Anchors In Sand Research Articles

A Study on Failure Surface of Helical Anchors in Sand by PIV/DIC Technique

Helical anchors have been used to carry tension loads in different applications including transmission tower foundation, pipeline anchors, foundation repair elements, and excavation bracing. There have been numerous changes in the shape and size of helical anchors and piles since their first usage. Several researchers have studied their failure mechanism to find their pullout capacity. In this paper, the uplift capacity of helical screw anchors has been investigated through laboratory testing. Half-cut double-helix anchors were tested in a sand tank by varying helix size, helix spacing, and relative density of sand. A series of images were captured during the process of anchor pullout. The images were used to obtain displacement and strain fields by Particle Image Velocimetry (PIV). Load–displacement curves have been presented and compared to the earlier works. Afterwards, pullout capacity factors were calculated from peak load values. PIV analysis results were used to study the effects of helix spacing, helix size, and relative density of sand on the displacement fields. The results showed that the effect of helix spacing and soil density in increasing pullout load is more than that of helix size. Moreover, failure surfaces were discussed through displacement and strain fields. The findings indicated that failure surface above the top helix of deeply embedded anchors is a truncated cone with the angle of approximately $${\raise0.7ex\hbox{$\phi $} \!\mathord{\left/ {\vphantom {\emptyset 3}}\right.\kern-0pt}\!\lower0.7ex\hbox{$3$}}$$, with the vertical and the failure surface shape between the two helices dependent on helix spacing.

Uplift resistance of horizontal strip anchors in sand: a cavity expansion approach

This paper presents an analytical cavity expansion theory-based method for predicting peak uplift resistance of shallow horizontal strip anchors buried in sand. On the basis of an analytical two-dimensional stress solution for loading analysis around a cylindrical cavity, the method was developed by assuming that the peak anchor uplift resistance can be approximated by the cavity breakout pressure. In the new cavity expansion model, the ultimate failure is reached once the plastic zone develops to the ground surface, and the biaxial state of in situ ground stresses is taken into account. A database consisting of 75 model tests on shallow strip anchors in sands was compiled to validate the new method. The predicted results and measured data are in reasonable agreement, with a mean over-prediction of the peak uplift resistance by 1·6%. The reliability of the new solution was also checked by comparing with other commonly used analytical solutions. It is shown that the current solution can provide a simple analytical tool for predictions of the peak uplift resistance of strip anchors in sand while a sliding-block failure mechanism dominates.

Pull-out behaviour of vertical plate anchor in granular soil

In this paper, the pull-out behaviour of a vertical plate anchor in sand is investigated. Load-carrying capacity and failure displacement of the anchor are found to be significantly influenced by its embedment depth and the density of the fill soil. It is observed that a vertical anchor at shallow depth exhibits clear failure with the failure plane reaching out to the ground surface, whereas at deeper embedment the soil is subjected to constrained plastic deformation leading to localised failure around the anchor. The critical embedment depth beyond which anchor behaviour tends to shift from general to local shear mode is found to be 7h in the case of dense soil and 5h in the cases of medium dense and loose soils, where h is the height of the anchor plate. The proposed regression model is reasonably good in predicting the load-carrying capacity of the vertical anchors under pull-out.

Experimental Study on Uplift Behavior of Group Anchors in Sand

Abstract This article presents an experimental investigation on sand deformation above a group of two plate anchors using a digital image correlation (DIC) technique. The test setup developed for this study includes a test frame with loading and deformation measurement, a computer connected to a high-resolution camera and equipped with DIC software, and a test mold to host scale model anchor plates. A series of physical model tests were conducted under different configurations, including different embedment depths, anchor spacings, and sand densities. Based on the sand displacement fields obtained from DIC, it was found that anchor group efficiency occurs when there is an overlap in soil displacement fields. This overlap is influenced by the combination of embedment depth, sand density, and anchor spacing. There is a critical spacing that defines the spacing at which group anchors act individually. An empirical formula is proposed for the relationship between group efficiency and critical spacing for sand at different density conditions. The formula compares favorably with experimental results published in the literature.

Model Test Research on Bearing Mechanism of Underreamed Ground Anchor in Sand

To improve the capacity of ground anchors, scholars and engineers worldwide have developed various types of underreamed anchors with expanded anchor parts. Underreamed anchors have a completely different mechanism from traditional shaft anchors. The expanded section of an underreamed anchor induces an end bearing force to endure the uplift force similar to a reversed pile. Therefore, the total resistance of an underreamed anchor includes friction and end bearing force. To clarify the bearing mechanism of underreamed anchors, a series of model tests were performed using fiber Bragg grating (FBG) sensors and the photogrammetry measuring method. Based on the tests, the distribution and development of the friction and end bearing force of the underreamed anchor model were acquired by the FBG sensors when being pulled out. Moreover, the deformation state of the soil around the anchor model was observed by the digital photogrammetry measuring method. Finally, the interaction mechanism between an underreamed anchor and surrounding sand was obtained, which was identified as the inherent reason for the distribution and development law of the resistance of the underreamed anchor in sand.

Seismic uplift capacity of shallow strip anchors: A new pseudo-dynamic upper bound limit analysis

Using a new pseudo-dynamic approach for the inclusion of seismic body forces, the seismic vertical uplift capacity of horizontally placed strip plate anchors in sand at shallow depths has been computed with the application of upper bound theorem of limit analysis. Unlike the earlier pseudo-dynamic approaches, in this study, the nonlinear variation of both horizontal and vertical acceleration along the depth of soil layer, and in the values of amplitude and phase of accelerations has been considered with the satisfaction of stress boundary condition at the ground surface. The seismic uplift factor Fγs due to the unit weight of soil for different combinations of seismic acceleration coefficients, internal friction angle of soil, and the embedment ratio of anchors has been obtained. The solutions indicate that the magnitude of Fγs decreases substantially with an increase in seismic acceleration coefficients; whereas as expected, increases with an increase in embedment ratio of anchors and soil friction angle. This study produces the least upper bound solutions in comparison to the earlier reported results in the literature.

Geocell reinforcement for performance improvement of vertical plate anchors in sand

In this paper influence of geocell reinforcement on performance of vertical plate anchors is studied. A series of model tests were carried out in a test bed-cum-loading frame assembly. The anchor used was a steel plate of size 100 mm × 100 mm. With geocell reinforcement the anchor could sustain deformations as high as 60–70% of its height when the load carrying capacity was increased by four fold. The optimum length, width, and height of geocell mattress giving maximum performance improvement are found to be 5, 3 and 2.8 times the anchor height respectively. For adequate performance improvement size of geocell pocket opening should be close to the anchor size. The load dispersion angle that depicts the rigidity of the geocell mattress tends to increase with increase in its width, height and reduction in pocket size. A numerical study using fast Lagrangian analysis of continua was carried out. The agreement between observed and computed results is found to be reasonably good.

Vertical uplift capacity of a group of shallow circular plate anchors in sand

ABSTRACTThe uplift capacity of a group of circular plate anchors buried horizontally in sand along a line has been determined. The uplift capacity of an interfering anchor is presented in terms of nondimensional uplift factors, Fγi and Fqi, due to components of soil unit weight and surcharge pressure acting on the ground surface, respectively. Theoretical solutions have been developed by applying the upper bound theorem of limit analysis based on a simple rigid wedge collapse mechanism. In the case of two and infinite number of anchors, closed-form solutions have been developed for computing the factor Fqi, whereas the factor Fγi is determined using a semianalytical approach. As expected, the interference of the anchors leads to a continuous reduction in the uplift resistance with a decrease in the spacing between the anchors, and the uplift resistance decreases with the increasing number of anchors at a given spacing. The results compare reasonably well with the available theoretical and experimental data from the literature.

Drained monotonic and cyclic capacity of a dynamically installed plate anchor in sand

The emergence of commercial offshore floating renewable energy devices that are expected to be deployed in large integrated arrays in relatively shallow water requires anchor types that are suited to sandy seabeds. This paper considers centrifuge test data that quantify the capacity of a novel dynamically installed plate anchor in sand under drained monotonic and cyclic loading. The monotonic tests investigated the role of the eccentricity of the load attachment point from the plate, the mooring line load inclination at the seabed and the plate embedment depth, whereas the cyclic tests examined the capacity mobilisation due to both regular and irregular drained cyclic loading for a fixed embedment depth and seabed load inclination. The test data indicate that an optimal anchor design would employ a load attachment point that is eccentric from the plate by at least 0.6 times the plate height, and that the anchor provides higher capacity when the seabed load inclination is horizontal (catenary mooring) than when it is vertical (vertical taut mooring). The centrifuge data show that drained cyclic loading does not degrade anchor capacity, and may be beneficial provided the cyclic loading involves a history of lower level cyclic loading that densifies the sand.

Cyclic and post-cyclic monotonic response of a single-helix anchor in sand

In recent years, the use of helical anchors for guyed transmission towers has increased considerably in Brazil. This type of structure is subjected to important environmental cyclic loadings, but their effects are typically ignored in the design process. Under this scenario, this paper presents the preliminary experimental results of an investigation of the cyclic and post-cyclic monotonic responses of a single-helix anchor in very dense sand. Three different combinations of high-level loading cycles, simulating the effects of high wind speeds on the anchors of the guyed towers, were evaluated using centrifuge tests on an instrumented model anchor. The results indicate that although a rapid degradation of shaft resistance was observed in all tests, the soil loosened by the anchor installation was densified due to the cyclic loading improving the monotonic post-cyclic response.

Scale effect in centrifuge tests of helical anchors in sand

Centrifuge modelling is an advantageous technique to simulate the behaviour of helical anchors in sand. The uplift capacity of a single-helix anchor is the sum of the helix bearing resistance plus the shaft resistance; therefore, recommendations to avoid the grain-size effect of previous model tests on plate anchors and on regular piles appear to be reasonable for helical anchor model tests. However, there are two important differences between centrifuge models of plate anchors and helical anchors: the ratio of the helical plate to the shaft diameter and the plate installation procedure. Consequently, the shaft friction and the plate bearing resistance of helical anchors can be affected by the scaling effect in a different manner. To clarify this uncertainty, this study was proposed to investigate the grain-size effect on the results of uplift capacity of helical anchor models in sand. Anchor models with different scales were tested to simulate the same prototype in very dense dry sand in a centrifuge. These experiments have shown that no scale effect was found for models with the ratio of the effective helical radius to the average grain size >58. The findings obtained here could guide future centrifuge model studies on helical anchors.

Load-Carrying Mechanism of Vertical Plate Anchors in Sand

AbstractAn experimental study on the load-carrying mechanism of vertical plate anchors is presented and discussed. It was found that, with a shallow anchor, the rupture surface reached the ground surface, leading to a general shear failure, whereas with deeper embedment, the rupture surface was localized around the anchor. Anchors placed close to the ground surface failed in linear rupture but, when embedded within the failure mechanism, were very close to the polar curve of a logarithmic spiral, which, however, tends to be nearly circular for deeper depths of embedment. The size of rupture surface increases with an increase in density of fill soil that mobilizes higher resistance, leading to enhanced anchor capacity. The critical embedment depth beyond which the anchor breakout factor does not change much is found to be approximately 7 times the anchor height for dense soil, whereas it is approximately 5 times the anchor height for loose soil. The anchor when placed below the critical depth, settlement, ...

Scaling Considerations for 1-g Model Horizontal Plate Anchor Tests in Sand

This paper addresses scaling issues related to small-scale 1-g model tests on plate anchors in sand under drained loading conditions. Previous centrifuge studies from the literature have suggested that the results of conventional 1-g model testing are inaccurate because of scale effects. Other studies have suggested, however, that scaling errors can be reduced in 1-g model tests if the results are presented in dimensionless form and the constitutive response of the model soil is representative of the prototype behavior. There are no experimental studies in the literature that have tested the validity of this approach for plate anchors. A simple 1-g scaling framework was developed for vertically loaded, horizontal plate anchors. Small-scale 1-g model tests were performed on square plate anchors in dry sand, and combined with existing centrifuge and 1-g model test data from the literature to test the scaling approach for both capacity and deformation. The 1-g model tests provided a reasonable representation of the full-scale prototype behavior when the scaling approach was applied.

MODEL TESTS OF DRAGGING HALL ANCHORS IN SAND

Hall anchors are often used as the main anchors in great medium ships. However, the methods for determining the holding capacity of Hall anchors when being pulled in soil are rarely proposed. In this study, a series of model tests was carried out using four scaled Hall anchor models to determine the kinematic behavior, trajectory, and development rules of the holding capacity of the anchors relative to dragging distances. Moreover, the holding force coefficients of the Hall anchors in sand were identified after the tests.

Analytical study of the failure mode and pullout capacity of suction anchors in sand

Suction anchors are widely adopted and play an important role in mooring systems. However, how to reliably predict the failure mode and ultimate pullout capacity of the anchor in sand, especially by an easy-to-use theoretical method, is still a great challenge. Existing methods for predicting the inclined pullout capacity of suction anchors in sand are mainly based on experiments or finite element analysis. In the present work, based on a rational mechanical model for suction anchors and the failure mechanism of the anchor in the seabed, an analytical model is developed which can predict the failure mode and ultimate pullout capacity of suction anchors in sand under inclined loading. Detailed parametric analysis is performed to explore the effects of different parameters on the failure mode and ultimate pullout capacity of the anchor. To examine the present model, the results from experiments and finite element analysis are employed to compare with the theoretical predictions, and a general agreement is obtained. An analytical method that can evaluate the optimal position of the attachment point is also proposed in the present study. The present work demonstrates that the failure mode and pullout capacity of suction anchors in sand can be easily and reasonably predicted by the theoretical model, which might be a useful supplement to the experimental and numerical methods in analyzing the behavior of suction anchors.

Analytical and experimental studies of dragging hall anchors through rock berm

An analytical method is proposed in this paper to calculate the maximum embedded depth of a dragged Hall anchor when passing through rock berm and to thus define a minimum buried depth of pipelines in rock berm to prevent pipelines from being damaged by dragging anchors. The movement of a Hall anchor in rock berm is interpreted based on the equilibrium conditions for resisting and driving moments acting on the anchor. To verify the accuracy of the proposed analytical method, model tests were carried out by using three scaled Hall anchor models and dragging them through rock berm. The comparisons between the two studies show that the average value of their differences for the stable embedded depth of a Hall anchor in sand and in rock berm are only 1.7% and 2.7%, respectively. The good agreements indicate that the proposed method is accurate enough to calculate the minimum buried depth of pipeline in rock berm during pipeline design.

Model studies of uplift capacity behavior of square plate anchors in geogrid-reinforced sand

An experimental investigation into the uplift capacity of horizontal square plate anchors in sand with and without geogrid reinforcement is reported. The parameters investigated are the effect of the depth of the single layer of geogrid, vertical spacing of geogrid layers, number of geogrid layers, length of geogrid layers, the effects of embedment depth, and relative density of sand. A series of three dimensional finite element analyses model was established and confirmed to be effective in capturing the behaviour of plate anchor-reinforced sand by comparing its predictions with experimental results. The results showed that the geogrid reinforcement had a considerable effect on the uplift capacity of horizontal square plate anchors in sand. The improvement in uplift capacity was found to be strongly dependent on the embedment depth and relative density of sand. A satisfactory agreement between the experimental and numerical results on general trend of behaviour and optimum geometry of reinforcement placement is observed. Based on the model test results and the finite element analyses, optimum values of the geogrid parameters for maximum reinforcing effect are discussed and suggested.

Numerical simulation of the penetration process of ship anchors in sand

AbstractIn order to study the burial depth of submarine power cables in the German North Sea, the penetration process of ships' anchors in seabed soil was simulated by using the coupled Eulerian–Lagrangian method in Abaqus/Explicit. In the 3D finite element model the sandy seabed was simulated with different densities under both drained and undrained conditions. The resulting penetration mechanism of a ship anchor in sand was verified by field test data. It showed that under specific conditions the penetration depth of a ship's anchor could be more than 1.5 m, which is suggested in “Bundesfachplan Offshore für die deutsche ausschließliche Wirtschaftszone der Nordsee 2012 und Umweltbericht” [8] as the standard burial depth of submarine power cables in the German North Sea by Bundesamt für Seeschifffahrt und Hydrographie (BSH).Numerische Simulation des Penetrationsprozesses eines Schiffsankers in Sand. Um die Verlegetiefe von Offshore‐Seekabeln in der Nordsee zu untersuchen, wurde der Penetrationsprozess eines Schiffsankers in den Meeresboden mittels der gekoppelten Euler‐Lagrangeschen Methode im Abaqus/Explicit modelliert. Im 3D‐FE‐Modell wird der sandige Meeresboden mit unterschiedlichen Lagerungsdichten unter dränierten sowie undränierten Bedingungen angesetzt. Der berechnete Eindringmechanismus des Schiffsankers im Sand wird durch Feldmessungen in der Nordsee bestätigt. Es zeigt sich, dass die Penetrationstiefe des Ankers unter Umständen größer ist als die im “Bundesfachplan Offshore für die deutsche ausschließliche Wirtschaftszone der Nordsee 2012 und Umweltbericht” [8] vom Bundesamt für Seeschifffahrt und Hydrographie (BSH) vorgegebene Verlegetiefe von 1,5 m.

Behaviour of Group of Helical Screw Anchors Under Compressive Loads

In this paper experimental investigations on the behaviour of single, double and triple helical screw anchors under the influence of vertical compressive loads are presented. The testing program was conducted on one, two, three and four numbers of model anchors in sand for different depths of embedment keeping shallow and deep mode of behaviour in mind. The experimental setup was instrumented to measure the compressive load and the downward displacement of the anchor. It was observed that the compressive load varied significantly with the installation depth of the anchor and the number of anchors. Based on experimental results, non-dimensional graphs were plotted normalizing the deformation and load axis both and simple equations are proposed correlating load and deformation. Ultimate compressive capacity of multiple anchors was plotted against that of single anchor and simple equations were proposed correlating them. Using these equations an anchor foundation for a particular structure can be easily designed. A 3-D finite element model, PLAXIS, was used to confirm the results obtained from laboratory tests and the agreement is excellent.

A Comparison of Pull-Out Capacity of Suction Anchors in Clay and Sand

With the oil and gas exploitation develop to the deep sea; offshore platform under extreme environment load needs more stable anchorage foundation. Based on the slender suction anchor of SPAR, three-dimensional numerical analysis method was presented to study the ultimate pull-out capacity. Based on the geological conditions from South China Sea, clay and sand was selected as soil conditions to make a comparison analysis. The effects of soil type, load positions, load angles and aspect ratio on the ultimate bearing pull-out resistance of the suction foundation were studied. The comparison analysis results indicated that the ultimate pull-out resistance of suction anchors in sand has a greater rise rate and achieve ultimate pull-out capacity need smaller displacement than in clay; load point and load angles have a great impact on the resistance and there is a critical aspect ratio under inclined loading in sand.