Drainage Piles -A Method for Increasing The Axial Capacity of Friction Piles

Abstract

This paper describes a method for increasing the axial capacity of friction piles embedded in clay soils. The proposed method consists of increasing the shear strength, and consequently the soil-pile adhesion, of the soil around a pile. This is accomplished by draining water from the surrounding soil into the pile. Equations are developed to predict the maximum pile capacity, and Terzaghi's theory of radial consolidation is used to predict the variation with time of the bearing capacity during the drainage process. Results from preliminary laboratory tests that demonstrate the feasibility of the drainage pile concept are described, and practical considerations that need to be investigated in greater detail are discussed. Introduction This paper introduces an unconventional method for increasing the load carrying capacity of offshore platform piles. The basic concept consists of increasing the axial capacity of friction piles embedded in clay soils by facilitating drainage of water from the soil formation into the pile. In this system, the drainage pile is analogous to a well point. The reduction in water content due to drainage causes the soil to consolidate around the pile, resulting in an increase in the soil shear strength. This leads to an increase in the soil-pile adhesion, and consequently in the axial pile capacity. The principle of driving water out of a soil layer in order to increase the soil strength locally is already a familiar one in the area of soil stabilization, and is the basis of sand drain technology and electro-osmosis. The conventional method of increasing the axial capacity of friction piles is to increase their diameter or embedded length. The pile dimensions, however, can only be increased until the penetration resistance equals the driving capacity of the largest pile driving hammers available. If additional bearing capacity is required to support the platform, the designer has to modify the design by either altering the platform configuration, changing the pile dimensions or resorting to the use of additional piles. Regardless of the alternative chosen, the overall cost of the platform can be expected to increase substantially. In such instances, the use of drainage piles to increase the bearing capacity without altering the design may prove to be an economically attractive alternative. Drainage piles may also find application in helping to increase the load-carrying capacity of existing platforms that fail to satisfy new or revised design criteria. In such platforms, the existing conventional piles could conceivably be converted into drainage piles, thereby increasing the load carrying capacity of the platform without increasing the pile dimensions.