Stability and Metacentric Height Analysis of Foam Concrete Pontoons for Floating Portable Toilets in Flood Prone Indonesia

Geographically, Indonesia has a tropical climate with relatively high humidity in most of its territory [1-3]. Due to this climate, Indonesia experiences extreme rainfall during the rainy season, which can have negative impacts such as frequent flooding [4-6]. In particular, high rainfall intensity can lead to excess water that cannot be absorbed, resulting in flooding in an area [7]. As a result, floods have become a severe natural disaster worldwide due to their high frequency, widespread impact, and potential for causing significant losses to human life and property [8, 9].

According to data from the National Disaster Management Agency (BNPB), floods are the most frequent natural disaster in Indonesia, with 1,520 of the 3,522 natural disasters recorded throughout 2022 [10, 11]. In particular, floods are also the most frequent natural disaster in Aceh Province. By early 2023, at least 16 flood incidents had occurred there [12]. This disaster occurs almost every year in Aceh due to a lack of management, which exacerbates the impact of flooding on communities [13]. Moreover, although flood mitigation programs require high costs and a long time, communities in flood-prone areas still need daily infrastructure [14].

When floods occur, conventional sanitation facilities are often damaged. At the same time, Goal 6 of the Sustainable Development Goals (SDGs) highlights the importance of adequate sanitation and clean water facilities [15, 16]. In fact, this goal ensures the availability and management of clean water and sanitation for all communities [17]. Despite this emphasis, providing sanitation facilities in areas affected by natural disasters remains a challenge [18]. For example, in flooded areas, obtaining clean water is very difficult, while sewage and wastewater treatment facilities tend to be clogged [19, 20]. Additionally, conventional portable toilets cannot operate in flooded areas and are difficult to distribute [21].

Existing emergency toilets commonly fail under flood conditions because they are inundated, displaced by flow, difficult to anchor, and vulnerable to capsizing or sewage leakage when water levels rise. These failures can be quantified through draft, buoyancy reserve, metacentric height (GM), off-center load response, and the system's ability to remain usable during inundation. The present study addresses these failure modes by proposing a floating portable toilet supported by foam concrete pontoons, with buoyancy assessed through Archimedes' principle and stability verified using GM analysis [22].

Indonesia's geographical characteristics include many areas prone to flooding, such as estuaries, swamps, and rivers, and have a high potential for flooding [23]. As a result, areas adjacent to these flooded regions tend to lack adequate sanitation facilities, which are characterized by various sanitation problems. Furthermore, these problems are influenced by several factors, including social, economic, and cultural conditions in the surrounding environment [24]. Specifically, issues such as a lack of sanitation facilities, uneven distribution of clean water supplies, and low public awareness and knowledge about maintaining cleanliness are prevalent. Additionally, environmental conditions in densely populated residential areas near beaches, swamps, and rivers pose challenges in providing sanitation facilities [25, 26].

Indonesia still lacks adequate sanitation facilities [27]. Based on data from the Directorate of Environmental Health, Ministry of Health of the Republic of Indonesia, the average national sanitation coverage is 73.9%, and there are 26.1% of sanitation problems that need to be addressed [28]. While these figures highlight the scale of the issue, the consequences are far-reaching. For example, inadequate sanitation facilities, especially those with open drains, directly cause health, environmental, poverty-related, socio-cultural, and educational problems [29]. In addition, many people living near rivers and reservoirs still use pit latrines, which contribute to water pollution. Research shows that more than 90% of Acehnese people use individual waste disposal systems or toilets that discharge directly into water bodies [30].

The high vulnerability to natural disasters and the lack of adequate sanitation facilities have led to the development of floating portable toilets, designed specifically for use in flooded areas [31]. Because currently available portable toilets cannot adapt to changing water levels or be placed above the water surface, communities affected by floods still lack suitable sanitation solutions. Additionally, distributing portable toilets in disaster-affected areas remains challenging [32]. As a result, floating portable toilets are intended for use by communities living near water bodies and evacuees in flooded regions. The design of these toilets requires careful buoyancy planning, as this is essential to maintain stability and balance on the water surface [33].

Various aspects are considered in designing a floating portable toilet, such as dimensions, septic tank operation, floats, materials, and budget estimation. However, the design results require further research regarding the stability of the structure while floating [33]. Consequently, the floating portable toilet is analyzed based on its buoyancy capacity and stability when it is on the water surface. In this regard, the analysis of buoyancy capacity and balance follows the same principles as ship stability. Notably, stability is a very important element in ship calculations, which requires both theoretical and experimental approaches to obtain accurate results regarding loads and disturbances to ship stability [34, 35]. Foam concrete was selected as the pontoon material because of its lightweight properties, high buoyancy, durability in wet environments, cost-effectiveness, and compliance with Indonesian National Standards (SNI 1729:2020 and SNI 03-3499-2002) [36, 37]. The specific dimensions (0.8 × 3.6 × 1 m) were chosen to balance septic tank capacity, user load requirements, and ease of distribution in flood-prone areas, thereby providing a practical and scalable design basis.

Table 5. Stability results for analytical load scenarios

3.2 Experimental validation and modification of lightweight concrete pontoon design

The lightweight concrete pontoon uses a hollow rectangular design with external dimensions of 0.8 m × 3.6 m × 1.0 m and a nominal wall thickness of 0.03 m. During prototype validation, the continuous 3 cm wall over a 3.6 m span was found vulnerable to bending under lateral water pressure, so the pontoon geometry was segmented for fabrication. This prototype modification is reported as a construction improvement only. For the analytical stability recalculation, the manuscript retains the documented substructure mass of 886.752 kg, so the mass budget remains consistent with the stated component values and does not mix design-version masses.

The maximum operating mass of the structure is calculated by adding the dry mass, fecal sludge load, and live load. The dry mass includes the documented substructure mass, superstructure, equipment, and sanitation utilities. No additional connection or installation allowance is included in the calculation.

$\begin{gathered}W_{\max}=W_{\text {dry}}+W_{\text {sludge}}+W_{\text {live}} \\ W_{\max}=3514.822+1072.5+260 \\ \mathrm{~W}_{\max }=4847.322 \mathrm{~kg}\end{gathered}$   (16)

The pontoon design was modified because the lightweight concrete used measured 3.6 meters in length and only 3 cm in thickness. This thin layer of concrete could not withstand the water pressure from the pontoon's sides. It was suspected of experiencing bending or even breaking. The failure resulted from the very thin design thickness and the relatively brittle nature of lightweight concrete. Therefore, the researchers modified the design by adding small supports and dividing the total length into six sections. This reduced water pressure and prevented the concrete from sagging under intense pressure. These changes led to the production of three types of lightweight concrete.

After dividing into three types, each pontoon needed two pieces of type A concrete, four of type B, and four of type C. The assembly process used bolts to connect the parts. Glue was applied to strengthen all components, as shown in Figure 3.

In addition to the mathematical analysis, the foam concrete pontoon was tested in a calm laboratory pool and then in the Titinanjang River, Alue Naga, Banda Aceh. The laboratory test used still-water conditions so that the observed draft and heel response could be compared directly with the static stability model. The pontoon was restrained with guide ropes during placement to prevent lateral drift, but the restraints did not support the vertical load.

Loading was applied gradually to represent the operating scenarios. Equivalent dead loads were positioned according to the assumed user and sludge layouts, including edge and asymmetric load positions. The draft was read after the water surface became calm at the port and starboard sides, and additional visual checks were used to confirm whether excessive heel, rotation, or local submergence occurred. The test was repeated by taking several readings at each loading stage rather than relying on a single instantaneous observation.

The measured draft under the representative loading condition was approximately 40 cm, which was close to the theoretical prediction of 41.7 cm. The deviation was less than 5%, indicating that the analytical displacement model provided a reasonable estimate for the prototype.

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Figure 3. Combination of 3 types of lightweight concrete, (a) combination design, (b) physical combination results

Field testing was then conducted on the Titinanjang River, Alue Naga, Banda Aceh, to examine flotation behavior outside the laboratory setting. The selected river segment had calm flow during testing and sufficient depth for the pontoon to float without touching the riverbed. The pontoon was lifted and lowered using a truck crane from the bridge. A support frame, wooden blocks, and steel slings were used so that lifting forces were distributed and the concrete surface was not damaged.

3.3 Stability testing of lightweight concrete pontoons

In addition to mathematical stability analysis, experimental testing of the floating portable toilet pontoon was conducted to validate the theoretical results. The prototype was tested in a controlled laboratory pool with calm water conditions, without waves or currents. The pontoon was carefully lowered into the pool. Loads were then gradually applied to simulate the weight of users and septic tank sludge. The loading procedure was carried out in stages, starting with no users and reaching a maximum scenario with four users and a full septic tank.

Following the staged loading in the laboratory pool, the pontoon draft was measured using calibrated instruments. The observed draft was approximately 40 cm, which was very close to the predicted value of 41.7 cm from the mathematical analysis. This small deviation (< 5%) confirmed the reliability of the theoretical calculations. The pontoon remained stable throughout the loading process, without excessive tilting or rotation. Visual documentation of the testing process was also recorded, showing the pontoon floating stably under various loading conditions, as presented in Figure 4.

Figure 4. Site planning, construction, and testing of a floating portable toilet pontoon on the Titinanjang River, Alue Naga

These laboratory tests demonstrate that the lightweight concrete pontoon can safely support the designed loads and remain stable under real-world conditions. The consistency between measured and predicted values provides strong evidence that the floating portable toilet design is technically feasible and practically reliable for use in flood-prone areas.

Building on the laboratory findings, field testing of the lightweight concrete pontoon was conducted on the Titinanjang River, Alue Naga, Banda Aceh. This location was chosen due to its calm water conditions and accessibility, allowing the use of a crane from the bridge. A support frame was constructed to facilitate lifting and prevent direct contact between the steel slings and the concrete surface. This preparation ensured that the pontoon could be safely transported and lowered into the river without damage.

During the river test, the slings remained attached as a safety restraint to limit drift caused by the current while still allowing the pontoon to float freely. This condition is reported as a restrained field-flotation test rather than an unmoored free-drift test. The procedure allowed buoyancy and draft response to be observed while reducing the risk of uncontrolled movement, as illustrated in Figure 5.

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Figure 5. (a) Lowering of the pontoon into the Titinanjang River, (b) documentation of the pontoon floating on the water surface

The pontoon was then lifted using a truck crane. Steel slings were carefully attached to the support frame, and wooden blocks were placed across the pontoon to prevent the steel cables from pressing directly on the concrete surface. This step was crucial to prevent structural damage during the lifting process.

From the bridge, the pontoon was slowly lowered into the river until it was fully afloat. The lowering process was carried out carefully to maintain stability, and the slings remained attached throughout the test to prevent movement due to strong river currents. This procedure ensured the pontoon entered the water in a controlled manner.

Draft was measured at the starboard and port sides after the pontoon reached a stable floating condition. The starboard side measured 45.5 cm, and the port side measured 17.5 cm during the restrained river placement, giving an average of 31.5 cm under that uneven external condition. In the later stability test without external lifting effects, the draft was approximately 40 cm, close to the theoretical prediction of 41.7 cm.

The buoyancy capacity of the final configuration was confirmed from the recalculated mass budget and displacement calculation. The maximum operating mass of the complete floating portable toilet is 4847.322 kg. The allowable support capacity after applying the minimum design factor of 1.2 is 5674.897 kg, which remains higher than the maximum operating mass. This corresponds to a gross buoyancy capacity of 6809.876 kg and an achieved gross buoyancy-to-load safety factor of 1.405. Thus, the design still meets the minimum structural safety criterion without adding unsupported residual mass.

After release, the pontoon floated stably. Although slightly tilted due to uneven external loads, it maintained buoyancy and did not sink. This shows that the lightweight concrete provides sufficient buoyancy in real conditions. Building on this initial assessment, further measurements were conducted to evaluate submersion depth. The draft measurement and stability validation are shown in Figure 6.

Figure 6. Draft measurement during pontoon testing

The draft was measured at several points. The starboard side measured 45.5 cm, the port side 17.5 cm, averaging 31.5 cm. In later stability tests without external loads, the draft was about 40 cm, which closely matched the theoretical 41.7 cm. This small deviation (< 5%) confirms the model's accuracy.

Measured and predicted draft values were consistent, with less than 5% difference. The pontoon stayed stable during loading, showing no excessive tilting or rotation. These results validate that lightweight foam concrete works as a buoyancy material in real-world conditions. This provides confidence in the design assumptions. Building on this validation, the next step focused on confirming the pontoon’s buoyancy capacity.

The buoyancy capacity confirmation is therefore based on the corrected maximum operating mass of 4847.322 kg, not on a back-calculated mass that includes unsupported residual loads. This correction keeps the analytical mass budget, buoyancy calculation, and GM results consistent.

Experimental testing of the lightweight concrete pontoons showed an actual draft of approximately 40 cm, close to the theoretical value of 41.7 cm. The deviation of less than 5% supports the reliability of the analytical model for calm-water conditions. This finding extends previous theoretical work on floating portable toilet stability by providing prototype-scale validation and by linking draft observation with a complete component mass budget [40].

The recalculated analytical stability results show positive GM values in all five operating scenarios, ranging from 0.222 m in the maximum-load condition to 0.775 m in the dry condition. Although the revised values are lower than the preliminary calculation because the total displaced volume is now applied consistently, they remain positive and therefore indicate stable equilibrium under small-angle static assumptions. Experimental pontoon tests also produced positive GM values for one- and two-pontoon configurations, supporting the suitability of the two-pontoon arrangement and reinforcing earlier material recommendations for floating portable toilet applications [50].

The achieved gross buoyancy-to-load safety factor of 1.405 exceeds the minimum threshold of 1.2 used in the design criterion [51]. This reserve is important because floating sanitation infrastructure may experience load uncertainty, uneven user positioning, partial sludge accumulation, and additional installation accessories during field deployment. The revised mass budget reduces the risk of overestimating reserve buoyancy because all major loads are now explicitly separated and no unsupported residual mass is inserted.

This study confirms that the revised hollow foam concrete pontoon system can support a floating portable toilet for flood-prone areas under calm-water static conditions. The recalculated maximum operating mass is 4847.322 kg, consisting of substructure mass, superstructure, equipment and sanitation utilities, full sludge load, and four users. The allowable support capacity after applying the minimum design factor of 1.2 is 5674.897 kg, while the corresponding gross buoyancy capacity is 6809.876 kg. This produces an achieved gross buoyancy-to-load safety factor of 1.405, exceeding the minimum design criterion of 1.2. Stability analysis across five load scenarios produced positive GM values ranging from 0.222 m to 0.775 m, confirming that the system maintains positive static stability under the evaluated loading conditions.

Experimental validation in the laboratory pool and in the Titinanjang River showed stable flotation and a measured draft close to the theoretical estimate, with less than 5% deviation. These findings indicate that foam concrete pontoons can function as a feasible buoyant foundation for emergency sanitation infrastructure, especially in communities where conventional portable toilets fail due to inundation, displacement, or deployment difficulty.

For practical deployment, pontoon dimensions should be standardized, the two-pontoon spacing should follow the validated stability model, and installation should include mooring lines, lateral bumpers, balanced access points, and routine inspection of bolts, adhesive joints, waterproofing layers, and septic tank connections. A basic maintenance schedule should include pre-deployment inspection, daily checks during flood operation, sludge-level monitoring, post-flood cleaning, and structural inspection before reuse. Future studies should evaluate hydrodynamic performance under currents, waves, debris impact, wind, and more severe off-center loading.