Design Considerations for Pile Groups Supporting Marine Structures with Respect to Scour

Yasser E. Mostafa

doi:10.4236/eng.2012.412106

Abstract

Piles supporting marine structures such as jetties, relieving platforms, quay walls and fixed offshore structures are subjected to lateral loads due to berthing and mooring forces, wind, waves, storm surges and current forces. This paper presents some factors that affect the design of pile groups supporting marine structures founded in cohesionless soils. Some main aspects that should be considered in the pile group design are addressed such as pile batter angle, pile group arrangement, pile spacing, pile slenderness ratio and magnitude of lateral static loading. Numerical analyses were conducted to investigate these design aspects with and without impact of scour. Different scour depths were considered to cover the possible root causes of scour around pile groups such as waves, current and ship propeller jets. The study revealed that scour has greater impact on lateral loading of pile groups compared to its impact on single piles. Pile groups with side-by-side arrangement exposed to scour are more critical than single piles and piles groups with tandem arrangement due to the combined effect of scour and pile-soil-pile interaction. It is also concluded that scour protection is not always required. More attention and considerations should be given to scour protection around piles especially if the piles are closely spaced, arranged side-by-side and if slenderness ratio is less than 12.5.

Highlights

Marine structures such as jetties, seawalls, relieving platforms, quay walls and fixed offshore structures are often supported on pile groups
A batter angle of 5 ̊ increases the lateral capacity by about 40% over vertical piles ( = 0)
A scour depth of three pile diameter causes a reduction in the lateral group capacity ranging between 29% and 35% for piles with tandem arrangement and side-by-side arrangement, respectively

Summary

Introduction

Marine structures such as jetties, seawalls, relieving platforms, quay walls and fixed offshore (jacket type) structures are often supported on pile groups. A very limited number of publications regarding the effect of scour on the behavior of pile groups is found in the literature. MOSTAFA luate the effects of scour on the behavior of pile groups Among this limited research, recent research focused on piles supporting bridge piers such as Lin et al [4]. Effect of scour on lateral loading of single piles has been investigated in a few recent publications such as Kishore et al [5], Lin et al [6], Mostafa [7] and Ni et al [8]. The combination of scour and pile-soil-pile interaction (i.e., group effect) can lead to a significant reduction in lateral pile capacity and may lead to the failure of marine structures. The decision to protect the piles from scour depends on the maximum anticipated scour depth based on the root cause of scour and depends on the pile, soil and loading conditions

Numerical Analysis and Validation with Previous Experiments

Effect of Batter Angle

Effect of Scour on Vertical Pile Groups

Scour Depth around Pile Groups

Effect of Pile Spacing

Effect of Pile Slenderness Ratio

Scour Protection

Findings

Conclusions