Effect of material quality on cemented sand and gravel (CSG) dam slope design of Cibeet

The construction of the Cibeet Dam is crucial to mitigate downstream flooding in the Citarum River. Initially planned as an embankment dam with a 1:3.50 upstream slope, a 1:30 downstream slope, and a height of 47 meters, challenges arose due to foundation and earthquake considerations. The excessive requirement of embankment material at the dam site, which was insufficiently available, prompted the exploration of a more resource-efficient alternative. Consequently, a slimmer type of dam, specifically a Cemented Sand and Gravel (CSG) dam, was deemed necessary. The CSG dam, designed with minimal cement (80 kg/m3) and a mixture of sand and gravel (105.5 kg/m3), boasts a more upright slope, reducing material demand. This research examines the impact of material quality on the CSG dam slope. The finite element method assesses the dam’s stability using dynamic seismic forces. To achieve a stable dam slope design with existing materials, an extensive analysis was conducted on various slopes upstream and downstream, ranging from 1:0.70 to 1:1.20. The results indicate that a slope of 1:1.10 with a cement compound of 105.5 kg/m3 offers optimal stability. Implementing the CSG Dam effectively minimizes material requirements, mitigating the need for excessive exploration of material supplies.


Introduction
The construction of the Cibeet dam, which is planned as an embankment-type dam with an upstream slope of 1:3.50, a downstream slope of 1:30, and a height of 47 meters, is experiencing problems with a shortage of embankment material.It means that it does not meet the construction requirements.This led to a change in the dam type due to materials availability at the planned dam location.
The dominant materials available at sediment locations are silty sand and gravel, so the existing sediment design should be adjusted to these materials.The type of dam suitable for the existing materials is the CSG (Cemented Sand and Gravel) type dam.This dam is the result of innovation in a new type of dam developed in various countries, and there has never been a dam of this type in Indonesia before.
The CSG-type dam uses sand and gravel from the excavation site without any material selection like other sediment types.This sediment comprises existing materials at the collection location in the form of sand and gravel without any complicated material barriers.It only needs to carry out a material enrichment process so that not more than 7 mm and the sedimentary material is mixed with cement with a concrete slump value of 0 and the total volume of concrete in a mixture of 1 m 3 of sand and gravel mixed with cement is 80 kg and also for sand and gravel mixed with cement is 105.5 kg.The safety control of CSG-type sediments currently in Indonesia has not been regulated in the existing sediment guidelines for planning CSG-type sediments using gravity concrete igneous rock molds [7], where the expected conditions are the tensile and compressive stresses of the concrete in the dam body that arises The resulting maximum forces with the compressive strength as 0.9 fc' Kpa and tensile strength as 2.855 (fc') 2/3 Kpa.This research step was carried out with the material gradation test stage at the Cibeet dam location to find out the composition of the existing material without sorting it and then mixing it with cement with a cement composition of 80 kg/m 3 and 105.5 kg/m 3 .From the results of the mixture after 28 days, a concrete compressive strength test was carried out only, and other parameters were taken from general concrete references.

Material and Methods
The materials for CSG are taken from the planned Cibeet dam location.At the initial stage of this test, a material gradation process was carried out to classify the materials for CSG.The results of the material gradation test are shown in Figure 1.The sieve analysis found that the material is silty sand (15%) and gravel (85%).The CSG materials from the planned Cibeet dam location, in the form of sand and gravel, are mixed with cement for the CSG dam materials with a cement composition set at 80 kg and mixed with cement 105.50 kg for every m 3 to obtain a certain quality of concrete at 28 days of age with cylinder specimens and tested for compression strength.These results are used to determine the quality of the CSG constituent concrete, while the composition of the concrete mixture is shown in Table 1.From the compressive strength test of the Cibeet material quarry with cement mixed of 80 kg/m 3 , it resulted in CSG mix with quality 0.92 Mpa (mix design A) and 3.81 Mpa for cement mixed of 105.5 kg/m 3 (mix design B).
The next stage is the stability analysis of the CSG dam carried out in 2 (two) dimensions with the help of Diana FEA software based on plane strain element to determine the maximum stress and deformation that occurred in the dam.To evaluate the effect of CSG dam slope on these parameters, stability analysis was conducted on several geometric models with slopes (vertical to horizontal) 1:0.70, 1:0.80, 1: 0.90, 1: 1.00, 1:1.10 and 1:1.20.Beforehand, it is necessary to prepare parameters for each material in CSG dam, such as conventional concrete materials in Table 2, CSG concrete materials in Table 3, and parameters for the Cibeet dam foundation in Table 4. Stability analysis begins with a dam geometric modeling consisting of the foundation, dam body (consisting of CSG and the Conventional Concrete Layer as protection with 30 MPa concrete quality 2m thick for the upstream, downstream, and bottom of the dam in contact with the foundation), and the boundary conditions are created in the form of support at the foundation base, as well as the loading in the form of a combination of deadweight, hydrostatic pressure, and earthquake loads (time history refers to the centro1940 earthquake) as shown in Figure 2.

Maximum Stress
The results of the maximum stress analysis on the CSG dam using Diana FEA software can be viewed from the X and Y axes, where the X axis describes the upstream direction of the dam, while the Y axis describes the up-down direction.Figure 3A to 3I illustrate the maximum stress that occurs at various dam slopes, for CSG running results with a grade 3.80 Mpa.In the figure above, the left side results from the analysis of the maximum stress that occurs on the X axis (upstream downstream), while the right side results from the analysis of the maximum stress that occurs on the Y axis (up-down).The minimum value (-) indicates compressive stress, while the maximum value (+) indicates tensile stress.The stress value mentioned in the drawing is in units of MN/m 2 or equivalent to MPa (Megapascal).In summary, the stress values shown in Table 5 are the results of CSG cement mixed with 3.80 Mpa (mix design B), while Table 6 is shown the analysis result of CSG mixed with 0.92 Mpa (mix design A). meet these criteria are 1:0.70,1:0.80, 1: 0.90, 1: 1.00, 1:1.10 and 1:1.20.The smallest stress was found in CSG dam with 1: 0.90 slope.Compression and tension strength at dam body with different slopes in the X-axis are shown in Figure 4 and Figure 5, while the strength in the Y-axis are shown in Figure 6 and Figure 7.

Maximum Deformation
The results of the maximum deformation analysis on the CSG dam can be viewed from the X and Y axes, where the X axis describes the upstream direction of the dam, while the Y axis describes the topbottom direction.Figure 8A 7, which is the analysis result of the CSG mix with 3.80 Mpa (mix design B), while Table 8 is the CSG mix with 0.92 Mpa (mix design A).Maximum deformation of dam body in various slope in X-axis and Y-axis at the upstream and downstream are shown in Figure 9 to Figure 12.

Conclusion
This study conducted stability analysis to determine the maximum stress and deformation values observed at the CSG Cibeet dam with different slopes, specifically slopes of 1:0.70, 1:0.80, 1:0.90, 1:1.00, 1:1.10 and 1:1.20.The dam was protected by a conventional concrete outer layer with a strength of 30 MPa.The analysis considered the condition of the sand and gravel material at the dam site, as well as the composition of the Based CSG mixed material with a cement volume of 80 kg/m 3 , resulting in a concrete quality of 0.91 Mpa, do not meet the requirement limit because the stress and strain in CSG concrete (mix design A) is higher.For CSG mix with cement volume 105.50 kg/m 3 (mix design B) resulted in a compressive strength of 3.80 Mpa, and the most optimal slope of the CSG dam to be used is 1:0.90 as it gives the lowest tension strength.

Figure 1 .
Figure 1.Sieve analysis result of Cibeet Dam materials

Figure 2 .
Figure 2. Boundary conditions and Dam Load

Figure 4 .
Figure 4. Compression strength at dam body with different slopes in the X-axis

Figure 5 .
Figure 5. Tension strength at the dam body with the different slopes in the X-axis

8 Figure 6 .Figure 7 .Figure 8I .
Figure 6.Compression strength at dam body with different slopes in the Y-axis

Table 1 .
CSG Material properties of Cibeet Dam

Table 2 .
Conventional Concrete Parameter

Table 4 .
Foundation Parameter

Table 5 .
Maximum Stress for CSG with 3.80 Mpa (mix design B)