Stability Control And Settlement Forecast Of Soft Roadbed Treated With Combined Vacuum-Surcharge Preloading

In recent years, highway is developing rapidly in our country. The settlement after work completion is the main problem in the point view of highway built on the soft soil region. Combined vacuum-surcharge preloading (CVSP) is a good method to reinforce soft foundation. The method has been used to improve soft foundation of speedway extensively. But the theory is deficient in the mechanism and stability control and settlement forecast of CVSP, especially the treatment in soft roadbed of super high fill (6-8 m). In order to improve the technology of design and construction and get the bottom of mechanism, it is necessary to study the method.The property of soft soil in Jiangmen region of the Xijiang River basin is studied in the paper associated with the project of"study on the method of improvement soft foundation and the consolidation mechanism of hydraulic fill soil"(49972089) and"study on the influence of organic matter in the progress of marine soft soil foundation consolidation"(40372122) sponsored by the national natural scientific fund and the project of"Soft roadbed treating engineering of Jiangmen offshore street"in Guangdong province. It was studied by field experiment for the reinforcement mechanism of CVSP. The solution and the controlling indexes of stability have been suggested for the stability control the soft roadbed of super high fill treated with CVSP. By the analysis of microstructure change characteristics of the soft soil, the elasto-plastic damage constitutive model of the soft soil was established by means of the pore change which has been taken as damage variable factor from macroscopic to microscopic. The model was proved by field trial. Finally, the controls of post-construction settlement and the time of executing pavement structures were solved by simulating the settlement of the soft roadbed of super high fill treated with CVSP based on the model.According to the analysis of collected data and field reconnaissance and laboratory test, the research of the soft soil in the field shows that:①the soft soil in the region mainly belonged to the quaternary marine and continental alternative deposition, the layer thickness is heterogeneously distributed between 3m and 30m. And the soft soil has many features, such as high water content, high porosity ratio, high compressibility, low strength, low permeability, etc.②according to the micro-structure picture, the soft soil that belongs to the transitional type between the mesh and flocculent structure, has high clay content. The clay minerals are preferably crystallized, contenting chlorite, illite-montmorillonite mixed-layer minerals and schistose minerals, while exceptional soil samples have diatomite particles and products after the feldspar weathering. The arrangements of the clay minerals possess directing property, and the contacts of the mineral particals belong to face-face contact and edge-face contact. Some samples has well nature fracture development, skeletal structure, and high grit content, the clay minerals are filled in the porous and covered the soil particals, so that the cementation occurs to strengthen the joint between the particals.③the soft soil in Jiangmen with high sensitivity belongs to structural soil, and the structural strength that has a mean value about 20 kPa, has low regularity along the depth. From the indoor experiment and field monitoring data, the soil damage mainly consistes of 2 forms as follows: first, the vane-shear strength is diminished 30% by the vibration caused by prefabricated vertical drains (PVDS) installation; second, when the top filling load exceeds the structure yield strengthσk, the soil structure is destroyed and the structure strength grandually diminishes, and the soil deformation is transferred from elasitic type to plastic type. Therefore, when implement the design of the soft roadbed reinforcement, the effect caused by structure strength to soil deformation and stabilization should be fully considered.On the basis of the designing experiences of the CVSP, aiming at the deficiency in the design, the paper indicates that:①To remedy the deficiency of the present criterion with a single standard (vacuum degree under membrance), a new idea of dual-index control method was created to assure construction quality;②airproof is the key to successfully implement the CVSP, the wax well can solve the shallow airproof problem triumphantly; aiming at the deficiency of the criterion, adding the seal sleeve to the vertical drainage bottom provide a new way to dispose the deep airproof problem.Based on the field monitoring data, aiming at the disputed problems in the CVSP consolidation mechanism, several conclusions are attained:①several basic concepts related to vacuum, such as vacuum, vacuum degree, excess pore water pressure, negative prloading, were discussed and distinguished. An analysis was executed to explain several conceptions of key parameters and their relation of the mechanism of vacuum preloading. Negative prloading consists of vacuum degree and excess pore water pressure (exclude excess pore water pressure caused by fill preloading);②The study of the theory and field experiment shows the transfer and distribution feature of the negative preloading agree with the rectangular distribution;③Base on the dynamism, the measuring method of the vacuum degree and pore water pressure were combined to creat a new way to discover the chang law of the groundwater table in vacuum preloading. Through theory and monitoring data, a proof is given to show the groundwater table is unchanged in vacuum preloading.④Under a certain condition, the consolidated time and the gain-in-strength of soil are direct proportional with vacuum energy which affects the reinforcement effect. The usual design (each vacuum pump control 800~1500 m2) is deficient, lower vacuum energy could affect the project duration and the roadbed stability;⑤On the basis of the field monitoring data, the author indicates that the effective influenced depth can reach 1m under the PVDS.⑥The CVSP seriously influences the surrounding environment. In this study, the results of monitoring and disposal show that the lateral influenced distance can exceed 20m at least. A rational configuration of cement mixing pile can reduce the influenced distance to 5m, which could provide experiences for the design of CVSP in the urban engineering.Analyzing the analogous roadbed landslide cases, the result of the stability study of CVSP shows that:①Under the same loading velocity, the comparison of two results of the CVSP and surcharge preloading shows the stability of the former is better than that of the latter, while the over-rapid loading velocity can lead to instability accident.②During the process of the CVSP, the strength of the foundation soil increases rapidly. When determine the loading velocity, a calculation must be executed to assure the roadbed stability and the gain-in-strength should be considered detailedly.③Aiming at the deficiencies of the stability control of the CVSP in criterion, employing the developing law of the soft soil deformation and the yield point, the data processing method of the Grey Theory (GT) was employed to forecast the roadbed stability, and determined the distinguished rule via a large amount of settlement data; The analogous roadbed landslide cases and the monitoring result verified the feasibility of the method which provided a new way to study the roadbed stability dealed with the CVSP.④the field stability control consists of three phases: firstly, before reaching the limited height, reinforcing the monitoring of the soil lateral displacement, roadbed filling should be done rapidly to save the project duration; then, after reaching the limited height, the filling rate can be raised rationally, the settlement rate and the lateral displacement rate are taken as the stability control indexes; finally, after getting the full load term, the vacuum energy in the soft soil must be held, the vacuum unloading should be implemented after the consolidation process experiencing a reasonable time.According to the characteristics of the soft soil in the study area and the consolidation mechanism of the CVSP, based on the masonry model developed by Shen Zhujiang, a updated elasto-plastic damage model for soft soil is given in the paper, the research result shows that:①Combined the Cam clay model, the elasto-plastic damage constitutive model of the soft soil was established by means of the pore change. The model assumes that the kernel yield surface don't only changc with the elasto-plastic cubic strain, but harden and soften with the time as well, during the rheologic process. Through the change of the yield surface, the control of the rheologic rate and stability are carried out;②Making use of the model, and employing the finit element to solve the Biot theory, this method is incorporated to the CVSP, and the result shows it is better than other methods. The calculation result of the incidence outside reinforcement area gets approximate to the monitoring data. Compared the results of diverse models, the model agrees with the monitoring data well.In the end, by the model in the paper, the forecast and control analysis result of post-construction settlements of the soft soil dealed with the CVSP shows that:①The road section disposed with the CVSP, needs a rationable preloading time. Especially for the high fill soft soil roadbed, the strength increase give the roadbed stability the assurance, and an efficient way to diminishe the post-construction settlement is applying enough preloading time. A reasonable overloading after the CVSP is a more efficient scheme for the road section that request lower post-construction settlement;②Aiming at the problem that can't precisively forecast the post-construction settlement via the CVSP, the feedback analysis was used to improve the forecast precision on the basis of monitoring data. The erro of the calculation was less than 10%. The result was used to get the controls of post-construction settlement and the time of executing pavement structures to be applied to the project, and it was provided for the decision-making and the planning concept.