Feasibility Study of Low-Environmental-Load Methods for Treating High-Water-Content Waste Dredged Clay (WDC)—A Case Study of WDC Treatment at Kumamoto Prefecture Ohkirihata Reservoir in Japan

Abstract

The effective and sustainable treatment of high-water-content waste dredged clay (WDC) remains a significant challenge in water conservancy engineering. In this study, we focused on the treatment of WDC produced by Kumamoto Ohkirihata Reservoir. The study examined the effect of two types of cement-based solidifiers, namely, ordinary Portland cement (OPC) and cement–fly ash agent (DF), on three clay samples collected from different locations. The cone index test was used to assess the samples’ properties. The dosage of cement required for effective improvement with DF was significantly reduced (by about 47–55%), compared to OPC. Moreover, the dewatering efficiency of WDC improved by the simple dewatering method of vertically placing environmental protection materials. Within seven days, the average water content of the WDC decreased to below the liquid limit compared with natural air drying. Finally, the dosage of DF required to stabilize the WDC under effective improvement conditions was reduced by 37–58%, which is higher than the dosage of OPC reduction (22–50%). The reduction in water content reduced the pore space of the soil particles, benefiting the internal bonding of DF-stabilized clay. Dewatering methods facilitate the use of DF solidifiers, facilitating sustainable and environmentally friendly improvement in WDC.

1. Introduction

In Japan, the annual amount of waste dredged clay (WDC) produced by dredging projects is about 2 × 10⁷ m³ [1]. Japan’s land per capita share is low, and the large-scale stacking of WDC has long been impractical. The accumulation of WDC will cause environmental problems, ecological problems, and even drinking water safety problems [2]. WDC usually has the characteristics of a natural water content higher than its liquid limit and high organic content, so its engineering performance is poor. Several environmentally friendly disposal methods for WDC have been identified. They mainly include: use in agriculture, forestry, and aquaculture; dumping on the coast for shoreline stability; and use in aggregate manufacturing and building materials [3].

To enable the large-scale resource utilization of dredged sludge, it is essential to focus on enhancing its mechanical properties. Adding cement or lime to soft clay can enhance its engineering properties by producing a compressive strength binding gel [4], and this method has been widely utilized in infrastructure construction in many countries [5,6,7,8]. However, the extensive use of these traditional solidifying materials has negative environmental impacts. For instance, the leaching of Cr⁶⁺ ions during the hydration process alters the groundwater quality and limits plant growth [9]. Additionally, cement manufacturing is a process that emits a considerable amount of CO₂ [10,11,12]. The annual CO₂ emissions from the cement industry account for approximately 10% of the global total [13]. Moreover, soils containing sulfate undergo excessive expansion and pavement damage after using large amounts of lime or cement stabilization [14]. Given these limitations, researchers have been exploring alternative technologies for stabilizing soil that are more sustainable and effective.

Fly ash is a byproduct of coal combustion in power plants and is extracted from flue gases. Fly ash particles typically consist of hollow spheres of oxides of silicon, aluminum, and iron, as well as unburned carbon [15]. Therefore, fly ash is commonly used in the production of ceramics, geopolymers, and other materials [16]. However, its generation exceeds its utilization. To increase the utilization rate of fly ash and reduce the use of traditional solidifiers such as cement, the use of industrial fly ash as a concrete admixture has been attempted [17,18,19,20]. In addition, the waste oyster shell calcined at high temperatures can be used as quick lime to stabilize WDC [21,22,23]. The effective use of fly ash and waste oyster shell powder will contribute to the sustainable development of this business.

Although environmentally friendly solidifiers such as fly ash and calcined oyster shells have advantages in stabilizing high-water-content WDC, their production technology and equipment costs are relatively high. Therefore, the extensive use of these solidifiers to obtain high-strength and stable clay is neither environmentally friendly nor economical. In order to reduce its environmental impact and improve economic benefits, dewatering treatment should be carried out on WDC before mixing it with solidifiers. Common dehydration methods for liquid clay include mechanical and natural dehydration. However, these methods have limitations. Mechanical dehydration is costly, and additional power consumption leads to greenhouse gas CO₂ emissions. Natural dehydration takes a long dehydration period and requires transportation and storage, bringing environmental problems and increasing management costs. Moreover, in recent years, more and more research has shown that electro-dewatering (EDW) can be used to treat high-water-content WDC [24,25]. This method promotes ion migration, dissolution, precipitation, and other reactions within the sludge by applying an electric field and current, achieving the purpose of treating and solidifying the sludge. However, this method has high energy consumption and technical complexity. For WDC with special properties, more environmentally friendly, cost-effective, and simple recovery methods are still needed.

The Mw 6.2 foreshock and Mw 7.0 mainshock of the 2016 Kumamoto earthquakes occurred at 21:26 JST on 14 April and at 01:25 JST on 16 April, respectively. The earthquake sequence significantly damaged buildings and infrastructure around Kumamoto City [26]. During the investigation after the Kumamoto earthquake, a fault was found directly below the Ohkirihata Reservoir. According to the “Kumamoto Restoration and Rehabilitation 4-Year Strategy issued” by the Kumamoto Prefecture of Japan in December 2016, it was decided to build a new reservoir 237 m upstream of the original Kumamoto Ohkirihata Reservoir. This process produces considerable amounts of WDC sediments, which are classified as waste [27]. During the field investigation, WDC showed a high liquid limit different from other clays, and its natural water content was higher than its liquid limit.

In this study, a scheme for the recovery and treatment of WDC was introduced, taking the waste dredged clay of an Ohkirihata reservoir in Kumamoto Prefecture, Japan, as a case. It aims to improve mechanical properties while achieving a low-carbon environmental protection economy. The accumulation of waste dredged clay to be treated in the field tests was conducted to determine the optimal treatment for the Ohkirihata Reservoir. Three samples of Kumamoto clay were collected from different locations within the reservoir, and they were treated with either cement or a cement–fly ash agent to improve their properties. The treated clay was then subjected to a cone penetration test to measure its cone index values. According to the research results, an on-site dehydration method is proposed to realize the evaporation of water in the clay with low energy consumption by inserting a circulating dewatering material into the on-site dredged clay. This treatment method has the advantages of low CO₂ emission and low cost and has reference value for recycling WDC.

2. Test Procedure

2.1. On-Site Sediment of WDC

To confirm the sediment depth containing WDC in the Kumamoto reservoir, clay water content was measured in the Nos.1–5 areas along both the transverse and longitudinal directions of the reservoir, as shown in Figure 1. Water content values were obtained through a water content test [28] conducted on the sampled clay. The distribution of water content with depth in the Nos.1–5 area is illustrated in Figure 2. In the shallow areas of the reservoir, the depth of clay deposition with water content exceeding 100% was approximately 0–1.2 m below the horizontal position. Notably, locations No.2 and No.3 showed the highest water content (approximately 250%) at a depth of 0.5 m. In the longitudinal positions represented by areas No.4 and No.5, the clay exhibited extremely high surface water content (approximately 200%). In addition, loss-on-ignition (LOI) tests were performed on the clays collected from five different locations (Nos.1–5) to determine their organic matter content after heating them to 750 °C [29]. The results showed that the LOI values of Nos. 1–5 were 28.68%, 30.10%, 35.80%, 22.98%, and 26.15%, respectively. Organic matter begins to burn at a calcination temperature of 200 °C and is completely depleted at 550 °C [30]. Therefore, LOI is considered an important parameter to characterize the content of organic matter [31]. Moreover, high-organic clay typically has an LOI value greater than 20% [32]. It is important to note that expandable clay minerals may release their adsorbed and structural water upon heating during the LOI test, leading to an apparent weight loss [33]. Hence, the possibility of expandable clay minerals should be considered in the sample. Our results indicate that many WDC samples, containing high water and organic matter content, are present within the depth range of 0–1 m in the impoundment area of the Kumamoto Ohkirihata reservoir and within the depth range of 0–1.5 m in the shoal area.

2.2. Preparation of Samples for Testing

Prior to processing on-site WDC, it is essential to conduct a preliminary assessment of its physical properties. In this study, clay samples (Samples A, B, and C) were obtained from three areas illustrated in Figure 1 to examine the physical properties of the WDC. The physical and chemical properties of the test clay samples are shown in Table 1. The test results showed that the clay samples had a high content of fine particles, a relatively high content of organic matter, and a high natural water content (the natural water content of all clay samples was higher than their liquid limit). The liquid limit of the samples was obtained by the plasticity index test of the viscous soil during the test [34].

The actual water content of the clay samples was determined through water content testing, which is equivalent to the natural water content in the initial field condition [28]. The clay was then mixed uniformly with a mortar mixer before adding the solidifying agent and mixing thoroughly until the sample achieved uniform consistency. To prevent moisture evaporation and absorption during testing, the sample was sealed with macromolecular polyethylene and stored under constant temperature (25 °C) and humidity (90%). The sample preparation was conducted following the standard procedure of the Japanese Geotechnical Society for preparing soil specimens using a rammer, known as “Practice for making and curing compacted stabilized soil specimens using a rammer” [35]. The cone penetration test conditions are detailed in Table 2. After 28 days of storage, the clay samples were fully compacted into the mold by allowing a 2.5 kg hammer to freely fall from a height of 300 mm. A compaction layer was created at 40 mm intervals within the mold, and the clay samples were pressed into the mold three times. Each layer was subjected to 25 compaction cycles using the JIS A 1210 A-c method, which has a compaction energy of approximately 550 kJ/m³ [36]. The cone penetration test was conducted immediately after compaction.

2.3. Quantitative Assessment of Sample Strength through Cone Index Test

Two cement-based binders were used to stabilize the WDC samples: ordinary Portland cement (OPC) and a cement–fly ash agent (DF). The main components of the DF binder were 30% cement and 58% coal fly ash. To inhibit the dissolution of natural heavy metals, such as hexavalent chromium ions, in the soil, a heavy metal leaching inhibitor composed of inorganic metals and chemicals was added at a content of 12%.

A cone penetration test, according to JIS A 1228 [37], was conducted to determine the cone index of the clays stabilized with solidifying agents. Table 2 summarizes the experimental conditions for the stabilized clay. The present study is a quantitative experiment on water content. All clay samples were in their natural water content state, i.e., w = w_n. Based on this water content condition, two solidifying agents (DF and OPC) were mixed at amounts of 100, 200, 300, and 400 kg/m³, respectively. The cone index was calculated by dividing the average penetration resistance of the test sample at 50, 75, and 100 mm by the cone bottom area.

It is worth noting that for OPC with a mixing amount of 400 kg/m³, the consolidation strength of all specimens exceeded the measurement range of the load measurement device. As a result, there are no corresponding data presented in the subsequent results. To ensure the accuracy of the test results, each mixing condition was tested at least three times under the same testing conditions. In most cases, the test results were reproducible.

3. Effect of OPC and DF Content on the Stabilization of WDC

To assess the mechanical performance of stabilized clay, the clay generated during construction undergoes classification and effective utilization based on its particle size distribution and strength characteristics, following the “Stabilized Clay Utilization Standards” issued by the Japanese Ministry of Land, Infrastructure, Transport and Tourism. The cone index serves as a strength criterion of the classification. For efficient utilization of stabilized clay, it must surpass the strength criterion of the fourth category of construction clay, where the cone index q_c should exceed 200 kN/m² [38]. However, it is widely recognized that soft clays can present challenges in construction, and various remedial measures have been proposed and adopted, including preloading, dynamic replacement and mixing, and electroosmosis. These measures aim to improve the strength and stability of soft clays and reduce their settlement potential. Further research could explore the effectiveness of these measures in conjunction with the use of solidifying agents, such as OPC and DF, for stabilizing WDC [39].

During the stabilization of high-water-content WDC with OPC, the hydration reaction produces calcium silicate hydrate (CSH) and calcium aluminate hydrate (CAH), and the production of these gel materials enhances the improvement effect of the sample [40,41,42]. In contrast to the stabilization process of WDC by OPC, the fly ash particles in DF are beneficial in reducing the voids in the mixed sample structure and making the microstructure of the sample denser, thereby improving the overall strength of the sample [43,44]. The results of the cone index test for WDC stabilized with solidifying agents under quantitative water content conditions are shown in Figure 3. The cone index of the stabilized soil with both OPC and DF solidifying agents rapidly increased when their respective contents exceeded 200 kg/m³ and 300 kg/m³.

The difference in improvement between the ordinary Portland cement (OPC) and cement–fly ash binder (DF) is primarily due to the sufficient hydration products that favor the embedding of fly ash particles into the gel material voids, thus enhancing the strength of the stabilized clay [45]. Therefore, the test results indicate that DF with only 30% cement content requires a higher mixing ratio to promote water and product generation. However, despite requiring a higher dosage than OPC to achieve a similar improvement effect, the amount of cement required to achieve an effective improvement with DF is significantly reduced (by about 47–55%) compared to OPC. Nevertheless, considering the reduction in CO₂ emissions due to the use of less cement in DF, it is still worth considering its applicability for stabilizing weakly cemented soils.

4. Application of a Simple Vertical Dewatering Device for on-Site Treatment of WDC

4.1. On-Site Dewatering Test

The research by Zhang et al. demonstrated that the stabilization strength of high-water-content dredged sludge using solidifiers is highly sensitive to water content, indicating that improvement in strength is easily affected by water content [46]. Although DF is advantageous in stabilizing high-water-content WDC, it is recommended to pair it with dewatering methods to achieve improvement under conditions of a low solidifier mixing ratio in WDC by reducing the natural water content. In assessing the change in water content under different physical properties, some studies have used liquid limits (w_L) as an important parameter [47]. Previous dewatering experiments have shown that the vertical placement of hemp rope and other dewatering materials can shorten the dehydration period of clay and significantly improve the dewatering efficiency compared to natural air-drying [48,49]. Therefore, the preliminary dewatering of WDC in the field was conducted under vertical drainage conditions using two types of polyester dewatering materials.

Two types of vertical drainage material were used in the tests: woven polyester for the Woven Polyester Drainage case and a chemical polymer mixture rope for the Chemical Polymer Rope Drainage case. The woven polyester was 0.60 mm thick, while the chemical polymer mixing rope had an 8.80 mm diameter. In the Woven Polyester Drainage case, the vertical drainage material with a size of 1.2 m × 1 m was connected to the steel rod with a spacing of 100 mm both horizontally and vertically. For the Chemical Polymer Rope Drainage case, a vertical drainage rope with a length of 1 m was connected to the steel rod with the same spacing. The steel rod was set at the upper end of the square woven polyester bag, which was surrounded by a wire mesh (as shown in Figure 4a). Field test photos of both drainage cases are shown in Figure 4b. Finally, wire meshes and stands with a 100 mm spacing of the drainage materials were incorporated into the sunshine dewatering method of NCPWT to create the simple vertical drain method for field dewatering conditions.

4.2. Effectiveness of Dewatering Test

The WDC used in the field dewatering test was taken from the vicinity of the Sample A sampling site shown in Figure 1, and excavated from the pond without adjusting its natural water content. Subsequently, the soft WDC was poured into two dehydration boxes with different vertical drainage materials. Figure 5 shows the dewatering of WDC under field conditions after some time. Initially, as depicted in Figure 5a,b, the degree of saturation of the clay was high, indicating a wet state. However, after one week of dehydration, the clay surface started exhibiting dryness, as illustrated in Figure 5c,d. In Figure 5c, it is discernible that the area where the dehydrated material is embedded in the clay has a visible gap due to faster water evaporation. After one month of clay dehydration, the clay surface showed an extremely dry state, as shown in Figure 5e,f.

To determine changes in water content in WDC, three sampling points were established at depths ranging from 0.05 m to 0.6 m at each point, with at least eight clay samples taken from different locations or depths to calculate the experimental average water content. Three sampling points were also established in the on-site 1 m high stacked soil of the same WDC, located at the top, middle, and bottom layers, respectively, and ranging in depth from 0.05 m to 0.65 m, to describe the dewatering efficiency of the WDC under natural drying conditions. The average water content was calculated based on the water content of all samples. The experimental results of the water content distribution with depth in Case 1 and Case 2 are shown in Figure 6a,b, respectively. The initial average water content after adding WDC was 145%, while after 7 days, it reduced to 108% in Case 1 and 114% in Case 2. After one month, the average water content decreased further to 84.3% in Case 1 and 86.9% in Case 2. The dehydration methods used in both cases reduced the average water content of the WDC to below the liquid limit within a week. In contrast, after 7 days of natural air drying, the water content of the WDC in the surface layer of the soil heap was close to the liquid limit, while that of the WDC in the middle and lower layers was far from reaching the liquid limit. The pressure head (pF) sensor reflects the soil’s water-holding capacity. After one month, the average water content of the WDC decreased by 35–38% below the liquid limit, and a significant decrease in water content was indicated by a large negative pressure of 20–30 kPa, measured by the pF sensor. This is an ideal result for achieving stability.

5. Evaluation of Improvement Effect of Solidifier-Stabilized Dewatered Clay (WDC)

In previous studies, Zhang et al. proposed a generalized empirical equation for K_L-q_c by introducing a new parameter, K_L. The K_L (K_L = C/(w/w_L)^dL) represents the solidifier correction value considering the influence of water content/liquid limit ratio (w/w_L) [46]. In the empirical equation for K_L, the water content influence factor dL balances the effect of water content and solidifier mixing amount on the improvement strength q_c. Based on this empirical equation, when the water content equals the liquid limit, i.e., w = w_L, K_L = C, the effects of water content and solidifier dosage on the stabilized soil mechanics are consistent. Using this theoretical basis, the linear relationship between strength development and water content at the liquid limit for the three clay samples under the solidifier dosage conditions shown in Figure 3a,b can be evaluated, as depicted in Figure 7a,b.

Figure 7a,b shows that when the water content of solidifier-stabilized dewatered WDC decreased from natural water content to the liquid limit state, and the same strength standard (q_c = 200 kN/m²) was reached, the dosage of DF in Sample A, Sample B, and Sample C decreased by about 190 kg/m³, 100 kg/m³, and 170 kg/m³, respectively, accounting for 58%, 37%, and 57% of the initial dosage. In addition, the dosage of OPC decreased by about 100 kg/m³, 40 kg/m³, and 100 kg/m³, respectively, accounting for 47%, 22%, and 50% of the initial dosage. Compared with OPC-stabilized WDC, reducing the water content of WDC is more beneficial to the strength development of DF-stabilized WDC. This is because when the water content decreases to the liquid limit and below, the loss of free water and some weakly bound water between soil particles makes the soil particle gaps smaller. After adding DF, the chemical bonds between soil particles and their surrounding strongly bound water and some weakly bound water are broken, and then the hydration reaction takes place. The small clay particle gaps are easily filled by fly ash, which enhances the overall improvement strength of stabilized WDC.

The reduction in water content of WDC to the liquid limit significantly reduces the necessary dosage of DF, and consequently greatly reduces the amount of cement needed, thereby reducing CO₂ emissions and construction costs. Therefore, for WDC with high water content, the method of preliminary dewatering using a simple vertical dewatering device before adding the solidifier is more environmentally and economically friendly than the traditional method of directly adding cement or other solidifiers.

6. Conclusions

Field tests revealed that a significant amount of WDC with high water and organic content exists within a depth range of 0–1.5 m from the surface layer. This research investigates the mechanical improvement in such WDC using a low-environmental-load method. The method involves adding cement-based solidification agents (OPC and DF) and using on-site simple vertical dewatering. The study used WDC from the Ohkirihata reservoir in Kumamoto prefecture, Japan, as a case study. The following conclusions were drawn:

• (1). In order to meet the minimum construction standard strength (q_c = 200 kN/m²), it is necessary to use a higher dosage of DF than OPC to stabilize WDC at its natural water content. However, since cement accounts for only 30% of DF, it can be concluded that DF is more environmentally friendly in terms of reducing cement use. In addition, reducing the amount of DF used can be achieved through dehydration.

• (2). Compared to the traditional sun-drying method, a simple on-site vertical dehydration method can reduce the water content of WDC to below the liquid limit within a week. After one month, the average water content of WDC decreases by approximately 35–38% below the liquid limit. Whether it is dehydrated for a week or a month, woven polyester achieved a better dehydration effect than chemical polymer mixed ropes.

• (3). Based on the empirical formula (q_c-C) of the cone index and solidifying agent dosage, the dosage of DF needed to stabilize WDC at the minimum construction standard strength (q_c = 200 kN/m²) decreases by 37–58% as the water content drops from its natural state to the liquid limit. Compared to the 22–50% reduction in OPC dosage, it can be concluded that reducing the water content is more beneficial for stabilizing WDC with DF. Moreover, the significant decrease in DF dosage further reduces CO₂ emissions and production costs.

The method is easy to operate, environmentally friendly, cost effective, and sustainable, with proven feasibility. It has engineering significance for addressing similar problems.

Author Contributions

Conceptualization, K.O.; Methodology, Z.Z.; Investigation, S.O.F.; Writing—Original Draft Preparation, Z.Z.; Writing—Review and Editing, C.L.; Supervision, K.O. and J.L. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The authors acknowledge the support of the Natural Science Foundation of Shandong Province (No. ZR2018LE008, No. ZR2022ME165). The authors acknowledge the financial support, facilities, and equipment provided by the Nagasaki University WISE Program Research Grant.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Tables

Figure 1. Kumamoto Clay Sampling and Test Locations.

Figure 2. Water content variation with depth in areas 1–5.

Figure 3. Under natural water content, the influence of solidifying agent content on the stabilized clay mechanical behavior of WDC.

Figure 4. Setup of field dewatering test of WDC.

Figure 4. Setup of field dewatering test of WDC.

Figure 5. Dewatering situations of waste dredged clay under field conditions.

Figure 5. Dewatering situations of waste dredged clay under field conditions.

Figure 6. Distribution of WDC water content with depth after 7 and 31 days of dewatering.

Figure 7. Evaluation of improvement effect of waste dredged clay.

Table 1

Summary of physical and chemical properties of samples.

Sample Type	Fine Particle Content Fc (%)	Liquid Limit wL (%)	Plastic Limit wp (%)	Plasticity Index Ip	Loss on Ignition LOI (%)	Density ρs (g/cm3)	Natural Water Content wn (%)
Sample A	64.77	121.98	82.12	39.86	23.4	2.32	160
Sample B	65.05	142.00	93.68	48.32	19.2	2.27	152
Sample C	67.46	158.00	96.92	61.08	17.3	2.55	208

Table 2

Experimental conditions for compaction and cone penetration testing.

Type of WDC	Kumamoto WDC (Sample A–C)
Solidifier	DF and OPC
Mold size	Inner diameter is 100 mm, and the capacity is about 0.001 m3
Rammer mass	2.5 kg
Falling height	300 mm (free fall)
Storage time	28 days
Cone penetrometer	Tip angle 30°, bottom area 320 mm2
Penetration rate	1 cm/s
Measuring method	The penetration resistance force was measured at the depths of 50, 75, and 100 mm during penetration, and the resulting average value was divided by the cone bottom area to obtain the cone index.
Measuring instrument	RZTA-1000N (IMADA)

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