Intelligent Soil Compaction Systems (2010)

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s U m m a r Y intelligent soil Compaction systems This report details the findings of NCHRP Project 21-09, “Intelligent Soil Compaction Systems,” undertaken to investigate intelligent soil compaction (IC) systems and to develop generic specifications for the application of IC in quality assurance (QA) of soil and aggre- gate base material compaction. The term intelligent soil compaction systems was defined (in the NCHRP Project 21-09 request for proposals) to include (1) continuous assessment of mechanistic soil properties (e.g., stiffness, modulus) through roller vibration monitoring; (2) automatic feedback control of vibration amplitude and frequency; and (3) an integrated global positioning system to provide a complete geographic information system-based re- cord of the earthwork site. An equally important term is roller-integrated continuous compac- tion control—defined by IC components (1) and (3). Roller-integrated continuous compaction control (CCC) technology was initiated in Eu- rope in the 1970s and has been used in European practice for nearly 20 years. The first Euro- pean specification for roller-integrated CCC was developed in Austria in 1990. Today, four European countries have soil compaction QA specifications using roller-integrated CCC (Austria, Germany, Sweden, and Switzerland) and U.S. states are beginning to implement pilot specifications (e.g., Minnesota). In European specifications the use of automatic feed- back control IC rollers is permitted during compaction but not during QA because the roller measurement values (MVs) can be strongly influenced by varying amplitude and frequency. The dependence of roller MVs on frequency and amplitude in particular was verified in this study (summarized below) and further determined to be quite complex and difficult to pre- dict. Accordingly, the recommended specifications developed here allow IC during compac- tion but do not permit the use of automatic feedback control IC during roller-based QA. Recommended Specifications for Roller- Integrated CCC in Earthwork QA Six options for QA of earthwork compaction using roller-integrated CCC were devel- oped as a result of this study to accommodate the diversity of earthwork site conditions, earthwork and QA practice, and agency needs observed throughout the United States. The six recommended specification options are distinguished into three principal categories. In Option 1, CCC is used to assist in QA, but acceptance is based on spot-test measurements. Options 2a and 2b acceptance is based on roller MVs, but no initial calibration of roller MV is required. Options 3a, 3b, and 3c acceptance is based on achieving a target roller MV over a specified proportion of an evaluation area. Target MVs are determined via various initial calibration techniques. Technically, the proposed specifications are end product based with methodological aspects that must be followed. None of the recommended options consti- tute performance-based specifications. Each specification option can be adopted as the sole  

method for QA; alternatively, two or more options can be combined to increase reliability. Option 1 is recommended as a beginning approach. Once personnel are comfortable with CCC technology, states can advance to more complex options. The recommended specifications were developed and pilot tested through field testing on active earthwork construction projects in Minnesota, Colorado, Maryland, Florida, and North Carolina. Extensive testing with IC and CCC rollers was conducted on granular soils, fine-grained soils, and aggregate base materials commonly used in subgrade, subbase, and base course construction. Smooth drum and pad foot drum IC and CCC roller compactors from Ammann, Ammann/Case, Bomag, Caterpillar, Dynapac, and Sakai were used through- out the study. Field investigations were conducted on more than 200 test beds across the five sites. Test beds involved single lifts of subgrade, subbase, and base course materials ranging in thickness from 150 to 300 mm (6 to 12 in) and, in some cases, multiple lifts and layered systems to depths greater than 1.5 m (4.9 ft). Single-lane test beds were constructed to con- duct detailed investigations of the relationship between roller MVs and measurements from commonly used spot tests (e.g., nuclear gauge, lightweight deflectometer, dynamic cone penetrometer). Full-width test beds were constructed to calibrate roller MVs to spot-test measurements and to examine the implementation of recommended specifications. Multi- ple lift and layered test beds were constructed with embedded instrumentation to investigate the relationship between roller MVs and in situ stress-strain-modulus, the measurement depth of roller MVs, and the influence of layered structures on roller MVs. Fundamentals of Roller Measurement Systems Each vibration-based roller MV investigated provides a measure of soil or foundation stiffness for an area the width of the roller [2.1 m (6.9 ft)] by a spatial distance in the direc- tion of roller travel that varies [0.06 to 1.0 m (0.2 to 3.3 ft)] across the different MVs. The reporting spatial resolution of roller MVs varied from 0.2 to 1.0 m (0.7 to 3.3 ft), and the resulting records provide complete coverage of the earthwork. For best results, real-time kinematic differential global positioning system (GPS) with an accuracy of 1 to 2 cm (0.4 to 0.8 in) is recommended. The position reporting accuracy of the roller-mounted GPS should be verified regularly. Repeatability testing of properly working CCC/IC rollers and roller measurement systems revealed a pass-to-pass roller MV uncertainty of ±10% (one standard deviation). Repeatability testing of vibratory pad foot measurement systems revealed pass- to-pass MV uncertainties in excess of 25%. A repeatability testing procedure was developed and is recommended for CCC specifications. Four vibration-based roller MVs were investigated—Ammann and Case/Ammann k s , Bomag E vib , Dynapac CMV D , and Sakai continuous compaction value (CCV). The various MVs correlated well with each other over a range of soft to stiff soil conditions. CCV and compaction meter value (CMV) were found to be insensitive to changes in soil stiffness below values of approximately 10. Many of the roller MVs employed by manufacturers were validated using independent instrumentation and implementation of published roller MV algorithms. This dispels the “black box” mentality that would inhibit implementation by the engineering community. Field testing revealed that vibration-based roller MVs vary with operating parameters such as excitation force amplitude and frequency, roller speed, and travel mode (forward/ reverse). The amplitude dependence of roller MVs was not predictable; MVs were found to increase, decrease, or remain the same with increasing amplitude depending on the soil and layering conditions. The complex variation of roller MVs with operational parameters indi- cates that operational parameters must be held constant when using roller-integrated CCC for QA. Local soil heterogeneity transverse to the direction of roller travel has a significant influence on roller MVs. Due to the nature of drum instrumentation, roller MVs are direc-

tionally dependent on heterogeneous soil. Bidirectional roller MVs were found to vary by 100% due to transverse soil stiffness variability. Spot testing should be conducted across the drum lane when correlating to roller MVs, and great care should be used when performing spatial statistical analysis of pass-to-pass data maps in the presence of heterogeneity. Relationship Between Roller-Measured Stiffness and In Situ Stress-Strain-Modulus Behavior Roller MVs measure to depths considerably greater than typical compaction lifts. For ver- tically homogeneous embankment conditions and the 11- to 15-ton smooth drum vibratory rollers used in this study, the volume of soil reflected in a roller MV is cylindrical in shape and extends to 0.8 to 1.2 m (2.6 to 3.9 ft) deep and 0.2 to 0.3 m (0.7 to 1.0 ft) in front of and behind the drum. The measurement depth of roller MVs was mildly influenced by vibration amplitude; that is, a 0.1-mm (0.004-in) increase in amplitude (A) yielded a 3-cm (1.2-in) increase in measurement depth. In situ stress-strain-modulus measurements at depths to 1 m revealed highly nonlinear modulus behavior within the bulb of soil reflected in roller MVs. In base, subbase, and subgrade structures, modulus varies widely from layer to layer and within layers. Modulus values increased by a factor of 2 with depth in vertically homogeneous embankment test beds. A change in vibration amplitude from low to high created a twofold change in modu- lus. Plane strain conditions exist under the center of the drum and do not exist under the drum edges. As a result, the soil under the drum center responds stiffer than the soil under the edge. Roller MVs are a composite reflection of typical base, subbase, and subgrade structures with a surface to top-of-subgrade thickness of less than approximately 1 m (3.3 ft). The contribution of each layer to roller MV is influenced by layer thickness, relative stiffness of the layers, vibration amplitude, and drum/soil interaction issues (contact area, dynam- ics). The contribution of sublift materials to roller MVs can be significant. The amplitude dependence of roller MVs—particularly stiffness measures such as E vib and k s —is a result of stress-dependent soil modulus, layer interaction, and drum/soil contact mechanics. For vertically homogeneous embankment conditions, granular soils that are governed by mean effective stress-induced hardening may exhibit a positive roller MV-A dependence (i.e., in- crease in A yields an increase in roller MV). Conversely, cohesive soils governed by shear stress–induced softening may exhibit a negative roller MV-A dependence (i.e., an increase in A yields a decrease in roller MV). The roller MV-A dependence of layered structures is more complex and is influenced by stress-dependent soil modulus (modulus function pa- rameters), layer thickness, relative stiffness of layers, and drum/soil interaction issues. Both positive and negative roller MV-A dependence is possible, even within the same material. The roller MV-A relationship is site dependent. Roller MVs were found to be insensitive to the compaction of thin lifts [15 cm (6 in)] of stiff base material placed directly over a soft subsurface. Roller MVs were sensitive to com- paction of 30-cm (12-in) lifts of the same stiff material over soft subgrade. The sensitivity of roller MVs to compaction of thin lifts improves as the modulus ratio of the overlying to underlying layers decreases. The extraction of mechanistic material parameters using roller-based measurements for performance-based specifications consistent with mechanistic-empirical–based design (e.g., AASHTO 2007 Pavement Design Guide) is possible. However, the extraction of appropriate parameters must account for the three-dimensional nature of the roller/soil interaction, the influence of layers, the nonlinear modulus of each involved material, and the dynamics of the drum/soil interaction.  

Evaluation of Automatic Feedback Control-Based Intelligent Compaction The current technology for IC involves automatic feedback control (AFC) of excitation force amplitude (Ammann, Bomag, Case/Ammann, Dynapac) and in some cases excitation frequency (Ammann, Case/Ammann). At a minimum level, each manufacturer controls the vertical excitation force amplitude to prevent unstable “jump” mode vibration of the roller. Bomag, Ammann, and Case/Ammann employ additional AFC in an attempt to improve compaction and uniformity. The influence of AFC-based IC on compaction efficiency and uniformity was investigated on granular base material. AFC-based IC did not produce in- creased compaction or improved uniformity compared to constant amplitude mode com- paction during this test case. The response distance of AFC was found to be approximately 1 m (3.3 ft), indicating that rollers in AFC mode can respond to relatively local changes in soil conditions. The dependence of roller MVs on A can provide a misleading record of soil stiffness when operating in AFC mode. Both positive and negative MV-A dependence were observed during testing and resulted in an artificial and misleading level of variability in soil stiffness. In addition, roller MV-A dependence can trigger AFC changes in A. This is particularly problematic when roller MVs hover around a target or limit MV. The current AFC-based method to IC is a first-generation approach. As the influence of vibration force, frequency, roller speed, and so forth on soil compaction is further developed, IC approaches will likely improve and advance the compaction process. Correlation of Roller Measurement Values to Spot-Test Measurements Field testing was performed with five roller MVs from smooth and pad foot rollers and spot-test measurements from 17 different nongranular subgrade, granular subgrade, and granular subbase/base materials. The results indicated that correlations are possible to dry unit weight, modulus, and California bearing ratio (CBR) with simple linear regression analysis for test conditions with homogeneous and relatively stiff underlying layer support conditions and MVs obtained under constant operation settings. A wide range of resulting R2 values is attributed to various factors, including sublift heterogeneity, moisture content variation, limited measurement range, transverse heterogeneity, and variation in machine operating parameters. High variability in soil properties across the drum width and soil moisture content contributes to scatter in relationships. Averaging measurements across the drum width and incorporating moisture content into multiple regression analysis, when statistically significant, helped mitigate the scatter to some extent. Correlations are generally better for low-amplitude vibration settings [e.g., A = 0.7 to 1.1 mm (0.028 to 0.043 in)]. The influence of soil moisture content, compaction layer lift thickness, underlying layer properties, and machine operation settings was statistically analyzed using multiple regres- sion analysis. Where heterogeneous conditions were evident below the compaction layer, the underlying layer properties (MVs and spot-test measurements) were often statistically significant in the multiple regression model. Regression relationships improved by incor- porating the underlying layer properties. Where compaction layer properties were strongly correlated with the underlying layer properties, compaction layer spot-test measurements were statistically not significant in the analysis. Moisture content was significant for two nongranular subgrade layer test beds and one granular base layer test bed, although generally moisture content was not statistically sig- nificant in the regression analysis for most of the test bed studies. Factors contributing to this observation were (1) moisture content did not vary enough over the length of the test

  strip; (2) spot-test measurements typically only measured moisture content to about 3 in below the surface, while the measurement depth of the roller is much greater; and (3) when correlating with elastic modulus–based spot-test measurements using multiple regression analysis, moisture content is co-linear (i.e., highly correlated to in situ measurement). Am- plitude variation was statistically significant for all cases in which a minimum amplitude variation of ±0.30 mm (0.012 in) was present in the data. An approach to empirically relate laboratory-determined M r and roller MVs was pos- sible for compaction layer material underlain by homogeneous and relatively stiff support conditions. Heterogeneous supporting layer conditions affected these relationships. The re- lationships improved by including parameter values that represented the underlying layer conditions through multiple regression analysis. Case Study Implementations of Recommended Specifications Implementation of the recommended specifications allowed a direct comparison of roller-based CCC options with each other and with existing (i.e., random spot-test-driven) QA practice. Specification Option 1—using roller-integrated CCC to identify the weak- est area(s) for spot testing—requires minimal changes to typical existing QA practices but may be more stringent than current random selection spot testing. Specification Option 2a—based on the percentage change in the mean roller MV from pass to pass—appeared to be less stringent than current practice. Specification Option 2b—based on the percentage change in spatial roller MV data— appeared to be more stringent than current QA practices. One major challenge to successfully implementing specification options that require initial calibration of the roller to spot-test measurements is ensuring that the calibration area is representative of the evaluation section. Calibration-based Options 3a, b, and c require a significant initial investment of time and careful, detailed analysis. QA personnel will require careful training to ensure they are familiar with both the roller MV systems and the analysis required for the various options. For these reasons, Option 1 is recommended as a begin- ning approach. Once personnel are comfortable with CCC technology, states can advance to more complex options. Construction traffic poses a challenge to implementing CCC-based QA. All of the options require careful, repeatable rolling patterns. Construction traffic often made it difficult to create uninterrupted and repeatable evaluation area roller MV maps and to perform mea- surements in the calibration areas. Performing correlation studies in a designated full-width calibration area requires a change in how the earthwork contractor places material. The pace of the production earthwork placement and compaction frequently limited the time the research team was able to spend in the calibration area. Correlations were developed in approximately 3 to 4 hours, though a time frame of 1 to 2 hours or less would be more con- sistent with production schedules. In typical production compaction practice, roller com- pactors are used throughout the hauling, placing, and grading operation. Careful planning and cooperation between the contractor and QA personnel, together with modifications in work flow, are critical for successful implementation of CCC-based QA. The quality of the constructed earthwork is critical to the performance of pavements. Roller-integrated measurement of soil properties holds significant promise in that it pro- vides an effective and efficient tool to comprehensively assess earthwork construction qual- ity. The complete coverage capability of roller-integrated CCC is a significant improvement over current spot-test-based QA. The use of roller-integrated CCC enables departments of transportation to enforce high expectations for earthwork quality. Successful implementa- tion of roller-integrated CCC for earthwork QA requires such high expectations, as well as

the coordination and thus buy-in from departments of transportation and contractors. QA personnel and contractors will need training on the capabilities and proper use of roller- integrated CCC. Intelligent compaction, currently implemented using automatic feedback control of vi- bration amplitude and sometimes frequency, is in its infancy and will likely evolve to in- corporate numerous ways in which the process of compaction is improved and made more efficient. The capabilities and friendliness of onboard computers and software will also likely improve significantly. Finally, the measurement systems, currently limited by the influence of operating parameters, local heterogeneity, and measurement depths that far exceed lift thickness, will also evolve to account for these factors. True performance-based assessment of earthwork materials using roller-integrated CCC is within reach. The results presented here illustrate reasonably complex but determinable soil behavior within the measurement volume of a vibrating roller.