Ground Anchors Research Articles

Local scour characteristics around offshore wind-turbine foundations under nonlinear waves and currents

In this study, we investigated the local scour around a large pile, termed the umbrella suction anchor foundation (USAF), under nonlinear waves and current. We developed a wave-foundation-seabed coupled model and analyzed the characteristics of the flow field, turbulence characteristics, and scour depth evolution, and the influence of wave height and anchor branches on local scour. The results indicate that the horseshoe vortex increases turbulence intensity around the offshore edge of USAF, amplifying the shear stress on the seabed, which causes local scour around the USAF upstream. The anchor branches protected the soil from scouring to some extent. The scour and accretion depths increased rapidly at the initial stage, and then, the scour rate decreased. After approximately 300 wave cycles, the scour and accretion depths gradually stabilized. The changing trend of the scour evolution with wave height converged, but higher wave heights resulted in larger scour and accretion depths simultaneously.

Design and Economy of Rock Anchor Foundation for Transmission Line

Design and Economy of Rock Anchor Foundation for Transmission Line

Evaluation of Anthropogenic Substrate Variability Based on Non-Destructive Testing of Ground Anchors

The purpose of this paper is to describe the variability of soil rheological properties based on research carried out using load tests of ground anchors under complex geotechnical conditions. The heterogeneity of soil should always be considered when designing geotechnical constructions. In the present case, the earthwork created at the Warsaw Slope revealed an embankment of anthropogenic origin, located in a geologically and geomorphologically complex area of the Vistula valley slope. Excavation protection was anchored mainly in soils of anthropogenic origin. When the acceptance tests of the ground anchor were completed, the subsoil randomness was confirmed using nondirect, geostatistical methods. A standard solid rheological model with nonlinear fitting to the data was used. This model was established to describe the creeping activity of the ground anchor more accurately. The characteristics of man-made embankments were described using the parameters obtained with the rheological model of the subsoil.

Influence of Pre-Stressing on Tieback Retaining Wall for Sandy Soils Excavations

Pre-stressed ground anchor systems or tieback systems are commonly used at wide and irregular-shaped excavations, with the advantage of lower cost and ease of construction compared to the braced excavations, but they come with the drawback on permits for excavations near buildings and tunnels. Research on tieback systems in sands was generally conducted. However, the studies on the correlation between the retaining wall deflection and pre-stress force are few. The objectives of this paper are to study the influence of pre-stress force, depth of excavation, wall embedment length, and soil shear strength that is represented by soil friction angle on the deflection and soil pressure acting on the retaining wall. The parametric study was conducted on an excavation in sand using the finite element method with the Hardening soil model. The results showed that a 50 kN/m increase in pre-stress force reduced the wall deflection on top of the wall by 0.005–0.083% of excavation depth. However, the pre-stressing influence in reducing wall deflection at excavations became less significant along with the sand density increase due to higher friction angle contribution to excavation stability. Moreover, the pre-stress force needed for stabilization of the wall with long embedment length is smaller than those on the wall with shorter embedment length, since the embedment length increase of 0.25 times of excavation depth reduces wall top deflection by 0.002–0.095% of excavation depth. Also, the increase of soil density reduces the need for wall embedment length, so at dense sand, the embedment length of 0.5 times of excavation depth is sufficient to support the excavation.

Scour Characteristics and Equilibrium Scour Depth Prediction around Umbrella Suction Anchor Foundation under Random Waves

A series of numerical simulation were conducted to study the local scour around umbrella suction anchor foundation (USAF) under random waves. In this study, the validation was carried out firstly to verify the accuracy of the present model. Furthermore, the scour evolution and scour mechanism were analyzed respectively. In addition, two revised models were proposed to predict the equilibrium scour depth Seq around USAF. At last, a parametric study was carried out to study the effects of the Froude number Fr and Euler number Eu for the Seq. The results indicate that the present numerical model is accurate and reasonable for depicting the scour morphology under random waves. The revised Raaijmakers’s model shows good agreement with the simulating results of the present study when KCs,p < 8. The predicting results of the revised stochastic model are the most favorable for n = 10 when KCrms,a < 4. The higher Fr and Eu both lead to the more intensive horseshoe vortex and larger Seq.

Influence Depth of the Tensile Capacity of Ground Anchors Folding Type in Cohesive Soils

A ground anchor is used to withstand tensile stresses caused by loading both the natural load of the soil and external pressures, then passing the force into the ground. It is generally used to retain tensile form and lateral power structures in constructing walls, foundations, transmission towers, pile, floating docks, mooring dolphin, and offshore buildings. Various ground anchors have been widely used, such as drag, helical, circular-shaped plates, and squares. Especially for massive plate anchors, installation is complicated because it must be excavated first before installation. A challenge in engineering is especially on cohesive soil conditions (soft soil). With these considerations, it is necessary to develop innovative models that are easier to install but still have a large enough tensile capacity. This research analyzes the tensile capacity by using ground anchor folding type to overcome building problems on cohesive soil. The test was tensile capacity with four leaves and a Variation of depth-the performance of this folding type anchor based on the tensile capacity and the anchor. The results obtained in this study; the greater area of the element occurred a significant increase in tensile capacity.

A case study on the design, numerical modelling and preliminary testing of tension piles in weak rock

Over the last twenty years several major tunnel projects have been constructed in the Copenhagen area in the Danian limestone, either in the Copenhagen Limestone or in the Bryozoan limestone; in parallel, developments have been made in the characterization of these rocks. The Danian limestones are weak sedimentary rocks with highly variable properties in terms of their strength, stiffness and in their mass permeability. Their properties are governed by their genesis resulting in a large variation in the carbonate content, their induration and a large variability of the fissuring and distribution of fissuring. The paper describes the experience gained from selected underground construction projects in the Copenhagen area, specifically for the passive uplift anchors/tension piles that are often uses as a means to resist uplift forces resulting from the porewater pressure exerted at the underside of the base slab of major underground structures. While tension piles are common in Copenhagen, as they are often used for basement construction too, the authors are not aware of a documented calculation method. The paper presents a summary of the case histories published for ground anchors in Danian limestone and proposes a calculation method for the unit shaft resistance of small diameter tension piles as well as the comparison with numerical modelling and preliminary testing undertaken for the Sydhavn metro in Copenhagen.

Observed Performance and FEM-Based Parametric Analysis of a Top-Down Deep Excavation in Soil-Rock Composite Stratum

This paper investigates the performance of a top-down deep excavation in soil-rock composite stratum. The behavior of the excavation bracing system, consisting of ground anchors and end-suspended piles, has not been well understood due to the lack of relevant research. Based on the observed data of a typical deep excavation case history for the May Fourth Square Station in Tsingtao, China, the characteristics of the horizontal and vertical pile displacements, ground surface settlements, building settlements, axial forces in ground anchors, earth pressure, and pore water pressure during excavation were analysed. Two-dimensional finite element simulations were carried out to further explore the deformation and internal force responses of end-suspended piles and to capture the effects of pile diameter, embedded depth, and rock-socketed depth on the horizontal displacement and bending moment distributions along the pile shaft. It was found that the pattern of the vertical pile displacements could be categorized into three types: rapid settlement, slow settlement, and rapid heave. The magnitudes of the ground and building responses can be well controlled within allowable limits by combining the top-down method with the adopted bracing system. Among the investigated parameters, pile diameter is dominant in affecting the horizontal pile displacement. The primary influence zone for pile bending moment varies, depending on the parameters. It is recommended that a combination of top-down method, ground anchors, and end-suspended piles be adopted for restraining excavation deformation and lowering construction costs of similar deep excavations in soil-rock composite stratum.

Performance of Ground Anchored Walls Subjected to Dynamic and Pseudo-Static Loading

This study investigates the response of pre-stressed anchored excavation walls under dynamic and pseudo-static loadings. A finite difference numerical model was developed using FLAC2D, and the results were successfully validated against full-scale experimental data. Analyses were performed on 10, and 20-m-height stabilized excavated slopes with 60° to 90° of inclination angle with the horizon to represent an applicable variety of wall geometries. In dynamic analysis, the statically stabilized models were subjected to 0.2 to 0.6g of the dynamic peak acceleration to evaluate the effect of ground acceleration on their performance. Furthermore, pseudo-static analyses were performed on the statically stabilized models with pseudo-static coefficients ranging from 0.06 to 0.22. The results revealed that ground anchored slopes generally showed acceptable performances under dynamic loading, while higher axial forces were induced to ground anchors in higher and steeper models. Furthermore, comparing the results of dynamic and pseudo-static analyses showed a good agreement between the two methods' predictions in the mobilized axial force along the ground anchors. Pseudo-static coefficients were then proposed to replicate dynamic results, considering the slope geometry and dynamic load peak acceleration. The results revealed that higher and steeper stabilized slopes required higher values of pseudo-static coefficients to match the dynamic predictions successfully. The results indicate that pseudo-static coefficient tend to increase with the increase in dynamic load peak acceleration in any given model. Doi: 10.28991/cej-2021-03091703 Full Text: PDF

Pullout resistance of inclined anchors embedded in geogrid reinforced sand

Transmission tower foundations experience large vertical uplift forces during their lifetime. These forces can be resisted by providing anchor foundations below the tower. In this study, laboratory experiments were conducted to investigate the pullout capacity of inclined plate anchors in dry sand. The capacity of inclined anchor significantly improved in the presence of geogrid reinforcement on top of the anchor plate. Besides, the pullout capacity increased with the inclination angle (30°, 45°, 60°) both in unreinforced and reinforced soil. It was observed that the ultimate pullout load is significantly influenced by the embedment depth of the anchor plate and relative density of the backfill material. The experimental results were compared with a three-dimensional numerical model developed in FLAC3D. The comparison shows good agreement between the experimental and numerical results, and the numerical model can predict the pullout behaviour of the inclined anchor plate both in unreinforced and reinforced soil. Comparison of uplift resistance of pile and anchor foundations are shown through a design example of a transmission tower foundation. Results indicate that inclined anchors embedded in reinforced soil can resist the uplift forces at shallow depth compared to piles.

Anchoring of a retaining structure with glass fibre strand anchors

In Copenhagen, Denmark, a large construction pit has been designed to facilitate the excavation for the basement of a new district in the city called “Postgrunden”. The retaining wall is constructed by sheet piles installed into the Copenhagen Limestone with the use of pre-drilling and with ground anchors in two levels. The construction pit as well as the anchors are temporary, however, both parts are left in the ground after construction of the basement has been concluded. Most of the anchors were constructed as normal steel strand anchors, however, at one location glass fibre anchors were found to be the best solution due to restrictions concerning installation under third party property. Therefore 82 ground anchors with glass fibre strands were installed. These anchors were the first of their type used in production in Denmark. However, in Denmark there is no Eurocode or National Annex on composite materials at the time of the design and therefore the designers had no direct standard to design by. The design of the glass fibre anchors was therefore based on the index of safety method from Eurocode 0 along with a set of fracture tests made in a laboratory. The tests were used for a statistical analysis that along with a required index of safety gave the characteristic ultimate resistance of the strands and the elastic modulus. During installation the contractor had to consider the different material behaviour of the glass fibre strands compared to the traditional steel strands, e.g. the brittle shear tolerance, that dictates the assembly of the glass fibre strands to steel strand couplers to allow for testing and prestressing of the anchors. Further, the glass fibre anchors are not as heavy and stiff as the traditional steel anchors, which added an extra challenge during installation below the ground water table. Anchor testing was performed using traditional anchor testing equipment. The hollow plunger jacks were chosen in respect to the long piston stroke required for the extra elongation of glass fibre strands due to a low modulus of elasticity. All anchors were tested with either suitability or acceptance test to a factor of 1.43 or 1.30 above the design load, respectively.

Permanent uplift Anchors in Copenhagen Limestone

On the former Postal Service Centre parcel in central Copenhagen a new urban area of 200,000 m2 is under construction, which accommodates service and retail trade, as well as apartments for housing. Below ground a parking basement in two levels is established with excavation to 8 m below ground level, corresponding to approximately 8 m below the design ground water level. To resist the uplift forces on the basement approximately 950 vertical ground anchors were needed. Because of the large number of ground anchors an initial test programme has been performed with the goal of minimizing the overall costs and to ensure a robust anchor project. The test programme consisted of 23 investigation tests on vertical ground anchors with varying anchor bond length, where the aim was to obtain failure in the interface between the grout and the limestone. All test anchors were installed with the anchor bond length in the Copenhagen Limestone. This paper describes the geology of the area, the concept for the preliminary anchor design, the test programme, and the results and conclusions. Furthermore, a description of the design and installation of the production anchors. The test programme showed that the anchor bond length could be reduced to (at least) 4 m for resisting a design anchor load of 1250 kN, corresponding to a measured shear resistance more than 70% greater than initially presumed. Based on this the production anchors were installed and tested by suitability and acceptance tests according to DS 1537, and all anchors passed the test.

Effect of Horizontal Multidirectional Cyclic Loading on Piles in Sand: A Numerical Analysis

Foundations of offshore and nearshore wind energy production systems are subjected to multidirectional and cyclic loads, due to the combined action of wind and waves and in the particular case of mutualized anchor foundations for floating wind turbines, to the phase shift between the loads generated in the adjacent anchored turbines. This article presents a three-dimensional numerical model developed with FLAC3D to analyse the impact of the change in direction of the horizontal load during the cycles. The typical case of a 1.7 m diameter and 10 m-long pile founded in a dense homogeneous sand is considered. A specific procedure has been implemented to apply force-controlled cycles with a change in lateral load direction. The results are compared to mono-directional lateral cyclic loads with the same average and cyclic forces. The results of the parametric study highlight the effect of the average value and amplitude of the cyclic loading on the accumulation of pile head horizontal displacements during the cycles. When a multidirectional cyclic loading is applied, it also leads to an accumulation of the deviated horizontal displacements, and the resulting accumulated horizontal displacements are larger than for a mono-directional cyclic loading of the same amplitude.

Influence of anchor cable failure on a supporting system of pile anchor foundation pit

The prestressed anchor cable plays an important role in controlling the deformation of the foundation pit. The failure of anchor cable will cause a series of safety problems of foundation pit. Aiming at this problem, with a deep foundation pit in Jinan as the engineering background, the deformation laws of the supporting structure caused by the failure of single-row anchor cables and local anchor cables are studied. For the failure of a single row of anchor cables, when the first row of anchor cables fails, the horizontal displacement of the pile body is the largest at the top of the pile, which has the most obvious impact on the deformation of the supporting pile. When the first row of anchor cables fails in a combination of 1, 2, 4, 9, 17, or 34 in the positive direction of the X-axis, the horizontal displacement of the pile top at the failure point will increase with the number of anchor cable failures. the greater the increase in displacement, the greater the axial force of the two anchor cables near the point of failure.

Development of a new nano modified cement based adhesive for FRP strengthened RC members

There is currently a high demand in the construction industry for high-strength and durable cement-based adhesives (CBAs) for use in a variety of applications such as ground anchors, ground improvement, concrete repair or as a substitute for epoxy resins to bond carbon fiber reinforced polymers (CFRPs) to concrete structures where increased fire performance is required. Limited research has been conducted on the use of nano-materials such as graphene oxide (GO), graphene nanoplatelets (GNPs) and nano silica (NS) to improve the mechanical properties and durability of CBA. This paper presents a new mix design comprising ordinary portland cement, GNP, GO and NS to improve the strength and durability of CBA. Furthermore, the paper presents bond-slip measurements between carbon fibre reinforced polymer (CFRP) and CBA embedded in concrete prisms. Single lap shear prisms are tested using different CBA mixes and structural epoxy to assess the bond for near surface-mounted (NSM) strengthened structures. The results show improvement in the compression and flexural strength of the CBA by up to 46% and 57% respectively by using NS and GO at low concentrations. The durability tests indicate that the modified CBA has 61% lower chloride penetration than standard CBA due to reduced porosity. The pull-out tests were conducted of FRP-to-concrete joints using the CBA materials and results of GNP-reinforced CBA show an improvement of 27% and 73% for CFRP laminate and textile, respectively.

Evaluation of Residual Tensile Load of Field Ground Anchors Based on Long-Term Measurement

Evaluation of Residual Tensile Load of Field Ground Anchors Based on Long-Term Measurement

Load transfer on instrumented prestressed ground anchors in sandy soil

abstract: This study evaluates load variations in instrumented prestressed ground anchors installed in a bored pile retaining wall system in sandy soil. Data were collected from instrumentation assembled in the bonded length of three anchors, which were monitored during pullout tests and during different construction phases of the retaining wall system. Instrumentation consisted of electrical resistance strain gauges positioned in five different sections along the bonded length. Skin friction distributions were obtained from the field load measurements. Results showed that the skin friction followed a non-uniform distribution along the anchor bonded length. The mobilized skin friction concentrated more intensely on the bonded length half closest to the unbonded length, while the other half of the bonded length developed very small skin friction. The contribution of the unbonded length skin friction to the overall anchor capacity was significant and this should be accounted for in the interpretation of routine anchor testing results. Displacements applied to the anchor head were sufficient to mobilize the ultimate skin friction on the unbonded length, but not on the bonded length. Performance of loading-unloading stages on the ground anchor intensified the transfer of load from the unbonded length to the bonded length. Long-term monitoring of the anchor after lock-off revealed that the load at the anchor bonded length followed a tendency to reduce with time and was not significantly influenced by the retaining wall construction phases.

Predicting load on ground anchor using a metaheuristic optimized least squares support vector regression model: a Taiwan case study

Abstract Failure of ground anchor is a major cause of landslides and severe natural hazards, especially in the highly developed mountainous areas such as New Taipei City. Accurately estimating load on ground anchors is thus essential for evaluating the stability status of slope to prevent landslide from happening. This study first employed correlation analyses to identify possible influential factors of load on ground anchors. Second, various artificial intelligence models were used to map the relationship of the found influencing factors with the current load on ground anchors. The results indicated that the symbiotic organisms search-optimized least squares support vector regression (SOS-LSSVR) model had the optimal accuracy by earning the smallest value of mean absolute percentage error (9.10%) and the most outstanding value of correlation coefficient (R = 0.988). The study applied the established inference model for the real case of estimating load on un-monitoring ground anchors. The analyzed results strongly advised administrators to conduct site surveying and patrolling more frequently to take early proper actions. In summary, the obtained results have demonstrated SOS-LSSVR as an effective alternative for the conventional subjective evaluation methods, which is able to rapidly provide accurate values of load on un-monitoring ground anchors.

The Effects of Anchor Configurations and Field Characteristics on Anchored Diaphragm Walls

Diaphragm walls are the ideal solution for productive utilisation of underground space to meet modern-day demands of infrastructural developments. Selection of appropriate wall and support configurations has substantial impact on economy, time and performance of diaphragm wall projects. Unsatisfactory implementation of such retaining systems during or after construction may cause heavy causalities such as loss of life and damage to adjacent infrastructures. Practical significance of excavation induced deformations is large due to its potential damage to adjacent structures. Hence, reliable estimates of excavation related responses are vital for construction and implementation of embedded diaphragm walls. This paper presents results of parametric analysis performed on anchored diaphragm walls evaluating the effects of anchor configurations and field characteristics. Numerical studies are carried out to comprehend the influence of factors like ground conditions, excavation geometry and anchor characteristics. Walls embedded in cohesive and non-cohesive soils with varying ground water locations are considered for analysis. Anchor features including length, inclination and axial prestress are varied in combination with the above parameters. The effects of these factors and their combined influence on the deformation, flexural strength and axial capacity of diaphragm walls are determined and evaluated. Numerical analyses are carried out using finite element simulations with Plaxis 2d. Comparative charts are drawn to demonstrate the variations of wall responses with different combinations of influencing factors. Optimal configurations of diaphragm wall and anchor system for different ground conditions can be perceived from the result charts.

A Technic for Ground Anchor Force Determination from Distributied Strain Using Fiber Optic OFDR Sensor with the Rejection of a Temperature Effect

Anchor systems are widely used to stabilize soil slope and suppress slope failure. Thus, monitoring conditions of an anchor system is important to prevent disasters due to slope failure. The slope condition can be indirectly monitored by sensing the tensile force applied to the anchor because the slope deformation directly affects the anchor force. Previously, we propose a way to monitor the tensile force of the anchor by measuring the strain field on a bearing plate using a distributed fiber optic sensor (OFDR) and experimentally demonstrate that the anchor force has a large correlation with the strain distribution on the bearing plate. However, it was found that a spatial variation of the strain and thermal strain due to temperature change makes it difficult to get a reliable correlation coefficient. In this study, we newly propose a way to get a reliable correlation coefficient between the anchor force and the strain field on the bearing plate. We install a distributed optical fiber sensor in two concentric circles on the bearing plate and measure circumferential strain distribution. We take average values of the strain field in each circle as representative strain values minimizing the spatial variation and takes a difference of the two strains to exclude the temperature effect. We experimentally demonstrate that the proposed method gives a reliable correlation coefficient between the anchor force and the strain field on the bearing plate. This technique can be applied to various anchor systems to monitor the anchor force and manage the anchor systems safely.