Research On Cement Mixing Pile Foundation Treatment Method In Road-culvert Transition Section Of Expressway On Dongting Lake Area Based On Post-construction Settlement

An-Ci expressway,which has poor geological conditions and various soft foundation treatment methods,crosses the Dongting Lake Plain of Hunan Province from east to west.In this case,different settlement during the operation period is prone to occur on the pavement in the road-bridge(road-culvert)section due to the effects including different foundation treatment methods.The different settlement affects directly the vehicle safety and the driving comfort.Therefore,it is urgent to study the connection schemes between different foundation treatment designs using the same foundation treatment method and between two different foundation treatment methods.On the based of the practical project,the finite element numerical model of the road-culvert transition section of An-Ci expressway in the soft foundation was established to obtain the settlement characteristics of subgrade and foundation.Meanwhile,the culvert’s stress distribution characteristic in the road-culvert transition section was also obtained.Researched the subarea-subsection foundation treatment method to put forward the suggestions on foundation treatment design of the road-culvert transition section in Dong-ting Lake district.The main research contents and conclusions were as follows.The conclusions could draw from the analysis of long-term deformation monitoring data of the road-culvert transition section,the settlement and the deep horizontal displacement of the composite foundation of cement mixing pile occurred in the mid to late period of roadbed filling,the settlement rate and the deep horizontal displacement rate of the foundation treated by plastic drainage plate were beyond the composite foundation all time,the final deformation was more than twice of composite foundation.The finite element numerical model of the road-culvert transition section of mixing cement pile composite foundation and the finite element numerical model of the road-culvert transition section whose foundation treatment method gradually changed from cement mixing pile to plastic drainage plate was established.Then,the process from the beginning of construction to the end of the operation was simulated by those models,the modeling results virtually coincided with the monitoring data,and the relative error was up to 15%.The maximum vertical tensile stress of the culvert of the plastic drainage plate was more than twice of the composite foundation,and the area under compression of the culvert was wider.Proposed the evaluate index of driving quality during the operation period according to the research results of running smoothness of vehicles,the subarea-subsection foundation treatment schemes design of the foundation were carried out with the single variable of the cement mixing pile(length,distance,and diameter)to put forward the suggestions on subsection foundation treatment design of the road-culvert transition section in Dong-ting Lake district:the effect of foundation treatment should be decreased from culvert section to general subgrade section step by step to avoid the vehicle bumping at the section between different foundation treatment schemes,the driving comfort could be effectively improved by increasing the length of pile within the range of the effective length of pile,the different settlement of road-culvert section could be effectively controlled by the composite foundation treatment scheme whose pile spacing was under 2m,the treatment scheme of variable pile diameter should not be adopted priorly due to the poor benefit from improving the pile diameter.Designed orthogonal experiments to clarify a conclusion that the effect of the length of composite foundation and plate spacing of the plastic drainage plate and surcharge preloading size of plastic drainage board on subgrade settlement during the operation period and ride comfort index were successively decreased.The experiment data revealed the scheme whose length of composite foundation ranged from 15m to 18m,the preloading size of plastic drainage board was 2.2m,the plate spacing of plastic drainage plate was 1.3m can control effectively the different settlement of road-culvert section under this foundation treatment scheme.Regarded post-construction differential subgrade settlement of the road-culvert transition section and maximum longitudinal slope difference as dependent variables.The regression equation between the above variables and three factors were fitted finally.