Anchor-soil Interface Research Articles

Anchor Shear Strength Damage under Varying Sand Content, Freeze-Thaw Cycles, and Axial Pressure Conditions

Sandy soil in the north of Hebei region of China is widely distributed, the temperature difference between day and night is large, the phenomenon of freezing and thawing is obvious, and the soil body before and after the freezing and thawing cycle of sandy soil slopes is affected by the changes. This paper takes the stability of a sandy soil anchorage interface under a freeze-thaw cycle as the research background and, based on the self-developed anchor-soil interface shear device, analyses the influence of changing sand rate, confining pressure, and the number of freeze-thaw cycles on the shear characteristics of an anchor-soil interface in anchorage specimens. The research findings indicate that, at 50–60% sand contents, the shear strength increases with a higher sand content and is positively correlated with confining pressure within a higher range. A higher sand content stabilises the anchoring body, but an excessively high sand content can lead to failure. Increasing the sand content, confining pressure, and freeze-thaw cycle number all result in a reduction in the shear displacement at the peak strength. After 11 freeze-thaw cycles, the shear strength of the anchoring body stabilises, with a reduction in strength of approximately 32%, and a higher sand content effectively reduces the reduction in strength.

An innovative interface friction algorithm for simulating dynamic installation of anchors using CEL approach

Dynamic installation of anchors is now commonly simulated using the coupled Eulerian-Lagrangian (CEL) approach in ABAQUS/Explicit. However, capturing anchor (Lagrange)-soil (Eulerian) interface frictional resistance during continuous penetration remains a challenge. This paper introduces an advanced frictional resistance algorithm called the Integrated Nodal Friction Algorithm (INFA) to improve the accuracy of capturing frictional resistance, and to increase computational efficiency. The innovative features of the method include automatic capturing of the anchor nodes in contact with the adjacent soil, estimating local shear strength of the contacted soil nodes, and calculating frictional resistance for the identified contacted areas based on the estimated local shear strengths. The algorithm was validated against centrifuge test data for a free-fall piezocone penetrometer, with good agreement obtained in terms of measured frictional resistance and end bearing profiles with penetration depth, and final tip embedment depth. Parametric analyses were then conducted for optimising mesh sizes and output frequency. Performance of the INFA method was compared with the existing two methods, demonstrating that the proposed algorithm is capable of improving the accuracy of capturing anchor-soil interface frictional resistance profile along the penetration depth.

Element-scale pullout test study on the mechanical behavior of grouted anchor–soil interface subjected to ground pressure

Element-scale pullout test study on the mechanical behavior of grouted anchor–soil interface subjected to ground pressure

Pullout capacity of vertical plate anchors in unsaturated clay considering variable surface flow conditions

Vertical plate anchors are placed in the soil to improve the lateral resistance of various geotechnical structures. Extensive research has been conducted to find out the pullout capacity of vertical anchors in saturated or dry soil. Despite all these attempts, no research is available to study the behavior of anchors in unsaturated soil where the shearing resistance of soil is governed by the matric suction depending on the surface flow conditions and water table positions. Since the soil in field is mostly in unsaturated state; thus, the pullout resistance of vertical plate anchor has been aimed to be evaluated in this study. Finite element limit analysis solutions in terms of a non-dimensional pullout factor have been presented to find the pullout capacity of vertical anchors. The solutions are inclusive of the effect of soil properties, embedment depth, height of anchor plate, depth of water table from the ground surface, flow conditions and flow rate under an immediate breakaway condition. The effect of variation of other factors like the degree of saturation, unit weight of soil, van-Genuchten SWRC parameters, specific gravity of soil solids, and anchor-soil interface conditions has also been evaluated.

Bond-slip model of anchor-soil interface and analysis of bearing capacity of anchor in frozen soil

It has been recognized that temperature and moisture have obvious influence on the bonding characteristics of anchor-frozen soil interface. Following the principle and method used to explore the interface performance of microelement anchor-soil, this research starts by performing the pullout tests of anchor-soil under different temperature and moisture content conditions. Then, we analyze the characteristics of interface bond stress-displacement and compare the effects of temperature and moisture content on interface mechanical properties. The test results show that the failure mode of anchor changes slightly when soil temperature and moisture content vary, wherein the shear failure which shows brittle characteristics at the anchor-frozen soil interface is dominant. Additionally, we have found that the effect of temperature on the peak bond stress is more significant than moisture. Under different soil temperatures and moisture content, the shape of bond stress-displacement curve of anchor-frozen soil interface is similar. Furthermore, the bond stress-displacement relationship of anchor-frozen soil interface can be fitted by a composite function which is composed of exponential and ideal elastic-plastic model function. Thus, this research recommends to propose a bond-slip model of anchor-frozen soil interface considering temperature effect. Combined with the load transfer method, the force and deformation equation of anchor in frozen soil is derived. Through example analysis, it is discovered that the pullout bearing capacity of anchor increases with the decrease of soil temperature. Moreover, the initial shear stiffness of anchor-soil interface is positively related to the bearing capacity of anchor. At the end, the influence of interfacial residual strength on the bearing capacity of anchor in the later stage of pullout is superior to that in the earlier stage.

Undrained uplift resistance of under-reamed open caisson shafts

Deep, large-diameter caisson shafts are a popular means of constructing underground storage and attenuation tanks and pumping stations for the water and wastewater industry. One of the key design concerns for these structures is resistance to flotation during periods when the tanks are only partially filled or empty. In this paper, two-dimensional numerical analysis is used to explore the undrained uplift resistance provided by under-reaming the walls of the caisson shaft to create an enlarged base. The primary aim of the study is to assess the influence of the taper angle of the anchor (i.e. the protruded base) on the resulting uplift resistance. The effects of the anchor–soil interface roughness factor, soil weight, surcharge pressure and caisson radius are also investigated. The results indicate that the effect of the taper angle on both the uplift bearing capacity and the developed horizontal reaction can be very significant. The numerical output informs the development of a closed-form approach for application in routine design. The new design method is shown to provide an excellent agreement with both finite-element and additional finite-element limit analysis calculations. By way of example, the proposed design method is applied to a hypothetical design scenario.

Study of the Load-Bearing Characteristics of Bolts under Asymmetric Freezing Conditions

To investigate the load-bearing characteristics of anchor rods under asymmetric freezing conditions such as those in cold regions and the effect of frost swelling, a model test device was developed using a controlled-temperature environmental box and a hydraulic actuator. Laboratorial pull-out model tests of anchor rods in soil layers were conducted at different moisture contents and freezing temperatures, the changes in anchor rod pull-out force before and after freezing were quantitatively described, load–displacement relationship curves were prepared, and the frost swelling displacement of anchor rods under asymmetric freezing conditions and the stress evolution of surrounding soil were observed and recorded. The shear strength of the anchor–soil interface increased with decreasing temperature, and the ultimate values of the pull-out force at 0, −3 and −7 °C were 4, 17.04 and 18.1 times greater than those at room temperature, respectively, for 10% water content. The pull-out force of the frozen anchor increased with increasing water content. The bolts were displaced by the freezing expansion force. Their lateral displacements at 0, −3 and −7 °C were 2.9, 3.2 and 3.5 mm, respectively, and their vertical displacements were 0.2, 3.5 and 4.3 mm, respectively, for 10% water content. The total displacement increased with increasing moisture content, and the maximum transverse displacements were 3.5, 3.65 and 3.8 mm for 10, 12 and 14% water contents, respectively, and 4.3, 5.1 and 5.5 mm in the vertical direction, respectively. The ultimate pull-out forces after freezing and thawing were 71, 68 and 52% of that before freezing for 10, 12 and 14% water, respectively.

Investigation of the installation process of drag-in plate anchors from LDFE modelling

The installation of drag-in plate anchors is investigated using advanced large-deformation finite-element (LDFE) analysis. A new modelling technique that can realise the continuously evolving loading angle on the anchor line is described. The entire anchor trajectory is simulated until the anchor reaches an ultimate state after dragging for a very large distance, typically in the range of 40–70 times the anchor width. An installation in simple uniform and non-sensitive soil is first presented with agreement against well-established plasticity analysis and an analytical solution offering verification of the techniques developed. The effects of anchor padeye position, seabed soil conditions, anchor line properties and the anchor–soil interface on the installation response are then discussed. The findings highlight that plasticity and analytical approaches are not necessarily conservative when the yield surface derived from deep embedded anchors is applied in shallow conditions. The LDFE results also demonstrate the importance of accounting for soil remoulding around both the anchor and anchor installation line. The results of this study provide insights for offshore engineers to consider and incorporate when estimating a drag anchor installation trajectory and holding capacity.

Numerical modelling on anchor-soil-water interaction for dynamic installation of gravity installed anchors

The dynamic penetration process of the gravity installed anchor is complex associated with very high shear strain-rate and large plastic shear strain-softening, potential of water entrainment along the anchor-soil interface, moving boundaries and many others. The present study conducted computational fluid dynamics (CFD) simulations to investigate the anchor-soil-water interaction during the anchor dynamic penetration. By setting very fine boundary layers in CFD simulations, the strength mobilisation of the soil at the anchor-soil interface was thoroughly investigated. The numerical results showed that the disturbed soil is very narrow beside the anchor fluke but expands a wide range below it. In CFD analyses, factors influencing the final penetration depth of the anchor were investigated, including the undrained shear strength, parameters of both strain-rate and strain-softening effects, impact velocities, booster weights, and interface adhesion factors. Based on the present simulation results, a theoretical model was proposed to predict the final penetration depth of the anchor.

An innovative booster for dynamic installation of OMNI-Max anchors in clay: Physical modelling

An innovative booster concept is proposed to increase both the kinetic and potential energies and hence to increase the penetration depth of the OMNI-Max anchor within the clayey soil with high strength gradient. This paper carried out 1g model tests to investigate the working efficiency of the booster on the anchor penetration depth both in normally consolidated (NC) and lightly over-consolidated (LOC) soils. The similarity relationships of the anchor dynamic penetration within the soil were first deduced, and then a series of dynamic penetration tests were performed to investigate the parameters that influence the anchor penetration depth. The parameters include the soil strength characterisation, impact velocity, booster weight, inclined impacting angle, and water drag and entrainment effects. The anchor dynamic penetration process within the soil was measured by a micro-electromechanical systems (MEMS) accelerometer, through which the soil strain-rate effect and the hydrodynamic aspects including the water drag force and potential water entrainment along the anchor-soil interface were analysed. Finally, energy-based and force-based prediction models were put forward to estimate the penetration depths of OMNI-Max anchors and hybrid anchors (i.e. an OMNI-Max anchor with a booster).

Assessing the penetration resistance acting on a dynamically installed anchor in normally consolidated and overconsolidated clay

Predicting the final embedment depth of a dynamically installed anchor is a key prerequisite for reliable calculation of anchor capacity. This paper investigates the embedment characteristics of dynamically installed anchors in normally consolidated and overconsolidated clay through a series of centrifuge tests involving a model anchor instrumented with a microelectric mechanical system (MEMS) accelerometer, enabling the full motion response of the anchor to be established. The data are used to assess the performance of an anchor embedment model based on strain-rate-dependent shearing resistance and fluid mechanics drag resistance. Predictions of a database of over 100 anchor installations — formed from this study and the literature — result in calculated anchor embedment depths that are within ±15% of the measurements. An interesting aspect, consistent across the entire database, relates to the strain rate dependence on frictional resistance relative to bearing resistance. The predictions reveal that strain rate dependency may indeed be higher for frictional resistance, although only if a soil strength lower than the fully remoulded strength is considered as the reference strength, which suggests that water may be entrained along a boundary layer at the anchor–soil interface during installation.

Generalized Design Approach for Inclined Strip Anchors in Clay

Abstract The estimation of the undrained pullout capacity of plate anchors is vital for the design of offshore floating facilities and has been an active topic of research for several years. Most of the previous studies have proposed empirical relationships to predict the undrained pullout capacities of inclined anchors. Generally, the buoyancy effect is added to the shearing resistance to obtain the total capacity, which is less than or equal to the maximum capacity. In this paper, an alternative mechanism-based simple design methodology is proposed to estimate the undrained pullout capacity of inclined anchors. A series of finite-element analyses are performed for a range of anchor inclinations in soils with uniform and heterogeneous shear strength. The effects of anchor embedment, soil unit weight, and anchor–soil interface tensile capacity are studied in a systematic manner. A relationship has been developed to estimate the depth at which the anchor undergoes a transition in the failure mechanism from...

Horizontal pullout resistance of a group of two vertical anchors in sand

The horizontal pullout capacity of a group of two vertical strip anchors placed along the same vertical plane in sand has been determined by using the upper bound finite elements limit analysis. The variation of the efficiency factor (ξγ) with changes in clear spacing (S) between the anchors has been established to evaluate the total group failure load for different values of (i) embedment ratio (H/B), (ii) soil internal friction angle (ϕ), and (iii) anchor-soil interface friction angle (δ). The total group failure load, for a given H/B, becomes always maximum corresponding to a certain optimal spacing (Sopt). The value of Sopt/B was found to lie in a range of 0.5–1.4. The maximum magnitude of ξγ increases generally with increases in H/B, ϕ and δ.

Seismic pullout capacity of vertical anchors in sand

The horizontal pullout capacity of vertical anchors embedded in sand in the presence of pseudo static horizontal earthquake body forces has been determined by using the lower bound finite element limit analysis. The variation of the pullout factor Fγ with changes in embedment ratio of the anchor plate for different values of horizontal earthquake acceleration coefficient (αh) has been established. The effect of the anchor-soil interface friction angle on the results has also been examined. The pullout capacity decreases quite significantly with an increase in αh. The pullout capacity becomes higher for greater values of embedment ratio and interface friction angle. The normal pressure distribution along the front surface of the anchor plate and the development of failure patterns have also been examined.

Vertical uplift resistance of two horizontal strip anchors with common vertical axis

With an application of the upper bound finite element limit analysis, the vertical pullout capacity of a group of two horizontal strip plate anchors, with the common vertical axis and placed in a cohesive-frictional soil, has been computed. The variation of the uplift factors <i>F_c, F_q</i> and <i>F_y</i>, due to the contributions of soil cohesion, surcharge pressure and unit weight, respectively, has been evaluated for different combinations of <i>S/B</i> and <i>H/B.</i> As compared to single isolated anchor, the group of two anchors generates significantly greater magnitude of <i>F_c</i> for Φ ≤ 20° especially with greater values of <i>H/B</i> and under fully bonded anchor-soil interface condition. The factor <i>F_c</i> attains almost the maximum value when the upper anchor plate is placed midway between ground surface and the lower anchor plate. The factors <i>F_q</i> and <i>F_y</i>, on the other hand, for a group of two anchors are found to remain almost equal to that of a single isolated anchor as long as the levels of the lower plate in the group and the single isolated anchor are kept the same.

Upper Bound Solution for Pullout Capacity of Vertical Anchors in Sand Using Finite Elements and Limit Analysis

The horizontal pullout capacity of vertical anchors embedded in sand has been determined by using an upper bound theorem of the limit analysis in combination with finite elements. The numerical results are presented in nondimensional form to determine the pullout resistance for various combinations of embedment ratio of the anchor (H/B), internal friction angle (ϕ) of sand, and the anchor-soil interface friction angle (δ). The pullout resistance increases with increases in the values of embedment ratio, friction angle of sand and anchor-soil interface friction angle. As compared to earlier reported solutions in literature, the present solution provides a better upper bound on the ultimate collapse load.

The modelling of anchors using the material point method

AbstractThe ultimate capacity of anchors is determined using the material point method (MPM). MPM is a so‐called meshless method capable of modelling large displacements, deformations and contact between different bodies. A short introduction to MPM is given and the derivation of the discrete governing equations. The analysis of a vertically loaded anchor and one loaded at 45° is presented. The load–displacement curves are compared to that obtained from experiments and the effect of soil stiffness and anchor roughness is investigated. The results of the vertically loaded anchor are also compared to an analytical solution. The displacement of the soil surface above the anchor was measured and compared to the numerical predictions. Convergence with mesh refinement is demonstrated and the effect of mesh size and dilatancy angle on the shear band width and orientation is indicated.The results show that MPM can model anchor pull out successfully. No special interface elements are needed to model the anchor–soil interface and the predicted ultimate capacities were within 10% of the measured values. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Seismic horizontal pullout capacity of vertical anchors in sands

The problem of finding the horizontal pullout capacity of vertical anchors embedded in sands with the inclusion of pseudostatic horizontal earthquake body forces, was tackled in this note. The analysis was carried out using an upper bound limit analysis, with the consideration of two different collapse mechanisms: bilinear and composite logarithmic spiral rupture surfaces. The results are presented in nondimensional form to find the pullout resistance with changes in earthquake acceleration for different combinations of embedment ratio of the anchor (λ), friction angle of the soil (φ;), and the anchor-soil interface wall friction angle (δ). The pullout resistance decreases quite substantially with increases in the magnitude of the earthquake acceleration. For values of δ up to about 0.25–0.5φ, the bilinear and composite logarithmic spiral rupture surfaces gave almost identical answers, whereas for higher values of δ, the choice of the logarithmic spiral provides significantly smaller pullout resistance. The results compare favorably with the existing theoretical data.Key words: anchors, earthquakes, failure, limit analysis, sands.

Development and Application of Anchor-Soil Interface Models

Understanding the anchor-soil interface behavior is essential to the determination of the anchor pullout capacity and prediction of the deformation of the anchor-reinforced system under working load conditions. Numerous anchor pullout tests under both lab and field conditions have generally yielded extremely high apparent interface strength, which cannot be explained by the conventional interface friction theory between the anchor material and the soil. In this paper, the mechanism and the phenomena of the anchor-soil interaction were studied, and the soil dilatancy due to shearing was regarded as the main factor contributing to the increase of the anchor-soil interface friction. Considering this effect, along with the adoption of a cylindrical shear deformation pattern of the soil, anchor-soil interface models have been developed for hardening and softening behavior, respectively. Using the developed interface model, the forward and backward calculation algorithms have been formulated and applied to predict the anchor performance for the given interface parameters and to determine the interface parameters from the given anchor pullout test, respectively. Furthermore, the prediction of the anchor pullout performance was compared favorably with two hypothetical cases, two laboratory test results, and one field case.

Displacement-Based Stability Analysis for Anchor Reinforced Slope

Combining the conventional method of slices for slope stability analysis with the previously derived analytical solutions for the anchor-soil interaction, a displacement-based approach for the stability analysis of anchor reinforced slope has been developed. The procedure for evaluating the working forces in the anchor, the global factor of safety, and the limits of the anchor working force is described in detail. Furthermore, the effects of reinforcement on the slope stability due to different parameters, such as soil properties, anchor characteristics and anchor-soil interface properties were studied. The numerical results demonstrate the progressively mobilized anchor forces and their distribution along a vertical section of the slope, and the expected global factor of safety at each stage of soil movement along the slip surface. Also, for a given anchor reinforced slope, the anchor working limit is generated along a vertical section of the slope, which controls the anchor resistance to the slope and limits the potential soil movement. Based on the results of the analysis, the general guideline for the design of an anchor reinforced slope is discussed.