Evaluating the Effectiveness of Geotextile and Sub-Drainage in Addressing Road Subsidence on Bumiayu–Sirampog Road, Brebes Regency

Road subsidence commonly occurs in highlands which have poor soil parameters, supported by a lack of proper treatment to prevent subsidence. Common problems are the unequal bearing capacity of the subgrade at each location and variations in settlement caused by the presence of soft soil layers beneath the subgrade. This is why using a geotextile approach is the first step. Like the road subsidence incident located in Sridadi Village, Kec. Sirampog, Kab. Brebes, to be precise at KM 116+100 to KM 116+250, was caused by ground movement which was influenced by a fault from the North East to the South West. Based on the collection and processing of primary and secondary data, the results of the technical evaluation of road repairs on soft soil were obtained, namely treatment with geotextile reinforcement, namely class 1 stabilizer type. Use of 5″ PVC pipe sub-drainage and Ø 1 m Armco to drain water below the ground surface. Through 2D V20 plaxis modeling, the condition of soil stability can be determined before and after the use of geotextiles and sub-drainage at that location. Namely, it can minimize the extreme value of land displacement (Utot) which was originally 5.126 m to 1,414 m. And the reduction value obtained based on field observations was smaller than the laboratory calculation, namely 0.700 m < 0.845 m.


Introduction
Roads play a crucial role in land transportation, serving as vital infrastructure to link different places together.As time progresses, the need for infrastructure facilities continues to grow.Hence, there is a significant requirement for effective road planning.This planning process should encompass geotechnical considerations to evaluate soil bearing capacity and stability.Geotextiles offer advantages in civil construction, including their ease of application on-site, availability of materials, and affordability.Moreover, geotextiles contribute significantly to soil reinforcement, particularly in the subgrade (Pratama et al., 2021).As reported on October 6, 2022, by murianews.com,the occurrence of subsidence on Bumiayu-Sirampog road in Brebes Regency resulted from the road's collapse due to soil instability, primarily triggered by heavy rainfall.The geological map analysis confirms that the area is susceptible to 1321 (2024) 012011 IOP Publishing doi:10.1088/1755-1315/1321/1/012011 2 landslides and is not recommended for human habitation.Nonetheless, despite this fact, the residents in the vicinity still express their contentment living in the area.The Central Java Provincial Government, through the Department of Public Works (Bina Marga and Cipta Karya), is continuously making efforts to address the subsidence issue on Bumiayu-Sirampog road.Various measures have been taken, including the installation of bamboo stakes, armco, and gabion baskets.In 2023, the implementation of geotextiles and sub-drainage in the area has been being considered as a means of intervention due to their easy and rapid application, which aids in monitoring soil subsidence.This research aims to determine the factors influencing the subsidence on Bumiayu-Sirampog road in Brebes Regency.Besides, the study seeks to identify the suitable technology to address the subsidence issue in that specific area and evaluate the implementation method of geotextiles and sub-drainage for mitigating subsidence on the road in the regency.

Theory of land subsidence
Based on a study conducted by Achenita et al., (2016), land subsidence, also known as subsidence of the land surface, is a dynamic process involving the downward movement of the land surface relative to a specific geodetic reference framework.This phenomenon is attributed to various factors that exhibit variability.Several studies on subsidence have been conducted and have successfully identified the underlying factors contributing to this phenomenon such as excessive groundwater extraction, consolidation resulting from building loads, natural consolidation processes occurring within the subsoil, and tectonic activities.(Makmur et al., 2016).Below are the three main types of landslides.See Figure 1 for detail: a. Rotational slips; these are landslides in which the shape of the surface collapses at the cut can be in the form of a circular arc or a non-circular curve.b.Translational slips; these are landslides that tend to occur when the soil layers at the boundaries are relatively shallow beneath the slope surface.c.Compound slips; these are landslides that occur when the soil layers at the boundaries are located at a greater depth.This typically happens when the failure consists of curved and planar sections collapsing together.Reported from murianews.comon October 6, 2022, due to the high rainfall during the end of 2022 and the beginning of 2023, there was soil movement at the location.This was evidenced by the deepening of the depressions on the road, making it difficult for passing drivers.Geotextile, as defined in the circular, is a type of geosynthetic material commonly utilized as a filter in subsurface drainage systems for roads.Geotextile filters are typically permeable to water and can be in the form of knitted, woven, or non-woven fabric, installed in conjunction with foundations, soil, rocks, or other geotechnical materials, forming an integral part of the overall structural system or human-made product.These filters are made of durable materials that exhibit resistance to chemical reactions, weathering, and aging processes.The long-term performance of the structure in permanent applications depends on the durability and resistance of the geotextile (Pratama et al., 2021).

b. Sub Drainage
One type of drainage used to handle road subsidence is underground drainage (Sub Surface Drainage).This is a drainage route that uses subsurface media (pipes) to drain air from surface runoff.orizontal drainage (horizontal drainage/sub-drainage) is made to drain water or lower the groundwater level in areas where slopes fail.This method can be used for large slope failures where the landslide plane is deep by making a half-horizontal hole until it reaches the water source.Water flows through pipes with a diameter of 5 cm or more with holes on the wall.The placement of the drainage pipe depends on the type of material whose groundwater level will be lowered.

c. Steel Sheet Pile
Steel Sheet Pile Wall is believed to be a new advancement in soil retaining construction.It is designed to prevent soil from sliding due to horizontal pressure and also beneficial for maximizing land utilization.The construction of this resembles a wall composed of multiple sheet piles driven into the ground.Typically, this construction is used as river embankments, soil retention in excavation projects, soil retention on highway slopes, and so on.When compared to the popular gravity wall sheets, the use of this pile tends to be more effective in terms of safety and land utilization.This is because they are thinner in profile than gravity wall sheets but still maintain a stable structure, safety, and construction integrity.
d. Corrugated Concrete Sheet Pile (CCSP) Corrugated Concrete Sheet Pile (CCSP) is a type of soil retaining wall that falls under the category of flexible walls.As the name suggests, CCSP is made of concrete.The use of CCSP for retaining walls offers several advantages, including relatively shorter construction time due to ease of installation.The depth of CCSP installation depends on the type of underlying soil at the construction site.The softer the soil, the deeper the CCSP needs to be embedded.Therefore, to ensure the safety of excavation works, an analysis of the appropriate depth of CCSP must be planned to meet stability requirements against overturning forces.
e. Vertical drainage Vertical drainage system is used to create channels that carry groundwater with the aim of accelerating the soil settlement process.The completion of vertical drainage process requires varying amounts of time depending on the porosity level of the soil being worked on.The time needed for this completion process is called the consolidation period.The use of vertical drainage can expedite the consolidation process by diverting water from the soil to the drainage area.By using drainage pumps, forced water flow can be increased.The faster the soil stabilizes at the construction site, the earlier the project can commence.Vertical drainage consists of plastic strips coated with pre-fabricated geotextile filters, and it features grooved channels.The filter layer prevents soil and debris from entering the drainage channels, resulting in a constant flow of water.

Overview of Slope Stability in Embanked Soil
Slope stability analysis needs to assume that the slope surface is a circular curve.The shear force along a circular curve that prevents sliding must be compared with the force that forces the soil mass above the curve to shift.Methods for Checking Slope Stability Slope stability analysis can generally be categorized into four groups: visual observation, calculations, graphics, and computer software such as PLAXIS, XSTABL, RHEOSTAUB, etc.

Research Method
The research had steps to follow according to the procedures to obtain reliable and high-quality research results.In this study, a descriptive analysis method was employed for the technical evaluation of the implementation of the road improvement on the soft soil using geotextile reinforcement (Sunarjono et al., 2021).
The research was located on the Bumiayu -Sirampog Road, Brebes Regency, specifically from kilometer 116+100 to kilometer 116+250, in Sridadi Village, Sirampog District, Brebes Regency.This site had experienced road damage due to ground subsidence or settlement caused by soil movements.The area is situated within the geological zone of Mount Slamet (Slamet Volcanic Quarter) with the Propagation Formation (Tmr).It has been influenced by a fault running from the northeast to the southwest direction.To obtain the primary data, the researcher conducted field data collection on soil investigation, laboratory testing, and chronological records of the site's handling.As for secondary data, the researcher conducted literature reviews to different sources such as reference books, online media articles, and journals related to the topic of the research.Data processing was performed by organizing the data into orderly, sequential, logical, non-overlapping, and effective sentences to facilitate understanding and interpretation.It included the following stages: editing, classifying, verifying, analyzing, and concluding.The data processing involved descriptive analysis aimed at providing a general overview of the data, as well as inferential analysis using basic statistical formulas.This allowed for obtaining calculation results that was used to draw generalized conclusions from the data.The use of the back analysis method helps estimate the possibility of landslides occurring again or not.
Namely by analyzing the estimated property value of slopes that could cause landslides.The slope parameter values such as cohesion (c') and internal shear angle (phi') are changed in various variations until the slope model experiences failure.

Results and Discussion
Based on the existing soil investigation results, it is explained that at a depth of 16 meters, there was clayey, silty, and hard to dense claysilt.This claysilt soil was very compact, but when it was saturated with water, it became very soft (muddy).This had an impact on the type of treatment that needs to be implemented.Class 1 Stabilization Geotextile was used to handle the road subsidence in Bumiayu-Sirampog area, Brebes Regency this year.The followings were the installation methods of geotextile in the project: 1.For high embankments (unstable slopes), the work began with clearing the area of plant roots and sharp rocks that might tear the geotextile.The geotextile was laid directly on the roadbed area after the process was completed.2. The fill soil could be placed on top of the geotextile and compacted according to compaction specifications every 50 cm until reaching the height of the next geotextile layer.3. The geotextile was laid on the subgrade in the direction perpendicular to the road axis.The geotextile joints were stitched together.The installation method of sub-drainage (5" PVC pipe) was carried out at points having the potential to be water sources.It involved digging to a certain depth until reaching the water source and preparing the pre-drilled PVC pipes and wrapping them with coconut fiber or cloth to prevent soil from entering the pipes and finally, covering and compacting the backfill.The installation method of sub-drainage (Armco Ø1m) was carried out at points having the potential to become water sources by digging to a certain depth until reaching the water source.Then, preparing and installing the Ø1m Armco as needed.After that, covering and compacting the backfill.In Figure 8, it can be seen that the soil displacement value resulting from modeling the basic soil conditions without geotextile reinforcement and the use of sub-drainage with adjustments to the cohesion value (c') and internal friction angle (phi') is 5.126 m with an SF value of 0.99.In Figure 9, it can be seen that the soil displacement value resulting from modeling the basic soil conditions with geotextile reinforcement and the use of sub-drainage as well as adjusting the cohesion value (c') and internal friction angle (phi') is 1.414 m with an SF value of 3.45.

Conclusions
Based on the analysis, the factors causing the subsidence on the Bumiayu-Sirampog Road, Brebes Regency included the natural subsidence, subsidence due to groundwater extraction, and subsidence due to building loads.Of the several treatment alternatives using existing technology, the treatments that were possible to carry out at the subsidence location were geotextiles and sub-drainage, because of the affordable costs and ease of material mobility.The geotextile installation method is carried out using the rule of compacting 50 cm of embankment soil until it reaches the height of the next geotextile layer and by sewing to connect the geotextiles together.Meanwhile, the installation of sub-drainage is carried out according to standards in locations where there is water flow.With plaxis modeling, the condition of soil stability can be known before and after the use of geotextiles and sub-drainage at that location through the resulting displacement values and safety factors.Namely, the ground displacement value which was originally 5.126 m with SF 0.99 < 1.5 (Not Safe) became 1.414 m with SF 3.45 > 1.5 (Safe).And the reduction value obtained based on observations in the field is smaller than the laboratory calculation, namely 0.700 m < 0.845 m.

Suggestions
Further monitoring is needed for the handling of the subsidence over a longer period to determine the extent of the subsidence.It is also necessary to conduct additional drilling at several other points to determine the physical and engineering properties of the soil, and compare them with the values obtained before and after the installation of the sub-drainage system.This will help determine the soil shear strength under saturated conditions.

Figure 1 .
Figure 1.The main typ es of landslides.

Figure 9 .
Figure 9.Total Displacement (arrows) is 1,414 m Subsidence due to building loads, which consists of: Consolidation decrease as a form of volume change in saturated soil due to the drainage of water from soil pores, and immediate settlement due to elastic deformation of dry, wet, and saturated soil without any change in moisture content.
2.3.Appropriate technologies to address subsidence issues a.Geotextile The Circular Letter of the Ministry of Public Works and Housing Number: 34/SE/M/2015 dated May 18, 2015 provides guidelines for the technical planning of subsurface drainage using geotextile filters.