Volumetric hardening in axial pipe soil interaction

Axial pipe soil interaction resistance is an important input into the design of pipelines, particular high pressure high temperature (HPHT) lines that are susceptible to 'axial walking'. This global movement of the pipeline can require costly mitigation through anchoring systems. This paper provides the results of a large scale axial pipe model test on soft clay showing the effect of volumetric hardening on the soil around the pipe and the resulting axial resistance. It has previously been hypothesised that the resistance to axial movement of pipelines on soft clay will increase over time due to the repeated shearing during expansion and contraction. This shearing causes failure, positive excess pore pressure and subsequent volumetric hardening leading to an increase in the undrained strength in the underlying clay. The resulting increase in axial resistance would reduce the pipe walking rate and the requirement for mitigation. The model test used natural reconstituted clay and a large scale (120 mm diameter) pipeline which was subjected to a series of axial sweeps with intermediate dissipation of excess pore water pressure. Over 13 sweeps and 30 days of testing it was observed that the residual axial pipe-soil resistance almost doubled. This was matched by a 19 % reduction in water content in the clay directly beneath the pipe and an almost 35 % increase in the T-bar-measured shear strength of the clay directly beneath the pipe. This dataset - quantifying both the changing axial pipe-soil resistance and the volumetric hardening of the underlying clay -provides strong evidence to support the hypothesis that axial resistance pipe-soil resistance on soft clay will progressively rise during the life of a pipeline. If this observation can be verified and scaled to field conditions and in situ seabed soils, it offers the possibility of less onerous axial walking mitigation requirements and resultant cost savings.