| A huge amount(more than several hundred million tons)of sediments is generated from ports,rivers,lakes and other water environments every year.Given the huge quantity,rapid and efficient treatment of these sediments has become one of the urgent geoenvironmental problems.These sediments often exhibit undesirable engineering properties such as high water content,high compressibility,low and even negligible bearing capacity.To accelerate the treatment process,an alternative approach to treating the dredged sediments is using prefabricated horizontal drain(PHD)assisted by vacuum preloading.However,very few studies have investigated the method of vacuum-assisted PHD so far.Therefore,a systematic investigation on the PHD-vacuum preloading method through model tests was carried out,and the performance and the consolidation theory of this method were investigated.The main contents and conclusions are as follows:(1)Experimental tests on PHD-vacuum preloading(PHDVP)and PHD-vacuum preloading-flocculation(PHDVPF)combined method were conducted with considering the effects of layered backfill,horizontal spacing of PHD and others.The dewatering efficiency and pore water pressure during the vacuum consolidation process were measured.Besides,the distribution of water content and shear strength of soil after vacuum preloading were measured and analyzed.The results show that the PHDVP method,compared to the natural sedimentation and consolidation method,can significantly improve the drainage efficiency(by 3 times)and consolidation rate of dredged sediments(by 4 times).The addition of APAM can significantly increase the free drainage rate of dredged sediments(by 40%),but its effect on the drainage efficiency of vacuum preloading is not significant.For the PHDVPF method,the water content distribution of soil is similar to that of PHDVP method.Compared with PHDVP model test,adding APAM can significantly increase the soil shear strength by 1-2 times.(2)The performance of PHD-vacuum preloading-solidification(PHDVPS)combined method with alkali-activated slag powder and cement as solidifying agent(different activators and content were used)was investigated through experimental test and compared with that of only vacuum preloading and only solidification method.The results show that the effects of flocculation and soil skeleton construction produced by the hydration products of solidifying agents in the early stage can significantly improve the permeability of sediments.Due to the increase in permeability,the drainage efficiency of vacuum preloading is significantly improved(by 60%-100%).Overall,the proposed PHDVPS method is more effective than the traditional methods in treating dredged sediment with high water content.(3)The mathematical model of the PHD-vacuum consolidation was established and the analytical solution was derived.Furthermore,the coefficient of seepage direction is proposed,which can improve the calculation accuracy of the analytical solution.The proposed analytical solution was verified by the results of Terzaghi’s two-dimensional theory(solved by Flex PDE).The calculation results of the two methods have a certain difference(less than 5%)because the assumptions of both methods are not completely consistent.However,the analytical solutions are very consistent with the results of Flex PDE in terms of the variation trend.In addition,the analytical solution was further compared with the field measurements,and the results showed that the analytical solution results were consistent with the results of field monitoring,and the difference was within the allowable range.(4)Based on the piecewise linear method,a plane strain PHD consolidation model(named VPSC herein)was established with considering the effects of large strain,selfweight,vertical and horizontal seepage and permeability anisotropy,nonlinear constitutive relationships,time-dependent vacuum pressure and surcharge load,non-Darcy seepage,horizontal spacing of PHD,and layered backfill.This proposed model was verified by the results of Flex PDE.Then,VPSC was used to analyze the experimental tests and field measurements.The results show that the results of VPSC and Flex PDE are completely consistent,and the calculation results of VPSC are also very consistent with the measurements of experimental test and field test. |
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