Faraz S. Tehrani

A Comparison of Density-Based and Modulus-Based In Situ Test Measurements for Compaction Control

This paper presents and compares the results from a series of in situ density-based and modulus-based compaction control tests that were conducted during construction of a coarse-grained soil embankment. To simulate current construction practices as closely as possible, these in situ tests were performed on an embankment that was constructed and compacted by a vibratory smooth drum roller in a series of lifts. During construction of the test embankment, the compaction process was monitored using the nuclear density gauge device and a number of alternative modulus-based devices, including the lightweight deflectometer, the dynamic cone penetrometer, and the soil stiffness gauge. Comparison of the in situ test results illustrates that point-to-point variability in measured values is quite common for each of these test devices, to varying degrees for the different devices that were examined. Consistent increases in measured soil properties from pass-to-pass of the compactor are considered critical for proper control of the compaction process, with some devices faring better than others in this area of performance. The measured modulus values correlated poorly to the nuclear density gauge dry unit weights, and also correlated poorly with other measured moduli when the results from different devices were compared. This lack of agreement was likely caused by a variety of factors including: variations in the magnitude of strain and rate of strain application between the different modulus-based devices, variations in the tested volume between the different devices, and variations in the local moisture content and matrix suction conditions. Finally, the effect of soil moisture content was shown to be critically important when interpreting the results from modulus-based tests, and the utility of multiple regression analyses was explored for including this effect.

The Effect of Water Content on Light Weight Deflectometer Measurements

The application of modulus-based in-situ testing methods has been widely increasing for control of the compaction quality of earthwork construction in recent years. Since their introduction to the QA/QC process, it has been observed that there are several factors that influence the measured modulus of the soil, such as moisture content, influence depth, and temperature. To achieve more reliable test results, these factors should be accounted for in interpretation of the data. As noted by previous researchers, water content is one of the most important properties that affects the modulus measurements of compacted soil. To explore the sensitivity of measured modulus-based in-situ test results to the effect of compaction water content, a field study was performed in the State of Delaware in the summer of 2008. Two Light Weight Deflectometers (LWDs) were used in the study to measure compacted soil modulus values, one with a 300 mm contact plate diameter and one with a 200 mm plate diameter. The fill material tested during this study was a poorly graded sand with silt (SP-SM). The purpose of the current paper is to demonstrate the sensitivity of the measured soil modulus values to fluctuation in soil moisture content in the field, and to discuss possible approaches for interpreting this type of variable LWD data.

Resilience of MSE Walls with Marginal Backfill under a Changing Climate: Quantitative Assessment for Extreme Precipitation Events

AbstractClimate change is expected to alter statistics of extreme events in the future. Adapting geotechnical infrastructure to a changing climate necessitates quantitative assessment of the potent...

Analysis of Axial Loading of Pile Groups in Multilayered Elastic Soil

AbstractThis paper presents a semianalytical solution for axially loaded pile groups in multilayered, linear-elastic soil profiles. The piles in the group can have circular, square, or rectangular cross sections. The soil displacement surrounding a pile group is linked to the axial displacement experienced by each of the piles in the group. The method is based on assigning a displacement decay function to every pile in the group and then summing up for all piles in the group the product of the axial displacement of each pile and its associated decay function. The governing differential equations describing the response of the soil and piles are derived by applying the principle of virtual work and calculus of variations to the pile–soil system. The governing differential equations predicting the response of the piles are solved analytically using the method of eigenvalues and eigenvectors, whereas the differential equations describing the soil decay functions are solved numerically using the finite-differ...

Comparison of Density-Based and Modulus-Based In Situ Tests for Earthwork Quality Control

A series of density-based and modulus-based quality control tests were performed to evaluate and compare the performance of a number of in situ testing methods for control of soil compaction. This paper highlights the results from the study that focus on three in situ testing tools for compaction control: the lightweight deflectometer (LWD), the dynamic cone penetrometer (DCP), and the nuclear density gauge (NDG). In situ tests conducted using these tools were performed during construction of an embankment built with a coarse-grained soil in a series of lifts. Comparison of the in situ test results shows that that there is a notable variability in measured soil properties for each of the testing methods that were performed. All three in situ tests showed an increasing trend in the measured properties from pass-to-pass of the compactor, which indicates the suitability of these tests for proper control of the compaction process. However, the soil moduli measured by the LWD and DCP correlated poorly to the NDG dry unit weights. This lack of agreement was likely caused by a variety of factors, including variations in the tested soil volume between these devices and variations in the local moisture content and matric suction conditions. The effect of soil moisture content was shown to be particularly significant when interpreting the results of LWD and DCP tests, and the usefulness of multivariate regression analyses was explored for including this effect.

Physical Modeling of Cone Penetration in Layered Sand

AbstractThis paper presents the results of cone penetration tests performed in layered, uniformly graded silica sand samples prepared inside a calibration chamber specifically designed for digital ...

Visualization of Active Mode of Failure behind Flexible Walls under Pure Rotation Using Digital Image Correlation

Determining the shape of the slip surface is an important step in the calculation of earth pressure on a retaining wall. Fundamental assumptions are made regarding the geometry of shear bands and slip surfaces in methods that are widely used in the analysis and design of retaining walls. Experimental visualization allows observation not only of how the slip surface develops but also how it evolves with wall movement and soil deformation. This paper presents results of experiments performed to study the formation of shear bands in sand retained by a flexible wall. Particular attention was paid to the effect of the initial sand density on the distribution of volumetric and maximum shear strains as a function of wall displacement. To achieve these goals, a small-scale test box was built to model a plane-strain condition for a retaining wall system. A model flexible wall was hinged to the bottom face of the test box. The wall was then backfilled with silica sand using the air pluviation method. The pluviator openings were such that different relative densities were achieved (Dr=15% and 77%). A displacement controlled mechanism was employed to apply outward displacement on the crest of the wall away from the backfill material to model an active-pressure mode of failure. The Digital Image Correlation (DIC) technique was used to observe shear band evolution with progressive outward movement of the wall. The results highlighted the suitability of this technique to visualize and characterize the formation of slip surfaces in laboratory-scale experiments.

Implementation of Limit States and Load Resistance Design of Slopes

A logical framework is developed for load and resistance factor design (LRFD) of slopes based on reliability analysis. LRFD of slopes with resistance factors developed in this manner ensures that a target probability of slope failure is not exceeded. Three different target probabilities of failure (0.0001, 0.001 and 0.01) are considered in this report. The ultimate limit state for slope stability (formation of a slip surface and considerable movement along this slip surface) is defined using the Bishop simplified method with a factor of safety equal to unity. Gaussian random field theory is used to generate random realizations of a slope with values of strength and unit weight at any given point of the slope that differ from their mean by a random amount. A slope stability analysis is then performed for each slope realization to find the most critical slip surface and the corresponding driving and resisting moments. The probability of slope failure is calculated by counting the number of slope realizations for which the factor of safety did not exceed 1 and dividing that number by the total number of realizations. The mean of the soil parameters is adjusted and this process repeated until the calculated probability of failure reaches to the target probability of failure. Optimal resistance and load factors are obtained by dividing the resisting and driving moments corresponding to the most probable ultimate limit state by the nominal values of resisting and driving moments. The main goal of this study was to provide specific values of resistance and load factors to implement in limit states and load resistance design of slopes in the context of transportation infrastructure. This report discusses the concepts of load and resistance factors, target probability of failure and the ultimate limit state equation in the context of slope stability analysis. It then presents a detailed algorithm for resistance factor calculation by using reliability analysis. Six cases of real slopes designed and constructed by the Indiana Department of Transportation (INDOT) are examined by using undrained shear strengths in order to illustrate the LRFD procedure and validate the recommended resistance and load factors.