Browsing by Subject "Expansive soils"
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Item Analytical, experimental, and field evaluations of soil-geosynthetic interaction under small displacements(2016-08) Roodi, Gholam Hossein; Zornberg, Jorge G.; Stokoe, Kenneth H; Gilbert, Robert B; Prozzi, Jorge A; Taleff, Eric MThe increasing use of geosynthetics in stabilization of pavement systems under traffic loads and environmental changes requires proper understanding of the mechanisms that govern the soil-geosynthetic interaction. Significant research has already been conducted on the soil-geosynthetic interaction under ultimate conditions, which is relevant to reinforcement of retaining walls and steep slopes. However, little research has been undertaken to investigate the properties and mechanisms that govern the soil-geosynthetic interaction under small displacements, which is relevant to applications such as the geosynthetic stabilization of pavement layers. While characterization of the maximum geosynthetic strength (e.g., tensile strength or pullout resistance) is relevant for the design of soil-geosynthetic systems under ultimate conditions, proper design properties in systems where geosynthetics are used to control deformations should involve characterization of the stiffness of soil-geosynthetic composite. The objective of this research is to develop a better understanding of the soil-geosynthetic interaction under small displacements using analytical, experimental, and field evaluations. Three studies were conducted on different aspects of soil-geosynthetic interaction under small displacements: (1) Analytical and experimental evaluations of the soil-geosynthetic composite (SGC) model using large-scale soil-geosynthetic interaction tests, (2) analytical and experimental evaluations of soil-geosynthetic interaction using small-scale soil-geosynthetic interaction tests, and (3) field evaluation of soil-geosynthetic interaction under small displacements. Each study provides lessons and conclusions on specific aspects investigated in that study. Collectively, they suggest that the analytical model proposed in this study provides a good basis towards predicting the general performance of geosynthetic-stabilized pavements. The analytical formulation of the SGC model indicates that soil-geosynthetic interaction under small displacements can be characterized by the stiffness of soil-geosynthetic composite ( ), which is the slope of the linear relationship defined between the unit tension squared (T2) versus displacements (u) in each point along the active length of a geosynthetic. The linearity and uniqueness of the relationship between the unit tension squared (T2) and displacements (u) throughout the active length of specimens tested in a comparatively large soil-geosynthetic interaction device were experimentally confirmed. Overall, the experimental results from the large-scale soil-geosynthetic interaction tests were found to be in good agreement with the adopted constitutive relationships and with the analytical predictions of the SGC model. Evaluation of experimental results from tests conducted to assess repeatability indicated that the variability of the estimated values for the constitutive parameters ( and ) and the stiffness of soil-geosynthetic composite ( ) are well within the acceptable ranges when compared to variations of other soil and geosynthetic properties. Suitability of the assumptions and outcomes of the model was also confirmed for a variety of testing conditions and materials. Evaluation of the experimental data obtained from a subsequent experimental program involving small-scale soil-geosynthetic interaction tests indicates that although the assumptions of the analytical model do not fully conform to the conditions in a small-scale test, experimental results confirm the linearity and uniqueness of the relationship between the unit tension squared (T2) and the displacements (u) throughout the specimen. Evaluation of the results obtained from small- and large-scale interaction tests on five geosynthetics with a range of properties indicates that both large and small testing scales can be used for comparative evaluation of the stiffness of soil-geosynthetic composite among geosynthetics. However, since the stiffness values obtained from the two testing scales were found to be different, the stiffness values from the large-scale soil-geosynthetic interaction tests should be suitable for design purposes, while values from the small-scale interaction tests should be suitable for specification and comparison purposes. Evaluation of the long-term performance of full-scale paved test sections under both traffic and environmental loads indicates that stabilization with geosynthetics contributes to improving the road performance under both loading conditions. The benefits derived from using geosynthetics under traffic loads were realized by reducing the total length of rut or rutting depth. On the other hands, the benefits from using geosynthetics under environmental loads in roads founded on expansive subgrades were realized by mitigating the percentage of longitudinal cracks appears on the road surface. The latter benefits were found to be more pronounced towards the end of dry seasons, when longitudinal cracks tend to develop. Comparison among the performances of geosynthetic-stabilized test sections under environmental loads indicate that the benefit provided by geosynthetics correlates well with the stiffness of soil-geosynthetic composite ( ) characterized in the laboratory. Geosynthetic products with comparatively larger were found to lead to a comparatively better field performance.Item Determination of potential vertical rise in expansive soils using centrifuge technology(2015-08) Snyder, Larson Mackenzie; Zornberg, Jorge G.; Cox, Brady RExpansive soils are a significant issue in Central Texas due to a high potential to shrink and swell which leads to cracking of roadways. A significant amount of research has been conducted on expansive soils, which has led to the development of direct and indirect methods to determine a soil’s swelling potential. The methods for direct measurement of the swell potential are typically both time consuming and expensive, which has led to the underutilization these methods. Indirect methods, which use index geotechnical properties to predict the swelling behavior of a soil, are empirically based correlations that are only approximations that don’t take into account variables such as the mineralogical composition of the soil and include the Texas Department of Transportation's (TxDOT) approach, Tex-124-E, which is based solely on Atterberg Limits and grain size distributions to determine the potential vertical rise of an expansive deposit beneath a pavement system. The purpose of this study is to develop an approach that both directly measures an expansive soil’s swelling potential using centrifuge technology (DMS-C) and determines a potential vertical rise (PVR) for use in site characterization. This study consists of eleven soils sampled from ten sites in Bexar, Atascosa, and Guadalupe Counties of the San Antonio TxDOT district to determine the PVR using the DMS-C and Tex-124-E approaches. Soil characterization tests were conducted including Atterberg Limits and compaction tests, as well as, over 300 specimens tested in the centrifuge testing program. The centrifuge testing program consisted of compacting samples into the double infiltration setup at initial conditions of 3% dry of optimum moisture content and 100% relative compaction and testing the samples at three separate artificial g-levels that correlate to three effective stresses to generate a swell-stress curve that was defined over a range of stresses typically found in the active zone. The results from the centrifuge tests for samples from each site are verified with the traditional free swell tests (ASTM D4546.) At each site, the swell-stress curve and stresses for the soil profile were used to determine the PVR for the DMS-C approach. From the results, seven of the sites received a high or severe degree of concern for potential damage to the pavement. Of these seven sites, six of the sites correlated to soils derived from the Navarro/Marlbrook Formation, which is a major geologic formation in both the San Antonio region as well as the rest of Central Texas east of the Balcones Fault zone. The same stresses, as well as, the liquid limit, plastic limit, and moisture content are used to predict the PVR with the traditional Tex-124-E approach. These results were analyzed and compared to the values to the PVR from the direct measurements taken in the DMS-C approach for each site. From the comparisons, the approximate prediction of PVR for Tex-124-E does not correlate to the direct measurements of swelling results to determine the DMS-C approach. Furthermore, the characterization the swell potential using the centrifuge for PVR calculation with the DMS-C approach was proven to be expeditious and can lead to a significant amount of savings by reducing maintenance and repair of damage. Thus, the DMS-C approach should be implemented into the protocol for the determination of potential vertical rise of expansive soils to more accurately determine whether a given location will be problematic.Item Effect of fabric on the swelling of highly plastic clays(2014-05) Armstrong, Christian Philip; Zornberg, Jorge G.Expansive soils are extremely problematic in transportation projects, and significant research has been done into examining the effect of moisture content changes and index properties on the swelling of soils. However, little has been reported on the effect of soil structure, or fabric, on swelling. The purpose of this study is to examine the effect of the soil fabric on swelling while, at the same time, validating a new set-up for a centrifuge testing program developed over the course of the project to allow for testing of undisturbed specimens. Testing to examine fabric was performed using two methods at the same effective stress, the conventional swelling test, ASTM D4546, and a new double infiltration approach in a centrifuge, on specimens of the Cook Mountain clay which were either compacted in the testing set-up or trimmed into cutting rings from soil compacted via ASTM D698, the Standard Proctor test. Specimens were compacted either dry of optimum to create a flocculated soil structure or wet of optimum to create a dispersed soil structure. Specimens were tested at their as-compacted moisture content or at a moisture conditioned moisture content to remove the effect of the initial moisture content. The results show that soils with a dispersed structure tended to swell more, over a longer time frame, and with a higher amount of secondary swelling in relation to soils with a flocculated structure when tested using the same initial moisture content. The strong influence of the initial moisture content on swelling was also verified. Further, soil specimens prepared at a comparatively high dry density for a given fabric and initial moisture content were found to swell more than soils prepared at a comparatively low dry density. The new centrifuge set-up, involving submerged specimens, was validated and was found to produce similar swelling results as those obtained from the ASTM D4546 tests. In addition, the new centrifuge approach was found to be more expeditious and results in less secondary swelling than the conventional ASTM approach.Item Predicting the behavior of a drilled shaft wall retaining highly expansive soil(2016-05) Helwa, Ali Mohamed; Gilbert, Robert B. (Robert Bruce), 1965-; Zornberg, Jorge G; Stokoe, Kenneth H; Tassoulas, John L; Wang, Shin-TowerA full scale drilled shaft retaining wall was constructed in the highly expansive soil of Manor, Texas, to advance our understanding of the behavior of walls in highly expansive soils. The wall was monitored for a total period of four years; during the monitored period the state of Texas experienced severe drought conditions and the retained soil was inundated via a manmade pond. The monitored wall did not experience a point of fixity, instead, the wall experienced global movement towards the excavated side. Analytical predictions of the wall during short-term and long-term conditions miss-predicted the deflection and bending moment profiles, and could not estimate the wall behavior during transition state towards the long-term conditions. The Reese wall was simulated in a numerical model using the Finite Element method. A framework is developed in this study that can describe the swelling behavior of soil. The framework relies on two soil properties, first, a relationship between effective degree of saturation and effective stress, second, a relationship between stiffness, effective stress and voids ratio. Comparison between measured and predicted deflection and bending moment profiles showed that the proposed framework could result in reasonable deflection and bending moment predictions during dry and inundated saturation conditions. The predicted short-term deflection and bending moment profiles best matched the measured profiles when a constitutive model that accounts for small strain stiffness nonlinearity was adopted. The numerical model was used to segregate the superimposed wall deflection profile obtained during long-term conditions. The study concluded that the short-term conditions accounts for 20%, dissipation of the excess pore-pressures accounts for 30%, the additional hydrostatic pressures accounts for 10%, saturation change related factors accounts for 15%, and change in soil properties on the excavation side accounts for 25% of the total deflection. Parametric analyses concluded that the short -term and long-term behaviors of the Reese wall are not very sensitive to building stiffer and deeper walls . The long-term behavior of the Reese wall is sensitive to construction season, the hardening properties of soil, and the relationship between effective stresses and effective degree of saturation .Item A subsurface investigation in Taylor clay(2011-08) Ellis, Trenton Blake; El Mohtar, Chadi Said; Gilbert, Robert B.A comprehensive field and laboratory investigation at the location of the Lymon C. Reese Research Wall is presented. Soil at the site is a stiff, fissured and heavily overconsolidated clay from the Taylor Group. Index properties such as Atterberg limits and clay fractions were used with common empirical guidelines to assess the qualitative swell potential. The soil's compressibility and strength characteristics were difficult to measure in the lab, owing to the stiff soil's secondary structure. Measured values were compared to well established correlations and test results from similar soils sampled from locations near the present test site. Cyclic swell tests were to predict the soil's lateral swell potential after multiple cycles of wetting and drying. Empirical guidelines indicated the soil has a "high" to "very high" swell potential. This was validated by the swelling that was observed during consolidation and cyclic swell tests. The soil's drained and undrained strengths were both rather large, often more typical of rock than soil. The stress history was not evident from consolidation results, either due to disturbance, cementation or extreme overconsolidation. The hydraulic conductivity was particularly elusive, again due to the soil's secondary structure.Item Use of geotextiles with enhanced lateral drainage in roads over expansive clays(2015-05) Garcia Delgado, Ivan Enrique; Zornberg, Jorge G.; Bhasin, AmitExpansive clays are very abundant across the central United States in general and in the state of Texas in particular damages induced by expansive clays have been reported to reach several billions of dollars per year. Volume changes in expansive soils due to change in their moisture content varies has caused significant cracking in roads and has resulted in costly maintenance projects over the lifetime of these roads. In Texas, expansive soils have been often treated with lime stabilization, which is not always possible, and in some cases by removing and replacing them with nonexpansive soils, which can be very costly. Recently, geosynthetic reinforcements have been incorporated in roads founded on expansive clays to make the structure stiffer and less prone to cracking. A new geotextile, which is capable of providing enhanced lateral drainage through capillarity has been recently develop. Facilitating moisture redistribution would be a feasible approach for roads on expansive clays as they may lead uniform vertical displacements resulting in minimized cracking in the asphalt layer. Eight test sections with different geotextiles were constructed on State Highway 21 in Bastrop, Texas. The road is founded on expansive clays. A number of geotextiles, including one with enhanced lateral drainage capabilities, were incorporated to 500 feet long test sections. All sections were equipped with sensors to monitor moisture beneath the geotextiles and were periodically surveyed to document pavement distresses. Results showed that the geotextile with enhanced lateral drainage was able to maintain a uniform moisture content along the length of the soil in contact with this geosynthetic. Condition surveys showed that the geotextile with enhanced lateral drainage prevented cracking in the portion of the pavement above it. As expected, cracks often developed in areas of the pavement section beyond the extent of the geotextile. This suggested that the geotextile was capable of providing enhanced lateral drainage, although placement of the geotextile over the full width of the road (and not only under the shoulder) would be necessary to minimize the development of longitudinal cracks. In conclusion, the geotextile with enhanced lateral drainage can deal with pavements on expansive clays by improving the pavements long-term performance.