.7 mm. Moreover, by applying gravel and geosynthetics type I (Case 3), the displacement was reduced up to 7.6 mm and 1.7 mm in the loose sand and dense sand conditions, respectively. Maximum results are shown on reinforcement with gravel and geosynthetics Type II, which the ground settlement lowered around 11.4 mm in loose sand condition and 1.8 mm in the dense sand state, compared to Case 1.
Furthermore, the differential settlement between non-liquefiable and liquefiable zones is compared, as shown in Fig. 10. In Case 1, the settlement difference is 15.3 mm, while in Case 2 is 13.2 mm, which means decreased 2.1 mm. The differential settlement is reduced up to 5.9 mm and 9.6 mm in Case 3 and Case 4, respectively.
The coherence of the gravel layer with its high permeability and high tensile strength provided by geosynthetics were considered as the main reason for this good result. Since the tension generated in the geosynthetics restrain the deformation of the gravel layer and integrally perform like a rigid plate with high permeability, this reinforcement could reduce the settlement that occurred on the ground surface. Since the tensile strength and the tensile stiffness of geosynthetics Type II that used in Case 4 is higher compared to type I, this type of geosynthetics could restrain the deformation of the gravel and sand better than Type I, resulting in lower ground vertical displacement compared to geosynthetics Type I that used in Case 3.
Based on the results obtained from laboratory testing, this proposed mitigation can be applied to overcome the liquefaction-induced ground settlement and the resulting damage, such as the impassable roads due to differential settlement appeared caused by the subsoil layer liquefy. This will result in substantial losses if this damage occurs on vital roads. Moreover, tilted houses and building also could be appeared due to liquefaction, for instance as happened in Kumamoto earthquake 2016, Japan, where it was reported that many residential houses and buildings were tilted due to liquefaction (Setiawan et al., 2017). The use of gravel and geosynthetics in those examples mentioned above will be able to lower the settlement and the related-damages caused by liquefaction.