Cyclic jacking of piles in silt and sand

Jacked piles are becoming a valuable installation method due to the low noise and vibration involved in the installation procedure. Cyclic jacking may be used in an attempt to decrease the required installation force. Small scale models of jacked piles were tested in sand and silt in a 10 m beam centrifuge. Two different piles were tested: smooth and rough. Piles were driven in two ways with monotonic and cyclically jacked installations. The cyclically jacked installation involves displacement reversal at certain depth for a fixed number of cycles. The depth of reversal and amplitude of the cycle vary for different tests. Data show that the base resistance increases during cyclic jacking due to soil compaction at the pile toe. On the other hand, shaft load decreases with the number of cycles applied due to densification of soil next to the pile shaft. Cyclic jacking may be used in unplugged tubular piles to decrease the required installation load. © 2013 Taylor & Francis Group, London.

The impact of sand slugs against beams and plates: Coupled discrete particle/finite element simulations

The impact of a slug of dry sand particles against a metallic sandwich beam or circular sandwich plate is analysed in order to aid the design of sandwich panels for shock mitigation. The sand particles interact via a combined linear-spring-and-dashpot law whereas the face sheets and compressible core of the sandwich beam and plate are treated as rate-sensitive, elastic-plastic solids. The majority of the calculations are performed in two dimensions and entail the transverse impact of end-clamped monolithic and sandwich beams, with plane strain conditions imposed. The sand slug is of rectangular shape and comprises a random loose packing of identical, circular cylindrical particles. These calculations reveal that loading due to the sand is primarily inertial in nature with negligible fluid-structure interaction: the momentum transmitted to the beam is approximately equal to that of the incoming sand slug. For a slug of given incoming momentum, the dynamic deflection of the beam increases with decreasing duration of sand-loading until the impulsive limit is attained. Sandwich beams with thick, strong cores significantly outperform monolithic beams of equal areal mass. This performance enhancement is traced to the "sandwich effect" whereby the sandwich beams have a higher bending strength than that of the monolithic beams. Three-dimensional (3D) calculations are also performed such that the sand slug has the shape of a circular cylindrical column of finite height, and contains spherical sand particles. The 3D slug impacts a circular monolithic plate or sandwich plate and we show that sandwich plates with thick strong cores again outperform monolithic plates of equal areal mass. Finally, we demonstrate that impact by sand particles is equivalent to impact by a crushable foam projectile. The calculations on the equivalent projectile are significantly less intensive computationally, yet give predictions to within 5% of the full discrete particle calculations for the monolithic and sandwich beams and plates. These foam projectile calculations suggest that metallic foam projectiles can be used to simulate the loading by sand particles within a laboratory setting. © 2013 Elsevier Ltd.

The dynamic response of edge clamped plates loaded by spherically expanding sand shells

The dynamic deformation of both edge clamped stainless steel sandwich panels with a pyramidal truss core and equal mass monolithic plates loaded by spherically expanding shells of dry and water saturated sand has been investigated, both experimentally and via a particle based simulation methodology. The spherically expanding sand shell is generated by detonating a sphere of explosive surrounded by a shell of either dry or water saturated synthetic sand. The measurements show that the sandwich panel and plate deflections decrease with increasing stand-off between the center of the charge and the front of the test structures. Moreover, for the same charge and sand mass, the deflections of the plates are significantly higher in the water saturated sand case compared to that of dry sand. For a given stand-off, the mid-span deflection of the sandwich panel rear faces was substantially less than that of the corresponding monolithic plate for both the dry and water saturated sand cases. The experiments were simulated via a coupled discrete-particle/ finite element scheme wherein the high velocity impacting sand is modeled by interacting particles while the plate is modeled within a Lagrangian finite element setting. The simulations are in good agreement with the measurements for the dry sand impact of both the monolithic and sandwich structures. However, the simulations underestimate the effect of stand-off in the case of the water saturated sand explosion, i.e. the deflections decrease more sharply with increasing stand-off in the experiments compared to the simulations. The simulations reveal that the momentum transmitted into the sandwich and monolithic plate structures by the sand shell is approximately the same, consistent with a small fluid-structure interaction effect. The smaller deflection of the sandwich panels is therefore primarily due to the higher bending strength of sandwich structures. © 2013 The Authors. Published by Elsevier Ltd. All rights reserved.

Sand column impact onto a Kolsky pressure bar

A laboratory-based methodology to launch cylindrical sand slugs at high velocities is developed. The methodology generates well-characterised soil ejecta without the need for detonation of an explosive; this laboratory-based tool thereby allows for the experimental investigation of the soil-structure events. The experimental set-up comprises a launcher with a cylindrical cavity and a piston to push out the sand slug. The apparatus is used to launch both dry and water-saturated sand slugs. High speed photography is used to characterise the evolution of the sand slugs after launch. We find that the diameter of the slugs remains unchanged, and the sand particles possess only an axial component of velocity. However, the sand particles have a uniform spatial gradient of axial velocity and this results in lengthening of the slugs as they travel towards their target. Thus, the density of the sand slugs remains spatially homogenous but decreases with increasing time. The velocity gradient is typically higher in the dry sand slugs than that of the water-saturated slugs. The pressure exerted by the slugs on a rigid-stationary target is measured by impacting the slugs against a direct impact Kolsky bar. After an initial high transient pressure, the pressure reduces to a value of approximately ρv 2 where ρ is the density of the impacting sand slug and v is the particle velocity. This indicates that loading due to the sand is primarily inertial in nature. The momentum transmitted to the Kolsky bar was approximately equal to the incident momentum of the sand slugs, regardless of whether they are dry or water-saturated. © 2013 Elsevier Ltd. All rights reserved.

Water supply to a geotechnical centrifuge for pile jetting in sand

The supply of water is often required during a centrifuge experiment. For the case of pile jetting, significant flow volumes and pressures are required from the water supply. This paper aims to detail the successful provision of water at high pressures and large flow rates to a centrifuge, using a novel water supply system. An impeller pump was used to pressurise the water in advance of the slip rings, with further pressure provided by the fluid accelerating along the centrifuge beam arm. A maximum pressure of 2 MPa and continuous flow rate of 6 litres per minute were achieved. The calculation of water pressure away from the measurement location is presented, offering a repeatable solution for the pressure at any point in the pipe work. © 2010 Taylor & Francis Group, London.

Response of clamped sandwich beams subjected to high-velocity impact by sand slugs

The dynamic response of end-clamped sandwich and monolithic beams of equal areal mass subjected to loading via high-velocity slugs of dry and water-saturated sand is measured using a novel laboratory-based method. The sandwich beams comprise aluminium face sheets and an aluminium honeycomb core: the effect of sandwich core strength and beam thickness on the dynamic beam deflection is investigated by varying the orientation and height of the anisotropic aluminium honeycomb core material. High-speed imaging is used to measure the transient transverse deflection of the beams and to record the dynamic modes of deformation. The measurements show that sandwich beams with thick, strong cores are optimal and that these beams significantly outperform monolithic beams of equal mass. The water-saturated sand slugs cause significantly higher deflections compared to the dry sand slugs having the same mean slug velocity and we demonstrate that this enhanced deflection is due to the larger mass of the water-saturated slugs. Finally, we show that the impact of sand slugs is equivalent to the impact of a crushable foam projectile. The experiments using foam projectiles are significantly simpler to perform and thus represent a more convenient laboratory technique. © 2014 Elsevier Ltd. All rights reserved.

Important aspects when modelling the interaction between surface structures and tunnelling in sand

Tunnelling in urban areas continues to increase and has highlighted the need for a better understanding of the impact of tunnel excavations on existing buildings. This paper considers the influence of surface structures on ground displacements caused by tunnelling in sand through finite element modelling and centrifuge testing. First, the importance of modelling assumptions is evaluated by comparing centrifuge modelling results to finite element modelling results for various soil constitutive models: both a Young's modulus that linearly increases with depth and a power law relation between the soil stiffness and stresses are considered. Second, the most effective soil constitutive model was used to perform a sensitivity study on the effect of different factors governing the structural response. In particular, the effect of the building stiffness and weight on the modification of soil displacements is investigated by introducing a simple surface structure. The use of a no-tension interface between the building and the soil was found to be essential to investigate the effect of weight on gap formation between the soil and the structure, as observed during the experimental tests. Results show the importance of considering the relation between the building weight and the relative stiffness between the building and the soil when assessing the structural response. © 2014 Korean Geotechnical Society.

Resistance of cast polyurethane elastomers to solid particle erosion

The rates of erosive wear have been measured for a series of eight polyester-based one-component castable polyurethane elastomers, with widely varying mechanical properties. Erosion tests were conducted with airborne silica sand, 120μm in particle size, at an impact velocity of 50 ms-1 and impact angles of 30° and 90°. For these materials, which all showed similar values of rebound resilience, the erosion rate increased with increasing hardness, tensile modulus and tensile strength. These findings are at variance with those expected for wear by abrasion, perhaps because of differences in the strain rate or strain levels imposed on the elastomer during erosion and abrasion.

Comparative study of the slurry erosion and free-fall particle erosion of aluminum

A study has been performed of the erosion of aluminium by silica sand particles at a velocity of 4.5 m s-1, both air-borne and in the form of a water-borne slurry. Measurements made under similar experimental conditions show that slurry erosion proceeds at a rate several times that of air-borne erosion, the ratio of the two rates depending strongly on the angle of impact. Sand particles become embedded into the metal surface during air-borne particle erosion, forming a composite layer of metal and silica, and provide the major cause of the difference in wear rate. The embedded particles giving rise to surface hardening and a significant reduction in the erosion rate. Embedment of erodent particles was not observed during slurry erosion. Lubrication of the impacting interfaces by water appears to have minimal effect on the wear of aluminium by slurry erosion.

Uplift resistance of buried pipelines at low cover-diameter ratios

Reliable estimates for the maximum available uplift resistance from the backfill soil are essential to prevent upheaval buckling of buried pipelines. The current design code DNV RP F110 does not offer guidance on how to predict the uplift resistance when the cover:pipe diameter (H/D) ratio is less than 2. Hence the current industry practice is to discount the shear contribution from uplift resitance for design scenarios with H/D ratios less than 1. The necessity of this extra conservatism is assessed through a series of full-scale and centrifuge tests, 21 in total, at the Schofield Centre, University of Cambridge. Backfill types include saturated loose sand, saturated dense sand and dry gravel. Data revealed that the Vertical Slip Surface Model remains applicable for design scenarios in loose sand, dense sand and gravel with H/D ratios less than 1, and that there is no evidence that the contribution from shear should be ignored at these low H/D ratios. For uplift events in gravel, the shear component seems reliable if the cover is more than 1-2 times the average particle size (D50), and more research effort is currenty being carried out to verify this conclusion. Strain analysis from the Particle Image Velocimetry (PIV) technique proves that the Vertical Slip Surface Model is a good representation of the true uplift deformation mechanism in loose sand at H/D ratios between 0.5 and 3.5. At very low H/D ratios (H/D < 0.5), the deformation mechanism is more wedge-like, but the increased contribution from soil weight is likely to be compensated by the reduced shear contributions. Hence the design equation based on the Vertical Slip Surface Model still produces good estimates for the maximum available uplift resistance. The evolution of shear strain field from PIV analysis provides useful insight into how uplift resistance is mobilized as the uplift event progresses. Copyright 2010, Offshore Technology Conference.

On the use of densification as a liquefaction resistance measure

The use of densification to improve the performance of shallow foundations during the centrifuge modeling of earthquake-induced liquefaction on level sand deposits is discussed. The densification of liquefiable ground provided protection against or significantly reduces liquefaction-related damage. Propagation of accelerations in the deposit exhibited considerable distinct features according to the relative density of the sand in the model. It was found that during the first couple of cycles, the dense soil amplifies the fundamental frequency component of the earthquake and preserves the higher frequency components.