Feasibility of Vacuum Consolidation in Managing Dredged Slurries with Wheat Straw as Drainage Channels

Guizhong Xu,Xiaojuan Yu,Fahong Wu,Yong Yin 대한토목학회 2017 KSCE JOURNAL OF CIVIL ENGINEERING Vol.21 No.4

It is suggested to use wheat straw as both vertical and horizontal drainage materials when using vacuum consolidation method to process dredged slurry in this paper. To verify the feasibility of this proposal, compression and hydrulic conductivity test on wheat straw and vacuum consolidation tests on slurry using wheat straw as drainage materials were conducted. The results show that the compression process of wheat straw is composed of instantaneous settlement and creep. The density of wheat straw changes from 20 kg/m3 to 200 kg/m3 as vertical stress varies from 5 kPa to 100 kPa. The creep rate increases to maximum when vertical stress reaches 60 kPa and then decreases with as vertical stress further increases. The hydrulic conductivity coefficient of wheat straw is changed between 104 cm/s and 101 cm/s, which decreases linearly as density increases in semi-logarithmic coordinates (hydraulic conductivity in log scale). In vacuum consolidation tests, the settlement, water content, and undrained shear strength of slurry proceeded by vacuum consolidation with wheat straw as drainage materials are similar to those obtained by using PVD and sand as drainage materials. All the results obtained by this paper show that it is feasibility to use wheat straw as vertical and horizontal drainage materials in vacuum consolidation.

Field Study on the Vacuum Preloading of Dredged Slurry with Wheat Straw Drainage

Chao Liu,Guizhong Xu,Bing Xu 대한토목학회 2018 KSCE JOURNAL OF CIVIL ENGINEERING Vol.22 No.11

In this paper, a new shallow reinforcement technique for dredged fill was proposed. In which wheat straw was used as vertical and horizontal drains in lieu of materials applied in the traditional vacuum preloading method. This was not only helpful to avoid permanent environmental damage caused by prefabricated vertical drains but also helpful to relieve the sand shortage and handle the problem triggered by noncontinuous sand cushion layer. On the other hand, this proposal was able to reduce the side effect to the environment triggered by improper handling method on agricultural associated waste greatly, for instance incineration and disposal. Based on prior indoor experimental findings, the feasibility and suitability of the vacuum drainage system composed by straw roller and straw blanket through field test was verified. After doing the comparison between new reinforcement technique and traditional method, it has been found that both the straw blanket and sand cushion performed good in vacuum transmission. Secondly, the vacuum degree of straw roller and prefabricated vertical drains decayed gradually along the depth, but the vacuum attenuation rate of straw roller was slightly smaller than PVD. Thirdly, the performance on the settlement, water content of reinforced soil and undrained shear strength of dredged fill obtained from new reinforcement technique were similar with that generated from traditional vacuum preloading method. Besides, as an agricultural associated waste, the wheat straw had great advantage on financial benefit. Most importantly, unlike PVD, this material is eco-friendly and degradable. Based on these findings, it was suggested that the new reinforcement technique is worth popularizing in real engineering projects.

Effect of Clogging on Large Strain Consolidation with Prefabricated Vertical Drains by Vacuum Pressure

Yupeng Cao,Jianwen Xu,Xia Bian,Guizhong Xu 대한토목학회 2019 KSCE JOURNAL OF CIVIL ENGINEERING Vol.23 No.10

Clogging will inevitably occur while using vacuum preloading method to process super-soft soils with high water content. How to quantitatively evaluate the effect of clogging on the consolidation behavior is a key issue to be solved. Based on NALSC (negative axisymmetric large-strain consolidation) model, considering the “internal clogging” and the “external clogging”, a calculation method by using parameter “clogging rate” to reflect clogging problem is proposed for single-well consolidation theory. Effects of clogging rate and clogging occurred time on Us (degree of consolidation defined by strain) and Up (degree of consolidation defined by stress) are investigated. Differences of the effect of clogging on consolidation degree are determined between large-strain and small-strain consolidation theory. The results show that, the larger the clogging rate, the smaller the degree of consolidation of the soil at the same consolidation time. Effect of clogging on Up is greater than Us. The earlier the clogging occurs, the smaller the degree of consolidation at the same clogging rate. Influences on the degree of consolidation for large-strain and small-strain theory are the same. Consolidation rate of soil is determined to a certain extent by the “generalized clogging parameter μ”. The “external clogging” has a greater influence on the degree of consolidation than the “internal clogging” at the same clogging rate.

A Large-Strain Vacuum-Assisted Radial Consolidation Model for Dredged Sludge considering Lateral Deformation

Yupeng Cao,Jing Zhang,Jianwen Xu,Guizhong Xu 대한토목학회 2020 KSCE JOURNAL OF CIVIL ENGINEERING Vol.24 No.12

A calculation model has been developed for large-strain radial consolidation incorporating lateral deformation in order to solve the problem of poor consolidation prediction for dredged sludge at high water content in vacuum preloading process. The differential solution for the proposed model is given, considering the effect of initial water content on the compression behavior. The model is suitable for unconsolidated soil and normally consolidated soil, which can take into account such factors as lateral deformation, varying compressibility and permeability coefficients, smearing effect, self-weight stress, negative pressure attenuation along the depth, etc. The proposed model is verified by comparing it with other existing theoretical solutions. Poisson's ratio (v) is used to reflect the influence of lateral deformation on soil consolidation. The results show that lateral deformation has an important influence on soil consolidation degree, especially on consolidation degree defined by stress (Up). The consolidation rate increases with the increase of v. The traditional equal-strain solution overestimates the consolidation rate of foundation. At the same normalized elevation, the void ratio increases while the excess pore water pressure decreases with the increase of v.

Effect of Vertical Flow on Consolidation Degree of Foundation with Vertical Drains in Large-Strain Consolidation Theory

Yupeng Cao,Jianwen Ding,Rui Zhang,Guizhong Xu 대한토목학회 2021 KSCE JOURNAL OF CIVIL ENGINEERING Vol.25 No.9

Vertical flow has an important effect on consolidation rate of ultra-soft soil, but the influence law and influence level on the degree of large-strain consolidation have not been quantitatively evaluated. Based on the negative axisymmetric large strain consolidation (NALSC) model, the variation laws of calculation error of consolidation degree ignoring vertical flow with consolidation time and strain are studied under different values of H/re (ratio of soil thickness to influence radius), w0/wL (ratio of initial water content to liquid limit) and kh/kv (ratio of horizontal permeability coefficient to vertical permeability coefficient). Taking 10% as the error threshold, the H/re values ignoring vertical flow are given, and the difference between them in the theories of large-strain and small-strain is discussed. The results show that the effect of vertical flow on degree of stress consolidation (Up) is greater than degree of strain consolidation (Us). The H/re values neglecting the effect of vertical flow on the degree of consolidation in large-strain theory are less than those in small-strain theory. The calculation errors affected by vertical flow decrease with the increase of H/re and kh/kv. Error of Us affected by vertical flowdecreases, whereas error of Up increases with the increase of w0/wL. The effect of vertical flow on the degree of consolidation is greater when well resistance is considered.