Effect of Local and Global Scour on Lateral Response of Single Piles in Different Soil Conditions ()
1. Introduction
Bridge piers and marine structures like jetties, seawalls and offshore structures can fail due to severe scour caused by currents and waves. Numerous publications are found in the literature for investigating scour around piles, especially in cohesionless soils and a smaller number of publications investigating scour in cohesive soils.
Coastal structures are often subjected to waves, storm surge, erosion and scour. Global scour refers to a general lowering of the ground surface over a wide area.
The ultimate scour depth depends on the diameter of obstruction, flow Froude number, shear stress and soil characteristics. It has been documented that local scour depth in sandy soils (ds) is 1.3 times pile diameter (d) with a mean of 0.7 [1]. In other words, the maximum scour depth is about 2 times the pile diameter (i.e., ds/d = 2). Global scour exposing piles due to storm surge events has been reported in the literature (for example, Robertson et al. [2]). Scour around piles in cohesive soils has been presented in [3-5]. For scour in soft clays, the ultimate scour depth ratio ds/d is 0.75 to 1 [6].
A very limited number of publications regarding the effect of scour on the behavior of piles are found in the literature. A thorough review of available literature on the topic showed that there is no research related to fullscale field or lab tests on scour effects on laterally loaded piles in sand.
Kishore et al. [6] and [7] conducted experimental tests on small diameter PVC and aluminum piles to study the influence of scour on laterally loaded single piles installed in soft clay. Lin et al. [8] investigated the scour effect on the response of laterally loaded single piles installed in sand considering the stress history of remaining sand.
This paper aims at reducing the gap of knowledge found in the literature in this subject. Different parameters were investigated such as soil types (medium dense sand, soft clay and stiff clay), scour depth, scour hole dimension, pile material (concrete piles and steel piles), submerged condition, magnitude of lateral load and load eccentricity. Finite element model (FEM) using the software program PLAXIS and Winkler model using the software program LPILE were used in the analysis. Comparison with experimental tests found in the literature for scour around piles installed in soft clay was conducted.
2. Mechanics of Scour
The flow velocity approaching a vertical cylindrical pile decreases from a maximum at the free surface to zero at bed level at the upstream pile face. If the pressure field induced by the pile is sufficiently strong, it causes a threedimensional separation of the boundary layers which roll up ahead of the pile to form the horse-shoe vortex [9]. Flow patterns in the wake of the cylinder also influence the scour. Scour occurs when the drag and lift forces applied by the eroding flow exceed the gravitational, frictional, and/or cohesive forces acting to hold the particles together [5].
Figure 1 shows a sketch of global scour and local scour. The vertical effective stress at a point depends on the soil weight above that point. In case of global scour, the effective soil pressure at all depths is reduced with the weight of scoured soil. In case of local scour depth, the effective stress near the pile and near the scour hole base is reduced. At a very large depth, impact of scour hole on the effective soil pressure diminishes [10].