Behaviour of buried pipes in unstable sandy slopes

Weiyuan Zhang,Amin Askarinejad

doi:10.1007/s10346-018-1066-1

Abstract

Significant forces can be applied to embedded pipelines in sloping grounds due to soil instabilities, which potentially might lead to leakage of hazardous fluids into the environment. The soil-pipeline interaction in sandy slopes has been investigated experimentally using small-scale physical models tested in geotechnical centrifuge. A novel method is developed in this paper to estimate the ultimate external forces, induced by slope failures, acting on buried pipes at various locations inside the slope. Instabilities were triggered by surcharge loading on the slope crest in the centrifuge tests. Six dense coarse sandy slopes were tested with different pipe locations with respect to the slope crest. Moreover, two medium dense fine sand slopes were tested in the same manner to study the effect of the grain size distribution on the soil-pipe interaction. The external forces on the pipe induced by the surrounding soil movements were calculated based on the measurements of four strain gauges installed on the pipe. The shape of failure surface and pipe movements were monitored with the aid of advanced image analysis techniques. The results indicate that a buried pipeline has the potential to affect the slope failure mechanism. Normalised force-pipe displacement relationships were derived and compared to the estimation methods suggested in previous studies, which were mainly done on pipes installed in flat grounds. A new prediction method is introduced in this study, which considers the pipe burial distance to the slope crest. Moreover, the slope angle effect on the ultimate force applied to the pipe is also investigated, and a generalised formula is developed. Finally, two examples of the application of the new method are presented for pipelines installed at the toe of two large-scale subarial and submarine slopes.

Highlights

Pipelines are widely used to transport fluids over long distances and may pass through various geological and topographic conditions
Reliable perdition of the external forces applied to the pipe as a result of slope instability is of utmost importance in the design or evaluation process
The maximum soil load acting on a pipe buried in a flat ground surface depends on the soil unit weight, pipe-embedment depth and the ultimate bearing capacity factor, which is a function of Hc/ D and soil friction angle (Trautmann and O'Rourke 1985, Guo and Stolle 2005)

Summary

Introduction

Pipelines are widely used to transport fluids over long distances and may pass through various geological and topographic conditions. Liu et al (2015) designed lateral pull-out tests to investigate the resultant forces on the pipelines both in sand and soft clay samples They claimed that the lateral soil resistance increases with increasing embedment ratio, Hc/D. The majority of the experimental work conducted to study the soil-pipeline interaction is performed on small or medium scale models at normal Earth gravity condition (1 g) This type of physical modelling technique, quite informative in a qualitative way, cannot be used reliably for quantitative analysis as the stress state of the soil in the model and prototype are different (Terzaghi 1943). Consolidated - drained triaxial tests at two different confining pressures of 25 and 100 kPa were performed on Geba Sand samples with relative density of 55% to determine the internal friction angle The grains of both sands are characterised as sub-angular based on image analysis techniques (De Jager et al 2017; Maghsoudloo et al 2017).

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Results and discussion

Selected data