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22 CHAPTER THREE LABORATORY METHODS FOR MATERIAL STIFFNESS MEASUREMENT Introduction Laboratory Tests Used for Resilient Modulus Chapters three, four, and five summarize the stiffness or Historical perspectives of the resilient modulus test proce- resilient moduli–related literature information collected dure and its evolution are well documented by Ishibashi et al. from various state DOTs. As a part of the literature review (1984) and Vinson (1989). This paper presented at a resilient collection, several research reports from various states modulus workshop in Corvallis, Oregon, compares simple including Texas, Virginia, Louisiana, Kentucky, and Wis- static load tests such as plate load test and CBR tests con- consin were acquired and reviewed. Library search engines ducted from the 1960s to the 1990s as well as current RLT- such as the Transportation Research Information Service related resilient modulus test procedures used after the 1980s (TRIS) and COMPENDEX were also used to collect other to characterize subsoils for the flexible pavement design. The available documented information on the resilient modulus use of RLT tests in the place of static tests was attributed of subgrades and unbound material. primarily to research efforts by Seed et al. (1962; 1967) the University of California, Berkeley. Other tests on aggregates The collected literature was summarized in chapters can be found in the references cited by Saeed et al. (2001). three, four, and five in the following categories: An earlier study conducted by Seed and McNeill (1958) • Laboratory resilient moduli tests; test results of differ- documented the differences between initial tangent elastic ent types of subgrades and unbound pavement mate- modulus and resilient modulus and the need to develop labo- rials; advantages and limitations of laboratory tests ratory procedures simulating in situ traffic loading condi- (chapter three). tions. After this study, several laboratory procedures were • Existing in situ nondestructive test methods for inter- developed to determine the resilient modulus of subgrades pretation of resilient properties of unbound bases and and base aggregates. A brief discussion of these tests to subgrades; test results from a variety of subsoils; com- determine MR property is summarized here. parisons with the laboratory-determined resilient mod- uli properties; and advantages and limitations of these Repeated Load Triaxial Test tests (chapter four). • Existing in situ intrusive test methods for resilient The Resilient Modulus test using RLT test equipment is property determination; test results for different sub- designed to simulate traffic wheel loading on in situ subsoils soils; development of correlations between in situ data by applying a sequence of repeated or cyclic loads on com- and moduli properties; and advantages and limitations pacted soil specimens. Figure 25 shows an RLT testing sys- of intrusive tests (chapter four). tem used to determine the resilient modulus of subgrades. • Direct expressions that correlate resilient moduli prop- erties with basic soil properties and in situ compaction parameters; statistics of the correlations; advantages and fallacies of these correlations (chapter five). • Indirect correlations that express model constant parameters (obtained from an analysis of various types of resilient moduli formulations including two- parameter “bulk stress and deviatoric stress” models to three- and four-parameter models) as functions of basic soil properties and compaction-related soil vari- ables (chapter five). • Matrix tables summarizing the literature findings (chapter five). FIGURE 25 Triaxial unit with data acquisition and control panel unit.
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23 The AASHTO T-307-99 method is currently followed the peak torsional displacement with frequency are then for determining the resilient modulus of soils and unbound recorded to obtain the frequency response curve. Peak dis- aggregate materials. Prior to this method, a few methods placements are recorded through an accelerometer attached (namely, T-274, T-292, and T-294) were used. The stress lev- to the drive plate. Test procedure details can be found in Sto- els for testing the specimens are based on the location of koe et al. (1990). the specimen within the pavement structure. The confining pressure typically represents the overburden pressure on the The frequency response curve generated during the test soil specimen with respect to its location in the subgrade. can then be displayed on the analyzer main screen for post- test data processing. The small-strain shear modulus G can The axial deviatoric stress is composed of two components, then be calculated as follows (Richart 1975): the cyclic stress (which is the actual applied cyclic stress) and a constant stress (which typically represents a seating load on the soil specimen). The constant stress applied is typically (1) equivalent to about 10% of the total axial deviatoric stress. The testing sequences employed for both granular and base or subbase materials and fine-grained subgrade soils are dif- ferent, and these details can be found in AASHTO test stan- (2) dard procedures. A haversine-shaped wave load pulse with a loading period of 0.1 s and a relaxation period of 0.9 s is used in the testing. A haversine load pulse is recommended in the where G is small-strain shear modulus, ρ is soil density, test procedure, which is based on the earlier AASHO road L is sample length, fr is resonant frequency, Fr is driver con- test research performed in the United States. stant, IR is polar moment of inertia of soil column, and Io is polar moment of inertia of driver system. The test procedure involves preparation of a compacted soil specimen using impact compaction or other methods, The small-strain shear modulus is then converted to resil- transfer of soil specimen into triaxial chamber, applica- ient modulus values by using the following Equation 3 with tion of confining pressure, and then initiation of testing by an assumed Poisson’s ratio. applying various levels of deviatoric stresses as per the test sequence. The test process requires both conditioning fol- E = 2 G (1+ µ) (3) lowed by actual testing under a multitude of confining pres- sure and deviatoric stresses. Barksdale et al. (1997) noted some concerns with the RC At each confining pressure and deviatoric stress, the resil- testing as a result of low axial strains applied during the test- ient modulus value is determined by averaging the resilient ing, which are assumed to be much smaller than the strains deformation of the last five deviatoric loading cycles. Hence, applied under heavy wheel loads applied near the surface. from a single test on a compacted soil specimen, several resil- Resilient strains applied under wheel loads are small and ient moduli values at different combinations of confining and comparable with the strains recorded in this test. deviatoric stresses are determined. From these values, the design resilient modulus value can be established by deter- Simple Shear Test mining the MR value at appropriate confining pressure and the deviatoric stress levels corresponding to the subgrade and It is well known that the stresses in subsoil undergo stress unbound base layer location within the pavement system. reversals owing to traffic wheel load movements, which can be seen in Figure 26. In a simple shear test, the soil specimen Resonant Column Test will be subjected to such state of stresses and hence consid- ered to be a more representative test for the determination The resonant column (RC) test was initially developed to of resilient moduli of soils. However, Barksdale et al. (1997) study dynamic properties of rock-like materials in the early noted that the stress paths of the soil specimen in the labora- 1930s. The technique has since been upgraded continually tory and the in situ soil in the field are different. for dynamic characterization of a wide variety of geologic materials (Huoo-Ni 1987). The test can be simulated as the In this test, a soil specimen is subjected to a shear stress in fixed-free system. The specimen rests on a pedestal and both both directions. Though this method was used for both resil- the top cap and torsional drive plate are securely attached ient moduli and permanent strain measurements, there are onto the top end of the specimen. During RC testing, the still some concerns that limit the adaptation of this method. drive plate is allowed to rotate freely such that a torsional These are the stress-induced anisotropy of the soil specimen excitation can be applied at the top end of the specimen with resulting from shear stress application, and the difficulty in a constant amplitude and varying frequency. Variations of applying uniform stress.