Some observations in soil–pipe model tests under vertical uplift loading in compacted clay

Abstract

Physical model tests are conducted to measure the soil resistance exerted on a steel pipe under vertical uplift loading at varying displacement rates in compacted clay. It is observed that the measured soil ultimate resistances are much lower (two to three times in magnitude) than those recommended by existing design guidelines for pipelines buried at shallow depths. This large discrepancy is attributed to the differences in failure modes observed in the tests and assumed in the guidelines. Under vertical uplift loading, the tensile failure mode is more dominant than the shear failure mode in compacted clay at shallow depths, which is not taken into account in the existing guidelines. Furthermore, the results illustrate that the soil–pipe system exhibits a time- or rate-dependent behaviour under vertical uplift loading – that is, ‘isotach’ behaviour. The soil resistance increases with increasing pipe displacement rate, or vice versa. The soil resistance drops by 10–30% when stress relaxation is allowed. Practical implications of this time-dependent behaviour of compacted clay on the performance of buried pipelines subjected to long-term ground movement are addressed.

Suction effects during uplift of steel pipes buried in compacted soil

“…Note that the uplift factor is in the same range as the results obtained by Wong et al [36] during uplift tests performed at different pulling rates on 150-mmand 250-mm-diameter pipes buried in compacted Regina clay. The soil tested by Wong et al [36] was compacted at 85-90% of its maximum dry unit weight (c dry-= 12.75-13.5 kN/m 3 ) and its water content was w = 30%, about 1.5% on the wet side of its optimum. According to the WRC presented by Wong et al, the initial suction in their experiments was approximately s = 100 kPa.…”

“…10, as well as in other relevant studies [2,12,16,35,37], demonstrate that the shape of the failure mechanism developing in dry sand during uplift is function of sand density and of the pipe embedment. Recently, Wong et al [36] postulated that a tensile-shear failure mechanism developed during uplift tests in compacted Regina clay. This mechanism is characterised by the formation of tensile cracks at the pipe springline during the initial stages of pipe uplift, which lead to the formation of a soil beam as pipe displacement increases and cracks propagate towards the surface.…”

“…Arguably there is also a lack of highquality experimental data to benchmark methods for deriving properties of Winkler springs for compacted backfill materials, where soil suction would be introduced as a parameter, and thus to consider its effects in pipeline engineering practice. An exception is the recent study of Wong et al [36], who investigated the effect of the uplift rate on the reaction developing on pipes buried in compacted Regina clay and provide insights on the failure mechanism that developed in the compacted backfill. However, Wong et al [36] have not studied parametrically the effect of the soil water content (i.e.…”

“…An exception is the recent study of Wong et al [36], who investigated the effect of the uplift rate on the reaction developing on pipes buried in compacted Regina clay and provide insights on the failure mechanism that developed in the compacted backfill. However, Wong et al [36] have not studied parametrically the effect of the soil water content (i.e. soil suction) of the backfill material or of the pipe embedment depth on the magnitude of the developing soil reaction.…”

Interpretation of uplift pipeline–soil interaction behaviour using acoustic emission measurements

“…A significant proportion of these assets are buried in soil for protection and support. Pipeline sections can experience upwards displacement through the surrounding soil due to: differential ground movement (e.g., fault-rupture or landsliding intersecting a buried pipeline); flotation of subsea pipelines (or pipelines buried under bodies of water) that have a very low combined pipe/fluid weight; the development of excess water pressures around the pipe; and upheaval buckling whereby axial thermal expansion forces, restrained by the pipe-soil interface friction and end connections, are relieved by pipe buckling (Cheuk et al 2008;White et al 2008;Byrne et al 2013;Williams et al 2013;Roy et al 2018;Wong et al 2021). Pipe uplift can result in significant strains and tensile or buckling failure, which can have catastrophic economic, environmental, and societal consequences, and the service of entire networks can be terminated (Karamitros et al 2007;Vazouras et al 2015;Robert et al 2016).…”