A study about group efficiency on barrette piles in clay soils

Rapidly growing urbanization and developing technology have brought the result of constructing structures under difficult ground conditions. One of the most frequently used types of deep foundation applications in these cases is pile foundations. In principle, the piles are structural elements that transfer the load from the superstructure to the soil. Apart from rare exceptions, pile foundations are mostly evaluated and applied as a group. In pile groups, an intersection occurs in the pile impact areas. Because of this intersection, the bearing capacities of the piles in the group will be different (usually less) than the bearing capacity of a single pile in the same conditions. It may be expected that settlement of a pile group will be different from settlement of a single pile in same conditions. There are numerous studies in the literature, especially for circular piles for each soil condition related to group efficiency of these piles. Al-Mhaidib (2007) finds that the group efficiency factor approaches 1 at 9 times of the pile diameter (9D) in the model experiments, that is, the group effect is close to zero. Prakash and Saran (1967) have shown that the group effect is not important when a gap of more than 6D is left between the piles (as mentioned in Toğrol and Tan, 2009). The purpose of this study is to define a new pile spacing approach which is multiple of long and short pile sides of the barrette piles and to analyze when the group interaction in the pile group is equal to zero and how it changes in the intermediate pile spacing values. In this direction, a pile group is modeled in a two layered soil profile consisting of soft clay and hard clay; bearing capacity and group efficiency factor values for different models with different pile spacings (varying in short edge and long edge directions) were recorded. Plaxis 3D Foundation ver. 2.1 software has been used in this study. With this study; A new pile spacing approach has been introduced for barrette piles which is multiple of long and short pile edges. In the direction of this new approach, in the group of piles modeled as 2x2 in clay soil, the spacing that the group interaction is equal to zero and the way it changes in the intermediate pile spacing values have been analyzed. Then, the load-displacement curves are plotted at the points determined at pile centers and the displacement value (19.54 cm) corresponding to 10% of the equivalent circular pile diameter has been accepted as the settlement limit. The load corresponding to this settlement limit has been determined as the pile bearing capacity based on analysis evaluations. In the classification of the analysis results at the first stage, a classification was made such that the pile spacing (B) in the long side b = 3.0 m direction was variable while the pile side was fixed at the short edge a = 1.0 m. Numerous trial analyzes have been conducted (on the largest pile group) to ensure that the pile group is minimally affected by the model boundary effects while model boundaries are defined with the need of keeping calculation times of the software at reasonable levels. Depending on all these limitations and requirements; model directions, 100x100 meters in the horizontal x and z directions and 60 meters in the vertical y direction, have been defined. Before going on with the calculation stage, points were chosen to provide curves for analysis purposes on the model. In order to obtain the load-displacement curves of the piles, the points corresponding to the pile center at the pile top were determined. Pile spacings used in short edge direction (A) are; 2.0a; 3.0a; 4.0a; 5.0a; 6,0a; 8.0a; 10.0a and 12.0a. Pile spacings used in the long edge direction (B) are; 2.0b; 3,0b; 4,0b; 5,0b; 6,0b; 8.0b and 10.0b. As a result of the studies, the pile bearing capacity value in the pile group at the pile spacing A = 12,0a - B = 10,0b (A = 12,0m - B = 30,0m) is close (almost the same) to the bearing capacity value of the single pile modeled under the same conditions. As the pile spacings were narrowed, a decrease in the bearing capacity of the piles has been seen as expected. In the analyzes made, the group efficiency factor values changed between 0.99 and 0.60. In the analyzes made on the pile spacing of A = 12,0a, the group efficiency factor was obtained between 0,99 and 0,83 as a result of pile spacing changes in the long side direction. In the analyzes made on A = 10,0a pile spacing, as a result of pile spacing changes in long edge direction, group efficiency factor is between 0.97 and 0.80; In the analyzes made on the pile spacing of A = 8,0a, as a result of the pile spacing changes in the long side direction, the group efficiency factor is between 0.95 and 0.77; In the analyzes made on the pile spacing of A = 6,0a, as a result of the pile spacing changes in the long side direction, the group efficiency factor is between 0,93 and 0,74; In the analyzes made on the pile spacing A = 5,0a, as a result of the pile spacing changes in the long edge direction, the group efficiency factor is between 0,91 and 0,71; In the analyzes made on A = 4,0a pile spacing, as a result of pile spacing changes in the long edge direction, the group efficiency factor is between 0,87 and 0,68; In the analyzes made on A = 3,0a pile spacing, as a result of pile spacing changes in the long edge direction, the group efficiency factor is between 0,82 and 0,65; In the analyzes made on A = 2,0a pile spacing as a result of pile spacing changes in the long edge direction, the group efficiency factor is varied between 0.75 and 0.60. The effect of pile spacing changes on the group efficiency factor has been categorized and evaluated in two ways: "short edge direction pile spacing A = constant" and "long edge direction pile spacing B = constant". As an additional result, it is analysed whether changing the pile spacing in the long or short edge direction have more influences the group efficiency factor value and presented by using graphs. In the case, it is seen that the pile spacing changing in the short side direction where the long side direction pile spacing (B) is considered as fixed, has more effect on the change of group efficiency factor compared to the other situation. In other words, changing the pile spacing in the short edge direction is more effective than changing the pile spacing in the long edge direction on the group efficiency factor.