Crack development in dredged sediment due to the deformation of vertical drains under intermittent vacuum preloading

doi:10.21203/rs.3.rs-4724796/v1

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Crack development in dredged sediment due to the deformation of vertical drains under intermittent vacuum preloading

https://doi.org/10.21203/rs.3.rs-4724796/v1

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The intermittent vacuum preloading in combination with the deformable drainage system method exhibited excellent performance in treating dredged sediment. The contraction and expansion of vertical drainage system caused the formation of cracks on the surface soil. To reveal the mechanisms of crack development, laboratory model box tests were conducted on dredged sediment with different initial water contents (w₀) under various expansion strains of vertical drainage system (ε). The study investigated the crack development patterns (e.g., crack number, crack length, maximum crack width, and crack area) on surface soil near the drainage system. The results indicated that the expansion of drainage systems gradually resulted in a radial distribution of soil cracks. The threshold strain values of vertical drainage systems corresponding to the initiation of soil cracks gradually increased, with the increase of w₀. The total crack length, total crack area and maximum crack width exhibited a decreasing trend as the w₀ increased. This study also identified a critical w₀ (w₀ < water content at liquid limit) for the formation of cracks on the surface soil. The crack development patterns on the surface soil could be divided into two stages: the slow development stage (ε ≤ 10.0%) and the rapid development stage (ε > 10.0%). The cracks could potentially increase the drainage area, which further enhanced the treatment effect of dredged sediment with high water content.

Physical sciences/Engineering/Civil engineering

Earth and environmental sciences/Environmental social sciences/Sustainability

crack development patterns

deformable vertical drainage systems

dredged sediment

initial water content

laboratory model box test

Dredging activities have significantly increased with the booming economy in recent years (e.g., land reclamation, channel dredging, river and lake management) (e.g., Cai et al., 2017; Zeng et al., 2018; Wan et al., 2022). Dredging is the typically hydraulic method by mixing natural sediments deposited at the beds of sea, river and lake with a large amount of water in China (e.g., Shi and Yin, 2018; Shi et al., 2021; Gao et al., 2023; Zeng et al.,2023). Open-air stockpiles are commonly used for treating dredged sediments. However, due to the high initial water content and low permeability of dredged sediments, the consolidation process is slow, causing an extended occupation of a significant amount of land resources (e.g., Bian et al.,2016; Zeng et al.,2020; Song et al.,2023). To address this issue, the vacuum preloading method has been developed as an efficient way to treat dredged sediment (e.g., Wang et al.,2017; Wu et al.,2021; Lei et al.,2023; Cao et al.,2023,2024). This is because the applied negative pressure to soil samples can accelerate the consolidation.

In practical engineering, plastic drainage boards are commonly utilized along with vacuum preloading to treat dredged sediment with high water content. On the one hand, dredged sediment often contains a high clay content, and the migration of pore water, which carries a large number of fine particles, can cause clogging in the drainage boards during vacuum preloading (e.g., Sun et al.,2022; Lei et al.,2023). On the other hand, dredged sediment exhibits high compressibility, causing substantial soil settlements and severe bending of plastic drainage boards. These two aspects above significantly reduce the efficiency of vacuum transmission in the treated slurry, hence hindering the consolidation (e.g., Xu et al.,2014; Cai et al.,2017). Additionally, after vacuum preloading, plastic drainage boards left in the soil are difficult to degrade over time. This not only affects subsequent ground reinforcement but also leads to environmental pollution ( e.g., Wiewel and Lamoree, 2016; Lin et al.,2023).

Environmentally friendly materials have been gradually replacing plastic drainage boards for treating dredged sediment in recent years (e.g., Liang et al.,2016; Tang et al.,2020; Xu et al.,2022). Natural pine wood can be used as a drainage body in combination with vacuum preloading to treat dredged sediment (Jian, 2022). It has been demonstrated that the inherent stiffness of pine wood can effectively reduce the bending degree in the drainage body, thereby improving vacuum transmission efficiency. Cotton stalks have also emerged as highly effective materials for dealing with dredged sediments, with treatment outcomes comparable to those achieved using plastic drainage boards (e.g., Lv, 2017; Yu, 2017; Yu et al., 2020). Additionally, the combination of wheat straw rolls and vacuum preloading has been proven feasible for treating dredged sediment (e.g., Xu et al.,2017; Liu et al.,2018; Xu et al.,2022). Straw drainage systems, serving as eco-friendly materials, align with the national "dual carbon" strategy by effectively converting agricultural waste straw into a valuable resource (e.g., Fang et al., 2019; Yang et al., 2023). This approach significantly contributes to waste reduction, carbon sequestration, and the promotion of ecological environment. Wheat straw rolls, derived from agricultural crop waste, are processed into rolls using a rolling machine. Short wheat straw (4 cm) wrapped into rolls serves as a substitute for plastic drainage boards in vertical drainage systems, while long straw (40 cm) woven into blankets replace sand bedding as horizontal drainage systems. Results indicate that straw drainage systems for reinforcing dredged sediment are essentially consistent with those using plastic drainage boards (e.g., Xu et al.,2017). Furthermore, compared to plastic drainage boards, straw drainage systems can alleviate clogging phenomena under vacuum pressure. The vacuum preloading method, along with straw drainage system, has exhibited excellent performance in treating dredged sediment (e.g., Xu et al.,2017; Liu et al.,2018).

To enhance the efficiency of vacuum preloading, the intermittent vacuum preloading with vertical drainage system method has been found effective in reinforcing super soft soil foundations (e.g., Wang, 2014; López-Acosta et al.,2019; Xu et al., 2020; Han et al., 2023). Intermittent vacuum loading is different from traditional vacuum loading methods. The traditional vacuum loading is a continuous vacuum loading method. In contrast, intermittent vacuum loading involves loading for a period followed by an interval. After the interval is complete, loading resumes. This method reduces the migration of soil particles, thereby alleviating the clogging in the drainage systems. An improved drain-to-drain vacuum preloading technology has been used to treat very soft clayey embankments, where cracks near the drainage system were observed (e.g., Cai et al., 2018; López-Acosta et al., 2019). Moreover, intermittent vacuum loading with a straw drainage system exhibited superior performance in dredged sediment treatment compared to traditional vacuum loading with prefabricated vertical drains (e.g., Xu et al., 2020). Similarly, cracks also appeared on the surface soil near the straw drainage system. Due to the deformability of the drainage system, when vacuum loading was applied, the drainage system contracted immediately, reducing its diameter. This led to a significant decrease in the water content of the soil around the drainage body, forming a dense sludge cake. When vacuum loading was removed, the drainage body rebounded quickly, compressing the nearby soil and resulting in crack formation in the sample. The formation of these cracks near the drainage system's surface soil is critical for effective dredged sediment treatment. However, the mechanisms of soil crack initiation and propagation near deformable drainage systems remain unclear. Therefore, it is necessary to explore the formation conditions and development patterns of soil cracks caused by the expansion of drainage systems. It can provide theoretical guidance for efficiently treating dredged sediment in engineering practice.

This study aims to investigate the influence mechanisms of deformable drainage systems on crack development patterns in dredge sediments. Firstly, laboratory model box tests were performed on dredged sediment with varying initial water contents under different expansion strains of vertical drainage systems. The soil crack development patterns near the vertical drainage systems were measured by the self-made crack monitoring device, including the crack number, crack length, maximum crack width, and crack area. Secondly, the influence of expansion strain of vertical drainage systems and initial water content of soil samples on crack development patterns was analyzed. Finally, the soil crack initiation mechanisms on the surface soil near the vertical drainage systems were further discussed. This research provides a scientific basis for the improved application of intermittent vacuum preloading methods with deformable drainage systems for treating dredged sediment with high water contents.

2.1 Test materials and test setup

The dredged sediment samples were collected from a storage site, located in Wuhe city, Anhui province, China. The liquid limit (w_L) and plastic limit (w_P) of the dredged sediment were measured using a fall cone penetrometer according to the standard procedure specified in BS1377-2:1990. Table 1 shows the basic physical properties of the dredged sediment. The values of w_L and w_P were 59.8% and 26.9%, respectively. The particle size distributions of the dredged sediment were determined by a hydrometer following ASTM D422-60 (2016). The contents of sand, silt, and clay were 1.4%, 77.4%, and 21.2%, respectively. The soil can be classified as high plasticity clay (CH) based on the Unified Soil Classification System in ASTM D2487 (2010).

Laboratory model box tests were conducted to analyze crack development patterns

Table 1 Basic physical properties of dredged sediment

Liquid limit

(%)

Plastic limit

(%)

Plasticity index

Sand

(> 0.075 mm)

Silt

(0.005-0.075 mm)

Clay

(< 0.005 mm)

59.8

26.9

32.9

1.4

77.4

21.1

of dredged sediment during consolidation. A self-made crack monitoring device with high resolution was installed at the top of the model box. Fig.1 illustrates the schematic diagram of the laboratory model box setup. The model box setup consisted of the model box, a deformable drainage system, a high-resolution camera, and a strain controller, as shown in Fig.1(a). It has been reported that the soil arching effect was most significant when the spacing between piles was 3 to 4 times the pile diameter (e.g., Wang and Sun, 2004; Li and Li, 2021). Hence, to minimize soil arching and boundary effects, the inner diameter and height of the model box, made of acrylic glass cylinders, were 30 cm and 50 cm respectively. Fig.1(b) shows the top view of the model box setup. The radial strain of drainage systems was set by a strain controller, subsequently causing the formation of cracks in the surrounding soil.

Fig. 2 shows the schematic diagram of the deformable drainage systems. These were self-designed acrylic transparent cylindrical tubes with an outer diameter of 7.6 cm and a length of 40 cm. The tubes were uniformly divided into four segments and secured with stretchable rubber bands. The high-resolution camera was used to capture the crack development patterns on the surface soil near the drainage system. The strain controller comprised M6-type nylon rods in cylindrical and conical shapes. During the test, different-sized nylon rods were inserted into the deformable drainage system to obtain the desired strain. The maximum designed strain value was 27%. The detailed dimensions of the nylon rods and their corresponding strain values of the deformable drainage system are provided in Table 2.

Table 2 Detailed dimensions of nylon rods and their corresponding strain values of deformable drainage system

Diameter of nylon rod (mm)	70.4	70.8	71.2	71.5	72.3	73.8	76.1
Strain of drainage system (%)	0.5	1	1.6	2	3	5	8
Diameter of nylon rod (mm)	77.6	79.1	81.4	82.9	84.4	86.7	90.4
Strain of drainage system (%)	10	12	15	17	19	22	27

2.2 Test programme and procedures

The detailed procedures for preparing the model test are listed as follows. Firstly, the dredged sediment was air-dried, crushed, and sieved. Varying amounts of water were then added to create dredged sediment with different initial water contents, which were uniformly stirred. Secondly, the bottom of the deformable drainage system was sealed to prevent any leakage of dredged sediment from the bottom. The drainage system was then wrapped in a filter cloth and fixed at the center of the model box. Thirdly, the dredged sediment with the target water content was poured into the model box in layers, with continuous stirring. For samples with lower water content, compaction was performed after each 5 cm layer was added, using a compaction machine following the quality control method (e.g., Hong et al., 2010). The final height of the soil matched that of the deformable drainage system. Once the dredged sediment filling was complete, the surface soil was smoothed.

A high-resolution camera was fixed above the deformable drainage system using a bracket, with its direction and lighting adjusted to capture clear photos of cracks. Both the camera and the lighting remained fixed during the test to maintain photo consistency. Vaseline was applied to the outer surface of nylon rods. During the test, the conical nylon rod was initially pressed into the bottom of the deformable drainage system, while the cylindrical nylon rod was placed at the top of the drainage system to vertically ensure the constant radial expansion of the drainage system. It was crucial to ensure that the nylon rods were inserted vertically and securely into the sample to avoid any disturbance to the test. After the test, photographs were promptly taken. The duration should not exceed one hour to avoid prolonged exposure of the surface soil, which could potentially alter the water content.

The high-resolution photos were processed using Photoshop (PS) and ImageJ software, as shown in Fig. 3. The image processing procedures included scale setting, grayscale conversion, binarization, noise reduction, and crack statistics. Firstly, the corresponding scale was established based on predefined scales within the images. Secondly, real-time color images were converted into grayscale images (8-bit). Threshold segmentation was then applied to transform the grayscale image into a binary image. The threshold value was chosen to effectively distinguish cracks and soil samples by converting the image into black and white. Through extensive image analysis, it was determined that setting the threshold at 60% of the maximum grayscale value provided optimal results for crack analysis. Thirdly, since soil may contain impurities, some isolated black dots may appear after binarization, which were not cracks. Therefore, manual removal of these artifacts was necessary. Finally, parameters such as the crack number, crack length, maximum crack width, and crack area were analyzed by ImageJ software. The crack width was defined as the maximum width of each crack.

3.1 Influence of deformable drainage system on soil crack development

Fig.4 shows the crack development patterns on the surface soil at w₀=0.7w_L. As the strain of the drainage system increased, the crack parameters such as the number, length, and area all gradually increased. The cracks on the surface soil were mainly distributed within an approximately 5 cm radius of the drainage system and appeared to be flattened and elongated. The direction of crack development was perpendicular to the drainage system. When the expansion strain of the drainage system (ε) reached 3.0%, cracks began to emerge on the surface soil. Notably, one observable crack measured 2.4 mm in length, 0.8 mm in width, and 1.1 mm² in area. When the values of ε were 3.0%, 5.0%, 8.0%, 10.0%, 12.0%, 15.0%, 17.0%, 19.0%, 22.0%, and 27.0%, corresponding crack numbers on the surface soil were 1, 1, 2, 2, 2, 5, 6, 8, 9, and 9, respectively.

Fig.4 shows that the majority of cracks initially appeared at the interface between the vertical drainage system and the sample. This phenomenon resembled stress changes observed in soil near a driven pile during pile-driving. As the pile penetrated the soil foundation, the soil adjacent to the pile progressively reached a plastic state. With the increase in stress, the plastic zone gradually expanded, while the soil beyond the plastic zone maintained its elastic state. Therefore, by assuming the soil was ideal elastoplastic, cavity expansion theory could be used to explain the observed results.

Fig. 5 shows the schematic diagram of the influence zone of adjacent soil near the expanded drainage system. According to the disturbance degree, the soil sample was divided into four regions: the strong disturbance region (A), the moderate disturbance region (B), the elastic compression region (C), and the undisturbed region (D). The strong disturbance region (A) was located close to the drainage system. As the drainage system underwent radial expansion, region A was subjected to the highest compression pressure. Consequently, an extremely high excess pore pressure was generated due to the sudden increase in horizontal stress. Over time, the excess pore water pressure gradually dissipated, leading to a gradual increase in tensile stress within the soil. When the tensile stress of the soil exceeded its tensile strength, it resulted in the formation of horizontal or vertical cracks within this region, ultimately leading to structural damage to the test sample (e.g.,Tian et al., 2022). The moderate disturbance region (B) also encountered significant excess pore water pressure due to horizontal compression. The elastic compression region (C) experienced a lesser impact from the radial expansion strain of the drainage system, with the soil maintaining its elastic state. The undisturbed region (D) of the sample remained unaffected by the deformable drainage system.

For the example with w₀=0.7w_L, when the ε ranged from 0.5% to 2.0%, the sample experienced lower horizontal stress, and the soil remained in its elastic state without exhibiting cracks. However, when the ε reached 3.0%, the soil underwent plastic deformation. Hence, the structure of the soil was disrupted, leading to the initiation of cracks. These cracks continued to expand as the ε increased beyond 3.0%. Upon removal of the drainage system, the crack width on the surface soil diminished, but they persisted as tiny cracks, as depicted in Fig.6. With increasing strain, these remaining tiny cracks reopened and extended outward from the crack tips, concurrently widening in width. The presence of these cracks could increase the water permeability coefficient of the soil (e.g.,

Zhang et al.,2009; Feng et al.,2023). It can be deduced that expanded drainage systems could enhance the permeability coefficient of the soil, facilitating the drainage of dredged sediment. This further indicated the feasibility of utilizing a deformable drainage system to enhance the treatment effectiveness of dredged sediment. As the values of ε increased, some cracks exhibited a reduction in width and even closed during the test, as shown in Fig. 7(a). Due to the amplified radial expansion of the drainage system, the stress in the sample near the drainage system increased. Consequently, the compressed soil expanded

outward, resulting in the closure of these small cracks that had not fully developed. Concurrently, some cracks gradually intersected with others during the test, as shown in Fig. 7(b). Since these cracks were linked to the drainage systems, they further increased the drainage area.

Figs.8 and 9 show the crack development patterns on the surface soil at w₀=0.8w_L and w₀=0.9w_L, respectively. The crack development patterns for these cases were similar to that of w₀=0.7w_L. As the ε increased, the crack parameters such as the number, length, and area of cracks also increased. Additionally, the cracks primarily clustered near the drainage systems. As the ε gradually increased, new branched cracks appeared along the main cracks, propagating in a direction perpendicular to them. This phenomenon was observed in samples with w₀=0.8w_L in Fig. 8 at ε=17.0%, ε=19.0%, and ε=22.0%. At w₀=0.8w_L, when ε was 3.0%, a single small crack appeared on the surface soil, measuring 3.2 mm in length, 0.6 mm in width, and covering an area of 1.2 mm². When ε values were 3.0%, 5.0%, 8.0%, 10.0%, 12.0%, 15.0%, 17.0%, 19.0%, 22.0%, and 27.0%, the corresponding crack numbers on the surface soil were 1, 1, 1, 3, 3, 3, 4, 5, 7, and 7, respectively. At w₀=0.9w_L, when the ε was 8.0%, three small cracks appeared on the surface soil. These cracks were measuring 2.4 mm, 3.6 mm, and 6.5 mm in length, 0.8 mm, 1.1 mm, and 0.9 mm in width, and 5.4 mm², 1.5 mm², and 3.1 mm² in area, respectively. When the values of ε were 8.0%, 10.0%, 12.0%, 15.0%, 17.0%, 19.0%, 22.0%, and 27.0%, the corresponding crack numbers on the surface soil were 3, 3, 5, 7, 9, 9, 9, and 9, respectively.

A heave phenomenon was observed on the surface soil near the drainage system, as shown in Fig.10. This phenomenon resembled the soil-squeezing effect observed during pile sinking (e.g., Ding et al., 2021). It resulted from the radial expansion of vertical drainage system, which disturbed the soil structure and induced plastic deformation in the soil near the drainage system. The expansion volume of the vertical drainage system displaced the soil sample and pushed soil particles upward, causing an uplift of the surface soil.

Fig. 11 shows the crack development patterns on the surface soil at w₀= 1.0w_L. As the radial strain of the drainage system increased, a ground heave phenomenon near the surface was observed. However, no cracks appeared on the surface soil, even at ε = 27.0%. It can be deduced that cracks would also not appear on the soil surface at ε = 27.0% when w₀ was larger than 1.0w_L. This speculative conclusion was consistent with the test results, where no cracks appeared on the surface soil near the drainage system when the value of w₀ was equal to 1.1w_L.

The threshold strain of the drainage system is defined as the strain value corresponding to the initiation of a single crack. Fig.12 shows the typical relationship curves between the expansion strain of the drainage system and the crack development patterns. The crack numbers gradually increased with the expansion strain of the drainage system, exhibiting a roughly quadratic distribution, as illustrated in Fig.12(a). This phenomenon occurred because the sample experienced higher stress, leading to gradual structural deterioration and entry into the plastic zone. When w₀was 0.7w_Land 0.8w_L,

During the initial stages of the test, the crack numbers remained relatively stable. However, as the strain approached 10%, there was a rapid increase in the number of cracks, followed by stabilization. When w₀＝ 0.7w_L, w₀＝ 0.8w_L, and w₀＝ 0.9w_L, the final numbers of observable cracks were 9, 7, and 9, respectively. Fig.12(b) and Fig.12(c) depict the curves between the total crack length, the maximum crack width, and the strain of the drainage system for different initial water contents. As the strain increased, both the total crack length and the maximum crack width gradually increased. When ε = 27.0%, compared to the samples with w₀= 0.8w_L and w₀= 0.9w_L, the total crack length of the samples with w₀= 0.7w_L increased by 80.9% and 113.4%, respectively. The maximum crack width increased by 29.3% and 63.0% respectively. Moreover, a lower initial water content resulted in more pronounced development of soil cracks near the ground surface.

The crack area serves as a crucial indicator for evaluating crack development patterns. Fig.12(d) shows the relationship curves between the total crack area and the strain of the drainage system for different initial water contents. It can be observed that as the strain of the drainage system increased, the crack area gradually increased, exhibiting a bimodal variation pattern. Taking the sample with w₀= 0.7w_L as an example, the crack development patterns on surface soil near the drainage systems can be divided into two stages:

1. When ε ≤ 10%, the cracks were in a slow development stage.

Although the sample near the drainage system underwent plastic deformation, most of the soil outside the plastic zone remained in an elastic state (e.g., Ding et al., 2021). This exhibited a relatively strong capacity to impede crack propagation. The transmission of stress caused by the deformable drainage system was hindered. This resulted in a gradual development of soil microcracks, typically numbering 2-3. During this stage, the total length of cracks increased from 2.4 mm to 42.6 mm, with the maximum crack width expanding by 2.5 times, from 0.8 mm to 2.3 mm. The crack area enlarged from 1.3 mm² to 10.4 mm², signifying an 8.1-fold increase.

2. When ε > 10%, the cracks were in a rapid development stage.

With the radial expansion strain of drainage system, region A was subjected to the higher compression pressure. Initially, the pressure was completely borne by the excess pore water pressure. Over time, the excess pore water pressure gradually dissipated, leading to a gradual increase in the tensile stress of the soil. The increased horizontal stress applied to the sample led to continuous expansion of the plastic zone. When the tensile stress of the soil surpassed its tensile strength, cracks were formed, ultimately causing structural damage to the test sample. At this stage, surface soil cracks developed rapidly, with the crack numbers increasing from 2 to 9. The total crack length expanded from 42.6 mm to 248.3 mm. The maximum crack width increased from 2.3 mm to 26.8 mm, representing a 10.5-fold growth. The crack area grew from 10.4 mm² to 506.8 mm², indicating a 47.6-fold increase. It can be inferred that soil crack development exhibited a threshold concerning the strain of the drainage system.

The above results indicate that cracks mainly initiated near the drainage system, displaying a radial distribution pattern. The crack ratio, a frequently employed parameter for evaluating crack progression (e.g., Tang et al., 2013), was used. The area within a 9 cm diameter from the perimeter of the drainage system was defined as the "crack zone". The method proposed by Tang et al. (2013) was employed to determine the crack ratio of soil samples with varying initial water contents and expansion strains of drainage systems. Fig.13 illustrates the relationship curves between the crack ratio on surface soil and the strain of the drainage system. When the expansion strain was less than 10.0%, the crack ratio of the soil increased slowly as the strain of drainage systems increased. The influence of the initial water content can be negligibly ignored. However, when the expansion strain exceeded 10.0%, the crack ratio on surface soil increased rapidly as the strain of the drainage system increased. The initial water content had a significant impact on the crack ratio of the specimen. Furthermore, when the strain of the drainage system reached 27.0%, the crack ratio on surface soil with w₀= 0.7w_L increased by 216% and 262%, compared to the cases of w₀= 0.8w_L and w₀= 0.9w_L, respectively.

3.2 Influence of initial water content of sample on soil crack characteristics

The initial water content of the sample is also a critical factor influencing crack development patterns. The crack development patterns on the surface soil at different initial water contents are illustrated in Figs.4, 8, 9, and 11. At the same strain value of the drainage system, a lower initial water content correlated with a faster growth rate in crack parameters. This indicates that the threshold strain value of the vertical drainage system for soil crack formation is influenced by initial water contents. For samples with w₀= 0.7w_L, and w₀=0.8w_L, one observable crack began to appear on the surface soil at ε = 3.0%. For the sample with w₀= 0.9w_L, three cracks began to appear at ε = 8.0%. Clearly, with an increase in initial water content, the threshold strain values of drainage system for the formation of soil cracks gradually increased. When w₀= 1.0w_L, no cracks developed on the surface soil due to the significant presence of free water in the sample. As a result, the sample essentially could not withstand shear stress. When the soil reached its liquid limit, it transitioned from a plastic state to a fluid state, and the deformation of the drainage system could not induce crack formation on the surface soil. Hence, the crack formation is related to the initial water content of the soil. For the dredged sediment in this study, w₀ < 1.0w_L was a necessary condition for crack initiation.

Fig.14 illustrates the typical relationship curves between soil crack development patterns on the surface soil and the initial water contents of soil samples. As the initial water content increased, there was a consistent decreasing trend in the values of total crack length, total crack area, and maximum crack width. After the test, when w₀= 0.7w_L, the specimen exhibited a total crack length of 248.3 mm, a total crack area of 500.0 mm², and a maximum crack width of 26.9 mm. Compared to the sample with w₀= 0.8w_L, these values increased by 81%, 193%, and 29%, respectively. Compared to the case of w₀= 0.9w_L, these values increased by 113%, 261%, and 64%, respectively. In the case of w₀= 0.9w_L, although there were slightly more cracks on the surface soil, their total length, total area, and maximum width were all smaller than those in other samples. For soil samples with relatively high initial water content, crack development was not significant and there were primarily micro-cracks and intermediate-sized cracks, as shown in Fig.15. It can be observed from Fig.14(b) that when w₀≥ 1.0w_L(w_L= 63.7%), an increase in the expansion strain of the drainage system would not cause the occurrence of cracks on the surface soil.

From Fig.14, it can be observed that when the strain of the vertical drainage system was relatively small, the initial water content had a minimal impact on the development of surface soil cracks. However, at higher strains, the initial water content significantly affected crack development patterns on the surface soil. For example, in the sample with ε ＝ 8.0%, when w₀= 0.7w_L, the total crack length measured 34.2 mm, with a maximum width of 1.9 mm. This represented a 157.4% and 60.4% increase, respectively, compared to the sample with w₀= 0.9w_L. For the sample with ε ＝ 22.0% at w₀= 0.7w_L, the total crack length extended to 210.7 mm, with a total area of 397.9 mm² and a maximum width of 23.0 mm. Compared to the sample with w₀=0.9w_L, it represented an increase by 107%, 221%, and 73%, respectively. These findings implied that the initial water content was a crucial factor in influencing crack development patterns on the surface soil. The crack development patterns on surface soil were closely linked to both the expansion strain of the drainage system and the initial water content of soil samples.

The laboratory model box tests were conducted on dredged sediments with varying initial water contents under different radial expansion strains of vertical drainage systems. The crack development patterns (e.g., crack number, crack length, maximum crack width, and crack area) on the surface soil near drainage systems were investigated. The mechanisms of strain on vertical drainage systems and initial water content of soil samples affecting on crack development patterns were discussed. The following conclusions are drawn:

(1) Surface soil cracks initiated at the interface between the vertical drainage system and the sample. The direction of crack development was approximately perpendicular to the drainage system. As crack development progressed, radial cracks gradually extended outward. With the increasing strain of the drainage system, new branched cracks appeared along the main cracks and developed perpendicular to them.

(2) All crack parameters gradually increased with the strain of the vertical drainage system. There was a threshold strain value for the vertical drainage system corresponding to the initiation of soil cracks. With an increase in the initial water content of samples, the threshold strain values increased. For the samples with w₀ = 0.7w_L, w₀ = 0.8w_L, and w₀ = 0.9w_L, the threshold strain values were 3.0%, 3.0%, and 8.0%, respectively.

(3) As the initial water content of samples increased, the total crack length, total crack area and maximum crack width exhibited a decreasing trend. Lower initial water content resulted in more pronounced development and faster growth of soil cracks. The formation of cracks on the surface soil depended on a critical initial water content (w₀ < 1.0w_L). When w₀ exceeded 1.0w_L, racks ceased to appear on the surface soil.

(4) The crack development patterns on the surface soil could be divided into two stages: a slow development stage and a rapid development stage. The slow development stage occurred when ε ≤ 10%, during which the number and size of cracks are relatively small. The rapid development stage occurred when ε > 10%, and all crack parameters increased with the horizontal strain of vertical drainage system.

(5) This study suggested that intermittent vacuum preloading, combined with deformable drainage systems, could be an effective treatment for dredged sediment. Deformable drainage systems had the potential to increase the drainage area by forming more cracks on the surface soil, enhancing the consolidation effect of dredged sediment.

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Author Contribution

Y.P. and G.Z. wrote the main manuscript text, M.Y. and Y.C. prepared figures. J.J. and Q.Y. analyzed the data. All authors reviewed the manuscript.

Acknowledgements

This study is supported by the National Natural Science Foundation of China (52078449, 52178347, 52308342), Fundamental Research Funds for the Central Universities (Grant No. RF1028623071), project ZR2021ME068 supported by Shandong Provincial Natural Science Foundation, the funding from the State Key Laboratory of Subtropical Building Science in South China University of Technology (Grant No. 2022ZC01).

Data Availability

The data that support the findings of this study are available on request from the corresponding author.

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Crack development in dredged sediment due to the deformation of vertical drains under intermittent vacuum preloading

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Abstract

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1. Introduction

2. Materials and methods

2.1 Test materials and test setup

2.2 Test programme and procedures

3. Interpretations of test results

3.1 Influence of deformable drainage system on soil crack development

3.2 Influence of initial water content of sample on soil crack characteristics

4. Conclusions

Declarations

Declaration of interests

Author Contribution

Acknowledgements

Data Availability

References

Additional Declarations

Status:

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