Common Unbound Soil/ Aggregate Tests Used in Transportation Systems

Posted: August 26th, 2021

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Common Unbound Soil/ Aggregate Tests Used in Transportation Systems

Globally, the use of common unbound soil is essential due to the ever-rising need forbuilding transportation systems using environmentally sustainable materials. Recycled materials are considered as having unique physical properties useful for pavement design. In the study of common unbound soil, researchers have zeroed on a full-depth reclaimed pavement material (RPM) as an industrial by-product exhibiting high stability for use in transportation systems. Therefore, it is vital to conduct various statistical methods of defining units of common unbound soils. The paper summarizes different historical parameter correlations that are relevant in establishing the reliability of the RPM using the following tests that entail chemical and physical analysis.

Section A: Selection of Design Unit Subgrade Support Areas

Statistical Approaches to Defining Units

Analytical approaches are applied in the labtesting to find the material resilient modulus relevant in the study ofa transportation engineering system. The regression-based M_R predictive model is incorporated in the lab as a way of defining the units of stress measurements in the resilient modulus (Camargo et al., 2012). The non-linear regression statistical predictive model is given by the following equation, as shown below.

Determining Reliability-Based Design Units

There exist two types of standard test methods of establishing the reliability of a resilient modulus. First, there is NCHRP 1-28A, a harmonized test technique associated with the determination of some levels of resilience in the modulus transportation system (Camargo et al., 2012). The method blends all the applicable test protocols, thus reinforcing on the reliability of the pavement areas vis-à-vis stress and stability.

Figure 1. The testing system, triaxle cell setup

            The standard reliability method is relevant in reducing the probability of test defects, only because it covers the entire degree of stress. Second, there is a Falling Weight Deflectometer (FWD) as a reliability method of back-calculation of the resilient layer modulus. With the FWD, it is possible to establish the reliability of the transportation system by reinforcing the cases of deflections occurring underneath the pavement (Gun et al., 2015). Through simulation of a dynamic load, the FWD system analyses the deviations by way of dropping a weight from a predetermined height onto the pavement surface. Indeed, the method is critical in establishing the reliability of ordinary unbound soil (Gun et al., 2015). Therefore, with the pavement response, it is easy to calculate the reliability of the transportation system by analyzing the frequency using non-destructive testing (NDT) technique.

Section B: Summary of the Historic Parameters Correlations

Plate Bearing, k -Westergaard Modulus of Reaction

According to Brown (2013), plate bearing is a test conducted to establish the bearing capacity of an in-situ loadof a solid, thus assessing the shape of the underneath soil. This test is aimed at achieving three significant objectives that include; estimation ofparameters of the design based on the bearing capacity of common unbound soilconformed bearing capacity of varied design stages and establishedfoundation settlement underneath the working in-situ load (Brown, 2013).

California Bearing Ratio (CBR) Test

CBR is a test ratio that determines the resistance of both a surface and underneath soil material relative to the permeation of a standardized plunger tool under measuredconditions of density and moisture. The test is set up by forcing a cylindrical plunger of the diameter of 50mm to enterinto a pavement surface at a rate of 1.25mm per minute (Brown, 2013). As a technique of measuring the shear strength, the CBR ratio is not primarilyprojected to correlate with stiffness or modulus of a transportation system. However, it has been widely applied to describe the subgrade soils and, therefore, is considered a low bearing ratio of testing both the stress and stiffness of a resilient common unbound soil (Brown, 2013).

Figure 2: A sample illustration of a CBR Test

R-Hveem Stabilometer, R-value

It is a test method for finding the stiffness of anRPM. Before implementing the R-value tests in the lab, it is essential to fabricate samples concerning their density and moisture conditions. It is vital to demonstrate the worst possible in-situ conditions for a compacted subgrade (Camargo et al., 2012). As the R-value is obtained based on the ratio of applied vertical pressure to the developed lateral force, the test also establishes the resistance of the RPM against the plastic flow (Camargo et al., 2012). From figure 2, the use of the Stabilometer apparatus is significant in obtaining the R-value.

Figure 2: An illustration of a Stabilometer devicefor establishing the R-values

Therefore, the equation below is moreefficient in calculating the R-value.Upon obtaining the measurements, the respective data values are analyzed in the comparison to find the value results, as shown below (Lusher, 2004).

}

Where R is the value of resistance; Pvis the applied vertical pressure calibrated at 160 psi; Phis thediffused horizontal pressure at the Pv= 160 psi; Dis the displaced Stabilometer fluid to raise the horizontal force to 100 from 5 psi (Lusher, 2004).

Non-Linear Resilient Modulus, Mr

Resilient modulus is an essential non-linear material quality associated with characterizing the common unbound pavement surface. This test technique is relevant in establishing the stiffness of material by calibrating a mean. According to Titi (2006), this non-linear resilient modulus coupled with equations has been so useful in determining the stiffness of a pavement surface material, especially under particular conditions like density, moisture, and level of stress.

Where Jd represents the applied axle deviator stress;Gr is the recoverable axle strain. The obtained values can be illustrated graphically, as shown in figure 3,based onthe repeated load axial tests.

Figure 3: A graph illustrating the resilient modulus terms of stress against strain

Dynamic Cone Penetrometer (DCP)

The DCPis a tool that measures the strength of an in-situ soil. More so, it establishes the thickness as well as the location of the common unbound subsoil. The whole operation helps determine the cone penetration. It is set up by considering qd as the cone penetration tip resistance, whereas A is the cone area (Nazzal, 2014). Therefore, conducting a DCP test encompasses raising and dropping the hammer drive of about 1.8 kg onto the cone area, measuring 2 to 4 cm^2,as shown in figure 4. Practically, this is done on the lower shaft via the underlying pavement surface (Nazzal, 2010). Typically, after each hammer blow of variable energy, denoted by E, the in-depth penetration onto the cone is determined and further recorded. Nonetheless, it is necessary to record the readings afternumerous hammer blows in case of stiffer soils.

Figure 4. Dynamic cone Penetrometer (DCP)

Conclusion

Resilient modulus is an outstanding measurement of stiffness associated with the underneath pavement surface. Indeed, the determination of resilient modulus measurement seems reliable for inputs of the MEPDG if the involved testing methods yield positive results. The quality of sub-soil material helps predict resilient modulus, which ranges from 0.79 to 0.97 in terms of measurement values (Ping & Dietrich, 2001).More so, the use of a back-calculated modulus coming from the FWD data offers a practical way of obtaining the inputs values of the design. Back-calculation is more efficient in bringing out a correlation relationship with the physical qualities of the tested common unbound soil. Therefore, an MR regression predictive model facilitates the conducting of the mechanistic analysis on the common unbound pavement.

Works Cited

Brown, Allen. Unbound aggregate base characterization for design. Practices for Unbound Aggregate Pavement Layers, 2013.

Camargo, Felipe et al. “Comparative Assessment of Crushed Aggregates and Bound/Unbound Recycled Asphalt Pavement as Base Materials.”International Journal of Pavement Engineering, vol. 14, no. 3, 2012, pp. 1-8, DOI: 10.1080/10298436.2012.655737

Gu, Fan et al. “Estimation of Resilient Modulus of Unbound Aggregates Using Performance-Related Base Course Properties.” Journal of Materials in Civil Engineering, vol. 27, no. 6, 2015, pp. 123-543, DOI: 10.1080/10298436.2012.655737

Lusher, Michael S. “Prediction of the Resilient Modulus of Unbound Granular Base and Sub-base Materials Based on the California Bearing Ratio and Other Test Data.” Master of Science in Civil Engineering, 2004.

Nazzal, Mohammad M. “Estimation of Resilient Modulus of Subgrade Soils for Design of Pavement Structures,” Journal of Materials in Civil Engineering, vol. 22, no. 7, 2010, pp. 726–734.

Ping, Yang Z., & Lyam Dietrich, “Measuring Resilient Modulus of Granular Materials in Flexible Pavements,” Transportation Research Record, no. 1778, 2001, pp. 81–90.

Titi, Elias M. “Determination of Typical Resilient Modulus Values for Selected Soils in Wisconsin.” Wisconsin Highway Research Program: Final Report 0092-03-11, 2006.