Dissertations / Theses on the topic 'Soil liquefaction. Silt'

Although silt and clay mixtures were mostly considered to be resistant to cyclic loading due to cohesional components of their shear strength, ground failure case histories compiled from fine grained soil profiles after recent earthquakes (e.g. 1994 Northridge, 1999 Adapazari, 1999 Chi-Chi) revealed that the responses of low plasticity silt and clay mixtures are also critical under cyclic loading. Consequently, understanding the cyclic response of these soils has become a recent challenge in geotechnical earthquake engineering practice. While most of the current attention focuses on the assessment of liquefaction susceptibility of fine-grained soils, it is believed that cyclic strain and strength assessments of silt and clay mixtures need to be also studied as part of complementary critical research components. Inspired by these gaps, a comprehensive laboratory testing program was designed. As part of the laboratory testing program 64 stress-controlled cyclic triaxial tests, 59 static strain-controlled consolidated undrained triaxial tests, 17 oedometer, 196 soil classification tests including sieve analyses, hydrometer, and consistency tests were performed. Additionally 116 cyclic triaxial test results were compiled from available literature. Based on this data probability-based semi-empirical models were developed to assess liquefaction susceptibility and cyclic-induced shear strength loss, cyclically-induced maximum shear, post-cyclic volumetric and residual shear strains of silt and clay mixtures. Performance comparisons of the proposed model alternatives were studied, and it is shown that the proposed models follow an unbiased trend and produce superior predictions of the observed laboratory test response. Superiority of the proposed alternative models was proven by relatively smaller model errors (residuals).

Liquefaction ls a phenomenon where a saturated soil can temporarily lose its shear strength during an earthquake as a result of the development of excess pore pressures. For the past 25 years since Iiquefaction phenomenon was first explained, it was thought to be mainly a problem with clean sand, and most of the research has focused on these soils. However, as case history information has come to light, it has become apparent that silty sands are commonly involved, and in some cases even silts. This has generated a need for knowledge about the response of silty sands and silts under seismic loading. Related to this issue is the question of how best to determine the Iiquefaction resistance of these soils in a practical setting. This research has the objectives of providing an understanding of the behavior of saturated silty sands under seismic loading, and developing a rational basis for the use of the Cone Penetration Test (CPT) to predict Iiquefaction resistance in these materials. The study is primarily experimental, relying on laboratory and field testing and the use of a unique, large scale calibration chamber. The calibration chamber allows the field environment to be duplicated in the laboratory where conditions can be closely controlled and accurately defined. One of the first problems to be overcome in the research was to determine how to prepare specimens of silty sands that would reasonably duplicate field conditions in both the small scale of the conventional laboratory tests, and the large scale of the calibration chamber. Out of four different methods explored, consolidation from a slurry proved to be best. Two silty sands were located which had the desired characteristics for the study. Field work, involving both the Standard Penetration Test (SPT) and CPT was done as part of this investigation. The behavior of the silty sands were determined in the laboratory from monotonic and cyclic loading tests. The test results show that the effect of fines is to reduce the cone penetration resistance, but not to affect the liquefaction resistance. The steady state shear strength of the soils seems to be correlated to the cone tip resistance, however, this correlation shows a higher steady state shear strength than those back figured from case history data. The results were also used to define state parameters for both of the soils tested. The state parameter was found to be a reliable index to the liquefaction potential and further study in this area is recommended.
Ph. D.

The presence of silt and clay particles has long been thought to affect the behavior of a sand under cyclic loading. Unfortunately, a review of studies published in the literature reveals that no clear conclusions can be drawn as to how altering fines content and plasticity actually affects the liquefaction resistance of a sand. In fact, the literature contains what appears to be contradictory evidence. There is a need to clarify the effects of fines content and plasticity on the liquefaction resistance of sandy soils, and to determine methods for accounting for these effects in engineering practice. In order to help answer these questions, a program of research in the form of a laboratory parametric study intended to clarify the effects which varying fines content and plasticity have upon the liquefaction resistance of sandy sands was undertaken. The program of research consisted of a large number of cyclic triaxial tests performed on two sands with varying quantities of plastic and non-plastic fines. The program of research also examined the applicability of plasticity based liquefaction criteria and the effects of fines content and plasticity on pore pressure generation. Lastly, a review of how the findings of this study may affect the manner in which simplified analyses are performed in engineering practice was made. The results of the study performed are used to clarify the effects of non-plastic fines content and resolve the majority of the inconsistencies in the literature. The effects of plastic fines content and fines plasticity are shown to be different than has been previously reported. The validity of plasticity based liquefaction criteria is established, the mechanism responsible for their validity is explained, and a new simplified criteria proposed. The effects of fines content and plasticity on pore pressure generation are discussed, and several recommendations are made for implementing the findings of this study into engineering practice.
Ph. D.

Stone column treatment is commonly used to mitigate liquefaction hazard in sandy soils. Research and experience indicate that this method is effective for clean sands but that it may not be effective for silts and sands with fines contents greater than 15-20%. An alternative to the stone column method involves supplementing stone column treatment with pre-fabricated vertical wick drains installed prior to the stone columns installation. Although this method is used in practice, there has not been a formal academic study of its effectiveness. This thesis evaluates seven different case histories where wick drains were used and one where wick drains were not used, for comparison purposes. The site locations varied as well as the soil properties and treatment plans. CPT testing was done at 3 sites and SPT testing was performed at the other 5 sites. CPT data were correlated to SPT data to facilitate comparisons. One of the case histories includes a unique study in which three different variations of the stone column treatment were applied at the same site, providing a direct comparison of the effectiveness of each method. A 26% area replacement ratio (Ar) with drains was determined to be more effective overall than a 26% Ar without drains and more effective in increasing low initial blow counts than the 34% Ar without drains. The areas with drains were more likely to exceed the minimum project criteria consistently throughout the site. Significant scatter were observed in the results and probable causes for the scatter are noted. Final blow count coefficients of variation ranged from 28% to 77%. Increased fines contents required increased Ar in order to maintain similar average final blow counts. Site improvements were evaluated separately and collectively. Individual site results were compared to clean sand curves developed by Baez (1995). Sites with average fines contents less than 20% which were improved using drains and an 11-15% Ar treatment were comparable to clean sand sites without drains and with 5-10% Ar. To achieve similar improvement at sites with 40-46% fines necessitated drains and Ar values of 23-26%. Design recommendations are provided.

The liquefaction resistance of undisturbed soil samples collected from a selected location in Carbondale, Southern Illinois was evaluated by conducting cyclic triaxial tests. Index property tests were carried out on the sample for identification and classification of the soil. Cyclic triaxial tests were conducted on undisturbed sample after saturation, undisturbed sample at natural water content and remolded samples prepared by compaction in the lab. The results were used to evaluate the effect of saturation and remolding on liquefaction resistance of the local soil. Effect of effective confining pressure on dynamic properties of soil (Young's Modulus and Damping ratio) was also studied. Forty five stress controlled cyclic triaxial tests were performed. Three different values of initial effective confining pressure (5 psi, 10 psi and 15 psi) were used and cyclic stress ratio was varied from 0.1 to 0.5 in order to apply different cyclic shear stresses. The results show that the cyclic load resistance of soil decreases as a result of remolding. Saturated undisturbed samples show increase in resistance to liquefaction with increase in initial confining pressure. Remolded samples were prepared by compaction in the lab keeping unit weight and water content equal to that of undisturbed samples. Remolded samples show increase in liquefaction resistance with increase in confining pressure. Undisturbed samples at natural water content show increase in resistance to develop axial strain with increase in confining pressure. Both the rate of excess pressure development and axial strain development increase significantly as a result of remolding. While investigating the effect of saturation of undisturbed samples on liquefaction resistance of soil, interesting observations were made. The excess pressure buildup rate was faster in case of saturated undisturbed samples compared to that in samples with natural water content. On the other hand, rate of strain development was significantly high in case of sample with natural water content compared to that in saturated sample. Also, results obtained from cyclic triaxial tests on saturated undisturbed samples were compared with results obtained from similar tests on Ottawa Sand (Lama 2014) sample. The comparison shows that the saturated undisturbed soil samples of the selected local soil have very high resistance to liquefaction both in terms of initial liquefaction and development of 2.5% and 5% axial strain. Modulus of Elasticity and damping ratio were studied as important dynamic properties of soil. Young's Modulus was observed to decrease significantly at higher strain levels for all three types of samples. Young's modulus increased with increase in effective confining pressure, the effect of confining pressure being large at low strain level and almost insignificant at higher strain level. Damping ratio was highest in undisturbed sample at natural water content and smallest in remolded sample and damping ratio for saturated undisturbed sample falls in between. The damping ratio did not show any definite correlation with strain and confining pressure at lower strain level. But, for strain higher than 1% double amplitude axial strain, damping ratio significantly decreases with increase in strain. Damping ratio increases with increase in confining pressure as observed at high strain for all samples.

碩士
國立中央大學
土木工程研究所
98
In this study, an effective stress based three dimensional finite element model is adopted to investigate the ground motion of a sandy deposits with a thin silt intra-layer at various depths. The nonlinearity of soil is assumed to follow the Cap model, and the pore pressure model based on Cap model, developed by Pacheco, is adopted to simulate the built-up of the pore pressure. The conclusion drawn from this studies are as follow: (a) the thin silt intra-layer in the sand deposit can reduce the extent of liquefaction and the surface settlement; (b) liquefaction occurs only in the sand beneath the thin silt layers near the surface. The ratio of the excess pore water pressure is larger than one in the sand just beneath the silt layer; (c) for the deeper thin silt intra-layer, liquefaction occurs not only in the sand beneath the thin silt layers but also in the sand near the surface; (d) the sand deposits with shallower thin silt intra-layer is danger.

Yes
When ocean waves propagate over porous seabed, they cause variations of the pore pressure within seabed, leading to the possible wave attenuation and soil liquefaction. In order to advance and improve our understanding of the process of wave-induced seabed liquefaction and its impact on wave propagation, systematical experiments are carried out in a wave flume with a soil basin filled with silt. Both the pore pressures and water surface elevations are measured simultaneously, while the seabed liquefaction is videotaped using a high-speed camera. Laboratory measurements show that the pore pressure in surface layer mainly oscillates over time, while the wave period averaged pore pressure has little change. In the deep layer, however, the wave period averaged value of the pore pressure builds up dramatically. The results show that the wave height decreases rapidly along the direction of wave propagation when seabed liquefaction occurs. Such a wave attenuation is greatly enhanced when the liquefaction depth further increases. The experiments also demonstrate that the conditions (wave height and wave period) of incident waves have significant impacts on the wave-induced pore pressures, liquefaction depth and wave attenuation in a silt bed.
National Natural Science Foundation of China (Grant No. 51479053), the 111 Project (Grant No. B12032), the marine renewable energy research project of State Oceanic Administration (GHME2015GC01), the Fundamental Research Funds for the Central University, China (Grant No. 2013B31614), the Colleges and Universities in Jiangsu Province Plans to Graduate Research and Innovation (Grant No. B1504708), and Open Foundation of State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University (Grant No: 2016491011).

The cyclic resistance of predominantly fine-grained soils has received considerable attention following ground and foundation failures at sites underlain by silt-rich soils during recent earthquakes. In several cases substantial ground deformation and reduced bearing capacity of silt soils has been attributed to excess pore pressure generation during cyclic loading. These field case studies are significant due to the occurrence of liquefaction related phenomena in soils that would be characterized as not susceptible to liquefaction using current geotechnical screening criteria. The most widely used of these criteria, the "Chinese Criteria" and its derivatives, are based solely on soil composition and they are essentially diagnostic tools that categorize the soil in a binary fashion as either liquefiable or non-liquefiable. The most significant limitations of these screening tools are that they fail to account for the characteristics of the cyclic loading. This investigation was undertaken to elucidate the potential for strain development in silts during cyclic loading, and to develop a practice-oriented procedure for evaluating the seismic performance of silts as a function of material properties, in situ stresses, and the characteristics of the cyclic loading. This dissertation presents the results of a multi-faceted investigation of the potential for seismically induced pore pressures and large strain development in silt soils. The primary focus of the research was on the synthesis of laboratory testing results on fine grained soils. Laboratory data from cyclic tests performed at Oregon State University and other universities formed the basis for enhanced screening criteria for potentially liquefiable silts. This data was supplemented with field data from sites at which excess pore pressure generation, liquefaction, and/or ground failures were observed during recent earthquakes. This investigation specifically addressed the behavior of silts during loading in cyclic triaxial tests due to the relative abundance of data obtained for this test. The data was used in conjunction with standard geotechnical index tests to enhance an existing energy based procedure for estimating excess pore pressure generation in silts. This pore pressure model can be used with the uncoupled, stress-based methods for estimating the post-cyclic loading volumetric strain developed in this investigation. The energy-based excess pore pressure model and empirical volumetric strain relationship were used to calibrate for applications involving silt soils a nonlinear, effective stress model for dynamic soil response (SUMDES). The SUMDES model was employed, along with the equivalent linear total stress model SHAKE, to estimate excess pore pressures generated at un-instrumented field sites that have exhibited evidence of liquefaction during recent earthquakes. A comparison of the SUMDES and SHAKE results highlighted the limitations of the latter model for simulating dynamic soil response at various levels of shaking and pore pressure response. The results of the SUMDES modeling at several well documented case study sites are presented in this dissertation. These comparisons are valuable for demonstrating the uncertainties associated with modeling of the effective stress behavior of silt during seismic loading.
Graduation date: 2005

碩士
國立中央大學
土木工程研究所
99
Strong earthquakes can cause serious damage to the failure of foundations and structures, which may result in loss of lives. From the geotechnical point of view, for a large earthquake one of the frequently occurred phenomenon is known as liquefaction. The common consensus about this phenomenon is that liquefaction may easily occur in a uniform loose sandy soil stratum. In reality, the presence of intralayers of silt may be found in the field. The smaller permeability of these silt layers may develop a water film at its bottom with a high pore water pressure, leading to failure of ground even long after the earthquake shaking stopped. In this study, the seismic responses of sandy soil stratum with silt layers were obtained by using nonlinear 3D effective stress finite element program. Verification and validation of the program was done first by comparing with centrifuge test results which are in good agreement. The parametric studies using nine numerical models were then conducted to investigate the behavior of liquefiable sand-silt stratum under strong earthquakes and to gain a better understanding of the mechanism of stone columns as a countermeasure in a liquefiable sand-silt stratum. The use of stone columns can delay and reduce the accumulation of excessive pore water pressure; although in some cases liquefaction cannot be avoided. The stiffening benefit from stone columns also reduces the ground settlement which is in parallel with the area of treatment; but the effectiveness of stone columns decreases as the more intralayers of silt are introduced to the stratum. The presence of intralayers of silt will reduce the extent of liquefaction and significantly reduce the ground settlement; however, the large pore water pressure beneath each silt layer forms the water film which requires longer time to dissipate.

博士
國立成功大學
土木工程學系
107
SUMMARY In this study, HsinHwa area in Taiwan was selected as research site to investigate the soil liquefaction properties of low-plasticity silty sand. A series of soil dynamic tests of high quality undisturbed low-plasticity silty sand soil specimens were performed.High quality undisturbed soil specimens were obtained by undisturbed sampling technique for laboratory tests, including the influence of void ratio, fines content, and the disturbance effect to post-liquefaction volumetric strain of low-plasticity silty sand were investigated. The applicability of the current soil liquefaction evaluation methods (include soil liquefaction potential and post-liquefaction settlement) in Taiwan is discussed based on relevant research results first, and then the formulation of analysis process of the current evaluation methods is proposed. Finally, modified analysis procedures of soil liquefaction potential assessment method and post-liquefaction settlement evaluation method were proposed based on the laboratory test results, case verification in Wufeng and Yuanlin area was carried out at the same time.Research progress presented here is in hoped to be helpful in understanding soil liquefaction behavior of low-plasticity silty sand in future engineering applications. Keywords: low-plasticity silty sand, post-liqeufaction volumetric strain, soil liquefaction potential assessment method, post-liquefaction settlement evaluation method. INTRODUCTION Low-plasticity silty sand with high fines content (SM or ML with PI 〈 4) extensively covers areas in the central to southern parts of the western Taiwan. In 1999, severe soil liquefaction disasters of low-plasticity silty sand were occurred by Chi-Chi earthquake in Wufeng and Yuanlin area of Taiwan. A lot of relative researches of soil liquefaction potential assessment were proposed, but the analysis results and accuracy were undesirable. According to the unique soil properties of low-plasticity silty sand layer, the influence of non-plastic fine was a major factor to soil liquefaction evaluation methods, it caused visible analysis deviation. Although current evaluation methods for soil liquefaction potential and post-liquefaction volumetric strain have been developed, they have focused only on clean sand, and soil fine aggregates have only been considered with respect to the plastic fines content or soil plasticity index. However, non-plastic fines can affect the soil structure and cause incorrect evaluation results, and therefore, the current methods of evaluation are inappropriate for use in areas of silty sand with a non-plastic fines content of more than 10%. For the convenience of use, the formulation of analysis process of the current evaluation methods is proposed in the proposed study. Modifications were made to the method used to evaluate the post-liquefaction settlement of silty sand, according to the laboratory test results and current assessment method principles, and results are verified using case examples. POST-LIQUEFACTION VOLUMETRIC STRAIN BEHAIOR OF LOW-PLASTICITY SILTY SAND In this research, a series of remolded soil specimens, with fines content equalled to 0, 10, 20, 30 and 40%, was utilized to investigate the influence of void ratio to post-liquefaction volumetric strain behavior of low-plasticity silty sand. The post-liquefaction volumetric strain would increase while maximum shear strain increased. When fines content was similar, post-liquefaction volumetric strain increased with void ratio increasing. When void ratio and relative density of test specimens were similar, the post-liquefaction volumetric strain, εv, increased with fines content increasing, and then achieved a constant value. In the same test boundary condition, post-liquefaction volumetric strain increased with maximum shear strain increasing. When void ratio and fines content were similar, post-liquefaction volumetric strain of remolded specimens was higher than it of undisturbed specimens. SUGGESTED MODIFICATIONS ON SOIL LIQUEFACTION POTENTIAL ASSESSMENT METHOD The common soil liquefaction potential assessment methods in Taiwan are Seed 97, T＆Y, and NJRA method. They considered that the existence of fine aggregate could increase soil strength, and more fines content get higher liquefaction resistance. NJRA method is the more common applications in Taiwan area, hence this study took NJRA method as main analysis method to investigate and verify the applicability of soil liquefaction potential assessment method, and then the suggested modifications of current soil liquefaction potential assessment method is proposed. The basis of modified assessment method is based on soil laboratory test data. In summary, the suggested modifications of soil liquefaction potential assessment method mainly divided into two parts as follows. 1. When plastic index of soil specimens is more than 4, it denotes that fine aggregate belong clayey particles, the cyclic resistance ratio should be calculated by original formulas. 2. If plastic index of soil specimens is less than or equal to 4, it denotes fine aggregate belong non-plastic fine particles, the cyclic resistance ratio should be multiplied by a reduction coefficient α, and calculated by suitable modified formulas according to the disturbance situation. In order to further verify the applicability of modified NJRA method, the Chi-Chi earthquake victims of Wufeng, Yuanlin area are used as verified cases in this study. According to the verified results, the judgment of modified NJRA method was more accurate at liquefied area, but it was conservative at unliquefied area. For the purposes of soil liquefaction region judgment, the liquefied area was determined as an unliquefied area which was more serious than unliquefied area was determined as liquefied area. It denotes that the conservative soil liquefaction potential assessment method was better than the others, therefore, the modified NJRA method which proposed in this study had ideal applicability of soil liquefaction potential assessment in Taiwan. PROPOSED POST-LIQUEFACTION SETTLEMENT EVALUATION METHOD Two approaches have previously been used to estimate post-liquefaction settlement (volumetric strain). One approach was proposed by Ishihara ＆ Yoshimine in 1992, and chart formulation of this post-liquefaction settlement evaluation method was presented in 2005. The other method was proposed by Tsukamoto, Ishihara, and Sawada in 2004. These two evaluation methods were based on the soil test results for clean sand or soil with low fines content, and have been constantly applied in related research over the past 10 years. However, earthquakes that have occurred around the world over the past few years have shown that the existence of non-plastic fine aggregate soil reduces soil liquefaction resistance and causes a greater amount of post-liquefaction settlement. In this study, we refer to the revised procedure proposed by Ishihara et al. (2016) to suggest modification of the post-liquefaction settlement method and its chart formulation. To improve the applicability of the current post-liquefaction settlement evaluation methods used in Taiwan, test results of post-liquefaction volumetric strain from low-plasticity silty sand specimens were collected in this study, with an aim of proposing suggested modification of the post-liquefaction settlement evaluation method and its chart formulation. Extending the research results proposed by Ishihara et al. (2016), the relationships between post-liquefaction volumetric strain, the void ratio range, and relative density were further analyzed. To simplify steps used in analysis and the tests conducted using the modified method in this study; test results obtained in the Hsinhwa area were compared with those proposed by Cubrinovski ＆ Ishihara (2002). This confirmed that the analytical charts had the same trends, and therefore, the series of analysis charts and evaluation procedures were utilized for modified post-liquefaction settlement evaluation in Taiwan. The effect of the Chi-Chi earthquake in the Wufeng area is used to verify the method presented in this study. A comparison of the analysis results of these evaluation methods indicates that the suggested modification method delivers a superior performance in the evaluation of post-liquefaction settlement. I＆Y1992 generally underestimated post-liquefaction settlement, whereas T.I.＆S.2004 overestimated it. The analysis results of the suggested modification proposed in this study are more consistent than those of the other method. CONCLUSION Soil engineering properties of low-plasticity silty sand was not conclusive in the past studies, and the influence of low-plasticity silty sand on soil strength was ignored generally. This concept affects the soil liquefaction assessment results of low-plasticity silty sand layer indirectly. Latest soil sampling technology was applied to obtain the high quality undisturbed soil specimens in this study, soil liquefaction properties of low-plasticity silty sand was realized though a series of laboratory dynamic tests, and the real engineering properties of low-plasticity silty sand was verified. This study is the first to extend and apply results of research conducted on undisturbed low-plasticity silty sand. The influence of the fine aggregates has often been overlooked in past soil liquefaction evaluation methods. In actual situations, the fines content and its plasticity are important factors influencing soil liquefaction properties. Additionally, the disturbance effect should be acknowledged and cannot be ignored. In this respect, when conducting soil liquefaction evaluations, the soil structure and engineering properties need to be firstly understood. In addition, the deposition history of the soil layer also needs to be clarified to select the appropriate assessment analysis mode. This study suggests that detailed soil tests should be conducted both indoors and outdoors to ensure that the true engineering properties of the soil layers are understood prior to building construction. To avoid unnecessary disasters related to any blind spots in this new evaluation method, it is advised that analysis results should not be solely relied on.

Thesis (Ph.D.)--State University of New York at Buffalo, 2006.
Title from PDF title page (viewed on Feb. 28, 2007) Available through UMI ProQuest Digital Dissertations. Thesis adviser: Thevanayagam, Sabanayagam. Includes bibliographical references.