| Since 2004, high-speed railways were developing rapidly in China, about a few high-speed railways were constructing and these design speed all above 250km/h, so subgrade should bearing more loads and be steadier than conventional railways'subgrade. However, considerable number of high-speed railways were located in where had many soft soil particularly in southeast of China the distribution ratio was higher. In order to ensure bearing capacity and stability of railway which roadbed constructed on soft soil foundation, it should been reinforced. Because of more superiority than other foundation treatment methods in the areas of cost, effects, environmental and duration, vacuum preloading method and vacuum combined surcharge preloading method have widely used in construction of port, terminal, civil, runway, high-speed railway and highway and so on. Further work should been done to clarify mechanism of vacuum combined surcharge preloading method, so that will improve design and construction level.In this paper, based on previous works, and relying the test section on a high-speed railway embankment, studies and analyses were conducted on mechanism of vacuum combined surcharge preloading method according to laboratory test, in-site test, theory analysis and numerical analysis. Main works in this paper were:(1) A kind of new equipment was developed to measure permeability coefficient under load of vacuum. According to many permeability tests in laboratory by permeability coefficient measuring instrument developed by our self, laws about influence on permeability by vacuum and soil compression were analyzed. It was discovered that permeability permeability change with soil compression and eventually stabilized. Change of permeability coefficient had observably decrease with settlement velocity which proved by computation of Flac5.0, and consolidation time with constant permeability coefficient is 1.5 times to that with changed permeability coefficient at the same consolidation magnitude, after that the influence of settlement velocity reduced with change of permeability coefficient. (2)According to in-site test, laws of settlement rate, settlement distribution, pore water pressure distribution etc. were analyzed. More significant phenomenons were:Although settlement rate reach to 52.2mm/d that was greater than lOmm/d in specification, but instability was not appearing. The settlement forms was different between vacuum preloading and vacuum combined surcharge preloading, under vacuum preloading settlement was uniform but under vacuum combined surcharge preloading settlement form was like bottom of pot. Distribution of pore water pressure isobar was like a funnel and it broadly in line with the principle of pumping wells, groundwater level dropped by 5.2m in where was 22.3m from bottom of slope as the groundwater has been drawn. Although depth of PVD was 18.5m, but in the range soil layer was all significantly compression.(3)Using unsaturated soil mechanics, changes of groundwater level was analyzed in vacuum combined surcharge preloading method, zero line of pore water pressure was descend,but groundwater level could descend, ascend or steadness with different factors. Forecast about layer settlement of roadbase was conduct by Grey Relation Analysis. And it was found that delamination configuration of soil had influence on forecast precision.(4)According to FEM analysis with ABAQUS, Constitutive relation for the soil was compared, it was discovered that using modified Cambridge model, plastic deformation could satisfied with compuding needs. The settlement results closed to in-site test, and change of pore water pressure basically fitted the distribution as in-site test.
|
PDF Full Text Request