Relief well as liquefaction mitigation option in Mpanau, Sigi, Central Sulawesi, Indonesia

The liquefaction disaster that occurred in Central Sulawesi in September 2018 was not only in the form of flow liquefaction but also a sand boil. Sand boils are caused by excess pore water pressure (PWP) which causes sediment ejecta due to water pressure within the ground. This research focuses on the use of relief wells to relieve excess PWP. During the 2018 disaster, the research location in Mpanau, Sigi, Central Sulawesi, had many sand boil points and the emergence of new water springs after the earthquake. At this point, the increase in PWP presumably resulted from a leaking groundwater basin, thus providing additional pressure. The method used was calculating the Ejecta Potential Index (EPI) and modelling with Seep/w. This calculation determined the amount of PWP that could be released to maintain the comparison between PWP and effective stress. This comparison is also known as the PWP (Ru) ratio and should be kept at less than 0.8, indicating the absence of liquefaction in the layer. This analysis was carried out with the initial conditions without wells and after the presence of wells, then assessing the distance of the wells to eliminate artesian pressure. It was concluded that this mitigation method is appropriate for areas that have groundwater basins with layers prone to aquifer leakage to reduce additional PWP.


Introduction
The liquefaction natural disaster began to be widely discussed in Indonesia after the earthquake and liquefaction occurred in Central Sulawesi, especially Palu City and Sigi Regency on September 28, 2018.However, the liquefaction disaster, or what the Kaili native people usually call "Nalodo", had happened before in that area based on the story of the previous ancestor.In general, this disaster involves three things: the presence of loose sand, the presence of water so that the ground is saturated, and the presence of vibrations which are usually earthquakes.
The Palu groundwater basin spans the Palu Valley and intersects a lot with the map (Figure 1).The existence of this groundwater basin was thought to be a support for increasing PWP in conditions of aquifer leakage.Despite the fact that there had been little rain prior to the inspection, spring waters were discovered in all affected locations.Interviews with local villagers and flow-slide witnesses revealed a wealth of evidence indicating the shallow groundwater in the impacted areas had been pressurised before to the earthquake [1].
Re-liquefaction can occur again, although it is not certain when it will occur.Locations that have liquefaction potential can be predicted to anticipate and carry out mitigation measures.Mitigation   2).The presence of groundwater basins could increase the influence of potential liquefaction, with the appearance of leaks and sand boils at some points after the 2018 earthquake [2].That a longer state of liquefaction was observed during the aftershocks.Finally, the different excess pore pressure responses of the four sand columns prove that the variability of geotechnical conditions plays an important influence on the liquefaction response [5].
The appearance of sand boils after earthquakes show that artesian affects liquefaction potential and soil conditions in the form of saturated loose sand [6].The research conducted at Mpanau involved examining 3 points within the Gumbasa area using Standard Penetration Test (SPT) data.The findings revealed that the water surface is shallow with loose sand, indicating a significant potential for liquefaction across all these points [7].
Liquefaction mitigation has been discussed by several studies on the Gumbasa Canal, one of which is the use of impermeable layers [8].Meanwhile, the influence of aquifers on liquefaction is hypothesized based on oral sources immediately after the earthquake [9].A seepage analysis is used to determine whether or not a relief well is required, and it also gives the requirements for the construction of a relief well system.Seepage study detects inlet and exit conditions and estimates substrate pressure [10].

Theoretical Framework
Research at three points that compared SPT and geoelectric data obtained Artesian Flow Potential results with a value of 2.89 m3 [2].This research can be developed to obtain the Ejecta Potential Index (EPI) with the method developed by [11].
A pressure relief well (or drain) that provides quick removal of excess water pressure buildup.Several case studies have demonstrated the success of this type of drainage using gravel columns or vertical geocomposite drainage with relatively large diameters [8].It has also been used to prevent "blowout" where the water pressure and vertical outflow gradient can become high, and has been incorporated in the design of flood control embankments, as in the Ejecta Potential theory which is related to the Artesian Flow Potential [11].
While it is difficult to quantify the contribution of aquifer leakage, causes of liquefaction involving different mechanistic processes operating together are likely to be non-linear and sporadic, but have the potential to be mitigated separately.[6].Liquefaction mitigation countermeasures can be classified into two types: One approach is to start with the liquefaction qualities of the foundation soil and work backwards, enhancing soil quality to boost the soil's anti-liquefaction capabilities early on.And the second one by changing the drainage characteristics of the liquefiable foundation soil so that excess PWP may be removed rapidly [12].Some suggested mitigations can be done with gravel channels, vertical gravel, or stone columns [13].
The proposed mitigation method is to use a relief well in which the relief well is filled with rock that can act as a stone column, an overview of the relief well is presented in Figure 3. Dewatering using relief wells can prevent liquefaction but does not cause seismic settlement [14].

Location
This research was located in Mpanau Village, Sigi, Central Sulawesi, Indonesia (Figure 4).In the 2018 earthquake, this location was not damaged by liquefaction because 1) the liquefaction only occurred in the form of sand boils at several points, 2) the contour conditions had a slope of 1.2%, and 3) the paddy rice fields dominated as the Gumbasa irrigation service area.

Ejecta Potential Index (EPI)
The AFP concept was designed to assess the required artesian pressure (i.e., hydraulic demand) by capturing post-shock water flows., represented by hexc, to produce an artesian flow that expels liquid sediments [11].AFP values and increased PWP at the Mpanau point based on geotechnical and geophysical data found that only one point experienced sediment ejecta (sandboil) [2] (Figure 6) AFP time is integrated over time to capture the important influence of duration to define EPI as Eq.( 1) [11].
Where t0 is the time when the earthquake acceleration reaches 0.05 g.On the side of the Gumbasa irrigation canal, two geoelectric tests were performed.Each geoelectric trajectory traverses the SPT locations.Because geoelectric results are interpreted based on resistivity, they can be categorised by soil type, as show in Figure  4.3.Seep/w SEEP/W is an advanced finite element software program for simulating groundwater flow in porous media [17].SEEP/W can simulate simple saturated steady-state situations as well as more complex saturated/unsaturated transient studies with atmospheric interaction at the ground surface [18].Calculations with geostudio are modeled in two stages, seep/w modeling after an increase in PWP without mitigation in the form of a relief well to determine the damage or impacts that arise, then application of relief wells to measure the excess reduction of PWP.

Excess PWP
The increase in the void ratio can be used to determine liquefaction conditions in the soil layers.The pore number ratio compares the pore water pressure () with the effective stress (').Ru with a value > 0.8 shows that the soil layer is experiencing liquefaction [19].So the initial conditions in modeling are done without using relief wells.
The increase in PWP, as an initial condition due to an earthquake, results in aquifer leakage.This leakage leads to pressure from the aquifer pushing onto the weak layer, causing a sand boil.This PWP change is shown in (Figure 7) so that a red layer appears at the leak point.The flow direction and pressure on the toe embankment can potentially reduce the effective soil stress.

Application of relief wells
As a potential solution for the research location, a relief well was chosen to release PWP in case of an earthquake and aquifer leakage.These cases cause additional pressure within the ground.In practice, this is difficult to measure precisely, but it can be learnt from empirical events to understand what factors affect it.Relief wells as liquefaction mitigation were also proposed by JICA in Palu [20].
The relief well modeled in this study was in the form of a well with a diameter of 1m, made of perforated concrete with a lid and a pipe for drainage, concrete blocks with a height of 1 can be arranged in 8 layers to get a total depth of 8m (Figure 8).

Figure 8.
Precast construction in the form of a concrete building that has holes in the walls for water dissipation, the construction can be arranged according to the design and needs, modified from [20] The relief well installation modeling is carried out using the scenario of changing the distance of the well to the toe of embankment.This scenario is used to obtain a distance that is close to optimal so that it can be adapted to field conditions if there are obstacles to constructing wells, the results of the seep/w modeling are shown in The conditions in Figure 9 show a clear change in reducing PWP in the presence of relief wells compared to conditions without relief wells.However, the difference was insignificant when compared to conditions with variations in distance.Therefore, the ideal choice that can be applied to this condition is a distance of 5m.This distance was chosen for ease of operation and maintenance because the relief well will be placed side by side with the inspection road next to the embankment with a width of 3m.The direction of the aquifer leakage flow is presented in Figure 10, and the PWP reduction value at this well distance is presented in The side towards the embankment flur was reduced towards the well (red line) while the side away from the embankment did not disturb stability.

Table 1.
Figure 10.The side towards the embankment flur was reduced towards the well (red line) while the side away from the embankment did not disturb stability.

Conclusions
Aquifer leaks can be triggered by earthquakes, thereby increasing PWP and reducing effective stress.This condition is commonly known as liquefaction.Liquefaction mitigation in this condition can be done by reducing the PWP by releasing or providing drainage channels.In mitigating irrigation canals in Mpanau, alternative mitigation is to use relief wells.The application of relief wells is considered effective in reducing excess PWP.The construction can use cylindrical precast concrete with a diameter and height of 1 meter.On the concrete walls, there are holes as a way for water to enter the well when there is an increase in PWP.In this case, the relief well is calculated to be 8 meters deep, meaning there are eight concrete layers.
The placement of relief well points is calculated based on the distance to the toe embankment.Distance modeling at the four scenario points did not produce a significant difference.However, the 5meter point showed the best PWP reduction.In a better view, the 5-meter modeling showed that the flux led to the well and decreased after passing through the well, which indicated that water was entering the well.At this point, it is also considered appropriate because it will be next to the inspection road to facilitate the operation and maintenance of the well.
1314 (2024) 012043 IOP Publishing doi:10.1088/1755-1315/1314/1/012043 2 measures can be taken to control one or several things that affect liquefaction.This research aims to use relief wells to release excess PWP.

Figure 1
Figure 1.Liquefaction Vulnerability in CentralSulawesi[2] modified from[3] 2. Literature Review Palu Groundwater Basin Palu has a piezometric surface ranging from shallow to deep.Shallow piezometric surface (<5.0-meterAGL) is in the eastern (East Palu) and western (South Palu and Marawola) parts.Deep piezometric surface (> 10.0 meter AGL) is in the east (South Palu and Biromaru) and west (West Palu)[4] (Figure2).The presence of groundwater basins could increase the influence of potential liquefaction, with the appearance of leaks and sand boils at some points after the 2018 earthquake[2].That a longer state of liquefaction was observed during the aftershocks.Finally, the different excess pore pressure responses of the four sand columns prove that the variability of geotechnical conditions plays an important influence on the liquefaction response[5].The appearance of sand boils after earthquakes show that artesian affects liquefaction potential and soil conditions in the form of saturated loose sand[6].The research conducted at Mpanau involved examining 3 points within the Gumbasa area using Standard Penetration Test (SPT) data.The findings revealed that the water surface is shallow with loose sand, indicating a significant potential for liquefaction across all these points[7].Liquefaction mitigation has been discussed by several studies on the Gumbasa Canal, one of which is the use of impermeable layers[8].Meanwhile, the influence of aquifers on liquefaction is hypothesized based on oral sources immediately after the earthquake[9].A seepage analysis is used to determine whether or not a relief well is required, and it also gives the requirements for the construction of a relief well system.Seepage study detects inlet and exit conditions and estimates substrate pressure[10].

Figure 2 .
Figure 2. Palu groundwater basin indicating the Mpanau study site is included in the delineation[4]

Figure 3 .
Figure3.The use of relief wells in dams to reduce PWP, modified from[15]

Figure 4 .
Figure 4. Map of the Mpanau research location in Sigi District, Central Sulawesi modified from[2,16]

Figure 6 .
Figure6.Stratigraphy shows weak locations as a basis for determining EPI points[2]

Figure 7 .
Figure 7. Aquifer leakage at a weak point causes the pore water pressure to increase and creates a sandboil on the ground surface

Figure 9 Figure 9 .
Figure 9. Simulation of the relief well distance from the toe embankment in several scenarios.(a) 3m scenario; (b) 5m scenario; (c) 10 m scenario; (d) scenario 20 m

Table 1 .
PWP reduction percentage after relief well installation based on depth