Coarse Skeleton Research Articles

Isotropic compression and triaxial shear behaviors of cement- and cement-gravel-treated granite residual soil for use as subgrade filling

The easy-disintegration nature and weak mechanical properties of granite residual soil (GRS) hinder its utilization as subgrade and embankment fillings. In this study, cement additive was used to introduce artificial interparticle cementation, and gravel additive was used to introduce coarse skeleton. Isotropic compression and triaxial shear tests were conducted on saturated cement-treated and cement-gravel-treated GRS to determine their compressibility and drained strength. Results show that, for the saturated cement-treated GRS, the post-yielding compression parameter (λpost-yielding) involved non-monotonic with the volumetric cement content (Cvi) and peaked at Cvi = 3%; its effective strength parameters (c' and φ') peaked at Cvi = 2 ∼ 3%. Adding gravel markedly decreased the compressibility and increased the drained strength of the cemented GRS. For the saturated cement-gravel-treated GRS, the pre-yielding compression parameter (λpre-yielding) involved non-monotonic with the volumetric gravel content (CvG) and troughed at CvG = 20 ∼ 30%; its c' and φ' peaked and troughed at CvG = 20 ∼ 30%, respectively. These non-monotonic correlations may be because macro-porosity of the cured specimens reached the lowest at Cvi = 2 ∼ 3% and a global gravel skeleton formed at CvG = 20 ∼ 30%. Hence, a cement dosage of Cvi = 2 ∼ 3% is suggested for treating the GRS (a silty sand) before it can be used as subgrade filling, and cement-gravel treatment with CvG = 20 ∼ 30% can be further considered to minimize the cement dosage and obtain higher geotechnical performance of the treated soil.

The influence of stone powder origin and superplasticizer type in the drying shrinkage of RPC

In the presented study, the combined effect of stone powder from mineral rocks (basic and acidic) and superplasticizers (naphthaleneand polycarboxylate-based) on drying shrinkage processes and moisture loss in Reactive Powder Concrete (RPC) is considered. Due to the peculiarities of the RPC structure, the moisture shrinkage is not restrained by the coarse skeleton and is completely determined by the characteristics of a hardening cement matrix modified with mineral additives. The paper shows that the balance of forces of internal interaction in the solid and liquid phases during the formation of the cement matrix is greatly influenced by stone powder and its origin. It has been established that the shrinkage of the RPCs with NF-based Polyplast SP-1 is higher than with PCB-based Glenium Ace 30. It was also found that shrinkage processes in RPC containing stone powder from granite and quartz sand do not always correspond to the internal moisture loss rate. Perhaps, due to the high strength of the RPC with stone powder from the listed rocks, a resistance to volumetric change increases during moisture loss, which in turn leads to a decrease in shrinkage deformations.

Internal Erosion Experiments on Sandy Gravel Alluvium in an Embankment Dam Foundation Emphasizing Horizontal Seepage and High Surcharge Pressure

For an internally unstable soil, fine particles can move in the pore channels between coarse particles along with seepage flow; this process is termed internal erosion. To evaluate the internal stability and internal erosion behavior of sandy gravel alluvium beneath the suspended cutoff wall in an embankment dam foundation, a series of horizontal seepage tests were carried out on the four representative gradations of the alluvium layer using a large-scale high-pressure erosion apparatus. The evolutionary trends of hydraulic conductivity, the erosion ratio of fine particles, and volumetric strain under stepwise increasing hydraulic loading were obtained. The results showed that the specimens of different gradations exhibited distinct properties in permeability, particle loss, and deformation, depending on the gradation continuity and fine particle content, which can be attributed to the difference in the composition of the soil skeleton and the arrangement of coarse and fine particles. For the specimens with continuous gradations or relatively high fine particle content, the surcharge pressure can significantly improve their internal stability. By contrast, in the situations of gap-graded gradations or low fine particle content, no considerable improvement was found because the stress was mainly borne by the coarse skeleton. The practical implications of the experimental results were demonstrated by evaluating the seepage safety of the zone beneath the suspended wall in the dam foundation.

Domain decomposition and upscaling technique for metascreens

PurposeThis paper aims to consider a multiscale electromagnetic wave problem for a housing with a ventilation grill. Using the standard finite element method to discretise the apertures leads to an unduly large number of unknowns. An efficient approach to simulate the multiple scales is introduced. The aim is to significantly reduce the computational costs.Design/methodology/approachA domain decomposition technique with upscaling is applied to cope with the different scales. The idea is to split the domain of computation into an exterior domain and multiple non-overlapping sub-domains. Each sub-domain represents a single aperture and uses the same finite element mesh. The identical mesh of the sub-domains is efficiently exploited by the hybrid discontinuous Galerkin method and a Schur complement which facilitates the transition from fine meshes in the sub-domains to a coarse mesh in the exterior domain. A coarse skeleton grid is used on the interface between the exterior domain and the individual sub-domains to avoid large dense blocks in the finite element discretisation matrix.FindingsApplying a Schur complement to the identical discretisation of the sub-domains leads to a method that scales very well with respect to the number of apertures.Originality/valueThe error compared to the standard finite element method is negligible and the computational costs are significantly reduced.

Microscopic mechanical analysis of sand production using a new arbitrary resolved-unresolved CFD-DEM model

A new discrete element method and computational fluid dynamics coupling model is developed by combining the resolved and unresolved method. To overcome the inaccuracy of fluid-particle drag force calculation and limitation on the size ratio of fine particles to fluid meshes, two aspects of improvements are made for sand production simulation over the existing models: the adoption of new drag force model to calculate the fluid-particle drag force considering irregular particle shape of sand and; integration of the Gaussian-based weighting function into the acquisition of the local void fraction. With these improvements of the new model, the fluid flow through the pores of coarse skeleton can be described in high resolution and accuracy with relatively low computational cost. And then, the new model is used to analyzed the sand production problem which is a tricky issue often encountered during hydrocarbon recovery in oil & gas industry. By this versatile tool, a series of characteristics related to the sand production are thoroughly described. Deeper insights into the mechanism of sand retention and pack permeability impairment are obtained particularly from a microscopic point of view. Such a microscopic insight will further facilitate the design of sand production prevention, particularly using gravel packing.

Study of the effects of fine coral powder and salinity on the mechanical behaviour of coral sand-seawater cement mortar

The coral sand and seawater are widely used as construction materials. The coral powder and salt contained in the coral sand-seawater cement mortar have a significant effect on the void distribution, cemented effect and mechanical behaviours. Thus, the mechanisms for the influences of coral powder content and salinity were investigated by a series of piezoelectric transducer (PZT) tests and uniaxial compressive strength tests. The cement mortar specimens with various coral powder contents and salinities were prepared and examined. The microstructure and pore characteristics of mortar under different conditions were analysed by scanning electron micrograph (SEM). The results show that the main ingredient in coral powder (i.e., CaCO3) would accelerates the cement hydration. The fine particles could fill the skeletal pores. Therefore, an appropriate content of coral powder can improve the strength and deformation properties of mortar. Salt had the dual effect of decreasing the stiffness of the cemented mortar specimen and intensifying the aggregation of coral powder particles. The aggregation of the coral powder particle in coarse skeleton is the key factor affecting the overall mechanical properties of the seawater-sea sand cement mortars. The coupling effects of coral powder and salt influence the microstructure and macroscopic mechanical properties of the mortar, and the effects of the two factors are mutually restricted. These research results provide a reference for guiding the optimisation of the mechanical properties of coral sand-seawater cement mortars and improving the safety and stability of structures engineered with these mortars.

Assessing inclination angles of tree branches from terrestrial laser scan data using a skeleton extraction method

Assessing the inclination angles (IAs) of tree branches is essential to evaluate their rainfall interception ability. However, depicting the IA distribution within a forest canopy is still a challenge. This study developed the WoodSKE method to extract skeletons from discrete point clouds of tree branches collected by terrestrial laser scanners (TLSs) for assessing their IAs. A TLS point cloud of tree branches was firstly contracted according to the pointwise local point distribution pattern to extract its coarse skeleton. Then, the coarse skeleton would be thinned and optimized by a noise filtering method. The IAs of tree branches were estimated based on their skeleton distribution. For each point cloud that has the reference skeleton, the average and root mean squared error (RMSE) of offset distance for its WoodSKE-extracted skeleton was less than 0.011 m and less than 0.019 m, respectively. Furthermore, the WoodSKE method was robust to process point clouds with noise. Comparing to the measured IAs at sample points selected from tested point clouds, the mean absolute error and RMSE of their skeleton-assessed IAs were 7˚ and 11.7˚, respectively. Over 86% of the number of sample points in each tested point cloud with IA assessment error lower than 15˚. Extracting skeletons from TLS point clouds of tree branches provides a base for assessing the distribution of IA-related structure features within a forest canopy.

Change in mechanical behaviour of gap-graded soil subjected to internal erosion observed in triaxial compression and torsional shear

Internal erosion can be a cause of failure in hydraulic structures such as levees, dikes and embankment dams. Suffusion is a type of internal erosion in which the fine particles migrate through the voids between the coarse particles under seepage flow, leaving behind the coarse skeleton. In this study the impacts of suffusion on the mechanical properties of soil in triaxial compression and torsional shear were investigated using a gap-graded soil mix of Silica sand No.5 and non-plastic silt. It was found that the erosion process varied depending on the initial density of the soil and higher density specimens showed relatively lower rates of erosion, degree of erosion, and compressive straining during erosion as compared with loose specimens. Effects on the mechanical behaviour​ of the soil were also dependent on the initial density of the specimen, with changes in the small strain stiffness, secant stiffness degradation, yield loci, small strain peak strength and medium strain strength in both drained and undrained conditions. In triaxial loading, force chains of coarse particles’ skeleton reinforced by fines developed in the suffusion process in the direction of major principal stress, then the stiffness at small strain increased. On the other hand, in torsional shear testing, the reinforced soil skeleton established by the erosion did not resist against shear loading. Increase in stiffness at small strain range was not observed.

Improvement of semi-resolved CFD-DEM model for seepage-induced fine-particle migration: Eliminate limitation on mesh refinement

The semi-resolved CFD-DEM model, proposed in our previous paper (Cheng et al., 2018) for the modelling of seepage-induced fine-particle migration through coarse skeleton in gap-graded soils, is improved through using Gaussian-based weighting function for fine particle-fluid coupling based on locally averaging theory. By doing so, it eliminates the limitation on fine particle-fluid mesh size ratio and thereby facilitates capturing detailed flow pattern through the coarse skeleton by refining fluid meshes. The improved semi-resolved CFD-DEM model is validated by Particle Image Velocimetry results of velocity field of fine glass beads migration through a simple coarse skeleton.

Interaction Mechanism of the Frost Heaving of the Filler and Skeleton Particles in a Micro-Frost-Heaving Filling Material

To address the adverse effects of the frost heaving of subgrades in regions with seasonal frozen soil on the smoothness of high-speed railway tracks, the effects of several factors—including filler, water content and external loading—on a micro-frost-heaving (MFH) filling material were studied through laboratory experiments and theoretical analysis. In addition, the interaction mechanism between the filler and skeleton particles during the frost heaving process was analyzed. The results show the following: The MFH filling material was composed of a coarse-grained skeleton, a filler between skeleton particles and the remaining unfilled voids. When the filling ratio of the filler was 0.25, the filling material underwent initial macroscopic frost heaving. An overlying load could inhibit the filling material from frost heaving. During the frost heaving process, the volume of coarse skeleton particles with high stiffness remained almost unchanged, whereas the filler expanded in volume and thus filled the remaining voids between skeleton particles and lifted the skeleton particles, resulting in macroscopic frost heaving of the filling material. When their ratio was relatively high, the remaining voids between skeleton particles had a strong absorptivity and weakened the capacity of the filler to lift the skeleton particles. The filler raised the skeleton particles, resulting in an increase in the gaps between them, which in turn facilitated the filling action of the filler. When the skeleton structure was stable, the filling action of the filler was more pronounced. The frost heaving of an MFH filling material is in fact a dynamic equilibrium process between the filling and lifting actions of the filler.

Capturing the human figure through a wall

We present RF-Capture, a system that captures the human figure -- i.e., a coarse skeleton -- through a wall. RF-Capture tracks the 3D positions of a person's limbs and body parts even when the person is fully occluded from its sensor, and does so without placing any markers on the subject's body. In designing RF-Capture, we built on recent advances in wireless research, which have shown that certain radio frequency (RF) signals can traverse walls and reflect off the human body, allowing for the detection of human motion through walls. In contrast to these past systems which abstract the entire human body as a single point and find the overall location of that point through walls, we show how we can reconstruct various human body parts and stitch them together to capture the human figure. We built a prototype of RF-Capture and tested it on 15 subjects. Our results show that the system can capture a representative human figure through walls and use it to distinguish between various users.

Distance Transform-Based Skeleton Extraction and Its Applications in Sensor Networks

We study the problem of skeleton extraction for large-scale sensor networks with reliance purely on connectivity information. Existing efforts in this line highly depend on the boundary detection algorithms, which are used to extract accurate boundary nodes. One challenge is that in practical this could limit the applicability of the boundary detection algorithms. For instance, in low node density networks where boundary detection algorithms do not work well, the extracted boundary nodes are often incomplete. This paper brings a new view to skeleton extraction from a distance transform perspective, bridging the distance transform of the network and the incomplete boundaries. As such, we propose a distributed and scalable algorithm for skeleton extraction, called DIST, based on DIStance Transform, while incurring low communication overhead. The proposed algorithm does not require that the boundaries are complete or accurate, which makes the proposed algorithm more practical in applications. First, we compute the distance transform of the network. Specifically, the distance (hop count) of each node to the boundaries of a sensor network is estimated. The node map consisting of the distance values is considered as the distance transform (the distance map). The distance map is then used to identify skeleton nodes. Next, skeleton arcs are generated by controlled flooding within the identified skeleton nodes, thereby connecting these skeleton arcs, to extract a coarse skeleton. Finally, we refine the coarse skeleton by building shortest path trees followed by a prune phase. The obtained skeleton is robust to boundary noise or shape variations. Besides, we present two specific applications that benefit from the extracted skeleton: identifying complete boundaries and shape segmentation. First, with the extracted skeleton using DIST, we propose to identify more boundary nodes to form a meaningful boundary curve. Second, the utilization of the derived skeleton to segment the network into approximately convex pieces has been shown to be effective.

Strong Interlocked Skeleton Dense Gradation for Cement-Fly Ash Stabilized Crushed-Stones

In order to optimize the material design for cement-fly ash stabilized crushed-stones, the strong interlocked coarse skeleton gradation was put forward based on the numerical experimentation method. In addition, the vibration method was employed to carry out the experimental study on the mechanical property of the cement-fly ash stabilized crushed-stones. Besides, the influencing regularities of the mixed amount of cement and fly ash on the cement-fly ash stabilized crushed-stones were researched. The result showed that the numerically simulated coarse aggregate gradation had great interlocked power, and the fine aggregate content of 31%±3% had stabilized mechanical property. Then, strong interlocked skeleton dense gradation of the cement-fly ash stabilized crushed-stones was put forward. Furthermore, when the 7d compression strength was used as the research indicator, the best proportion of the cement and fly ash was 1:0.5~1:1, and when the ultimate strength was used as the research indicator, then, the best ratio of the cement and fly ash was 1:1~1:1.5. Therefore, the cement dosage was advised to be 3~4% and the mixture amount of fly ash was 4~5%.

Connectivity-Based Skeleton Extraction in Wireless Sensor Networks

Many sensor network applications are tightly coupled with the geometric environment where the sensor nodes are deployed. The topological skeleton extraction for the topology has shown great impact on the performance of such services as location, routing, and path planning in wireless sensor networks. Nonetheless, current studies focus on using skeleton extraction for various applications in wireless sensor networks. How to achieve a better skeleton extraction has not been thoroughly investigated. There are studies on skeleton extraction from the computer vision community; their centralized algorithms for continuous space, however, are not immediately applicable for the discrete and distributed wireless sensor networks. In this paper, we present a novel Connectivity-bAsed Skeleton Extraction (CASE) algorithm to compute skeleton graph that is robust to noise, and accurate in preservation of the original topology. In addition, CASE is distributed as no centralized operation is required, and is scalable as both its time complexity and its message complexity are linearly proportional to the network size. The skeleton graph is extracted by partitioning the boundary of the sensor network to identify the skeleton points, then generating the skeleton arcs, connecting these arcs, and finally refining the coarse skeleton graph. We believe that CASE has broad applications and present a skeleton-assisted segmentation algorithm as an example. Our evaluation shows that CASE is able to extract a well-connected skeleton graph in the presence of significant noise and shape variations, and outperforms the state-of-the-art algorithms.

Dissimilarity between two skeletal trees in a context

Skeletal trees are commonly used in order to express geometric properties of the shape. Accordingly, tree-edit distance is used to compute a dissimilarity between two given shapes. We present a new tree-edit based shape matching method which uses a recent coarse skeleton representation. The coarse skeleton representation allows us to represent both shapes and shape categories in the form of depth-1 trees. Consequently, we can easily integrate the influence of the categories into shape dissimilarity measurements. The new dissimilarity measure gives a better within group versus between group separation, and it mimics the asymmetric nature of human similarity judgements.

Simple Reconstruction of Tree Branches from a Single Range Image

3D modeling of trees in real environments is a challenge in computer graphics and computer vision, since the geometric shape and topological structure of trees are more complex than conventional artificial objects. In this paper, we present a multi-process approach that is mainly performed in 2D space to faithfully construct a 3D model of the trunk and main branches of a real tree from a single range image. The range image is first segmented into patches by jump edge detection based on depth discontinuity. Coarse skeleton points and initial radii are then computed from the contour of each patch. Axis directions are estimated using cylinder fitting in the neighborhood of each coarse skeleton point. With the help of axis directions, skeleton nodes and corresponding radii are computed. Finally, these skeleton nodes are hierarchically connected, and improper radii are modified based on plant knowledge. 3D models generated from single range images of real trees demonstrate the effectiveness of our method. The main contributions of this paper are simple reconstruction by virtue of image storage order of single scan and skeleton computation based on axis directions.

Roots of pioneer trees in the Amazonian rain forest

On low fluvial terraces of the Caqueta River, Araracuara region, Colombia, tree root systems were ex- amined with regard to their structure, underground strati- fication and biomass distribution. Excavations of skele- ton roots and microscopic observation of terminal fine roots included ten species belonging to the Cecropia, Vismia, Miconia, Goupia, Clathrotropisand Brosimum genera. Roots of particular species varied in the external features of their periderm and structure of freshly cut slash. Coarse skeleton roots differed in the shape and po- sition of the plagiotropic branches and positively geotro- pic taproots or sinkers. None of the observed species could be identified as a shallow-rooted tree, in spite of the general fine root concentration in the upper soil hori- zon. In the two seasons of study, the terminal roots dis- played prevailingly secondary anatomical structure and did not maintain much primary anatomical tissue in their apices. Neither ectomycorrhizas nor endomycorrhizas were detected in the samples. In a set of regeneration stages the amount of tree roots linearly increased with the age of growth. However, fine roots below 2 mm in diameter shared 80-90% of all roots in the upper 20 cm layer of all sample plots. In an old-growth forest, the to- tal tree root biomass amounted to 39 ton/ha, thus being comparable to the underground biomass observed in sim- ilar tropical forests.