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Technicalpaper: The significance of soil conditions and trench preparation in relation to pipeline performance

By Trevor R Smith, Stanton and Staveley

This paper was first publsihed in GE’s April 1976 issue


t is not so long ago since the most senous problems facing the pipeline engineer were to get his pipe in the ground, jointed and covered with the minimum of fuss. Provided the pipeline did not leak and as long as it remained covered up, all was well. Pipes at the turn of the century were, in general, very thick and rigid and the demands placed upon them were not very high. Today’s pipeline engineer is faced with more exacting operational requirements which include higher pressures, greater- traffic-induced loading, tighter leakage control and minimum disturbance to the environment. In addition to the more exacting technical requirements, design decisions are significantly influenced by financial aspects.

As pipe manufacturing technology has improved, so higher strength materials have become available and pipe economy has been achieved by utilising these material properties to allow reduction in pipe wall thickness. Previously, the very thick walled pipe has necessitated consideration to be given to the supporting effect of the soil.

Pioneer work on the application of soil mechanics to pipeline design was undertaken in America at the Iowa State College by Spangler, Marston and Schlick and although the passage of time has led to various alternate hypotheses and modifications being proposed, the original work by these three still remains valid today. This pioneering work indicated how the bedding around a pipe can influence its structural strength and quickly led to the introduction of design parameters such as bedding factors or soil modulus.

The first part of this paper outlines the influence of these factors on the performance of rigid and flexible pipe(ines and is thus able to give a guide on trench preparation and selection of embedment materials. However, this is only one part of the pipeline design which is influenced by soil conditions. The mechanical properties of the soil can also be a significant factor in anchor block design. In addition to the soil mechanics aspects of pipeline design, appropriate attention has to be given to the influence of soil chemistry as this can have a bearing on the selection and suitability of pipeline materials or give guidance to the type of corrosion protection which is both adequate and economically attractive. Both these additional aspects are also discussed in this paper. 

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