Clogging effect of prefabricated horizontal drains in dredged soil by air booster vacuum consolidation
Introduction
Dredging is necessary for maintaining navigable waterways or channels, increasing a channel's water-carrying capacity or allowing larger ships access, and reclaiming soil-contaminated areas. Large amounts of mud slurry are produced in dredging projects. Several hundred billion tons of dredged soil is generated worldwide annually, and this amount has been increasing according to statistics (Dahl et al., 2018). Furthermore, this type of dredged soil has poor physical and mechanical properties, such as a high initial water content, high compressibility, low bearing capacity, and rheological properties (Berilgen et al., 2006). Rapid and effective treatment of this dredged soil is needed to help conserve land resources and protect the environment.
Vacuum preloading is a well-established technology for dredged soil improvement (Bergado et al., 1993, 2002; Chai et al., 2008; Chu et al., 2004; Indraratna et al., 2011; López-Acosta et al., 2019; Quang and Giao, 2014). The combination of vacuum preloading with prefabricated horizontal drains (PHDs) was proposed by Chiba et al. (1992), and this method have been applied to accelerate the consolidation of embankments with clayey backfills (Chai and Duy, 2013; Nagahara et al., 2004) and the vacuum pressure-induced consolidation of dredged soil (Shinsha and Kumagai, 2014). Importantly, the thickness of the soil layer between the PHDs decreases continuously during the consolidation process, when compared to the use of prefabricated vertical drains (PVDs), which not only avoids the PHD bending caused by soil settlement deformation but also shortens the drainage path and accelerates the dewatering and consolidation of dredged soil. However, a typical problem in applying the conventional vacuum preloading (CVP) method to dredged soil (particularly newly dredged soil) is that the drainage is impeded, and permeability is reduced as the improvement progresses; this is called clogging.
A clogging phenomenon mainly occurs near a drain. This area is also called the clogging zone and is characterised by local denseness and low permeability (Wang et al., 2018; Yuan et al., 2018). Some scholars have noted the clogging effect caused by impregnated soil particles in the filter openings (Abuel-Naga and Bouazza, 2009; Lei et al., 2017; Palmeira and Gardoni, 2002; Tai et al., 2017; Wang et al., 2020). Owing to the filter clogging effect, the effective drainage area of the filter surface and drainage capacity of the filter decrease to a certain extent (Chai and Miura, 1999). Furthermore, in addition to filter clogging, recent studies have demonstrated that vacuum-induced fine particle migration and accumulation in the inner zone around the drain might also play an important role in the apparent clogging. This migration is considered to be the relative movement between particles of smaller and larger dimensions, and this relative movement results in significant changes in particle size distribution (Deb and Shiyamalaa, 2016; Indraratna et al., 2013). Some studies have proposed that the blinding effect induced by particle migration around the drain also remarkably affects the radial consolidation rate. Deng et al. (2019) further studied the particle migration of drain consolidation experimentally and argued that particle migration is the reason behind the apparent clogging phenomenon. Moreover, more recent studies have indicated that the apparent clogging effect cannot be fully attributed to the filter clogging and blinding effects; non-uniform consolidation also has a significant impact on the overall consolidation rate (Chai and Zhou, 2018; Chai et al., 2020; Zhou and Chai, 2017). After the commencement of consolidation, the soil near the drain consolidates faster, resulting in a dramatic reduction in the void ratio and permeability of this zone, thereby slowing down the final consolidation rate of the entire soil domain.
In recent years, some in situ field tests have indicated that the air-booster vacuum preloading (AVP) method can satisfy the requirements of a more rapid consolidation effect of the dredged soil (Shen et al., 2011, 2012), which has the following two functions. First, the pressurised air increases the pressure difference between the soil and the PHD and speeds up the drainage rate (Anda et al., 2020; Xie et al., 2020). Second, the pressurised air creates microcracks in the soil and forms a new drainage channel, which helps to solve the clogging problem (Lei et al., 2020). If air booster through PHDs to treat the dredged soil with a high moisture content, PHD bending can be avoided, the clogging problem can be alleviated, and the dredged soil dewatering efficiency can be considerably improved. However, only a few studies have focused on the PHD combined with AVP. Therefore, the application of this method should be researched further.
This study accordingly presents a novel method in which air booster through PHDs under vacuum preloading to reinforce newly dredged soil. The results with respect to the settlement, water discharge, dissipation in pore water pressure, water content, and vane shear strength were obtained, and the clogging problem was quantitatively analysed using the aforementioned indices.
Section snippets
Soil specimens
The dredged soil used in this study was obtained from the Oufei Tideland Reclamation Project of Wenzhou City, a major coastal city in southeast China (Fig. 1). The basic physical properties were determined according to standard laboratory testing. The plastic and liquid limits of the clay were determined to be 23.5% and 50.2%, respectively, and the specific gravity of the soil particles was 2.72. The basic physical indices of the soil specimens are presented in Table 1. The particle size
Vacuum pressure
As the PHDs were vertically laid out layer-by-layer in these tests, each PHD was directly connected to the vacuum pump; thus, the vacuum pressure of each group was nearly the same. The vacuum pressure was directly measured using a vacuum gauge installed on the water–air separation bottle. Fig. 7 shows the relationship between vacuum pressure and time during the tests; the vacuum pressure remained nearly steady at approximately 90 kPa, with only small fluctuations over time.
Fig. 8 presents the
Conclusions
This paper reported a series of laboratory model tests and proposed a novel method that can be applied to the newly dredged soil in the Oufei Tideland Reclamation Project of Wenzhou City, China. Based on a comparison of the results obtained using the proposed method and the CVP method, the following conclusions are drawn:
- 1.
Compared with the conventional method, the pressurised air broke through the clogging layer and created many tiny cracks, thereby providing new drainage channels for free
Declaration of competing interest
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.
Acknowledgements
This study was supported by the National Natural Science Foundation of China (51778501, 51778500, 51878512), the Zhejiang Province Natural Foundation Projects of China (LR18E080001), and the Key Research and Development Program of Zhejiang Province (2018C03038).
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