Experimental Study On Engineering Characteristics Of Soft Rock Filling Subgrade For Heavy-haul Railway

The heavy-haul railways are characterized by large capacity,high efficiency and low transportation cost,developing rapidly all over the world,it has become an important symbol of the modernization of cargo transportation in most railway countries.Based on China's industrial layout and energy distribution,the basic pattern of coal transportation from north to South and from west to East will exist for a long time,heavy-haul railway is the most effective way to solve the transportation of large quantities of goods such as coal.The Menghua Heavy-Haul Railway is currently the largest coal transportation special line under construction in China,the Yueyang-Ji'an section belongs to hilly and low mountain areas with undulating topography,the strata along the line are mainly weathered or easily weathered soft rocks,distribution section accounts for about 70%of the total length of the main line,there is a huge amount of slag discarded from cutting excavation and tunnel,if it can be used for roadbed filling,it can solve the problem of large amount of waste soil and soil acquisition,reduce damage to the local ecological environment,it produces enormous economic benefits.Due to the suitability of soft rock fillers in roadbed filling mostly focuses on high-speed railway or passenger dedicated line,for heavy-haul railways with heavy axle load,large capacity and high traffic density,there is a lack of in-depth analysis on the improvement and filling techniques of soft rock fillers.Therefore,further research on weathered soft rock filler is carried out,exploring the feasibility of heavy-haul railway subgrade filling,it can lay the foundation for perfecting the design and construction technology of railway subgrade under special conditions,has important practical significance.Based on a summary of the existing research results,relying on the new Menghua Heavy-Haul Railway project,for fully weathered soft rock filler distributed along the line and soft rock blocks with different degrees of weathering,through indoor and field tests,from the engineering properties and road performance of the filler,the post-construction settlement and dynamic stability characteristics of subgrade are studied in depth,the main work and conclusions are as follows:?1?Equivalent area method for load strength of irregular soft rockUniaxial compressive strength is an important index for rock strength classification and rock mass weathering zoning,it is the most basic physical property of rock,usually obtained by a compression test using a standard cylindrical specimen,but the process of specimen preparation is complex,for weak,severely weathered and jointed rocks,because the complete core can not be removed normally or can not be processed into standard specimen,it is difficult to use standard rock test methods to measure strength,the point load test provides an efficient and quickly way.Based on the mechanical essence of point load strength,by introducing the width ratio“area factor”of the actual fracture section to the minimum section of the specimen,the“equivalent area method”and the unified expression with the minimum cross-sectional area as the basic parameter are constructed,using irregular soft rock samples,the variation characteristics of area coefficient with different weathering degree are discussed,the influence of shape factor and loading point spacing on point load strength is studied.Tests show that:the area coefficient is highly skewed,the median eigenvalue increases slightly from 1.40 to 1.46 with the increase of weathering degree,when calculating the soft rock point load strength,the average value can be 1.43;the point load strength I_s decreases gradually to a stable state with the increase of shape factor?and loading point spacing D,with the weathering degree decreases,the influence of?increases correspondingly,but that of D does not change much.For strong,weak and slightly weathered soft rock,taking the relative deviation of I_s is less than 40%as the sampling standard,it is suggested that the?of sample should not be less than 0.4,0.5 and 0.6,and D should be greater than35mm.?2?Strength control standard of Soft rock block as stone filling for railway subgrade fillingThe line structure of railway base section from top to bottom is as follows:ballast bed,surface layer of the foundation bed,bottom layer of the foundation bed and embankment below the foundation bed,under the action of train,the dynamic load gradually decreases along the depth of subgrade,the requirement of each structural layer of roadbed on the property index of filling can be reduced accordingly.The soft rock blocks with saturated uniaxial compressive strength of 5²5MPa are divided into 5 groups according to the grade difference of 5MPa,experiments on physical,hydraulic and mechanical properties were carried out,the basic engineering properties of soft rock blocks are mastered,the relationship between crushing value CA,Los Angeles wear rate LAA and saturated uniaxial compressive strength _cR of soft rock with different strength grades is established.When soft rock block is used as stone filler for the surface layer filling of foundation bed,it bears strong dynamic loads,the material index of filler should be in accordance with the Interim Code for Design of Railway Subgrade Limit State Method?Q/CR 9127-2015?,determined on the passenger and cargo collinear railway subgrade bed surface grade gravel CA<16%higher standard,according to the established relationship,_cR>24.2MPa and LAA<39.5%can be obtained.Because the dynamic load acting on the bottom of the foundation bed is relatively weak,it can be determined according to the lower standard of graded gravel LAA?50%,corresponding _cR?18.4MPa,CA?18.7%.For embankments under subgrade which are mainly subjected to static load,the material index of filling materials can be determined according to the standard of graded gravel CA?30%,corresponding _cR?13.5MPa,and CA<20.9%and LAA<58.7%in railway.Accordingly,the recommended strength standard values of soft rock blocks used as stone filler in railway subgrade filling are put forward,i.e.when used for surface filling of subgrade,_cR?25MPa,CA<16%,LAA<40%;when used for bottom filling of subgrade,the corresponding _cR?20MPa,CA<18%,LAA>50%;when used for subgrade filling below subgrade,_cR?15MPa,CA>20%,LAA?55%.?3?Engineering properties and filling technology of fully weathered soft phyllite and cement modified soil fillerThe engineering properties of filling materials are important factors affecting the quality of roadbed filling,the test shows that the mineral components in the filling materials of fully weathered soft rock are mainly quartz,iron-rich muscovite,glacial feldspar,kaolinite and chlorite,and no montmorillonite is detected in clay minerals.Among the 24 chemical constituents,quartz is the most abundant,accounting for 69.5%of the total mass,while Nb is the least,accounting for 0.001%.The experimental study on the hydraulic properties shows that the expansion rate,expansion force and shrinkage coefficient of the fully weathered soft rock filling are small,and when it is used for embankment filling under the Heavy-haul Railway bed,there will be no obvious wet expansion and dry shrinkage phenomenon.Embankment filling is silty clay with high liquid limit,which belongs to group D.When it is used for embankment filling under Heavy-haul Railway Foundation bed,it should be strengthened or improved.Through unconfined compressive strength tests of different mixing ratios,compaction coefficients and curing ages,the optimum mixing ratio of cement-modified soil fillers is 3.5%.Furthermore,the mechanical properties of the fully weathered soft rock and the cement-modified soil filler with 3.5%mixing ratio were systematically tested,and the strength index of the filler improved by cement was significantly improved.The field filling test shows that the compaction coefficient of embankment does not change obviously with the increase of the thickness of loose pavement.When the moisture content increases,the compaction coefficient increases first and then decreases,while the foundation coefficient decreases gradually.When embankment is filled,the thickness and moisture content of loose pavement should be strictly controlled.For fully weathered soft rock fillings,it is suggested that the thickness of loose pavement h should be 0.35m,and the moisture content w should be controlled in the range of w_opt-3%?27?w?27?w_{op t},the construction should be carried out by means of static pressing 2 times+weak vibration 1time+strong vibration 1 time+static pressing 1 time+static pressing 2 times.For cement improved soil fillings,the thickness of loose pavement h should be 0.4m,and the moisture content w should be controlled in the range of w_{op t}?27?w?27?w_{op t}?10?2%,and the rolling process should be static pressing 2 times+weak vibration 1 time+strong vibration 1 time.?4?Long-term settlement observation and water-bearing state of roadbed filled with fully weathered soft phyllite and cement-improved soilBased on the analysis of the mathematical characteristics of two-parameter hyperbola and exponential curve models,a three-parameter power function model for predicting the post-construction settlement of subgrade is proposed by introducing the characteristic quantity of"half-life deformation rate of settlement"and according to a large number of field settlement observation data.The reliability of the three models for predicting post-construction settlement is comprehensively compared and analyzed by selecting the judgment coefficient,mean square deviation and correlation index.The results show that the judgment coefficient and correlation degree of the power function model are the largest and the mean square deviation is the smallest.It shows that the regression curve of the three-parameter power function model is the closest to the measured curve and the prediction effect is the best.The maximum post-construction settlement of the test section is 37.25mm,which is far less than the limit of 200mm for Heavy-haul Railway subgrade,and the post-construction settlement of the soft rock subgrade meets the requirements of the code.The measured moisture content of embankment filling presents three stages:initial adjustment,slow growth and basic stability with time.The first stage occurs in the initial stage of sensor embedment,the second stage is in the period of rainwater and weak sunshine from autumn to winter to spring,and the third stage is basically stable,with no significant change in season.In the horizontal direction of embankment,the water content of filling is small in the middle and large on both sides;in the vertical direction,the water content of embankment filling is small in the upper part and large in the lower part.?5?Simulating the dynamic response of embankment and cutting subgrade structure under long-term load of heavy-haul railway trainUnder the load of heavy haul railway freight train,the stress superposition effect of roadbed is remarkable.Based on this,the dynamic stress of roadbed is calculated by using the four-axle load mode of two adjacent bogies.Under the condition of 30t axle load and 80km per hour,according to the equivalent principle of elastic deformation in the range of 3.0m subgrade structure,the dynamic stress of subgrade surface under common load is calculated to be 67.7kPa,and the ultimate load is 80.5kPa.Using self-developed dynamic test equipment,the field excitation test of Subgrade under simulated Heavy-haul Railway train load was carried out,and the dynamic response characteristics of embankment and cutting subgrade structure under different dynamic loads were tested.The test results show that the measured dynamic stress,vibration acceleration and vibration velocity of roadbed gradually decay along the depth direction,and gradually increase with the increase of dynamic load.Under dynamic load,the vibration displacement at the bottom of loading plate is the largest,and the trend along the horizontal direction of roadbed surface is gradually reduced.The closer the distance from the edge of loading plate,the more obvious the attenuation amplitude,and the vibration displacement along the longitudinal direction of roadbed.The significant influence range is about 1 times the edge length of the loading plate.Under normal loads,the vibration displacement of the bottom of the plate loaded on the embankment section is 0.724mm;under the ultimate load,the vibration displacement of the embankment and the cutting section are 0.833mm and 0.871mm,respectively,which are less than the control value of the code 1.0mm,indicating that the embankment has good dynamic stability characteristics.