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Experimental Study On Clogging Behavior Of PVD In Nansha Soft Clay Improved By Vacuum Preloading

Posted on:2016-04-08Degree:MasterType:Thesis
Country:ChinaCandidate:J H LiFull Text:PDF
GTID:2272330479993744Subject:Structural engineering
Abstract/Summary:PDF Full Text Request
For the reason of specific sedimentary environment, clay and clayey soil in coastal region is easy to form flocculent or alveolate structure that lead to high value of void ratio, moisture content and compressibility. The foundation with such properties often shows low shear strength and low bearing capacity, ground improvement should be conducted before regular engineering construction. So far, vacuum preloading method combining prefabricated vertical drains(PVD) has its greatest use in practical improvement engineering. However, as engineering proceeding with, fine particles in soil will gradually accumulate on filter and in mandrel of PVD, which apparently affect foundation consolidation, causing delay of construction duration and raise in engineering cost.With typical soft clay in Nansha area selected as research subject, through gradient ratio test and single-drain model test, clogging behavior occurred in PVD during particle engineering is simulated. To investigate main effect factors and index of clogging process, filters with varied physical-mechanical properties are chosen for testing. Also Environmental Scanning Electron Microscope(ESEM) is applied to observe clogged filter, that accumulation pattern and microscopic mechanism of fine particles are analyzed during seepage process.According to test results derived from above, it is found that pore characteristics of filter has a close relationship with the clogging behavior. When the pore size of filter is greater than a certain level, fine particles in soil will migrate into drain’s mandrel in large quantities, which causes clogging inide the PVD; while the pore size is too small, fine particles will accumulate in or on the surface of filter, that also causes severe clogging. Compared with cohesiveless soil, apart from mechanical clogging caused by particles blocked in mandrel or filter pore, soft clay particles have strong electrochemical effect with each other that drive them absorbing and condensing on filter surface, which causes microscopic physical-chemical clogging. When the clay appears as gap-graded particles or high clay particles content, severe clogging in PVD will be very likely developed under vaccum preloading, alternative ground improving method should be adopted with high priority. Due to diminution of vaccum pressure caused by bengding and clogging of PVD, the vaccum preloading method exists an effective improved depth, whose value approximately maintains at 15~18m in high compressive soft clay soil.Based on test results and analysis above, modified clogging criteria and modified consolidation theory for vertical drains with consideration of clogging behavior along depth direction of PVD are proposed. Compared with test results, the theory calculation results have produced data essentially in agreement with trends of foundation consolidation and excess pore pressure distributing pattern in soil, that the validity of modified consolidation theory is verified. In addition, the proposed criteria and theory can provide theoretic guide for the III period Nansha Habor foundation improvement engineering, and available design parameters for similar ground improvement engineering in coastal region at future construction.
Keywords/Search Tags:soft soil in coastal region, clogging, vaccum preloading, microscopic study, effective improved depth, modified consolidation theory for vertical drains
PDF Full Text Request
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