Uplift Resistance of Buried Pipelines at Low Cover-Diameter Ratios

Abstract

Reliable estimates for the maximum available uplift resistance from the backfill soil are essential to prevent upheaval buckling of buried pipelines. The current design code DNV RP F110 does not offer guidance on how to predict the uplift resistance when the cover:pipe diameter (H/D) ratio is less than 2. Hence the current industry practice is to discount the shear contribution from uplift resitance for design scenarios with H/D ratios less than 1. The necessity of this extra conservatism is assessed through a series of full-scale and centrifuge tests, 21 in total, at the Schofield Centre, University of Cambridge. Backfill types include saturated loose sand, saturated dense sand and dry gravel. Data revealed that the Vertical Slip Surface Model remains applicable for design scenarios in loose sand, dense sand and gravel with H/D ratios less than 1, and that there is no evidence that the contribution from shear should be ignored at these low H/D ratios. For uplift events in gravel, the shear component seems reliable if the cover is more than 1-2 times the average particle size (D 50 ), and more research effort is currenty being carried out to verify this conclusion. Strain analysis from the Particle Image Velocimetry (PIV) technique proves that the Vertical Slip Surface Model is a good representation of the true uplift deformation mechanism in loose sand at H/D ratios between 0.5 and 3.5. At very low H/D ratios (H/D < 0.5), the deformation mechanism is more wedge-like, but the increased contribution from soil weight is likely to be compensated by the reduced shear contributions. Hence the design equation based on the Vertical Slip Surface Model still produces good estimates for the maximum available uplift resistance. The evolution of shear strain field from PIV analysis provides useful insight into how uplift resistance is mobilized as the uplift event progresses.

Reliability functions for buried submarine pipelines in clay subjected to upheaval buckling

“…It is important to note that the typical values of H in practice range from 0.5 m to 2 m [2]. In this work, the cover height was represented using a lognormal distribution with a mean cover height of 1 m and COV equal to 10% as seen in Table 1.…”

“…It has been widely recognized the uncertainties associated with the parameters involved in both pipe-soil uplift capacity and vertical driving force (Friis-Hansen [1]; Wang et al [2,3]). …”

Upward Pipe–Soil Interaction for Shallowly Buried Pipelines in Dense Sand

“…The uplift resistance offered by soil (Fv) depends on upward displacement (v) and generally comprises three components: (i) the submerged weight of soil being lifted (Ws); (ii) the vertical component of shearing resistance offered by the soil (Sv); and (iii) suction under the pipe (Fsuc). The component Fsuc could be neglected for a drained loading condition at low uplift velocities (Bransby and Ireland 2009;Wang et al 2010). The force-displacement behaviour is generally expressed in normalized form using Nv = Fv/HD and ̃ = v/D, where  is the effective unit weight of soil, which is the dry unit weight in physical model tests and FE modelling of uplift behaviour presented in this study.…”

“…The load-displacement curves obtained from model tests evolve from complex deformation mechanisms and the stress-strain behaviour of soil above the pipe. To understand these mechanisms, the particle image velocimetry (PIV) technique (White et al 2003) has been used in recent model tests (Cheuk et al 2008;White et al 2008;Thusyanthan et al 2010;Wang et al 2010). When the peak uplift resistance mobilizes in medium to dense sand, two inclined symmetric slip planes form in the backfill soil, starting from the springline of the pipe (White et al 2008).…”

“…In the former one, the slip plane forms at an angle  to the vertical, while  = 0 in the latter one. Experimental studies show that the vertical slip plane model is primarily applicable to loose sand at medium ̃(White et al 2001;Wang et al 2010). For dense sand, two symmetrical inclined slip planes form from the springline of the pipe at  ~ (White et al 2008;Huang et al 2015).…”