Experiment Study On Engineering Characteristics Of Cinder Subgrade For Addis Ababa-djibouti Railway

The subgrade is a type of earth structure made from loose soil material.Due to its long-term exposure to the natural environment,such as repeated influence from rainfall,climate,and train loads,the strength and stability of which are varying with times.To meet the stability requirements of the track superstructure,as well as the safety and comfort during the railway operation,clear requirements are made for the strength,stiffness,stability,and durability of subgrade in the railway system.Cinder gravel,also known as scoria,is a lava stone generated from the volcanic eruption.As cinder gravel in gaseous form coming out from the volcanic vent,it rapidly cools down and condenses into solids and breaks into irregular particle shapes with interconnected pores as it hits the ground.Given its special forming conditions,cinder gravel is mainly distributed in East Africa,the Pacific Rim,the mid-ocean ridge,and the Mediterranean volcano zone where earthquakes occur frequently.China is located on the edge of the Pacific Rim of volcanoes.Cinder gravel is widely distributed in Jilin,Hainan,Yunnan,Xinjiang,and Taiwan.With the development of railway construction,the"4 vertical lines and 4 horizontal lines"for the main China HSR network will be formed by 2035.China has a vast territory and a huge length of railway line,and the railway will inevitably pass through the area where cinder gravel widely.Cinder gravel has features such as porous particles and light-weight nature.Using the special geomaterials for subgrade construction can save investment and protect the environment.At the same time,it also has important practical significance for the alternatives of railway subgrade fill material in China.Compared with other methods and relying on the construction of the Addis Ababa-Djibouti Railway in Ethiopia,given that the special geotechnical materials are widely distributed along the railway line,the paper has studied and analyzed the physical and mechanical properties of blended cinder gravel in terms of the compaction characteristics,strength and deformation characteristics,dynamic characteristics,water stability,and seepage properties.Combined methods such as investigation,laboratory testing,field compaction,and theoretical analysis are also adopted for the project.The selection criteria of raw materials for subgrade of the structural form and construction method of subgrade structure have been put forward as follows:1)The Physical and Mechanical Properties of Cinder Filling and ParticlesCinder gravel is a lava block generated from a volcanic eruption.It cools and condenses during the eruption in the volcanic vent.Cinder gravel has irregular shapes and numerous honeycomb-like interconnected pores.Provided its special forming conditions,the cinder gravel formed at different eruption sites has significant differences in terms of physical and mechanical properties during different eruption stages.Their engineering properties also are varying compared with those of general terrestrial and marine earth rock fills.Nineteen samples were collected from 15 sites along the railway line stretching from the DK21+000?DK204+700 section.Through the physical and mechanical experiments,the results show that the gravel size of the cinder is smaller than that of the ordinary ones,but the grain surface is rougher.With the decrease of the gravel size,the surface roughness increases.The crushing value of the anti-crushing capacity increases with the increase of the porosity.The minimum crushing value is 29.1%and the maximum value is 58.3%.The porosity value ranges from 14.5%to 35.1%.The maximum dry density of cinder gravel in compaction is smaller and the porosity is larger.However,the clearance rate after deducting the pore volume of the cinder fill is similar to that of the traditional gravelly soil.L.A.abrasion loss and crushing value are the main indexes to measure the anti-crushing capacity.The paper considered the dynamic load levels of the railway subgrade,as it is closely related to the ballast layer and the surface strengthened layer of the high-speed railway subgrade.Therefore,it is suggested that L.A.abrasion loss be used as an evaluation index for the anti-crushing capacity.2)Compaction Characteristics of Cinder Gravel BlendedThe paper takes DK76+000 cinder(Z8)fill as a research object and carried out the field construction test of the stabilized cinder aggregate.The following conclusions are drawn:the variation of K30 of blended cinder gravel increases first and then decreases with the addition of the sand content.When the volumetric mixing ratio is 2:1,the maximum value is 154MPa/m.which meets the requirements of the existing specifications for the railway subgrade surface,the internal pores of the cinder gravel with good crushing resistance have little influence on the compaction performance of the subgrade fill.Compared with porosity,the clearance rate after deducting the opening surface of the granular surface is more reasonable than that of porosity as the compaction index of blended cinder gravel.Compared with cinder fill at a natural moisture content,the strength of the 2:1 and 3:1 blended cinder gravel fill soaked by rains is 35%lower,but it can still meet the compaction,strength,and anti-deformation requirements of conventional railways as the surface and bottom fill of the subgrade bed,and has good water stability.3)Static and Dynamic Characteristics of Cinder Gravel BlendedThe most important characteristics of cinder gravel are its irregular shape,internal pores,and great porosity variability.Its resistance to breakage under a static and dynamic load of the track superstructure and its strength and deformation characteristics are the key issues that affect whether it can be used as railway subgrades.The paper conducted static and dynamic triaxial tests on the stabilized aggregates within mixing ratios of 2:1 and 3:1 to analyze the particle crushing degree of cinder particles under long-term dynamic loading.The influence law of confining pressure and water content on the static and dynamic characteristics of the blended cinder gravel was discussed.Based on the negative power ratio evolution and power discriminant criterion of cumulative residual strain rate,three thresholds were obtained to distinguish the four forms of rapid plastic deformation,which are fast stability,slow stability,slow failure,and rapid destruction.The results showed that:In the process of sample preparation,blended cinder gravel will produce more significant gravel breakdown and enhance with the increase of compaction degree,yet decrease with increasing input of soil gravel.Under the condition of compaction,the relative crushing rate of blended cinder gravel under static loading and long-term cyclic loading is less than 3%,and gravel breakage is not significant.The influence of water content on maximum dynamic elastic modulus and maximum damping ratio is more significant than that of confining pressure.The dynamic stress thresholds of blended cinder gravel at rapid stability,long-term stability,and rapid destruction are 0.030?0.065,0.315?0.445,and 0.80?0.95 times static strength respectively.4)Permeability and Seepage Stability Characteristics of Cinder Gravel BlendedThe seepage stability problem of blended cinder gravel with rainfall infiltration is a key problem affecting whether it can be used as a subgrade.Under the blend ratios of 2:1 and 3:1,the influence of compaction degree on the permeability of the blended cinder gravel is studied through field and laboratory seepage tests.The seepage stability of the blended cinder gravel under the condition of rainfall infiltration is discussed.The following conclusions are drawn:The permeability coefficient of the blended cinder gravel decreases with the increase of compaction degree.The blended cinder gravel within volume ratios of2:1 and 3:1 is highly permeable soil under the condition of 80%.At 85%,90%and 93%compaction degree,the soil is moderately permeable.The critical hydraulic gradient increases with the increase of compaction degree;Under the same compaction degree,the critical hydraulic gradient of blended cinder gravel at ratios 2:1 is greater than the 3:1.The hydraulic gradient of rainwater is lower than the corresponding critical hydraulic gradient when it is infiltrated at the ratios of 2:1 and 3:1 stabilized subgrade,the hydraulic gradient is1.15 and 1.14 respectively.The results show that the seepage failure of the blended cinder gravel subgrade at ratio of 2:1 and 3:1 under rainfall will not occur.5)Site Dynamic Tests and Stability Evaluation of Cinder Filler Subgrade.To study the dynamic characteristics of the cinder gravel subgrade under the train-induced dynamic loading,site dynamic tests were carried out on the Addis Ababa-Djibouti railway.The dynamic parameters such as dynamic stress,dynamic deformation,vibration acceleration,and acceleration were obtained under different train running speeds.The results show that:The dynamic stability of subgrade with traditional geomaterials and cinder fills can meet the requirements,and the dynamic stability index of cinder fill is better than that of traditional material.The dynamic stress,dynamic deformation,vibration acceleration,and acceleration increase with the increase of train running speeds.Under the condition that the train axle load is 25 t and the train running speeds is not more than 100 km/h,the maximum dynamic stress,maximum dynamic deformation,maximum speed effect index is 70.079 k Pa,0.255 mm and 0.0036(km/h)^-1,respectively,which meet the requirement for railway subgrade.6)Evaluation of Service Condition of Cinder Filler SubgradeBy carrying out on-site vehicle and rail inspection tests on cinder gravel fill and traditional geo-materials,the difference between cinder gravel fill subgrade and traditional geo-materials subgrade is analyzed.The service condition is also evaluated in the experiment.The following conclusions are drawn:The TQI frequency accumulation curve of cinder gravel fill is steeper than ordinary filler,and the average of TQI in which cinder gravel fill is much smaller than ordinary filler.The deterioration rate of line condition which can evaluate the degree of line condition deterioration,cinder gravel fill is smaller than ordinary filler.cinder gravel fill subgrade is better than ordinary filler.The high and low level,level value,track twist irregularity value frequency accumulation curve of cinder gravel fill is steeper than ordinary filler,and the average of high and low level,level value,track twist irregularity value which cinder gravel fill is much smaller than ordinary filler.The paper conducted the contrast test of the deterioration rate of high and low level,level value,track twist irregularity value on both cinder gravel fills and ordinary fills,and the results show that cinder gravel fills subgrade perform better.7)Structural Design of Subgrade with Cinder FillerA method for determining the thickness of the subgrade under the condition that the embankment in the state of no time effect is established.Based on the dynamic triaxial test results of the blended cinder gravel,the accumulated residual deformation of the subgrade is in the weak time effect state as the control target.To satisfy the structural dynamic strength,cyclic deformation,and long-term stability as the design principles,the structural design technology of cinder subgrade is studied.The following conclusions are drawn:The thickness of the railway subgrade is not only related to the trainload but also depends on the properties of the embankment.When the K₃₀ of the embankment filler increases from 70MPa/m to 135 MPa/m,the thickness of the subgrade reduces from 3.3 to 1.8 m in the condition of passenger transport and the thickness of the subgrade reduces from 3.7 m to 2.1m in the condition of freight transportation.To meet the requirement of embankment in no time effect state,the thickness of the railway subgrade adopts 2.3m in passenger transport and 2.6 m in freight transportation.When the thickness of the subgrade is 2.5 m,the K₃₀ of embankment?120 MPa/m in mixed passenger and freight.Based on subgrade fillers in a weak time effect,the paper put forward technical standard and suggestion for mixed passenger and freight subgrade as follows:The thickness of subgrade adopts 2.6 m and subgrade surface adopts 0.6 m within blend ratios of 2:1 stabilized aggregates,which requires K₃₀?150 MPa/m;Subgrade bottom adopts 2 m within blend ratios of 3:1,2.5:1,and 1.5:1 stabilized aggregate,which requires K30?120 MPa/m;Embankment requires K₃₀?110 MPa/m at 4:1 and 3.5:1 by volume.8)Key Engineering Technology of Cinder Filler SubgradeThrough field construction test,static and dynamic triaxial tests,field and laboratory seepage tests,site dynamic tests,in situ vehicle track inspection tests,the paper deals with the influence of cinder gravel's porous structure on the compaction characteristics,strength and deformation characteristics,dynamic characteristics,seepage stability.Based on the above research results,the selection criteria of raw materials for normal railway subgrade,the quality control standard of subgrade compaction,the structural form and construction method of subgrade structure,and the construction technology of subgrade are proposed.