Design Considerations for Pile Groups Supporting Marine Structures with Respect to Scour ()
1. Introduction
Marine structures such as jetties, seawalls, relieving platforms, quay walls and fixed offshore (jacket type) structures are often supported on pile groups. The foundation piles usually comprise a large portion of the marine structure cost. These piles are usually subjected to large lateral loads induced from waves, currents, vessel berthing and mooring forces. Also, these piles are subjected to scour due to waves, current and ship propeller jets.
Marine structures and bridge piers supported on pile groups can fail due to severe scour. Numerous publications are found in the literature for investigating the scour around piles for bridge piers and a smaller number of publications investigating the scour around marine structures. Moreover, a very limited number of publications regarding the effect of scour on the behavior of pile groups is found in the literature. Vertical pile capacity is composed of friction along pile length and end bearing at pile toe while pile lateral capacity highly depends on the soil conditions surrounding the top one third of pile length. Therefore, scour impact on lateral pile capacity is more significant than the scour impact on vertical pile capacity.
Global scour refers to a general lowering of the ground surface over a wide area. Figure 1 shows global scour around a fixed offshore structure supported on piles.
Scour around piles varies due to the root cause. For single piles, scour depth in sandy soils (ds) is 1.3 times pile diameter (d) with a mean of 0.7 [1]. In another meaning, the ultimate scour depth is about 2 times the pile diameter (i.e., ds/d = 2). However, this value can be different for scour due to waves only or due to ship propeller jets only. Some research has been performed to examine the scour around pile groups due to waves and currents such as Sumer and Fredsøe [2], Mostafa and Agamy [3]. However, very limited research has examined how to eva-
Figure 1. Global scour: wide depression around a jacket structure (after Whitehouse [10]).
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]. Mostafa [7] reported that scour has significant impact on single piles installed in sand and less significant impact on piles installed in clay. Also, Mostafa [7] reported that ultimate lateral capacity for single pile subjected to global scour is found to be about 50% to 70% of ultimate lateral capacity of pile subjected to local scour depending on the scour hole dimension.
As piles are usually installed in groups, it becomes necessary to study the effect of scour on the behavior of pile group not just single piles. The combination of scour and pile-soil-pile interaction (i.e., group effect) can lead to a significant reduction in lateral pile capacity and consequently may lead to the failure of marine structures.
This paper presents the impact of global scour around batter and vertical pile groups installed in medium dense sand. The software program GROUP V.7.0 [9] was used in the analyses. For calibration, results from the software were compared with experimental tests on batter pile groups found in the literature.
Different design parameters were investigated in this paper such as pile batter angle, pile group arrangement, pile spacing and pile slenderness ratio. The impact of scour depth on all these parameters was investigated. Only global scour was considered in this study as it has more significant impacts compared to local scour.
Based on the results of the numerical analyses, this paper also provides general recommendations and guidelines on the necessity of using scour protection. Scour protection using riprap or geotextile may be necessary sometimes and may be a waste of money in other cases. The decision to protect the piles from scour depends on the maximum anticipated scour depth based on the root cause of scour and also depends on the pile, soil and loading conditions.
2. Numerical Analysis and Validation with Previous Experiments
In this paper, numerical analyses were conducted to investigate several parameters affecting the design of pile groups supporting marine structures subjected to scour. The software program Group [9] was used in the analysis. GROUP is a 3D software program for analyzing pile groups subjected to axial and lateral loads. A solution requires iteration to accommodate the nonlinear response of each pile in the group model. The program GROUP solves the nonlinear response of each pile under combined loadings. For closely-spaced piles, the pile-soilpile interaction is taken into account by introducing reduction factors for the p-y curves used for each single pile. These reduction factors or called “p-multipliers” are generated based on results of laboratory and field experiments published in the literature.
A comparison between the results from the computer program Group [9], results from the program Piglet [11] and experimental tests of batter pile groups performed by Zhang et al. [12] was conducted. Batter piles are widely used to support marine structures especially for structures subjected to relatively large lateral loads. Zhang et al. [12] carried out 18 different lateral load tests in the centrifuge on 3 × 3 and 4 × 4 fixed head battered pile groups to investigate the effects of vertical load on the group lateral resistance in cohesionless soils. Zhang et al. [12] proved that designs based on standard lateral load tests with small vertical dead loads would be on the safe side.
Figure 2 shows a sketch of the prototype 3 × 3 battered pile group simulated by the centrifuge models. Two pile arrangements were simulated for the 3 × 3 pile group (Zhang et al. [12]). In the first arrangement, the side pile rows were battered forward at 1:8 slope and the middle row was battered reverse at 1:4 slope (referred to as the 6F3R arrangement). In the second arrangement, the side pile rows were battered reverse at 1:8 slope and the middle row was battered at 1:4 slope (3F6R arrangement). The piles were square aluminum with 304 mm in length and 9.5 mm in width. In prototype scales, the width, total length and embedded length of the piles were 0.43, 13.7 and 10.4 m, respectively and the free length from the ground surface to the point of lateral load application was 2.7 m. The piles were three-diameter spaced and rigidly attached to the pile cap. The soil comprised medium dense sand with relative density (Dr) of 55%.