Study Of Consolidation Behavior Of Soft Clay With Structure Characteristics And Application In Practice

Deep & thick soft clay formed from sea facies sediment extensively distributing near the littoral in southeast China,due to its low shear strength and high compressibility,results in many difficulties in design & construction of groundwork. Especially,post-construction settlement control problem has been to be solved desirably. In this dissertation,the consolidation and settlement mechanism of thick structural clay improved with partially penetrated vertical drains and partially penetrated mixing cement piles,is discussed. And the choose of the reasonable treatment method is analyzed.First,the experimental study on the structure property of thick Wenzhou clay is carried out. At the same time,a new method to revise compression curve is proposed. By controlling the stress value of under-lying layer less than tits structure yield stress value,a new design method to decrease composite ground settlement is suggested. And the composite foundation settlements of 12 buildings with the deep mixing cement piles is calculated,which is in good accord well with the measurement in field.Second,considering the influence of well resistance,smear effect and structure breakage of thick soft clay,the equivalent calculation method is proposed. That is,the partially penetrating sand drain ground is viewed as the double-layered ground,and its consolidation is analyzed with 1-D consolidation theory,through the transformation from 3-D consolidation to 1-D consolidation in the region of sand drain. This can overcome the disadvantage that the excess pore water is discontinuous between the region of sand drain and under-lying soft layer. To the double-layered ground,based on the consolidation mechanism under surcharge,the excess pore water pressure calculation formula is deduced under the condition of multilevel equably loading or unloading,which perfects the consolidation theory. According with the characteristics of pore water seepage and soil deformation,the 1-D rebound anti-consolidation model is set up. Then the anti-consolidation differential coefficient equation is deduced and the corresponding analytical solution is obtained. In succession using the above theory,the influence to effective stress area ratio of some factors,such as preloading time,surcharge's magnitude and the depth of vertical drain. And the feasibility that effective stress area ratio is used as the guideline of unloading surcharge,is studied.Finally,associated with the certain airport runway engineering,the three ground treatment methods,which is vertical drains preloaded by equiponderance,vertical drains preloaded by surcharge and deep mixing cement piles,is compared and contrasted. It indicts that,in the deep & thick soft ground improved by preloading or surcharge loading with sand drain partially penetrating,post-construction settlement mostly results from the dissipation of remained excess pore water pressure and the proportion of compress occurred in under-lying soft layer to total post-construction settlement is becoming larger with the time. Thus the preloading method is applicable to project,which is not strict in the post-construction settlement or is not with the thick ground clay. In addition,Deep mixing cement piles method is the most effective method to reduce the post-construction settlements and different settlements.