Eshan Ganju

Matric suction measurements of compacted subgrade soils

The mechanical response of compacted subgrade soils is highly influenced by matric suction. This paper proposes a detailed procedure for matric suction determination, using the filter paper technique, in samples compacted in the laboratory using the standard Proctor procedure (ASTM D698-12). Special attention is paid to the quantification of errors associated with the measurement of matric suction in compacted soils using the filter paper technique. The procedure is illustrated through suction measurements made for two fine-grained soils obtained from Indiana road sites. Soil samples were compacted at the optimum water content (OWC) and also both wet and dry of optimum. Test results indicated that the matric suction was substantially different for soils compacted on the dry side of the OWC and at the OWC. Matric suction measurements approached zero for soils compacted at water contents greater than the OWC + 2%. At the OWC, the soil with the lowest plasticity index but highest clay content developed higher matric suction than the soil with the highest plasticity index. The proposed test procedure facilitates the estimation of matric suction of subgrade soils in the laboratory.

Experimental investigation of matric suction in compacted fine-grained soils

ABSTRACT This paper presents the results of laboratory tests performed to study the effect of compaction properties on the development of matric suction in four different soils (three soils were classified as silty clay and one was classified as silty sand). The soils were compacted at the optimum water content (OWC) and wet and dry of optimum according to the standard Proctor procedure (ASTM D698-12). For each sample, matric suction measurements were made using the filter paper technique. When the silty clay soils were compacted at a water content less than the OWC, a significant increase in matric suction was observed. For all the soils tested, matric suction values were very low when the compaction water contents were greater than the OWC. Plots of matric suction vs. compaction water content [ranging from (OWC-2%) to OWC] were developed for the compacted silty clays. The test results indicate that there is a direct relationship between the compaction properties (i.e. compacted dry unit weight and water content) and the matric suction values retained in compacted silty clays. Preliminary relationships were proposed between the matric suction of compacted silty clays and water content based on the data collected in this paper.

Quality assurance and quality control of subgrade compaction using the dynamic cone penetrometer

Abstract The Dynamic Cone Penetrometer (DCP) is a device that is used in the construction industry for the assessment of in situ soil compaction quality. Over the past few decades, numerous correlations have been developed between the DCP test results and soil strength and stiffness parameters. This paper proposes a comprehensive set of criteria and recommendations for quality control (QC) of compacted subgrade that take into account the inherent statistical variability of DCP test results. For the development of the QC criteria, a new statistical methodology is used to extract representative test values from the raw field DCP test data. In order to use the proposed QC criteria, soils are first classified into two broad categories (fine-grained and coarse-grained) depending on their fabric and response to compaction efforts. Test results indicate that (i) for fine-grained soils, the DCP test values have good correlation with the plasticity index (PI), which is indicative of the type and amount of clay content of the soil and (ii) for coarse-grained soils, the DCP test values have good correlation with the optimum water content of the soil, which is directly related to its in situ density. DCP blow count correlation equations are presented for both soil categories. Recommendations for field DCP testing and data analysis are also provided to highlight the significance of the statistical distribution of the DCP test results in QC testing of compacted subgrade.

Moisture-Strength-Constructability Guidelines for Subgrade Foundation Soils Found in Indiana

One of the most important factors in earthwork-related design is the correct estimation of the water content of in situ soil because its mechanical response to loading and construction activities depends strongly on its water content at the time of construction. However, because the time between site investigation, design and construction phases varies substantially for any given project, initial estimates of soil water content may no longer apply. Occurrence of excessive soil moisture in the in situ soil at the time of construction leads to low strength, which inevitably results in constructability problems, particularly for fine-grained soils. If the strength of the foundation soil is too low, it is not capable of sustaining the loads due to construction activities. Changes in soil moisture over time have led to a larger number of change orders for INDOT. Once a change order is seen as necessary, INDOT engineers and contractors working at a jobsite have to then agree on how to proceed and spend extra time and effort to bring the water content of the in situ soil to the desired level or redo the design for current conditions before the construction process can actually begin. This report presents a methodology that can be used to estimate the water content of fine-grained soils (A-4, A-6 and A-7-6 according to the AASHTO classification system) found in Indiana near the ground surface (within the top 5 ft. [150 cm]) and to assess the impact of changes in water content of fine-grained soils on their constructability.

QA/QC of Subgrade and Embankment Construction: Technology Replacement and Updated Procedures

The Dynamic Cone Penetrometer (DCP) is a device that is used for the estimation of in situ compaction quality of constructed subgrades and embankments. It is a relatively inexpensive, light-weight and easy to use device that measures the dynamic penetration resistance of the compacted soil, from which an estimate of soil strength and stiffness characteristics can be made. Owing to its ease of use, many Departments of Transportation (DOTs) in the U.S. have employed the DCP in their compaction quality control procedures, and over the past few decades, extensive research has been carried out on the development of correlations between the results of the DCP test and the results of strength and stiffness tests performed on compacted soils (e.g., California bearing ratio, and resilient modulus). The objectives of this research are to refine DCP-based quality assurance (QA) and quality control (QC) correlations for compaction quality control developed by previous research studies carried out at Purdue for the Indiana Department of Transportation, especially focusing on i) grouping of the soils based on their mechanical response to the DCP loading, and ii) limiting the in situ moisture range of the soils used for development of correlations within -2% of the optimum moisture content of the tested soil. The factors outlined above are studied, and in particular, soil grouping is examined critically. The American Association of State Highway and Transportation Officials (AASHTO) (‘A-based’) classification employed previously for classification of soils is replaced by a new classification criteria specifically developed for the DCP test. Soils are grouped into one of the two categories of coarse-grained or fine-grained soils on the basis of the size of the dominant particle in the soil. The criteria developed for the classification of soil into one of these two categories is based on index properties of the soil, such as the standard Proctor maximum dry density, optimum moisture content, plasticity index (PI) and fines content (percentage passing 0.075 mm sieve size). For the purpose of refinement of the QA/QC correlations, extensive field and laboratory tests (more than 750 DCP tests) were carried out on soils found in Indiana to add to the existing database of DCP test results. The database was then statistically analyzed for extraction of the representative DCP test value (number of DCP blows required for a specific depth of penetration into the compacted soil) for different types of soil. Results show that the DCP test results for fine-grained soils have a good correlation with the PI, which is indicative of the clay content of the soil, while the DCP test results for coarse-grained soils have good correlations with the optimum moisture content of the soil, which is indicative of the targeted in situ density of the soil. Furthermore, a statistical analysis of the distribution of DCP blow counts in the field revealed that the mean of a minimum of 7 closely spaced tests is required to get a representative blow count of the compacted soil at a given location. More targeted testing is needed to assess the frequency of DCP testing required for larger areas.

Assessment of Site Variability from Analysis of Cone Penetration Test Data

Soil property values for use in geotechnical design are often estimated from a limited number of in situ or laboratory tests. The uncertainty involved in estimating soil properties from a limited number of tests can be addressed by quantifying the variability within individual soundings and of the collection of soundings at a site. It has been proposed that factors of safety or resistance factors used in design be linked to site variability. Site variability can be assessed by studying the correlation structure of in situ test data. The cone penetration test (CPT), which is a reliable and widely‐accepted in situ test, can be used for this purpose. Soil behavior type (SBT) charts are often used to obtain the subsurface soil profile from CPT parameters such as the cone resistance and the sleeve friction. A soil profile generation algorithm was developed in this research to generate a soil profile from an individual CPT sounding using two modified SBT charts. Soils are variable in both the vertical and horizontal directions. A vertical variability index (VVI) was defined to quantify variability in a CPT sounding. The average of the VVIs for all CPT soundings performed at a site is the site VVI. A site horizontal variability index (site HVI) was also developed, based on cross‐correlation between cone resistances, the cone resistance trend differences and the spacing between every pair of CPTs considered, to quantify the soil variability of a site in the horizontal direction. A site variability rating (SVR) system, integrating the vertical and horizontal site variability, was developed to assess the overall site variability. Depending on the SBT chart selected, the soil profile generated using the soil profile generation algorithm may be slightly different; however, the SBT chart effect on the variability indices that compose the SVR index is small. Close agreement was found between the SVRs obtained using the two SBT charts selected for this research. In order to illustrate the use of the algorithms for VVI and HVI calculations and SVR of sites, CPTs from across the state of Indiana were analyzed. CPT data were obtained from Purdues own database, Indiana Department of Transportations (INDOT’s) data repository and the U.S. Geological Survey (USGS) website. Site variability is calculated for specific depths of interest. For example, that depth of interest will be shallower for shallow foundations than for deep foundations. Site variability rating maps (SVR maps) for various depths of interest were constructed for the state of Indiana, illustrating the potential use of the site variability assessment methodology. An optimal sounding spacing calculation methodology was also developed to make the site investigation process more efficient, cost‐effective and reliable.