Uplift Resistance Research Articles

Bearing capacity of plate anchors subjected to average and cyclic loads in clays

Plate anchors has been widely adopted to provide uplift resistance for offshore floating facilities. The research on the bearing capacity of plate anchors under cyclic loads is still relatively rare compared with the monotonic loads. In the present paper, the bearing capacity of plate anchors under the combination of average and cyclic loads in normal consolidation clays with linearly increasing shear strength is investigated by performing finite element analysis. A more reasonable expression is proposed to determine the cyclic loading factor Ucy by modifying the formula suggested by Det Norske Veritas. A global cyclic loading factor Ucy∗ is defined to determine the cyclic anchor resistance considering the difference of average stress level for each soil element around the anchor. The method to determine Ucy∗ by finite element analysis is suggested, and then the Ucy∗ for any given normalized average loads and the equivalent number of cycles is determined. The effects of the embedment depth and inclination angles on Ucy∗ are also investigated by performing a series of numerical simulations. An expression of Ucy∗ associated with normalized average loads and the equivalent number of cycles are proposed based on numerical results, which can provide some reference for the engineering design of plate anchors under cyclic loads.

Rate effects on the uplift capacity of pipelines embedded in clay: Finite element modelling

This paper describes a finite element study of the uplift behaviour of a plane strain pipe segment embedded in modified Cam clay soil. The primary aim of this study is to explore the role of rate effects on pipe uplift capacity and the transition between drained and undrained behaviour using coupled-consolidation analyses. The velocities considered in the modelling cover six orders of magnitude allowing a rigorous assessment of the effect of loading rate. The effects of pipe embedment depth, soil strength profile and pipe-soil interface roughness are also investigated. The results indicate that the range of uplift velocities for which the soil response can be considered ‘partially drained’, as determined from the peak uplift resistances, is significant and exceeds bounds established for full flow penetrometers previously reported in the literature. The data also suggests that excess pore pressures determined locally at the pipe-soil interface may not be a reliable means of determining whether the overall response is ‘drained’ or ‘undrained’. To address some limitations associated with design guidelines currently used by industry, a new approach for the prediction of pipe uplift capacity as a function of loading rate is proposed and applied to a hypothetical design scenario.

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.

Uplift resistance of strip plate anchors in cohesive-frictional sloping ground

ABSTRACT This paper presents numerical solutions for determining the uplift capacity of strip plate anchors embedded at different positions either from the toe or crest of cohesive-frictional soil slope. Finite element method based on lower bound theorem of limit analysis in combination with second-order cone programming has been used for obtaining the solutions. Dimensionless uplift factors are used for estimating the ultimate uplift resistance of anchors due to contribution of cohesion, unit weight and angle of internal friction of soil. For different edge-distance of anchor either from the crest or toe of the slope, computations are carried out for different combinations of internal friction angle of soil, angle and height of sloping surface, normalised strength of soil, and embedment ratio of anchors. The uplift capacity has been found to remain the same within a zone extending from the middle of the slope towards both toe and crest of sloping ground.

Experimental Study on Uplift Mechanisms of Pipes Buried in Sloping Medium Dense Sand

Experiments are conducted to study the uplift behavior of buried pipes in medium dense sand with ground slopings of 0°, 10°, 20°, and varying burial depths. Test results show that the peak uplift resistance reduces with the increasing sloping, and up to 10% reduction is observed for 20° sloping. The displacement of pipe where the peak is mobilized, δp, also reduces for 20° sloping. The uplift resistance experiences a quicker reduction after reaching the peak due to the ground sloping. The stress level increases on the uphill side and decreases on the downhill side, resulting in lower and higher dilatancy, respectively. Consequently, the angle of slip plane to the vertical direction become smaller on the uphill side and larger on the downhill side. The slip plane extends to the ground and a complete slip mechanism develops at a pipe displacement of 2δp. The postpeak softening behavior of uplift resistance oscillates at an amplitude increasing with the burial depth and decreasing with the sloping, due to the arching effect as soil moving around the pipe periphery.

Towards the modelling of uplift resistance of skirted offshore mudmat foundations

A well-planned retrieval or removal operation of deep-water subsea foundations, such as mudmats, resting on soft soil requires estimation of the uplift forces required for the execution of the retrieval. In this paper, mathematical functions that are tuned to describe the uplift resistance due to skirt-soil friction and suction are proposed. Furthermore, possible benefit from inclined pull is deduced.

The Effectiveness of Ensemble-Neural Network Techniques to Predict Peak Uplift Resistance of Buried Pipes in Reinforced Sand

Buried pipes are extensively used for oil transportation from offshore platforms. Under unfavorable loading combinations, the pipe’s uplift resistance may be exceeded, which may result in excessive deformations and significant disruptions. This paper presents findings from a series of small-scale tests performed on pipes buried in geogrid-reinforced sands, with the measured peak uplift resistance being used to calibrate advanced numerical models employing neural networks. Multilayer perceptron (MLP) and Radial Basis Function (RBF) primary structure types have been used to train two neural network models, which were then further developed using bagging and boosting ensemble techniques. Correlation coefficients in excess of 0.954 between the measured and predicted peak uplift resistance have been achieved. The results show that the design of pipelines can be significantly improved using the proposed novel, reliable and robust soft computing models.

Effect of Different Soil Saturation Conditions on The Ultimate Uplift Resistance of Helical Pile Model

Helical piles have many properties over the other types of piles systems include high tensile capabilities, the possibility of fast installation, applying load immediately after installation, and suitability for most soil's condition. In addition to that, helical piles have relatively less noise during installation; they represent a cost-effective alternative to conventional pile types. The use of helical piles grows in the world in the last fifty years. Many studies concentrate on the performance of this type of piles in fully saturated and dry soils. The achievement of the helical pile in unsaturated soils is rarely studied. So, to cover this small-scale demand model of the helical pile with double helices has been tested. Twenty tests were performed on three different models (pile with single helix, pile with double helices, and pile with triple helices) and pile shaft only, embedded in different conditions of soil saturation (fully saturated, partially saturated, and dry soils) under uplift loading. Three different matric suction of partial saturation were used of 6.5, 7.4, and 9.6 kPa. The results obtained from the tests showed that the highest value in the unsaturated soil was at suction 6.5 kPa compared to other soil saturation conditions. The results mention that model piles embedded in dry soil have lower values of ultimate uplift capacities. The increment in uplift resistance of additional helices of single double and triple helices than that of shaft pipe embedded within dry soil shows an increment by approximately about 170, 240, and 282% respectively, for fully saturation soil, the increment about 342, 463, and 585% respectively, and by about 400, 429, and 475% respectively for matric suction of 6.5 kPa.

Numerical finite element modelling of soil resistance against upheaval buckling of buried submarine pipelines

Submarine high-temperature and/or high-pressure (HT/HP) pipelines used to transport oil and gas in different areas of the world are susceptible to global buckling. If the pipelines are simply laid on the seabed, buckling will likely occur in the horizontal plane, while if buried in a trench, buckling will probably arise in the vertical direction. The latter buckling mode may lead to catastrophic pipeline failure potentially associated with massive oil leakage and severe environmental damage. Concerns about upheaval buckling have motivated a number of researchers to investigate the uplift resistance of buried pipelines. The objective of the work presented in this paper is to evaluate the effects of various parameters on the resistance to upheaval buckling of buried submarine pipelines. A numerical approach was adopted to build representative and reliable 3D models of 100 m long embedded pipelines, using the specialized finite element software ABAQUS. Starting with a baseline model of a pipe buried in a medium dense sand with fines, the effects of pertinent parameters on the uplift resistance were explored for two scenarios: (1) uplift of the full pipeline length, which in essence captures the plane strain 2D response and (2) uplift of a central middle length of 20 m. Specifically, the effects of the apparent cohesion of the soil due to the presence of fines, pipeline diameter, embedment depth and diameter to wall thickness ratio were investigated. The uplift forces obtained were normalized in reference to actual pullout and/or effective pipe lengths. The results indicated that the normalized uplift forces obtained in the 3D analyses at high pipeline displacements were in close agreement with those yielded by plane strain models. The model analyses indicated that the contribution of soil apparent cohesion to the uplift resistance was significant. The work indicated that as the level of confinement increases (due to an increase in embedment depth and/or pipeline diameter) the uplift resistance increased, whereas the relative contribution of soil cohesion to the resistance decreases. The paper includes a comparison of the FE results obtained in this study with the available predictive/analytical methods, along with a discussion of which best captured the failure mode and resistance to uplift values of the numerical model.

Experimental Investigation of Skirted Foundation in Sand Subjected to Rapid Uplift

Abstract This paper reports results from 1g model tests carried out under single gravity on a skirted foundation installed in sand and subjected to a rapid uplifting force. The effects of displacement rates ranging from 5 mm/s to 450 mm/s on the ultimate capacity, suction pressure inside the skirt compartment, and time of extraction were investigated. Test results indicate that the displacement rate significantly affected the magnitude of uplift resistance, as well as the magnitude of suction under the foundation lid, but had little effect on the relationship between stress and the displacement of the foundation. The shapes of the uplift capacity-displacement curve and the suction-displacement curve were similar for all experimental displacement rates.

Numerical Analysis of Pipeline Uplift Resistance in Frozen Clay Soil Considering Hybrid Tensile-Shear Yield Behaviors

Pipeline uplift resistance plays an essential role in the design of buried pipelines in permafrost regions or artificially frozen ground. The uplift resistance of a frozen soil is dependent on the soil’s mechanical properties, which are needed to characterize plastic zones in the frozen soil that surrounds pipes. Owing to the presence of unfrozen water, frozen soils (especially frozen clay) display complex mechanical properties that vary with temperature. There are limited amount of research studies with a focus on frozen soil–pipe interactions that consider temperature change within the literature. In this study, numerical modeling is conducted to investigate frozen soil–pipe interactions at different temperatures. A pipe is simulated with vertical displacement-controlled or vertical load-controlled conditions. The temperature-dependent mechanical parameters of frozen clay soil are examined in the simulation. Both tensile yield and shear yield behaviors are included in the modeling with the consideration of temperature-dependent failure criterions. Simulated stress paths in monitoring points are collected and displayed. The results from the Mohr–Coulomb model with the Rankine tensile cut off, and the hyperbolic Drucker–Prager model are compared and analyzed. The results indicate that hyperbolic Drucker–Prager model is effective for analyzing soil plastic zones surrounding a pipe. The applied hyperbolic Drucker–Prager model with reduced strength parameters can produce a more conservative uplift resistance–displacement relation when compared with that from the Mohr–Coulomb model. Thermally induced plastic strain may have a significant impact on the uplift resistance analysis, since a significant thermally induced uplift displacement is expected if there is a heat transfer process from a buried pipe to the surrounding frozen soil.

Performance of anchor in sand with different forms of geosynthetic reinforcement

This article investigates the uplift resistance of a model anchor embedded in sand by varying its density at four embedment depths (L/D = 1, 2, 3, and 4). Such different configurations of geosynthetic reinforcement as single-layer planar, double-layer planar, geocell, and geocell with basal reinforcement were used and the results were compared. The inclusion of reinforcement in all configurations led to greater values of the peak and residual pullout load than the anchor in sand at different densities. Of the reinforcement configurations used, a geocell placed at the top of the anchor was the most efficient. The shape of the uplifted soil mass by the anchor is a frustum of a cone with an apex angle approximately equal to ϕ′ of sand both in the sand bed without and with a geocell-encapsulated layer, except for the enlarged size of the uplifted mass for the anchor in the geocell-reinforced bed. The enlarged size is attributed to the wide slab response of the combined geocell composite anchor system. The hyperbolic stress–strain relation was used to predict the pre-peak phase of the load–displacement response and the relation thus obtained compared well with experimental data irrespective of the embedment ratio and relative densities of the sand bed.

Investigation on determining uplift capacity and failure mechanism of the pile groups in sand

Foundations of some structures such as transmission towers, mooring systems for ocean surface or submerged platforms, tall chimneys, jetty structures are subjected to uplift loading. In such cases, to increase the uplift resistance of piles, pile foundations like anchored piles, batter piles and micro piles are extensively employed depending on in-situ conditions. This paper deals with the contribution of pile surface to the uplift capacity and the failure mechanism obtained for the pile groups in sand, and the main goal of the study is to investigate the effect of the pile surface roughness on the uplift capacity for different spacing ratios between piles. Therefore, the paper presents some findings of a single pile and pile groups embedded in sand under uplift loading. The variables investigated include the pile surface, pile embedment ratio and the spacing ratio between piles. Experiments were conducted to study foregoing parameters, and three-dimensional analyses were performed using the finite element method to illustrate the failure mechanism of pile groups and the interaction between piles for different spacing ratios between piles. The results show that the finite element simulation results indicate good agreement with the experiment results, and all variables significantly affect the uplift capacity of piles.

Study on the Uplift Resistance of a Joint Board Cable Foundation Considering the Critical Burial Depth

Joint board cable foundation is a kind of foundation form proposed by the author which is applied to transmission lines. In the study of pullout resistance of a joint board cable foundation, the most important factor is the critical depth of the anchor plate. Theoretical analyses, numerical simulations, and field tests were carried out to study the impact of the critical burial depth on the uplift capacity. Test results were corrected by the back-stepping cohesive force method. The theoretical analyses, numerical simulations, and field test results are consistent. It is found that the development of the plastic failure zone in the foundation is directly controlled by the shear strain. Through the field test, the pullout resistance of the anchor plate is studied, and it is found that the depth ratio has the most influence on the pullout capacity of the anchor plate. The field tests also provide valuable data for theoretical calculation, numerical simulation, and design of joint board cable foundations. The field test data analysis and numerical simulation results show that the optimal burial depth ratio of the joint board cable foundation is about 2.5–3.0. Therefore, in the design of the joint board cable foundation, 2.5 can be adopted as the critical depth ratio of the joint board cable.

MPM simulation of uplift resistance of enlarged base piles in sand

Piles with enlarged base have been widely employed in the civil and coastal engineering due to its high resistance to uplift. Many researchers have investigated the bearing capacity of enlarged base piles in case of small strains, but the investigations of enlarged base piles with variable geometries and large deformations are very limited. In this work, Material Point Method (MPM) is adopted to investigate the uplifting behavior of enlarged base piles with variable geometries and large deformations. As a continuum-based particle method, the MPM is well-suited to model the dynamic large deformation problems, for which the conventional Finite Element Method (FEM) is limited due to the mesh distortion. Centrifuge uplift tests of enlarged base piles in dense and loose sand are simulated using MPM and the calculated results are compared with the corresponding experimental data. The influence of various parameters such as bell angle, shaft/bell diameter ratio and the contact friction angle between the soil and pile, on the uplift capacity in sand have been investigated.

Seismic stability of obliquely loaded circular plate anchors

The vertical uplift resistance of obliquely loaded circular plate anchors buried in sand subjected to seismic loadings has been obtained in this paper. The pseudo-dynamic approach based on kinematic theorem of limit analysis has been applied for performing the analysis in which the nonlinear acceleration distributions are incorporated considering standing shear (S) and primary (P) wave propagations within the soil medium. Results are obtained in terms of seismic uplift factor (Fγs) due to soil self-weight component for a wide range of parameters. Through a parametric study, the effects of different parameters including normalized frequencies of Shear (S) and Primary (P) waves and damping ratio (ξ) of soil are highlighted. On account of amplified accelerations effects, the value of Fγs has been found to drastically reduce with an increase in load inclination (θ), horizontal and vertical seismic acceleration coefficients (kh and kv). The results show that the influence of damping ratio (ξ) of soil becomes more dominant particularly for frequencies close to the fundamental frequency of P wave.

Vertical uplift resistance of an innovative plate anchor embedded in sand

An innovative plate anchor, increasing its bearing section area during uplift is introduced in this paper. In this experimental study, the influence of the embedment ratio (depth of embedment/anchor width) of the plate anchor and anchor width (120, 150, 180, 250 mm) on the anchor pullout capacity and anchor displacement during an uplift test is investigated. Particle Image Velocimetry (PIV) is used to study soil deformation around the anchor at the maximum pullout load’s instant. The pullout loads predicted by prevalent theoretical methods have been compared with obtained experimental values. According to PIV analysis, two failure surfaces are observed and two equations are presented to estimate the ultimate pullout load of the shallow and deep opening plate anchor. It was observed that the pullout capacity ( was significantly influenced by anchor embedment ratio, but the anchor displacement corresponding to the maximum pullout load ( increased slightly as the embedment ratio increased. As the anchor width (B) increased, and increased, but the breakout factor (dimensionless pullout load) decreased. For shallow anchors, the sand deformation zone lines formed a curve that extended to the soil surface. For deep anchors, the sand deformation zone formed a truncated cone in the soil.

Efficacy of Geopile–Anchors in Controlling Heave of Expansive Clay Beds

This paper presents the efficacy of a new technique in the form of geopile–anchors in arresting heave of expansive soils. Heave behaviour of a remoulded expansive clay reinforced with vertical cylindrical cells made of geogrid and filled with geomaterials (geopiles) with a central anchor inside is presented. A geogrid cylinder installed vertically in an expansive soil (forming a geopile), with an anchor inside is a geopile–anchor. In a geopile–anchor, foundation is anchored to an anchor plate at the bottom of the geopile with the help of an anchor rod. The upward movement of foundation due to swelling of clay is resisted by anchorage and by friction mobilized at the interface formed by the fill material (expansive soil)–geogrid–expansive soil. Filling the geopile with granular material can augment uplift resistance. Tests were conducted on expansive clay beds reinforced with geopile–anchors varying geopile diameter and type of geopile-fill material. In the case of group of geopile–anchors, spacing was varied and its effect on heave studied. The experimental results indicated that geopile–anchors reduced heave of expansive clay beds. With expansive clay fill, a maximum reduction in heave of 78.50% was obtained. When gravel fill was used, a percentage reduction of 92.50% was obtained. At a radial distance of 60-mm the reduction in heave obtained was 50%, 58%, 67% and 73% for single geopile–anchor, and 69%, 73%, 81% and 88% for two group geopile–anchors installed at a spacing of 2d when the fill materials were clay, fine sand, coarse sand and gravel respectively.

Wind Resistance and Fragility Functions for Wood-Framed Wall Sheathing Panels in Low-Rise Residential Construction

AbstractThe wind uplift resistance of wood roof sheathing panels has been the subject of many engineering studies due to the high probability that roofs are subjected to large uplift loads during i...

Uplift Capacity of Light-Frame Rafter to Top Plates Connections Applied with Elastomeric Construction Adhesives

AbstractThe effects of the application of elastomeric construction adhesives on the wind uplift resistance of light-frame wood connections were investigated and are presented in this paper. Previou...