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Preloading with wick drains
Published in Jay Ameratunga, Sivakugan Nagaratnam, M. Das Braja, Soft Clay Engineering and Ground Improvement, 2021
The ground improvement with or without wick drains with conventional preloading is generally cost effective compared to many other ground improvement techniques. However, sometimes it requires a significant height of preload when,the clay thickness is high; and/orthe target time for preloading is low; and/orstringent settlement criteria apply.High surcharges create issues of instability and excessive lateral displacements. Hence, some other means of treatment is required to mitigate such issues. One of the methods that could be adopted is vacuum consolidation in conjunction with reduced surcharge. The main difference between the conventional preloading and vacuum preloading is that the former will increase the total stress while the latter will reduce the pore pressure while maintaining a constant total stress as illustrated in Figure 9.19. The main advantages of the vacuum consolidation technique are:Increased stability;Significantly reduced lateral displacements;Reduction in the required surcharge by 3.5 m to 4.0 m if a vacuum pressure of 70 to 80 kPa could be maintained thereby reducing the fill transport costs significantly.The fundamental procedure of vacuum consolidation process consists of removing atmospheric pressure from a confined sealed (membranes/cut of wall) medium of soft soils to be consolidated and maintain the vacuum pressure for a predetermined preloading time. A typical vacuum system is illustrated in Figure 9.20.
Rapid treatment of high-water-content dredged slurry using composite flocculant and PHD-facilitated vacuum
Published in Marine Georesources & Geotechnology, 2022
Dibangar Khoteja, Yang Zhou, Hefu Pu, Youfu Pan
A substantial amount of water can be drained out just in the sedimentation stage for Cases B and C due to the flocculation effect. Cases B and C have similar dewatering trends in the entire test duration, with no supernatant above the soil surface. However, in Case A, a much smaller amount of water is drained out during the sedimentation stage due to the slow settling process of the natural, chemically untreated clay slurry in Case A due to the absence of a vacuum (in sedimentation stage) as well as flocculant. In the vacuum consolidation stage, the dewatering in Case A is also slower than that in Cases B and C, creating supernatant above the soil surface. In the vacuum consolidation stage, the vacuum pressure yields a greater consolidation effect near the PHD and a lesser consolidation effect on the soil domain farther away from the PHD, because of non-uniform consolidation effect (Zhou and Chai 2017). This creates a gap between the soil mass and the inner wall of the model box in the lower soil zone, and this gap develops with time and ultimately extends to the soil surface, after which the supernatant above the soil surface is quickly drained out. In Case A, no flocculant is used and thus water-soil separation occurs slowly and a considerable amount of supernatant accumulates above the soil surface as time elapses, which later is suddenly removed from the gap when the gap extends to the soil surface, manifest as a sudden change in the dewatering rate (Figure 7). In Cases B and C, however, a substantial amount of supernatant is formed during the short period of sedimentation stage (due to flocculation effect) and is then removed before the vacuum consolidation stage begins, after which no supernatant is accumulated above the soil surface, and therefore no sudden change is observed. Dewatering is the fastest in Case C and the slowest in Case A because flocculation forms larger flocs and thus creates larger voids that increase the flow rate of the pore water (Wang et al. 2017, 2018). Additionally, flocculation prevents fine particles from accumulating at the filter of the PHD (Hogg 2000), which reduces clogging and thus results in faster dewatering. Dewatering by flocculation and vacuum pressure is very fast in Cases B and C (taking only 247hours and 207hours, respectively), whereas it takes 411hours for Case A. This indicates that the use of PAM enhances the dewatering rate and that the addition of lime can further increase the dewatering rate and ultimately result in a much shorter treatment duration.