Dissipation Of Excess Pore Water Pressure Research Articles

Green’s function analytical solution for free strain consolidation of soft soil improved by stone columns subjected to time-dependent loading

This paper proposes an analytical solution in terms of Green’s function formulations for axisymmetric consolidation of a stone column improved soft soil deposit subjected to time-dependent loading under free strain condition. The mathematical derivations incorporate the pore water flows in radial and vertical directions in stone column and soft soil synchronously. The capabilities of the proposed analytical solution are evaluated via worked examples investigating the influences of three common time-dependent external surcharges (namely step, ramp and sinusoidal loadings) on consolidation response of the composite ground. The examples show that a faster increase of load from an initial surcharge to an expected loading might generate more significant excess pore water pressure to be dissipated during the early stages of consolidation, but the dissipation rate in soft soil would speed up significantly afterwards. The column and soil settlements along with the differential settlement between them also proceed faster corresponding to the acceleration of loading – unloading processes. Finally, the proposed analytical solution is employed to evaluate the excess pore water pressure dissipation rate at an investigation point in soft clay of a case history foundation. The calculation results exhibit a reasonable agreement with field measurement data when various constant values of stress concentration ratio are substituted into the solution to reflect the increase of stress concentration ratio with consolidation time in real practice.

Laboratory Model Test and Analysis of Excess Pore Water Pressure Dissipation of Pile Driven by Tapered - Perforated Tubular Pipe Pile

Through methods based on the indoor model test, the use of static pressure pile with three different hole arrangement and three different kinds of taper cone - bore log shape composite pipe pile bored into soil by static pressure,and after observing the excess pore water pressure caused by pile driving when testing, and analyzing the process of various pipe pile static pressure, the change rule of the excess pore water pressure varying with taper hole arrangement, size, radial distance, pile depth has been gotten. It is found that stellate shape has the best dissipation rate of excess pore water pressure under the condition of hole arrangement and taper 1/70, which is of certain guiding significance for promoting the engineering application of tapered - perforated tubular piles.

A large-strain consolidation model incorporating vacuum pressure growth propagation

A large-strain consolidation model incorporating vacuum pressure growth propagation (VPGP) under vacuum preloading is proposed. A finite-difference method was developed to predict the vacuum consolidation behaviour. The proposed model can be degenerated into the existing model when the vacuum pressure growth factor reaches 10/day. The rationality of the proposed model was verified and compared with experimental results. The effect of VPGP on the consolidation behaviour of two typical soils was then analysed. A growth in vacuum pressure has a significant effect on consolidation, especially for a growth factor in the range 0–0.5/day. The radial and vertical propagation of vacuum pressure obviously slowed down the dissipation of excess pore-water pressure (EPWP). The maximum difference reached 48% and 37.5% when the radial attenuation factor and the vertical residual factor were in the range 0–0.1/cm and 0.25–1.00, respectively. The maximum value of the EPWP difference of two typical soils reached 65 kPa.

Full scale consolidation test on ultra-soft soil improved by prefabricated vertical drains in MAE MOH mine, Thailand

The reclamation of slurry pond with ultra-soft soil deposit using the prefabricated vertical drains (PVDs) with preloading technique in the Mae Moh mine, Lampang, Thailand is a challenging work and is illustrated in this paper. Geotextile reinforcement was used to strengthen bearing capacity of the soil foundation prior to the installation of sand platform. The delay of excess pore water pressure dissipation at the early loading stage occurred despite the occurrence of large settlements was a distinct behavior of the ultra-soft soil. Within the delayed time, the calculated average degree of consolidation based on measured settlements, Us increased while the calculated average degree of consolidation based on measured excess pore water pressures, Ue and the undrained shear strength, Su remained unchanged. Beyond the delayed time, both Ue and Su increased significantly with time and when Us > 90%, the difference between Ue and Us was observed to be small. It was suggested to use Us for approximation of Su when Us > 90% based on the SHANSEP's method. The successful installation and application of PVD to improve ultra-soft soil in this research is applicable for the ground improvement of similar problematic ultra-soft soils in international land reclamation projects.

Mathematical characterization of pile–soil interface boundary for consolidation analysis of soil around permeable pipe pile

A permeable pipe pile is proposed to accelerate the dissipation of excess pore-water pressure through drainage holes around the pile circumference into the pile’s cavity. This investigation generalizes the permeable pile–soil interface as an impeded boundary, based on which a mathematical model for soil consolidation around the permeable pile is derived. Subsequently, the influence of opening pattern, drainage hole-to-pipe pile radius ratio, and opening ratio on the consolidation efficiency is numerically investigated. A total of 240 numerical cases with different drainage hole-to-pipe pile radius ratios, opening ratios, and hydraulic conductivity ratios are calculated to determine the permeable pile–soil interface parameter using back-analysis, and an approximate expression for interface parameter is obtained. Comparing against experimentally measured excess pore-water pressure dissipation, the proposed technique with impeded drainage boundary can provide good evaluations for drainage characteristics at the permeable pile–soil interface and consolidation behavior of the surrounding soil. It is found that the spatial distribution pattern of drainage holes has negligible influence on the consolidation efficiency. Increasing the opening ratio (with constant opening size) or reducing the drainage hole-to-pipe pile radius ratio (with constant opening ratio) can both accelerate the consolidation efficiency.

Numerical prediction of settlement due to the Pohang earthquake

On 15 November 2017, an earthquake with a magnitude of 5.4 caused geotechnical damage over a wide area of Pohang, South Korea. In this study, the UBCSAND constitutive effective stress model is calibrated by cyclic direct simple shear (CDSS) tests conducted on a type of sand from liquefied paddy fields at Pohang under confining stresses of 100 and 200 kPa. The calibrated UBCSAND model is then used to assess the liquefaction and calculate the ground settlement based on standard penetration test (SPT) data near the epicenter of the Pohang earthquake. The liquefaction characteristics from the UBCSAND model matched those from laboratory tests. According to the numerical results, liquefaction mostly occurred within the uppermost 10 m, and ground surface settlement from excess pore water pressure dissipation varied from 3.86 to 10.95 cm. These settlements were higher than the surface displacement estimated by satellite image analysis with interferometric synthetic aperture radar (InSAR) and lower than the ground subsidence observed solely by visiting the sites.

X-ray CT based clogging analyses of pervious concrete pile by vibrating-sinking tube method

Pervious concrete piles (PCP) have high permeability and strength. During installation by vibrating-sinking tube construction, liquified soil migrates into PCP, leading to clogging. To study PCP clogging, model tests and X-ray computed tomography analyses were carried out. The clogging depth, porosity, and excess pore water pressure dissipation were characterized. The effect of geotextile on PCP anti-clogging was evaluated. The results show an increase in target porosity and sampling depth significantly increased the PCP clogging depth. A preliminary model was proposed to evaluate the clogging of PCP. The results show that clogging can be solved by surrounding PCP with geotextile.

Delayed response of excavations in structured clays

In this paper, the behaviour of an idealized excavation carried out in a clay sensitive to destructuration is studied through a series of finite element analyses, employing an advanced bounding surface model developed for structured clays. Several cases are examined to investigate the influence of factors including the velocity of destructuration, damage to the soil–wall contact produced by the wall construction, and width-to-height ratio of the excavation. The case of a soil deposit insensitive to microstructural damage is also studied for comparison. Results of the numerical analyses show that the progressive dissipation of excess pore-water pressures generated during the excavation stage can damage the clay microstructure severely enough to trigger an instability phenomenon. If the clay structure deteriorates rapidly, the instability is concurrent with the dissipation of excess pore-water pressures. However, for a clay less sensitive to microstructural damage, the instability can occur towards the end of the consolidation process, or even be preceded by a deceptively stable time interval, during which small redistributions of pore-water pressure can trigger an important destructuration and collapse of the excavation. In a final part of the paper, results of the numerical analyses are used to provide indications about the most appropriate quantities to monitor to provide an effective early warning of the instability phenomenon.

Tsunami-induced pore water pressure response of unsaturated soil beds: Numerical formulation and experiments

Tsunamis attack coastal areas and cause significant damage to coastal buildings and infrastructure. Many coastal areas are dominated by fine, poorly-graded sands, and in many cases, the upper portion of the nearshore sand bed is unsaturated before tsunami attack. During tsunami runup, inundation of water into the unsaturated sand bed occurs, and the sand bed can eventually become pressurised under the correct conditions. During tsunami drawdown, the total stress is relieved from the sand bed surface, and excess pore water pressure dissipation process can cause sediment instability via fluidisation. The foregoing physical description is complicated by the large bed shear stress caused by tsunami loading, which can motivate significant sediment transport and scour around coastal structures. Herein, we present a coupled seepage-deformation model to estimate the temporal and spatial pore water pressure changes in sand beds during tsunami loading. The numerical formulation is described in detail, and then numerical experimentation is performed to demonstrate modelling capabilities. The results of the numerical experimentation show that tsunami drawdown can induce large pore water pressure head gradients and the speed of the infiltration front is important for examining the potential for tsunami-induced sediment instability.

Study of the 1‐D consolidation behavior of two‐layered soft soils: Parametric studies using a rheological model with viscoplastic body

SummaryThe governing equations for the coupled processes of consolidation and creep of two‐layered soft soils are established. The Nishihara rheological model is adopted to simulate the elasto‐viscoplastic characteristics of soft soils, disregarding the effects of the soil self‐gravity. A semi‐analytical theory combined with numerical and analytical methods is introduced to solve the governing equations of the one‐dimensional rheological model. The computational procedure and the approximate solutions for two‐layered soft soils subjected to surface loading are obtained for two drainage conditions. The solutions and the computational procedure are used to study the effects of the two layers and constitutive parameters on rheological consolidation behavior of soft soils. It can be concluded that two layers affect the rate of excess pore water pressure dissipation and settlement development. The parametric studies show that when the parameters of the upper layer remain constant, increases in the permeability and elastic modulus in the lower layer accelerate the dissipation of the excess pore water pressure, and meanwhile increases in the viscosity coefficient and viscoplastic limit slows down the dissipation of the excess water pressure.

Evaluation of local soil-pile friction in saturated clays under cyclic loading

This paper presents laboratory measurements of mobilized local friction along piles subjected to very large numbers of axial loading cycles. The experimental approach used here is of the physical modelling type and consists of testing an instrumented prototype pile-probe installed and loaded in specimens of saturated clay reconstituted in a calibration chamber. The procedure developed for evaluating the local friction mobilized upon monotonic loading and the further evolution of local friction during the application of displacement-controlled cycles is described. After the installation of the probe, a succession of monotonic and cyclic displacement-controlled loading phases, carried out on a reference kaolinite, is presented and analysed. During the cyclic sequence, carried out up to 105 cycles, an initial phase of friction degradation is observed, followed by a reinforcement phase, which continues until the end of the test. A coefficient of evolution is defined which allows for the quantification, during the application of the cycles, of the evolution of mobilized friction in terms of the degradation or reinforcement of friction. The evolution of the friction mobilized during the application of the cycles is interpreted in terms of the combination of excess pore water pressure generation and dissipation. A comparison is made between the maximum static shear mobilized before and after the cycles, which shows the influence of the cyclic sequence on this quantity. Elements are finally given on the repeatability of the test, showing a fairly good level of repeatability.

Model Test Analysis of Excess Pore Water Pressure during the Sinking of a Pipe Pile with a Group of Holes

To investigate the law that governs changes in excess pore water pressure during the sinking of a pipe pile with a group of holes, excess pore water pressure is monitored during the process of sinking a pile between a pipe pile without holes and three types of pipe pile with holes based on model test. The influences of the time-space dissipation of excess pore water pressure are analyzed by considering the factors of sinking pile sequence and disposal hole mode. The variation rules for excess pore water pressure dissipation with time, depth, radial distance, and sinking pile sequence are also established during the process of sinking the pile. On the basis of comparison and analysis, the pipe pile with holes, which are penetrated in a radial manner, achieves the lowest excess pore water pressure peak and the fastest dissipation rate, which are the most favorable conditions for reducing the squeezing effect of the pile, and the best pile driving effect is achieved. Moreover, the results can provide a reference for the application of pipe piles with holes in engineering practice.

Effect of Soil Layering on Shorter-Term Pile Setup

Four medium-scale experiments were conducted in a laboratory setting to better understand the effect of soil layering on the increase in pile resistance as a function of time known as pile setup. Each experiment entailed compacting a soil profile consisting of cohesionless and/or cohesive soil in a large manhole and driving a steel pile model instrumented with strain gauges and pile driving monitoring system sensors. Piezometers were placed in selected soil layers to monitor pore-water pressure. Two experiments performed were entirely cohesive and cohesionless soil, and the other two experiments were conducted with mixed soil profiles simulating single- and double-drainage conditions. Time-dependent pile resistances were estimated by pile restrikes using the monitoring system with subsequent signal matching analyses. Static load tests (SLT) were also performed at the end of the last restrike. Cone penetration tests (CPT) were performed before pile installation and after the static load test. This study showed that pile setup occurred in cohesive soil layers, followed the logarithmic time scale, and is contributed mainly from the gain in shaft resistance. No pile setup was experienced in cohesionless layers, regardless of their locations along a test pile and the soil stratigraphy. The presence of a double-drained condition in an intermediate cohesive soil facilitates the dissipation of excess pore-water pressure and pile setup. The pile setup trend follows the pattern of pore-water pressure dissipation in a cohesive soil.

Evaluation of train-induced settlement for metro tunnel in saturated clay based on an elastoplastic constitutive model

In this study, a two-dimensional (2D) soil–water coupling dynamic finite element (FE) analysis is conducted to investigate the effect of repeated train vibrations on the long-term settlement of a metro tunnel in saturated clay. Particular attention is paid to the leakage problem of the metro tunnel by assuming different permeability conditions, namely fully permeable, fully impermeable, and partially permeable, on the periphery of the tunnel for simplicity. The train vibration load is first evaluated using a rail–fastener–tunnel–subgrade model and averaged over a characteristic length for 2D numerical analysis. Cyclic Mobility model is used to simulate the mechanical behaviors of saturated soft clay in the FE analysis. Excess pore water pressure (EPWP) and associated tunnel settlement in trial operation and normal operation are calculated using the FE code DBLEAVES for different permeability conditions. It is found that a very low EPWP is generated in the trial operation, which then increases rapidly to peak values at the early days of normal operation. Afterward, the EPWP diminishes gradually as the train vibration continues. The permeability of the tunnel lining plays a significant role in the distribution of EPWP around the tunnel but produces a minor influence on the development of tunnel settlement. The train-induced tunnel settlement is mainly caused by the static settlement resulting from the EPWP dissipation during train interval, while the dynamic settlement arising from dynamic consolidation in each train vibration only accounts for a small portion. According to the 2D dynamic FE analysis, the final train-induced settlement of the metro tunnel in saturated clay is estimated to reach 160 mm while the peak EPWP value can reach 26.55 kPa. The settlement discrepancies between the numerical method and empirical method are discussed in detail.

A study into the behaviour of the formation level of an excavation under different unloading patterns in soft deposits

The construction of basements in urban areas is often associated with the possible damage to existing structures and services. The varying construction processes inevitably lead to different stress unloading patterns and therefore the dissipation of these excess pore-water pressures may lead to non-standard deformation profiles. The three main types of basement construction processes are layered excavation (LE), basin excavation (BE) and island excavation (IE). The effect of the various unloading patterns has been investigated by a three dimensional (3D) effective stress analysis method using the developed computer program 3DBCPE4.0. An excavation of length 50 m, width 50 m and depth 9 m in a certain homogenous and isotropic saturated soft soil was modelled. This included a diaphragm wall of 800-mm thickness embedded 18 m deep into the soft soil. The different excavation deformation profiles under different excavation patterns were related to the different unloading process, the exposure time of excavation face and the dissipation of negative excess pore-water pressures. The most favourable process for controlling the horizontal deformation of a retaining wall or the heave deformation of the formation level is suggested. The ground water potentials within the formation level are also presented.

Numerical modelling of pile installation and set-up effects on pile shaft capacity

Soil set-up effects significantly contribute to the increase in shaft bearing capacity of prefabricated displacement piles installed in saturated clay. This paper quantitatively distinguishes the effects of two contributing factors, (1) excess pore water pressure dissipation and (2) aging. A large-deformation coupled analysis using a finite-element scheme is used to carry out the analyses. The model only captures excess pore water pressure effect and the differences in skin friction between field measurements and numerical predictions are considered as aging. The results show that the coefficient of lateral earth pressure adjacent to the pile skin increases over time to 2.5 times of the initial coefficient (at rest). Numerical simulation of load–displacement response of static load tests at different times show that the contribution of the first factor which increases the shaft resistance is initially more significant. As time passes, the magnitude of the second factor (aging) becomes more significant..

Ageing and cyclic behaviour of axially loaded piles driven in chalk

This paper reports a programme of static and cyclic loading tests on seven open steel tubes driven in low- to medium-density chalk at a well-characterised test site, describing their response to driving, ageing in situ and loading under both static and cyclic conditions. Back analysis of dynamic monitoring identifies the distributions of notably low shaft resistances that develop during installation, showing that these depend strongly on the relative pile tip depth (h/R). The shaft capacities available to ‘virgin’ piles are shown to increase markedly after driving, following a hyperbolic trend that led to a fivefold gain after 250 days. Pre-failed piles do not follow the same trend when re-tested. Pile exhumation confirmed that driving remoulded the chalk, creating a puttified zone around the shaft. Excess pore water pressure dissipation, which is likely to have been rapid during and after driving, led to markedly lower water contents close to the shaft. Axial cyclic testing conducted around 250 days after driving led to a range of responses, from manifesting stable behaviour over 1000 cycles to failing after low numbers of cycles after developing sharp losses of static capacity. The dependence of permanent displacement on the cyclic loading parameters is explored and characterised. The experiments provide the first systematic study of which the authors are aware into the effects of undisturbed ageing and cyclic loading on previously unfailed piles driven in chalk. Potential predictive tools may now be tested against the reported field measurements.

Experimental Study to Evaluate the Effect of Consolidation Behavior on Pile Setup

Abstract This paper presents the field experimental results of static load tests (SLTs) and dynamic load tests (DLTs) that were conducted on instrumented prestressed concrete (PSC) test piles to characterize the pile setup phenomenon for individual soil layers along the piles' length. For this purpose, three PSC piles were driven at a bridge construction site in Lake Charles, LA, that were tested at different times after installation. The piles were instrumented with sister bar strain gauges to measure the load transfer along the piles' length, and pressure cells and piezometers to measure the total stress, effective stress and excess pore water pressure (PWP) with time and hence the setup for different soil layers along the piles' length. Multilevel piezometers were installed in the surrounding soil to evaluate the spatial distribution of excess PWP and to measure the influenced zone caused by pile driving. The load tests showed that the side resistance increased by 129 %, 99 %, and 91 % for the three test piles during the last load test. The case pile wave analysis program (CAPWAP) analysis of the DLTs and the load distribution plots from SLTs were used to compute the resistance of individual soil layers along the piles with time. The results showed that the soil layers with low undrained shear strength and high value of generated excess PWP exhibited higher amount of setup; while soil layers with low permeability and low coefficient of consolidation exhibited higher rate of setup compared to other soil layers. The results also showed that setup increased at a higher rate before the dissipation of excess PWP and the rate became much slower after the end of dissipation process. The dissipation of excess PWP (or consolidation data) was used to quantify the setup by its contributing factors (i.e., consolidation and aging effect). The results showed that a majority of setups was because of the consolidation effect. The results of multilevel piezometers indicated that the influenced zone because of pile driving was almost 3B (B = pile width) radial distance from pile face.

Resilient Modulus Characterization of Compacted Cohesive Subgrade Soil

Soil investigations concerning cyclic loading focus on the evaluation, in particular, of design parameters, such as elastic modulus, Poisson’s ratio, or resilient modulus. Structures subjected to repeated loading are vulnerable to high deformations, especially when subgrade soils are composed of cohesive, fully-saturated soils. Such subgrade soils in the eastern part of Europe have a glacial genesis and are a mix of sand, silt, and clay fractions. The characteristic of, e.g., Young modulus variation and resilient modulus from repeated loading tests, is presented. Based on performed resonant column and cyclic triaxial tests, an analytical model is proposed. The model takes into consideration actual values of effective stress p′, as well as loading characteristics and the position of the effective stress path. This approach results in better characterization of pavement or industrial foundation systems based on the subgrade soil in undrained conditions. The recoverable strains characterized by the resilient modulus Mr value in the first cycle of loading was between 44 MPa and 59 MPa for confining pressure σ’3 equal to 45 kPa, and between 48 MPa and 78 MPa for σ’3 equal to 90 kPa. During cyclic loading, cohesive soil, at first, degrades. When pore pressure reaches equilibrium, the resilient modulus value starts to increase. The above-described phenomena indicate that, after the plastic deformation caused by excessive load and excess pore water pressure dissipation, the soil becomes resilient.

Numerical Simulation of Soil Stress State Variations due to Mini-Pile Penetration in Clay

During installation of prefabricated piles in saturated clayey soils, excess pore water pressure (EPWP) is generated around the pile shaft and tip. The developed excess pore pressure is extremely important for evaluation of changes of stress state in clay and subsequent pile bearing capacity. Therefore, the main objective of this paper is to present a numerical finite-element model (FEM) to simulate a mini-pile installation and subsequent pore water pressure dissipation over time. The mini-pile is inserted from the ground surface into the soil to the desired depth. The numerical model is validated using the results of a physical model in which a piezocone is penetrated into a consolidation chamber containing saturated clay. The effects of Over-Consolidation Ratio (OCR), coefficient of lateral soil pressure (K 0), penetration rate (PR), and soil hydraulic conductivity (K i ) on soil stress state are investigated. The verification results have shown that the numerical model has successfully simulated the penetration of mini-pile, generation, and subsequently dissipation of the excess pore water pressure (EPWP). The parametric study revealed that increasing the OCR and K 0 has resulted in increase of the effective radial stresses on the pile skin compared to its initial value, after dissipation of EPWP. The results indicate that the variations in radial stresses at the end of dissipation are independent from the penetration rate. The dissipation of EPWP is more correlated with horizontal hydraulic conductivity of the soil, because the dissipation rate is accelerated in the radial direction.