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It is well known that carbonate sands possess weak mineralogies, complex particle morphologies and porous microstructures. These characteristics lead to very distinct mechanical properties of carbonate sands, such as low shear strength, high crushability and high permeability. This paper presents a novel investigation of the recognition and tracking of intact carbonate sand particles using a deep learning method called PointNet++. The capability of PointNet++ to extract the global and local features of the porous structures of carbonate sand particles enables it to excel in the pattern recognition of porous granular materials. First, for the reconstruction of carbonate sand particles, a set of two-dimensional raw images obtained from X-ray microtomography scanning were handled by a series of image processing techniques such as median filter, segmentation and thresholding algorithms. In particular, a special technique previously developed by the authors was used to treat the abundant intra-particle pores and surface concavities of carbonate sand particles to avoid the problem of image over-segmentation. Second, to prepare the training datasets to be used in the PointNet++ deep learning exercise, a strategy of sampling and grouping was proposed to divide the initial point set of each sand particle into several groups. Next, PointNet++ was utilised to capture the global and local context features of the sand particles at different length scales and shown to successfully recognise and track all of the particles. Finally, a comprehensive comparison between several particle tracking methods reported in the literature was made, and the outstanding advantages of the deep learning-based particle tracking method were summarised. [ABSTRACT FROM AUTHOR]

This paper describes a centrifuge study using a range of penetrometer tests (T-bar, piezocone and free-fall piezocone) to explore strength changes in a reconstituted, normally consolidated, natural calcareous silt. Various penetrometer test procedures were applied to measure the penetration resistances including monotonic, cyclic and twitch-type movements as well as pauses for pore pressure dissipation. These mobilised combinations of partial or full remoulding, strain softening, consolidation and viscous rate effects. The penetrometer resistance – representing a proxy for strength – reduced by a factor of 4·1 from drained to undrained conditions (at the lowest fully undrained penetration rate). In undrained conditions, viscous enhancement of the penetration resistance raised the tip and shaft resistance in free-fall piezocone tests by ∼2·8 and ∼3·6 times, respectively. The 'restart' resistance immediately after the dissipation tests was ∼2·5 times higher than the resistance prior to dissipation, giving an indication of consolidation-induced strength gain. The 'twitch' test (using sequential steps decreasing the velocity) captured drainage and viscous rate effects, and also gave a 'restart' resistance that showed even greater consolidation effects than from a dissipation test. Overall, the different penetrometer test types and procedures measured resistances in the same soil sample that varied by a factor exceeding 20 from highest to lowest, resulting from different penetration rates and history, due to strain rate, strain level (or remoulding) and consolidation. An expression for the monotonic penetration resistance combining drainage and viscous rate effects was fitted to the response of all tests, spanning >7 orders of magnitude in strain rate. [ABSTRACT FROM AUTHOR]

This paper details analysis of deformation behaviour of silica and carbonate sands under a rectangular foundation subject to uniaxial vertical load based on results from a series of centrifuge model tests. A multiscale particle image velocimetry/digital image correlation (PIV/DIC) technique was used to record and analyse the foundation tests with high resolution and measurement precision. Cone penetrometer and pressuremeter tests provide in situ soil characterisation of the tested sand sample in the centrifuge environment. The soil behaviour is analysed through foundation load–settlement response and the observed soil deformation measurements. Different soil deformation mechanisms and strain behaviours were observed in the different sands tested, and the particle shape effect is considered, with data from scanning electron microscopy, to explain the differences. The results and analyses contribute towards better understanding of different soil behaviours under shallow foundations in different sands. [ABSTRACT FROM AUTHOR]

This paper focuses on the properties of a crushable, uncemented, carbonate sand, retrieved from an artificial island to be used for petroleum production offshore of the United Arab Emirates. The sand was dredged from an offshore borrow area, placed hydraulically within engineered bunds to create the island and densified by vibroflotation. The results of a comprehensive laboratory testing programme are summarised in this paper, providing a set of mechanical properties for carbonate sand. The laboratory programme included static, cyclic and dynamic tests, the results of which are interpreted in a critical state soil mechanics framework, including stress dilatancy. The results of this testing programme contribute to on-going research on the behaviour of carbonate sands under static and cyclic loading conditions. The results are also compared with selected data for other carbonate and siliceous sands, and highlight the behaviour of crushable, carbonate sands used as hydraulic fills. [ABSTRACT FROM AUTHOR]

This paper describes a project-based cone penetration test (CPT) calibration on a carbonate sand fill that was hydraulically placed and subsequently densified by vibrocompaction. The project involved the development of an artificial island constructed offshore from the United Arab Emirates for oil and gas production. Carbonate sands have crushable grains and can be significantly more compressible than silica sands, so it was determined that the semi-empirical CPT-based correlations for silica sands were not applicable and a soil-specific calibration was needed for post-densification characterisation of density, shear strength and compressibility. The CPT calibration investigation was primarily undertaken in a centrifuge, and then checked with supplementary tests in a large calibration chamber. In this paper, analysis of the calibration data follows three different threads for comparison with more typical siliceous sands: ( a) relationships between CPT tip resistance, void ratio and vertical effective stress; ( b) a relationship between CPT tip resistance and state parameter; and ( c) an approach based on cavity expansion theory, which provides predictive capability for the CPT once soil properties have been measured in the laboratory. The test results are compared with silica sands, in terms of void ratio or the state parameter ψ. [ABSTRACT FROM AUTHOR]

Shelly carbonate sands proliferate in regions of the world where construction of offshore structures is in high demand. These structurally weak sands have high intra-granular voids and complex angular grain shapes. To improve the understanding of the mechanical properties of the material, a detailed morphological quantification is required. This paper presents a three-dimensional characterisation of the morphology of shelly carbonate sands based on analyses of X-ray computed tomography images. Two sands from the Persian Gulf with distinct grading were investigated. An adaptive watershed segmentation technique was developed to identify the individual grains for subsequent grain-scale analysis, which overcomes the challenges posed by the intricate microstructure of these sands. Non-invasive measurements of size, intra-granular void and various shape parameters were carried out, and statistical analyses were conducted, to characterise the grains. The results help to better understand the mechanisms of grain interlocking, and the role of grain angularity and intra-granular void ratio on the mechanical behaviour of shelly carbonate sands. [ABSTRACT FROM AUTHOR]

Challenges associated with dynamically installed anchors include prediction of the anchor embedment depth, which dictates the anchor's holding capacity. This is particularly true for calcareous sediments, as very little performance data exist for this anchor type in these soils. This paper reports results from a series of model tests undertaken to provide insight into the behaviour of a torpedo anchor during dynamic installation and monotonic pullout in lightly overconsolidated calcareous silt. The tests were carried out in a beam centrifuge, varying the drop height and consequently the impact velocity, and the consolidation period prior to anchor pullout. The mudline load inclination was also varied to encompass various mooring configurations. The centrifuge model test data were used to calibrate: () an analytical dynamic embedment model, based on conventional bearing and frictional resistance factors but with strain-rate-dependent undrained shear strength for the soil; and () an analytical quasi-static vertical pullout capacity model, accounting for reverse end bearing and frictional resistance. A total energy based expression, appropriate for calcareous silts, was proposed for predicting anchor embedment depth for a given anchor geometry, mass and impact velocity. For assessing anchor vertical holding capacity, a piezocone based direct design approach was also proposed, deriving anchor end bearing and frictional resistance from cone tip resistance and sleeve friction, respectively. Anchor capacity under inclined loading was presented as failure envelopes expressed in terms of dimensionless vertical and horizontal components of anchor net resistance, which agreed well with a finite-element based envelope developed for embedded foundations. The regain of anchor capacity was found to be in good agreement with predictions based on the cavity expansion framework. [ABSTRACT FROM AUTHOR]

This paper presents data from ring-shear and one-dimensional compression tests, which have been used to cause breakage under large shear strains and large compressive stresses respectively. A comparison of the behaviour of carbonate sand in monotonic shear and compression is reported, focusing on void collapse, grading properties and particle shape evolution. Trends in breakage behaviour under compression and under shear were similar; however, ultimate steady-state properties differed significantly. The higher effect of shear breakage was evident, in that the steady-state gradings for sheared material had higher fractal dimensions, indicating that the gradings were finer, and morphological changes in particle shape were more significant. Steady-state gradings tended to be multi-fractal in nature rather than mono-fractal, and showed that a unique monotonic steady-state grading does not exist. Two types of ring-shear device were compared, and the differences between the results highlighted a dependence of steady-state properties on test apparatus configuration. [ABSTRACT FROM AUTHOR]

Chemical and biological processes, such as dissolution in gypsiferous sands and biodegradation in waste refuse, result in mass or particle loss, which in turn lead to changes in solid and void phase volumes and grading. Data on phase volume and grading changes have been obtained from oedometric dissolution tests on sand-salt mixtures. Phase volume changes are defined by a (dissolution-induced) void volume change parameter (). Grading changes are interpreted using grading entropy coordinates, which allow a grading curve to be depicted as a single data point and changes in grading as a vector quantity rather than a family of distribution curves. By combining contours with pre- to post-dissolution grading entropy coordinate paths, an innovative interpretation of the volumetric consequences of particle loss is obtained. Paths associated with small soluble particles, the loss of which triggers relatively little settlement but large increase in void ratio, track parallel to the contours. Paths associated with the loss of larger particles, which can destabilise the sand skeleton, tend to track across the contours. [ABSTRACT FROM AUTHOR]

Reliable prediction of the embedment of untrenched subsea pipelines is of increasing importance as hydrocarbon developments progress into deeper waters, located further from shore. Pipeline design issues such as hydrodynamic stability, lateral buckling and axial walking require accurate assessment of the pipe embedment, in order to assess correctly the pipe-soil resistance forces and the thermal insulation provided by the soil. This study presents a detailed back-analysis of the laying process and the as-laid condition of a pipeline on carbonate sediments. The pipe embedment is linked to the relevant soil properties, metocean conditions, vessel motions, and lay geometry along the route. A cycle-by-cycle framework is proposed for the development of embedment as the pipe is subjected to oscillations during laying. The calculations use parameters obtained from standard in situ tests, and are applied across a range of soil and lay conditions along this particular pipeline route. The proposed calculation framework incorporates the effect of the lay rate and the pipeline catenary on the embedment process. It offers a significant improvement on the current practice of applying empirical multiplicative factors to the calculated static embedment in order to account for dynamic lay effects. [ABSTRACT FROM AUTHOR]

Mobile drilling platforms perform the majority of the world's drilling in water depths up to approximately 120 m. The installation of their large, inverted, conical spudcan footings into fine-grained silt material is considered problematic by an industry that has usually defined the conditions of installation as either undrained in 'clays' or drained in 'sands'. This is reflected in current SNAME industry guidelines. Little guidance is provided for calculating the penetration resistance of spudcans in partially drained conditions. The use of full flow-round penetrometers may provide the opportunity to directly correlate spudcan predictions with in situ measurements. Previous centrifuge testing of T-bars and spudcans has shown a similar bearing pressure for installation under undrained conditions in kaolin clay. However, to provide correlation between a penetrating spudcan and penetrometers in soils displaying partially drained conditions, relationships between size, shape and penetration rate are required. This technical note provides an experimental comparison of the 'in-flight' installation of a T-bar penetrometer and a typically shaped spudcan footing in reconstituted calcareous silt. The experiments were conducted at an acceleration of 100 times that of Earth's gravity (known as 100) in the drum centrifuge at the University of Western Australia. The primary variable was the rate of installation. The experimental aims were first to accurately measure the change in capacity with normalised velocity for both the T-bar and spudcan, second to compare these silt results to other published experimental data on kaolin clay soils, and third to investigate the relationship between bearing pressure on a T-bar and a spudcan footing. [ABSTRACT FROM AUTHOR]

The results of drained cyclic triaxial tests performed on Dogs Bay sand (a carbonate sand) are discussed in this paper. For this sand it has been previously demonstrated that the critical state exists and is unique. The relationship between volumetric strains due to cyclic loading and the critical state of this material under monotonic conditions is explored here. Drained cyclic tests on loose samples have been carried out, applying different amplitudes of cyclic loading for samples that generally have initial states on the wet side of critical. The trends of behaviour are summarised, pointing out the relationship between initial state parameter, amplitude of cyclic loading and final change in the state parameter at equilibrium. Experimental evidence is also given to demonstrate that the sand has a memory of the volumetric strain that has previously occurred when different patterns of cyclic loading are consecutively applied and equilibrium has not been reached. [ABSTRACT FROM AUTHOR]

Maximum working loads in the existing grand assembly area at Drydocks World - Dubai have been increased following three-dimensional finite-element analysis of the large ground-bearing slab and underlying carbonate sand. To obtain good calibration data, precise monitoring of the slab was undertaken during live launch operations using automated total station and precise levelling techniques. The uncertainty in these measurements was much improved at night in the absence of solar radiation and heat shimmer effects. The monitoring data revealed a higher stiffness of the carbonate sands than expected, which was the main reason for allowing working loads to be increased. It is recommended that more research be undertaken into the engineering properties of the calcareous soils of this region in order to develop more efficient geotechnical designs. [ABSTRACT FROM AUTHOR]

Dynamic loading of embankment, foundation and pavement structures results in particle breakage of the constituent granular materials, when the stresses imposed on their particles exceed their strength. This paper presents the results of a number of drained cyclic triaxial tests on loose, uniformly graded samples of Dogs bay carbonate sand. It is observed that particle breakage is dependent on stress level, cyclic stress ratio, and creep and is directly related to volumetric strain. Drained cycling increases volumetric strain and therefore more breakage occurs when larger numbers of cycles are imposed. The increase in particle breakage from one cycle to the next indicates that while particles may not be loaded to their full capacity in a given cycle, they can be crushed in subsequent cycles without any variation in the amplitude of cyclic loading. [ABSTRACT FROM AUTHOR]

The current paper documents an experimental study conducted to evaluate the internal stability of sands subjected to seepage at high stresses. A custom-made one-dimensional compression cell with fluid flow capabilities was used to load and flush saturated specimens composed of selected sands with heterogeneous mineral composition. The degree of internal stability (i.e. the extent to which a soil can retain its mobile particles) was quantified with both the size distribution curves and the relative amounts of fragments collected in the effluent. Results indicate that: (a) uniform internally stable sands become internally unstable at high stresses, albeit temporarily; and (b) internal instability features are dependent on mineral composition and the extent of particle breakage. Particle-level force analyses suggest that the internal stability of crushed sands is dependent on the hydrodynamic conditions imposed. [ABSTRACT FROM AUTHOR]

The paper presents a study of the undrained behaviour of an uncemented carbonate sand (Quiou sand) under simple shear loading conditions. The experimental study was conducted through cyclic and monotonic undrained/constant volume simple shear tests carried out on reconstituted specimens prepared by using the sedimentation in water (WS) method. Tests were carried out on specimens reconstituted at two void ratios (i.e. loose and dense) and different effective consolidation stresses. Furthermore, to account for the effect of non-zero mean shear stress level, cyclic simple shear tests were performed under both symmetrical and non-symmetrical cyclic loading. Two types of failure modes have been observed in cyclic tests, that is 'cyclic liquefaction' or 'cyclic mobility', depending on whether or not they were conducted under shear stress reversal conditions. A unified framework seems to exist whereby undrained monotonic and cyclic response can be comparatively analysed. In particular, normalising the cyclic liquefaction resistances obtained from symmetrical tests by phase transformation strengths determined in corresponding monotonic tests, provides a cyclic liquefaction resistance curve which was found to be unique, irrespective of initial void ratio and vertical effective stress. Undrained cyclic shear strength of the tested sand appears to be affected by the presence of a non-zero mean shear stress, following a pattern of behaviour, which is similar for both loose and dense specimens. Furthermore, the normalised stress-strain curves of the cyclic tests show back-bone curves that are practically coincident with the equivalent monotonic curves. [ABSTRACT FROM AUTHOR]

A series of pile load tests was carried out in a centrifuge model, investigating the influence of the in situ volume-stress state on the load-deflection behaviour. Two sands of different mineralogies were tested at a variety of accelerations and sample densities. In each case a good correlation was found between the load-deflection behaviour and the in situ state quantified by the distance of the initial state from the critical state line in the v-ln p' plane. How the soil arrived at its initial state, whether by compaction or overconsolidation, did not have a significant impact on the load-deflection behaviour. Repeated loading with a zero head load condition maintained between each cycle was not found to influence significantly the capacity or stiffness of response, but loading the pile to its full tensile capacity was found to reduce significantly the stiffness of response in compression. [ABSTRACT FROM AUTHOR]

The underlying mechanisms governing the behaviour of displacement piles in sand are not well understood, leading to unreliability in design methods. A series of plane- strain calibration chamber tests has been conducted in order to quantify the penetration mechanism around the pile tip, and the response of the interface layer adjacent to the shaft during further penetration. A series of eight tests is reported, examining the influence of soil type, initial state, pile breadth and the use of a driving shoe. A novel image-based deformation measurement technique has been used to observe the displacement and strain paths, which are found to be relatively independent of soil type. The measured strain paths are similar to predictions made by the strain path method, and contrast sharply with assumptions implicit in cavity expansion solutions. An interface zone adjacent to the pile shaft comprising fine broken soil particles was observed to contract while shearing along the pile-soil interface. This mechanism offers an explanation for the degradation of shaft friction at a given soil horizon with increased pile penetration ('friction fatigue'), and a subsequent recovery of capacity over time ('set-up'). [ABSTRACT FROM AUTHOR]

A series of ring shear tests was conducted to investigate the development of particle breakage with shear strain for a carbonate sand. It was found that at very large displacements the soil reached a stable grading, but that the final grading was dependent on both the applied normal stress and the initial grading. The particle breakage caused a volumetric compression, which again ceased only when the stable grading had been attained, emphasising that critical states as observed at much smaller strains in triaxial tests are not rigorously defined. Despite the severe degradation of the soil particles the mobilised angle of shearing resistance was found not to change significantly. [ABSTRACT FROM AUTHOR]

The behaviour of three calcareous sediments—a muddy silt, a silt and a sand from offshore areas on the North- West Shelf of Australia—is studied through undrained simple shear tests. In monotonic tests, the muddy silt shows contractive behaviour and the silt and sand show dilative behaviour. In cyclic tests, a cyclic mobility failure mode is observed for the three soils. Responses in non-symmetrical cyclic tests were significantly different from those in symmetrical cyclic tests. A 'cyclic phase transformation state', which separates undrained cyclic stress paths into contractive and dilative phases, was identified: this was found to be different from the phase transformation line obtained from monotonic tests, and it is unique for a particular soil. The three soils show strain hardening behaviour in plots of stress ratio against shear strain, which can be characterised by a backbone curve and hysteresis loops. [ABSTRACT FROM AUTHOR]

The article presents an analysis of suction and strength in heavily compressed carbonate sand. When oven-dried powdered clays are heavily compressed they acquire tensile strength, which is variously attributed to bonding or to suctions in water not evaporated during oven drying. The tests described show that, after compression under stresses of the order of 100 MPa, oven-dried carbonate sand develops tensile and compressive strengths of the order of 1 MPa due to relatively large suctions in small quantities of water released from within the pores of the carbonate material. Although the sand samples were initially oven dried they had small measurable water contents after heavy one-dimensional compression.