The Mechanism of Pile Plugging in Sand

Abstract

ABSTRACT An understanding of the mechanism of pile plugging in granular soils is important for design and evaluation of new and existing piles. A mechanism of pile plugging is introduced, based on the arching phenomenon. The proposed mechanism is supported by reinterpretation of available data and recent model pile testing results. INTRODUCTION Open pipe piles are widely used for offshore construction. During the initial stage of installation, soil enters the pile at a rate equal to the pile penetration. As penetration continues, the inner soil cylinder may develop sufficient frictional resistance to prevent further soil intrusion, causing the pile tobecome "plugged". Although technically the inner soil can be referred to as a "plug" only when it prevents entry of additional soil during penetration, the term "soil plug" is commonly used in reference to any soil mass inside the pile, regardless of its state during installation. When plugged, an open-ended pile penetrates in a mode characteristic of a closed-ended pile. The mode of pile penetration significantly controls the soil-pile interaction during and after installation. This affects the ultimate static bearing capacity (especially in granular material), the time dependent pile capacity (in clays), and the dynamic behavior and analysis of the piles1. This significance is enhanced by the fact that plugging has been shown to be of frequent occurrence, in contrast to the currently accepted view2. Plugging of piles in clays has a negative effect. Their driving is designed, therefore to eliminate its occurrence. One way of accomplishing this is by choosing driving equipment capable of overcoming the worst case (in which the soil plug/pile acts in full compatibility)3. Plugging of piles in granular soils has a desirable effect. The understanding of its mechanism is important for:The static capacity analysis of open-ended piles, either for design or for evaluation of existing structures (e.g. adding loads to offshore platforms).Developing models to enable dynamic analysis during driving. This paper is aimed at the investigation of the mechanism underlying the plugging of piles in granular material. AVAILABLE DATA In spite of the frequent occurrence of plugging and the importance of its effects, only limited attention has been given to the subject, resulting in limited data availability. This can be attributed to:Misidentification of plugging, caused by referring to the average plug length rather than to incremental changes.Complications that plug measurements cause in pile driving operations.Unawareness of the consequences of pile plugging. The mechanism of pile plugging can be identified by using measurements of soil intrusion, relative to pile penetration. Three types of data sources are used for this purpose:soil samplers;small-scale models andfull-scale piles. SOIL SAMPLERS Considerable data exist concerning the different factors which control soil penetration during sampling operations. In order to validate the use of that data, an analogy between thin walled samplers and offshore piles has been established2. Qualitative data is presented, which aids in identifying the mechanism controlling plug development.