Naji Khoury

Correlation Between Resilient Modulus, Moisture Variation, and Soil Suction for Subgrade Soils

In recent years, interest in determining the influence of moisture changes on the resilient modulus (MR) of subgrade soils beneath a pavement has increased. This is because the 1993 AASHTO Guide for Design of Pavement Structures recommends using a single MR value. The design MR is expected to account for the seasonal variation in subgrade moisture content. This study was undertaken to evaluate the variation of MR with postcompaction moisture content and suction of selected subgrade soils in Oklahoma. A sandy soil (S-Soil) and a clayey soil (C-Soil) were used for laboratory testing. The C-Soil specimens to be subjected to wetting were prepared by a modified compaction method. The proposed method was expected to enhance the flow of water in a specimen during the wetting process. New laboratory procedures for wetting and drying of specimens were also introduced and were used to establish correlations among MR, moisture variation, and suction. Results indicate that MR-moisture content relationships for C-Soil exhibit a hysteretic behavior due to wetting and drying. A similar behavior was observed for S-Soil. The C-Soil was more susceptible than the S-Soil to moisture variation, as expected. It was also observed that changes in MR values and suction were influenced by the initial (compaction) moisture content.

Effect of Wet-Dry Cycles on Resilient Modulus of Class C Coal Fly Ash-Stabilized Aggregate Base

A laboratory study was undertaken to investigate the effect of wet-dry (W-D) cycles on low-quality aggregates stabilized with Class C coal fly ash (CFA). Resilient modulus (M r ), unconfined compressive strength, and elastic modulus were used to evaluate this effect. Cylindrical specimens stabilized with 10% CFA, cured for 3 and 28 days, and subjected to different W-D cycles were tested. The M r values of 28-day-cured specimens increased as W-D cycles increased up to 12, beyond which a reduction was observed. For 3-day-cured specimens, M r increased with the number of W-D cycles. W-D action produced a greater detrimental effect on 28-day-cured specimens than on 3-day-cured specimens. The M r values of 28-day-cured specimens subjected to 30 cycles were approximately 5% lower than the corresponding M r values of specimens without any W-D cycles. The M r values of 3-day-cured specimens subjected to 30 W-D cycles, however, increased approximately 55% compared with the corresponding M r values of specimens with no W-D cycles. Also, it was found that 12 to 30 W-D cycles could be considered adequate to have a noticeable negative effect on 28-day-cured specimens; however, more than 30 cycles are needed for 3-day-cured specimens. Additionally, the positive effect of curing time was more dominant on 3-day-cured specimens, and the detrimental effect of W-D cycles was more influential on 28-day-cured specimens.

Durability of stabilized base courses subjected to wet–dry cycles

A laboratory study was undertaken to evaluate the durability of cementitiously stabilized aggregate specimens subjected to wet–dry (W-D) cycles, representing a base course in a pavement structure. Specifically, cylindrical specimens of aggregates stabilized with 15% cement kiln dust (CKD), 10% class C fly ash, or 10% fluidized bed ash were prepared, cured for 28 days, and then subjected to W-D cycles prior to testing for resilient modulus (M r). Four aggregates were selected in this experimental study: (1) Meridian, (2) Richard Spur, (3) Sawyer and (4) Hanson. Results showed that the M r values decreased as W-D cycles increased up to 30, except for Sawyer specimens stabilized with 15% CKD. These specimens exhibited an increase in M r values with W-D cycles up to 8, beyond which a reduction occurred. The performance of the stabilized specimens is believed to depend upon SAF (Silica, Alumina and Ferric Oxide compounds) and free lime of the stabilizing agents. Also, the optimum moisture content (OMC) and maximum dry density (MDD), were found to be a good indicator of performance pertaining to stabilization and W-D action. A regression model correlating M r with W-D cycles, SAF, free lime, OMC and MDD offered statistically promising results.

Effect of Cyclic Suction History (Hydraulic Hysteresis) on Resilient Modulus of Unsaturated Fine-Grained Soil

This study explored the effect of hydraulic hysteresis on the resilient modulus (Mr) of subgrade soils. For that purpose, suction-controlled Mr tests were performed on compacted specimens along the primary drying and wetting and secondary drying and wetting paths. First, Mr tests were performed continuously on the same sample at various suction values along all of the paths (drying, wetting, etc.). A relationship between Mr and matric suction was obtained and identified as the resilient modulus characteristic curve (MRCC). MRCC results indicated that Mr increased with suction along the drying curve. Results along the primary wetting curve indicated a higher Mr than along primary drying and secondary drying curves. The second series of tests, referred to as virgin tests, were performed at suction values without subjecting the sample to previous Mr testing. Results indicated that Mr values were lower compared with results from the other type of test (i.e., with continuous Mr testing), and that Mr results were influenced by the continuous stress loading. However, Mr values along the wetting curves were still higher than for the drying curves for these virgin tests and therefore indicated that results were also influenced by hydraulic hysteresis (i.e., cyclic suction stress loading). New models to predict the MRCC results during drying and wetting were proposed on the basis of the soil–water characteristic curve hysteresis. The models favorably predicted the drying and then the wetting results with the use of the soil–water characteristic curve at all stress levels.

Modeling Resilient Modulus Hysteretic Behavior with Moisture Variation

AbstractThe changes in subgrade moisture conditions play an important role in the in-service performance of a pavement. These conditions are sensitive to environmental conditions, such as a rise in the water table, precipitation, freeze-thaw cycles, and wet-dry cycles, among others. Several studies have been conducted to develop relationships between the mechanical properties, namely, the resilient modulus (MR) of subgrade soils and moisture conditions. No studies, to the authors’ knowledge, have directly addressed the hysteretic behavior of MR with moisture conditions. This study evaluates the effect of moisture hysteresis on MR values of a subgrade soil following two different paths: (1) drying-wetting-drying path and (2) wetting-drying path. Initially, cylindrical specimens were prepared at approximately optimum moisture content and maximum dry unit weight. The moisture content in each compacted specimen was altered to achieve the target moisture content. Specimens were then tested for MR values. The l...

Effect of gradation and source properties on stability and drainability of aggregate bases: a laboratory and field study

A combined laboratory and field study was undertaken to evaluate the effect of gradation on resilient modulus (M R) and falling weight deflectometre (FWD) modulus (M FWD) as measures of stability and coefficient of permeability (k) as an indicator of drainability of aggregate bases. Aggregates from three different sources in Oklahoma and three gradations consisting of M-AASHTO #57, ODOT Type A and OKAA Type M were used. M R and k tests on compacted aggregate specimens were conducted in the laboratory. Also, a 152.4-m long test section was constructed with the aforementioned gradations and FWD and in-situ falling head permeability tests were conducted periodically. Results showed that denser gradations resulted in higher M R and M FWD and lower k values, both in the laboratory and in the field. M FWD and field k values were found to increase and decrease, respectively, with time, possibly due to traffic-induced compaction. Regression models were developed to estimate M R and k in terms of gradation and source properties of aggregates.

Soil Fused with Recycled Plastic Bottles for Various Geo-Engineering Applications

The emphasis of the present work is to study the mechanical properties of a new generation of geo-plastic products that can be used in various aspects of geo- engineering applications. The plastic-soil based geoplastic-material (PS) is produced by heating and mixing simultaneously recycled plastic bottles with soil at a specified temperature to melt the thermoplastic material so that a uniform fused mix is attained. This study is also directed to assess the potential of PS as an additive to stabilize open graded aggregate base. Laboratory results showed that PS properties varied with the types of soil. PS produced with sand exhibited unconfined compressive strength and indirect tensile strength values higher than PS produced with lean clay. Generally, specimens exhibited unconfined compressive strengths as high as ordinary concrete specimens. In addition, permeability and strength results of open-graded aggregates stabilized with PS indicated that such technology produces a drainable and a stable base for a better pavement performance with ecological and economic implications.

Performance of a Stabilized Aggregate Base Subject to Different Durability Procedures

This study examined the effects of two freeze-thaw (FT) laboratory procedures, FT-1 and FT-2, on stabilized aggregate specimens. Cylindrical specimens were stabilized with 10% Class C fly ash (CFA) cured for selected periods and then subjected to either FT-1 or FT-2 cycles. FT-1 and FT-2 procedures consisted of freezing specimens at −25°C for 24 h and thawing them at 21.7°C for another 24 h with a high relative humidity; the only difference was that FT-1 required a membrane around each specimen, while FT-2 required no membranes during freezing and thawing. After being subject to freezing and thawing actions, specimens were then tested for resilient modulus ( MR ) and unconfined compressive strength (UCS) values. Results showed that the MR values of 28-day cured specimens increased as FT-1 cycles increased up to 12, beyond which a reduction in MR values was observed. For 3-day cured specimens the MR increased with FT-1 cycles up to 30. The UCS values of 28- and 3-day stabilized specimens also exhibited the...

Variations of resilient modulus of subgrade soils with postcompaction moisture contents.

Environmental conditions such as temperature and moisture can significantly influence the performance of a pavement structure. These conditions vary within the pavement layers, degrading or enhancing the properties of pavement materials. The AASHTO 1993 design guide accounts for this influence by incorporating a single resilient modulus (MR) value that represents the effective roadbed resilient modulus; the revised AASHTO Mechanistic–Empirical Pavement Design Guide (MEPDG) uses models that predict changes in modulus due to moisture, temperature, or both. The effect of changes in postcompaction moisture content (i.e., wetting or drying) on the resilient moduli of subgrade soils is evaluated. The effects of wetting and drying were examined by conducting resilient modulus tests on specimens compacted at various initial moisture contents: optimum moisture content (OMC), 4% drier than OMC (OMC – 4%), and 4% wetter than OMC (OMC + 4%). Results showed that MR–moisture content (MRMC) relationships caused by drying exhibited higher values than the corresponding MRMC curves for specimens subject to wetting. The most significant finding was that changes in MR values depended on the initial compaction moisture contents; the MR values compacted at OMC exhibited a different drying and wetting trend than the ones compacted at OMC ± 4%. Finally, the MEPDG MR–moisture model was modified in this study to better predict the variations of resilient modulus with moisture changes.

Environmental influences on the engineering behavior of unsaturated undisturbed subgrade soils: effect of soil suctions on resilient modulus

Abstract This study evaluated the effect of soil suction on resilient modulus of undisturbed subgrade soil specimens collected from a section located on I-40 in Canadian County, Oklahoma. A total of 18 undisturbed specimens were tested for resilient modulus (<i>M_R</i>) and soil suction. AASHTO 2002 Design guide <i>M_R</i> predictive model was used and evaluated for its reliability in predicting <i>M_R</i> values. Findings from <i>M_R</i> and soil suction tests showed that <i>M_R</i> increased as total and matric suctions increased. The variation of <i>M_R</i> with both suctions revealed the same trends. Additionally, an existing <i>M_R</i>-stress-suction model was modified to evaluate the variation of <i>M_R</i> with suction and stresses. The modified model was a reliable predictor of <i>M_R</i> values of unsaturated subgrade soils.