Subsurface Characterization of Flexible Pavements Constructed Over Expansive Soil Subgrades and Selection of Suitable Rehabilitation Alternatives

Expansive soils present significant engineering challenges, with annual costs associated with repairing structures constructed over expansive soils estimated to run into several billion dollars. Volume changes in expansive soil deposits induced by fluctuations in the moisture content can result in severe damage to overlying structures. A flexible pavement section near the Western Border of Idaho has experienced recurrent damage due to volume changes in the underlying expansive soil layer; traditional stabilization methods have provided partial success over the years. The main objective of this research effort was to characterize the problematic soil layer contributing to the recurrent pavement damage and propose suitable rehabilitation alternatives. An extensive laboratory test matrix was carried out to characterize soil samples collected from underneath the problematic pavement section. Laboratory tests showed that the problematic expansive soil deposit was often at depths greater than 6 ft. (183 cm) from the pavement surface. Potential Vertical Rise (PVR) values calculated for ten boreholes strategically placed along the problematic pavement section closely matched with the surface roughness profile observed in the field. Liquidity Index (LI) calculations indicated that the active-zone extended to a depth of least 11 ft. (335 cm) from the pavement surface, and therefore, most of the heaving likely originates from soil layers as deep as 11 ft. (335 cm) from the pavement surface. Clay mineralogy tests indicated the presence of high amounts of Montmorillonite that can lead to significant volume changes. Moreover, high sulfate contents were detected in soil samples obtained from several of the boreholes, indicating a potential for sulfate-induced heaving upon chemical stabilization using calcium-based stabilizers. Based on findings from the laboratory testing, it was concluded that chemical stabilization or shallow treatment alternatives are not likely to be successful in mitigating the recurrent differential heave problems. A mechanical stabilization approach using geocells was proposed as a likely rehabilitation alternative for this pavement section. By dissipating the heave-induced stresses over a wider area, this reinforcement configuration was hypothesized to significantly reduce the differential heave. Finite-element models of the pavement section comprising six alternative geocell-reinforced configurations were prepared using the commercially available package, ABAQUS®. Moisture swelling and suction properties for the expansive soil deposit were established in the laboratory and were used in the numerical model to simulate the swelling behavior. Results from the numerical modeling effort established that placing two layers of geocell within the unbound granular base layer led to the highest reduction (~60%) in the differential heave. Placing a single layer of geocell, on the other hand, reduced the differential heave magnitude by approximately 50%. A single layer of geocell was therefore recommended for implementation to achieve the optimal balance between pavement performance and construction costs

Characterization of the Line Configuration in Wired Communication Networks

This thesis presents an algorithm to identify the full configuration of a wired transmission line from its frequency response. It is assumed that the line can have up to two bridged taps. Each bridged tap divides the main line to two segments, and with two bridged taps there will be at most three segments in the main line. Furthermore, each segment of the main line and the bridged taps can have three different gauges. The problem of characterizing the line configuration is concerned with identifying each segment (main line segments and bridged taps) in terms of its length and gauge. The problem is solved in two phases: initialization and optimization. The algorithm can be used as single ended line testing, which means the line can be characterized by performing a simple test from the central office. Simulations demonstrate the accuracy of the proposed method

Effect of the Field-Stress State on the Subgrade Resilient Modulus for Pavement Rutting and IRI

The new Mechanistic-Empirical Pavement Design Guide (MEPDG) uses the subgrade resilient modulus (MR) as the key input parameter to represent the subgrade soil behavior for pavement design. The resilient modulus increases with an increase in confining pressure, whereas, for an increase in deviatoric stress, it increases for granular soils and decreases for fine-grained soils. The value of MR is highly stress dependent, with the stress state (i.e., bulk stress) a function of the position of the materials in the pavement structure and applied traffic loading. Applying excessive vertical stress at the top of the subgrade without knowing the appropriate stress state can result in permanent deformation. In situ stress must be calculated so the correct resilient modulus can be determined. To facilitate the implementation of MEPDG, this study develops a methodology to select the appropriate subgrade resilient modulus for predicting rutting and IRI. A comprehensive research methodology was undertaken to study the effect of in situ or undisturbed subgrade MR on pavement performance using the MEPDG. Results show that MR obtained from in situ stress is approximately 1.4 times higher than the MR estimate from NCHRP-285. Thus, the in situ stress significantly affects the calculation of subgrade MR and, subsequently, the use of MR in the predicted rutting, with IRI using the AASHTOWare pavement mechanistic-empirical design. Results also show that the pavement sections were classified as in “Good” and “Fair” conditions for rutting and IRI, respectively, considering in situ MR

Estimation of Resilient Modulus for Coarse-Grained Subgrade Soils from Quick Shear Tests for Mechanistic-Empirical Pavement Designs

The resilient modulus represents the subgrade soil stiffness, and it is considered one of the key material inputs in the Mechanistic Empirical Pavement Design Guide (MEPDG). The resilient modulus is typically estimated in the laboratory using a repeated load cyclic triaxial test, which is complex and time consuming to perform. Technical ability is also required to prepare the test specimens, particularly for coarse-grained soils. Therefore, there is a need to estimate the resilient modulus of coarse-grained soils from other simpler tests. In this study, correlations of resilient modulus with soil index properties and quick shear (QS) test results (quick shear strength, stress at 1% strain and tangent modulus) were developed for remolded coarse-grained soils, collected from different geographic regions in South Carolina. The developed models showed good correlations of resilient modulus to tangent modulus and soil index properties. The average tangent, modulus obtained from 30% and 50% of maximum stress of the QS tests, moisture content, optimum moisture content, dry unit weight, and maximum dry unit weight showed a statistically significant effect on estimating the resilient modulus for coarse-grained subgrade soils. The validation study confirms that the developed models can be used for predicting the resilient modulus for South Carolina coarse-grained soils

Forensic Investigations into Recurrent Pavement Heave from Underlying Expansive Soil Deposits

Pavement sections constructed over expansive soil deposits often exhibit excessive distresses due to volume changes in the underlying soil strata. Differential movements within these deposits resulting from fluctuations in the moisture content manifest themselves in the form of localized heaves on the pavement surface. A pavement section near the western border of Idaho has experienced recurrent damage due to volume changes in the underlying expansive soil layer; traditional treatment methods such as lime stabilization and moisture barrier installation have provided partial relief over the years. A recently concluded forensic research study at Boise State University investigated the causes for failure of earlier treatment methods. This study involved extensive laboratory characterization of expansive soil samples collected from underneath this pavement section to identify location of the problematic soil strata, and to propose suitable rehabilitation measures. Laboratory characterization included tests such as moisture content, Atterberg limits and One-Dimensional swell test to determine the potential vertical rise (PVR) and establish approximate active zone. Laboratory test results indicated that the most expansive soil deposits were at a depth of at least 1.83 m from the pavement surface. PVR values calculated closely matched with the surface profile trends observed in the field. In addition, the soluble sulfate tests performed on various soil samples indicated that sulfate heaving could be a problem for calcium-based stabilizer. Based on the findings, the research team proposed that the pavement section be reinforced using a flexible mechanical system that dissipates the swell pressures originating from the underlying clay layers