Dense Sand Deposits Research Articles

Lateral loading response of monopiles in sand considering installation effect

The effect of pile installation is a critical issue in the analysis and design of pile foundations but it is commonly ignored due to various numerical difficulties. In this paper, the installation process of an open-ended pipe pile into a sand deposit is simulated by using the zipper technique combined with the Arbitrary Lagrangian-Eulerian (ALE) method in the framework of three-dimensional (3D) finite element modeling. The subsequent lateral load response of the pile is also simulated and compared with that without consideration of the pile installation. The state dependent behavior of sand in associated with the pile installation and the subsequent lateral load is described using an advanced constitutive model. The study shows that for a relatively loose sand deposit, the sand surrounding the pile exhibits contraction and the disturbance zone takes the form of a trapezoid, whereas for a dense sand deposit, the sand near the pile will dilate and the disturbance zone takes the form of an inversed trapezoid. Furthermore, the deformation mode of the surrounding sand under the lateral load differs to some extent from that without consideration of the installation effect. Particularly, the location of the rotation point for the laterally loaded pile tends to move downwards when the pile installation process is considered.

On the In Situ Cyclic Resistance of Natural Sand and Silt Deposits

This study presents cyclic resistances of an instrumented medium dense sand (i.e., the Sand Array) and medium plastic silt (i.e., the Silt Array) deposit deduced from in situ dynamic testing using the controlled blasting test method. Particle velocity records were used to calculate the cyclic resistance ratios, CRRs, and convert the transient blast-induced ground motions into their equivalent number of shear stress cycles, Neq, through consideration of the cyclic resistance observed from stress-controlled, constant–volume, cyclic direct simple shear (DSS) tests. The CRR-Neq relationship developed for the medium dense sand deposit demonstrated that the in situ cyclic resistance is larger than that (1) expected from cyclic DSS test specimens reconstituted to the in situ vertical effective stress, relative density, and shear wave velocity, Vs; and (2) calculated using case history-based, penetration-, and Vs-based deterministic formulations of liquefaction triggering models. Differences between the in situ cyclic resistance and that computed using probabilistic liquefaction triggering models reduced somewhat when considering probabilities of liquefaction exceeding 50% and 85%, depending on the model. Partial drainage during dynamic loading of the Sand Array appears to have contributed to the cyclic resistance of the sand deposit, with an increase of 6% to 27% compared to that estimated for fully undrained conditions. Differences between the cyclic failure criteria used to interpret the cyclic resistance of intact laboratory specimens of silt result in significantly different interpretations of the in situ CRR; the use of maximum excess pore pressure-consistent criteria appears to provide the best representation of the in situ, stress-based cyclic resistance when high quality, intact silt specimens form the basis for conversion of transient seismic waveforms to uniform shear stress loading cycles. The investigation described herein suggests that the reduction of cyclic resistance for plastic soils to account for multidirectional shaking ranges from 0% to 7% over Neq of 1 to 100.

Permanent accumulated rotation of offshore wind turbine monopile due to typhoon-induced cyclic loading

In order to study the effect of typhoons on the accumulated deformation of monopile foundations for offshore wind turbines, a series of 1-g laboratory model tests with a geometrical scale of 1:100 were carried out. Through the horizontal static and cyclic loading tests of a stiff pile embedded in a medium dense sand deposit, the relationship between the accumulated rotation of the pile and the number of loading cycles under different loading conditions was obtained. The results show that the final accumulated rotation is mainly caused by the typhoon load series and is not affected by the loading sequence. Based on these results, a method is presented to predict the accumulated rotation of the monopile foundation during its service life, and a case study of a 6 MW wind turbine supported by a monopile at a water depth of 30 m in sand is conducted by using the method. The results show that the permanent accumulated rotation of the monopile throughout the design life is mainly contributed by cyclic loading induced by typhoons and the contribution of cyclic loading with small amplitudes can be ignored.

Three-Dimensional Finite-Element Analysis of Pressure–Settlement Response of Sand Compaction Pile-Treated Cohesionless Deposits

Abstract Strength characteristics of loose to medium dense sand deposits improved by sand compaction piles (SCPs) are assessed by performing three-dimensional (3D) finite-element analysis using PLA...

Correction: Spatial variability of medium dense sand deposit

Correction: Spatial variability of medium dense sand deposit

Spatial variability of medium dense sand deposit

This paper presents a study of the spatial variability of the cone resistance in a medium dense sand deposit in Norway. Spatial variability studies have been done earlier for clay deposits, but there are few results available for sands. To bridge this gap and to compare the spatial variability of sand and clay, the in situ cone resistance measured with the piezocone at the Oysand benchmark site near Trondheim was studied. Corrected cone resistances qt derived from the measured cone resistance and pore pressure measurements were used to establish the autocorrelation structure of the Oysand deposit. The qt data were examined visually and conclusions were drawn on the data that should be analyzed. A depth interval of 7 to 12 m was selected for spatial variability further analysis. The scale of fluctuation was identified. Several autocorrelation functions were considered, and the single exponential function was found to be the one offering the best autocorrelation. The spatial variability in the vertical and horizontal directions was analyzed statistically, using three different approaches, the auto-correlation fitting, maximum likelihood estimation and simplified Vanmarcke method. The results indicate short autocorrelation distances of 3 m or less in the horizontal direction, suggesting a very variable sand at the Oysand site. These horizontal autocorrelation distances are much shorter than those obtained for clays. In the vertical direction the scale of fluctuation was less than one meter, as obtained for other soils.

Bearing capacity of ring footing on weak sand layer overlying a dense sand deposit

ABSTRACT The present paper deals with the numerical investigation on the ultimate bearing capacity of ring footing resting on loose sand overlying a dense sand deposit. The study followed the use of lower and upper bound finite element limit analysis in conjunction with second-order conic programming. The analysis was carried out by varying the thickness as well as the angle of internal friction of the top and the bottom sand layers for the ring footing with the variation in the radius (ratio of internal radius to external radius) ratio. The results were expressed in terms bearing capacity ratio, which was found to decrease with an increase in the thickness and the friction angle of the dense sand layer. Further, the results obtained from this study compared well with the data available from the literature.

Penetration response of spudcans in layered sands

The behaviour of spudcan foundations during the installation and preloading in two-layer sand sediments was investigated through large deformation finite element (LDFE) analyses. The LDFE analyses were carried out using the coupled Eulerian-Lagrangian approach, modifying Mohr-Coulomb soil model to capture hardening and subsequent softening effects of sand. Parametric analyses were undertaken varying the top layer thickness, relative density of sand and spudcan diameter. Both loose to medium dense-over-dense and dense-over-loose to medium dense sand deposits were explored. The results showed that, for the investigated relatively thin top layer thickness of ≤ 5 m, spudcan behaviour was dictated by the bottom sand layer with a minimal influence of the top layer. For assessing the penetration resistance profile in two-layer sands, the performance of the ISO, SNAME, InSafeJIP, and other existing theoretical design methods were evaluated.

Numerical investigation of dip-slip fault propagation effects on offshore seabed sediments

An enhanced understanding of the phenomenon of dip-slip fault propagation through young, unconsolidated offshore sediments is essential for the evaluation of the response of shallow offshore structures and subsea oil/gas pipelines. Substantial and permanent seafloor deformation constitutes a severe geohazard for these marine structures and associated offshore elements/components. This paper addresses the response of seabed sand deposits subjected to underlying dip-slip fault movement. The resulting propagation of the faulting offset in seabed sediments is explored using 2D finite element modeling in Abaqus©, while accounting for nonlinear soil behavior with strain softening. A 2D plain strain FEM seabed model is adopted to investigate the effect of different seabed soil properties and layer thicknesses on the extent and magnitude of seabed surface deformation induced by normal and reverse faults with different dip angles. The results indicate that (1) the adoption of sand properties that depend on depth is necessary to realistically model the fault propagation mechanism, particularly for fault propagation through thick and dense sand deposits, (2) the presentation of fault propagation results in a normalized manner (with respect to sand cover thickness) does not eliminate the dependency on the actual layer thickness, and (3) the deformation of the beds in the cover as a result of basement faulting can be modeled realistically using trishear kinematic approaches.

Numerical study on accumulated deformation of laterally loaded monopiles used by offshore wind turbine

The design of an offshore monopile is generally governed by its accumulated response to lateral cyclic load, e.g., loads induced by winds and waves. In order to investigate the characteristic of this accumulated response, a user subroutine of degradation stiffness model (DSM) is developed and incorporated into a commercial finite difference program. Based on this program, the effect of load character, pile embedded length, and load eccentricity on the displacement development of monopile is quantified, and the applicability and reliability of the two most used models, power function model, and logarithmic function model, for the prediction of accumulated pile displacement are evaluated. Based on the numerical results, a design model which accounts for the influence of number of loading cycles, load amplitudes, and pile embedded length on the accumulated pile displacement is proposed. The proposed design model is validated against measurements from the field test on scaled monopile driven in dense sand deposit, which proves the validity of the recommended design in this paper.

Field validation of fibre Bragg grating sensors for measuring strain on driven steel piles

In recent years, fibre Bragg grating (FBG) sensors have emerged as a relatively new strain sensing technology for civil engineering applications. This paper presents a field trial to assess the feasibility of using FBG sensor arrays to measure strain in driven steel piles. Two FBG arrays were installed in grooves within the wall of an open-ended steel pile such that the finished profile was completely flush with the pile shaft. The pile was then driven into a dense sand deposit using an impact hammer to provide the required installation energy. The FBG gauges were monitored throughout driving in conjunction with accelerometers to quantify the scale of the hammer impacts. The FBG sensors were subjected to hammer blows that yielded pile accelerations between 500 g and 1400 g during installation. The fibre optic sensors were measured throughout driving, where they were observed to respond to the hammer impacts, showing a rapid increase in strain and a return to their initial values between hammer strikes. After installation, a lateral load test was performed with independent load measuring devices. Excellent agreement was observed between the measured moments and those inferred from the FBG strain output. The output of this trial demonstrates that FBG strain sensors are a viable means of measuring load transfer in foundation systems and are suitably robust to withstand high pile driving accelerations.

Liquefaction resistance of Babolsar sand reinforced with randomly distributed fibers under cyclic loading

This research explores the performance of randomly distributed fibers in enhancing the liquefaction resistance and shear modulus of loose and medium dense sand deposits. Thirty stress-controlled cyclic triaxial tests were performed on saturated samples with and without reinforcements under undrained conditions. The effect of parameters such as fiber content, fiber length, relative density and confining pressure on liquefaction behavior of unreinforced and reinforced specimens was studied. Test results indicated that the fiber inclusions significantly increased liquefaction resistance of sand specimens. Upon increasing the fiber content and fiber length, the number of loading cycles leading to liquefaction increased. The reinforcement effect in medium dense samples was found to be more significant than that of looser samples. Further, the confining pressure had a considerable effect in reducing the liquefaction susceptibility. Shear modulus of unreinforced and reinforced samples was also evaluated; the results revealed that the shear modulus increases with increasing fiber content.

Experimental validation of a numerical model for the interaction of dip-slip normal fault ruptures, sand deposits, and raft foundations

An incomplete understanding of the failure mechanisms in fault rupture propagation has led to inconsistent and insufficient regulations in building codes. In the present study, a sophisticated numerical model is calibrated and validated in order to clarify a complex problem involving the interaction of fault ruptures, medium dense Fontainebleau sand deposits, and existing structures across the fault plane. Calibration is performed using direct shear test data. Repeatable centrifuge models of dip-slip normal faults with a dip angle of 60° in the free field condition and light and heavy rigid strong raft foundations are used for the validation. The present numerical model satisfactorily simulates the centrifuge models. Rigid rafts divert the shear bands so as to bypass the rafts, rather than rip them apart. The rafts tilt on the foundation soil during fault rupture. The raft-tilting increases as the raft bearing pressure on the soil decreases.

The effect of ageing on the axial capacity of piles in sand

Open-ended steel pipe piles are used to support jacket structures in the offshore wind sector. These piles experience significant tensile axial loading in-service and the tensile shaft resistance thus governs their design. Although pile ageing (increased shaft resistance with time) has been noted by a number of workers and incorporation in design could lead to significant efficiencies, there is a dearth of high-quality field test data that measure its effects. This paper presents the results from an experimental investigation designed to examine the effect of ageing on the tension shaft resistance developed by open-ended piles in sand. As part of this investigation, four 340 mm diameter open-ended steel piles are driven 7 m into a dense sand deposit in Blessington, Ireland. Each pile is subjected to a series of static axial tension load tests at different time intervals after driving and the effects of ageing are assessed. The tension capacity of the piles is seen to increase by as much as 185% over a period of 7 months after driving.

Modelling the Cone Penetration Test in sand using Cavity Expansion and Arbitrary Lagrangian Eulerian Finite Element Methods

The paper considers two techniques to model the Cone Penetration Test (CPT) end resistance, q c in a dense sand deposit using commercial finite element programmes. In the first approach, Plaxis was used to perform spherical cavity expansion analyses at multiple depths. Two soil models, namely; the Mohr–Coulomb (MC) and Hardening Soil (HS) models were utilized. When calibrated using simple laboratory element tests, the HS model was found to provide good estimates of q c . However, at shallow depths, where the over-consolidation ratio of the sand was highest, the relatively large horizontal stresses developed prevented the full development of the failure zone resulting in under-estimation of the q c value. The second approach involved direct simulation of cone penetration using a large-strain analysis implemented in Abaqus/Explicit. The Arbitrary Lagrangian Eulerian (ALE) technique was used to prevent excessive mesh deformation. Although the Druker–Prager soil model used was not as sophisticated as the HS model, excellent agreement was achieved between the predicted and measured q c profiles.

Seismic response evaluation of an urban overpass

AbstractThe seismic response of one section of a 23 km strategic urban overpass to be built in the so‐called transition and hill zones in Mexico City is presented. The subsoil conditions at these zones typically consist on soft to stiff clay and medium to dense sand deposits, randomly interbedded by loose sand lenses, and underlain by rock formations that may outcrop in some areas. Several critical supports of this overpass are going to be instrumented with accelerometers, inclinometers and extensometers, tell tales and end pile cell pressures to assess their seismic performance during future earthquakes and to generate a database to calibrate soil–structure interaction numerical models. This paper presents the seismic performance evaluation of the critical supports located in one section of the overpass. Sets of finite elements models of the soil–foundation–structure systems were developed. Initially, the model was calibrated analyzing the seismic response that an instrumented bridge support exhibited during the June 15th, 1999 Tehuacan (Mw = 7) Earthquake. This bridge is located also within the surroundings of Mexico City, but in the lake zone, where highly compressible clays are found. The computed response was compared with the measured response in the free field, pile‐box foundation and bridge deck. Once the model prediction capabilities were established, the seismic response of the critical supports of the urban overpass was evaluated for the design earthquake in terms of transfer functions and displacement time histories. Copyright © 2010 John Wiley & Sons, Ltd.

Liquefaction resistance of sand reinforced with geosynthetics

This paper describes an investigation into the performance of geosynthetics in enhancing the resistance to liquefaction of medium dense (Dr = 50%) sand deposits. Cyclic torsional shear tests were carried out on 99 laboratory test specimens under a confining pressure of 98 kPa and a loading frequency of 0.1 Hz. A series of shear stress ratios was used to evaluate the behaviour of sand under cyclic loading conditions. Different types of geotextile (woven, nonwoven) and different arrangements of geotextiles (one, two, three and four layers) were tested. Test results revealed that the liquefaction resistance of sand deposits can be significantly increased by geosynthetic reinforcement. Nonwoven geotextiles were found to perform better than woven geotextiles as reinforcement materials. The results indicate that both the type and the arrangement of reinforcements are crucial in increasing the liquefaction resistance of medium dense sands, and there can be significant variations in the behaviour, depending on the arrangement of the reinforcement.

Investigation of liquefaction and pore water pressure development in layered sands

This paper presents the results of shaking table model tests which were carried out to investigate the pore water pressure generation and related liquefaction mechanism in layered sand deposits. The experiments were performed on uniform sand columns, silt interlayered sand columns and two layered sand columns deposited at various relative densities and subjected to different input excitations. During the experiments excess pore water pressures were measured by pore pressure transducers installed at three different depths and, surface settlements and thickness of water film developed under less permeable inclusions were measured by a digital camera. The experimental results are discussed and compared to demonstrate the effects of relative density, input acceleration and presence of a silt seam on the generation of excess pore water pressure in sand deposits subjected to dynamic loading. The results showed that the presence of a less permeable silt interlayer within the sand deposit and existence of a loose sand layer underlying dense sand deposits can have significant effect on the pore water pressure generation mechanism.

Use of State-Dependent Strength in Estimating End Bearing Capacity of Piles in Sand

The pressure and density dependence of the shear strength of sand poses a tricky problem in pile foundation design. In this study, a correlation is suggested to link the effective friction angle of sand with its initial confining pressure and relative density, and a simple approach incorporating this correlation is presented for predicting pile end bearing capacity. Assessment of the approach against pile load tests shows reasonably good agreement between predictions and measurements. It is also shown that the effect of the state-dependent strength is particularly important in cases where long piles are installed in dense sand deposits and the use of critical state friction angle will produce a conservative prediction in such cases.

Pore-water pressures in soft to firm clay during driving of piles into underlying dense sand

During driving of steel piles through soft, sensitive clay into very dense sand and gravel, pore-water pressure responses were monitored. As a result of the large length of the piles and also because of the high sensitivity of the soft clays, the piles were driven in two stages. During the initial stage of driving in the soft clay, only very small pore-water pressure increases were recorded together with very low pile driving resistances; however, during the second stage of driving, high pore-water pressure increases were observed in the clay as soon as the piles penetrated into the underlying very dense sand and gravel. It was concluded that the clay deposit was loaded from below, as the piles were driven into very dense sand. The total stress changes and the resulting pore-water pressure changes in the clay were analyzed, assuming that the pile driving load was equivalent to a flexible load acting on the surface of an elastic half-space, which represents the soft clay deposit. This interpretation of the pore-water pressure increases is important for the assessment of the bearing capacity of engineering structures affected by piles driven through soft soils into very dense deposits. The potential for high pore-water pressure increases in the clay during undrained loading as well as for volume increases in the dense sand due to pile driving can be predicted from piezocone test data. Key words: pile driving, pore-water pressure, piezocone testing, soft sensitive clays, dense sand deposits.