Potential deformation assessment of Semantok Main Dam in North Nganjuk Region at East Java Indonesia

Semantok Dam was constructed using a zone l embankment type and required safety measures for deformation control. This study aims to understand material specifications, geological conditions, and deformation behavior. The results of the material specifications indicate that the core material and fine filter material meet the design criteria. The geological conditions comprise sand, sandstone, and claystone. The rock’s bearing capacity and settlement in the core zone are considered safe. The elastic vertical deformation of the foundation, analyzed using a flexible foundation approach by Lord, is estimated to be between 4.36 and 6.10 cm. Dynamic analysis is also performed using the Plaxis 8.6 version software to estimate deformation. The software input includes soil properties from the borrow and quarry areas surrounding the dam site. Earthquake design criteria are based on the Operating Basis Earthquake (OBE) with a 100-year return period and the Maximum Design Earthquake (MDE) with a 10,000-year return period. The dynamic analysis results indicate that the maximum deformation during a 100-year return period earthquake is 7.37 cm (horizontal deformation) and 12.68 cm (vertical deformation). In the case of a 10,000-year return period earthquake, the maximum deformation is 83.41 cm (horizontal deformation) and 175 cm (vertical deformation) during rapid drawdown.


Introduction
The Indonesian government, through the Ministry of Public Works, has been implementing a program in the field of water resources from 2014 to 2024.This program involves the construction of 61 dams to provide raw water, irrigation, hydroelectric power generation, and flood control [1].
Semantok Dam is one of the infrastructures built to capture the flow of the Semantok River, which is part of the Brantas River Basin.Its construction is intended to provide 0.31 m 3 /sec of raw water, supply irrigation to 1900 hectares, and reduce floods by 137 m 3 /sec.
In its planning, the dam must meet design criteria and safety requirements against hydraulic failure, filtration failure, and structural failure [2].Based on the specifications of the available fill materials and the foundation conditions of the Semantok Dam, a study has been conducted on the geological conditions and foundation improvement of the Semantok Dam, as well as an analysis of the stability of the Semantok Dam, including stability against bearing capacity and settlement, filtration flow, and dam slope.The main goal of this study is to obtain an overview of the safety of landslide hazards and deformations based on a dynamic analysis of dam body deformation under Operating Basis Earthquake 1311 (2024) 012008 IOP Publishing doi:10.1088/1755-1315/1311/1/012008 2 (OBE) and Maximum Design Earthquake (MDE) earthquake conditions will be conducted.

Study Location and Dam Site Layout
Semantok Dam is located in Dusun Kedungpingit, Desa Sambi Kerep, Kecamatan Rejoso Kabupaten Nganjuk Provinsi Jawa Timur.Astronomically, the dam is situated at coordinates 111°53'25.68"East Longitude and 7°29'41.90"South Latitude.Semantok Dam has a catchment area of 54,032 km 2 and a dam length of 3,005 km.The layout of the dam can be seen in Figure 1 below.

Workflow
After collecting the data, the discussion concerning the safety of earthfill dam construction commences with an examination of embankment material data, which includes the core material and fine filter material.Additionally, geotechnical data at the dam's foundation site, such as vertical stress and settlement in the core zone, is investigated.Subsequently, the foundation bearing capacity and settlement analysis are performed for both the embankment and foundation.It encompasses vertical stress, settlement of the core zone, and elastic foundation settlement.This is followed by an examination of the stability of filtration flow that covers phreatic line pattern, seepage low rate, piping, and boiling phenomena.
As the final output, this study primarily concentrates on assessing the stability of the construction and the deformation of the main dam's embankment body, covering the determination of seismic loads and slope stability and deformation.The stages of this discussion are illustrated in Figure 2

Required data
The required data includes the fill material data for Semantok Dam, a summary of fill material gradation, a summary of fill material testing results, borehole logs for the dam's foundation, trial grouting results data, geological data for Semantok Dam, average flow data for the Semantok River, technical and geometric data for the Semantok Dam structure, and the 2017 Earthquake Map.The cross-sectional view of the dam is shown in Figure 3.

Embankment Material Specification
The specifications for the embankment material in this study are specifically focused on the impermeable core material and fine filter material.The impermeable core material specifications include regrading of the base soil [3], plasticity, compaction criteria, and permeability [4].The fine filter material specifications include determining material gradation limits [3], material properties, and permeability [4].

Geological Study and Foundation Improvement
An investigation into the geological and geotechnical conditions was conducted to ascertain the properties of the bedrock beneath Semantok Dam.Additionally, an analysis of the trial grouting results was performed to elucidate the foundation soil characteristics and explore alternative foundation methods beyond grouting.

Vertical Stress and Foundation Bearing Capacity
The rock foundation is considered safe if its bearing capacity meets the requirements for supporting the vertical stress of the dam.Vertical stress is calculated using the Boussinesq's theory for uniformly distributed trapezoidal loads [5].The bearing capacity of the rock can be estimated based on the rock class using the "Rock Mass Classification and Rock Parameters" table by Kikuchi [6].The designed strength of the rock bearing capacity needs to be correlated with the Rock Quality Designation (RQD) [7].

Settlement of the Dam Core Zone
The settlement of the dam depends greatly on the characteristics of the materials used.The settlement magnitude can be calculated using an empirical formula [6].The allowable settlement is 2%, and if the dam's body height is >30 meters, an additional 1% is added to account for settlement due to earthquakes [2].

Elastic Settlement of Foundation Rock
The calculation of elastic settlement is used to determine the magnitude of settlement in compressible foundation soil layers.The approach was developed by Lord using the Shape Factor from Whitman and Richart and the Depth Factor from Pells Turner and Meigh [8].

Filtration Flow Capacity
The analysis of filtration flow is conducted using Geo-Studio SEEP/W 2018, referencing "Stability Modelling with SEEP/W" by Geo-Slope International Ltd.The analysis is performed for floodwater, normal, minimum, and rapid drawdown conditions.The discharge capacity or seepage rate is obtained by multiplying the flow rate (SEEP/W) by the length of the dam.Seepage discharge is considered safe when Q Seepage < 1% of the Average River Flow [9].

Piping and Boiling
Seepage through the foundation and body of the dam must be controlled to prevent excessive uplift forces [10].The vertical component of the outflow velocity or critical velocity is theoretically developed by Justin [4].If the seepage velocity () is less than the critical velocity (), then no piping phenomenon occurs [11].In sand, there is no friction between particles, and they have no strength, so if γ'/γw or the critical hydraulic gradient (icr) increases, the soil surface will experience a 'boiling' condition.Clay still retains strength when the effective normal stress is zero; therefore, if the critical gradient for clay reaches γ'/γw, the 'boiling' phenomenon does not occur [12].

Determination of Earthquake Load
The earthquake return period criteria analyzed depend on the dam's risk class [13].For the return period to be analyzed, the magnitude of the bedrock acceleration (SB) is obtained from the 2017 Earthquake Map [14] and the website pgacal.pusair-pu.go.id.

Slope Stability and Deformation Analysis
Slope stability analysis is conducted using the Plaxis version 8.6 software.Plaxis is finite-element (FE) software used for the analysis of deformation and stability in geotechnical engineering and rock mechanics.In this study, we utilize this software to investigate the deformation of the dam body in both horizontal and vertical directions, both with and without earthquake conditions.
Critical conditions that need to be reviewed include Post-Construction, Normal Water Level, Minimum Water Level, and Rapid Draw Down.In each condition, the Safety Factor must meet the required Safety Factor [15].

Specifications of Semantok Dam Materials
Specifications were carried out on the regrading results and the testing results of the impermeable core material.Table 1 shows the investigation results from the core material samples meeting the criteria for use as impermeable core fill material.Before specifying the fine filter material, the determination of the gradation range limit for the filter material is conducted.Table 2 shows the investigation results from the filter material samples meeting the criteria for use as filter fill material.3. A comparison between the total embankment stress and the foundation rock design strength is shown in Table 4. Since σz total < qu design, the foundation rock is SAFE and meets the requirements of the dam's support.The dam's geometric approach is divided into several sections.The division scheme of the planes for vertical stress calculations is shown in Figure 4.The value of I obtained from the graph and analytical methods is relatively close.To perform a more detailed calculation, the analytical method is used.Vertical stress (σzpart 1) = q x I = 223.74x 0.328 = 73.447kN/m² Note: 1.The same calculation is performed for Parts II, III, and IV, and then the values are summed to obtain the total σz. 2. The calculations are carried out at each depth to be reviewed using the same steps.

Settlement of the Core Zone of the Dam
The consolidation process causes settlement in the core zone of the dam.The known data is as follows: • Coefficient of Compressibility (mv) = 1.762 x 10 -5 cm²/g • Saturated Soil Bulk Density (γsat) = 1.776 g/cm³ • Dam Height (H) = 30.56m = 3056 cm • Settlement Coefficient (T) = 0.4 (taken from the range of 0.3 to 0.5) • Consolidation Coefficient (Cv) = 1.07 x 10 -2 cm²/sec • Time Factor (T90) = 0.848 (for 90% consolidation degree) The magnitude of settlement (∆H) in the core zone of Semantok Dam is as follows [4]: Because the settlement value is 1.91%, which is less than the permissible limit of 3%, Semantok Dam can be considered SAFE against settlement.

Elastic Foundation Settlement
Considering the presence of three sub-layers in the foundation beneath Semantok Dam, namely sand, sandy rock, and claystone, it is estimated that elastic soil settlement may occur.Elastic soil settlement typically occurs in compressible soil, as illustrated in Figure 5 and described in the borehole position map shown in Figure 6 below.With the flexible type of foundation for Semantok Dam, elastic deformation is calculated using the Settlement on Flexible Foundations Method with an approach developed by Lord [8].This approach considers a single crucial parameter: the foundation depth, denoted as (L) and (D).The relationship between these parameters is expressed as follows: L/B = D/B = 3.36 / 17.46 = 0.192 Consequently, the values for the shape factor (I) obtained from Figure 7 and the depth factor (μ0) obtained from Figure 8 are as follows: I = 0.9 (with respect to the IM line) μ0 = 0.7 (with respect to the Pells and Turner line).The elastic foundation deformation using the Settlement approach (Lord) is obtained to be in the range of 4.36 to 6.10 cm.

Filtration Flow Capacity
Using the SEEP/W Geo-Studio 2018 software, the phreatic line patterns were obtained for Minimum Water Level (MWL), Normal Water Level (NWL), Flood Water Level (FWL), and Rapid Drawdown conditions.With SEEP/W, the flux and flow rate values were determined for each condition.It is known that the average flow rate of the Semantok River is 1.13 m³/s.The dam seepage must not exceed 1% of the average annual runoff of the Semantok River, which is 1.13 m³/s x 1% = 0.0113 m³/s.The values of Semantok River seepage flow rate are shown in Table 5.The results show that the seepage flow rates under various conditions are < 1% of the average river flow rate, thus confirming that Semantok Dam is SAFE against seepage.

Piping and Boiling
The seepage velocity value is obtained from the water flux downstream of the impermeable core heel using SEEP/W Geo-Studio 2018, at Normal Water Level (NWL) +90.14 m.Based on the simulation results, the seepage velocity (Vs) is determined to be 8.58 x 10 -8 m/second.The known data is as follows: • D50 (Core Zone) = 0.0053 mm = 0.00053 cm The hydraulic gradient value was also determined through analysis using SEEP/W Geo-Studio 2018 at NWL +90.14 m.The hydraulic gradient is assessed at the downstream toe of the dam core, with the core material being clay, which possesses cohesive properties.Due to the cohesive nature of the clay material, the critical gradient value is exceptionally high, resulting in a very low potential for boiling.Therefore, Semantok Dam can be deemed safe against boiling phenomena in terms of hydraulic gradient.

Determination of Seismic Load
Semantok Dam has a Total Risk Factor value of 34, classifying it as Risk Class IV (Extreme).In light of this classification, the dam's stability under seismic loads will be assessed for two return periods: 100 Years and 10,000 Years.
For the analysis, bedrock acceleration values (SB) are sourced from the 2017 Earthquake Map, with values of SB = 0.098 g for a 100-year return period and SB = 0.518 g for a 10,000-year return period [14].

Dam Slope Stability and Deformation with Plaxis Software
The required inputs for this software include dam geometry, material characteristics, and seismic parameters (amplitude and frequency) [16].Then, the Safety Factor obtained from the simulation is compared with the minimum Safety Factor criteria.
Examples of the Plaxis [16] results are presented in Figures 9 to 11.For the dam under full reservoir conditions without seismic activity, the slope stability safety factor is 1.973, horizontal deformation is 3.374 cm, and vertical deformation is 2.599 cm.   6.Based on these results, the slope stability under non-seismic conditions and under OBE (Operating Basis Earthquake) with a return period of 100 years complies with the permissible safety factor or is

Conclusions
The specifications for impermeable core material and fine filter material, in terms of gradation and material properties, have met the criteria as per the guidelines and references available.
In general, the geotechnical condition of Semantok Dam is sufficiently compact, but the seepage is significant.The results of the grouting trial were deemed ineffective, and alternative foundation improvement plans include Secant Pile Cut-Off Wall and Cut-Off Trench.
The bearing capacity of the foundation rock of Semantok Dam is safe and meets the requirements for dam support.The material in the core zone of Semantok Dam is safe against settlement.Meanwhile, elastic soil settlement in the compressible foundation layer of Semantok Dam is relatively very small.
The capacity for filtration flow or seepage discharge of Semantok Dam is safe.The seepage velocity of Semantok Dam is safe from piping phenomena.The hydraulic gradient of Semantok Dam is safe from boiling phenomena.
Under non-seismic conditions and OBE (Operating Basis Earthquake) with a return period of 100 years, the slope stability of Semantok Dam is safe, and the maximum horizontal deformation is 7.37 cm, and vertical deformation is 12.68 cm.However, under MDE (Maximum Design Earthquake) conditions with a return period of 10,000 years, the slope stability of Semantok Dam is unsafe, with a maximum horizontal deformation of 83.41 cm and vertical deformation of 175 cm (Rapid Draw Down).

Figure 3 .
Figure 3. Cross-section of Semantok Dam 1311 (2024) 012008 IOP Publishing doi:10.1088/1755-1315/1311/1/0120085 material 96.34% -100% retained on sieve No. 200 (Sand and Gravel) Pass Particle gradation of material within the filter zone range Falls within the filter zone range Pass 0.10 mm < D15F ≤ 0.70 mm 0.10 mm < D15F = 0.24 mm ≤ 025 x k core material) k = 0.00596 cm/s > 25k core = 1.81 x 10 -5 cm/s Pass 3.2.Vertical Stress and Foundation Bearing Capacity With the dam foundation base situated at an elevation of +63.53 m, according to borehole log BH-14, the sub-layers of the foundation consist of CL (class low), D (decomposer), CL, and CM (class medium) layers.The calculation of the total embankment stress is reviewed for each rock layer.The foundation's design strength values, considering RQD, are presented in Table

Figure 4 .
Figure 4. Scheme for the Division of Vertical Stress Analysis on Semantok Dam

Figure 6 .
Figure 6.Borehole position map Foundation stress (q) = 429.18kN/m² Foundation width (B) = 17.46 m Young's Modulus (E) = 95000 kN/m²With the flexible type of foundation for Semantok Dam, elastic deformation is calculated using the Settlement on Flexible Foundations Method with an approach developed by Lord[8].This approach considers a single crucial parameter: the foundation depth, denoted as (L) and (D).The relationship between these parameters is expressed as follows: L/B = D/B = 3.36 / 17.46 = 0.192Consequently, the values for the shape factor (I) obtained from Figure7and the depth factor (μ0) obtained from Figure8are as follows: I = 0.9 (with respect to the IM line) μ0 = 0.7 (with respect to the Pells and Turner line).

Table 1 .
Results of Impermeable Core Material Specifications

Table 3 .
Calculation of the Design Bearing Capacity of Foundation Rock

Table 4 .
Comparison of Vertical Stress to Foundation Rock Bearing Capacity

Table 5 .
Summary of Seepage Flow Rates under Various Conditions

Table 6 .
Safety Factor and Deformation of Dam Slope Stability

Horizontal deformation (cm) Vertical deformation (cm) Remark and Allowable Safety Factor (SFa)
However, under the MDE (Maximum Design Earthquake) condition with a return period of 10,000 years, it does not comply with the permissible safety factor or is NOT SAFE. 14SAFE.