Key factors affecting prediction of seismic pore water pressures in silty sands based on damage parameter

The paper provides insight into factors affecting the prediction of seismic pore-water pressure build up in clean sands and sand–silt mixtures for modeling purposes. Laboratory pore pressure measurements were conducted using stress-controlled undrained cyclic simple shear (CSS) tests carried out on both reconstituted and undisturbed specimens of silty sands under different initial conditions (density state, effective vertical stress, initial fabric and fines content). Test results were interpreted by using a damage concept-based model which is actually implemented for clean sands in non-linear time domain site response analysis codes. In the present work, such a model was properly modified for sands having fines contents higher than 35%. The general applicability of the modified procedure for predicting pore water pressure response of silty sands under irregular shear stress loading using data from stress-controlled CSS tests was also verified and all factors affecting calibration parameters of the model were throughly analyzed.     

Cyclic undrained behavior of silty sand

Laboratory cyclic triaxial tests were performed to investigate the effect of fine content on the pore pressure generation in sand. Strain-controlled, consolidated undrained tests have been performed with a cyclic shear strain range of 0·015-1·5%. These tests were carried to 1000 cycles or to initial liquefaction, which ever occurred first. Triaxial tests were performed on pure sand silt specimens and specimens with silt additions of 10, 20, 30, and 60% by weight. Two types of silt, a non-plastic silt and a low plasticity silt (PI 10) were used as control materials. The main parameters varied in this study were the amount of silt, the plasticity index of silt, and the void ratio where the observed parameter was the pore pressure generation. For all silt contents, silt plasticity and the number of loading cycles have no significant effect at strain levels below 0·01%. Therefore, threshold strain for silty sands have approximately the same value as sands. For both non-plastic and low plasticity silts, there is a significant increase in the generated pore pressure at high strain levels.     

Cyclic shear response of channel-fill Fraser River Delta silt

The cyclic shear response of a channel-fill, low-plastic silt was investigated using constant-volume direct simple shear testing. Silt specimens, initially consolidated to stress levels at or above the preconsolidation stress, displayed cyclic-mobility-type strain development during cyclic loading without static shear stress bias. Liquefaction in the form of strain softening accompanied by loss of shear strength did not manifest regardless of the applied cyclic stress ratio, or the level of induced excess pore water pressure, suggesting that the silt is unlikely to experience flow failure under cyclic loading. The cyclic shear resistance of the silt increased with increasing overconsolidation ratio (OCR) for OCR>1.3. The silt specimens that experienced high equivalent excess cyclic pore water pressure ratios (r_u>80%) resulted in considerable volumetric strains (2.5%–5%) during post-cyclic reconsolidation implying potentially significant changes to the particle fabric under cyclic loading.     

沙土在孔隙水注入实验中渗透剪切破坏

前人曾指出液化后伴随着超孔隙水压重新分配的渗透会引起流体破坏的可能性。为了研究这一现象,利用实验室三轴试验将孔隙水注入土壤检测了土壤的渗透剪切破坏。该实验是在各项异性的固结作用后保持差应力,使用孔隙水控制装置在体积不变的应变控制条件下将孔隙水注入。实验中所用的材料是在1995年神户地震时被液化的常规洁净细砂和风化的花岗岩土壤。本文以实验结果为基础,讨论了由孔隙水注入引起的渗透剪切破坏判据和导致后液化行为的剪切应变发展特征。     

Evaluation of temperature and freeze–thaw effects on excess pore pressure generation of fine-grained soils

The November 3, 2002 Denali-Alaska earthquake (M_w=7.9) caused significant liquefaction associated damage to various infrastructure built on fine-grained soils. The seismic response, liquefaction potential, and excess pore pressure generation of soils in cold regions, especially those of fine-grained nature, have not been studied thoroughly and therefore are not well-understood. This paper presents results from an extensive laboratory study on the characteristics of excess pore pressure generation and liquefaction potential of fine-grained soils. Laboratory-constituted soils specimens were tested in four categories: (1) tests on specimens subjected to no thermal conditioning or freeze–thaw cycles; (2) tests on specimens conditioned at 24, 5, 1, 0.5, and −0.2 °C; (3) tests on specimens subjected to 1–4 freeze–thaw cycles; and (4) tests on specimens conditioned at near-freezing temperatures of 0.5 and −0.2 °C through different freeze–thaw paths. Strain-controlled, undrained, cyclic triaxial tests were performed at shear strain levels of 0.005–0.8%. Specimens conditioned at different temperatures were found to generate significantly different pore pressures with cyclic loading. The excess pore pressure generation at near or slightly below freezing was found to change dramatically. A transitional change in the dynamic soil behavior, attributed to unfrozen- or frozen-dominant pore water, was discovered. The threshold shear strain was also found to be influenced by the temperature. Subjecting the soil specimens to 1, 2 and 4 freeze–thaw cycles caused a reduction in excess pore pressure generation and slight change in the threshold shear strain. The temperature conditioning path to reach the target temperature was found to be important on the development of excess pore pressure at near and slightly below-freezing temperatures.     

Mismatch between variations of solar indices, stratospheric ozone and UV-B observed at ground

In solar cycles 22–23, all solar indices showed maxima near 1990 and 2000 and minima in 1996. The maximum to minimum variation was only 1–2% in the UV range 240–350 nm. Dobson ozone intensities did not show any clear relationship with solar cycle and ozone variations were less than 10%. The UV-B (295–325 nm) observed at ground by Brewer spectrophotometers at some locations had variations of 50–100% for 295–300 nm, and 20–50% for 305–325 nm. The maxima were in different years at different locations (even with separations of only 300 km), did not match with the solar cycle, and were far too large to be explained on the basis of ozone changes (1% decrease of ozone is expected to cause 2% increase of UV-B). Thus, if the data are not bad, the UV-B changes do not match with solar activity or ozone changes and must be mostly due to other local effects (clouds, etc.?). When data are averaged over wide geographical regions, UV-B variation ranges are smaller (10–20%, probably because localised, highly varying cloud effects get filtered out), and are roughly as expected from ozone variations.     

复杂应力条件下松砂振动孔隙水压力与体变特性的试验研究

利用新研制的土工静力-动力液压三轴-扭转多功能剪切仪,在5种初始主应力方向角与5种中主应力系数相组合的初始固结条件下,对饱和松砂进行了不排水循环扭剪试验。讨论了初始固结条件对不排水条件下饱和松砂孔隙水压力变化规律及对剪胀、剪缩、卸荷体缩等体积变化过程的影响。试验研究表明：（1）分别以稳定残余孔隙水压力和破坏时循环次数归一化后的残余孔隙水压力比和循环次数比之间的关系可以用双曲线模式表达。其参数主要依赖于初始主应力方向,中主应力系数对参数的影响并不显著。归一化后的孔隙水压力比与广义剪应变之间的关系也可以用双曲线模式表达,其中的2个待定参数依赖于初始主应力方向,与中主应力系数无关;（2）在三向非均等固结条件下的不排水循环扭剪试验中,饱和松砂表现出卸荷体缩特性,不同初始主应力方向时,饱和松砂剪缩、剪胀、卸荷体缩呈现出不同的交替变化模式。     

Undrained monotonic and cyclic behavior of sand-ground rubber mixtures

In this study the stress–strain characteristics of sand-ground rubber mixtures are investigated in the sandlike zone,at different confining pressures,using hollow cylinder specimens subjected to torsional monotonic and cyclic loading.Under monotonic loading a mixture of sand-ground rubber with 10% and 25% rubber content show more contraction behaviour than that observed in a pure sand specimen.Phase transformation point in these mixtures are located on a larger shear strain.As expected,the shear strength of specimens decreases with increase of ground rubber content.However,with increasing of effective confining pressure,the loss in shear strength of the mixture is decreased.In addition,a mixture with 25% ground rubber shows a smaller loss in shear strength compared to a mixture with 10% ground rubber mixture.Under cyclic loading mixtures with 10% and 25% ground rubber have similar liquefaction resistance,especially at confining pressures of 110 k Pa and 260 k Pa.Therefore,by using of the mixture with 25% ground rubber,a larger volume of scrap tires could be recycled.The addition of ground rubber to sand would affect the shear strain variation and excess pore water pressure trends,and this effect was further intensified with increasing ground rubber percentage.     

Are fiddler crabs potentially useful ecosystem engineers in mangrove wastewater wetlands?

The effect of different organic-rich sewage concentration (0%, 20% and 60% diluted in seawater) and absence or presence of mangrove trees on the survival, bioturbation activities and burrow morphology of fiddler crabs species was assessed. After 6 months, males of both species always showed higher survival (80%) when compared to females (20%). Crabs inhabiting pristine conditions achieved higher survival (67–87%) than those living in sewage-exposed mesocosms (40–71%). At 60% sewage loading, fiddler crabs processed less sediment (34–46%) during feeding and excavated slightly more sediment (45–80%) than at pristine conditions. While percent volume of the burrow chambers increased (13–66%) at contaminated mesocosms for both vegetation conditions, burrows were shallower (33%) in bare cells loaded with sewage. The results show that fiddler crabs presented moderate mortality levels in these artificial mangrove wetlands, but mainly in sewage impacted cells. However, they still function as ecosystem engineers through bioturbation activities and burrow construction.     

Effects of plastic fines on the instability of sand

The effects of plastic fines on the instability of sand were studied in this article. For this purpose, the results of undrained monotonic triaxial compression tests conducted on specimens of sand with variation in fines content from 0% to 30% are presented. The specimens were prepared in two different initial dry unit densities and were subjected to two different confining pressures. The results of the tests are shown in four groups. They demonstrate that an increase in plastic fines leads to an increase in the instability, followed by a decrease with a further increase in fines content. It is also seen that the slope of the steady-state lines in p’-q space increases with increase in fines content, but after certain fines content, it begins to decrease. A reverse trend is observed for the slope of instability lines; it decreases at lower fines content, followed by an increase with a further increase in fines content.     

Settlements of saturated clean sand deposits in shaking table tests

Shaking table tests were conducted on saturated clean Vietnam sand in the large biaxial laminar shear box (1880 mm×1880 mm×1520 mm) at the National Center for Research on Earthquake Engineering (NCREE), Taiwan. The settlement of sand specimens was measured and evaluated during and after each shaking test. Without liquefaction, the settlement of sand caused by shaking is very small. Significant volume changes occur only when there is liquefaction of sand. The volumetric strain of liquefied sand was calculated according to the measured settlement and the thickness of liquefied sand in the specimen. Relations between volumetric strain after liquefaction and the relative density of saturated clean sand were developed for various shaking durations and earthquake magnitudes. They are not affected by the shaking amplitude, frequency, and direction (one- or multidirectional shaking).     

Strain energy based evaluation of liquefaction and residual pore water pressure in sands using cyclic torsional shear experiments

In this study, cyclic hollow cylinder torsional tests were conducted on the reconstituted specimens of Toyoura sand in a practical range of initial density and stress states. The results were employed to evaluate the liquefaction resistance and residual pore water pressure of sand using the strain energy concept. A simple pore water pressure (PWP) model with two calibration parameters was developed for the prediction of residual pore pressure as a function of cumulative strain energy density and the capacity energy of sand. Capacity energy is defined as the cumulative strain energy that is required for liquefaction onset. Based on the results of the tests, an equation is then presented for the estimation of capacity energy in terms of relative density and initial effective confining pressure of sand. This equation is shown to work well as a state boundary curve, which can discriminate between the liquefied and non-liquefied field case histories. Several extra tests were also performed to investigate the effect of initial static shear stress on the proposed PWP model and capacity energy. The results show that initial shear stress has a minor effect on the trend of the proposed PWP model; however, it definitely affects the capacity energy. The final part of the paper aims to confirm reasonable performance of the proposed PWP model by the available observations of seismically induced pore water pressure in shaking table, centrifuge, and real site conditions.     

Measurement of small strain shear modulus of clean and natural sands in saturated condition using bender element test

Bender element (BE) tests of saturated sand have increased interest to researchers currently. However, the measurement of small strain modulus from BE tests shows large difference between saturated and dry conditions. In this study, BE tests of a type of clean sand (Fujian sand) and two types of natural sands (Hangzhou sand and Nanjing sand) were performed. For the purposes of comparison, resonant column (RC) test and torsional shear (TS) test were also carried out on the same specimen. The factors that influence the determination of the travel time of shear wave in BE tests are discussed and a reliable method for the determination of the shear-wave velocity is obtained. It is found that the shear-wave velocities V_s of saturated Fujian sand (clean sand) and Hangzhou sand (natural sand) obtained from BE tests are 5–10% greater than those obtained from RC and TS tests. However, the V_s of saturated Nanjing sand (natural sand) obtained from BE, RC and TS tests show good agreement with a maximum difference of about 3%. Sands with various fines contents were also tested in an attempt to explain the differences between the two saturated natural sands. Biot׳s theory accounting for the dispersion of shear wave was employed to interpret the results of BE tests. The results indicate that the fines content of natural sand plays an important effect on the hydraulic conductivity, which affects the relative motion between soil particles and fluid when a high frequency shear wave propagates in the specimen. Based on this, a method for the determination of small strain shear modulus in BE test was proposed for both saturated clean sands and natural sands.     

Clinoform mechanics in the Gulf of Papua, New Guinea

The largest islands of the Indo-Pacific Archipelago are estimated to account for 20–25% of the global sediment discharge to the ocean, and much (>50%) of this sediment is supplied to wide (>150 km) continental shelves. These conditions are conducive to creation of large-scale morphologic features known as clinoforms—sigmoidal-shaped deposits on the continental shelf. The Gulf of Papua (GOP) receives 3.84 ×10⁸ tons of sediment annually from three principal sediment suppliers, the Fly, Kikori and Purari Rivers, and its prograding clinoform is the focus of this study. During three research cruises, 80 cores and 37 CTD/optical backscatter casts were collected, and an instrumented tripod was deployed twice. Sedimentological and radiochemical results indicate that the GOP clinoform has characteristics similar to those seaward of other major rivers (e.g., Amazon, Ganges–Brahmaputra), specifically sand/mud interbedding on the topset, rapidly accumulating muds on the foreset, and siliciclastic mud mixed with carbonate sand on the bottomset.Using core data and field observations, the mechanics of clinoform progradation are examined. Discrete, large sedimentation events are identified as processes building the clinoform feature. X-radiographs from foreset cores reveal thick beds (>5 cm) between bioturbated sections. Detailed ²¹⁰Pb and grain-size data indicate that low activities and increased clay contents are associated with these beds. They are hypothesized to be formed by fluid–mud deposition in response to periods of large wave-tide bed shear stresses, more likely during the SE-tradewind season, and their regular occurrence produces high rates of mean accumulation (4 cm/y). Bed preservation is determined by the rates of sediment accumulation and bioturbation.To assess the influence of physical oceanographic factors on clinoform shape, bottom shear stresses from tides and surface waves were calculated using available wave and tripod data. This effort reveals that the depth range (25–40 m) of the clinoform rollover point (seaward edge of the topset region) is roughly consistent with the sediment-transport regime. Furthermore, calculations corroborate the core data that suggest possible seasonal sediment storage in the inner topset region (<15-m water depth, during the NW-monsoon winds) with subsequent transfer to foreset beds (more probable during SE-tradewind conditions).A 100-yr sediment budget created with accumulation rate data suggests approximately 20% of the total sediment supplied to the GOP accumulates on the clinoform (creating the clinoform morphology). Less than 5% is believed to escape to the adjacent slope, and much of the remaining 75% is likely trapped on the inner-topset region (<20 m water depth) and within the mangrove forests and flood/delta plains of the northern GOP.     

Flow deformation of liquefied sand under constant shear load and its application to analysis of flow slide of infinite slope

This paper intended to evaluate the behavior of saturated sand and sloped ground subjected to flow failure with seepage of pore water in the ground after earthquake and the resultant liquefaction. Triaxial compression tests of sand with constant deviator stress but changing of pore pressure and volume of the specimens were conducted in this study. It was revealed that the relation between the volume change and the amount of shear strain during deformation depended on the initial density of the sand but it did not much depend on shear stress and initial confining stress levels. Based on this test results and numerical analysis of the seepage of pore water in liquefied ground, a methodology was proposed to predict the deformation of inclined ground due to liquefaction.     

Influence of fines content on the undrained cyclic shear strength of sand–clay mixtures

A series of undrained cyclic triaxial tests were performed on sand–clay mixtures with various sand–clay mixing ratios. Prior to the primary tests, the threshold fines content was examined by consistency tests, which was found to be approximately 20%. For sand–clay mixtures with a sand-matrix (fines content less than the threshold fines content), the cyclic shear strength of low-density mixtures increases and that of high-density mixtures decreases with increasing fines content. However, for sand–clay mixtures with a fines-matrix (fines content greater than the threshold fines content), there exists a unique correlation between the cyclic shear strength and global void ratio for different fines content. The equivalent granular void ratio is introduced in this paper to account for the contribution ratio of the fines to soil skeleton. As a result, a unique relationship between cyclic shear strength and equivalent granular void ratio was observed for pure sand and sand–clay mixtures with a sand-matrix.     

Evaluation of static and dynamic properties of sand–fines mixtures through the state and equivalent state parameters

The results of an experimental investigation on sands with low plastic fines content are presented. Specimens with a low plastic fines content of 0%, 15%, 30%, 40%, 50% and 60% by weight were tested in drained and undrained triaxial compression tests. The soil specimens were tested under three different categories: (1) at a constant void ratio index; (2) at the same peak deviator stress in a triaxial test; and (3) at a constant relative density. By a combination with our published experimental data in recent years, the critical state line and various state parameters have been proposed and discussed for a further understanding the behavior of sand–fines mixtures. Results indicated that a unique critical line was obtained from drained and undrained triaxial compression tests for each fines content. The effects of fines content on critical state line (CSL) were recognized and discussed. In addition, the results revealed that normalized peak undrained shear stress, cyclic resistance ratio, and compression index were found to be a good correlation with state parameter Ψ as well as equivalent state parameter Ψ*. An increasing state parameter decreased the normalized peak undrained shear stress, and cyclic resistance ratio; however, the compression index increased with an increase in state parameter. Finally, there were no correlations such as the coefficient of consolidation–state parameter and maximum shear modulus–state parameter due to the different testing condition.     

黄土液化与砂土液化的差异浅析

初步分析了黄土与砂土在液化机理、孔压增长模型、体积压缩系数以及渗透系数方面的差异以便认识黄土液化具有的孔压增长快、消散慢和沉降大的特性。