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Article

Optimal Mixing Design and Field Application Protocol of Lightweight-Foamed Soils with Waste Fishing Nets

by
Gil-Lim Yoon
1,
Sun-Bin Kim
1 and
Jinung Do
2,*
1
Coastal Development and Ocean Energy Research Center, Korea Institute of Ocean Science and Technology, Busan 49111, Republic of Korea
2
Department of Ocean Civil Engineering, Gyeongsang National University, Tongyeong 53064, Republic of Korea
*
Author to whom correspondence should be addressed.
Appl. Sci. 2025, 15(18), 10207; https://doi.org/10.3390/app151810207
Submission received: 5 August 2025 / Revised: 11 September 2025 / Accepted: 16 September 2025 / Published: 19 September 2025
Browse Figures
Versions Notes

Abstract

Lightweight-foamed soils are mixed soils with foam and cement to enhance the solidity and lightness of soils. Marine wastes, especially waste fishing nets, can be additives to reinforce the engineering properties of lightweight-foamed soils. In this paper, lightweight-foamed soils reinforced with waste fishing nets were investigated. Dredged soil and waste fishing nets were collected and pre-processed for testing. For optimization, the water content, foam ratio, cement ratio, net ratio, net conditions, and curing days were evaluated with respect to workability, unit weight, and strength. The variables were narrowed down based on the performance criteria. The results found that a water content of around 100%, cement ratio of 20%, foam ratio of 5%, and net ratio of 4% with shredded nets provide the best engineering performance of lightweight-foamed soils. The use of nets presented a superior increase in critical strength rather than an obvious increase in peak strength. A normalized factor was used to predict the required strength of lightweight-foamed soils. Finally, this study proposes field implementation methods in terms of the initial conditions of soils and optimal conditions of soils, resulting in the depletion of waste fishing nets.

1. Introduction

Lightweight soils refer to soils with an increased lightness and solidity, achieved by mixing lightweight materials, such as rubber, expanded polystyrene (EPS), and waste tires, and adding cementing agents such as synthetic cement. Lightweight soils provide less settlement and reduce the earth pressure on a structure. Therefore, lightweight soils can be effectively used as backfill materials for structures such as retaining walls and quay walls. Using air foam as a lightweight material is referred to as lightweight-foamed soils (LWFS). Environmental concerns exist over such materials as rubber, EPS, and waste tires, while LWFS has no harmful effects.
LWFS has been studied from the laboratory scale [1,2,3,4,5,6,7] to the field scale [8,9,10,11,12,13]. Moreover, LWFS exhibits an improvement in compressive strength but insufficient tensile strength. Therefore, many researchers have tried to reinforce the engineering properties of LWFS by adding additives. Kim et al. [14] investigated the characteristics of LWFS mixed with waste fishing nets according to the cement ratio, water content, foam ratio, curing days, and net ratio. They confirmed the maximum strength development when the net ratio was 0.25%. Kim and Kang [15] and Yoon et al. [16] mixed LWFS with tire shreds to induce a performance improvement. They also found that a higher compressive strength was developed when waste tire shreds were added compared to LWFS with no tire shreds. Yun and Kim [17] found an improvement in compressive strength when pre-coated waste fishing nets were added to LWFS. Hilal et al. [18] confirmed that the fluidity and strength were further improved by adding silica fume or fly ash compared to conventional methods. Sukkarak et al. [19] and Jiang et al. [20] examined the flexural strength of LWFS using polypropylene (PP) fibers and confirmed that the flexural strength and critical strength increased through the fiber addition. Jiangbo et al. [21] also examined the properties during triaxial shearing by adding PP fibers to LWFS and identified the relationships among the fiber content, adhesion, and internal friction angle. Ren et al. [22] added processed cotton stalks to lightweight soils with EPS and proposed an optimal mixing design. They revealed that cotton fibers most significantly affected strength development, followed by soil, EPS, and cement. Li et al. [23] showed the potential of PP and polyester fibers for the improvement of LWFS.
As a possible fiber for the improvement of LWFS’s tensile strength, waste fishing nets can be considered. The number of marine debris has been increasing every year (Figure 1) [24]. In South Korea, plastic debris takes the largest portion out of other classifications such as metal, wood, paper, rubber, etc. Waste fishing nets, especially, account for 14% out of the total marine debris. Waste fishing nets consist of fibrous plastic threads, which can be effective tensile fibers. In this circumstance, waste fishing nets could be an attractive option for improving the tensile strength of LWFS. Waste fishing nets require no extra material cost except for the pre-treatment process. The recycling of waste fishing nets is closely related to the environmental solution of marine waste. Dredged clayey soils are suitable for LWFS as foams can be easily entrapped within the soil matrix [25]. Thus, the use of dredged soils with waste fishing nets creates a great synergy as both dredged soils and waste fishing nets originate from marine waste. Except for a few studies [14,26], the use of waste fishing nets for reinforcing LWFS does not seem to be deeply explored so far.
This study aims to derive the optimal mixing design of LWFS reinforced by waste fishing nets in view of the engineering criteria, and present a protocol for field applications. The soil type, water content, foam ratio, cement ratio, net ratio, net conditions (e.g., net type and length), and curing days were evaluated as key variables. The optimization criteria were workability, unit weight, economy, and strength. A series of laboratory tests were performed. At the end of this study, a field application protocol was proposed. Note that this study is the extended version of the recent work by Do et al. [26], having more cases and an implementation protocol. The paper aims to seek the possible utilization of waste fishing nets in the area of geotechnical fields.

2. Materials and Methods

2.1. Test Soil

For the base soil of LWFS, dredged soil from the dredging reclaimed land at the Busan New Port construction site in South Korea was collected and transported to a laboratory. The dredged soil showed inconsistent natural water contents of 20% to 30%. Therefore, the collected soil was oven-dried, and pulverized thoroughly using a soil grinder. Basic properties of the soil were measured and summarized in Table 1 [27,28,29]. The test soil has a specific gravity (Gs) of 2.58 and plastic and liquid limits (wP and wL) of 29.7% and 44%. The test soil is classified as a silty soil with low plasticity (ML) based on the unified soil classification system (USCS).
The dry unit weight (γd) of the soil was measured according to water content. The sample showed γd = 9.44 kN/m3 when dried; γd exhibited a peak as 10.34 kN/m3 at w = 45.86%, and then decreased as w increased. At w = 376.27%, γd was 2.47 kN/m3. Refer to Do et al. [26] for further details regarding the dredged soil.

2.2. Waste Fishing Nets

Waste fishing nets collected near a local harbor were pre-treated before testing through washing, natural drying, removal of severe dirt, etc. The deterioration of fishing net by natural weathering takes place with the degradation of material properties [30,31]. For consistent experiments, however, only nets with no severe deterioration based on visual observation were utilized. Table 2 summarizes the basic properties of test waste fishing nets. There are various factors concerning the conditions of the waste fishing nets, such as net types (original or fragmented), the presence of nodes, length, whether they were twisted or untwisted, etc. The factors were considered in the experiments. Note that the degree of nets’ deterioration would influence the experimental results; however, only relatively fresh nets were used in this research for the repeatability. The effect of deterioration needs future research.

2.3. Air Foam and Cement

An air form generator developed by Korea Institute of Ocean Science and Technology (KIOST) was used. The foam was generated using a synthetic resin. The Gs of the foam was 1.03, and the unit weight of the foams was 0.46 to 0.75 kN/m3 with pH = 7.1. Foam has a certain level of resistance to collapse, but the quality of the foam decreases as time passes. Therefore, only the foam generated immediately before the test was used for the consistent quality of the test foam. For the cementing agent, Portland cement was used.

2.4. Evaluations

2.4.1. Fluidity

A flow test was conducted prior to the mold preparation to evaluate the fluidity of the LWFS with waste fishing nets [32]. The mold for the flow test was fabricated by processing a PVC pipe with a diameter of 8 cm and a height of 8 cm. The sample was placed inside the mold, and the level of spreading was measured after the mold was vertically lifted. The average value of the maximum and minimum diameters measured using a rectangular ruler was defined as a flow value of the sample. Note that the construction specification for the pumping and transportation of the mixture is approximately 20 ± 5 cm [33].

2.4.2. Unit Weight

The unit weight (γ) of LWFS is an important criterion as γ should be higher than the unit weight of water with γ that is not too high for lightness [34]. A preliminary test found the γs before and after curing were almost identical, so the γ of samples were measured before the test and defined as the unit weight of the samples. Each mixture was placed in a cylindrical mold with a diameter of 10 cm and a height of 20 cm. Once the surface was flattened, the specimens were cured for strength testing.

2.4.3. Uniaxial Compression Test

After 7, 14, and 28 days of curing, the specimens were taken out from the mold and moved to the pedestal of a uniaxial compression tester (Daeyoung High-tech, Busan, Republic of Korea). The strain of 1% per minute (i.e., 1%/min: 2 mm/min) was used in the experiment. An area correction was made for the test data. In the stress–strain relationship, the peak strength was defined as a uniaxial compressive strength (UCS).

3. Results

3.1. Experimental Variables and Criteria

In the recipe optimization of LWFS reinforced by waste fishing nets, several variables were associated such as the soil type, water content, foam ratio, cement ratio, net ratio, net types, and curing days. Moreover, the workability, unit weight, economic feasibility, and strength were considered as the criteria with which to evaluate the performance (Figure 2). Each component should be understood based on the theoretical and experimental findings.
As the numbers of tests were high and the variables were very complex, the test was denoted as follows: ‘W’ represents the water content, ‘C’ is the cement ratio, ‘F’ is the foam ratio, and ‘N’ is the net type with 4% ratio. ‘N1F’, ‘N4F’, and ‘N4P’ are the net types with a 1 cm filament, 4 cm filament, and 4 cm piece. Details on the selection of net types will be discussed later. ‘NX’ means no addition of nets. The number behind ‘N’ indicates the curing days and the experiment number. All the ratios are based on the soil mass. For example, ‘W88-C20-F5-N1F-28-1’ indicates an LWFS consisting of 88% water content, 20% cement ratio, 5% foam ratio, 4% net ratio with a 1 cm filament, 28 curing days, and #1 specimen. Considering the continuity of the research, refer to Do et al. [26] for further experimental procedures.

3.2. Flow of Optimization

3.2.1. Net Type

A single line of waste fishing nets has a diameter of ~2 mm and 42 filaments when unthreaded. One filament has a diameter of 0.23 mm. The behavior of mixtures would be significantly different whether twisted or untwisted. In this study, only the fragmented net was used, assuming using entire nets is less efficient in a quality perspective.
The length of the net piece was expected to provide a significant impact on the results [35]. Preliminary studies found the following: First, the waste fishing nets are generally composed of squares with a length of ~5 cm. To reduce variables, the lengths of the nets were set to 1 cm and 4 cm. In the absence of nodes, the 4 cm net represents the longest net piece, while the 1 cm net is the shortest. In the case of the 4 cm piece, the twist was maintained. The case in which the twist was maintained was defined as P (piece), while the untwisted nets were defined as F (filament). In the case of 1 cm, however, the piece was not maintained and split into filaments once fragmented. Therefore, for the waste fishing net, the variables were confirmed as 1 cm filaments (N1F), 4 cm pieces (N4P), and 4 cm filaments (N4F) (Figure 3), with no net (NX).

3.2.2. Mixing Sequence

The laboratory experiment results confirmed that the mixing sequence, especially the mixing sequence of foam, has a significant impact on the results [36]. For example, when the foam was mixed before adding water, a rapid foam-collapsing happened during the water-mixing phase. When significant foam-collapsing occurs, the lightness of LWFS is greatly reduced due to the considerable increase in unit weight. When water was mixed first and then foam was added, the foam-collapsing was minimized. Therefore, solid materials (the soil, cement, and net) were mixed first. Water was then added and uniformly mixed with the other materials. The foam was added in the final step. Figure 4 shows the mixing sequence adopted in the experiment.

3.2.3. Foam Ratio

Foams can be merged during the mixing phase, resulting in an increasing bubble size. When the foam–soil mass ratios of 2.5 and 5% were compared, the 2.5% ratio exhibited higher foam-collapsing during the mixing process compared to the 5% ratio. Therefore, the foam ratio was fixed at 5% in most experiments. A foam ratio of more than 5% was not considered due to the lower mixing efficiency (e.g., lightness, material segregation, etc.).

3.2.4. Flow Value According to Water Content

The dredged soil had various initial water contents. In the case of the dredged soil immediately after dredging, the water content can be 100% or higher [37], but a water content of 20 to 30% was observed when the soil was naturally drained after a certain period [38,39]. The test soil showed a natural water content of less than 20%. Under the judgment that it was difficult to reflect various field conditions, we decided to oven-dry the sample and add a certain water content. Since the water content based on a liquid limit (wL) was widely accepted, such an approach (e.g., 1.5wL, 2wL, etc.) was applied in this study [40,41,42].
At the initial water contents of 66% (1.5wL) and 88% (2wL), the flow values were 10.5 cm and 16.9 cm, respectively. At an initial water content of 44% (1wL), the water content was so low to mix. Considering an acceptable workability (20 ± 5 cm), an initial water content of around 88% was mostly used in the experiment.

3.2.5. Flow Value According to Net Type

The workability of the mixture varies depending on the net type. Figure 5 shows the change in flow value according to the net type for the mixture with a water content of 88%, a cement ratio of 20%, a foam ratio of 5%, and a net ratio of 4%. For NX, N1F, and N4P, the flow values were 17.5, 16.4, and 16.6 cm, respectively, which met the flow criterion of 15 cm (Figure 5). In the case of N4F, however, the flow value was 11.7 cm, which could not meet the criterion.

3.2.6. Flow Value According to Net Ratio

As long as the engineering properties are improved, using as many waste fishing nets as possible is beneficial [43]. Figure 6 shows the flow value according to the net ratio for a water content of 88%, a cement ratio of 20%, a foam ratio of 5%, and 1 cm net filaments (N1F). At up to a ~5% net ratio, insignificant flow reduction occurred; however, after a ~6% net ratio, the flow values were less than 15 cm. Beyond 6%, the flow value significantly decreased to 10.8 cm (8%) and 8.8 cm (10%). Based on the experiment results, a net ratio of 6% or less can meet the quality criterion to some extent, but using a net ratio of 6% could be risky considering the uncertainty in the field. Therefore, a net ratio of 4% can be considered as the optimal net ratio.

3.2.7. Unsuitability of Long Net Filament

When the waste fishing nets were cut into a 4 cm length and untwisted into filaments, significant problems were observed in the sample preparation phase. First, excessive time was required to untwist the waste fishing nets. It took approximately three hours to prepare 4% 4 cm net filaments (~60 to 80 g) for a single specimen. This is practically very inefficient unless specific equipment is available. Second, even if N4F was prepared in spite of the difficulty, severe aggregation of the mixing materials, especially by filaments, was observed during the mixing phase. Third, the aggregation caused a low unit weight of mixtures, ~9.3 kN/m3. The low unit weight affected the strength development as well. The uniaxial compressive tests with W88-C20-F5-28 showed 180 kPa for N4P and 60 kPa for N4F at 4% axial strain. Even the flow value for the N4F recipe does not meet the construction specification (Figure 5). Consequently, the cases of N4F are no longer considered.

3.2.8. Cement Ratio

The cement content is a key factor that significantly affects the economic feasibility of LWFS. When the cement ratio is 20% or higher, the economic feasibility significantly decreases [44]. Therefore, the 10, 15, and 20% cement ratios were examined. First, the change in flow value according to the cement ratio was evaluated with the conditions of water content = 88%, a foam ratio = 5%, and net ratios = 0 and 4% (NX, N1F, and N4P). In terms of the results, flow values of 18, 17.8, and 16.6 cm were observed for the cement ratios of 10, 15, and 20%, respectively, meaning there was no significant difference depending on the net type.
The strength of LWFS significantly increases as the cement ratio increases (Figure 7). According to the field quality criterion, a UCS of 100 kPa or higher is required [45]. A value close to 100 kPa was observed at a cement ratio of 15%, but it was judged that a cement ratio of at least 20% could meet the strength criterion, considering the uncertainty in the field. Considering that the economic feasibility is not secured if the cement ratio exceeds 20%, a cement ratio of 20% was confirmed as the optimal cement ratio in this study.

3.2.9. Unit Weight of Mixtures

The γ of LWFS must be lower than the γ of typical soil (15 to 20 kN/m3) and higher than the γ of water (9.8 kN/m3) for stability against buoyancy. Therefore, it is important to secure a certain level of γ, for example, 10 to 12 kN/m3. Figure 8 summarizes the γ of the mixtures according to the water content. In general, the γ decreased as the water content increased. There is no significant difference depending on the net types. Considering the benefit of the weight reduction from LWFS, a γ of ~14 kN/m3 seems unattractive at a water content of 66%. When the water content was between 88% and 110%, the γ ranged from 10 to 12 kN/m3, which was located in the appropriate γ range of LWFS.
Figure 8 showed the γ rapidly increased at a water content of 154%; however, careful observation is required herein. When the water content increases up to 154%, severe material segregation occurred after the specimen preparation. As shown in Figure 9, the thin-cemented surface was observed at the top of the specimen at w = 154% and the surface was easily deformed and broken by a small touch. When the weak surface was removed, a water layer was placed. Below the water layer, finally, a cemented layer existed. As the γ was measured from the cemented layer, Figure 8 showed a higher γ than the actual value. On average, the height reduction happened at 27.3% (16 to 45.5%). Overall, a high water content is inappropriate for the use of LWFS due to the quality degradation by material segregation.

3.2.10. Final Summary with Respect to Water Content and UCS

By confirming a cement ratio of 20% (C20), a foam ratio of 5% (F5), a net ratio (4%), and the net type (NX, N1F, and N4P), the UCS according to the water content (W) was evaluated (Figure 10). The range of the water content was limited from 66% (1.5wL) to 154% (3.5wL). At a relatively low water content of 66%, a high UCS of 500 kPa or more was measured. Compared to NX and N4P, which showed an insignificant increase in strength, N1F exhibited a significantly high strength of 1165 kPa. This indicates that short net filaments had a significant impact on strength development at a water content of 66%. Afterward, the strength sharply decreased as the water content increased.
It is necessary to find an optimal water content that meets both the workability and strength criteria (Figure 10). In the workability perspective (e.g., flow value ≥ 15 cm), a water content of 66% was shown to be unfavorable and the criterion was met at w = 88% or higher. In the strength perspective (e.g., UCS ≥ 100 kPa), the criterion was met at w = 110% or less. Therefore, it is concluded that water contents between 88% and 110% met both conditions and can be chosen for optimal water contents in the mixing design of LWFS reinforced by waste fishing nets.

3.3. Behaviors of LWFS Reinforced by Waste Fishing Nets

3.3.1. Behavior Change According to Water Content and Curing Days

UCS generally increases as the curing day increases and the water content decreases. In the following, the strength characteristics with a cement ratio of 20%, a foam ratio of 5%, a net ratio of 4%, no net (NX), 1 cm net filaments (N1F), and 4 cm net pieces (N4P) were summarized.
For NX, at w = 66%, a clear peak strength and rapid brittle failure behavior were observed (Figure 11a). As the water content increased to 88%, a peak strength of 350 to 550 kPa changed to a peak strength of 50 to 150 kPa (Figure 11b). As the number of curing days increased, the position of the behavior rose. This tendency became more obscure as the water content increased (e.g., 110 and 154%; Figure 11c,d). In particular, when the water content was 154%, the reliability of the behavioral results decreased as the material segregation occurred.
Figure 12 shows the behaviors of N1F. In the case of N1F, a UCS of up to 900 kPa was already observed at 14 days of curing when the water content was 66% (Figure 12a). When the water content was between 88% or 110% (Figure 12b,c), the peak strength was not significantly different from that of NX. The results of a water content of 154% were also not reliable (Figure 12d). Figure 13 shows the behaviors of N4P. The results exhibited an insignificant difference from those of NX.

3.3.2. Strength Characteristics According to Net Types

The behaviors of NX, N1F, and N4P with the same design were compared in Figure 14. At w = 88% (Figure 14a), there was no significant difference in the peak strength according to the net types. While it can be said that the peak strengths of NX and N4P were similar, however, N1F was ~40 kPa higher. It is worth noting that, for NX and N4P, a slow increase in normal stress was observed initially, and a linear increase began from a strain of ~1%. In the case of N1F, however, it can be seen that the linear region, i.e., elastic region, is immediately entered from the beginning of strain. This appears to be because the reinforcement of LWFS with 1 cm net filaments prevented the development of microcracks [46].
At w = 110% (Figure 14b), there was almost no difference in peak strength among the cases. However, the critical strength was shown to be different. For NX, a rapid stress softening phenomenon occurred at a strain of ~5%, and then the stress continuously decreased. At a high strain of 15%, a critical strength of less than 50 kPa was observed. In the case of N4P, rapid stress softening occurred at a strain of ~10%. At a strain of 15%, the critical strength was less than 30 kPa, which was lower than that of NX. Considering the levels of the values, however, it is difficult to expect a large difference in the critical strength between NX and N4P. N1F, however, showed a different behavior. Similar stress softening occurred at a strain of ~11%, but its magnitude is not large compared to NX and N4P. At a strain of 15%, the critical strength was less than 90 kPa, indicating that a significant proportion of the peak strength (110 kPa or less) was maintained. Figure 14 shows that the N1F recipe is favorable whether the water content is low (88%) or high (110%). The efficiency of net reinforcement becomes high when the w is low, while it becomes low when the w increases.

4. Discussions

4.1. Engineering Benefits of Using Waste Fishing Nets

As shown in Figure 10, there is no significant improvement between the addition and non-addition of waste fishing nets. There is also no clear difference between the N1F recipe and N4P recipe. However, the difference appears when the overall behavior of the specimens is examined. For Figure 14a, similar UCS values of up to 200 kPa were observed in all three cases. As the strain increased, the stress-softening tendency became significantly different. For NX, after the peak strength, the normal stress dropped to less than 100 kPa due to the typical stress-softening phenomenon. In the case of N4P, the shear resistance was significantly maintained without rapid stress softening after the peak strength. It exhibited a critical strength of less than 150 kPa at a strain of 15%.
The N1F cases also exhibited the excellent maintenance of shear resistance. It showed a normal stress of smaller than 200 kPa at a strain of 10% and a normal stress of ~170 kPa even at a strain of 15%. This appears to be because the short fiber fragments with a length of approximately 1 cm significantly impacted maintaining resistance after the peak strength by reinforcing the microcracks generated during shear deformation. Therefore, the N1F recipe shows an improved behavior compared to the N4P recipe from an engineering perspective.

4.2. Further Optimization

4.2.1. Practical Benefits of Using Shortly Shredded Nets

Considering the pre-process of waste fishing nets, the use of 4 cm net pieces is unrealistic. There is a net shredder, which makes net-like materials shredded. Through this industrial machine, waste fishing nets is divided into 1 cm or less filaments. The 4 cm net piece was valid for experiments, but not suitable from an industrial perspective. Moreover, several engineering superiorities of N1F were verified by Figure 14. Therefore, it is reasonable to consider shredded short net filaments for the optimal mixing design, rather than using long net pieces.

4.2.2. Water Content Optimization

As shown in Figure 10, water contents between 88% and 110% met the field quality criteria. This section proposed a further optimization for the water content selection through a self-weight consolidation test of the test soil. Figure 15 shows the results of reducing the water content from the initial w = 200% during the self-weight consolidation for 12 days. As reported in Kim et al. [37], that the w of dredged soil was more than 100% immediately after dredging, the initial w = 200% was selected as the starting point. The experiment results showed that the w was reduced to approximately 130% after one day and to 80% or less after three days. After that, the w reduction slowed down. The w was approximately 50% after approximately 12 days.
For the collected dredged soil, it is expected that the water content will drop to 80% or less, even under self-weight consolidation for two days after dredging. This indicates that it is inefficient to use a w of 110%. It seems wise to decrease the water content of the dredged soil after dredging to 80% or less through self-weight consolidation for a few days and increase the w to the optimal level (80 to 90%) at the time of preparing the LWFS. Overall, a level of w = 80 to 90% is proposed as the optimal water content of LWFS.

4.3. Solving Environmental Problems Through LWFS Using Waste Fishing Nets

A net ratio of 4% is optimal with LWFS. Then, it is necessary to see how much 4% is impactful in the environmental perspective. The specimen with a water content of 88%, a cement ratio of 20%, a foam ratio of 5%, and a net ratio of 4%, which was obtained as the optimal design, has a unit weight of 1140 kg/m3. Accordingly, a soil mass of 525.2 kg/m3 and waste fishing nets of 21 kg are required per cubic meter.
Assume that the proposed design is used for a quay wall, a backfill material, or a roadbed material. Marine debris weighing 95,632 tons was collected in 2018, in South Korea. Of the total debris, waste fishing nets comprised 14.1%, i.e., 13,484.11 tons. Assuming that the target ground has a width of B, a height of H, and a length of L. LWFS reinforced by waste fishing nets is used as the ground material; waste fishing nets corresponding to 21 kg/m3 × B × H × L can be used. If an area with a height of 5 m and a width of 10 m is assumed based on the backfill material, all of the fishing net waste generated per year can be used in a construction area with a length of ~13 km. The calculation would be a realistic solution to the waste fishing net, considering the construction scale in South Korea. The utilization of waste fishing nets as ground material is significantly effective in solving immediate environmental issues.

4.4. Field Application Protocol of LWFS Reinforced by Waste Fishing Nets

For the field application of LWFS reinforced by waste fishing nets, there are two approaches: a method using field conditions and a method using the optimal design. The following addresses how to use the methods.

4.4.1. Method Based on Field Conditions

Yoon and Kim [2] reported that UCS could be predicted through the main three factors of LWFS, i.e., water content (W), foam ratio (F), and cement ratio (C), by a normalized factor (NF), as in Equation (1):
N F = W · F C
In this study, the NF was used to quantitatively compare the results of this study with those of previous studies [2,3,6,14]. Only the data in which the values of the variables used and UCS (curing for 28 days) were confirmed were used. In addition, only the cases of dredged soil or clay soils were selected for a consistent comparison. The relationship between the NF and UCS is shown in Figure 16. Note that Kim et al. [14] used unfragmented waste fishing nets. The results are divided into UCSAll, which includes all data, and UCSWFN, which considers only the values with waste fishing nets obtained in this study.
The relationship between NF and UCSAll is presented in Equations (2) and (3):
U C S A l l   [ k P a ] = 7.7 N F − 1.75
N F = 3.21 U C S A l l − 0.57
Here, a mixing design can be derived using Equation (3). For example, when the design UCS value required in the field is assumed to be 200 kPa on the safe side [45], assuming W = 0.9 (90%) and F = 0.05 (5%), the C (cement ratio) is calculated as 0.29 (29%). The prediction indicates that a UCS of 200 kPa can be obtained at a cement ratio of 29% by dredged soil (ML), a water content of 90%, and a foam ratio of 5%.
However, Equation (2) has a low reliability, R2 = 0.42. Consider UCSWFN in Equations (4) and (5):
U C S W F N   [ k P a ] = 8.9 N F − 2.11
N F = 2.82 U C S W F N − 0.47
Equation (4) has a higher correlation, R2 = 0.95, as the results are limited to this study. Assuming the same conditions, for example, UCS = 200 kPa, W = 90%, and F = 5%, C is calculated as 19.2%. It is concluded that a UCS of 200 kPa can be obtained using a cement ratio of 19.2% when a net ratio of 4% is used. Note that the proposed approach includes many uncertainties and limited data. Therefore, a conservative determination is required to use the approach.

4.4.2. Method Based on the Optimal Design

The key results of this study showed that it is possible to produce the optimal LWFS design using dredged soil, a water content of 88%, a cement ratio of 20%, a foam ratio of 5%, a net ratio of 4%, and short shredded waste fishing nets (1 cm). A field application protocol for the use of this optimal design is presented as follows:
  • Dredged soil collection and water content control
For the dredged soil collected, it is necessary to decrease the natural water content to 80% or less through self-weight consolidation for at least two days. The decreased water content is increased to the optimal water contents (e.g., ~88%) by measuring the initial water content before use.
  • Cement/waste fishing nets mixing
When the water content is 88%, the dredged soil content is approximately 500 to 600 kg/m3. After determining the volume of the target ground, a cement ratio of 20% and a net ratio of 4% are mixed first. The waste fishing nets are recommended to be shredded before mixing. A commercial mixer can be used to mix the materials; however, the machine should be large enough to accommodate the quantity of the materials.
  • Foam mixing
A foam ratio of 5% based on the calculated soil mass is finally mixed with the pre-mixed materials. Before field application, the flow value of the mixture is measured to secure 15 cm or higher. An empirical guideline such as for the mixing speed, time, amount of batch, etc. should be suggested for the quality control of mixtures with foam.
  • Field implementation
The mixture is applied to the target site as quickly as possible to prevent foam-collapsing. For the applied LWFS reinforced by waste fishing nets, the quality of the mixture needs to be checked through field evaluation methods (e.g., the plate load test, penetration testing, etc.) at 28 days of curing.

5. Conclusions

This study investigated an optimal method for reinforcing lightweight-foamed soils (LWFSs) with waste fishing nets. Various experimental attempts were made and invested in the design of mixing optimization. The results are summarized as follows:
  • It is favorable to use shredded nets for reinforcing LWFS. Long pieces of waste fishing nets have low effectiveness and are not favorable in terms of performance. The optimal net ratio is 4%. A net ratio of up to 5% can be acceptable, but the workability problem occurs from 6%.
  • Air foam needs to be added at the end of the mixing process to minimize foam-collapsing. If added at the beginning of the process, the unit weight of the mixture is increased by excessive foam-collapsing, resulting in reducing the lightness. The optimal foam ratio is 5%.
  • The cement ratio is recommended as 20% in the consideration of economic feasibility. At a cement ratio of 15% or less, the strength of the mixture significantly decreases.
  • When the cement ratio is fixed at 20%, the water content plays the most critical role in making the final decision on LWFS. The strength and workability significantly vary depending on the water content. This study found 88 or 110% to be acceptable water contents. It is recommended that we reduce the water content through self-weight consolidation and meet the optimal water content of 88% by adding water before mixing.
  • It is possible to calculate the water content, foam ratio, and cement ratio for an optimal mixing design using the normalized factor. Since the proposed relationships significantly varies depending on the mixing method, skilled workmanship is essentially required.
  • In fact, LWFS reinforced by waste fishing nets is not absolutely attractive for strength improvement. However, reinforcement with waste fishing nets for LWFS has two benefits: the first is an increase in the critical strength, and the second is the recycling of waste fishing nets. The researchers associated with this study hope the proposed idea can be considered in future construction.

Author Contributions

Conceptualization, G.-L.Y., S.-B.K. and J.D.; methodology, G.-L.Y., S.-B.K. and J.D.; formal analysis, J.D.; data curation, J.D.; writing—original draft preparation, J.D.; writing—review and editing, G.-L.Y. and S.-B.K.; visualization, J.D.; supervision, G.-L.Y.; project administration, G.-L.Y. and S.-B.K.; funding acquisition, G.-L.Y. and S.-B.K. All authors have read and agreed to the published version of the manuscript.

Funding

This research work was supported by the project titled “Development of Advanced Technology for Ocean Energy, Harbor and Offshore Structure (PEA0321)” funded by the Korea Institute of Ocean Science and Technology (KIOST), Korea.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Some or all of the data, models, or code that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Collected marine wastes and their contents in South Korea. Waste fishing nets represent 14.1% of the total waste (source: www.meis.go.kr by Korea Ministry of Oceans and Fisheries).
Figure 1. Collected marine wastes and their contents in South Korea. Waste fishing nets represent 14.1% of the total waste (source: www.meis.go.kr by Korea Ministry of Oceans and Fisheries).
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Figure 2. Variables and criteria for designing LWFS reinforced by waste fishing nets.
Figure 2. Variables and criteria for designing LWFS reinforced by waste fishing nets.
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Figure 3. Three net types used in the test. N1F is 1 cm untwisted net filament, N4F is 4 cm untwisted net filament, and N4P is 4 cm twisted net pieces.
Figure 3. Three net types used in the test. N1F is 1 cm untwisted net filament, N4F is 4 cm untwisted net filament, and N4P is 4 cm twisted net pieces.
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Figure 4. Sequence of mixing (in order of soil–cement–net–water–foam).
Figure 4. Sequence of mixing (in order of soil–cement–net–water–foam).
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Figure 5. Change in flow value according to net types. N4F shows thickened behavior due to entangling of 4 cm filaments.
Figure 5. Change in flow value according to net types. N4F shows thickened behavior due to entangling of 4 cm filaments.
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Figure 6. Flow value according to net ratio.
Figure 6. Flow value according to net ratio.
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Figure 7. UCS of LWFS according to cement ratio (water content of 88%, a foam ratio of 5%, and net ratios of 0 and 4%).
Figure 7. UCS of LWFS according to cement ratio (water content of 88%, a foam ratio of 5%, and net ratios of 0 and 4%).
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Figure 8. Unit weight according to water content and net types.
Figure 8. Unit weight according to water content and net types.
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Figure 9. (a) Serious material segregation observed on the surface at a water content of 154%, (b) water layer on the surface, and (c) cemented zone below the soft zone on the surface.
Figure 9. (a) Serious material segregation observed on the surface at a water content of 154%, (b) water layer on the surface, and (c) cemented zone below the soft zone on the surface.
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Figure 10. UCS according to water contents (a cement ratio of 20%, a foam ratio of 5%, and a net ratio of 4%) and net types (no net (NX), 1 cm filaments (N1F), and 4 cm pieces (N4P)).
Figure 10. UCS according to water contents (a cement ratio of 20%, a foam ratio of 5%, and a net ratio of 4%) and net types (no net (NX), 1 cm filaments (N1F), and 4 cm pieces (N4P)).
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Figure 11. Behaviors of no net specimens (NX) according to water content/curing days.
Figure 11. Behaviors of no net specimens (NX) according to water content/curing days.
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Figure 12. Behaviors of 1 cm net filament specimens (N1F) according to water content/curing days.
Figure 12. Behaviors of 1 cm net filament specimens (N1F) according to water content/curing days.
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Figure 13. Behaviors of 4 cm net filament specimens (N4P) according to water content/curing days.
Figure 13. Behaviors of 4 cm net filament specimens (N4P) according to water content/curing days.
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Figure 14. Behaviors of NX, N1F, and N4P with same recipe: (a) w = 88% and (b) w = 110%.
Figure 14. Behaviors of NX, N1F, and N4P with same recipe: (a) w = 88% and (b) w = 110%.
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Figure 15. Dewatering of test soil during self-weight consolidation.
Figure 15. Dewatering of test soil during self-weight consolidation.
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Figure 16. UCS according to normalized factor [2,3,6,14].
Figure 16. UCS according to normalized factor [2,3,6,14].
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Table 1. Basic properties of test soil.
Table 1. Basic properties of test soil.
GsFine Content [%]wP [%]wL [%]USCS
2.5882.629.744.0ML
Table 2. Basic properties of test waste fishing net.
Table 2. Basic properties of test waste fishing net.
MaterialFilament Diameter [mm]Number of FilamentFiber Diameter [mm]Section Area [mm2]Tensile Strength [MPa]Density [g/cm3]
HDPE0.234221.74303.810.95
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Yoon, G.-L.; Kim, S.-B.; Do, J. Optimal Mixing Design and Field Application Protocol of Lightweight-Foamed Soils with Waste Fishing Nets. Appl. Sci. 2025, 15, 10207. https://doi.org/10.3390/app151810207

AMA Style

Yoon G-L, Kim S-B, Do J. Optimal Mixing Design and Field Application Protocol of Lightweight-Foamed Soils with Waste Fishing Nets. Applied Sciences. 2025; 15(18):10207. https://doi.org/10.3390/app151810207

Chicago/Turabian Style

Yoon, Gil-Lim, Sun-Bin Kim, and Jinung Do. 2025. "Optimal Mixing Design and Field Application Protocol of Lightweight-Foamed Soils with Waste Fishing Nets" Applied Sciences 15, no. 18: 10207. https://doi.org/10.3390/app151810207

APA Style

Yoon, G.-L., Kim, S.-B., & Do, J. (2025). Optimal Mixing Design and Field Application Protocol of Lightweight-Foamed Soils with Waste Fishing Nets. Applied Sciences, 15(18), 10207. https://doi.org/10.3390/app151810207

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