Marika Santagata

Liquefaction Mitigation Using Bentonite Suspensions

AbstractOttawa sand specimens premixed with 0, 3, and 5% bentonite by dry mass of sand were tested under undrained static and cyclic loading to investigate the effects of bentonite on the static and cyclic shear strength of the sand. The results show that allowing the bentonite to hydrate within the sand pore space increases the cyclic resistance of the sand. For the same skeleton relative density and cyclic stress ratio, cyclic tests on specimens with sufficient hydration times showed a significant increase in the number of cycles required for liquefaction compared with clean sand. When the specimens were allowed an extended postconsolidation aging period, the cyclic resistance increased further. Resonant column and cyclic triaxial tests showed that this is a result of the delay in the generation of excess pore pressure in the presence of the bentonite suspension in the pore space. The improvement in cyclic behavior does not occur at the expense of the static resistance of the soil under working loads be...

Cyclic Response of a Sand with Thixotropic Pore Fluid

Saturated specimens of Ottawa sand prepared with 0%, 3% and 5% bentonite by dry mass of sand are tested under cyclic loading to investigate the effects of bentonite on the cyclic response. For the same skeleton relative density and cyclic stress ratio (CSR), the cyclic tests on the sand-bentonite mixtures show a significant increase of the number of cycles required for liquefaction compared to the clean sand. This is caused , as observed in resonant column tests, by an increase of the elastic threshold due to the presence of bentonite, which delays the generation of excess pore pressure. Such behavior can be explained by the rheological properties of the pore fluid. Oscillatory tests conducted with a rheometer on bentonite slurries show that for shear strains as large as 1% these materials exhibit elastic behavior with a constant shear modulus. Moreover, due to the thixotropic nature of the bentonite slurries, their storage modulus shows a marked increase with time. This observation is consistent with the increase in the liquefaction resistance of the sand-bentonite mixtures with time also observed in cyclic triaxial experiments.

Combined Resonant Column and Cyclic Triaxial Tests for Measuring Undrained Shear Modulus Reduction of Sand With Plastic Fines

This paper investigates the undrained shear stiffness of sand-bentonite specimens (with 0 %, 3 %, and 5 % bentonite by dry mass of the sand) prepared at the same skeleton void ratio (Drsk ¼ 35 % to 40 %) using a dry pluviation technique. The experimental program consisted of (1) small strain tests using a resonant column apparatus and (2) large strain tests using a cyclic triaxial apparatus. The resonant column tests were performed at three confining stress levels (50, 100, and 193 kPa) under drained and undrained conditions. A comparison of the shear modulus reduction with shear strains for both drained and undrained conditions is presented; the effects of changes in effective stresses and the rate of mod- ulus reduction as a function of the effective stress are discussed to describe the discrepancy between the two sets of data. The results show a mar- ginal decrease in Gmax for specimens with bentonite, which is attributed to the presence of bentonite at the sand grain contacts. However, the presence of bentonite increases the linear elastic threshold, particularly in the case of undrained tests, in which a noticeable delay in excess pore pressure generation was measured. The strain level required in order to initiate excess pore pressure generation increased with increasing bentonite content. A similar trend was noted in cyclic triaxial tests, in which, for a given strain, specimens with bentonite generated lower excess pressure than sand specimens tested under similar conditions. Finally, a combined normalized G/Gmax curve from both tests is presented for specimens with 0 %, 3 %, and 5 % bentonite at 100 kPa.

Microstructure of Sand-Laponite-Water Systems using Cryo-SEM

This paper describes the use of cryo-scanning electron microscopy (cryo-SEM) to characterize the microstructure of concentrated laponite suspensions (3% by mass of the water), which are being considered for treating liquefiable deposits. In cryo-SEM a sample is cooled rapidly in nitrogen slush, sublimated to remove the unbound water and imaged under cryogenic conditions (~ -130 °C); hence it remains close to its natural state and dehydration is avoided. This is ideal for materials such as laponite suspensions, which have very high water content (>3000%) and a delicate structure that would be damaged as a result of shrinkage. Cryo-SEM observations at magnifications varying from 250x to 40kx indicate that laponite suspensions prepared with deionized water have a cellular microstructure formed by elongated cells of a size several orders of magnitude greater than the clay particles, consistent with the structure of an attractive gel. Images of suspensions prepared with water of increasing ionic strength (10 -4 M NaCl – 1 M NaCl) show an increased number of cross-links and more densely packed walls, and ultimately the formation of clay aggregates. The paper also presents cryo-SEM images of sand-laponite mixtures prepared using two different methods: a) dry-mixing sand and laponite and then permeating the resulting specimen with water; and b) permeating a clean sand specimen with a laponite suspension. The images highlight differences between the two specimens and provide direct insight into the micro-mechanism responsible for the observed macroscale geotechnical properties.

Identification of Low-Organic-Content Soils: An Engineering Approach

From a geotechnical engineering perspective, the presence of organic matter in soils can often be a concern, because of its negative impact on many mechanical properties and its potential interference with soil stabilization reactions. For this reason, many regulating agencies have strict limits on the maximum allowable organic content in subgrade soils and backfills, requiring that it fall below a threshold value in the 2 %−7 % range. Methods currently used in practice for the identification of organic soils and for the quantification of organic matter have shortcomings when applied to soils with organic matter content less than ∼10 %−15 %. For such soils, the loss on ignition often overestimates the true organic content, and the criteria employed by the ASTM and the American Association of State Highway and Transportation Officials classification systems are generally insensitive to the presence of these amounts of organic matter. This paper presents the results of a study conducted to identify a practical approach for the identification of soils with organic content in the 3 %−15 % range. The study explored the relationship between true organic content, as determined through the dry combustion test, and the results of three tests: loss on ignition, Atterberg limits (with and without oven drying), and the colorimetric test. Tests were conducted on a number of natural soil samples, select clay minerals, and three types of laboratory-prepared soils. It was found that the combined use of these tests is effective in screening soils for the presence of percentages of organic matter in the 3 %−15 % range. Results of thermal gravimetric analyses, differential scanning calorimetry, and x-ray diffraction analyses performed on select tests provide an improved scientific understanding of the results.

Project Implementation: Classification of Organic Soils and Classification of Marls—Training of INDOT Personnel

This is an implementation project for the research completed as part of the following projects: SPR‐3005 ‐ Classification of Organic Soils and SPR‐3227 – Classification of Marl Soils. The methods developed for the classification of both soils have been incorporated in Indiana Department of Transportation (INDOT) standard specification 903.05 and 903.06 respectively. Both projects included recommendations for implementation that reflected input from the project administrator and study advisory committee. A specific recommendation from both projects was that INDOT soil technicians be trained to perform the required tests and classify soils based on the revised classification systems. This project was initiated to carry out the implementation of those recommendations. The project scope includes development of training material for instruction about the performance of the revised classification tests and methods, training to pertinent INDOT personnel, integration of the revised classification system into INDOT’s standards, and establishment of a resource database for future training of INDOT personnel. Within the general scope outlined above,the specific objectives of the proposed work were to: 1) administer training to select INDOT personnel and interested representatives from the geotechnical consulting/construction community; and 2) develop training materials to be used by INDOT to train additional personnel. These two general objectives were accomplished through four specific tasks: 1) Collection of Sample Soils for Testing and Classification; 2) Development of Training Material (a PowerPoint presentation with concise instructional handouts; supporting classification examples from a variety of soils; and a short manual summarizing the classification system for both soils with supporting examples); 3) Delivery of Training Sessions for INDOT personnel, as well as representatives from select geotechnical consultants and contractors; and 4) Production of a Training Video.

Verification of the Enhanced Integrated Climatic Module Soil Subgrade Input Parameters in the MEPDG

At the beginning of 2009, the Indiana Department of Transportation (INDOT) adopted the MechanisticEmpirical Pavement Design Guide (MEPDG) method to study long-term pavement performance. The implementation of this new design approach led to difficulties for the pavement to pass the INDOT performance criteria: in particular, pavement roughness (International Roughness Index (IRI)) for hot-mixed asphalt (HMA) and faulting for jointed plain concrete pavement (JPCP) when A-6 or A-7-6 soils were considered as foundation soils. This study focuses on investigating the influence of the soil input parameters in the Enhanced Integrated Climatic Model (EICM) on the prediction of the soil resilient modulus (MR) in the MEPDG. A total of four sites located around the state of Indiana are used to propose/validate the observations and conclusions made in the research.

Engineering Properties of Marls

The term “marl” is used to designate soft, carbonate‐rich, fine‐grained soils, which pose concerns related to both settlement and stability. Despite the prevalence of marls in Indiana and the concerns associated with their behavior, very limited work has been done to study the engineering properties of these soils. This was the motivation for this research project, which involved two primary activities: a) the creation of a map and database of existing information on marl deposits in Indiana; and b) an in‐depth characterization of the properties of a marl deposit in Daviess County, which was considered representative of similar deposits encountered in Indiana. The marl database was generated using ArcGIS 10.0.from information available at the Indiana Department of Transportation (INDOT), and involved mining data from over five thousand boreholes. The second part of the project involved field tests (seismic cone penetration tests, standard penetration tests, field vane shear tests), and laboratory experiments (index tests, incremental and constant rate of strain consolidation tests, and K₀‐consolidated undrained triaxial tests) conducted on high quality Shelby tubes samples. Additionally, the mineralogy and the microstructure of the soil were studied in detail. The laboratory tests reveal that the deposit was not homogeneous as was initially anticipated, but was, instead, formed by two types of soils that repeat in horizontal thin layers. These two soils, referred to as ‘soil M’ and ‘soil C’, are both characterized by very high calcium carbonate contents but show distinct index and engineering properties, that may be ascribed to differences in mineralogy and composition. This stratification is not detected by the field tests. The consolidation tests show that the deposit has an overconsolidation ratio (OCR) less than 2 and compressibility parameters markedly dependent on stress level, as typical of sensitive soils. K₀‐consolidated undrained compression triaxial tests show that both soils exhibit normalized behavior, and that the relationship between strength and stress history is well described by the SHANSEP equation (although the SHANSEP parameters differ for the two soils). Comparison of the field data and laboratory results provides the means to validate published correlations for interpretation of the geotechnical properties of marls from field results. For the site examined, correlations to estimate shear wave velocity, stress history, and undrained strength from cone penetration test (CPT) results are identified. Implementation recommendations are provided for soil identification, sampling and specimen preparation, interpretation of filed data, and preliminary design.