Rail Transit Induced Foundation Settlement And Mechanistic-Empirical Design Method Based On Service Performance

Train loads induced cumulative settlement on track structure is a complicated issue to be solved in practical transit engineering.The settlement of embankment will cause the irregularity of upper track structure,which then can intensify the wheel-rail dynamic interaction,influence the riding comfort of passengers or even jeopardize the safty of train operation.The embankment will also deteriorate under the long-term development settlement and shorten the serviceability of track structure.Focused on this issue,comprehensive and deep research is continuously undertaken around the world.Based on the practical transit issue and related research,a series of static and dynamic soil element tests are conducted in this paper to gain a clear understanding of settlement pattern in railway transit embankment,according to which a mechanistic-empirical calculating model for soil cumulative deformation is established.Then,combined with numerical analysis,physical model test and field measurement results,this calculating model is discussed and verified.In combine with the calculating model,a Mechanistic-Empirical design method for Chinese high-speed railway embankment considering long-term serviceability is proposed.the specific research work carried out in this paper and related results obtained are as follows:(1)Through research review on traffic load induced cumulative settlement of transit embankment,the deformation mechanism and main influence factors are obtained,and the development of calculating method of cyclic accumulated deformation of soil is clarified.The existing mechanistic-empirical calculating models still have flaws on lack of deformation development mechanism.The code for design of high-speed railway embankment in China is still based on empirical elastic calculating method.(2)The dynamic response in the subgrade soil is the direct inducing factor on the development of subgrade settlement.Through a 3-D finite element model on high-speed railway train-track-roadbed,dynamic response of roadbed surface is calculated and verified by field measurement data.The distribution and attenuation pattern of dynamic stress among the roadbed is clarified.(3)Static and cyclic dynamic triaxial shearing tests on unbound granular of ballast are conducted under different confining pressure and degree of compaction.Under same degree of compaction,higher confining pressure results in less amount of plastic strain in soil element sample.Compared with lower degree of soil compaction,higher degree of soil compaction results in less amount of plastic strain in soil element sample.The test results serve as a reference for the establishment and calibration of proposed mechanistic-empirical model.(4)A new mechanistic-empirical model for calculating cyclic permanent deformation of unbound granular material is proposed.This calculating model is based on the concept of soil hardening under cyclic load and the influence of relative distance between stress path and soil failure line on the generation of soil plastic strain.The proposed model innovatively replaces the empirical part of cyclic load number N related expression which describes the development of soil plastic strain in existing traditional empirical models with an expression that describe the development soil hardening,making the model with more strength in soil deformation mechanism.The related parameters in this proposed model is derived from laboratory tests.The fitting results is in good consistency with test results.(5)A full-scale physical model test of ballasted high-speed railway track is conducted.Attention is focused on the settlement development of ballast layer which occupies the main part of embankment settlement.Test results show that under train speed of 144 km/h and 216 km/h,only 0.45 mm of ballast layer settlement is observed after totally 200,000 of carriage load.This is mainly because the high degree of ballast layer compaction.As the train speed increases to 300km/h,the settlement of ballast layer develops rapidly,reaching about 8.9 times the result at train speed of 216 km/h,while the dynamic stress at the ballast layer shows only 10.2%increase compared with that at train speed of 216 km/h.It may be the high intense of ballast particals vibration which lossens the lateral constraint of ballast layer that causes high development of ballast layer settlement.When axle load increases from 17t to 22t,an obvious increase of dynamic stress in ballast layer is observed.However,the dynamic stress still increases slowly with train speed,and the settlement development under each train speed is limited.(6)Based on the new mechanistic-empirical calculating model,an optimal design method for Chinese high-speed railway embankment considering long-term serviceability is proposed.Different from the traditional design method,the optimal design method uses train-track coupling model and numerical method to calculate the wheel-rail contacting force and the dynamic stress attenuation in the roadbed.This is an improvement in the mechanistic basis of design method.Based on this design method,the settlement pattern of each roadbed layer under different design condition is analyzed.The settlement calculating based design method can satisify the long-term serviceability of high-speed railway track under large amount of train moving load.(7)Based simplified train-track-trackbed coupling model,train-track dynamic responses on different settlement curves and train speeds are analyzed.With the increase of deflection ratio H_s/L_sand train speed,contact force between wheel and rail as well as car body acceleration become intense.Train speeds related uneven settlement control criterion expressed by H_s/L_sare proposed based on train’s operation safety and passengers’riding comfort.A more flexible control standards considering both the influence of train speeds and settlement lengths can be proposed.Thus,the limitations could be more intelligent and economical in design and maintenance.(8)Through a series of stastic and dynamic soil element tests on soft clay,the developing pattern of cyclic deformation and pore water pressure is discussed.Based on these results,a modified mechanistic-empirical calculating model is developed applicable to soft clay.The modified model is verified through existing test results.Meanwhile,Field settlement monitoring on a section of Ningbo Metro Line 1 tunnel is conducted continuously for 2 years after it finished construction phase and started to operate.The contact channel between left and right line is constructed through freezing method,by which the subsoil is totally disturbed,so the settlement at this cross section reaches to 19.5 mm after 2 years of train operation.At the cross section away from contact channel,settlement develops rapidly at first 4 months,then gradually settles down and reaches to about 7 mm after 2 years of train operation.Through a series of labortary tests on Ningbo natural and remolded soft clay,related parameters for calculating model is obtained,based on which soft clay settlement considering soil disturbance degree can be calculated.Finally,through proposed calculating model,settlement of Ningbo Metro Line 1 tunnel is predicted and compared with measure value,which verified the reliability of proposed model.