Uplift Capacity Research Articles

Prediction of Uplift Capacity of Cylindrical Caissons in Anisotropic and Inhomogeneous Clays Using Multivariate Adaptive Regression Splines

The uplift capacity factor of cylindrical suction caisson in anisotropic and inhomogeneous clays considering the adhesion factor at the interface is investigated in this paper. The finite element limit analysis based on lower bound and upper bound analyses is used for analyzing purposes. The anisotropic undrained shear model is employed to describe the anisotropic and inhomogeneous clay. The impact of these dimensionless parameters on the ratio of inhomogeneity or strength gradient ratio, the adhesion factor, the ratio of depth over diameter, and the ratio of anisotropic undrained shear strengths on the uplift resistance and the collapse mechanisms of suction caisson foundations are determined. The multivariate adaptive regression splines technique is employed to access the sensitivity of all considered dimensionless parameters on the uplift capacity factor and to propose an empirical design equation as an effective tool for predicting the uplift capacity factor. The results presented in this paper can be guidance for the preliminary design of suction caissons in anisotropic and non-homogeneous clays that are useful for engineering practitioners.

Embedment depth and uplift capacity of a cryogenic pipeline dynamically laid in a trench

A cryogenic pipeline is a pipe-in-pipe with larger outer diameter, and is heavier and stiffer, compared to conventional deepwater steel flowlines or export pipelines. Cryogenic pipelines are being considered to be placed on shallow water sandy seabeds for transporting liquefied natural gas to shore. This paper reports a series of centrifuge tests to assess the as-laid embedment of a cryogenic pipeline during the dynamic laying process, and the uplift capacity of a trench-laid pipeline. Two seabed conditions were tested – a pipe laid onto the base of a trench and a flat silica sand deposit. A total of 12 dynamic pipe laying tests were conducted varying vertical force and pipe oscillation amplitude, frequency and number of cycles; encompassing typical laying processes. The embedment depth was found to be less than 0.5 diameters, owing to the resultant effect of lateral ploughing and sand densification. For the trench-laid cases, after the completion of simulating the pipe laying process, the trench was filled with a layer of sand and a layer of rock armour; varying the thickness of the layers. The pipe was then extracted vertically to evaluate the uplift resistance, which was shown to increase with increasing the thickness of the rock layer.

Uplift resistance of buried pipelines: The contribution of seepage forces

Pipelines are commonly buried, and can buckle upwards when heated if there is insufficient soil uplift capacity. Interface tension beneath the buried pipe significantly influences the uplift capacity at shallow embedments. Conventional design approaches, which consider either zero or unlimited interface tension, do not assess and quantify the effect of interface tension on uplift capacity. The present study bridges the gap between conventional “no tension” and “full tension” capacities. Mobilisation of interface tension is governed by seepage forces which in turn directly control the formation of a gap beneath the pipe. A large deformation finite element approach, which simulates this phenomenon of gap formation using a thin layer of gap elements below the pipe, is adopted to study the soil response for various cases of uplift velocity, embedment and soil weight. The enhancement in undrained shear strength of soil at higher uplift velocities due to strain rate effects has also been considered. The interface tension mobilised at these different velocities and embedments varies systematically in a way that is expressed by modifying Hvorslev's intake factors. The proposed expressions may be used with the existing methodologies to assess pipe stability during operation, demonstrated here through a design example.

Study on Vertical Piles Batter Piles and Pile Groups Under Uplift Load Conditions in Non-Cohesive Sub-soil

Model study was carried out in the laboratory to know the effect of uplift loads on vertical piles, batter piles and pile groups. Mild steel piles having 22 mm outer diameter and 1 mm thickness were considered for the study. Different slenderness ratios of 18, 28, and 38 and various angles such as 0°, 15°, 25°, 35°, and 45° were considered for the study. Sand was poured at a fixed relative density in the mild steel tank. The experimental study shows that the pile and pile group attain a maximum value of uplift load capacity when the batter angle reaches to 35°. Further, increase in batter angle causes a reduction in the uplift capacity of the pile. It is also noticed from the experiment that the uplift capacity increases with the slenderness ratio of the pile but its variation is not linear. Pile group of four piles with two vertical piles having L/D ratio 38 and two batter piles of L/D ratio 28 offers same resistance as of the same pile group with all the piles having L/D ratio 38. Numerical modeling was performed using Plaxis-3D to analyse the behavior of vertical piles, batter piles and pile groups under uplift loads. A good correlation between Plaxis-3D results and experimental results was observed.

Accounting for Soil Spatial Variability in Plate Anchor Design

AbstractThe effect of soil spatial variability on the undrained vertical uplift capacity of plate anchors has been examined using the three-dimensional random finite-element approach. Multiple dire...

Lower bound limit analysis for an axisymmetric problem in rock mass using power cone programming

AbstractBy using the lower bound finite elements limit analysis (LB‐FELA) and the power cone programming (PCP), a methodology has been introduced to solve an axisymmetric stability problem with the usage of the generalized Hoek and Brown (GHB) yield criterion‐ applicable for rock mass. The formulation involves the application of the GHB yield criterion in its native form without any smoothing and it does not require any assumption associated with the circumferential stress (σθ). The efficacy of the proposed formulation in a rock mass has been demonstrated by determining (i) the bearing capacity of a circular footing, (ii) the stability numbers for an unsupported vertical cylindrical excavation, and (iii) the vertical uplift capacity of a circular horizontal anchor. The obtained solutions have been compared with that reported in literature. Failure patterns for each problem have also been explored. For the anchor problem, for no solution is reported with the usage of the GHB criterion, the influences of different material yield parameters (GSI, mi, and σci), unit weight of rock mass (γ), and the embedment ratio (H/B) of the anchor on the uplift capacity have been comprehensively examined.

Experimental Study on Bearing Capacity of Normal‐ and High‐Strength Steel Screw Anchors

In order to study the bearing capacity of high‐strength steel screw anchors, six groups of screw anchors with different parameters are carried out in in situ construction and tensile capacity test, and the load‐displacement curves of screw anchors with different parameters are obtained. The bearing mechanism, design method, and key design parameters of screw anchors are analyzed. Meanwhile, the test results are compared with theoretical calculation and numerical simulation, respectively. The results show that the drawing capacity of high‐strength screw anchor calculated by the existing standard theoretical calculation formula is conservative. The load capacity of high‐strength steel screw anchor is significantly higher than that of ordinary strength steel screw anchor. Taking steel strength, buried depth, quantity, distance, and diameter as the influencing factors, parametric analysis is conducted on the uplift capacity of high‐strength steel screw anchor in this paper. The embedding depth of screw anchor, the number, spacing, and diameter of anchor plates have significant influence on the drawing capacity of screw anchor. The construction technology of high‐strength steel screw anchor is reliable, and the bearing capacity is good, which can be applied in the soft‐soil power transmission line projects, such as bog and coastal beach. Meanwhile, the research results can provide reference for the design and construction of screw anchor foundation.

Seismic vertical uplift capacity of horizontal strip anchors embedded in sand adjacent to slopes using finite element limit analysis

Seismic vertical uplift capacity of horizontal strip anchors embedded in sand adjacent to slopes using finite element limit analysis

A large deformation finite element analysis of uplift behaviour for helical anchor in spatially variable clay

Helical anchors are emerging as an innovative foundation design option for offshore applications to resist uplift loading. Numerous studies have been conducted to investigate the uplift behaviour of helical anchor under different helical bearing plate configurations in clay, using field or model tests and numerical analyses. However, almost all of them are limited to deterministic analyses, ignoring the influence of inherent spatial variability of soil. This work conducts a 3D large deformation random finite element analysis to examine the uplift behaviour of helical anchor in spatially variable clay. Both the stochastic and deterministic trends are reflected in the 3D non-stationary random field adopted. It is found that the spatial variability of soil strength significantly influences the failure mechanism of helical anchor. Parametric studies indicate that the coefficient of variation and vertical scales of fluctuation of soil strength can much influence on the uplift capacity, while the effect of horizontal scales of fluctuation is trivial. The goodness-of-fit test of probability distribution suggests that random uplift capacities follow log-normal distribution, whereby the relationship between factor of safety and probability of failure is established. The findings of this paper might facilitate the safety design of helical anchor.

An experimental and numerical comparative study on the uplift capacity of single granular pile anchor and rough pile in sand

ABSTRACT Granular pile anchor (GPA) is a relatively new foundation technique that is found to be effective in resisting uplift forces in expansive soils. GPA can be utilized as an economical substitute to other methods to resist the uplift loads in sandy soils. In this study, the performance of GPA and rough piles installed in the sand bed under uplift was examined using laboratory-scale model tests and numerical modelling under field-scale conditions. The laboratory-scale model tests were performed on GPA and rough pile installed in loose, medium dense, and dense sand beds by varying the embedment ratio. A numerical study was also done to check the findings from the experimental study and GPA suitability under field-scale conditions. The laboratory-scale and numerical study revealed that GPA performs better than bored pile under uplift force and can be used as an effective ground anchoring alternative.

Control of screw pile installation to optimise performance for offshore energy applications

Screw piles can be used as foundations for offshore energy applications, thanks to their silent mode of installation and considerable uplift capacity, although a significant upscaling of onshore dimensions is necessary. However, the crowd (vertical) force necessary to install upscaled screw piles was previously shown to be far too great for practical installation. Although guidance recommends that the pile's vertical displacement must be one helix pitch (helix height) per each pile revolution, it is shown in this paper that a lower vertical displacement per revolution can significantly reduce the necessary crowd force during installation or even generate some pull-in. In addition, it was shown that the uplift stiffness and capacity of the pile were enhanced by this installation process, at a shallow (relative) depth in sand. This paper gathers 19 centrifuge tests, with varying screw pile geometries (shaft diameter, base shape), sand relative density and advancement rates. A predictive framework for the pull-in potential of a given pile geometry was proposed to assess its ability to be installed with a reduced crowd force.

Consolidation effects on uplift capacity of shallow horizontal plate anchors in dilating sand

This paper examines the effect of consolidation on a shallowly embedded horizontal plate anchor in medium dense and dense sand using centrifuge tests, where the loading rate was varied over four orders of magnitude. The experimental results show a 4·8 to 5·5 times increase in anchor capacity as the consolidation condition changes from drained to undrained, driven by a steadily increasing negative excess pore pressure with increasing loading rate. At the highest loading rate, under undrained conditions, the measured maximum negative excess pore pressures reach a steady limit, suggesting the occurrence of cavitation. This increase in anchor capacity with dimensionless velocity is captured using a ‘backbone curve’ interpretation framework that describes the change in capacity between the limiting values of drained and undrained anchor capacity. Calculation of drained anchor capacity is straightforward relative to the more challenging problem of calculating undrained capacity, particularly during cavitation. This was addressed separately through a numerical parametric study (pure undrained analysis) using a bounding surface soil model involving different water depths (cavitation potential), densities and embedment ratios. The numerical results are then synthesised into a simple extended analytical solution to allow estimation of undrained anchor capacity under different densities and water depths.

Numerical model for suction caisson under axial cyclic loadings

Jacket foundation with multi-caissons has been recommended for the offshore wind turbines (OWTs) with water depth exceeding 30m. For the jacket supported OWT, the response of suction caisson under axial cyclic loadings is critical for the engineering design. In this study, a numerical model is proposed to estimate the differential pressure, displacement accumulation and loading stiffness of suction caisson under axial cyclic loadings. In the proposed model, novel cyclic t-z and Q-z spring models are established and included to account for cyclic loading effects. The proposed model is validated by the centrifuge tests of suction caisson. Afterwards, a series of numerical studies are performed to study the effects of key factors (i.e., cyclic loading amplitude, frequency, soil permeability and loading direction) on the suction caisson responses. It is found that for the suction caisson in typical fine sand seabed, the threshold amplitude of cyclic tension loadings generally ranges between 0.4Pu_d and 0.5Pu_d (Pu_d is the uplift capacity of suction caisson under fully drainage condition). The contribution of differential pressure to the suction caisson capacity can reach up to about 20% for the stable state. The failure risk of suction caisson in permeable seabed will be obviously increased for low-frequency cyclic tension loadings.

Effects of scour-hole depth on the bearing and uplift capacities of under-reamed pile in clay

Scouring causes reduction in the load-carrying capacity of pile foundation. The load-carrying capacity of a pile foundation can be increased by increasing the pile dimension. In the present study, under-reamed piles are considered under scour condition. In this study, bearing and uplift capacities of under-reamed pile in clay under scour condition are obtained numerically using lower bound finite element limit analysis. The effects of scour-hole shape, bulb shape, and embedment ratio are studied. The non-dimensional bearing capacity factor (Ncus) and uplift capacity factor (Fcus) are presented through design charts. Ncus and Fcus of under-reamed pile are found to be significantly higher than those of pile without bulb under scour condition. Ncus and Fcus are found to decrease significantly with scour depth. Ncus and Fcus are found to increase with the increase in under-ream angle (β). From the axial force distributions, it is found that bulb carries a significant portion of the ultimate load in axial compression and tension. With the consideration of stress history, bearing and uplift capacities of under-reamed pile are found to reduce significantly. Also, the extent of plastic shear failure in the soil is found to increase with decrease in scour depth and increase in embedment ratio.

Investigation on the ultimate uplift capacity for asteroid exploration in drilling anchoring process: Numerical modelling and DEM simulation

Due to the microgravity environment on the asteroid surface, the asteroid probe can not stay on the asteroid surface for a long time for in situ exploration. Therefore, an anchoring device is required to provide enough ultimate uplift capacity to hold the asteroid probe on the asteroid’s surface. In this work, based on the circular arc failure surface hypothesis, a numerical model is proposed to predict the ultimate uplift capacity of the anchor rod of the anchoring devices during the anchoring process. The discrete element method (DEM) simulation is adopted to calculate the ultimate uplift capacity and analyze the geomaterial behavior. To accurately simulate material properties, the parameters of three typical geomaterials are calibrated by the calibration tests. The anchoring experiment verifies the DEM simulation results, and the error between the anchoring experiment results and DEM simulation results is less than 14%. The distribution of loaded particle forces in the geomaterials under various anchoring angles(30°–60°) and the anchoring depths(20 mm–40 mm) in the simulation are compared and analyzed. Compared with the DEM simulation results, the ultimate uplift capacity’s numerical model prediction results have an error of 5%. This work provides a new approach to predict the ultimate uplift capacity of the anchor rod of the anchoring devices during the asteroid exploration process.

Evaluating the behavior of expandable multi-plate mechanical anchors in granular soils

Many offshore and onshore platforms are constructed widely, especially in the energy supply sector. So far, various anchors with different shapes have been introduced for these applications to reinforce and stabilize the superstructures and related facilities. This study deals with the novel Expandable Multi-plate Mechanical Anchors (EMPMAs) and investigates their performance using physical modeling tests. Previous studies have focused on the behavior of single anchors, and there is no comprehensive experimental research on the multi-plate anchors, which is the subject of the present study. This research examines uplift capacity regarding the effect of five soil densities for loose, medium, and dense conditions and the vertical distance between two plate anchors from 0.5B to 2B. The obtained results were compared with the single plate anchor with equivalent areas. Based on the results, the highest bearing capacity is related to the EMPMA with an optimum distance of 1B in 80% relative density. In addition, by decreasing the distance between two plates, the relative density becomes a more dominant bearing capacity parameter. Furthermore, the Particle Image Velocimetry (PIV) method results are presented, and a new formula is proposed. Eventually, a comparative study is conducted between the results of PIV and an analytical formula.

Vertical Uplift Capacity of Rectangular and Square Plate Anchors in Granular Soil under Seismic Forces

ABSTRACT Earthquakes have unfavorable effects of reducing uplift resistance of plate anchors. Present work investigates the seismic vertical uplift resistance of shallow buried square and rectangular plate anchors in granular soil by using the upper bound theorem of limit analysis in combination with pseudo-static approach. The influence of shear transfer at the interface between anchor plate and overlying soil has been taken into account. The uplift resistance was evaluated in terms of dimensionless factors due to component of soil weight and surcharge pressure on the ground surface for varying combinations of parameters involved in the present analysis. Results revealed that seismic uplift factors increase continuously with increasing soil friction angle and embedment ratio of anchors; whereas, the same reduce with increasing acceleration coefficients, shear factor and aspect ratio of anchors. A significant reduction was observed for higher shear factor and earthquake acceleration coefficients, especially for lower soil friction angle and embedment ratio, and for higher aspect ratio of anchors.

Vertical Uplift Capacity of Circular Anchors in Clay by Considering Anisotropy and Non-Homogeneity

Vertical Uplift Capacity of Circular Anchors in Clay by Considering Anisotropy and Non-Homogeneity

Causes of Failure and Strengthening Measures of a Pile Foundation Supporting Transmission Line Tower

The present paper describes the failure of an under-reamed pile foundation supporting a transmission line tower. Assessment of in-service pile conditions, i.e., in-built length, depth of bulb, uniformity of cross-sectional area, and length of the pile, are estimated through low strain integrity test. Integrity test results reveal that improper bulb formation and shorter as-built length than the designed are the leading causes of failure. Different performance criteria (uplift, compressive, and lateral capacities) of the in-service pile foundation are estimated through extensive finite element analysis and conventional approaches. Different geometric characteristics, i.e., straight shaft, different diameter of the bulb, and pile length, are simulated to represent the in-service pile conditions in the field. The estimated performance criteria are compared with the initial design values to evaluate the required degree of strengthening measures. The paper presents the initial design criteria of pile, soil condition, low strain integrity test, finite element modeling, and strengthening measures of the pile foundation to ensure the long-term stability of the transmission tower.

Analysis of Under-Reamed Piles Subjected to Different Types of Load in Clayey Soil

Underreamed piles with one or more bulbs have been widely used in almost all types of soil to support a range of structures. In some cases, in addition to vertical compressive or uplift loads, piles must withstand a considerable lateral load. A 3-D finite element study using ABAQUS software was conducted to examine the behavior of underreamed piles in clay soil under pure lateral, pure uplift, and combined uplift and lateral loads. In this study, pile (L/D) ratios of 11.66, 15, 20, and 25 were considered by adjusting the pile length to simulate the behavior of rigid and flexible piles. The piles were modeled as a linear elastic material, and the soil behavior was simulated using the Drucker-Prager constitutive model. The findings show that the lateral resistance of piles with (L/D) ratios of 11.66 and 15 increased slightly when underreamed piles were used. However, no change in lateral resistance was observed for underreamed piles with (L/D) ratios of 20 and 25 compared with straight piles. The uplift capacity of underreamed piles was significantly greater than that of a straight pile. The lateral capacity was marginally influenced by the prior uplift loading, such that it decreased for a rigid underreamed pile, and increased for a flexible underreamed pile.