Residual Excess Pore Water Pressure Research Articles

Experimental study of liquefaction strength affected by residual excess pore water pressure induced by earthquake

Experimental study of liquefaction strength affected by residual excess pore water pressure induced by earthquake

Three-Dimensional Numerical Investigation on Seismic Response of Subway Station in Liquefied Soil by the Loosely Coupled Effective Stress Model

ABSTRACT Three-dimensional numerical simulation for clarifying the seismic response characteristics of an underground structure considering the influences of dynamic interaction with the surrounding liquefied soil is still a huge challenge. Using the proposed generic loosely coupled nonlinear effective stress method for site response of liquefiable soil, the authors developed an effective stress seismic response analysis of the underground subway station structure. Based on the effective stress method, the responses of the model underground subway station with three floors and three spans in model liquefiable foundation in the shaking table test are simulated. The comparison between the measured and simulated results proves that the developed method can effectively capture the response characteristics of the liquefiable foundation，by which the generation of residual excess pore water pressure and the effect of cyclic stiffness softening and degradation can be taken into good consideration, and the measured responses of model underground structure can be well reproduced. The results also demonstrate the availability of the shaking table test simulating the dynamic interaction between the underground structure and the surrounding liquefied soil.

Seabed Liquefaction around Pipeline with Backfilling Trench Subjected to Strong Earthquake Motions

As an indispensable part of the lifeline for the offshore gas and oil industry, submarine pipelines under long-term marine environmental loadings have historically been susceptible to earthquakes. This study investigates the impact of trench backfilling on the residual liquefaction around a pipeline and the induced uplift of a pipeline under the combined action of an earthquake, ocean wave and current loading. A fully coupled nonlinear effective stress analysis method, which can consider the nonlinear hysteresis and the large deformation after liquefaction of the seabed soil, is adopted to describe the interaction between the seabed soil and the submarine pipeline. Taking a typical borehole in the Bohai strait as the site condition, the nonlinear seismic response analysis of the submarine pipeline under the combined action of seismic loading and ocean wave and current is carried out. The numerical results show that trench backfilling has a significant impact on the seismic response of the pipeline. The existence of trench backfilling reduces the accumulation of the residual excess pore water pressure, so that the seabed liquefaction around the pipeline is mitigated and the uplift of the pipeline is also decreased.

The influence of geotextile on the characteristics of railway subgrade mud pumping under cyclic loading

The mud pumping beneath railways is both widespread and harmful and seriously affects the stability of railroad tracks and their operational safety. This paper presents a series of mud pumping tests, and analyses the influence of geotextile on the characteristics of saturated subgrade mud pumping. Our results show that the addition of geotextile between subgrade and gravel layers can significantly reduce accumulated axial strain by 40% to 76%, reduce residual excess pore water pressure by 33% to 92%, and reduce soil moisture content at the interface between the subgrade and gravel layers. However, we find that geotextile has no significant effect on soil dynamic stress. This suggests that the geotextile can act as a separator between the subgrade and gravel layers thus significantly reducing the thickness of an interlayer. Additionally, geotextile can also act as a filter and reduce the displacement caused by the migration of fine particles. In general, placing geotextile between the subgrade and gravel layers can mitigate railway subgrade mud pumping under cyclic loading, but with an increase in cyclic loading amplitude, the mitigating effectiveness of geotextile on mud pumping gradually weakens.

Poro-elastic and poro-elasto-plastic modeling of sandy seabed under wave action

The seabed could be liquefied due to the wave-induced accumulation of residual excess pore water pressure (EPWP), resulting in the instability of offshore and marine structures. Thus, it is of importance to consider both the buildup of EPWP and the associated deformation behavior of the seabed. In this study, a dynamic poro-elasto-plastic seabed model, integrated with a state-dependent plasticity model, is developed in COMSOL Multiphysics. The numerical model of seabed is validated against the analytical solutions and model tests. The elliptical trajectories of soil particle at different depths of the poro-elastic seabed under the progressive wave are illustrated. The ratio of the amplitude of vertical displacement to that of horizontal displacement generally decreases as the depth increases for the poro-elastic seabed. In the poro-elasto-plastic seabed, however, the soil particles tend to move following the direction of the wave propagation. Moreover, the soil particles at the shallow part of the seabed go downwards before the initial liquefaction, while the soil particles at the lower position initially go upwards and then turn to go downwards when the initial liquefaction occurs. Particularly, the surface seabed with lower relative density is more prone to lose the stability, resulting in a dramatic upward vertical displacement.

Re-liquefaction resistance of lightly cemented sands

The re-liquefaction resistance of cemented sands under multiple liquefaction events such as pre-shock, main-shock, and after-shock earthquakes is a complex phenomenon because the response may alter due to bond breakage. A series of multistage liquefaction–re-consolidation soil element tests under undrained stress-controlled cyclic loading condition using cyclic triaxial were carried out to assess the liquefaction and re-liquefaction resistance of cemented sands with varying degrees of cementation. Lightly cemented specimens were reconstituted using Sydney sand and high early strength Portland cement with cement content ranging from 0.25% to 1% and unconfined compression strength from 15 to 80 kPa. The results showed that the re-liquefaction resistance of cemented sands with different amounts of cement decreased after the first liquefaction event and then increased for succeeding liquefaction events. While the trend of residual excess pore water pressure ratio and cyclic stiffness degradation index of untreated sand under successive liquefaction events remained consistent, the corresponding responses for cemented sands altered for the second to the fifth liquefaction events. In fact, the residual excess pore water pressure ratio and cyclic stiffness of cemented sand increased and degraded faster during the early cycles of loading for the second to fifth liquefaction events.

Evaluation of the effects of initial deviatoric stress and cyclic stress amplitude on liquefaction potential of loose and medium-dense sands: An energy-based method

The energy-based method is a promising tool for evaluating soil liquefaction potential. In this study, the effects of initial deviatoric stress (qs) and cyclic stress amplitude (qcyc) on soil liquefaction potential in undrained cyclic triaxial tests were evaluated using the energy-based method. The testing results indicate that the required dissipated energy for the onset of liquefaction is influenced by both qcyc and qs for a loose sand, while it is only affected by qs for a medium-dense sand. For both loose and medium-dense sands, there exists an approximately unique relationship between the residual excess pore water pressure ratio and the normalized dissipated energy (Wn) during cyclic loadings, independent of qcyc and qs. The relationship between the double-amplitude axial strain and the Wn is influenced not only by initial conditions but also by loading conditions. The evaluation of the effects of the various factors on the liquefaction potential based on the energy-based method provides insight into the liquefaction resistance for both loose and medium-dense sands.

Large wave flume tests on wave-induced response of sandy seabed adjacent a water intake

The response of near-field seabed under wave loads is important to the safety of subsea structures and foundations. This study investigates the wave-induced response of sandy seabed adjacent a water intake through a series of large scale model tests. The tests are conducted at currently the world's largest wave flume. The spatial and temporal distribution of dynamic sandy seabed response under regular and irregular waves is investigated. The change in excess pore water pressure and effective stress within the sandy seabed is studied through direct test measurement and combination of measurement and analytical solutions. The influences of wave height, wave period, and overlaid armor stones on the response of soil close to the water intake are analyzed. Attenuation and phase lag of oscillatory pore pressure in the seabed with respect to depth is observed in the tests, and the presence of the water intake amplifies the excess pore pressure in the upstream direction. Analysis of the effective stress shows that the principal axes of stress in the soil is subjected to continuous rotation under wave loading. Residual excess pore water pressure is generated in the tests of irregular waves with relatively large height and regular waves with relatively high frequency.

Excess Pore-Water Pressure Generation in Cyclic Undrained Testing

AbstractA new strain-based model is presented in this paper for assessing the residual excess pore-water pressure (ue) buildup in fully saturated sands. In this model, the generation of residual ue...

Liquefaction and post-liquefaction assessment of lightly cemented sands

Post-liquefaction response of lightly cemented sands during an earthquake may change and become similar to uncemented sands due to bonding breakage. In the current study, the effect of degree of cementation on liquefaction and post-liquefaction behaviour of lightly cemented sands was studied through a series of cyclic and monotonic triaxial tests. Portland cement with high early strength and Sydney sand were used to reconstitute the lightly cemented specimens with unconfined compression strength ranging from 25 to 220 kPa. A series of multi-stage soil element tests including stress-controlled cyclic loading events with different amplitudes and post-cyclic undrained monotonic shearing tests were carried out on both uncemented and cemented specimens. Furthermore, a series of undrained monotonic shearing tests without cyclic loading history on different types of specimens was conducted to investigate the effect of cyclic loading history on the post-cyclic response of the specimens. The results show that residual excess pore-water pressure is correlated to the cyclic degradation of lightly cemented sands during cyclic loading. In addition, optical microstructure images of the cemented specimens after liquefaction showed that a major proportion of cementation bonds remained unbroken, which resulted in a superior post-liquefaction response with respect to initial stiffness and shear modulus in comparison to the uncemented sand.

The influence of non-plastic fines on pore water pressure generation and undrained shear strength of sand-silt mixtures

An extensive experimental study was undertaken to investigate the influence of non-plastic fines on pore water pressure generation and undrained cyclic shear strength of sand–silt mixtures. For this purpose, stress-controlled undrained cyclic simple shear (CSS) tests were carried out on reconstituted specimens of Ticino silica sand under different combinations of fines content (FC) and global void ratios (e). The applicability of several models suggested in literature for predicting cyclic residual excess pore water pressure generation in non-plastic silty soils was analyzed, and revised correlations for assessing model parameters were proposed. Finally, the study confirmed that the equivalent granular void ratio (e*) is an effective and appropriate parameter for characterizing the behavior of sand with fines up to a threshold value of fines content of sand-silt mixtures, and cyclic resistance ratio (CRR) of the mixtures can be well correlated with e*, irrespective of their fines contents and void ratios.

Nonlinear seismic behavior of pile groups in cement-improved soft clay

Centrifuge tests were performed to investigate the effects of ground improvement on the seismic behavior of pile groups in soft clay. The soil profile consisted of four lightly overconsolidated clay layers overlying a dense layer of sand. The pile groups had a symmetrical layout consisting of 2×2 piles spaced at 3.0 pile diameters and were driven into both unimproved soft clay and soft clay improved by a simulated Cement Deep Soil Mixing (CDSM) method. The centrifuge model was subjected to seven different earthquake events with peak accelerations ranging from 0.03 to 0.66g. The foundation level motions of the improved pile groups were different than the surface free-field motion. The foundation level motion for the unimproved pile group was, however, identical to that in the free-field. Higher peak accelerations were observed in the pile cap of the group with smaller CDSM block (GIS) compared to the unimproved pile group (GU) and the group with the largest CDSM block (GIL). Higher pile cap to the soil surface spectral ratios were also obtained for the GIS group in both short and long periods. Cement-Deep-Soil-Mixing was effective in reducing the peak displacements of the GIL pile cap. The peak displacements of the GIS pile cap remained about the same as the GU pile cap. As the size of the ground improvement increased, the fundamental period of the pile groups reduced. The estimated fundamental periods of the GIS and GU pile groups were, however, close to each other. Acceleration and displacement response spectra of the foundation level motions in comparison to the fundamental periods of the pile groups provided insight into the observed acceleration and displacement responses. The adhesion between soft clay and CDSM blocks helped to reduce the soft clay settlement in the vicinity of CDSM blocks compared to the free-field and the vicinity of unimproved pile group. More residual excess pore water pressure was, however, generated in the vicinity of CDSM blocks compared to the free-field and the corresponding location in the unimproved pile group, likely due to vibrations of the CDSM blocks and the piles.

Strong ground motion characteristics observed in the 13 June 2011 Mw6.0 Christchurch, New Zealand earthquake

This paper examines the salient characteristics of near-source strong ground motions recorded from the 13 June 2011 Mw6.0 Christchurch, New Zealand earthquake. 26 strong motions were recorded within Rrup=30km of the source, with maximum geometric mean horizontal shaking exceeding peak accelerations and velocities of PGA=1.5g and PGV=100cm/s, respectively. The polarity of strong motion recordings with Rrup<6km illustrate the complexity of the seismic source, supporting the two-fault model of Beavan et al. [4]. Directivity in the observed ground motions in eastern and central Christchurch is dominated by the secondary rupture of a right-lateral ENE-trending fault plane, which produces polarization in a predominantly NS direction. As seen in previous events in the Canterbury earthquake sequence, significant basin-generated surface waves led to strong long period ground motions with relatively long significant durations. The installation of strong motion instruments in the Port Hills following the 22 February 2011 Mw6.2 Christchurch earthquake resulted in five new observed ground motions on rock or shallow soil sites, relative to past events, with the PARS and GODS stations on top of the causative faults recording very large amplitudes, inferred as a result of site response and topographic effect coupling. Significant nonlinear site response is evident at several locations in eastern Christchurch, resulting in truncation of acceleration amplitudes, with liquefaction-induced cyclic mobility clearly evident at two locations. Pore pressure recordings at five locations in eastern Christchurch illustrate that significant excess pore water pressure (EPWP) generation in a Mw5.3 event, which occurred 80min prior to the Mw6.0 event, resulted in residual EPWP's that had a measurable influence on high frequency ground motion amplitudes at soft soil sites in the latter event. Finally, the observed ground motion amplitudes are compared with empirical models for response spectra and significant duration.

파랑에 의한 방파제 케이슨 침하 경향 및 원인 분석: 침하 계측자료를 중심으로

지금까지의 방파제 침하와 관련된 연구는 주로 해석위주의 수치모형실험 또는 축소모형을 이용한 실내수조모형실험을 통해 이루어졌다. 현재까지 실제 방파제 구조물에서 계측된 침하를 이용한 연구는 이루어지 않았다. 본 연구에서는 실제 케이슨 방파제에서 장기간 계측된 침하 자료를 분석해 정성적인 측면에서 파동에 의한 케이슨의(하부지반 포함)침하 경향과 그 원인을 분석하였다. 분석 결과, 케이슨 침하에 파랑의 영향이 있음을 분명하게 확인할 수 있었다. 특히 태풍과 같은 고파랑 조건에서는 그 경향이 뚜렷하게 나타났다. 케이슨 침하는 파랑에 의한 해저지반에서의 진동과잉간극수압과 잔류과잉간극수압의 합으로 표현되는 과잉간극수압의 증가에 의한 지반의 액상화와 축적된 과잉간극수압의 소산에 따른 지반의 고밀도화 과정을 통해 발생된다. 케이슨 하부 지반의 과잉간극수압 거동은 전적으로 케이슨 거동에 지배된다. 고밀화과정을 경험한 지반은 동급의 또는 그 보다 작은 파랑 조건에서는 액상화 발생 가능성이 현저하게 줄어들어 결과적으로 침하 발생도 감소된다. So far, studies on the settlement of breakwater have mainly been conducted through numerical model tests focusing on an analysis or through the laboratory wave tank tests using a scaled model. There has not been a study on the settlement that is measured in an actual breakwater structure. This study analyzed the data of settlement that has been measured in an actual caisson breakwater for a long time and the characteristics and causes of wave-induced settlement in the caisson (including beneath ground), based on qualitative aspect, were examined. The analysis revealed that wave clearly has an effect on the settlement in caisson, especially in the condition of high wave such as typhoon. Caisson settlement is caused by the liquefaction of ground, which is due to the increase of excess pore pressure, the combination of oscillatory excess pore pressure and residual excess pore water pressure, and the solidification process of ground due to dissipation of the accumulated excess pore pressure. The behavior of excess pore pressure in the ground beneath the caisson is entirely governed by the behavior of the caisson. Ground that has gone through solidification is not likely to go through liquefaction in a similar or a smaller wave condition and consequently, the possibility of settlement is reduced.

Influence of cyclic loading history on small strain shear modulus of saturated clays

Small strain shear modulus Gmax is an essential parameter in soil dynamics, and it is usually estimated based on the Hardin and Richart equation. However, many previous researches on sands have indicated that the Hardin and Richart equation does not consider the influences of cyclic loading history on Gmax. In this paper, effects of cyclic loading history on Gmax of saturated clays under undrained conditions are studied using a combination device of piezoelectric-ceramic bender element system and cyclic triaxial apparatus. The dynamic pre-loading includes both relatively high amplitudes of cyclic stresses and cyclic strains. Gmax without cyclic loading history is also investigated for the comparison purpose. Test results show that, at the same effective stress, both cyclic strain history and cyclic stress history will induce reduction of Gmax compared to the corresponding Gmax values with non-cyclic loading effects. In strain-controlled tests, the reduction of Gmax is slight and relatively stable; while in stress-controlled tests, the reduction of Gmax increases suddenly and remarkably when the effective stresses degrade to a certain degree. The comparison between double amplitude axial strain and residual excess pore water pressure behaviors show that the remarkable reduction of Gmax can demonstrate the cyclic failure of saturated clays.

Developing characteristics of standing wave-induced residual excess pore water pressure in the seabed

The interaction among wave, seabed and marine structure is an important issue in geotechnical engineering as well as coastal engineering. It is important for civil engineers who have to design the foundation for various marine structures and verify the instability of the seabed to understand variations of pressures and stresses in the seabed induced by waves. Previous studies showed that there are two different mechanisms in the seabed instability, particularly, in the case of wave-induced liquefaction. The first is caused by the transient or oscillatory nature of excess pore water pressure, and is accompanied by damping of the amplitude and lagging of the phase in the variation of pore water pressure. The second is caused by the residual or progressive nature of excess pore water pressure, which appears after a certain number of cyclic waves loading. This study numerically analyzed the generation of excess pore pressure in the seabed induced by a standing wave. Particularly it investigated the developing characteristics of residual excess pore pressure in the excess pore pressure generated in the seabed below the antinode and the node of a standing wave. In addition, the numerical results are compared to experimental ones. They exhibited good agreement qualitatively in the developing characteristics of the residual excess pore pressure.

해저지반에서 계측된 잔류과잉간극수압에 대한 비교 연구

파랑과 해저지반 그리고 해안 해양 구조물과의 상호작용은 지반공학뿐만 아니라 해안공학 분야에서도 중요한 이슈중의 하나이며, 파랑에 의해 해저지반 내부에 발생하는 응력 및 간극수압 거동의 파악은 다양한 해안 해양 구조물의 기초 설계 및 해저 연안 지반의 불안정성 검토에 있어서 중요한 과제이다. 해저지반의 불안정에 대한 문제 중, 파랑에 의한 해저지반의 액상화는 기존의 연구를 통하여, 두개의 메커니즘이 존재한다는 것이 밝혀졌으며, 이는 각각 파랑에 의해 해저지반 내부에 발생하는 과잉간극수압의 변동 특성 및 잔류 특성에 따른 것이다. 본 연구에서는 일본 시코쿠 코치(高知)현에 위치하고 있는 코치항에서 채취한 토사에 대한 동적 특성을 고려하여, 파랑에 의해 해저지반 내부에 발생하는 침투류에 의한 잔류과잉간극수압에 대하여 해석을 하였으며, 더 나아가, 코치항에서 계측된 값과 비교 분석을 하였다. The interaction among wave, seabed and marine structure is an important issue in coastal engineering as well as geotechnical engineering. Understanding variations of stresses and pore water pressures generated in seabed induced by waves is important for civil engineers who have to design the foundation for various marine structures and verify the instability of seabed. In the matters on seabed instability, particularly, in the case of wave-induced liquefaction of seabed, it is turned out there are two different mechanisms through previous studies. These are caused by the transient or oscillatory nature and the residual or progressive nature of excess pore water pressure generated in seabed, respectively. In this study, it is analyzed dynamic characteristics of soils sampled in seabed around the port of Kochi, Japan, through the dynamic triaxial tests and the residual excess pore water pressure in the seabed induced by seepage force of wave. In addition, the calculated residual excess pore water pressures were compared with the field data observed in the port of Kochi.

파압에 의한 해안구조물-해저지반의 침하거동에 대한 수치해석

Seabed settlement underneath a coastal structure may occur due to wave loading generated by storm surge. If the foundation seabed consists of sandy soil, the possibility of the seabed settlement may be more susceptible because of generation of residual excess pore-water pressure and cyclic mobility. However, most coastal structures, such as breakwater, quay wall, etc., are designed by considering wave load assumed to be static condition as an uniform load and the wave load only acts on the structure. In real conditions, however, the wave load is dynamically applied to seabed as well as the coastal structure. In this study, therefore, a real-time wave load is considered and which is assumed acting on both the structure and seabed. Based on a numerical analysis, it was found that there exists a significant effect of wave load on the structure and seabed. The deformation behavior of the seabed according to time was simulated, and other related factors such as the variation of effective stress and the change of effective stress path in the seabed were clearly observed.

相対密度に着目した防波堤周辺地盤の波浪による液状化に関する水路模型実験

A flume model experiment is newly developed to simulate the wave-induced liquefaction in seabed on a reduced scare of 1: 100, and examine influential factors such as the relative density of seabed on the mechanism of waveinduced liquefaction. This experiment was carried out under these conditions; the depth of water, 17cm, the wave period, 1.0s, the incident wave height, 5.5cm, the thickness of permeable layer, 35cm, and the relative density, Dr (%), 20-60. In order to satisfy similarity law in 1g gravitational field, the polymer fluid was used to change the permeability of seabed. The main conclusions obtained from this study are as follows; 1) When the relative density is about 20-40%, wave-induced liquefaction due to the increase in residual excess pore water pressure were observed in deep seabed, and shear failure due to wave force were observed in shallow seabed. 2) When the relative density is about 50-60%, wave-induced liquefaction due to the increase in residual excess pore water pressure were not observed in the present experimental condition. 3) Vertical deformation of seabed decreases with increasing density while the shape of seabed surface is similar regardless of the relative density.

Physical modelling of consolidation of Hong Kong marine clay with prefabricated vertical drains

In this study, a small-scale physical model with full instrumentation was set up and used to measure the full consolidation process of a cylinder of clay with one prefabricated vertical drain (PVD) in its middle. The model test produced several interesting phenomena: (i) a delayed pore-water pressure increase under a constant pressure during loading stages, (ii) a delayed pore-water pressure decrease under a constant pressure during unloading stages, and (iii) residual excess pore-water pressures at the end of loading. Two analytical solutions accounting for the well resistance and a smear zone with reduced permeability are used to calculate the variation of average excess pore-water pressure with time, and these are compared with the measured variation. The well resistance of the PVD has a larger influence on the consolidation process than the permeability of the smear zone, especially during the latter stages of consolidation. The analytical solutions are able to show the influence of the well resistance of the PVD strip, but they cannot satisfactorily simulate the pattern of the residual excess pore-water pressure with time.Key words: consolidation, vertical drain, marine clay, physical modelling.