## 2 Behavioral Properties of Soil and Reinforcement  
### Theoretical Background  
Many studies have been conducted on the pullout test of MSE walls to find pullout characteristics between the soil and the reinforcement. There are many pullout test data obtained for various reinforcements, features of pullout test systems and test conditions. However, each country has its own test methods and regulations. In particular, many block-type MSE walls appropriate for the conditions of Korea are employed, rather than the panel-type MSE walls, and geogrid reinforcements are used in most cases. Conducted pullout test studies showed that the confining stress ranges between 30 kPa and 200 kPa, and Figure 1 shows the pullout force depending on the influence of confining stress in the pullout test. Therefore, the confining stress, which is one of various factors in the pullout test, has great influence, and in particular, the pullout behavioral properties between the soil and reinforcements are different depending on the types and shapes of reinforcements and their tensile strength. In particular, non-extensible reinforcements do not experience damage by confining stresses, and their pullout force following pullout displacement linearly increases. However, geogrid reinforcements tend to have pullout force concentrating mostly on their front depending on the pullout load in the case of high confining stresses, and have a smaller pullout force in the back thereof. Moreover, with smaller confining stresses than approximately 50 kPa, the pullout force is exerted across the whole area of the reinforcements. There is no method specified for confining stresses in the pullout test so far; therefore, pullout tests with only three or four types of confining stresses were conducted. The result of pullout testing geogrid reinforcements showed no pullout displacement occurs in a pullout box with a high confining stress in some cases. [PERSON], et al. [17] suggested an average resistance method for evaluating shear strength between the soil and the reinforcements by using pullout resistance obtained from the pullout test result to consider the pullout resistance properties of a geogrid reinforcement. The average resistance method uses the average in consideration of a pullout force distribution depending on reinforcement length under maximum pullout force for calculating the shear strength at the interface between the soil and the reinforcement, and is mainly used for limit equilibrium analysis of the MSE wall. The average resistance method is classified into the whole area method, the effective area method, and the steepest descent method depending on the average-taking process. The whole area method assumes that pullout resistance acts on the whole area of a reinforcement; the effective area method assumes that pullout resistance acts just on the section where actual strain occurs; and the steepest descent method calculates an average resistance between the soil and the reinforcements from the gradient of a tangent at the point where the curve gradient of a tensile force distribution curve is the greatest. Among the three aforementioned methods, the effective area method is considered the most effective, and the result of calculation  
Figure 1: Relationship between the pullout load and displacement in geogrid at different confining stresses (Mabrouk [16]). gives intermediate values. However, with the effective area method, it is hard to calculate an effective area, and shows a big difference between the pullout resistances depending on the effective areas. In this study, the nodal displacement of the geogrid installed in the soil tank for applying pullout parameters was measured to determine the confining stress at which the whole reinforcement area was pulled out as an effective confining stress, and then pullout parameters were calculated. The pullout test was conducted with different reinforcement tensile strengths and confining stresses to compare the results.