Compaction Of Road Embankment And Subgrade Geocomposite Drainage Layers

Embankments provide foundations for much of our transportation infrastructure. Quality construction maintains smooth riding pavements and provides slope stability. Proper selection of soil, adequate moisture control, and uniform compaction are required for a high quality embankment.The compaction of embankment is a familiar but complicated problem to civil engineers because of many factors have influence on it. So it seems simple but may cause rut, crack, and settlement on pavement, and even shear failure of the whole pavement. These may deteriorate the riding performance and service life of the road and will cause great economic lose. There are many reasons are responsible for these, such as the level of construction and administration, and also the inadequate standards or codes. Because of the complexity of factors affecting compaction, and the non-uniform and anisotropical properties of soil, and effectiveness from environment and repeated traffic load on pavement in service, in order to know the engineering properties of compacted embankment and to maintain its long-term stability, the studies on mechanism of compaction and how to apply adequate standards in construction and methods of compaction controlling (including laboratory and field tests), and technical measures of maintaining the strength and stability of embankment should be taken.Studies are in focus on several primary and complicated factors affecting compaction and the related standards and codes in domestic and overseas. Because water in pavement system is one of principal reasons of pavement distress, the use of geocomposites as drainage layers in roadway base and subgrade are studied too. The principal contents are as follows:1. The development of compaction methods and the theories of compaction mechanism are summarized at first, and the reasons for deformation of embankment are analyzed. Then, the importance of the compaction water content of soil and its degree of compaction corresponding to the engineering properties of embankment is pointed out.2. According to the analysis of quality problems on embankments at present, it is showed that the standards and codes of identifying and selecting earth fill are not workable properly in practice and not suitable for rapid development of large amount of embankment construction, because of the field technicians are lack of necessary soil identification skills and the limits of test apparatus on one side, and soil identification methods in many standards are sophisticated and cost much time on the other side. Combined with the standard in our country and the new published one (EPC) in Iowa, U.S.A., the examples on identification and selection of earth fill in the both standards are compared and analyzed. It shows that methods of EPC are more explicit on the classification of earth fill only by the tests on limit of liquidity, plastic index and fine content. EPC method is easy to be executed and has the advantage of simplicity and applicability.Moreover, the moisture content control figure about non-cohesive and cohesive soil in EPC is workable in practice and have some advantages that we can use for reference. The difference of moisture content controlling range between new published standard in Iowa and Technical Specification for Construction of Highway Subgrades (JTJ 033-95) in our country are compared by some practical engineering examples. Generally speaking, the range of permitted compaction moisture on the wet side of optimum content for clay soils is broader in new published standard in Iowa, and therange of permitted compaction moisture content for non-cohesive is broader too, which includes any moisture content except those in the range of bulking moisture content and those above up-limit moisture content. It is good for shortening time limit for project and saving its cost.It is showed that the particle distribution of soil must be considered,when the compaction standards are drawn up, besed on comparing the codes of our country, Japan and U.S.A.3. Calculations are done by means of data obtained from a 41m high road embankment compacted by dynamic compaction method in Yuncheng-Sanmenxia Highway and the results show that the high degree of compaction is not always beter, no matter for engineering properties or construction cost of embankment. Overcompaction may cause many negative effects such as increasing settlement of high embankment on soft subgrade and decreasing slope stability and descending squeeze resistance stability of foundation soil. So proper compaction is promoted here. If the compaction effort is increased more than the bearing capacity of soil mass, it will make grain excessively broken and worn and torn, and cause frost/thaw failure and bearing capacity decreasement in compacted soil. On the dry side of optimum moisture content of compacted soil, the more compaction effort increases the density of soil, but the expansive potential will be higher. On the wet side of optimum moisture content of compacted soil, there is a critical density corresponding to the maximum strength, above it soil strength will decrease as the increment of density.4. For granular soil, Gyratory Compaction Test is a more reliable testing method in laboratory. This thesis introduces Gyratory Compaction Test procedure conducted with Servopac Gyratory Compactor with 200kPa vertical pressure, 1.25 degree gyration angle, 90 gyrations, and 20 gyrations per minute, and the test results show considerable promise to be construction specification for the quality control of field compaction for granular soil. For clay soils, it would probably be adequate if the gyratory compaction procedure can produce the maximum dry unit weight that is equivalent to the one produced by the modified Proctor test. Because in practice the modified Proctor compaction method has been working just fine for field compaction of clay soils.5. When the degree of compaction of field fill is evaluated, the consistency between soil samples in-situ and in lab must be paid attention to. Different methods to adjust the maximum dry density (P dmax) in lab will cause it obviously diverse for the same kind of earth fill by calculating on some examples here. The results of adjusted maximum dry density by lab standard tests can be compared with those measured in-situ directly, but the ones of the elimination method show the compacted dry densities of suitable fine particle, not the real compacted dry densities of field earth fill(including oversize particle), so it can be said that adjusted maximum dry density method is better than elimination method. But in some special situations such as the compacted dry density of suitable fine particle need to be controlled, the elimination method is the more suitable one.6. Three indistinct concepts about collapse are discussed in this thesis, and some conclusions are presented here:(1) Collapse potential doesn't increase without limit as increasing vertical stress.(2) Soils compacted at a water content wetter than standard Proctor optimum water content may also collapse heavily. But those compacted at critical degree of saturation will not collapse. This critical degree of saturation varies with overburden pressure.(3) There is a critical degree of compaction (critical dry density or stable density)in non-expansive soil. No matter in what moisture conditions there is very little collapse or no collapse when the soil compacted at critical degree of compaction. And this critical degree of compaction will increase as increasement of overburden pressure. So the degree of compaction must be higher if the pressure on fill is greater. Though the collapse potential of expansive soil can be controlled via compacting to a higher dry density, the expansion potential will increase as the dry unit density increases.Analyses of many examples show that whatever highway or railway embankment or earth dam, as long as the collapse occurs, their compacted dry density and the degree of saturation are low. So controlling the compacted dry density and the degree of saturation is the key to avoid or reduce collapse potential.7. To keep enough strength and modulus of base and subgrade the most important thing is to avoid surface water and ground water invading the base or drain off the invading water quickly to keep the base and subgrade off softening or frost heave. Results from an analysis program for stress and displacement of elastic layered system (APBIH . FOR) in this thesis show the disadvantageous effects caused by decreasing of the base and subgrade modulus, and the importance of subgrade drainage and keeping enough strength and modulus of base and subgrade are emphasized here. New type of three-dimensional geocomposite drainage material overcomes the shortcomings of common drainage materials. Their three-dimensional structure guarantees high compressive strength, strong drainage ability and long-term effect on capillary resistance. They have the effect of reinforcement like geogrid and show great prospect on base and subgrade drainage.At the end of this thesis suggestions for further research and development have been mentioned.