도로 성토로 인한 연약지반의 측방유동에 관한 연구

Abstract

To investigate the characteristics of shear strength and soil deformation in soft grounds, in which various vertical drains were placed, two hundreds field monitoring data of embankments performed in thirteen road construction sites at west and south coastal areas of the Korean Peninsula were collected. At first, the relationship between settlement and lateral displacement was investigated into three stages, in which embankment construction works were divided into initial filling stage, final filling stage and stage after complete filling. And then, the relationship of surcharge pressures and embankment heights with undrained shear strength of soft grounds were investigated. The investigation on settlement and lateral displacement illustrated that the increment of lateral flow to the increment of settlement was low during initial filling stage, but increased gradually with filling and showed largest during final filling stage. After complete filling, the lateral displacement was converged, even though the settlement was increased continuously. Therefore, most of lateral flow was occurred during embankment filling. The ratio of the lateral displacement increment to the settlement increment was 20% for initial filling stage, which coincided with the one presented by Tavenas et al.(1979), but became 50% for final filling stage, which was half of the one presented by Tavenas et al.(1979). However, the ratio reduced to 1% to 9%, which was quite lower than the one presented by Tavenas et al.(1979). Shear deformations, even shear failures, were predicted in soft grounds under initial undrained shear strength, since the design heights of embankments were higher than the yield height in all the sites. However, embankment construction would be possible since the yield height became higher than the design height due to improvement of shear strength of soft grounds with application of the vertical drains. In order to perform safely embankments for road constructions, the embankment loads should be designed not to exceed 5.14 times the initial undrained shear strength of soft grounds and to be less than 3.0 times the undrained shear strength improved with application of vertical drains in soft grounds. In this study, the effect of the embankment scale on the lateral flow was investigated. The thicker soft soils and the wider bottom of embankments produced the more horizontal displacements in soft grounds. Especially, if thick soft grounds were placed, the embankment scale, which was given by the ratio of thickness of soft ground to the bottom width of embankments, became larger and in turn large horizontal displacement was produced. And also the higher filling velocity of embankments induced the more horizontal displacements in soft grounds. The other major factors effected on the lateral flow in soft ground were the thickness and undrained shear strength of soft grounds, the soil modulus and the stability number. The more maximum horizontal displacement was induced by the less undrained shear strength and soil modulus of soft grounds. Also the more stability number produced the more maximum horizontal displacement. The stability number was less than 3.0 and the safety factor of bearing was more than 1.7 when the shear deformation was not developed. However, if the stability number was more than 5.14 and the safety factor of bearing was less than 1.0, the unstable shear failure was developed in soft ground. 50mm can be recommended as a criterion of the allowable maximum horizontal displacement to prevent the shear deformation in soft ground, while 100mm can be recommended as a criterion of the allowable maximum horizontal displacement to prevent the shear failure in soft ground. Finally, some methods were proposed to predict lateral flow due to embankments for road constructions on soft grounds, in which vertical drains were placed. For analyzing the relationship between the safety factor of embankment slope and the horizontal displacement in soft grounds, it was reliable to apply the maximum horizontal displacement in soft ground instead of the horizontal displacement at ground surface. In safe fields where the maximum horizontal displacement were developed within 50mm, lateral flow would not happen since shear deformation was not appeared. On the other hand, shear failure would happen in the fields where the maximum horizontal displacement were developed more than 100mm. In such fields, embankments might be continued after some appropriate countermeasures should be prepared. Safe embankments can be performed on soft grounds, in which the stability number is less than 3.0 and the safety factor for bearing is more than 1.7. However, if the stability number is more than 4.28 and the safety factor for bearing is less than 1.2, shear deformation would begin and even shear failure would happen.