Static and Pseudo Static Study of Loose Clay Layer Effects on Steel Sheet Pile Walls Behaviors

Introduction Usually, the construction of new multi-story buildings require deep supported excavation. Steel sheet pile walls are being widely used in civil engineering projects for excavation support systems. Many researches have performed about various problems of steel sheet piles, like steel sheet pile behaviors; using of sheet pile as a permanent structure; long term performance of sheet piles; Vertical bearing capacity; construction of steel sheet pile walls on sloping ground; performance of steel sheet pile wall for supporting an excavation in urban environment. Soil is not uniform in depth, sometimes loose soil layer may exist in various depth and situations. This issue can cause different effects on ground surface displacements, forces and moments acting on sheet pile and struts during excavation procedure, compared with status that soil is uniform in depth, especially in seismic conditions that must be considered in design of sheet piles and struts. Methodology In this study a deep excavation by using finite element method is analyzed. Excavation’s depth is divided to three clayey layers. One of three layers is loose clay layer that its positions is modelled in three different situations, top, middle and bottom. Obtained results are compared with excavation without loose layer. Since excavation support system may be a permanent structure, long term stability must be considered. Pseudo static analysis is performed by applying 0.3g horizontal acceleration. Models are analyzed in different situations, dry and saturation. Width and depth of considered excavation are 10 and 12 meters, respectively. The first strut is installed beneath one meter of ground surface and subsequent struts are modeled in 3 meters spacing from each other so that finally, four struts are considered along the depth of excavation. Change percent of different parameters in models with clay loose layer is calculated in comparison with model without clay loose layer. Results According to comparative obtained results it can be concluded that: 1. Existence of a loose clay layer on two stiff clay layers that thicknesses of all three layers are same, generally has reducing effects on soil and sheet piles deformations, forces and bending moments of sheet piles. 2. With increasing depth of loose clay layer, lateral deformation, shear force and bending moment acting on sheet piles are increased. In depth equal to two times of loose layer thickness, these parameters have maximum values. In fact, with increasing sheet piles horizontal displacement, bending moments are increased. 3. Maximum horizontal displacement of the soil is in condition that loose clay layer is bottom layer. 4. When loose clay layer is located on the middle or bottom layer, shear forces acting on sheet piles are greater due to loose layer located on upper layer. 5. Generally, it can be said, with increasing depth and location of loose clay layer, affecting parameters on sheet piles and struts behaviors are increased. 6. Existence of loose clay layer changes axial force of struts. This issue must be considered in design of sheet piles, especially in design of middle struts. 7. Saturated situations increase axial force of struts.