| Newly hydraulic reclamation mud was formed, in a suspended or fluid state, after the dredged mud was hydraulic remodeled and re-sorted. The weight of soil particls deposition has not been completed, the grain structure very loose, the moisture content very high ranging from 85% to 150% or being even more than 150%. At present, the newly hydraulic reclamation mud foundation was often improved by a secondary treatment technology. The secondary treatment technology was that when the mud foundation was pre-improved for 30 days by the rapid treament technology of shallow mud foundation, the fine(very fine) sand was hydraulic reclamed with the thickness of 1.0m and then the medium coarse sand was mechanical layed or hydraulic reclamed with the thickness of abhout 0.5m; it can been unloaded when it was pre- improved for 45 days, and then the pre-improved mud foundation can be a working platform for the PVD construction equipment; finally, the deep soil of the mud foundtion can be improved by the traditional vacuum preloading technology or vacuum combined with surcharge preloading technology.The two main reasons were for using a secaondary treatment technology that:(1) the bearing capacity was zero of the newly hydraulic reclamation mud foundation, which cannot directly bear the construcion machinery and equipment, such as bulldozers, flapper,etc.(2) the content was very high of the soil particle with the diameter less than 0.005 mm, and then the deposition was slowly; the content of the strongly hydrophilic mineral was very high,and the content was high of the adsorbed bound water. It can be not approprate to the inherent engineering characteristic for the related performance indicators of the vertical drains including drainage performance indicators and material strength indicators. It can be shown in engineering practice that the latter reason directly resulted in(1) the clogging of vertical drains was very serious, and the drain efficiency was drastically reduced;(2) soil strength was very uneven between vertical drains, and it increased very limited;(3) the effective improved depth was small, and the bearing capcity was low;(4) many soil columns were formed around vertical drains, and the ground differential settlement was very signficant. Therefore, it was not fully applicable to the newly hydraulic reclamation mud foundation for the classical consolidation theory and calculation method of the soil strength increased for sand drains ground.Firstly, it was systematically and comprehensively summarized and analized for the engineering characteristics of the newly hydraulic reclamation mud in different regions of Chinese mainland, and then the field study with the different forms of vacuume drainage system was carried out based on the actual project to discuss the transfer characteristics and distribution model of vacuum pressure and a corresponding vacuum consolidation theory was established; then, it was deduced of calculation formulas of the soil strength increase considering the consolidation characteristics of such foundation, and the rationality of the formulas was verified in the actual engineering; finally, based on the above two reasons of the secondary treatment, the field study and many indoor model experiments were carried out to develop a kind of large area sand cushion technology and new technology of improving the newly hydraulic reclamation mud foundation, which provided a new idea of one-time improving the newly hydraulic recalmation mud foundation.The conclusions can be summarized as follows:(1) newly hydraulic reclamation mud in different regions of Chinese mainland was formed during the hydrodynamic deposition and the hydrostatic deposition. It was deposited in the vicinity of and away from the outlet, consisted of fine soil particles and very fine soil parcles. Its pores was mainly constisted of isolated pores and intergranular pores, and its microstructure was mainly consitsted of turbulence –like structure and granular mosaic-like structure. The content of clay soil particles(clay & rubber) and strong hydrophilic minerals(illite & smectitle) were high. Its moisture content was usually larger than 90% and more than 1.5 times greater than the liquid limit; its void ratio was larger than 2.5; its plasticity index was larger than 20; its liquid index was larger than 2.0. The poor engineering properties resulted in a big difference for traditonal vacuum preloading technology(including shallow rapid improvement technology) improving newly hydraulic reclamation mud foundation and general soft soil foundation, which macroscopicially was showed that the PVDs were seriously bent and a serious soil column phenomenon occurred around PVDs during vacuum consolidation.(2) when trational vacuum preloading technology(including shallow rapid improvement technology) improved newly hydraulic reclamation mud foundation, vacuum negative pressure was reduced by up to 57% in the process of transferring to the bottom of vertical drains, and also the loss of vacuum negative pressure was very serious in the transfer path from vertical drain to soil, at least 67%, which was the main reason why soils strength was increased only a little after improved by no-horizontal-sand-drainage vacuum preloading technology(NSVPT). Moreover, during the vacuum preloading, the negative pressure was non-uniform at the boundary of drainage that it was time-varying linear attenuation model when transferring in the horizontal drainage cushion, but it was time-varying nonlinear attenuation model when transferring in the vertical drains, depending on the site condition. In view of this, it was estabished of vacuum consolidation theory with the non-uniform distribution boundary condition of negative pressure, and then it was analized of advantages and limitations of its analytical solution.(3) the increase mechanism of soil stength was qualitatively analized based on the effective stress method and the stress path method. It was concluded that the development of soil shear strength under groundwater level was different from that of above groundwater level. And then the calculations formula of soil strength growth was deduced based on the theory considering the uniqueness of the effective stress during the drainage consolidation of newly hydraulic reclamation mud foundation. The formula gave fully consideration of the vacuum pressure and its decay in the depth direction, the preloading, and groundwater levels and other factors. It can be verified based on the actual project that the average error was 8.0% considering the undrained shear strength and the average error was 3.2% considering the strength on the slide section. Therefore, the accuracy can meet the engineering requirements.(4) new type of large area sand quilt cushion was a large style 3-D spatial and permeable structure, made from flat structure, lateral composite flexible structure and columnar structure( be absent in the fluid mud foundation). The 3-D structure combined with the underlying foundation was an integral and high bearing capacity system which can been performed as a working platform for constructing deep PVDs. Its working mechanism was mainly reflected in the shell effect, the lateral confinement effect, and drainage consolidation & vertical reinforcement effect(be absent in fluid mud foundation). Its failure model had punching failure characteristics. The scheme 7(7/) with 280g/m2 woven geotextile was best. A estimates formula calculation of bearing capacity was derived, and the critical edage load crP of the scheme 7/was 146.4k Pa based on the formula. It should be noted and advised that in the engineering practice of the scheme 7/, it was necessary to clear mud and make drainage measures during the vertical drains construction.(5) the performance indicators related to the existed PVD was not appropriate to the inherent engineering properties of newly hydraulic reclamation mud, and the very last loading rate of vacuum pressure was used in design of vacuum preloading technology which resultd in serious “soil column phenomenon” around the PVDs. Both can result in that after rapid improvement of newly hydraulic reclamation mud foundation, the soil strength increased very limit, the effective improved depth was small and the bearing capacity was still unable to meet the design demand. For one-time improving newly hydraulic reclamation mud foundation, it can been showed based on indoor model experimental studies that the vertical drains should been made of the medium characteristic opening size(such as O95=0.15mm) filter and the high flexural strength core, and a step loading of vacuum pressure should be advised with the relatively fast loading rate. |
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