Evaluating the liquefaction and reliquefaction behavior of a carbonate sand

Despite extensive amount of research on liquefaction analysis of sands, reliquefaction behavior and cyclic liquefaction resistance of cohesionless soils under a repeated cyclic load have not received the same amount of consideration. This is important as most soil deposits in high seismic areas have been subjected to repeated number of earthquakes. This paper presents series of laboratory cyclic simple shear tests on specimens of a local carbonate sand from London, Ontario. Sand specimens are reconstituted at relative densities of 25%, 45%, and 65% and subjected to effective vertical stresses of 50, 100, 200, 400, and 600 kPa. Reconstituted samples are subjected to two consecutive cyclic loads and sand behavior and liquefaction resistance are examined and compared following both cyclic loads. Reliquefaction is simulated by unloading the specimens after the first cyclic load and re-consolidating the specimen under the same initial vertical stress. A similar cyclic load is then re-applied on the sand specimen. The results show a moderate increase in relative density after re-consolidation, which is greater for loose sand specimens. However, a complicated change in cyclic liquefaction resistance is observed for loose and dense specimens which also varies with stress level. Despite the larger increase in relative density, the loose specimens exhibit a dramatic decrease in cyclic resistance following the first cyclic load. The reduction in cyclic resistance increases with decreasing effective stress (from 600 to 50 kPa). On the other hand, dense samples experience an increase in cyclic resistance, particularly at lower stress levels

Prediction of Static Liquefaction Landslides

Static liquefaction failure of sloping grounds has resulted in significant damages to built structures and even loss of lives. The principal aim of this research is to relate static liquefaction behavior of cohesionless soils to a measurable threshold from the field. Based on a very large number (893) of undrained laboratory shear tests on cohesionless soils collected from the past literature, a threshold triggering excess pore water pressure is introduced in this study above which static liquefaction failure occurs. The effect of variations in the direction and relative magnitudes of principal stresses associated with different modes of shear and ground slopes on static liquefaction failure of cohesionless soils is characterized by empirical relationships of the triggering excess pore water pressure ratio with these variables. The triggering pore pressure ratio can be employed as a more precise criterion for detecting liquefaction triggering and landslide warning in instrumented slopes of saturated cohesionless soils

The Importance of Mineralogy and Grain Compressibility in Understanding Field Behavior of Failures

In this paper, we examine the role of grain mineralogy and compressibility, sample preparation, and shear strain/displacement levels on the shearing behavior of sands using undrained triaxial and constant volume ring shear tests in an attempt to explain some discrepancies observed between field and laboratory behavior. As expected, preparation by moist tamping can produce specimens that are contractive throughout shear, while counterparts prepared using pluviation exhibit dilative behavior at intermediate shear strain/displacement levels (i.e., after initial yield). However, both triaxial and ring shear tests illustrate that some sands consisting of more compressible minerals can exhibit entirely contractive behavior regardless of the sample preparation method. These preliminary tests suggest that laboratory testing of pure quartz sands may result in potentially misleading conclusions regarding the field behavior of mixed mineral soils involved in many liquefaction flow failures and long run-out landslides. Furthermore, grain crushing at larger displacements (larger than those that can be achieved in the triaxial device) results in net contractive response regardless of the sample preparation method or the grain mineralogy. Grain crushing has been observed in shear zones formed during a few well-documented long run-out landslides. The combination of these factors: grain mineralogy and compressibility, particle damage and crushing, and shear zone formation may help to explain some discrepancies observed between field and laboratory behavior of sands

Characterization of a Carbonate Sand based on Shear Wave Velocity Measurement

Numerous studies have been carried out on the dynamic behavior of sands. However, few studies have investigated the dynamic characteristics of carbonate sands. This paper presents series of laboratory simple shear tests on specimens of a local carbonate sand from London (ON). Besides monotonic and cyclic shearing, the dynamic behavior of the sand is also characterized by measuring the velocity of shear waves travelling through the specimens. Drained and undrained shearing behavior of specimens with a wide range of relative density and consolidation stresses are tested. Shear wave velocity is found to vary with effective overburden stresses by an average power of 0.25. Maximum shear modulus (Gmax) is also computed from the shear wave velocity measurements and a correlation is developed between Gmax, effective stress, and void ratio for a carbonate sand. The critical state line of the carbonate sand established from the simple shear tests is used for determining the state parameter of each specimen and this is related to the shear wave velocity measured in the same specimen. Such a relationship can be employed for measuring the in-situ state of this carbonate sand. Cyclic resistances of the sand specimens are determined from cyclic shear tests. Combined with shear wave velocity data, these are compared with current liquefaction triggering curves

Earthquake Induced Excess Pore Water Pressures in the Upper San Fernando Dam During the 1971 San Fernando Earthquake

The excess pore water pressure developed in the Upper San Fernando Dam during the 1971 San Fernando Earthquake has been evaluated in several studies. Almost all of these studies indicate large excess pore pressure ratios developed only in the upstream and downstream shells which are not consistent with the limited deformation of the dam and the piezometer responses during the earthquake. In this paper, the construction and field observations of the behavior of the Upper San Fernando Dam are reviewed and a simple approach involving Newmark’s (1965) and Makdisi-Seed’s (1978) permanent deformation and limit equilibrium slope stability analyses are used to estimate the excess pore water pressures developed in the core and downstream shell areas during the earthquake for comparison with field measurements. The major differences of this analysis with previous studies lies in the assumptions regarding the selection of the failure plane, liquefiable zones, and mobilized shear strengths. The results explain the field piezometric observations and the limited displacement of the dam

Development of a new ring shear apparatus for investigating the critical state of sands

The present study investigates the shearing behavior of sands pertinent to liquefaction and critical state soil mechanics using an improved ring shear apparatus designed and constructed at the University of Illinois. Undrained/constant volume and drained triaxial compression and ring shear tests (sheared to about 30 m of shear displacement) were performed on two clean sands and one silty sand. Shear displacements localized when the peak friction angle was mobilized and subsequent shear displacements occur only within the shear band which was (10–14)×D50 thick. Considerable particle damage and crushing was observed within the shear band, particularly for dilative specimens, which led to volumetric contraction in the shear band. The critical state line (corresponding to the critical state after particle damage and crushing was complete) was much steeper and plotted below conventional critical state lines in e – log σ' space measured using triaxial tests. Both dense and loose sands reached this final state. Accordingly, two definitions of critical state of sands with and without particle damage are proposed. The critical state friction angle from the ring shear tests was independent of the initial sand fabric and decreased only slightly with stress level, becoming essentially constant at stresses larger than 100–200 kPa. Particle crushing induced in the ring shear tests increased the critical state friction angle by a few degrees by producing a wider grain size distribution and more angular particles. However, because some of the triaxial specimens likely did not reach a critical state, the mobilized friction angle from triaxial tests was influenced by the initial fabric of the sand. A constant critical shear strength was achieved at very large shear displacements (>750 cm) in the ring shear tests, and before this the shear resistance of sands was dependent on the amount of shear displacement and particle crushing. Yield strength ratios of contractive specimens ranged from 0.15 to 0.31, while the critical strength ratios of both contractive and dilative specimens decreased from a range of 0.04–0.21 (for the original sand) to 0.01–0.07 (for the crushed sand)

Reply to the Discussion submitted by Karim Kootahi on “Accuracy of determining pre-consolidation pressure from laboratory tests”

N/AThe accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Development of a New Ring Shear Apparatus for Investigating the Critical State of Sands

423 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2009.The present study investigates the shearing behavior of sands pertinent to liquefaction and critical state soil mechanics using an improved ring shear apparatus designed and constructed at the University of Illinois. Undrained/constant volume and drained triaxial compression and ring shear tests (sheared to about 30 m of shear displacement) were performed on two clean sands and one silty sand. Shear displacements localized when the peak friction angle was mobilized and subsequent shear displacements occur only within the shear band which was (10--14)xD50 thick. Considerable particle damage and crushing was observed within the shear band, particularly for dilative specimens, which led to volumetric contraction in the shear band. The critical state line (corresponding to the critical state after particle damage and crushing was complete) was much steeper and plotted below conventional critical state lines in e -- log sigma' space measured using triaxial tests. Both dense and loose sands reached this final state. Accordingly, two definitions of critical state of sands with and without particle damage are proposed.The critical state friction angle from the ring shear tests was independent of the initial sand fabric and decreased only slightly with stress level, becoming essentially constant at stresses larger than 100--200 kPa. Particle crushing induced in the ring shear tests increased the critical state friction angle by a few degrees by producing a wider grain size distribution and more angular particles. However, because some of the triaxial specimens likely did not reach a critical state, the mobilized friction angle from triaxial tests was influenced by the initial fabric of the sand.A constant critical shear strength was achieved at very large shear displacements (>750 cm) in the ring shear tests, and before this the shear resistance of sands was dependent on the amount of shear displacement and particle crushing. Yield strength ratios of contractive specimens ranged from 0.15 to 0.31, while the critical strength ratios of both contractive and dilative specimens decreased from a range of 0.04--0.21 (for the original sand) to 0.01--0.07 (for the crushed sand).U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD

ACCURACY OF DETERMINING PRE-CONSOLIDATION PRESSURE FROM LABORATORY TESTS

Shear strength and compressibility of fine-grained soils is strongly influenced by its stress history and the maximum (pre-consolidation) pressure (σ'p). Accurate determination of σ'p is thus critical for settlement and stability analysis involving fine-grained soils. Many graphical techniques are available for estimating σ'p from the interpretation of soil compression in laboratory consolidation (oedometer) tests. However, the accuracy of these methods has not been extensively proven or compared with each other. A series of 30 laboratory oedometer tests is carried out in this study based on controlled-rate of strain and incrementally-loaded testing techniques. Several Canadian clay specimens are subject to cycles of one-dimensional compression loading and unloading in order to produce a known stress history and σ'p. The imposed σ'p are compared with the predictions of 11 methods for determining σ'p. The accuracies of these methods are subsequently evaluated by comparing their predictions with σ'p imposed during the consolidation experiments. While these methods mostly overestimate σ'p, it is determined that a graphical approach based on the slopes of the virgin compression and recompression segments of soil consolidation curve provides the most accurate predictions of σ'p. The potential ranges of errors associated with the application of each method are also presented.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author