Document Type : Article
Authors
1
Dept. of Civil Engineering\r\nUniversity of Tabriz
2
Dept. of Civil Engineering\r\nUniversity of Tabriz
3
Dept. of Civil Engineering\r\n University of Tabriz
Abstract
Many buildings and structures require the use of deep foundations to utilize the bearing capacity of stronger soil layers. Pile groups are one particular type of deep foundation commonly used for large structures. In addition to vertical loads that must be sustained by the piles, significant lateral loads may be present and must be accounted for in the design. These lateral loads can come from a variety of sources, such as wind forces, collisions, wave or ice impact, earthquake shaking, liquefaction and slope failure. A pile group subjected to lateral loading is nfluenced by the existence of neighboring piles. This pile-soil-pile interaction is strongly dependent on the pile-to-pile spacing, which dominates the pattern of stress and deformation around a pile group. Full-scale tests are generally believed to provide the most accurate results, but are rare due to high costs. Therefore, many studies are available concerning centrifuge and model testing. In this paper, the behaviour of soil around laterally loaded pile groups in sand for different cases were studied and interpreted in the laboratory on small scale physical models, using the Particle Image Velocimetry (PIV) method. The group effect and interaction between piles in a pile group were also studied. A wooden box was used as the test box. A transparent sheet was used in place of a fourth wooden wall. Plexiglass was used to allow observation of the deformations in the soil sample. The soil was fine, dry sand. The soil was placed in the box using a sand raining system, in order to obtain a uniform and homogeneous medium. Rectangular piles were selected, in order to apply the similitude law. The smallest dimensions of piles were in contact with the Plexiglass. The 2$times$ 1 pile group model, with different pile spacing, was used in the direction of the load. An aluminous cap was used to connect two pile heads. Each pile was joined by a hinge (free rotation) to the cap. An electromotor with uniform velocity was used to apply lateral load statically to the pile group cap. A LVDT displacement sensor and a sensitive load cell were installed in order to record the pile head displacement and applied force. A digital camera was used for visualization of the soil movements and for image processing. The tests were performed by varying the pile centre-to-centre spacing from 1 to 6 times the pile widths (B). One test was performed on a laterally loaded single pile. Soil displacement vectors, soil shear strain and the soil displacement field around a laterally loaded single pile and pile groups, were obtained by image processing using the PIV method. Also, the load --- deflection curve for pile heads was obtained. It is found that, although a pile-group strengthens overall lateral load resistance it can weaken the individual pile response of the piles in the group. The overall lateral load is divided among each of the piles in the group. Each pile pushes against the soil behind it, creating a shear zone in the soil. These shear zones begin to enlarge and overlap as the lateral load increases. More overlapping occurs if the piles are spaced very closely together. In the case of 1B space between piles, the soil between the two piles has very low resistance against the horizontal displacement of the trail pile. In the case of 6B space between piles, the displacements of soil around each pile have no overlaps. It can be concluded that the interaction between piles decreases with increasing pile space. Also, in the case of 1B space between piles, strain wedges are overlapped, which shows the interaction of two piles. In this case, the created strain wedge is very similar to a single pile with larger width. This means that when two piles are very close to each other, the behaviour of the pile group would be similar to a single pile. With increasing pile spacing in a group, the displacement field between piles decreases and, also, shear strain created in the soil around the lead pile increases. In front of laterally loaded piles, the horizontal displacements are generally larger than vertical ones, and these two values decrease with an increase in the distance from the soil surface.
Keywords
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