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Monday, March 16, 2009

Bitumen coating on pile to educe skin friction


The settling soil imposes dragload on piles and may cause excessive settlement of pile foundation. The large magnitude of dragload may necessitate higher pile cross section and/or deeper pile penetration which increase the cost. The pile design must ensure that the dragload is accommodated without causing any structural distress and excessive settlement of pile. In the past various methods have been adopted to reduce the dragload depending upon the field condition. Coating the pile with bitumen is the most economical method for reducing the negative skin friction (Baligh et al., 1978).

The effectiveness of slip layer in reducing dragload depends on the characteristics of the pile, the type of soil strata through which pile passes and the properties of coating material itself. In case of fine grained soils, the shearing behavior depends on the average rate of soil settlement. In case of coarse grained soils, soil particles may penetrate into the coat during pile driving. The particle penetration may adversely affect the efficiency of coat in reducing the skin friction. The ideal coating material should have low viscosity to permit the slippage of soil surrounding the pile shaft and at the same time it should have adequate strength to adhere the pile shaft during storage and pile driving. The cost of coated pile can be much higher than that of uncoated pile (Briaud and Tucker, 1997).

The granular soil penetrates into coat during pile driving and may result in scrapping off the coat and higher skin friction. Therefore it is important to study the effectiveness of coating material in reducing the interface friction between pile material and granular soil. The selection of type of coat and thickness forms an important aspect of pile design for dragload mitigation and overall economy of the project. In the case of uncoated pile surface the frictional resistance is found to increase with the normal stress (or radial stress) whereas for the coated surface the frictional resistance is practically independent of the normal stress.

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