VALIDATION AND REFINEMENT OF CHEMICAL STABILIZATION PROCEDURES FOR PAVEMENT SUBGRADE SOILS IN OKLAHOMA – VOLUME II (FHWA-OK-11-02(2))

For projects involving a chemically stabilized layer as a part of the structural design of the pavement, it is typical to conduct a mix design to assess the additive content needed to achieve a certain unconfined compressive strength (UCS) and to determine the resilient modulus (MR) of the stabilized soil. However, there is considerable uncertainty regarding whether the strength and resilient modulus of the field stabilized soil are consistent with design values determined in the laboratory. A purpose of this study was to compare results of field tests and laboratory tests on chemically stabilized soil at different curing times to assess whether a relationship exists between field and laboratory measurements. The goal was to determine if a field testing method could be used to assess whether the strength and stiffness in the field are consistent with laboratory measurements used for design. In addition, numerous other physical and chemical tests were conducted on the soils with an aim to enhance interpretation of UCS and MR and comparisons to field tests. Field testing included three devices that are portable, quick, and easy to use. These devices include: the Dynamic Cone Penetrometer (DCP), the PANDA penetrometer, and the Portable Falling Weight Deflectometer (PFWD). Laboratory testing was conducted to determine the unconfined compressive strength (UCS) and resilient modulus (MR) of laboratory specimens prepared using additive contents that were similar to samples taken from field test locations. To estimate the additive contents in the field samples, a mineralogical test method known as “whole rock analysis” using x-ray fluorescence (XRF) was investigated. Samples mixed in the laboratory were tested to determine the UCS and MR after curing times of 1, 3, 7, 14, & 28 days. Field tests were conducted at each of the five test sites after curing times that fell within the 1 to 28 day time frame; however, because of construction logistics and weather conditions it was not always possible to match the curing times of laboratory tests or conduct field tests over the full 28 days at every site. Nevertheless, sufficient field data was collected to make meaningful comparisons with laboratory test data. Mineralogical, electrical, chemical, physical and index property testing (Atterberg Limits, linear shrinkage, Total Specific Surface Area (SSA), etc.) was conducted on the natural soils and the stabilized cured samples to observe the relationship of these properties to stabilized soil strength and stiffness. The effect of curing temperature on stabilized strength gain of soils was also examined. The UCS samples were cured at both room temperature (68°F) and at 40°F, which is the minimum ambient temperature specified for chemical stabilization of subgrades. Correlations were examined and involved basic soil measurements (mineralogical, electrical, chemical and index properties) and mechanical properties (UCS and MR), and field test results (DCP, PANDA, and PFWD). Some of the various correlations developed show promise as methods for predicting UCS and MR based on more simply measured soil properties. Relationships between field and laboratory tests also show promise as a means to evaluate strength and stiffness gains in field stabilized soils. Additionally, lower curing temperatures were observed to have an adverse affect on more reactive clayey soils.Final Report, October 2007-December 2010N

VALIDATION AND REFINEMENT OF CHEMICAL STABILIZATION PROCEDURES FOR PAVEMENT SUBGRADE SOILS IN OKLAHOMA – VOLUME I FINAL REPORT (FHWA-OK-11-02 2207)

Additions of byproduct chemicals, such as fly ash or cement kiln dust, have been shown to increase the unconfined compression strength (UCS) of soils. To be considered effective, the soil must exhibit a strength increase of at least 50 psi. Many current design methods base chemical additive percentage recommendations on the results of Atterberg Limit tests which do not always properly characterize the soil stabilization response. For example, Atterberg limit tests may reveal the same AASHTO classification of soil at two different sites, but one site may require more than twice the additive percentage of a chemical to achieve the desired UCS increase. This study examined the relationship between soil physico-chemical parameters and unconfined compression strength in various fine-grained soils to determine if other soil parameters have significant effects on predicting the strength of a soil treated with a given additive and additive content. The results of this study suggest that the surface area and shrinkage properties of the soil, combined with the Atterberg limit results, present a better picture of a given soil and will allow for better predictions of the amount of chemical stabilizer needed to adequately stabilize the soil.Final Report, October 2007-December 2010N

REAL TIME MONITORING OF SLOPE STABILITY IN EASTERN OKLAHOMA

There were three primary objectives of the proposed research. The first was to establish a comprehensive landslide database, the second was to create a first- cut regional landslide map and the third was to relate safe and stable constructed slope geometry to soil type and geologic setting with site-based in-situ monitoring and modeling experiments. Accomplishing the project objectives involved collecting historical and current landslide information from around the state, as well as climate, rainfall history, geology and topography information for recorded landslide sites. From this comprehensive database, a landslide susceptibility map was derived. In addition, in situ measuring equipment was used to monitor a selected slide to verify a site-based landslide modeling system.Final report, October 2011-December 2013N

Using X-Ray Fluorescence to Assess Soil Subgrade Stabilization Competency During Construction Inspection

ODOT SPR Item Number 2310A large portion of transportation corridor projects use lime and other calcium-based stabilizers to reduce soil plasticity, increase shear strength, reduce compressibility, and reduce volume changes when subjected to variations in water content. While design and construction practices for subgrade stabilization have been standardized, there is no extant quality control measures, particularly measures that are timely, cost effective, and accurate. Therefore, this study was undertaken to evaluate the ability of portable handheld X-ray fluorescence (PXRF) to detect calcium stabilizers in soil samples. The accuracy of Whole Rock XRF (WXRF) was evaluated and used to verify a variety of PXRF measurement techniques, including scan duration, particle size, and sample type. Additionally, the viability of PXRF in measuring soil sulfate content was investigated. It was concluded that the WXRF method is highly accurate in detecting both calcium and sulfate in soils, with average magnitude of deviation between WXRF-determined stabilizer content and actual stabilizer content of 0.3%. While the PXRF method found to be less accurate than WXRF, the average magnitude for clay samples was 2.1% and 10.6% for sand samples. Moreover, the existing calibration libraries do not require slope correction for calcium contents in lower ranges, but do not accurately measure calcium content in higher ranges. The calibration libraries for sulfate detection are able to accurately detect sulfate contents in the range of 0 to 8%, but require individual calibration coefficients

Scale effects of shallow foundation bearing capacity on granular material

This project investigated the scale effects of shallow foundation bearing capacity on granular materials to evaluate the observed decreasing trend of the bearing capacity factor, Nγ, with increasing footing width, B, for a given sand at a given density. Model and prototype scale square and circular footing tests ranging in size from 25.4 mm to 914.4 mm, were performed on two compacted sands at three relative densities. Results of model scale footing tests indicate that the bearing capacity factor, Nγ , is dependent on the absolute width of the footing for both square and circular footings. Direct shear box tests were performed in three different size shear boxes. Results of the direct shear tests indicate that there is a substantial scale effect; i.e., as the box size increases the friction angle, &phis;, decreases. The results of the direct shear box tests also show that the Mohr-Coulomb strength envelope is non-linear. The curvature shows higher strengths at low normal stresses and lower strengths at high normal stresses. High strengths at low confining stresses found in the direct shear box can help to explain the scale effect seen in small footings. Both the footing tests and direct shear tests were modeled using a lower and higher order Finite Element model. New Nγ values versus friction angle were generated using Drucker-Prager (1952) plane strain conditions. The values match the results of the footing tests from this study extremely well, while all other previously published solutions overpredict Nγ at high friction angles. The higher order model utilized the parameter, internal length, which has been shown to be a function of panicle shape, degree of interlocking and binder stiffness. The internal length parameter accurately predicted the direct shear, model and prototype footing test results. Although the scale effect of shallow foundation bearing capacity on granular materials still cannot be quantified, it can be predicted with the addition of the internal length parameter and the understanding that particle interlocking at low stresses significantly influences the results of the strength of a small scale footing and of a shear test

Calcium-Based Stabilizer Induced Heave in Oklahoma Sulfate-Bearing Soils (FHWA-OK-11-03 2210)

The addition of lime stabilizers can create problems in soils containing sulfates. In most cases, lime is mixed with expansive soils rendering them non-expansive; however, when a certain amount of sulfate is present naturally in expansive soils, the lime reacts with gypsum to create an expansive mineral ettringite and causes the soil to become more expansive. The goal of this study was to provide a more accurate sulfate determination method and determine the physical, chemical, and mineralogical characteristics of Oklahoma soils that may predict vulnerability to adverse reactions from calcium-based stabilizers and attempt to relate these characteristics to free swell. Through this project, it was found that the current method of testing soil sulfate, Oklahoma Highway Department’s (OHD) L-49, resulted in substantial sulfate solubility issues and did not in all cases accurately determine sulfate concentrations in soils. Several bench studies were performed to understand the solubility problems and modifications were proposed.Final Report, October 2008-December 2010N

A method to predict desiccation crack depth in a compacted clayey soil

Estimation of crack depths due to desiccation of clayey soils is needed to predict changes in mechanical or hydraulic properties in the cracked layer. Desiccation cracks are associated with increasing suction due to moisture loss accompanied by restrained shrinkage, which results in tensile stresses in near surface soil layers. A simple analytical method is presented to predict crack depths in compacted clayey soil due to changes in matric suction with depth. The model equation is based on the Hookean elastic equation relating incremental strain to incremental stress and incorporates two stress state variables including net normal stress and matric suction. Input to the model includes the tensile strength and elastic parameters, and to complete the prediction of crack depth, the suction change profile of interest is needed. The method validity was investigated by comparing predicted crack depths to those observed in soil compacted in a bench scale apparatus for studying desiccation cracking. Tensile strength and elastic properties were determined from tests conducted on soil during desiccation under approximate uniaxial conditions. Predicted crack depths were obtained based on changes in suction interpreted from water content sensors at various depths in the soil bed and compared favorably to observed desiccation crack depths.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

Evaluation and Field Verification of Strength and Structural Improvement of Chemically Stabilized Subgrade Soil (FHWA-OK-07-09 2195)

Often subgrade soils exhibit properties, particularly strength and/or volume change properties that limit their performance as a support element for pavements. Typical problems include shrink-swell, settlement, collapse, erosion or simply insufficient strength. A common approach to subgrade soil support or stability problems involves chemical modification or stabilization with additives such as lime (hydrated or quick), fly ash (Class C from lignite coal), cement kiln dust (CKD) or Portland cement. Other additives· are available, but this group constitutes the major products or by-products used on roadway construction in Oklahoma. The type and amount of chemical additive is dependent on the purpose or function of the treated material (i.e., improved physical properties or improved strength) and selection is based on aceepted or standardized procedures. Questions then arise with regard to chemically treated subgrade soils about the rate of development and ultimate value of improvement. The purpose of this research is to develop relationships between rate of development and magnitude of strength (or physical property) improvement for chemically treated subgrade soils. The purpose of this interim report is to summarize the results of recent research into any existing relationships and current experience with chemically treated soils as used in the AASHTO-MEPDG. No active or recently completed research involved with characterizing Level 2 and 3 design input parameters for AASHTO-MEPDG was found. The closest research topics involved the use of field and laboratory data to calculate structural layer coefficients. The research confirmed that when chemically treated subgrade soil layers were included in the structural design of the pavement, the layer coefficients were conservative and the resulting pavement layer thicknesses were reduced, as was the construction cost. Most of the recent research evaluation of performance of various chemical additives. Most of the evaluations were between different chemicals, i.e. lime vs. fly ash vs. CKD, etc. The research reaffirmed that chemicals such as lime, fly ash, CKD, and cement generally perform well in most fine-grained soils as long as the type and amount of the chemical are selected to meet the purpose of the treated layer and problem soils are identified and considered in the selection process.Final reportN