Khalid Farrag

Geosynthetic Reinforcement-Cohesive Soil Interface During Pullout

The influence of soils physicochemical property (cohesion) on the pullout behavior of different geosynthetic reinforcements was studied in this paper. This was accomplished by considering the pullout load-displacement curves measured at different points along the reinforcement. One woven geotextile and four geogrids with different stiffnesses and geometries were studied. The measured load-displacement and the deduced load-deformation curves were examined to determine the interface strength parameters (interface adhesion and friction). The influence of the interface adhesion on the load-deformation curve for a given segment of the reinforcement was indicated by an inflection point that corresponds to a compatibility force. The compatibility force is the force required to produce displacements at both ends of the segment. The load-deformation curves were bilinear for relatively weaker reinforcements and nonlinear for stronger reinforcements. The compatibility forces were used to back-calculate the reinforcement-soil interface adhesion and the angles of interface friction. The angles of interface friction were found to be inversely proportional to the squared root of the confining (overburden) stress level of the tests.

Comparison of Field and Laboratory Pullout Tests on Geosynthetics in Marginal Soils

The increasing use of geosynthetics to reinforce soils in many geo-technical engineering applications requires better evaluation of the soil-geosynthetic interface properties for use in the design and analysis of the different reinforced soil structures. The soil-geosynthetic interaction can be determined from direct shear tests or pullout tests. Until now, most research was focused on investigating the interface properties between geosynthetics and granular materials. Interest has increased in the use of marginal soils as backfill materials in mechanically stabilized earth (MSE) walls and embankments because of the cost-effectiveness of using the available cohesive soils. This strategy requires the evaluation of interaction parameters between geosynthetics and marginal soils. For this purpose, a series of laboratory and field pullout tests was carried out with different types of geosynthetic reinforcements in marginal silty clay soil of medium plasticity. Four types of geogrids and three types of geotext...

Controlled Low-Strength Material Used Around Buried Pipelines

Controlled low-strength material (CLSM) is commonly used as an alternative to traditional soil backfill materials around buried pipes. Laboratory and field tests were performed on various CLSM mixtures to evaluate their performance for the following: short- and long-term compressive strength, bearing resistance, and resilient modulus; setting time; settlement under repeated traffic loads in comparison with other backfills and adjacent pavement; susceptibility to freeze–thaw cycles; and freeze depth in the backfill. Several mix ratios based on the standard specifications of various state highway agencies were evaluated in the testing program. The strength properties of CLSM and its susceptibility to freeze–thaw were highly dependent on the amount and type of fly ash, cement content, and the amount of water added to the mix. Test results of samples obtained from the field did not compare well with those of laboratory-mixed samples of similar mix ratios. This difference was mainly due to the addition of water in the field to achieve flowability and lack of field quality control procedures to ensure proper mix ratios. The results demonstrated the need for well-defined quality control procedures to prevent variability in the mix ratios in the field. Tests of pavement sections under repeated traffic loads showed that CLSM backfill had lower settlement under repeated traffic loads than did other compacted soil backfills. However, CLSM with high fly ash had lower frost heave and thawing resistance than soils had. The frost heave susceptibility was negligible in mixes with an air-entraining mix without fly ash.

Bearing and Frictional Contributions to the Pullout Capacity of Geogrid Reinforcements in Cohesive Backfill

The interface resistance between the geogrid reinforcements and cohesive backfills are dependent upon the failure mechanism involved, the properties of the backfill, the test conditions and the geometry and stiffness of the geogrids. In the current study, the results of field and laboratory pullout tests, conducted by Farrag and Morvant, were compared to the pullout resistances predicted using the relationships readily available for calculating the frictional and bearing resistances of the reinforcements. The calculation of the bearing resistances was based on the relation recommended by Peterson and Anderson, whereas the frictional resistances were calculated based on the effective (solid) frictional areas of the geogrids with two interface (shear strength) factors of (0.8 and 1.0). The field pullout tests were conducted in a 20 ft (6 m) high wall constructed at the Pavement Research Facility (PRF) of the Louisiana Transportation Research Center (LTRC), while the laboratory tests were conducted in a test box in the geosynthetic engineering laboratory of the LTRC. Four geogrids were used in this study: Stratagrid-500, UX-750, UX-1500, and UX-1700. The tests were conducted at different levels of stress and different reinforcement lengths. Based on the comparison of the total predicted and the laboratory tests results, the relationship recommended by Peterson and Anderson was found to conveniently predict the contributions of the bearing resistances of the transverse members of the reinforcement. However, the same relationship was found to have more significant errors in predicting the bearing contributions of the pullout strips in the field. The contributions of the bearing resistances to the pullout capacities of geogrids were found to vary from 8% to 40%, depending on the types, geometry and stiffness of the geogrid.

Multiresolution Change Analysis Framework for Postdisaster Assessment of Natural Gas Pipeline Risk

Natural disasters, such as hurricanes and floods, pose significant threats to the integrity of natural gas pipelines. In an emergency situation following a disaster, thorough pipeline safety assessments must be performed to avoid costly postdisaster damage and to ensure the safe and reliable delivery of energy resources. This paper presents a multiresolution change analysis approach for detecting emerging threats to natural gas pipeline systems after a natural disaster. For aboveground pipeline facilities, the change analysis focuses on displacement of buildings as an indicator of whether a pipeline segment or gas meter needs repair or replacement. For underground pipeline facilities, the change analysis focuses on detecting soil movement and storm surges, all of which cause additional stress on the pipeline facilities. This approach can also be applied to estimate the likelihood of failure of buried pipelines by measuring terrain change and soil movement.

Long-Term Evaluation of the Bonding Strength of Composite Repairs

The long-term performance of composite repair systems depends on their structural integrity and interaction with the carrier pipe. The adhesives used in the composites are critical components that not only bond the repair to the pipe, but also bond the individual layers of the repair to one another. The durability of the inter-laminate adhesive bond is required to ensure adequate load transfer between the pipe and the composite layers over the predicted lifetime of the repair.A testing program was performed to evaluate the shear strength of the adhesives used in composite repairs. The testing program evaluated the performance of seven commercially-available composite repair systems and it consisted of short-term and long-term shear tests on the adhesives and cathodic disbondment tests on the repair systems. The long-term shear tests were performed for 10,000 hours on samples submerged in a water solution with pH value of 9 and at various loading levels at temperatures of 70°F, 105°F and 140°F.The results of the long-term tests at elevated temperatures were extrapolated to predict the shear strengths at longer durations. The 20-year shear strengths of the composites were estimated using: (a) direct extrapolation of the best-fit curves and (b) the application of the rate process procedure. The results demonstrated the significant effect of temperature on the bond strength of the composites and provided a comparative analysis to evaluate the long-term shear strength and cathodic disbondment of the composite repair systems.© 2012 ASME

Mechanical Damage of Pipelines at Low Operating Pressure

The paper presents the results of a testing program to characterize mechanical damage (dents and gouges) to pipelines at low operating pressures (i.e., at stress levels below 40% of the Specified Minimum Yield Strength, SMYS of the pipe material). The testing program was performed on pipelines of different sizes and grades; and the pipes were subjected to various gouges and dents when pressurized at 40% SMYS. The results of rupture tests on the pipes were compared with the ‘European Pipeline Research Group (EPRG) Simplified Model’ criterion. The results show that the model is sufficiently conservative to be used for evaluating mechanical damage of low-stress gas pipelines. The results provide guidelines for gas utilities to assess the damage at these stress levels. These guidelines allow a pipeline operator to assess the repair needs of a pipeline based on its operating pressure and damage level.Copyright

External Corrosion Growth-Rate From Soil Properties

External corrosion growth rate is an essential parameter to establish the time interval between successive pipe integrity evaluations. Actual corrosion rates are difficult to measure or predict. NACE Standard RP0502 [1] recommends several methods including comparison with historical data, buried coupons, electrical resistance (ER), and Linear Polarization Resistance (LPR) measurements. This paper presents a testing program and procedure to validate the use of the LPR and ER methods to enhance the estimation of corrosion growth rates and improve the selection of reassessment intervals of gas transmission pipelines. Laboratory and field tests were performed using the LPR and ER technologies. The evaluation of soil parameters that affect localized corrosion included its type, moisture content, pH, resistivity, drainage characteristics, chloride and sulfite levels, and soil Redox potential. The results show that the LPR device provides instantaneous measurement of corrosion potential and it may be used to reflect the variations of corrosion rates with the changes of soil conditions, moisture, and temperature. However, LPR measurements are more efficient in saturated soils with uncertainty about its validity in partially and totally dry soils. Consequently, seasonal changes in soil conditions make it difficult to estimate total corrosion growth rate. On the other hand, the measurements using the ER method provided consistent estimates for long-term corrosion growth rates. Corrosion growth rates were also evaluated from a previous study by the National Institute of Standards (NIST) [2]. A procedure was developed to correlate soil properties to corrosion rates from the ER measurements and NIST data. The procedure was implemented in a computer program to provide an estimate of corrosion rate based on the soil input data and allows the operator to use the ER probes to improve the reliability of corrosion rate estimates.Copyright

DEVELOPMENT OF DATABASE FOR LOUISIANA HIGHWAY BRIDGE-SCOUR DATA

Scour of highway bridges in Louisiana has been collected and monitored since 1970. Approximately 120 bridges are being monitored at a frequency of one to several times per year with an average of six cross sections for each bridge. Consequently, a tremendous amount of scour data in the format of traditional paper files already exists for these bridges. Manual and visual analysis of data has been tedious work, and there existed a need to develop a computer program and a database system to perform scour-data management. Database-management software has been developed for storage, retrieval, and analysis of scour data of Louisiana highway bridges. The system stores the data in a database system within the network. A computer program was developed as a front-end to allow the user to access the database, retrieve the data via several search routines, plot the scour data for analysis, and perform data updates. The program compiles the on-site data collected during scour survey and the bridge data relevant to scour properties. It can be used in (a) determining local scour at the piers and contraction scour at the bridge, (b) analyzing the long-term changes in riverbed elevations at various locations in the site, (c) providing the major parameters that are commonly used in bridgescour models for scour prediction, and (d) evaluating site conditions and soil properties for scour repair. The program tabulates and plots the bridge information, pier data, and coordinates of the river cross sections at the survey locations. It displays survey maps and soil-boring data. The analysis of the bridge-scour data is performed using plots of the cross sections, longitudinal sections, and contour lines of the scour data. The history of the scour at any specific location is plotted in time plots. Further implementations of the database system are suggested for more accurate determination of potential scours in highway bridges.