Experimental Investigation of PipelIne Stability In Very Soft Clay

ABSTRACT The stability of pipes embedded in clay at water content above the liquid limit was investigated. Pipe sections remain stable within a weight range depending on the clay moisture content. Soil resistances to flotation and settlement decrease, with the increase in clay water content, to zero at about twice the liquid limit. Soil resistances 'can be estimated as the clay strength times one-half the pipe surface area. A pullout test is proposed to determine the yield strength of clay. The flotation force is considered equal to the buoyancy calculated using the saturated unit weight of clay. Introduction Significance. Pipelines are essential means of transporting a wide range of products. Long sections of pipe are buried under various soil conditions, in different environments. These pipelines must be stable against any flotation or settlement leading to costly damage. Stability of buri'ed pipelines is influenced by several factors which have not been fully investigated. Objectives and Scope. The main objectives of this paper are to analyze the stability of pipe sections embedded in very soft clay, and to consider the effects of pertinent soil and pipe characteristics. Laboratory flotation and settlement tests were performed in which short pipe sections of different diameters were embedded in two clays prepared at four water contents above the liquid limit. Pullout tests were performed as a means of determining the yield strength of the experimental clays for use in pipeline stability analysis. The experimental research was conducted to study the short-term response of buried pipe; the effect of clay thixotropy was not considered. Criteria for pipeline stability were developed on the basis of experimental results and theoretical considerations. Review of Earlier Research. An ASCE task committee (1) reported that flotation of a pipe buried in clay is potential at a moisture content which can be determined from the results of liquid limit tests by extending the straight line relationship between number of blows and water content to 0.01 blows. Tests performed by the ASCE research council (2) indicated that flotation and settlement of buried pipe depend on the difference between the unit weights of soil and pipe, and the shear strength and liquid limit of soil. Reese and Cosbarian (3) studied the stability of pipe immersed in uniform suspension of soil grains in water, and of pipe buried in consolidated soil with shear strength. Bonar and Ghazzaly (4) proposed a pipeline stability analysis as a result of their laboratory investigation of the flotation of pipe sections embedded in cohesive soils. Theoretical Considerations Clay Consistency and Rheology. Problems of instability are more likely to be associated with pipelines buried in weaker clays. The strength of a clay is a function of its water content. Clays with water content above the liquid limit, referred to as liquid clays in this paper flow upon application of external forces and their mechanical properties can, therefore be investigated on the basis of rheology.