Assessment of Pavement Foundation Stiffness using Cyclic Plate Load Test Mark H. Wayne & Jayhyun Kwon Tensar International Corporation, Alpharetta, U.S.A. David J. White R.L. Handy Associate Professor of Civil Engineering, Department of Civil, Construction, and Environmental Engineering, Iowa State University, Ames, Iowa ABSTRACT: The quality of the pavement foundation layer is critical in performance and sustainability of the pavement. Over the years, various soil modification or stabilization methods were developed to achieve sufficient bearing performance of soft subgrade. Geogrid, through its openings, confine aggre- gates and forms a stabilized composite layer of aggregate fill and geogrid. This mechanically stabilized layer is used to provide a stable foundation layer for roadways. Traditional density-based QC plans and associated QA test methods such as nuclear density gauge are limited in their ability to assess stiffness of stabilized composite layers. In North America, non-destructive testing methods, such as falling weight de- flectometer (FWD) and light weight deflectometer (LWD) are used as stiffness- or strength-based QA test methods. In Germany and some other European countries, a static strain modulus (Ev2) is more commonly used to verify bearing performance of paving layers. Ev2 is a modulus measured on second load stage of the static plate load test. In general, modulus determined from first load cycle is not reliable because there is too much re-arrangement of the gravel particles. However, a small number of load cycles may not be sufficiently dependable or reliable to be used as a design parameter. Experiments were conducted to study the influence of load cycles in bearing performance. The influence of confining pressure on the in-situ re- silient modulus was also investigated in this field study. Based on cyclic plate load tests, in-situ resilient modulus of the unstabilized and stabilized layers have been determined. The in-situ resilient modulus can be utilized within pavement design procedures. This paper presents the findings from an automated field plate load test. Keywords: In-situ resilient modulus, Modulus of deformation, Geogrid, mechanical stabilization 1 INTRODUCTION The static plate load test has been widely used in different geotechnical engineering fields and particularly in the characterization of pavement foundation of rigid pavement. In Europe, the strain or deformation modulus, Ev2, is commonly used in pavement design instead of resilient modulus. The deformation modulus, EV2 is calculated from the second loading cycle using the Boussinesq solution and secant method (Reference a standard?). In contrast, resilient modulus is determined from the stiffness of a stress depend- ent paving materials under repeated load condition or after many stress cycles. Resilient modulus can be obtained from the laboratory triaxial test. However, due to the complexity of the laboratory triaxial test, the resilient modulus of pavement foundation materials is often obtained from correlations between resili- ent modulus and other properties such as California Bearing Ratio (CBR) or Hveem R-value. In-situ re- silient modulus is also predicted from non-destructive QC tests which can include the falling weight de- flectometer (FWD). One of the limitations of these non-destructive tests is the lack of a conditioning stage prior to testing. During pavement construction, pavement foundation materials are subject to relatively high loads from construction traffic and compaction equipment. In response to these loads, aggregate particles rearrange themselves resulting in higher density and stiffness and for mechanically stabilized layers, greater inter- lock and aggregate confinement. For this reason, it is important to apply conditioning load repetitions pri- or to testing for in-situ resilient modulus. Once surface paving is complete, the pavement foundation be- low is confined by overlying pavement layers. Knowing that the response of a pavement foundation to subsequent repeated traffic loading in both nonlinear and stress-dependent means that the effect of con- finement is an important condition to consider in a field based resilient modulus test. In response to this need, the influence of load cycles and confining pressure on resilient modulus and permanent deformation of the pavement foundation is examined in this paper. 2 AUTOMATED PLATE LOAD TEST (APLT) EQUIPMENT For rapid field assessment of critical performance parameters, Automated Plate Load Test (APLT) equipment was developed by Dr. David J. White (U.S. and International Patents Pending). The APLT equipment was specifically developed to perform rapid field testing of pavement foundations, embank- ments, stabilized materials. The APLT equipment is capable of measuring the following: • Modulus of subgrade reaction • Confining stress dependent resilient modulus • Undisturbed tube sampling and extrusion • Stress controlled wheel rutting simulation • Bearing capacity • Shear wave velocity/modulus • Cone penetration testing • Borehole shear testing Figure 1 shows the test equipment mounted on a trailer unit and Figure 2 is an example of the data out- put including the stress cycles, cyclic and permanent deformation, stress-displacement relationship, num- ber of load cycles, and in-situ resilient modulus. Figure 1. APLT test system. Resilient Modulus Permanent Deformation Cyclic Deformation Cyclic Stress Stress-Deformation Figure 2. Example output from APLT test system. The in situ resilient modulus is calculated as the ratio of the cyclic stress divided by the recoverable displacement (during unloading) using the following equation: Where Mr = in situ resilient modulus, δc is the recoverable deflection during the unloading portion of the cycle, ν is the Poisson ratio (assumed as 0.4), ∆σp is the cyclic stress, r is the radius of the plate and f is the shape factor selected as 2 assuming a uniform stress distribution under a circular rigid plate. In reality, Poisson’s ratio will vary between test sections due to the aggregate stabilization mecha- nism(s) and loading conditions. Several papers in the literature demonstrate that this value can vary from 0.1 to 1+ due to the stress level and volume change characteristics (e.g., Brown et al. 1975, LeKarp et al. 2000). 3 EXPERIMENTAL STUDY 3.1 Field Test Program The field performance testing program involved a series of cyclic plate load tests. The cyclic plate load test is used to determine field strength and deformation values. Several APLTs were performed using dif- ferent plate sizes, stress conditions, and load cycles. The test site was prepared by excavating the topsoil. A bulldozer with a laser level was used to exca- vate the area. A tractor with a blade and a skid steer were used for fine grading. No fill materials were used in an effort to keep the ground conditions as uniform as possible. Subgrade soils at the site were classified as CL with the following properties: Clay content (0.002 mm) = 23.4%, silt content = 39.6%, sand content = 37.0%, LL = 32.5, PI = 10.0, pH = 6.6, and CEC = 22.5 (milliequivalents/100 grams). Once the subgrade was prepared, 150-mm of crushed limestone fill was placed on all the test sections to stabilize the subgrade. Figure 3 provides information on the crushed limestone aggregate used in this study. 100 80 60 no. 4 40 Percent Passing Percent 20 no. 200 0 100 10 1 0.1 0.01 Particle size (mm) Figure 3. Crushed limestone aggregate gradation. Dynamic cone penetration (DCP) tests were performed in accordance with ASTM D6951. California bearing ratio (CBR) values were estimated from the DCP results as shown in Figure 4. The DCP profiles of the geogrid stabilized sections show that the aggregate particles are confined within the geogrid aper- tures, creating a stiffer composite layer. The profile also shows that the GG2 TX geogrid created a deeper zone of influence than that of the GG1 BX geogrid. CBR 0 5 10 15 20 25 30 0 5 10 15 20 25 30 0 5 10 15 20 25 30 0 0 0 200 200 200 400 400 400 Depth(mm) 600 600 600 Control GG1BX1200 BX GG2TX160 TX 800 800 800 Figure 3. Dynamic Cone Penetrometer profiles at test locations. Tables 1 and 2 provide details for the APLT configurations, load cycles, and stresses. Note that the as- sumption of Poisson’s ratio being constant for all sections and for all loading conditions is an oversimpli- fication which warrants further investigation. Table 1. Summary of plate tests and configurations Target Stress Test Number of Load Cy- Range (kPa) Plate Configuration Designation cles Min Max A 250 34.5 345 300-mm diameter, flat 20 cycles each x 15 200-mm diameter, flat with 610-mm diameter B See Table 2 stress levels confining plate Table 2. Summary of plate tests and configurations Constant Surface Confinement Max Stress Cyclic Stress Number of Sequence Stress (kPa) (kPa) (kPa) Cycles (kPa) condition 0 69 138 69 1 86 17 2* 14 138 69 3* 207 138 4* 276 207 5* 23 345 276 6 86 17 7* 138 69 69 138 8* 46 207 138 9* 276 207 10* 345 276 11* 69 138 69 12* 207 138 13* 276 207 92 14* 345 276 Note: * indicates sequences used in calculating in situ resilient modulus model parameters The influence of geosynthetics on in-situ resilient modulus of the stabilized pavement foundation was also evaluated in the test program. Two of these test sections contained geogrid between the subgrade and fill while one section served as a control. Table 3 provides material mechanical properties of the geogrids used in the field testing program. Mean Radial Stiffness is the arithmetic mean of the secant stiffness in the four test directions (0 MD direction, 30, 60, 90 degrees) measured at 0.5% strain. Radial Stiffness Ra- tio is the quotient of the minimum and maximum stiffness at 0.5% strain of the four test directions. Table 3. Summary geogrid material mechanical properties Geogrid Type Mechanical Properties Rectangular aperture GG1 BX Tensile Strength: 6.0 kN/m at 2% strain and 19.2kN/m ultimate geogrid Triangular aperture Radial Stiffness*: 300 kN/m at 0.5% strain GG2 TX geogrid Radial Stiffness Ratio ** > 0.6 Note: * The secant stiffness in the four test directions (0 MD direction, 30, 60, and 90 degrees) measured at 0.5% strain.