Pressure Dissipation Research Articles

Evaluation of Negative Skin Friction on Bridge Abutment Piles

Piles are used to transfer loads of structures to deeper and stronger soil layers through skin friction and/or end bearing. Surcharge loads, site grading, or dewatering may induce downward movement of soil adjacent to piles installed in a compressible medium. This movement creates negative skin friction stresses acting downward at the pile-soil interface, which applies additional loads “drag forces” to the pile causing a maximum axial load in the pile shaft at the “neutral plane”. To evaluate the development of drag forces, a comprehensive field monitoring program was conducted over four years for three instrumented abutment H-piles as part of a three-span bridge project. The soil settlement and changes in pore water pressure in the soil adjacent to the piles due to the construction of an approach embankment were monitored using multiple-point extensometers and vibrating wire piezometers. The piles’ elastic settlement and strains were measured using single-point extensometers and vibrating wire strain gauges. The field measurements are presented and discussed in terms of responses time histories and load distribution along one pile shaft. In addition, the calculated forces from vibrating wire strain gauges are compared with the unified design method prediction considering the total stress method (α-method) for cohesive soils. The results show that the maximum drag force was developed after the complete dissipation of excess pore water pressure and that the location of neutral plane varied during the embankment construction stages. Employing the total stress method in the unified design method provided a reasonable prediction of the drag force and the neutral plane’s location.

An experimental study of the dynamic compaction method based on relay drainage in foundation treatment of the coastal backfill area

The studies of foundation treatment methods are carried out for the special and complex foundation in the fill, saturated silty fine sand and silt in the coastal backfill area of the coastal zone of the Shandong Peninsula. Based on the economical and efficient dynamic compaction method, the concept of integrated combined drainage is put forward. The relay drainage system of the shallow and deep vertical drainage channels and their relay horizontal drainage channels are designed, and field tests are undertaken. The monitoring data show that under the dynamic compaction load, the relay drainage system can coordinate drainage as a whole, which can quickly discharge the groundwater to dissipate the excess pore water pressure. The rise of groundwater level and dissipation of pore water pressure caused by dynamic compaction can be basically eliminated in about 7 hours. With continuous precipitation, the surface settlement is 0.7%−2.0% of the thickness of the upper soil mass. Under the dynamic load of dynamic compaction, the surface soil is compressed to 8.7%-10.9% of the thickness of the upper soil. The soil settlement is about 5 ‰ and 3 ‰ of the soil thickness at the buried depth of 3−7 m, and 2 ‰ of the soil thickness is at the buried depth of 7−10 m. The test data show that in the effective depth of dynamic compaction, the effect of foundation treatment is obvious, the engineering properties and the degree of consolidation of foundation are improved. The surface bearing capacity and deformation modulus meet the design requirements, the average bearing capacity of silt below 4 m is slightly lower than the design requirements, and the average degree of consolidation of mucky soil is 77%. The monitoring data of one month after tamping shows that the surface subsidence is within 25 mm, which tends to be stable, the layered settlement and pore water pressure are stable and slightly decreased.

Prediction on Flow and Thermal Characteristics of Ultrathin Lubricant Film of Hydrodynamic Journal Bearing.

This paper focuses on the flow and thermal characteristics of the lubricant film in the micro clearance of a hydrodynamic journal bearing (HJB) at high rotating speed. A thermohydrodynamic (THD) method consists of the Reynolds equation coupled with energy and viscosity-temperature equation with considering the cavitation is put forward. The 3D surface diagrams of the lubricant film thickness, pressure, temperature, liquid mass fraction, flow rate and heat dissipation distributions under different geometric, operating, slip and no-slip boundary conditions are systemically exhibited and analyzed. The results show that with the rise of eccentricity or length diameter ratio, the maximum peaks of pressure, temperature and heat dissipation are rapidly increased, the cavitation is aggravated, and the flow rate is accelerated in different extent. As the bearing speed accelerating, the maximum peak of temperature is strongly increased, whereas, the distinction between peaks of flow rate and heat dissipation is magnified and reduced, respectively. It provides a fruitful inside view of the inner flow and thermal characterizations of HJB for further understanding its flow-thermal interaction mechanisms and offers theoretical support for improving its working performance.

A novel implicit coupled hydro-mechanical SPFEM approach for modelling of delayed failure of cut slope in soft sensitive clay

Modelling of delayed failure of cut slopes in soft sensitive clays is a significant challenge because of three major features taking place simultaneously: large deformation of the slope up to failure, pore water pressure generation and dissipation in the slope, and complicated time-dependent deformation of sensitive clay. This paper proposes a novel fully hydro-mechanical coupled edge-based strain smoothed particle finite element (ES-PFEM) modelling approach for delayed failure of cut slope with three outstanding features: (1) the particle finite element method with low-order edge-based strain-smoothed element for large deformation analysis, (2) an efficient stabilization method for pore water pressure to circumvent the Babuška-Brezzi stability restriction and (3) the elasto-viscoplastic model considering time-dependency, anisotropy and destructuration of soft sensitive clays. The accuracy and efficiency of the proposed approach are first examined by simulating consolidation test and strip footing. Furthermore, an adaptive substepping algorithm is proposed for the elasto-viscoplastic model. Finally, a typical case of delayed failures of cut slopes with different angles in soft sensitive clays is simulated and analysed, demonstrating the predictive ability of the proposed approach. The relationship between slope angle, sensitivity of clay and time of failure occurring is obtained for predicting the slope failure time in practice.

Liquefaction and post-liquefaction resistance of sand reinforced with recycled geofibre

The present study provides an insight into the effect of recycled carpet fibre on the mechanical response of clean sand as backfill material subjected to monotonic loading and cyclic loading as well as post-liquefaction resistance of both unreinforced and carpet fibre reinforced soils. To achieve these goals, a series of multi-stage soil element tests under cyclic loading event resulting in liquefaction followed by undrained monotonic shearing without excess pore water pressure dissipation as well as a series of monotonic undrained shear test is conducted. All the specimens are isotropically consolidated under a constant effective confining stress of 100 kPa by considering the effect of cyclic stress ratio and carpet fibre content ranging from 0.25% to 0.75%. The obtained results revealed the efficiency of carpet fibre inclusion in increasing the secant shear modulus and ductility of clean sand under monotonic shearing without previous loading history. The impact of carpet fibre inclusion on the trend of cyclic excess pore water pressure generation and cyclic stiffness degradation was minimal. However, adding carpet fibre significantly improved both liquefaction and post-liquefaction resistances of clean sand. The liquefaction resistance of clean sand, at a constant 15 loading cycles, improved by 26.3% when the soil was reinforced with 0.75% recycled carpet fibre. In addition, the initial shear modulus of the liquefied specimen significantly increased by adding recycled carpet fibre.

Field monitoring of bearing capacity efficiency of permeable pipe pile in clayey soil: A comparative study

For clayey soil, an effective solution to accelerate pore water pressure dissipation after pile installation is using permeable pipe piles. But the differences in bearing capacity efficiency between permeable and normal piles are still unclear. In this paper, a series of load tests were conducted to compare their performance. The bare fiber Bragg grating (FBG) sensors were used to monitor and explore the pile-soil interaction process, together with pore water pressure and earth pressure cells. The FBG-based performance evaluation system provides an effective tool for the analysis of piles. The results show that the time required for the permeable pile to reach the ultimate bearing capacity was shorter than that of the normal one. The drainage holes did not lead to quality problems of the pile. The positive effect of drainage holes on the end-bearing resistance was gradually evident with time. This study provides valuable insight into the working mechanism of permeable pipe piles.

Theoretical analysis of long-term setup of driven piles considering soil anisotropy, destructuration and creep effects

The soil adjacent to the pile suffers a severe destructuration during pile installation and then regains its strength during the setup stage. This paper presents a theoretical approach to evaluate the long-term setup of driven piles in soft sensitive clays. An advanced elastic-viscoplastic model that accounts for fabric anisotropy, particle bonding and viscosity, is used in both stages of pile installation and setup. A semi-analytical solution for the undrained cylindrical and spherical cavity expansion in soft sensitive clays is developed to capture the pile installation effects. The changes in the stress state and fabric anisotropy of the soil adjacent to the pile caused by pore pressure dissipation and sustained service loads are analyzed. The evolution of soil mechanical behavior is incorporated into the load-transfer models of shaft friction and base resistance. Comparisons between measured and predicted results of field tests at three different sites demonstrate the applicability of the proposed approach. The calculated results illustrate that the ultimate bearing capacity increases steadily with time during the long-term setup stage. An extensive study is carried out to evaluate the effects of soil's initial anisotropy and bonding on the load-settlement behavior of driven piles.

Seismic analysis of a model tension leg supported wind turbine under seabed liquefaction

The paper presents numerical analyses of a pile supported Tension Leg Platform (TLP) wind turbine, during seismic loading and seabed liquefaction, taking consistently into account the pile-tendon-platform interaction. The emphasis is on the system response when subsoil liquefaction is extensive, leading to degradation of the pseudo-static safety factor against pile pullout well below unity. It is shown that the pile resistance to pullout failure decreases drastically during shaking, but fully recovers during the following dissipation of earthquake-induced excess pore pressures and even exceeds the initial (pre-shaking) resistance. Pile head displacements develop steadily both during shaking and dissipation, but only while the static pullout safety factor remains less than unity. Due to the high tensional stiffness of the tendons relative to the buoyancy stiffness of the platform considered in this study, the pile head pullout is mostly transmitted to the platform with a relatively small part corresponding to reduction of tendon elongation. The resulting loss of buoy stability and tendon pretension may prove detrimental for the short-term operation of the platform. However, except possibly from the rare scenario of concurrent extreme environmental loads, both effects are recoverable, as they are unlikely to cause irreparable damage to the foundation and the superstructure.

Analytical solutions for excess pore water pressures generated by TBM tunnelling in a semi-confined aquifer

Tunnel-boring machine (TBM) tunnelling through an aquifer will generate excess pore water pressures in the soil around the tunnel face. Accurately predicting the magnitude of the excess pore water pressures is significant because it directly determines the effective face support pressure. In this study, an analytical solution for transient cylindrical flow caused by TBM tunnelling considering the elastic storage of the aquifer is developed. Furthermore, a quasi-static solution to couple the effect of slurry infiltration with the elastic storage is presented. It is shown that the pore water pressures derived from the analytical solution match quite well with the measurements obtained in proximity to as well as far away from the TBM. The coupled quasi-static solution also agrees with the measurements for the places close to the tunnel face, but underestimates the observations far away from the tunnel face. The results also show that a larger infiltration distance leads to a slower dissipation of excess pore water pressures, which does not make a difference during excavation (when excavation velocity is faster than the pore fluid velocity).

Gas Production from Fractured Hydrate Reservoirs: Numerical Modeling and Evaluation

Fractured natural gas hydrate (NGH) reservoirs are widely tested all over the world. Existence of fractures will aggravate the basin‐scale heterogeneity in both NGH distribution and seepage properties and, therefore, inevitably challenge some of our former understanding on gas production. However, gas productivities of the fractured NGH reservoirs have not been evaluated so far due to the rare accessible logging data for detailed characterization of fractures. Gas production from the fractured NGH reservoirs is modeled and the differences made by position, width, and length of the single conducive fracture and the single hydrate‐filled fracture (HFF), respectively, are systematically analyzed. Sequentially, randomly distributed fracture networks are also considered in the gas productivity estimation. Results show that the conducive fractures in the overburden influence water production dramatically. When hydrate‐bearing layers (HBLs) are characterized with HFFs, there will be obvious secondary formation near the HFFs, which together with its initial high NGH saturation hinders the pressure dissipation and the gas migration, generally shortening the production duration and reducing the accumulated gas. When the overburden and the HBLs are characterized with a randomly distributed fracture network, input fluid from upward could enhance gas production to some extent and lead to a fluctuated gas productive rate.

An approach for modelling spatial variability in permeability of cement-admixed soil

This paper presents a framework for modelling the random variation in permeability in cement-admixed soil based on the binder content variation and thereby relating the coefficient of permeability to the unconfined compressive strength of a cement-admixed clay. The strength–permeability relationship was subsequently implemented in random finite element method (RFEM). The effects of spatial variation in both strength and permeability of cement-admixed clays in RFEM is illustrated using two examples concerning one-dimensional consolidation. Parametric studies considering different coefficient of variation and scale of fluctuation configurations were performed. Results show that spatial variability of the cement-admixed clay considering variable permeability can significantly influence the overall consolidation rate, especially when the soil strength variability is high. However, the overall consolidation rates also depend largely on the prescribed scales of fluctuation; in cases where the variation is horizontally layered, stagnation in pore pressure dissipation may occur due to soft parts yielding.

Experimental Study on Silt Liquefaction by Shaking Table Test

Based on the shaking table test, the liquefaction of silt under earthquake load and the earthquake damage after liquefaction are studied. The test results show that the lower silt is relatively dense, the pore pressure increases slowly, and the pore pressure ratio can not reach the initial liquefaction state. However, under the stronger earthquake action, the dense silt may also produce liquefaction, which indicates that the increase of pore water pressure has a great relationship with the compactness and earthquake intensity under the earthquake load. Because the permeability coefficient of silt is relatively low, the increase and dissipation of pore pressure are relatively slow. After the earthquake, he pore pressure of silt continues to rise, and the liquefaction will be further aggravated.

Changes in sand mesostructure under repeated seismic liquefaction events during centrifuge tests

Previous studies have revealed that past shaking can substantially affect sand reliquefaction resistance. Researchers have attributed these effects to the formation of stable or vulnerable sand mesostructures during previous shaking events. However, no experiment has been performed to directly visualize how sand particles evolve into stable or vulnerable mesostructures. In this study, a sand deposit was shaken by four repeated seismic motions in centrifuge model tests, and simultaneously, mesoscopic images of sand particles were recorded by a newly developed microimage acquisition system. The mesoscopic images showed that pore fluid seepage caused particles previously in contact to be separated and form large voids during sand reconsolidation stages. The sand with large voids quickly contracted during the subsequent shaking event, exhibiting a decreasing sand reliquefaction resistance. The sand reliquefaction resistance gradually increased as the seepage damage effects attenuated, and the number of sand particle contacts increased with sand densification under multiple shaking events. The testing results suggested that influences of the excess pore pressure dissipation on the sand mesostructure should be considered in assessing sand reliquefaction resistance.

Sleeping at the switch.

Sleep slow waves are studied for their role in brain plasticity, homeostatic regulation, and their changes during aging. Here, we address the possibility that two types of slow waves co-exist in humans. Thirty young and 29 older adults underwent a night of polysomnographic recordings. Using the transition frequency, slow waves with a slow transition (slow switchers) and those with a fast transition (fast switchers) were discovered. Slow switchers had a high electroencephalography (EEG) connectivity along their depolarization transition while fast switchers had a lower connectivity dynamics and dissipated faster during the night. Aging was associated with lower temporal dissipation of sleep pressure in slow and fast switchers and lower EEG connectivity at the microscale of the oscillations, suggesting a decreased flexibility in the connectivity network of older individuals. Our findings show that two different types of slow waves with possible distinct underlying functions coexist in the slow wave spectrum.

Strip load on transversely isotropic elastic double porosity media with strong permeability contrast

In double porosity media, fluid could flow through the distributed micro-fractures at a larger scale as well as through the nanopores of the organic matrix at a smaller scale, while fluid mass could be transferred between them. Besides, the solid deformation acts as an important coupling term and leads to the two-way coupled process. However, these processes are not well presented in previous studies. In this work, a new coupled modeling approach was proposed for saturated double porosity media with strong permeability con-trast. Specifically, the transversely isotropic equivalent fracture permeability is considered for the preferentially oriented micro-fractures at the micro-scale or meso-scale; while at the nano-scale, the non-Darcy flow and mass transfer equations are employed to consider the interaction between fluid particles and the solid pore wall. Finally, the stabilized finite element method is developed to investigate the settlement and pressure dissipation behaviors of the strip load problem. Cited as: Zhang, Q. Strip load on transversely isotropic elastic double porosity media with strong permeability contrast. Advances in Geo-Energy Research, 2021, 5(4): 353-364, doi: 10.46690/ager.2021.04.02

Semi-analytical solutions to the consolidation of unsaturated composite ground enforced by stone columns under equal strain condition considering smear effect

Composite ground is widely employed in geotechnical engineering as a ground reinforcement method, which can be applied to either saturated or unsaturated natural ground. There is limited existing research on consolidation behaviour of the unsaturated composite ground with stone columns, particularly when smear effect is considered. The smear zone is inevitable due to the disturbance to the ground when constructing stone columns, and the flow rate of either air or water through the smear zone significantly reduces, leading to a slow dissipation of the excess pore pressures. This paper proposes semi-analytical solutions in the equal strain conditions for consolidation problem of unsaturated composite ground with stone columns considering the smear effect. Based on the flow rule of air and water phases in the axisymmetric case and the basic relationship of unsaturated composite ground, we first derived the governing equations of excess pore-air and pore-water pressures. Then, the semi-analytical solutions are obtained with the joint use of expansion of Fourier series, Laplace transform and inverse Laplace transform. For a degenerated case, we compare the proposed solutions with the existing ones in literature for the verification purpose. Furthermore, we perform a series of parametric studies by using the proposed solutions and find that the consolidation rate is underestimated when ignoring the smear effect of the unsaturated composite ground.

Characteristics of small-scale motions in a dual-plane jet flow

In this study, direct numerical simulations are performed to investigate the spatial evolution of a dual-plane jet flow with Ld/L0 = 6, where Ld and L0 are the separation distance between the two jets and the jet width, respectively. The formation mechanism of the turbulent kinetic energy along the centerline can be quite different. In the reversal flow region, the transport equation of turbulent kinetic energy can be seen as the combination of the transport, convection, production, pressure, and viscous dissipation terms, whereas in the downstream combined region, it can be further simplified as the combination of convection and viscous dissipation terms. The gain of the centerline TKE mainly occurs in the vertical direction through the turbulence transport effects. Concerning the strain product process, in all three regions the strain self-amplification term is the dominant source term, whereas the enstrophy production and viscous dissipation terms are main sink terms. Two important aspects of small-scale motions are considered, one being the correlation between the vorticity vector and the corresponding eigenvectors of strain rate tensor and the other being the characteristics of invariant of the velocity gradient. The joint PDFs of the strain self-amplification and enstrophy production are highly squeezed in the vertical direction in the converging region. In contrast, in the reversal flow region, where the flow is still highly intermittent, the contour lines already take similar albeit slightly squeezed shapes as those in HIT. The joint PDFs of the second and third invariants in the reversal and highly intermittent flow region and the subsequent fully turbulent regions acquire the well-known ‘teardrop’ shape. It is demonstrated that even in the highly intermittent region, the characteristics of small-scale motions are already close to the fully developed flow. These above findings contribute to a better understanding of the dual-plane jet flows.

Performance based design of Tension Leg Platforms under seismic loading and seabed liquefaction: A feasibility study

The response of pile supported Tension Leg Platforms during seismic loading and seabed liquefaction is analyzed numerically, taking consistently into account the pile-tendon-platform interaction. The emphasis is on the system response when liquefaction in the subsoil is extensive, leading to degradation of the pseudo-static factors of safety against pullout failure of the pile well below unity. It is shown that the pile resistance to pullout failure decreases drastically during shaking due to subsoil liquefaction, but fully recovers during the following excess pore pressure dissipation phase and may even exceed the initial (pre-shaking) resistance value. Pile head displacements develop steadily during shaking and the following dissipation phase, but only during the finite time period when the static pullout factor of safety of the pile remains less than unity. Thus, final pile head displacements may increase considerably when this time period is elongated. Due to the generally high tensional stiffness of the tendons, relative to the buoyancy stiffness of the platform, the pile head pullout is mostly transmitted to the platform, with only a small percentage corresponding to reduction of tendon elongation. For the cases examined herein, the potential loss of platform buoyancy and tendon pretension may prove detrimental but is unlikely to threaten the safety and functionality of the platform. On the other hand, initially minor tilting of the platform (e.g. due to long period lateral loading and differential tendon pretension) may be magnified during seismic loading above allowable limits.

Determination of the ultimate consolidation settlement of jack-up spudcan footings embedded in clays

When jack-up platforms are deployed for long-term services such as those functioning as production units, the consolidation settlement of spudcan footings is an issue of particular concern. Unpredicted and thus unprepared settlement may do harm to the serviceability and safety of jack-up rigs. Through centrifuge experiment and large deformation finite element calculation, this paper first illustrates the inherent correlation between the excess pore pressure dissipation inside soil and spudcan settlement development. With the measured and calculated settlement time histories, subsequent efforts are directed towards exploring how to predict the ultimate consolidation settlement with accuracy. Particular attentions are placed on investigating the applicability and advantages of two observational methods: Asaoka's method and Hyperbolic approach. Their prediction performances for spudcan with various soil, loading, embedment and geometrical conditions are symmetrically evaluated through numerical parametric investigation. The upper and lower limits of their prediction errors are established. Explicit recommendations are given on the selection of appropriate prediction approaches for different consolidation conditions. The practical significance of these research outcomes lies in 1) providing guidance on the selection of settlement prediction approaches; 2) establishing the limits of prediction errors involved. These are expected to be particularly useful for the field monitoring of spudcan settlement.

Analysis on flow structure and improvement of heat transfer in 3D circular tube with varying axial groove turbulator configurations

AbstractUsing passive devices are an efficient method to enhance streamline behavior when liquid flows through the circular pipe. The interrupted structure groove is usually used to change the flow patterns. In this analysis, a heat performance numerical technique is applied to study the characteristics of fluid flow and heat transfer of the circular pipe using different axial groove geometrical configurations with different axial groove numbers, including 2, 3, and 4, under different conditions. The number of annular grooves and circumferential positions are the important parameters to analyse with varying operating conditions, with the Reynolds number (Re) range from 1500 to 23,000. A three‐dimensional coordinate pipe system is applied using tetrahedron grids. The discretization equations are obtained by deriving algebraic approximations to integral conservation equations. Results observed that using this type of passive method has a low effect on pressure dope compared to the normal one (smooth pipe). The flow change occurs near and closed to the axial groove parameters. Moreover, the Nusselt number (Nu) value for the groove turbulators was higher than the normal one, about 14.5%–21%. The friction factor (f) value for the groove turbulators was higher than the normal one, were about 7.5%–24%. Most friction losses are caused by dynamical pressure dissipation owing to more viscous losses closed to the wall surfaces. The improvement of heat performance using this type of passing method was more than 1.2%.