Assessment of Pavement Foundation Stiffness Using Cyclic Plate Load Test

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 aggregates 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 deflectometer (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 resilient 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 dependent 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 resilient modulus and other properties such as California Bearing Ratio (CBR) or Hveem R-value. In-situ resilient modulus is also predicted from non-destructive QC tests which can include the falling weight deflectometer (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 interlock 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 confinement 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 output including the stress cycles, cyclic and permanent deformation, stress-displacement relationship, number 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 different 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

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. ** Ratio of the minimum value to the maximum value of radial stiffness at 0.5% strain.

3.1.1 In-situ Resilient Modulus Cyclic plate load tests were performed on sections to measure in-situ resilient modulus using a 300-mm diameter plate. Figure 5 compares in-situ resilient modulus and accumulated permanent deformation of the test items. It is important to note that the in-situ resilient modulus is not constant until approximately 100 cycles. This finding supports the need for cyclic in-situ testing and partially explains why traditional plate load testing, Falling Weight Deflectometer (FWD) testing, and Light Weight Deflectometer (LWD) tests do not reliable predict in-situ resilient modulus. The in-situ resilient modulus obtained from cyclic plate load tests better represent the actual field conditions for pavement loading and number of load cycles compared to traditional testing. The inclusion of Geogrids showed a substantial improvement to the in-situ resilient modulus values of the stabilized pavement foundation. The result shows that the control section exhibited a permanent displacement of 15-mm whereas geogrid stabilized sections have only 5 to 7 millimeters of permanent displacement at the end of cyclic loading. The result clearly demonstrates that resilience of the stabilized foundation was increased due to the presence of geogrids. 180 20 TX160 18 GG2 TX BX1200 160 GG1 BX 16 Control

140 14

12 120 10

100 8

6 80 Permanent Deformation (mm) Deformation Permanent In-situResilient Modulus (MPa) 4 35 to 345 kPa cyclic stress 60 2 0 50 100 150 200 250 0 50 100 150 200 250 Number fo Load Cycles Number of Load Cycles

Figure 5. APLT results for Test A.

3.1.2 Influence of Confinement on Resilient Modulus The pavement section, which will be placed on the stabilized foundation, will directly affect the resilient modulus of the pavement foundation by providing the overburden pressure. A unique aspect of the APLT system includes the use of a confining plate system to evaluate the influence of confinement on resilient modulus. The confining pressure is adjusted to represent the overburden pressure on the pavement foundation. To investigate the effect of confining pressures on the in-situ resilient modulus, cyclic plate load tests were performed using a 200-mm diameter loading plate with a 610-mm diameter confining plate. The bulk stress model used to generate model parameter values was calculated as follows:

2 = 1 + 𝑘𝑘 𝑀𝑀𝑀𝑀 𝑘𝑘 �𝜎𝜎𝑝𝑝𝑝𝑝 𝑘𝑘𝜎𝜎𝑐𝑐 𝑐𝑐� Where k1 and k2 are empirical parameters, σp = the maximum cyclic plate contact stress, σc = plate confining stress, and k = lateral earth pressure coefficient.

Figure 6 compares the in-situ resilient modulus values with the bulk stress level of the pavement foundation. The results demonstrate a substantial improvement to the in-situ resilient modulus at low stresses where geogrids are used to stabilize the aggregate. The influence of geogrids on resilient modulus is reduced at high stress levels. However these stress levels are much greater than conditions found in the actual constructed pavement.

400 1800

Bulk Stress = 276 kPa 1600 Bulk Stress = 138 kPa 300

1400

200 1200

1000 100 In-situResilient Modulus (MPa) In-situResilient Modulus (MPa) 800

0 600 ControlControl GG1BX1200 BX GG2TX160 TX ControlControl BX1200GG1 BX GG2TX160 TX

Figure 6. Comparison of resilient modulus values at various bulk stresses

Using a bulk stress resilient modulus model, the results show increasing in-situ resilient modulus with decreasing stresses. This contradicts most laboratory tests on aggregate. This in-situ response is a signifi- cant finding that may impact how pavement foundations are designed in the future. It has been known that laboratory resilient modulus tests suffer from boundary condition problems which occur during compaction and testing. The resulting particle arrangement/interlock also differs from field conditions. The APLT does not suffer from the limitations of laboratory testing. At low stresses, it is believed that the benefits of aggregate interlock and geosynthetic stabilization are realized during cyclic loading. Small displacements during cyclic loading are generating very high in-situ resilient modulus values (over 500 MPa due to small in-situ deformations). Further, the in-situ tests revealed cyclic rebounding wherein following high vertical stress; low vertical stress cycles produce cyclic upward movement. Boundary conditions do not permit rebounding to occur in laboratory testing.

3.2 Predicting Trafficking Performance At two test locations (Control and GG2 TX section), cyclic stress tests were performed for 10,050 cycles. Performance prediction testing consisted of 50 cycles of high stress (550kPa) preconditioning followed by 10,000 cycles of moderate stress (310 kPa). The initial 50 cycles of high stress simulates truck axle load during pavement construction and the subsequent 10,000 cycles of moderate stress simulates vertical stress on the pavement foundation from in service traffic loading. The advantage of testing to 10,000 cycles is to better predict trafficking performance at higher cycles. Cycle times were controlled at 5 second intervals and each test lasted approximately 14 hours. In general, the subgrade will deform under construction traffic as shown in Figure 7 (a). Subgrade rutting resulting from construction traffic is filled be- fore paving as the subgrade rutting could cause premature pavement rutting if not filled prior to placement of the trafficking course. Therefore, the deflection was set to zero prior to initiation of the subsequent 10,000 cycle testing. N @ 25mm Control = 9,935 N @ 25mm Control = 66 N @ 25mm GG2TX = 4,858,972 N @ 25mm GG2TX = 1221

GG2 TX GG2 TX Control Control

Figure 7. Deformation versus load cycles (left) preconditioning and (right) 10,000 cycles

In Figure 8, results from the repeated load test were used to develop a permanent deformation performance prediction model in order to predict the number of cycles to achieve 12.5 mm of permanent deflection of the subgrade (considered as the onset of subgrade rutting failure). The log (N) permanent deformation model was then used to predict the number of cycles to generate the same levels of deformation for the control and the GG2 TX geogrid stabilized sections.

δp - Control

δp – Control = -26.79 + 4.27 ln (N)

δp – GG2 TX = -4.23 + 1.09 ln (N)

δp – GG2 TX

Figure 8. Rutting performance prediction models (left) preconditioning and (right) 10,000 cycles

4 SUMMARY

A summary of the key observations for resilient modulus evaluation through APLT are as follows:

• Test results demonstrate that the resilient modulus obtained from a small number of load cycles is not reliable. Conditioning of the pavement foundation using cyclic loading is therefore required to obtain reliable in situ resilient modulus values. • Resilient modulus is sensitive to the applied stress conditions. For paved roadway structures, stresses on the pavement foundation are about 35 to 100 kPa (with confinement), whereas higher stress is ex- pected for unpaved granular surfaced roads. In-situ resilient modulus QC testing should be determined based on the in-service stress conditions on a project specific basis. • Permanent displacement is an important parameter that should be recorded along with in-situ resilient modulus and is increasingly being considered as a design input. In these tests geogrids reduced the permanent strain accumulation at all stress levels. However, not all geogrids provide an equal benefit. The APLT can be used to evaluate the performance benefit of using a geogrid. • The compaction stresses likely play a role in the magnitude of in-situ resilient modulus values. Test- ing conducted at this site was performed on aggregate that was compacted with a relatively light compactor (Wacker RD11A tandem vibratory compactor: 1,173 kgf). Additional testing is needed with different compaction equipment and should be coupled with in ground vertical and lateral stress measurements. The lateral stress measurements could be used to validate the appropriate values for lateral earth pressure coefficients in the bulk stress calculation for the confining plate load tests. • Additional models can be explored for expressing the resilient modulus as a function of stress and using layered elastic analysis (e.g., a two layer model) and developing relationships between permanent deformation and load cycles. • The Barksdale permanent deformation model has been shown as a potential means of predicting service life for both unstabilized and mechanically stabilized pavement foundation layers.

REFERENCES

Brown, S., and Hyde, A. (1975). “Significance of cyclic confining stress in repeated-load triaxial testing of granular material.” Transportation Research Record, (537). LeKarp, F., Isacsson, U., and Dawson, A. (2000). “State of the art. I: Resilient response of unbound aggregates.” Journal of transportation engineering, American Society of Civil Engineers.