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Zhou, Y.;  Chen, S.;  Guo, W.;  Ren, Y.;  Xu, G. Improvement of Vacuum Preloading System. Encyclopedia. Available online: https://encyclopedia.pub/entry/38851 (accessed on 19 May 2024).
Zhou Y,  Chen S,  Guo W,  Ren Y,  Xu G. Improvement of Vacuum Preloading System. Encyclopedia. Available at: https://encyclopedia.pub/entry/38851. Accessed May 19, 2024.
Zhou, You, Shuli Chen, Wei Guo, Yuxiao Ren, Guizhong Xu. "Improvement of Vacuum Preloading System" Encyclopedia, https://encyclopedia.pub/entry/38851 (accessed May 19, 2024).
Zhou, Y.,  Chen, S.,  Guo, W.,  Ren, Y., & Xu, G. (2022, December 16). Improvement of Vacuum Preloading System. In Encyclopedia. https://encyclopedia.pub/entry/38851
Zhou, You, et al. "Improvement of Vacuum Preloading System." Encyclopedia. Web. 16 December, 2022.
Improvement of Vacuum Preloading System
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This entry is adapted from the peer-reviewed paper 10.3390/su142113897

The vacuum preloading system consists of vertical drains, horizontal vacuum pipes embedded in a layer of the sand blanket, membranes, and vacuum pumps. Studies have confirmed that vacuum preloading can effectively accelerate the consolidation process of soft soil. Further improvement in the efficiency of this method is still the continuing goal of scholars.

soil improvement vacuum preloading soft soil consolidation

1. Introduction

The land requirement for the construction of housing, industry and airports and some other infrastructures in coastal areas of the world is being rapidly increased in recent years. Marine clay dredged from the seabed using a cutter suction dredger has been widely used as a filling material for land reclamation in China [1][2]. The vacuum preloading method is often adopted to improve the engineering properties of the newly pumped marine clay slurry due to its advantage of being more environmentally friendly and having lower energy consumption [3][4][5][6]. The conventional vacuum preloading method uses atmospheric pressure as a temporary surcharge and prefabricated vertical drains (PVDs) to shorten the drainage path to accelerate the consolidation process. Compared with the surcharge preloading method using the equivalent loads, the vacuum preloading method is cheaper and faster [7].

2. Improvement of Vacuum Preloading System

2.1. Improvement of PVDs

The method of vacuum preloading combined with short PVDs is used to form a working platform for future soil improvement work at a land reclamation site in Tianjin, China [8]. Short PVDs were installed in square grids with spacings of 0.4 m and 0.6 m, and short PVDs connected using embedded vacuum pipes installed at 0.4 m spacing present a better effect in forming a working platform on the surface of the dredged slurry. The degree of consolidation (DOC) after 60 d of vacuum preloading combined with short PVDs at 0.4 m and 0.6m spacing are 96.2% and 85.1%, respectively.
To improve the newly dredged marine clay layer together with the bottom sediment clay, the sand-free vacuum preloading method can combine with short and long PVDs. Pilot tests were conducted at a construction site in Tianjin port, China, to investigate the performance of this method [9]. The long PVDs are installed into the sediment soil layers in a square pattern with a grid of 0.8 × 0.8 m. The short PVDs are installed in the dredged clay layer with their positions in-between the long PVDs. The vane shear strength profile of the soil improved by the vacuum preloading combined with short and long PVDs was more uniform than those improved by that combined with only long PVDs to improve top dredged clay together with the bottom sediment clay. The maximum undrained shear strength in the depth of 0–10 m from the soil surface can reach 40 kPa. The average DOCs on the 30, 60, and 110 d were estimated as 52.3%, 83.7%, and 90.1% based on pore water pressure data, respectively. A similar layout of short and long PVDs was adopted by Liu et al. [10] to improve the newly dredged fills, and two lengths of PVDs can be simultaneously processed. The improved synchronous and alternate method increases the water discharge by 27.9% compared with the traditional vacuum preloading. The average vane shear strength of 49 kPa with the newly improved method is achieved at the surface. In addition, the better effect of consolidation achieved by the newly improved method is also presented from MIP data.
Considering environmental impacts, a method of using wheat straw as degradable vertical drains and blankets has been proposed in China. The feasibility of this method has been verified by the model tests conducted by Xu et al. [11], and the field test by Liu et al. [12]. The wheat straw drain was fabricated using short straws with their length ranging from 5 to 20 cm and covered by a layer of geotextile. The permeability coefficient of this wheat straw drain ranges from 10−2 to 10−5 mm/s [11]. The fabricated wheat straw drain with a density of 0.121 g/cm3, a diameter of 12 cm, and a length of 3 m were installed in a square pattern with a grid of 0.6 × 0.6 m. The blanket was layered by the wheat straw mat with dimensions of 20 m in length and 3 cm in thickness. It is suggested to weave the wheat straw mat using the wheat straw with a length of more than 50 cm. Four layers of wheat straw blankets were placed across vertically with a total thickness of 12 cm. Two layers of membranes with a thickness of 0.25 mm were covered to provide airtight condition. The field test results showed that the vacuum pressure in the wheat straw blanket can be maintained at 80 kPa after 12 h of vacuum pumping.

2.2. Sand-Free Vacuum Preloading

The surface drainage system of the vacuum preloading method connects the top of the vertical drains to the vacuum pump. It consists of a sand blanket layer and horizontal vacuum pipes. To save the cost of the sand blanket layer in the conventional vacuum preloading method, a sand-free vacuum preloading method has been proposed by Sun et al. [9]. The horizontal vacuum pipes are connected to the end of PVDs by using plastic cable ties and embedded in the excavated trenches with a depth between 30 to 50 cm. The PVDs are installed into the soft clay in a square pattern with a grid of 0.8 × 0.8 m. The edges of the airtight membrane are embedded below the soft clay surface of not less than 1.0 m in the excavated trench and backfilled with in situ clay. The results showed that the sand-free vacuum preloading method of using an airtight tubing system to connect PVDs and horizontal vacuum pipes was effective for improving the 20 m-thick soft clay. The DOCs of 86.4% and 90.1% can be achieved based on settlement and pore water pressure data, respectively. A similar method of using this system was adopted by Zhu et al. [13] to improve the dredged marine clay slurry at a reclamation site at Leqing port zone, China. Field tests show that the average water contents of clay were approximately 50.1 % and 54.4 % in the depth of 0–1.5 m and more than 1.5 m from the soil surface, respectively. The average DOC was more than 88% based on the settlement data. Wang et al. [14] proposed a membrane–connector vacuum preloading method where airtight membranes over the surface of the dredged slurry are used to maintain the vacuum pressure and PVDs are connected to a horizontal non-porous vacuum pipe by a sealed connector. In the model tests, the membrane–connector method with spacings of 0.8 m and the traditional PVD-hose method with spacings of 0.7 m has a similar effect. About 13% cost can be reduced by the decrease in the number of PVDs.

2.3. Membrane-Free Vacuum Preloading

The airtightness of the vacuum preloading system strongly influences the applied vacuum pressure and thus the efficiency of the soil treatment [15]. The conventional vacuum preloading method uses 2–3 layers of airtight PVC membranes to cover the treatment site and imbed their edges in the peripheral trench excavated at least 0.5 m-deep below the ground surface. To save the membrane and avoid the dike constructions in each subsection of the site for a large land reclamation project, the membrane-free vacuum preloading technique using mud to cover the site surface to form the airtight system was proposed by Sun et al. [16]. Because of the low permeability of the covered clay at a land reclamation site in Tianjin, China, a vacuum pressure above 80 kPa can be maintained for a long time under the mud-covering. At the end of the pilot test, the average DOCs of 85.1% and 83.5% can be achieved based on settlement and pore water pressure data, respectively. Field vane shear tests show that the average undrained shear strength of the soil in the very soft marine clay layer and the soft clay layer increased from 5.6 kPa and 14 kPa, to 20 kPa and 30 kPa, respectively. However, there are several disadvantages to these approaches. Firstly, it may be difficult to ensure that each drain is operating at the same vacuum pressure. Secondly, vacuum can only be transported to the soil by vertical drainage [17]. Thirdly, the absence of membranes and deformation of the sealing slurry layer are the main reasons that low pressure was maintained [18].
The sand blanket is an alternative method of forming the drainage system in the membrane-free vacuum preloading system. The PVDs can be connected to the horizontal vacuum pipes directly to enable each vertical drain to work independently and thus in turn eliminate the requirement of a sand blanket [17]. There are several ways to connect PVDs to horizontal vacuum pipes. One method is to wrap PVDs around horizontal vacuum pipes directly. Although vacuum pressure can be transferred from horizontal vacuum pipes to PVDs, this is not a direct connection system. Alternatively, the PVDs can connect to the horizontal vacuum pipes through a specially designed connector to form the airtight tubing system. However, the larger requirements of the connectors greatly increase the connection time and construction cost. The fish-bone shape connecter to connect the drainage pipes and PVDs was proposed by Sun et al. [8].

2.4. Horizontal Drain Vacuum Preloading

A horizontal drain method presented by Park et al. [19] and Kim and Shin [20], which installs plastic drains in the reclaimed ground in the horizontal direction, combined vacuum or gravity preloading to improve the bearing capacity of dredged clay. To reinforce and consolidate the soft soil, a new method of sewing prefabricated vertical drains onto a layer of geotextile sheet, or so-called geotextiles with horizontal drains (GHD), was proposed by the third author to improve the reclaimed ground in the horizontal direction. Using GHDs, the installation of vertical drains is no longer required. The GHDs can be placed in the fill materials, layer by layer, during the fill placement process. Once a layer of GHDs is placed, vacuum pressure can be applied immediately through GHDs for soil consolidation. This is a huge advantage compared to the use of vertical drains where the vacuum or surcharge load can only be applied after all the fill materials have been placed. Furthermore, the fill materials placed on top of the layer of GHDs can now contribute to the surcharge preloading. A 1D consolidation solution was proposed by Zhou et al. [21] considering the vacuum boundary and critical flow gradient.
Another prefabricated radiant drain (PRDs) was proposed by Lei et al. [22] to improve the consolidation effect of the dredged clay. The PRDs were fabricated by horizontally attaching several short PVDs, so-called PHDs, onto a vertically installed PVD. The PRDs were arranged according to the design requirements before filling, or PRDs were rooted into the dredged clay which was in the fluid–plastic state by using manual installation method. The heads of PVDs are directly connected to the horizontal vacuum pipes through a specially designed connector to form the airtight tubing system. It is found by Lei et al. [22] that the effect of vacuum preloading combined with PRD is better than that combined only with PVDs. The PRDs fabricated with horizontal drainage spacing of 0.5 m and installed into the soft clay in a square pattern with the grid of 0.8 × 0.8 m provide the best consolidation effect. Compared with conventional vacuum preloading, the maximum settlement of vacuum preloading with PRD was increased by 88.9 %. However, this method also has the following several disadvantages. For example, it may be difficult to ensure each PHD to be horizontal after installation especially in relative hard soil. Although vacuum pressure can be transferred from PHDs to PVD, this is not a direct connection system that reduces the water flow efficiency.

2.5. Multiple-Step Vacuum Preloading

When using the conventional vacuum preloading technique to consolidate the newly pumped dredged clay, there is often a reduction in drainage capacity due to the clogging of the filter sleeves of PVDs. A multiple-step vacuum applied method has been investigated in recent years through laboratory model tests [23][24][25] and field cases [26][27]. The vacuum pressure could be applied in two-step of 40 and 80 kPa, in three-step of 20, 40, and 80 kPa, and even in five-step of 10, 20, 40, 60, and 80 kPa [27]. The DOC was 88.2% and 94.7% for the one-step and two-step vacuum preloading method based on final settlement data [23], and the average DOC reached 97.8% for the three-step method based on settlement data [25].
Compared with the traditional vacuum preloading method that applies the vacuum pressure in one go, the multiple-step vacuum preloading method could effectively improve the soil strength, delay the formation of clogging by reducing the non-uniform settlement and the radial movement of soil particles [28]. SEM images show that a large number of fine particles accumulate on the surface and inside of filter sleeves resulting in severe clogging in the conventional vacuum preloading method, however, the number of flow channels in filter sleeves increases significantly with the increase in vacuum steps. Moreover, particle analysis tests show that more vacuum steps are beneficial to reduce the movement of fine particles to PVDs under negative pressure [25]. The permeability tests show that multiple-step vacuum preloading can delay the decrease in the permeability coefficient of filter sleeves, thus reducing the aggregation of fine particles around PVDs [25]. However, more vacuum steps require more consolidation time which has to be fully considered in the design stage.

2.6. Air Booster Vacuum Preloading

The air booster vacuum preloading technique was proposed to solve the shortcoming of the conventional vacuum preloading technique in that the PVDs’ drainage capacities were often reduced due to the clogging of the filter sleeves. The effectiveness of this method was verified by the field practices [29][30] and laboratory model tests [31][32][33]. This method includes vacuum and air-booster systems, both of which are sealed by the membrane. The vacuum preloading system has to connect the PVDs directly to the vacuum pipes using the airtight connectors. The air booster system consists of a high-pressure air pump and injection PVC pipe. The high-pressure air applied through the injection PVC pipe washes the fine particles accumulated on the PVD surface, increases the hydraulic gradient, and creates the rapid free water flow into the PVD. The high air pressure may generate fractures in soil which significantly increase the coefficient of permeability and thus speed up the consolidation process. After the high-pressure air pump was activated, the DOC increased from 80% to 85.7%. For adapting the conventional vacuum preloading method, the DOC only increased from 69.3% to 70.1% [30]. However, the disadvantage of this method is that the injected air may reduce the vacuum pressure. The excess pore water pressure in soil reported in the literature is mostly limited to 60 kPa and variable once the high-pressure air is injected into the soil. The booster activation time critically affects the vacuum preloading process with the optimum time to start the air booster system at the soil DOC of 60% [34].

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Contributors MDPI registered users' name will be linked to their SciProfiles pages. To register with us, please refer to https://encyclopedia.pub/register :
You Zhou
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Shuli Chen
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Yuxiao Ren
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Guizhong Xu
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Update Date: 16 Dec 2022
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