Three-Dimensional Large Deformation Finite-Element Analysis of Plate Anchors in Uniform Clay

Three-dimensional large deformation finite-element (FE) analyses were performed to investigate plate anchor capacity during vertical pullout. The remeshing and interpolation technique with small strain approach was expanded from two-dimensional to three-dimensional conditions and coupled with the FE software, ABAQUS. A modified recovery of equilibrium in patches technique was developed to map stresses after each remeshing. Continuous pullout of rectangular plate anchors was simulated and the large deformation results for strip, circular, and rectangular anchors were compared with model test data, small strain FE results, and plastic limit solutions. Interface conditions of no breakaway (bonded) and immediate breakaway (no tension) were considered at the anchor base. The effects of anchor roughness, aspect ratio, soil properties, and soil overburden pressure were investigated. It was found that the anchor roughness had minimal effect on anchor performance. For square and circular deep anchors under immediate breakaway conditions, the maximum uplift capacity increased with soil elastic modulus, which suggests that lower bound limit analysis and small strain FE analysis may overestimate the capacity. The soil beneath the anchor base separates from the anchor at a certain embedment depth near the mudline, once tensile stresses were generated. The ratio of separation depth to anchor width was found to increase linearly with the ratio of soil undrained shear strength to the product of soil effective unit weight and anchor width and was independent of the initial anchor embedment depth.     

Numerical Simulation of Vertical Pullout of Plate Anchors in Clay

The behavior of strip and circular plate anchors during vertical pullout in uniform and normally consolidated clays was studied in this paper by means of small strain and large deformation finite-element analyses. Both fully bonded (attached), and “vented” (no suction on rear face), anchors were considered. The current numerical results were compared with existing laboratory test data, finite-element results, and analytical solutions. This study showed that, in small strain analysis, the scatter of existing data was mainly due to the effect of soil stiffness. In large deformation analysis, when soil and anchor base were attached with suction, the pullout capacity factor formed a unique curve independent of the soil strength (su), soil effective unit weight (γ′) and anchor size (B=width of strip anchor and D=diameter of circular anchor). The transitional embedment depth ratio, HSD/B or HSD/D, (where HSD=transition depth between shallow and deep embedment) was 1.4 for a strip anchor and 0.75 for a circular anchor. The ultimate pullout capacity factors (Nc) for deep embedment were 11.6 and 11.7 for smooth and rough strip anchors and 13.1 and 13.7 for smooth and rough circular anchors, respectively. However, when the anchor base was vented, the soil stayed attached to the anchor base for deep embedment, and the pullout capacity was therefore the same as for the attached anchor. The separation depth ratio, Hs/B or Hs/D, (where Hs=embedment depth at which the soil and anchor base separated) was found to increase linearly with the normalized strength ratio, su/γ′B or su/γ′D.     

Ultimate Uplift Capacity of Multiplate Helical Type Anchors in Clay

In recent years, the use of helical anchors has expanded beyond their traditional use in the electrical power industry. The advantages of rapid installation and immediate loading capability have resulted in their being used in more traditional civil engineering infrastructure applications. Unfortunately, our current understanding of these anchors is unsatisfactory, and the underlying theoretical framework adopted by engineers has proven to be largely inappropriate and inadequate. A better understanding of helical anchor behavior will lead to increased confidence in design, a wider acceptance as a foundation alternative, and more economic and safer designs. The primary aim of this research is to use numerical modeling techniques to better understand multiplate circular anchor foundation behavior in clay soils. A practical design framework for multiplate anchor foundations will be established to replace existing semiempirical design methods that are inadequate and have been found to be excessively under- or overconservative. This framework can then be used by design engineers to confidently estimate the pullout capacity of multiplate anchors under tension loading.     

Pullout Behavior of Granular Pile-Anchors in Expansive Clay Beds In Situ

Granular pile anchors (GPA) are one of the recent innovative foundation techniques devised for mitigating the problems posed by swelling clay beds. In a granular pile anchor, the footing is anchored to an anchor plate at the bottom of the granular pile. This makes the granular pile tension resistant and enables it to absorb the tensile force caused on the foundation by the swelling clay. An understanding of the amount of uplift resistance offered by the GPA is important in the design of granular pile-anchor foundations in field situations causing tensile forces on foundations, such as in expansive clay beds. This paper presents the results of a field-scale test program conducted to study the pullout response of GPAs embedded in expansive clay beds. Pullout load tests were conducted on GPAs of varying lengths and diameters. It was found from the field pullout load tests that granular pile anchors of larger surface area resulted in higher pullout capacity. Of the various single granular pile anchors with l/d values between 2.5 and 10, the GPA of length 1000?mm and diameter 200?mm (l/d = 5) showed the best pullout load response when tested alone, resulting in a failure uplift capacity of 14.71?kN. Increase in diameter and length of granular pile anchor increased the uplift capacity. When the length of the GPA was increased from 500 to 750 and 1000?mm, the percentage increase in the uplift load required for an upward movement of 25?mm was 33.3 and 55.5% respectively. The pullout load of the GPA when tested under group was 18?kN as against a 12?kN for the GPA when tested single.     

Undrained Capacity of Plate Anchors under General Loading

Under general conditions of loading, a plate anchor is subjected to six degrees of freedom of loading, three force components and three moment components. Prediction of the anchor performance under general conditions of loading requires realistic estimates of the anchor pullout capacity for each individual load component as well as the interaction effects when these loads are applied in combination. This paper presents an analysis of plate anchor capacity under these general conditions of loading. The study considers a range of plate width-to-length ratios ranging from 1:1 to 2:1. The anchor capacity estimates and interaction relationships were developed based on finite-element studies and upper bound plastic limit analyses. Interaction relationships developed from the numerical and analytical studies were fitted to a simple six degrees-of-freedom yield locus equation.     

Undrained Stability of Braced Excavations in Clay

Short-term undrained stability often controls the design of braced excavations in soft clays. This paper summarizes the formulation of numerical limit analyses that compute rigorous upper and lower bounds on the exact stability number and include anisotropic yielding, typical of K0-consolidated clays and bending failure of the wall. Calculations for braced cuts bound the actual failure conditions within ±5%, and highlight limitations of existing basal stability equations. The analyses clarify how wall embedment and bending capacity improve the stability of well braced excavations. Careful selection of mobilized strengths at shear strains in the range 0.6–1.0% are necessary to match the predictions of anisotropic limit analyses with nonlinear finite-element predictions of failure for the embedded walls. Two example applications from recent projects in Boston highlight the practicality of the numerical limit analyses for modeling realistic soil profiles and lateral earth support systems, but also focus attention on the need for careful selection of undrained strength parameters. Credible estimates of stability have also been obtained in reanalyzing a series of case studies reported in literature using isotropic strength parameters derived from field vane or laboratory simple shear tests.     

Vertical Stability of Anchored Concrete Soldier–Pile Walls in Clay

The problem of vertical stability in flexible anchored retaining walls is analyzed and the pattern of the behavior under conditions of poor vertical support is described, on the basis of results from case histories, small-scale tests, and numerical modeling. The possibility of shear stress mobilization in the soil-to-wall interface of anchored concrete soldier–pile retaining walls is discussed. A finite element procedure to model excavations supported by soldier–pile retaining walls is described and applied to a numerical case study. Finite element analyses are performed, emphasizing the consequences of vertical instability due to buckling of the soldier piles and the role of interface resistance in vertical equilibrium. The understanding of some results of the numerical analyses, which are highly influenced by the complexity of the interaction between the different parts of the structure, is obtained by reassessing the vertical equilibrium issue in the light of limit analysis. This approach makes it possible to estimate the pile resistance corresponding to the limit situation of excavation collapse. The finite element model is used to confirm this resistance. Some conclusions are drawn.     

Loss in Anchor Embedment during Plate Anchor Keying in Clay

Vertically installed plate anchors have been investigated in this paper by numerical analysis and centrifuge modeling. In the numerical analysis, the large deformation finite-element method (remeshing and interpolation technique with small strain) was used to simulate strip plate anchor rotation. In the centrifuge model tests, transparent soils were used to observe square anchor rotation. The loss in anchor embedment during anchor keying was assessed for anchors in uniform and normally consolidated soils with anchor pullout angle varying from 30° to 90° to the horizontal. It was found that the loss in anchor embedment during anchor keying may be expressed in terms of a nondimensional anchor geometry factor, which is a function of the eccentricity of the padeye, angle of loading, and the net moment applied to the anchor at the stage where the applied load balances the anchor weight. However, once the anchor geometry factor reaches a certain value, the loss in anchor embedment stabilizes at 0.25–0.5 times the anchor width. The loss in anchor embedment decreases linearly with decreasing pullout angle. Simple formulae and design procedures have been proposed to estimate the loss in anchor embedment during keying.     

Uplift Behavior of Blade-Underreamed Anchors in Silty Sand

To evaluate the uplift behavior of anchors installed by the blade underreaming system, a numerical model for anchors in silty sand has been developed in this study and the calculated results are compared to the results of full scale anchor pullout tests. Although the blade-underreamed anchor tends to be irregular in shape due to possible collapse of the borehole, the excavated anchor showed an underreamed body of approximately multiple-stepped shape. Despite the difference in shape, the numerical results indicate that the difference between the load–displacement curve of the multiple-stepped anchor and that of the conical shaped anchor is small. In addition, the anchorage behavior of conical shaped anchors calculated from this numerical model was in good agreement with those of full scale anchor tests. No sign of progressive soil yielding along the underreamed body was found from the numerical analysis. So, the pull-out capacity of this underreamed anchor increases more than linearly with the length of the underream. Since only a small underream angle is needed to generate a substantial increase in anchor pull-out resistance, the ultimate pull-out capacity of the blade-underreamed anchor is found to be higher than that of straight shaft anchor in silty sand.     

Base Stability of Circular Excavations in Soft Clay

The base stability of circular excavations in soft clay is expected to be larger than rectangular excavations due to the shape effect. Up to the present, however, the base stability of circular excavations is usually evaluated with two-dimensional methods although some modification has been made to consider the shape effect. In this analysis, the finite-element method with reduced shear strength is used to evaluate the base stability of circular excavations. The base stability of circular excavations significantly increases with the presence of the hard stratum close to the base of the excavation, and with the depth of the wall inserted into the soft clay layer below the base of the excavation. A design chart for the base stability of circular excavations is provided.     

Performance of Tension and Compression Anchors in Weathered Soil

Anchor pull-out tests were performed on seven instrumented full-scale low-pressure grouted anchors installed in weathered soil at the Geotechnical Experimentation Site at Sungkyunkwan University. Anchors were 165 mm in diameter and embedded 9 to 12 m in weathered soil. Four were compression type and three were tension anchors. Performance tests, creep tests, and long term relaxation tests were performed and the results are presented. The characteristics of grout-soil and grout-strand interface were investigated and presented. From the measurements, a load transfer mechanism for tension and compression ground anchors was investigated and evaluated by a simple beam-spring numerical model.     

Numerical Analysis of T-Bar Penetration in Soft Clay

The penetration resistance of a cylindrical T-bar penetrometer in soft clay is affected by features such as anisotropy, high strain rates, and gradual strain-softening during passage of the T-bar. In order to evaluate these effects, a detailed numerical study has been undertaken, comprising: (1) finite-element analysis; and (2) a strain path approach within the upper bound plasticity mechanism. These studies showed that the T-bar factor is relatively insensitive to the degree of strength anisotropy, provided the penetration resistance is normalized by the average shear strength. Strain rates were found to be six or seven orders of magnitude greater than typical laboratory testing rates, and about three orders of magnitude higher than in a standard vane test. However, the effect of high strain rates is partly compensated by remolding of the soil, where average strains of 400% are imposed on the soil. Charts are presented showing how the separate effects of high strain rates and partial softening may be combined to derive a T-bar factor for a given soil. The paper concludes with a discussion of the measurement of remolded shear strength using cyclic T-bar tests, and interpretation of the T-bar resistance in fully remolded soil.     

Calibration of Reliability-Based Resistance Factors for Flush Drilled Soil Anchors in Taipei Basin

The goal of this research is to calibrate the reliability-based resistance factor of flush drilled soil anchors for their ultimate pullout capacities based on in situ anchor pullout test data in the alluvial soil underlying the Taipei Basin. Efforts are taken to quantify the uncertainties with a full probabilistic analysis approach. The resistance factor is calibrated based on the in situ test results of 46 anchors with a rigorous theoretical approach which constructs the relationship between the resistance factor and failure probability. With this relationship, the reliability corresponding to the code regulation can be verified. From the results of the analysis, it is found that the borehole enlargement due to the flush drilling is quite significant: the actual diameter of the fixed anchor end may be much larger than the nominal diameter of the drilling casing. Consequently, the safety factor of 3.0, recommended by most anchor codes, is found to be too conservative. The results should be valuable for reliability-based design of flush drilled soil anchors in the Taipei Basin.     

Setup Following Installation of Dynamic Anchors in Normally Consolidated Clay

This paper describes a series of centrifuge model tests designed to assess the increase in capacity of dynamic anchors due to setup in normally consolidated clay. The tests involved measurement of the vertical capacity of 1:200 reduced scale model anchors following various periods of postinstallation consolidation. The short-term capacity was shown to be dependent on the anchor impact velocity. Cavity expansion solutions for consolidation around a solid driven pile were found to provide agreement with the experimental results. A simplified capacity calculation technique predicted higher friction ratio values than is typically observed for driven piles; however, these calculations were complicated by the unusual dynamic anchor load–displacement response and uncertainty regarding the true sample shear strength. Dynamic anchor consolidation proceeds at a slower rate than for suction caissons and open-ended piles of similar equivalent diameter. However, the results indicate that depending on the site conditions, dynamically installed anchors remain a viable alternative to conventional deep-water mooring techniques.     

Effects of Electrode Configuration on Electrokinetic Stabilization for Caisson Anchors in Calcareous Sand

The study is on the electrokinetic strengthening of caisson anchors embedded in offshore calcareous sand. The effects of electrode configuration on the effectiveness of electrokinetic treatment are investigated based on electric field analysis and are verified by results from a series of large scale laboratory tests on caisson models of 200?mm diam and 400?mm height, embedded in calcareous sand submerged under seawater. The electrokinetic treatment generates cementation of soil solids as well as bonding between soil and caisson shafts, which leads to increases in the side resistance and overall pullout resistance. The effectiveness of electrokinetic treatment is directly related to the electric field intensity. A linear relationship is observed between the increase in the side resistance and energy consumption. The study shows that the effectiveness of electrokinetic treatments can be maximized by the optimization of the electric field distribution through the electrode configuration.     

Tensile Capacity of GFRP Postinstalled Adhesive Anchors in Concrete

This paper presents the results of an experimental study conducted on the pullout capacity of glass fiber reinforced polymer (GFRP) postinstalled adhesive anchors embedded in concrete. A total of 90 adhesive anchors were installed using sand-coated GFRP reinforcing bars and tested under monotonic tension loading in accordance with ASTM E-488-96 in 1996. The test parameters were: (1) the GFRP bar diameter (25.4, 15.9, and 6.4?mm); (2) the embedment depth (5, 10, and 15 db where db=bar diameter); (3) the adhesive type (epoxy-based and cement-based adhesives); and (4) installation conditions (wet or partially submerged and dry holes). The tested GFRP adhesive anchors were installed in concrete slabs measuring 3,750?mm long, 1,750?mm wide, and 400?mm deep. The test specimens were kept outdoors for 7?months to be subjected to real environmental conditions including freeze-thaw cycles, wet and dry cycles, and temperature variations. The experimental results indicated the adequate performance of GFRP adhesive anchors installed in wet or partially submerged condition using epoxy-based adhesive. Similar behavior was observed for those installed with cement-based adhesive in dry conditions as well. The capacity of the GFRP bars installed with both adhesive types was achieved at an embedment depth ranging from 10 to 15 db.     

Three-Dimensional Effects for Supported Excavations in Clay

This paper presents the results of 150 finite-element simulations conducted to define the effects of excavation geometry, i.e., length, width, and depth of excavation, wall system stiffness, and factor of safety against basal heave on the three-dimensional ground movements caused by excavation through clays. The results of the analyses are represented by the plane strain ratio (PSR), defined as the maximum movement in the center of an excavation wall computed by three-dimensional analyses normalized by that computed by a plane strain simulation. A simple equation for PSR is presented based on excavation geometry, wall system stiffness, and factor of safety against basal heave. This PSR equation reasonably represents trends in results of the 150 simulations as well as those simulations reported in literature. When the excavated length normalized by the excavated depth of an excavation wall is greater than 6, results of plane strain simulations yield the same displacements in the center of that wall as those computed by a three-dimensional simulation.     

Three-Dimensional Analysis of Nonhomogeneous Slopes

Presented is a method of three-dimensional slope stability analysis for homogeneous and nonhomogeneous symmetrical slopes based on the upper-bound theorem of the limit analysis approach. A rigid-block translational toe, above-the-toe or below-the-toe collapse mechanism is considered, with energy dissipation taking place along planar velocity discontinuities. The approach can be considered as a modification and extension of the procedure proposed by Michalowski in 1989. An effective iterative algorithm is applied to find the optimum (least) upper bound in constrained or unconstrained problems. The present procedure removes some essential limitations of limit analysis methods in two- and three-dimensional stability analysis of nonhomogeneous slopes.     

Three-Dimensional Responses Observed in an Internally Braced Excavation in Soft Clay

Several three-dimensional effects were observed in the performance monitoring data collected during excavation for the Ford Engineering Design Center in Evanston, Illinois. The elevations of the soil around the excavation varied and the excavator removed the soil in a nonuniform excavation process, both of which contributed to the observed three-dimensional (3D) effects. This paper describes the excavation support system and subsurface conditions at the site, summarizes the construction procedures, and presents the lateral soil movements measured by inclinometers, ground-surface movements measured by an automated total station, tilt of components of an adjacent structure, and forces in internal braces. These responses are compared with expected responses from current design methods. The 3D nature of the excavation resulted in smaller movements on the side of the excavation, where the retained soil was lowest, an unexpected pattern of axial strut loads and very slight damage to an exterior wall that paralleled one of the excavation walls.     

Stability Analysis of Complex Soil Slopes using Limit Analysis

The limit equilibrium method is commonly used for slope stability analysis. Limit equilibrium solutions, however, are not rigorous because neither static nor kinematic admissibility conditions are satisfied. Limit analysis takes advantage of the lower- and upper-bound theorems of plasticity theory to provide rigorous bounds on the true solution of a stability problem. In this study, finite-element models are used to construct both statically admissible stress fields for lower-bound analysis and kinematically admissible velocity fields for upper-bound analysis of soil slopes. While limit analysis of relatively simple slopes, typically homogeneous and of simple geometry, has been done previously, limit analysis of slopes with complex geometries, soil profiles, and groundwater patterns could not be effectively done in the past. In this paper, the theoretical basis and procedure for limit analysis of such slopes is presented. Various examples of slopes are selected from the literature and analyzed using both limit equilibrium and limit analysis. Factors of safety from limit equilibrium and limit analysis are compared. A comparison is also made, for each example, between the critical slip surfaces from limit equilibrium with the velocity field and plastic zone from the upper-bound solution and with the stress field from the lower-bound solution.