12,027 research outputs found
Rapporteur’s report – innovative geotechnologies for energy transition
No full textThe 9th Society for Underwater Technology (SUT) International Conference on Offshore Site Investigation and Geotechnics (OSIG) closed with a Rapporteur’s report given by the author. This paper provides a record of that report, transcribed from a video recording. The presentation slides are shown as Figures.</p
PSD measurement using the single particle optical sizing (SPOS) method
No full textA new technique for measuring the particle size distribution (PSD) of a soil, known as single particle optical sizing (SPOS), has been evaluated. A series of tests was conducted to compare the results obtained from sieve analysis and the SPOS method, using both uniform and well-graded sands of varying particle size and angularity. It was found that the SPOS method oversizes particles compared with sieving by 20-30%, as predicted by theoretical analysis. Measurements of particle size gathered from sieving and SPOS methods are not equivalent. However, as neither of the definitions of particle size implicit in each method can be considered as 'correct', this exercise does not represent a 'validation' or otherwise of the SPOS method. The repeatability of the SPOS method was demonstrated by sizing multiple samples obtained by riffling from the same batch of sand. The accuracy of the SPOS method was demonstrated by the high correlation between the calculated and measured PSD of mixed samples. Finally, a series of tests showed that the system can easily resolve a small (<1%) introduction of fine material into a coarser sample. As the technique requires only a small sample volume, it is particularly suited to the assessment of changes in PSD during geotechnical model or element testing.</p
Contributions to Géotechnique 1948-2008: physical modelling
No full textThis paper reviews the major contributions to Géotechnique that relate to physical modelling, including developments in modelling technology, important experimental observations, and the resulting advances in geotechnical engineering. An increasing proportion of the papers published by this journal involve physical modelling, conducted either at Ig or in a geotechnical centrifuge. Over the 60 years since Géotechnique was first published, experimental techniques have advanced significantly, improving the realism of small-scale simulations, and raising the quality and detail of the measurements that can be made. These techniques are reviewed, and some of the consequent advances in relation to foundations, tunnels, retaining walls and slopes are highlighted, as reported in the pages of Géotechnique.</p
A general framework for shaft resistance on displacement piles in sand
No full textThe stress history of a soil element adjacent to the shaft of a displacement pile in sand leads to a general relationship for predicting shaft resistance. The stages in the stress history are linked to the penetration process at the pile base, and subsequent 'friction fatigue' along the pile shaft. The stress history gives each part of the relationship physical significance, providing a rational basis for selecting design parameters. Recent field and laboratory data is used to illustrate each stage of the stress history, with emphasis on the influence of cyclic loading on 'friction fatigue' and the resulting mean unit shaft resistance.</p
Centrifuge modelling of the pushover failure of an electricity transmission tower
No full textCentrifuge model tests were conducted to examine foundation failure mechanisms during rapid horizontal pushover of an electricity transmission line support tower, simulating a broken transmission line response or wind gust loading. A model transmission tower supported on four pad foundations in clay and backfilled with sand was loaded horizontally and the loads at each foundation were measured during fast and slow pushover. The tests examined the influence of tensile resistance mobilized at the underside of the footings, which is difficult to reliably incorporate within design practice due to a lack of accepted quantitative design methods. The measured performance of the tower footings was compared with results from a series of tests where a single footing is subjected to purely vertical loading in compression and tension and was found to be in good agreement. The tower response was back-analysed as a simple push–pull model and the calculated uplift capacity of the footing backfill provided a close match to the observed response of the tower footings subjected to slow pushover. During fast pushover, the additional capacity mobilized due to tensile resistance (suction) created by the reverse bearing capacity beneath the base of the footings subjected to uplift was quantified using a suction capacity factor
Reply to the discussion by McCarron on “Modelling spatial variability in as-laid embedment for high pressure and high temperature (HPHT) pipeline design”
No full textN/AThe accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author
Volumetric hardening in axial pipe soil interaction
No full textAxial 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.</p
Modelling the dynamic embedment of seabed pipelines
No full textThe as-laid embedment of a seabed pipeline is an important design parameter. As a pipe is laid on the seabed it oscillates, owing to vessel motion and hydrodynamic loading of the hanging pipe. This movement significantly increases the pipe embedment beyond the theoretical value related to the static pipe weight, even when corrected for any stress concentration caused by the hanging catenary. Dynamic lay effects are either ignored in practice, or are accounted for by scaling up the static embedment by an empirical factor, leading to significant uncertainty in this important design parameter. A series of centrifuge model tests has been conducted using two clays - kaolin and a high-plasticity natural clay - to simulate the dynamic embedment process. The results indicate that only a few cycles of small-amplitude oscillation (60.05D) are required to double or triple the pipe embedment, owing to the combined effect of lateral ploughing and soil softening. In these experiments the pipe embedment increased to up to eight times the static embedment after 100 cycles of motion, which represents a typical lay process. A model is proposed for the cycleby- cycle embedment of a pipeline under a given sequence of small-amplitude oscillations at a given applied vertical force. The trajectory of the pipe movement is assessed using a flow rule derived from plasticity-based yield envelopes. The effect of soil remoulding is explicitly captured by linking the accumulated disturbance to the decay in soil strength. Using input parameters derived from theoretical considerations and T-bar penetrometer tests, the model captures the essential features of the dynamic embedment process. With modest optimization of the model parameters, the mean discrepancy between the calculated and measured embedment is only 12% for both clays. The ultimate states predicted by this cycle-bycycle model also provide a rough estimate of the maximum pipe embedment for fully remoulded conditions, which include some degree of water entrainment caused by the lay process, evident in the optimised parameters. This ultimate embedment is governed by the remoulded soil strength and the pipe weight (augmented by any stress concentration). The amplitude of the cyclic motion affects the rate of softening, and hence the rate of settlement. This model provides a framework for assessing the as-laid embedment of seabed pipelines on a more rigorous basis than current practice.</p
The mechanism of steady friction between seabed pipelines and clay soils
No full textThe large-amplitude lateral soil resistance between an on-bottom pipeline and the seabed is an important design parameter in assessing pipeline behaviour during lateral thermal buckling or under the impact of a submarine slide. This paper describes a series of centrifuge model tests that shed light on the underlying behaviour during large-amplitude lateral pipe movement. It is shown that at large displacements the lateral response is governed predominantly by the passive resistance of the growing berm of soil ahead of the pipe. Using a new analysis of this growing soil berm, based on conservation of volume, the 'local' embedment of the pipe relative to the top of the idealised soil berm is defined. In this way, the normalised lateral pipe-soil resistance, H/suD, from tests encompassing a range of pipe weights and initial embedments follows a single trend line. This idealisation of the response is more consistent than the usual terminology of a pipe-soil friction factor.</p
Shallow penetrometer penetration resistance
No full textShallow penetrometers-such as the hemiball and toroid-were conceived as potential in situ testing devices with the ability to measure: (1) soil strength parameters during vertical penetration, (2) soil consolidation characteristics during dissipation tests postpenetration, and (3) interface friction during torsional loading. Knowledge of the response of soil to such tests is critical to the design of subsea pipelines and the ability to measure the response of soil to all three types of test using a single device in situ from a mobile testing platform, such as a remotely operated vehicle (ROV), would be highly advantageous. Potential benefits of the employment of such devices could include significant time and cost savings and improved spatial measurement density, since more tests could be conducted along the route of a pipeline if an ROV is used as a mobile in situ testing platform. This paper presents an assessment of the ability of the hemiball and toroid to measure soil strength parameters directly from their response to vertical penetration. A large deformation finite-element approach was employed to model the penetration process and initial simulations were validated against small-strain analyses published in the literature. A comprehensive parametric study was then conducted investigating the impact on normalized penetration resistance of soil unit weight, shear strength gradient and penetrometer-soil interface friction. A forward model was derived from the parametric analyses and its inverse performance (i.e., the ability to infer soil parameters from force-displacement response) was assessed using additional large deformation analyses with randomly assigned material parameters within realistic bounds. Both variants of shallow penetrometer investigated are found to be well suited to inferring soil strength parameters directly from their response to vertical penetration.</p
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