Browsing by Subject "Soil liquefaction"
Now showing 1 - 6 of 6
- Results Per Page
- Sort Options
Item Development of constitutive models for linear and nonlinear shear modulus and material damping relationships of uncemented soils(2018-06-22) Wang, Yaning, Ph. D.; Stokoe, Kenneth H.; Cox, Brady R.; Rathje, Ellen M.; El Mohtar, Chadi S.; Gilbert, Robert B.; Wilson, Clark R.For many decades, hundreds of soil samples recovered from North America and other parts of the world have been tested in the Soil and Rock Dynamics (SRD) Laboratory at The University of Texas at Austin using combined resonant column and torsional shear (RCTS) equipment. Dynamic soil property relationships determined in this study include: (1) linear relationships of log G[subscript max] - log σ₀ and log D[subscript min] - log σ₀, and (2) nonlinear relationships of G/G[subscript max] - log γ and D - log γ. These relationships are important in geotechnical design, especially for geotechnical earthquake engineering and soil dynamics. More recently, higher resolution measurements have been made over shear strains ranging from about 1×10⁻⁶ to 0.3 %. Most recently, measurements of the initiation of pore-water pressure generation in nearly to saturated poorly-graded sands and low-plasticity silts have been conducted during torsional shear (TS) testing. A significant database has been developed involving all types of uncemented soil specimens tested in the SRD Laboratory since the late 1980's. The effects of soil type, index properties, density, confining state and strain level on the dynamic properties have been quantified through multivariable regression analyses performed on the database. Four sets of empirical models for log G[subscript max] - log σ₀, G/G[subscript max] - log γ, log D[subscript min] - log σ₀ and D - log γ relationships are presented in this study. These models are composed of simple equations that incorporate the key parameters controlling linear soil behavior as well as nonlinear soil behavior. The process used to develop the parameters in the models employed a residual analysis procedure in a staged manner. Both the mean values of the empirical models and the uncertainty associated with these values are presented. Those empirical models show significant improvements in accuracy and applicable range compared to previous models. The application ranges of these models and sparse portions of the associated database are also discussed for the sake of appropriate utilization and future updates. The influence of number of loading cycles, confining pressure and soil type (sand or low-plasticity silt) on the initiation of pore-water pressure generation during TS testing are briefly discussed. A preliminary model for the r[subscript u] - log γ relationship is presented. This model focuses on determination of the threshold strain at which pore pressure generation is triggered, γ[subscript t][superscript pp], and the early stages leading towards soil liquefaction.Item Direct in-situ evaluation of liquefaction susceptibility(2014-05) Roberts, Julia Nicole; Stokoe, Kenneth H.Earthquake-induced soil liquefaction that occurs within the built environment is responsible for billions of dollars of damage to infrastructure and loss of economic productivity. There is an acute need to accurately predict the risk of soil liquefaction as well as to quantify the effectiveness of soil improvement techniques that are meant to decrease the risk of soil liquefaction. Current methods indirectly measure the risk of soil liquefaction by empirically correlating certain soil characteristics to known instances of surficial evidence of soil liquefaction, but these methods tend to overpredict the risk in sands with silts, to poorly predict instances of soil liquefaction without surface manifestations, and fail to adequately quantify the effectiveness of soil improvement techniques. Direct in-situ evaluation of liquefaction susceptibility was performed at a single site at the Wildlife Liquefaction Array (WLA) in Imperial Valley, California, in March 2012. The project included a CPT sounding, crosshole testing, and liquefaction testing. The liquefaction testing involved the measurement of water pressure and ground particle motion under earthquake-simulating cyclic loading conditions. The objective of this testing technique is to observe the relationship between shear strain in the soil and the resulting generation of excess pore water pressure. This fundamental relationship dictates whether or not a soil will liquefy during an earthquake event. The direct in-situ evaluation of liquefaction susceptibility approach provides a more accurate and comprehensive analysis of the risks of soil liquefaction. It also has the ability to test large-scale soil improvements in-situ, providing researchers an accurate representation of how the improved soil will perform during a real earthquake event. The most important results in this thesis include the identification of the cyclic threshold strain around 0.02% for the WLA sand, which is very similar to results achieved by other researchers (Vucetic and Dobry, 1986, and Cox, 2006) and is a characteristic of liquefiable soils. Another key characteristic is the 440 to 480 ft/sec (134 to 146 m/s) shear wave velocity of the soil, which are well below the upper limit 656 ft/sec (200 m/s) and an indication that the soil is loose enough for soil liquefaction to occur. The third significant point is that the compression wave velocity of the sand is greater than 4,500 ft/sec (1,370 m/s), indicating that it is at least 99.9% saturated and capable of generating large pore water pressure due to cyclic loading. These three conditions (cyclic threshold strain, shear wave velocity, and compression wave velocity) are among the most important parameters for characterizing a soil liquefaction risk and must all be met in order for soil liquefaction to occur.Item Field evaluation of large-scale, shallow ground improvements to mitigate liquefaction triggering(2017-10-26) Roberts, Julia Nicole; Stokoe, Kenneth H.; Andrus, Ronald D.; Cox, Brady R.; Rathje, Ellen M.; Wilson, Clark R.Much of the devastation wrought by the 2010-2011 Canterbury Earthquake Sequence (CES) in Christchurch, New Zealand, was caused by extreme levels of liquefaction-induced damage to structures with shallow foundations. In response to this disaster, the New Zealand Earthquake Commission (EQC) funded a large study known as the Ground Improvement Trials to evaluate and identify shallow ground improvement methods that are not only effective at increasing the soil’s resistance to soil liquefaction, but are also cost effective and practical to build for lightweight structures. Of the nine ground improvement methods included in the trials, three were selected for extensive analysis in this dissertation. These three ground improvement methods are the Rapid Impact Compaction (RIC), the Rammed Aggregate PiersTM (RAP), and the Low-Mobility Grout (LMG). At three test sites along the Avon River in Christchurch neighborhoods that were among the worst hit by liquefaction-related damage, full-scale test panels of natural soil and ground-improved soil were constructed and evaluated using a variety of in situ test methods. The analysis in this dissertation primarily relies on data from excavation trenching, cone penetrometer testing (CPT), direct-push crosshole testing (DPCH), and shake testing with T-Rex. These tests capture changes in density and stiffness, and therefore liquefaction resistance, due to the ground improvement methods in comparison to the natural soil. Shake testing with T-Rex is further able to define the relationship between cyclic shear strain and the generation of excess pore pressure that ultimately determines whether or not a soil will liquefy under cyclic loading. Under this framework, the effectiveness of each of the three ground improvement methods is evaluated and discussed.Item Improvements to field liquefaction testing with large mobile shakes(2021-04-06) Zhang, Benchen; Stokoe, Kenneth H.; Cox, Brady R.; Rathje, Ellen M.; Wilson, Clark R.; Mo, Yi-LungEarthquake-induced soil liquefaction can cause devastating damage to buildings and infrastructure such as bridges, embankment dams, and retaining structures. For the past 15 years, large, mobile shakers operated by researchers at the University of Texas at Austin have been used to investigate the liquefaction susceptibility of granular and fine-grained soils directly in the field. The original test procedure involved dynamically loading the surface of a natural soil deposit horizontally with one shaker (T-Rex) in stages of increasing load levels. In each loading stage, simultaneous measurements of dynamic motions and pore-water pressures at multiple depths in the soil were performed with a sensor array embedded in the soil below the loading baseplate of T-Rex. The objectives of the field shaking test were to determine excess pore-water pressure generation and nonlinear shear moduli as functions of number of loading cycles over a wide range of induced cyclic shear strains. During recent field liquefaction studies, three new improvements have been developed and incorporated into the existing test procedure to improve the capability, success, and accuracy of the field shaking tests. The first improvement to the field shaking test is focused on increasing the maximum, shaking-induced strain level and is composed of two new techniques. The first technique involves shaking at or near the resonant frequency, f [subscript r], of the shaker-and-site system. The second technique is to combine a second shaker (Rattler) with the first shaker (T-Rex). The two shakers are parked side by side and synchronized to apply dynamic loading simultaneously so that a larger range of shear strains can be induced. The second improvement to the field shaking test is a new test method named liquefaction screening test. The liquefaction screening test is designed to quickly identify the existence and extent of potentially liquefiable soil at the site. A liquefaction screening sensor is pushed to a series of increasing depths and, at each depth, measurements of the horizontal particle velocity (used to estimate shear strain) and excess pore-water pressure generation at one or more force levels of shaking are conducted. This approach greatly enhances proper placement of liquefaction instrumentation array. The third improvement to the field shaking test aims to better characterize the linear and nonlinear shear moduli during the shaking tests. Two mobile shakers, Rattler and Thumper, are used to excite the instrumented soils at different loading frequencies simultaneously, which permits measurement of the small-strain shear modulus to be performed many times during each larger strain cycle. These new approaches have been developed and validated in recent field liquefaction projects at multiple locations in the United States. The results of the improved field shaking tests are presented and discussed in this dissertation.Item Laboratory investigation into evaluation of sand liquefaction under transient loadings(2015-12) Kwan, Wing Shun; El Mohtar, Chadi Said; Kramer, Steven L; Rathje, Ellen M; Cox, Brady R; Kallivokas, Loukas FThe current laboratory procedures for evaluating liquefaction potential are still the same as 40 years ago, with minor updates. The complex seismic loading motions are simplified to a series of uniform harmonic sinusoidal loading cycles with amplitudes related to the maximum amplitude of a given ground motion; liquefaction resistance is then evaluated as the load generating liquefaction in a predefined number of harmonic loading cycles. The simplified methods of loading and resistance characterizations are a crude proxy and provide limited information in predicting the time of liquefaction triggering and therefore, the expected effects/damage of seismic events. Specific details of the time of soil liquefaction within a ground motion can be better understood from laboratory testing. Among the available element-level types of cyclic testing, cyclic simple shear (CSS) tests are the most popular and commonly used. The CSS tests provide a satisfactory simulation of seismic induced in-situ stresses. A testing program consisting of a series of multi-stage undrained direct simple shear tests was performed using the hydraulically-actuated GCTS cyclic simple shear apparatus. The apparatus had been modified and upgraded so that it is capable of applying user-specified, transient loading histories to Nevada Sand soil samples. Reconstituted specimens were prepared by the wet pluviation method at two different densities, 40% and 70%, followed with back-pressure saturation and K0 consolidation. The shearing phase was conducted in three distinctive stages: (1) Scaled transient stress histories, (2) modulated sinusoid with a taper-up shape stress histories, (3) static monotonic loads. All shearing stages were performed under continuous undrained conditions. This research program had two major motivations. The first motivation is to provide element-level tests subjected to transient loadings, so that the soil responses of excess pore pressure generation and shear strain along the time domain can be measured. The transient loading was selected from a suite of ground motions with different spectral and temporal characteristics to cover a wide range of possible ground motions. The second motivation is to investigate the performances of four Intensity Measures (IMs): CAV5, Arias Intensity, Normalized Energy Demand and PGA magnitude. These IMs were proven to be more efficient predictors of soil responses than peak acceleration. The experiments provide a database that can systematically illustrate the response of liquefiable materials subjected to transient ground motions before and after liquefaction; such a database was virtually non-existent prior to this study. Therefore, the data generated in this study supports the development of improved and more informative procedures for the evaluation of liquefaction potential, the effects of liquefaction, post-liquefaction responses, and more accurate constitutive models for liquefiable soils.Item Pore pressure generation characteristics of sands and silty sands: a strain approach(2005) Hazirbaba, Kenan; Rathje, Ellen M.Liquefaction of saturated granular soils during earthquakes has been one of the most important problems in the field of geotechnical earthquake engineering. It is well established that the mechanism for the occurrence of liquefaction under seismic loading conditions is the generation of excess pore water pressure. Most of the previous research efforts have focused on clean sands. However, sand deposits with fines may be as liquefiable as clean sand deposits. Previous laboratory liquefaction studies on the effect of fines on liquefaction susceptibility have not yet reached a consensus. This research presents an effort to find a unified picture regarding the effect of fines content on excess pore water pressure generation. Different from earlier studies that placed an emphasis on characterization of liquefaction in terms of the induced shear stress required to cause liquefaction, this study adopted a strain approach because excess pore water pressure generation is controlled mainly by the level of induced shear strains. This approach was first proposed by Dobry et al. (1982). Multiple series of strain-controlled cyclic direct simple shear and cyclic triaxial tests were used to directly measure the excess pore water pressure generation of sands and silty sands at different strain levels. The soil specimens were tested under three different categories: a) at a constant relative density, b) at a constant sand skeleton void ratio, and c) at a constant overall void ratio. The results from each of these groups were examined. In addition, laboratory measured pore water pressures of clean sands were compared to in situ measured values. The findings from this study were used to develop insight into the behavior of silty sands under undrained cyclic loading conditions. In general, beneficial effects of the fines were observed in the form of a decrease in excess pore water pressure and an increase in the threshold strain. However, pore water pressure appears to increase when enough fines are present to create a sand skeleton void ratio greater than the maximum void ratio of the clean sand. The comparison between laboratory and in situ measurements indicated that larger pore water pressure was generated in situ.