Research On Engineering Characteristics Of Collapsible Loess And Subgrade Treatment Technique In West Area Of Liaoning

Based on project of Fuxin-Chaoyang expressway, which is a section of Tieling to Chaoyang highway, the engineering characteristics of loess of west area in Liaoning is studied in this paper by the methods of field investigation, tests in-situ and tests in laboratory. So a lot of survey works and triaxial tests in laboratory have been done, then some advanced methods such as age test with 14C, X radial diffraction, test technique of scan electron microscope (SEM), computer image process technique, technique of artificial nerve network, theory of fraction geometry and modelling method based on disturbed state concept, are used to study and discuss systemically on loess distribution and its engineering properties. Furthermore, a great deal field tests are also studied on subgrade treatment of collapsible loess. Some technical parameters are obtained for collapsible loess subgrage treatment by contrasting the indexes after treatment and before treatment, such as collapsibility and physical and mechanical properties. All these results can provide scientific basis for future projects in this area.The main studies and creative results of this paper are shown in the following:(1) The loess in west area of Liaoning is mainly distributed in three unit of physiognomy, namely, the physiognomy of river alluvial, alluvial and flood plain and front area of mountain, the physiognomy of low mountain upland by tectonic erosion and the physiognomy of low mountain by tectonic erosion. The loess belongs to Malan loess. Loess geomorphology is characterized by no typical Yuan, Liang and Mao, which is significantly different from loess geomorphology of northwest in China. The loess in different location all have collapsibility, but there are some differences between different geomorphological units. From the view of origin, the loess is formed by winding and gravitational conveying and flow transporting, therefore the loess in this area can be regarded as secondary loess by alluvial-flood and slope-flood deposition.Grain composition of loess consists mainly of silt particle, in which coarse silt grain is great. The content of clay grain varies in more large extent. So there is different in grain composition from northwest loess. Clastic minerals are mainly quartz and feldspar, heave mineral is little. The main clay mineral is illite and mixture layer of illite and montmorillonite, secondary composition is chlorite, and kaolinite is little in the loess. The average quality content of easy-soluble salt in loess is 0.047%, medium-soluble salt is 0.029% and difficult-soluble salt (CaCO3) is 2.46%. The value of PH varies between 6.91 and 8.06, and the average of PH is 7.47, that is, loess is in a state of alkalescence.(2) Framework grain in loess is in a state of coexistence of three types of single, aggregate, and clot grain, and single grain silt and aggregate is more which cemented by colloidal and fine clastic particle. When content of clay mineral is low, framework grain will be in a type of bracket-inlay contact, and which will change into disperse type with decreasing of coarse grain and increasing of clay grain, so the shear strength and stability of loess will be improved. The number of large and medium pores of loess in a shallow depth is much more than in a deep depth, and the number of small and micro pores much more than shallow loess. It shows that shallow loess is looser, in which exists a lot of bracket pores and other type pores, on the other hand, deep loess is denser, in which exists more lots of micro and small pores. The pore of loess has a characteristics of self-comparability and its distribution has a characteristics of fraction geometry. Fraction dimension of pore distribution varies between 1.22 and 1.41, and this index in shallow loess is slightly greater than in deep loess. In depth of 20m, four main types of structure are (A) bracket and large pore with weak-cemented structure, (B) bracket large pore and inlay and micro pore with half-cemented structure, (C) flocculent-cemented structure and (D) clotted-cemented structure respectively. The structure of loess is looser in shallow depth, in which is in structure of A, and denser in deep layer, in which is in type of C and D. Therefore, it is shown that with the depth increasing, the structure tends to dense, the number of pores is getting to decrease, the type of grain is getting to change from single to aggregate, cemented characteristics among particles varies from weak-cemented into cemented type, so stability of soil enhances and collapsibility reduces gradually.(3) The variabilities for physical property indexes of loess are less than mechanical property indexes, that is, the great scatter distribution for the latter. Further analysis results express that there is neither a very good statistical relationship among physical property indexes, nor relation between physical and mechanical property indexes, and comparatively the former is slightly better. The coefficient of permeability for undisturbed loess sample after saturated is about from 4.13×10-5cm/s to 9.13×10-5cm/s in vertical direction and from 1.67×10-5 cm/s to 5.33×10-5 cm/s in horizontal direction. The ratio of kz to kx is between 1.54 and 4.65. There is a power relation between coefficient of permeability and seepage pressure. The vertical permeability coefficient decreases more obviously with depth.(4) The loess has basic reasons which lead to collapse, namely, special composition and special structure characteristics. It doesn't have a very obvious statistical relation between coefficient of collapsibility and physical property indexes. It is found that technique of artificial nerve network (ANN) can used to predict reasonably the collapsibility of loess according to physical indexes. Here an expression which based on method of secant modulus also suggest to calculate rationally collapsible deformation of loess subgrade.(5) The indexes of shear strength determined from different test conditions are different. Results show that the c andφfrom the unconsolidated-undrained shear test (UU) are all larger than the indexes from consolidated-undrained shear test (CU). So indexes of effective shear strength is suggested to adopt in practical project. The stress-stain relation is mainly in a stain-soften pattern under UU condition, but will be transmitted to a strain-harden pattern when consolidated pressure is large. The stress-stain relation is mainly in a little stain-soften and little strain-harden pattern under CU condition, and it is a stain-soften relation under consolidated-drained test condition. These characteristics of stress-strain relation are influenced by structure, degree of saturation and drainage condition. Then a constitutive model, which can describe the characteristics of strain-harden and strain-soften, is established which can calculate deformation and stability of loess subgrade.(6) Impact-rolling method is an effective method to treat shallow collapsible loess which can increase dry density of soil and improve bearing capacity of subsoil, and eliminate the collapsibility of loess. The effect of impact-rolling method is related with property of collapsible loess and impactive effort, so the treatment effect should be determined by tests in a section of highway with similar geological condition. For Chaoyang collapsible loess the effective influence depth of impact-rolling can reach about 0.90m under impacting and rolling 40 times in the velocity of 12km/h with triangle impact-rolling machine in power of 25kJ.As a effective ground treatment method, dynamic compaction is propitious to treat the collapsible loess of west area in Liaoning. In the same field condition and test equipment, different compacted times, effective influence depth is different. The more of compacted times, the greater of influence depth, but in small variation. The value of dry density of soil can be greater than 1.50g/cm3 after collapsibility is eliminated. So the dry density can be reference index to examine the compacted effect. And there is a better relationship between number of dynamic penetration and degree of dense, so the number can be used as a technical index to determine the effective depth.Lime soil pile can effectively eliminate collapsibility of loess, but its effect relates with equipment and pile spacing. Treatment effect of this method can also be judged by comparing number of dynamic compaction before compact and after. The value of dry density of soil can be greater than 1.50g/cm3 after treating with lime soil pile, and can take dry density as a reference to examine collapsibility which is eliminated or not.