Chapter 14: Motivation -- Theory and Practice

University of GuyanaFaculty of TechnologyDepartment of Civil Engineering

1The Effective Road Bed Soil Resilient Modulus MR

Course: CIV 4105 (Highway Engineering)Lecturer: Mr. Ronald Roberts

University of GuyanaFaculty of TechnologyDepartment of Civil Engineering

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Presented in Partial Fulfilment for CIV 4105 (Highway Engineering) by: NameRegistration NumberJaikeshan Takchandra13/0933/1323Stephen Liu13/0933/2205Presentation Outline3IntroductionDefinitionStressDeformation & StrainStiffness not StrengthImportance of the Resilient Modulus (MR)How is MR Determined MR Typical Values Use of MR in Pavement Design Conclusion References

1.0 Introduction4The design and evaluation of pavement structures on base and subgrade soils requires a significant amount of supporting data.Currently, empirical correlations developed between field and laboratory material properties are used to obtain highway performance characteristics (Titi, et al., 2006).These correlations do not satisfy the design and analysis requirements since they neglect all possible failure mechanisms in the field. 1.0 Introduction5Recognizing this deficiency, the 1986 and the subsequent 1993 AASHTO design guides recommended the use of the resilient modulus (MR) for characterizing base and subgrade soils and for designing flexible pavements. The resilient modulus accounts for soil deformation under repeated traffic loading with consideration of seasonal variations of moisture conditions.Accurate knowledge of the resilient modulus of materials within the pavement layer facilitates the determination of how the system would react to traffic loadings (Buchanan, 2008).2.0 Definition62.1 Stress7Stress is determined by dividing the applied load by the contact area of the material in question. The stress essentially provides a method of standardising load and area for testing and designing purposes. As long as the applied load remains in full contact with the soil mass, the stress will be equal regardless of the soil type. However, if a wheel load is applied to a pavement structure, specific locations under the load will experience different levels of stress which is based on their respective depths from the pavement surface and distance away from the applied loading.2.1 Stress8

Figure 1: Resilient Modulus Stress States.Taken from: Buchanan, S., 2008. Resilient Modulus: What, Why, and How?, Birmingham, Alabama: Vulcan Materials Company.2.2 Deformation & Strain9

Figure 2: Types of deformation.Taken from: Siddiki, N., 2015. Importance of Resilient Modulus and its Interpretation, Purdue: Geotech Construction & Tech Support Engineer, INDOT.2.2 Deformation & Strain10The observed magnitude of the soil deformation as a result of the applied loading will vary. The deformation will vary dependent on the soil properties.As long as the applied load is constant, a portion of the deformation may be recoverable or resilient while the remainder is unrecoverable or plastic.

2.2 Deformation & Strain11

Figure 3: A typical specimen response during a loading and unloading cycle.Taken from: Buchanan, S., 2008. Resilient Modulus: What, Why, and How?, Birmingham, Alabama: Vulcan Materials Company.2.3 Stiffness not Strength12It must be noted that the resilient modulus is a stiffness measurement and not the strength of the material. The resilient modulus can be determined at many combinations of applied loading and confinement. The ultimate strength or stress is the point at which failure occurs under applied loading.

3.0 Importance of Resilient Modulus13The resilient modulus is utilised to characterise pavement materials under loading conditions that will not result in failure of the pavement system. This parameter is utilised to directly determine the structural coefficient of the untreated base and subbase layer. Based on the structural number (SN) value, the pavement layers and their dimensions are designed in accordance with the AASHTO Pavement Design Guide. By varying the pavement layers thicknesses and stiffness, the system can be designed to carry the design axle load applications during its service life.4.0 How is MR Determined14The resilient Modulus can be determined from one of the following three methodsLaboratory Testing From material properties soil information is used to estimate MR values Back-calculation from deflection data of a particular pavement design

(AASHTO, 1993-i)

4.1 Laboratory Testing 15Lab testing is accomplished through a variant of the Triaxial TestPrinciple: repeated axial and cyclic stresses of fixed magnitude and duration and is usually electronically monitoredThe most common methods are AASHTO T 274 and AASHTO T 307 Determining the resilient modulus of soil aggregate materials (AASHTO, 1993-ii) Generally, the methods involves an axial stress applied through a haversine load and rest method

4.1 Laboratory Testing 16Cylindrical samples of 4 D by 8, or 6 D by 12 Samples are placed in the apparatus Linear variable deformation transducer (LVDTs) are installed to measure deformation Sample is pre-tested for failure using a haversine 0.1 and 0.9 sec, or 0.2 and 0.8 sec

Fig 4.1: Triaxial CellFrom:BALADI (2009) Pavement Subgrade Mr Design Values for Michigans Season Changes. Michigan State University, USA. Pp. 5-136

4.1 Laboratory Testing 17Samples are loaded for a minimum of 1000 cycles The test loads are specified at 3-4 psi for base and sub-base materials, and 1-10 psi for road bed materials. If permanent vertical strain reaches 5% during pre-test, the process is terminated Testing: the test is performed using the load sequenceThe results from the triaxial test is represented on a Mohrs Circle for further analysis

4.1 Laboratory Testing 18

Fig 4.2: Mohrs Circle From:HARRIGAN. E (2004) Laboratory Determination of Resilient Modulus for Flexible Pavement Design. National Cooperative Highway Research Program, Transportation Research Board. Pp. 1-52

4.1 Laboratory Testing 19

2654.1 Laboratory Testing 20

Figure 4.3: Regression analysisFrom:Buchanan, S., 2008. Resilient Modulus: What, Why, and How?, Birmingham, Alabama: Vulcan Materials Company.

4.1 Laboratory Testing 21

(Buchanan, 2008)4.2 Material Properties 224.2 Material Properties 234.2 Material Properties 244.3 Back-calculation 255.0 Typical MR Values 26Unbound aggregate base materials: 15,000 psi 60,000 psi

6.0 MR use in Pavement Design 27Pavement materials are not exclusively elastic, and the resilient modulus models the approximate materials behavior Layered elastic analysis becomes easier when using the resilient modulus and Poissons Ration since one is able to predict a linear elastic analysis Strain at the bottom of an asphalt system could lead to fatigue, which would eventually cause cracking in the surface layer due to deformation of the subgrade 7.0 Conclusion 28MR is a key characterization parameter for granular materialsIt closely models the resilient behavior of a granular material, recognizing that the behavior is difficult to generalize The resilient modulus is highly dependent on the stress state (i.e. bulk stress and confining pressure)Each pavement design should have its specific calculated resilient modulus based on its structure and load application QUESTIONS?29THANK YOU FOR LISTENING! References30AASHTO, 1993. AASHTO Guide for Design of Pavement Structures 1993. 2nd ed. Washington D.C.: American Association of State Highway & Transportation Officials.Buchanan, S., 2008. Resilient Modulus: What, Why, and How?, Birmingham, Alabama: Vulcan Materials Company.Pavement Interactive, 2007. Resilient Modulus. [Online] Available at: http://www.pavementinteractive.org/article/resilient-modulus/[Accessed 19 December 2015a].Siddiki, N., 2015. Importance of Resilient Modulus and its Interpretation, Purdue: Geotech Construction & Tech Support Engineer, INDOT.Titi, H. H., Elias, M. B. & Helwany, S., 2006. Determination of Typical Resilient Modulus Values for Selected Soils in Wisconsin, Milwaukee: Department of Civil Engineering and Mechanics, University of WisconsinMilwaukee.

References31AASHTO (1993-i) Standard Specification for Transportation Materials and Methods of Sampling and Testing. Washington, D.C. pp. 1-624AASHTO (1993-ii) Guide for Design of Pavement Structures, American Association of State Highway and Transportation Officials. Washington, D.C. pp. 33-114BALADI (2009) Pavement Subgrade Mr Design Values for Michigans Season Changes. Michigan State University, USA. Pp. 5-136HARRIGAN. E (2004) Laboratory Determination of Resilient Modulus for Flexible Pavement Design. National Cooperative Highway Research Program, Transportation Research Board. Pp. 1-52YODER. E. I & WITCZAK. M. W. (1975) Principles of Pavement Design. 2nd Edition. London: John Wiley & Sons Inc.