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.     

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.     

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 Lower Bound Solutions for Stability of Plate Anchors in Clay

Soil anchors are commonly used as foundation systems for structures that require uplift or lateral resistance. These types of structures include transmission towers, sheet pile walls, and buried pipelines. Although anchors are typically complex in shape (e.g., drag or helical anchors), many previous analyses idealize the anchor as a continuous strip under plane strain conditions. This assumption provides numerical advantages and the problem can be solved in two dimensions. In contrast to recent numerical studies, this paper applies three-dimensional numerical limit analysis to evaluate the effect of anchor shape on the pullout capacity of horizontal anchors in undrained clay. The anchor is idealized as either square, circular, or rectangular in shape. Estimates of the ultimate pullout load are obtained by using a newly developed three-dimensional numerical procedure based on a finite-element formulation of the lower bound theorem of limit analysis. This formulation assumes a perfectly plastic soil model with a Tresca yield criterion. Results are presented in the familiar form of break-out factors based on various anchor shapes and embedment depths, and are also compared with existing numerical and empirical solutions.     

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.     

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.     

Installation of Torpedo Anchors: Numerical Modeling

Torpedo anchors are used as foundations for mooring deep-water offshore facilities, including risers and floating structures. They are cone-tipped cylindrical steel pipes ballasted with concrete and scrap metal and penetrate the seabed by the kinetic energy they acquire during free fall through the water. A mooring line is usually connected at the top of the anchor. The design of such anchors involves estimation of the embedment depth as well as short-term and long-term pullout capacities. This paper describes the development of a computational procedure that leads to prediction of torpedo-anchor embedment depth. The procedure relies on a computational fluid dynamics (CFD) model for evaluation of the resisting forces on the anchor. In the model, the soil is represented as a viscous fluid and the procedure is applied to axially symmetric penetration of the seabed. The CFD approach provides estimates of not only the embedment depth but the pressure and shear distributions on the soil-anchor interface and in the soil.     

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.     

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.     

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.     

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.     

Termination Criteria for Jacked Pile Construction and Load Transfer in Weathered Soils

Pile jacking is a piling technique that provides a noise- and vibration-free environment in the construction site. To improve termination criteria for pile jacking and to better understand the behavior of jacked piles, two steel H piles were instrumented, installed at a weathered soil site, and load tested. A set of termination criteria was applied to the test piles, which includes a minimum blow count from the standard penetration test, a specified final jacking force, a minimum of four loading cycles at the final jack force, and a specified maximum rate of pile settlement at the final jacking force. The two test piles passed all required acceptance criteria. Punching shear failure occurred at the failure load for both piles and the shaft resistance consisted of approximately 80% of the pile capacity. Based on the results of field tests in Hong Kong and Guangdong and several centrifuge tests, a relation between the ratio of the pile capacity Pult to the final jacking force PJ and the pile slenderness ratio is established. The Pult/PJ ratio is larger than 1.0 for long piles but may be smaller than 1.0 for short piles. A regression equation is established to determine the final jacking force, which is suggested as a termination criterion for jacked piles. The final jacking force can be smaller than 2.5 times the design load for very long piles, but should be larger than 2.5 times the design load for piles shorter than 37 times the pile diameter.     

Tensile Behavior of FRP Anchors in Concrete

Strengthening of concrete structures using fiber-reinforced polymer (FRP) systems has become a widely accepted technology in the construction industry over the past decade. Externally bonded FRP sheets are proven to be a feasible alternative to traditional methods for strengthening and stiffening deficient reinforced or prestressed concrete members. However, the delamination of FRP sheets from the concrete surface poses major concerns, as it usually leads to a brittle member failure. This paper reports on the development of FRP anchors to overcome delamination problems encountered in surface bonded FRP sheets. An experimental investigation was conducted on the performance of carbon FRP anchors that were embedded in normal- and high-strength concrete test specimens. A total of 81 anchors were tested under monotonic uniaxial loading. Test parameters included the length, diameter, and angle of inclination of the anchors and the compressive strength of the concrete. The experimental results indicate that FRP anchors can be designed to achieve high pullout capacities and hence can be used effectively to prevent or delay the delamination of externally bonded FRP sheets. The results also indicate that the diameter, length, and the angle of inclination of the anchors have a significant influence on the pullout capacity of FRP anchors.     

Rapid Pullout Test of Soil Nail

This technical paper describes the rapid pullout response of soil nail embedded in dry clean sand. In the rapid pullout test, soil nail is pullout by a tensile impulse load with loading duration that is long enough to eliminate the influence of the stress wave propagation phenomenon. The results of these experiments showed the influence of loading rate on pullout response is highly dependent on the roughness condition of the nail surface. For rough nail, the prepeak rapid pullout response was significantly stiffer in the load-displacement characteristic and higher in peak pullout strength when compared to the corresponding quasi-static pullout response. While for a smooth nail, a negligible difference between rapid and quasi-static pullout response was noticed. In light of these limited experimental results, the radiation damping effect appears to be the dominant contributor to the enhancements in prepeak rapid pullout response of rough nail. “Actual” damping coefficient that quantifies the damping resistance mobilized in a rapid pullout test was found not to be constant but to decrease with the increase in pullout displacement.     

Mechanics of Drag Embedment Anchors in a Soft Seabed

This paper presents an analysis of the load capacity and trajectory of a drag embedment anchor in a soft seabed. Anchor capacity relationships are developed for an idealized anchor comprising a rectangular fluke and a cylindrical shank. Geometric variables considered for the anchor include fluke length, fluke thickness, shank length, angle between fluke and shank, and shank thickness. Parametric studies are presented investigating the effect of these variables on anchor capacity and performance. A method of anchor trajectory prediction during drag embedment is developed by considering anchor behavior in conjunction with the mechanics of the anchor line. The anchor trajectory simulations indicate that an equilibrium condition rapidly develops during embedment in which the rate of anchor rotation is identical to the rate of change in the anchor line uplift angle at the shackle point. At the equilibrium state, the anchor load capacity normalized by soil strength remains constant and the anchor is in a state of incipient rotation. The anchor line angle at the seabed also influences anchor trajectory. This angle varies throughout embedment according to the mechanics of the anchor line in both the seabed and the water column.     

Development of Residual Forces in Long Driven Piles in Weathered Soils

A large-scale field-monitoring program for studying residual forces in long-driven piles is described. Eleven steel H-piles, 34.2–59.8?m in embedded length, were instrumented with vibrating-wire strain gauges, installed and subjected to static loading tests in a building site in Hong Kong. The residual forces in these piles during and after pile installation were recorded. The development of residual forces as it relates to the pile penetration depth during construction, and in time after the piles were installed, is presented. The measured load transfers in the piles from static loading tests are reported and the effect of the residual forces on the interpretation of load-transfer behavior is studied. The field measurements show that residual forces increase approximately exponentially with penetration depth. The residual forces continue to increase with time after pile driving due to secondary compression of disturbed soils around the pile shaft and other factors. The large residual forces in the long piles significantly affect the interpretation of the pile load distributions. The effect of residual forces on the shaft resistance is significant at shallow depths. Bearing-capacity theory tends to overpredict the true toe resistance of the long piles founded in weathered soils.     

Pullout Behavior of Geogrid by Test and Numerical Analysis

This paper presents a study of geogrid pullout behavior in laboratory pullout tests and finite element modeling of the laboratory pullout tests. The pullout tests and the finite element method (FEM) analyses were carried out on two geogrid types with different stiffness values in dense sand under different overburden pressures. The pullout test results show that the geogrid behavior can be categorized into three types based on the bond stress distributions. The FEM results show reasonable agreement not only with the pullout force against the geogrid displacement, but also with the distributions of geogrid displacements, strains, tensile forces, and bond stresses along the geogrid length during deformation. This research demonstrates that the deformation characteristics of geogrids play an important role in the pullout tests while the interface properties play a significant role in the FEM simulations of geogrid pullout behavior. A method to obtain suitable interface parameters for designing of actual reinforced structures from the laboratory pullout tests is provided.     

Numerical Analysis of Geosynthetic-Reinforced and Pile-Supported Earth Platforms over Soft Soil

Geotechnical engineers face several challenges when designing structures over soft soils. These include potential bearing failure, intolerable settlement, large lateral pressures and movement, and global or local instability. Geosynthetic-reinforced and pile-supported earth platforms provide an economic and effective solution for embankments, retaining walls, and storage tanks, etc. constructed on soft soils; especially when rapid construction and/or strict deformation of the structure are required. The inclusion of geosynthetic(s) in the fill enhances the efficiency of load transfer, minimizes yielding of the soil above the pile head, and potentially reduces total and differential settlements. A numerical study has been conducted to investigate pile-soil-geosynthetic(s) interactions by considering three major influence factors: the height of the fill, the tensile stiffness of geosynthetic, and the elastic modulus of pile material. While current methods have not fully addressed important effects of the geosynthetic stiffness and pile modulus on the soil arching ratio, numerical results suggested that the stress concentration ratio and the maximum tension in geosynthetic increase with the height of the embankment fill, the tensile stiffness of geosynthetic, and the elastic modulus of the pile material. The distribution of tension force in the geosynthetic reinforcement indicated that the maximum tension occurs near the edge of the pile.     

Effects of Long-Term Dynamic Loading and Fluctuating Water Table on Helical Anchor Performance for Small Wind Tower Foundations

The application of this study is to use helical anchors as a foundation system for small wind tower (1–10?kW) guyed cables. Helical anchors are currently used to anchor guyed cables of cell or transmission towers. However, the increased dynamic vibrations a wind turbine adds to the tower and foundation system under working loads, as well as extreme environmental conditions (e.g., straight line winds, ice load, or sudden furling shocks), require additional knowledge about the behavior of helical anchors. These field conditions were simulated in this study from tower-instrumented field data on wind speed and tower response. These tower responses were then transmitted to the helical anchors through an extensive, large-scale testing program that included monitoring the performance of the helical anchor foundation under dynamic loads, subject to natural variations in both wind regimes, precipitation (water level) and variations in helical anchor geometry. This paper compares the uplift prediction methods used in helical anchor design as well as discusses the effects of long-term dynamic loading and fluctuating water table on helical anchor performance.     

Design and Evaluation of Fiber-Reinforced Polymer Bond-Type Anchorages and Ground Anchors

This paper presents laboratory tests on a bond-type anchorage system and a full-scale ground anchor with fiber-reinforced polymer 9-bar tendons. A cement-based mortar filled steel tube was used to simulate a rock mass environment. Resistance strain gauges, fiber optic sensors, embedded strain gauges, and linear variable displacement transducers were used to monitor the behavior of the anchor and to study load transfer mechanism of the tendon to surrounding rock mass. The test results show that the developed bond-type anchorage performs well for post-tensing applications with the tendons. The tested anchor presents an acceptable tensile behavior for each loading stage investigated (up to a load level of 0.6ffu, where ffu=guaranteed capacity of the tendon) in accordance with existing codes. The anchor is similar in strain distribution profile to conventional steel anchors, except giving a shorter load transfer length. It is expected that the anchor require a minimum anchor bonded length of 2,000?mm with plain cement grouts.