Simple house foundation models in potential landslide area (case study: Bojong Koneng Village, Babakan Madang Sub-District, Bogor District)

. Bogor Regency is a land area that has varied geomorphological conditions, mostly in the form of plateaus, hills and mountains with constituent rocks dominated by volcanic eruptions. This study was conducted to determine the topographic and geotechnical conditions at the Bojong Koneng location to determine the prediction of landslide areas, obtain the foundation planning criteria for simple houses in vulnerable areas, and design a foundation model suitable for simple houses in erosion-prone areas based on field survey in Curug Village, Bojong Koneng Village, Babakan Madang, Bogor Regency, West Java With a slope ranging from 10°-25° sloping to the north which is a soft black clay layer with the slope of the layer in the direction of the slope. In the existing slope model without Strauss piles, the safe slope stability factor (SF) is only around 1.02, so the slope is not fully declared safe. The installation of Strauss piles fondation theoretically helpbaas to improve slope stability for hard soil conditions at a relatively shallow depth from the ground surface or in this study at a depth of 5.20 meters. The SF value at the tip of the strauss pile is only attached to the hard soil layer, almost approaching the condition of the strauss pile penetrating 1.00 m deep hard soil. However, when viewed from the sliding plane of the Strauss pile fondation tip which is only attached to the hard soil layer, it behaves like a pile so this needs to be avoided in planning.


Introduction
Bogor Regency is a land area that has varied geomorphological conditions, most of which are plateaus, hills and mountains with constituent rocks dominated by volcanic eruptions which result in relatively large rainwater infiltration [1].The type of cover soil is dominated by loose volcanic material which is rather sensitive and very sensitive to erosion.High intensity rains have the potential to cause landslides, especially in hilly areas and highlands.The characteristics of rainfall that trigger landslides have been used to establish relationships between rainfall and landslides in various parts of the world including shallow landslides [2].Several factors trigger landslides, namely slope, geological conditions, soil type, land use, drainage patterns, rainfall, and human activities.Landslides that often occur in Indonesia are weathered volcanic rocks with a sloping to steep topography with a slope angle of 15o-45o with very heavy rain intensity of more than 100 mm a day [3].
Based on the results of a field survey by the Center for Volcanology and Geological Hazard Mitigation in 2022, the morphological conditions in Curug Village, Bojong Koneng Village, Babakan Madang, Bogor Regency, West Java are in the form of a horseshoe basin which is a water accumulation zone.The slope of the slope ranges from 10°-25° which is sloping towards the north.Outcrops of grey-black black clay are found; the character of the claystone softens relatively when interacting with water.Reported water conditions found seepage of new groundwater springs at the bottom of the avalanche crown.The seepage is a waterway zone which is part of the Cikeruh River sub-basin.In addition, there was no adequate water flow along the road at the disaster site.So that when high-intensity rain occurs, water accumulates in the middle and flows following the topography of the basin.In addition, based on the forecast map for ground movement in Bogor district in September 2022, it is included in the potential for high ground motion, as shown in Fig. 1 and Fig. 2. In this zone, ground motion often occurs, especially in areas bordering river valleys, escarpments, road cliffs or slopes that are disturbed, while old and new soil movements are still actively moving due to high rainfall and strong erosion [4].
The house foundation structure used by residents is a continuous type of foundation using river stones which only stand on the surface soil.During the rainy season the foundation structure easily shifts slowly following the direction of movement of the soil mass on the slope.The ground movement that occurs in Kampung Curug is a creeping type of ground movement, that is, the movement of the ground moving at a slow speed [5].In the incident in Kampung Curug found layers of black clay which are soft with the slope of the layers in the direction of the slope.Geological factors coupled with topographical characteristics and poor drainage channels make water saturate the soil because clay stone does not allow water to pass through.The contact between the ground and the black clay stone becomes a slip plane where the materials move.The creep zone can be mapped based on the distribution of cracks and the Cikeureuh sub-watershed.The mapped crawl zone reaches up to 37.54 ha.The creep zone or soil movement is in the Cikeruh River sub-watershed zone.This explains that the zone is a waterway/water accumulation zone.These factors support each other and influence the occurrence of soil movement when high rainfall/water infiltration is too high so that the soil becomes very saturated with water [6].Thus, the impact of the ground movement caused as many as 278 heads of households or 1,020 people to be hit.There needs to be a technical solution to ease the burden on the residents in conditions where the relocation efforts are uncertain and the residents also have no other choice to live.Foundation design solutions that are in accordance with the conditions of the problems faced by the residents of Curug village so that a foundation system is produced that is safe and friendly to soil movement on the slopes by taking advantage of the community's cooperation which still feels.There needs to be a technical solution to ease the burden on residents in conditions of uncertain relocation efforts and residents also have no other choice to live.
Foundation design solutions that are in accordance with the conditions of the problems faced by Curug residents with designs based on the characteristics of slope conditions.Design buildings that can be built by residents themselves with simple technology.
The goal to be achieved in this study is to obtain more comprehensive information about the topographical and geotechnical conditions at the study site to determine the prediction of the landslide that will occur [7].Obtain planning criteria for simple house foundations in landslide-prone areas based on previous studies, and based on topographical and geotechnical conditions in accordance with field conditions [8].Designing a suitable foundation model for simple houses in landslide-prone areas through a parametric study of finite element method numerical modeling using Plaxis 8.6 and GeoStudio software [9].creeping, namely damage to roads, houses, and resulted in damage to the surrounding land and even resulted in the breaking of bridges as traffic channels for the movement of people and goods.The condition of the damaged road due to soil movement in Curug village can be seen in (Fig. 2) Curug village is at an elevation of 650-750 masl asl, the main escarpment is a landslide at an elevation of 700 masl.

Types of research
According to Koh and Owen [10] this research is survey research with descriptive methods.Survey research is a research method that aims to collect large amounts of data in the form of variables, units, or individuals at the same time.which with this research has collected soil yield data, and has also conducted direct observations in the Bojong Koneng area, Babakan Madang, Bogor district.

Research flowchart
The steps of this research are made to facilitate the research, then a research flow chart is made as shown in Fig. 3.This research begins with surveying the location in the Bojong Koneng area, Babakan Madang then proceeds to collect soil data to support this research.then looking for parameters used to determine modeling.then the slope stability analysis is carried out and the slope reinforcement analysis is carried out using Plaxis 8.6 software if the research has been analyzed then the research has been completed.

Survey and topography
At the time of carrying out a detailed survey of the condition of each house, the season was the dry season.Almost all the existing house structures are masonry structures with shallow foundations.Meanwhile, houses that use a wooden masonry structure are houses No. 1, 2, 5, 10,12, and 17 (Fig. 4-5).
In Fig. 6, the condition of the house which is no longer symmetrical due to the movement of the soil mass is still visible.The movement of this soil mass occurs slowly so that it does not collapse the house in an instant.Every year the residents work together to repair the house by jacking up the house and repairing the structural elements.
For houses using masonry walls, the response to the movement of the soil mass in the building is in the form of diagonal cracks as in houses number 12 and 16.The response of the house to the gradual movement of the soil mass indicates that the structure experiences unequal deformation at each joint (joints) sloof, beam and column elements or in other words the structure is less rigid in the lateral direction [11].

Geoelectrical results
The resistivity method (or commonly called geoelectric) is a geophysical method that uses subsurface electric potential fields as its main observation object.The resistivity contrast in the rock will change the subsurface electric potential so that an anomaly can be obtained in the area studied [12].
Geoelectrical measurements are carried out by making 2 tracks (Fig. 7) to determine subsurface conditions in areas affected by ground movements.The configuration used in this survey is the Wenner-Schlumberger Configuration.This configuration is not very sensitive to horizontal changes but has deep current penetration, so it is good for depth surveys and can be used to determine slip planes [13].This first track has a South-North direction, using 48 electrodes with a distance between electrodes of 10 meters so that the total length of the track is 480 meters.From the results of the subsurface cross-section of the first track, there is a water-saturated layer starting from a depth of 10 meters below the ground surface (Fig. 8).The low resistivity value in this layer indicates that this area is mostly composed of claystone with a resistivity value between 1-100 Ωm.In the upper layer there are rocks with quite high resistivity values up to above 100 Ωm, which indicates that this layer is composed of quite hard rocks.From the subsurface section above, it can be estimated that the slip surface will start at a depth of 5-10 meters.
The second track is south of the first track and is trending west-east.This track uses 48 electrodes with a distance between electrodes of 5 meters, so the total length of this track is 240 meters.In the subsurface cross-sectional image of the second track, there is a water-saturated layer on the west side up to the 100th meter with a depth of 10-15 meters (Fig. 9).This section is right at the crown of the avalanche.The topmost layer on this track is volcanic rock, this can be seen from its high resistivity value.The contact between the hard layer and the water-saturated layer indicates the slip surface, so it can be estimated that the slip surface is at a depth of 10-15 meters.

Slope stability analysis
Slope stability analysis begins by knowing how the condition of slope stability is on a wider scale.In Fig. 10, the top layer is clay silt and the bottom layer is hard soil.The slope model that will be analysed for slope stability observes the depth of the Strauss pile.In modeling piles using Plaxis software, the use of "beam" elements is generally not suitable for modeling the effects of rejection or drag between piles.This is because the beam element in Plaxis is generally used to model beam or plate structures with a certain flexibility, not to represent the complex resisting effects between piles and the surrounding soil.To model the effect of rejection or drag between the pile and the soil in Plaxis, you can use the "interface" element provided by the software.Interface elements make it possible to model the contact between two different materials, such as between a pile and the surrounding soil.Here are the general steps in determining Strauss parameters in a Plaxis model: Define the pile geometry: Define the size and shape of the pile to be modelled.This involves the length, diameter, and location of the pile in the model.Define the pile material parameters: Select appropriate material parameters to represent the mechanical properties of the pile, such as stiffness (modulus of elasticity) and shear strength.Define the soil material: Define the material parameters for the soil around the pile.This involves soil characteristics such as density, inner friction angle, and coefficient of elasticity.
Create a numerical model: Create a numerical model in Plaxis using appropriate elements, including an "interface" element to represent the contact between the pile and the soil.Determine the size and distribution of elements considering the detail and complexity of the pile and soil system.Determine the Strauss parameters: Strauss parameters in the Plaxis model are generally related to the contact arrangement at the interface element.can further specify parameters such as contact stiffness, contact distribution, and direction of repulsion or attraction between the pile and soil.These parameters will affect the response of the pile system to loads and surrounding soil conditions.
Verification and calibration: Verify and calibrate the model using field data or laboratory test data if available, and be able to compare the model response with the actual response to ensure that the Strauss model is providing appropriate results.It is important to note that Strauss parameters can be highly dependent on specific soil conditions and the properties of the soil around the pile.Therefore, the above steps should be tailored to the specific needs and characteristics of the pile project being modelled.
Mesh creation is done by Generate Mesh on a soil geometry model that has input soil and reinforcement parameter data, refine the clusters on the model for more accurate results as shown in Fig. 11.Soil parameters on slopes are presented in Fig. 12 and structural element parameters are presented in Fig. 13.The groundwater level (m.a.t) for both models is at an elevation of ±0.00 meters from the slope surface.Based on the results of the analysis of slope stability, the value of the factor of safety for slope stability (SF) and the prediction of the slip plane that occurs on the slope is presented in Table 1.As for the existing results that occur in the results of the safety factor and the description of SF_pole 1-4 in Fig. 14.In the existing slope model, the slope stability factor (SF) is only around 1.02, so the slope is not fully declared safe.Strauss piles theoretically help increase slope stability for hard soil conditions at relatively shallow depths from the soil surface or in studies This is at a depth of 5.20 meters.

Conclusion
An acceptable conclusion is that the response of the house building to the gradual movement of the soil mass indicates that the structure experiences unequal deformation at each joint of the sloof, beam and column elements or in other words the structure is less rigid in the lateral direction.The natural slope angle at the study site is in the range of 10° to 25° causing the house to move down the slope in accordance with the direction of the dominant soil mass movement.

Location No Name
Type The ground motion that occurs in Village Curug, Bojong Koneng, Babakan Madang is a slow or creeping type of ground motion.In the existing slope model, the slope stability factor of safety (SF) is only around 1.02, so the slope is not fully declared safe.The installation of strauss piles theoretically helps to increase slope stability for hard soil conditions at relatively shallow depths from the ground surface or in this study at a depth of 5.20 meters.
Research on the strauss pile foundation model bound by sloof for simple houses in landslide-prone areas in KP.Curug, Bojong Koneng, Babakan Madang, Bogor needs to be developed to apply the foundation model in the field.Observations of the deformation of the foundation model are carried out at the end of each rainy season to study the extent of the lateral stiffness of the foundation relative to the movement of the soil mass that occurs, so that no further soil movement occurs.

Fig. 2 .
Fig. 2. The condition of roads and houses was damaged due to land shifts.

Fig. 4 .
Fig. 4. The condition of the houses in the study location for house numbers 01-06.

Fig. 5 .Fig. 6 .
Fig. 5.The condition of the houses in the study location for house numbers 07-14.

Table 1 .
Slope stability factor of safety (SF).