Intelligent Compaction Control of Highway Embankment Soil

Mechanistic pavement design procedures based on elastic layer theory require characterization of all pavement layer materials including subgrade soil. This paper discusses the subgrade stiffness obtained from a new compaction device called the Intelligent Compaction (IC) roller on highway embankment projects in Kansas. Three test sections on two routes were compacted using a single, smooth steel drum Bomag Variocontrol (BVC) intelligent roller that compacts and at the same time, measures stiffness values of the compacted soil. Traditional compaction control measurements like density testing, in-situ moisture content, soil stiffness measurements using Geogage, surface deflection tests using Light Falling Weight Deflectometer (LFWD) and Falling Weight Deflectometer (FWD), and penetration tests using a Dynamic Cone Penetrometer (DCP), were also done. The results show that the IC roller was able to identify the locations of lower soil stiffness in the spatial direction. In general, IC roller stiffness showed sensitive to the field moisture content. No universal correlation was observed among the IC roller stiffness, Geogage stiffness, backcalculated subgrade soil moduli from the LFWD and FWD deflection data, and the California Bearing Ratio (CBR) obtained from the DCP tests. TRB 2007 Annual Meeting CD-ROM Paper revised from original submittal. Rahman, Hossain, Hunt, and Romanoschi 3 INTRODUCTION During last two decades, pavements have been designed using several rational design procedures. Design of pavements using these new tools requires detailed inputs on material response and damage properties. Example of such a tool is the newly developed MechanisticEmpirical Pavement Design Guide (M-EPDG) of the National Cooperative Highway Research Program (NCHRP) (1). Foundation layer modulus, strength, and permeability are considered as the most critical material response parameters under repeated traffic load. Pavement performance is highly influenced by the above mentioned subgrade characteristics as well as by the ease and permanency of compaction. Subgrade compaction increases strength, decreases permeability, and reduces undesirable settlement. Current compaction quality control methods are fully based on the results of the laboratory compaction tests. The in-situ dry density of the soil is measured after compaction and compared with the laboratory maximum dry density. A number of methods such as, sand cone, rubber balloon and nuclear gage are used to measure the in-situ density. The in-situ moisture is also measured by the nuclear gage and other measurement methods, and controlled. Intelligent Compaction Control FHWA (2) defined the intelligent Compaction (IC) technology as “Vibratory rollers that are equipped with a measurement/control system that can automatically control compaction parameters in response to materials stiffness measured during the compaction process. The roller must also be equipped with a documentation system that allows continuous recordation, through an accurate positioning system, of roller location and corresponding density-related output, such as number of roller passes and roller-generated materials stiffness measurements.” The Intelligent Compaction (IC) is made possible because of the ability of a vibratory roller to first sense the material response of soil under loading, to process this information and compare it to the input requirements, and then to “decide” how to adjust compaction parameters to most efficiently compact the material. Since none of these features are available on conventional vibratory rollers, IC represents a major innovation in soil compaction technology (2). Currently, this technology is marketed by BOMAG from Germany, AMMANN from Switzerland, and DYNAPAC from Sweden. CATERPILLAR has also a system available for demonstration (3). Stiffness Measured During Intelligent Compaction “Material Stiffness” is the conceptual basis for intelligent compaction (2). During this compaction process, the stiffness is measured with a vibratory roller equipped with an accelerometer-based measuring system. The stiffness (roller vibratory modulus) is calculated continuously (30 to 60 times per second) as a function of the acceleration of the roller drum (or force) and the deformation of the compacted soil while the vibratory roller moves down the roadway. The soil-drum-interaction-force (FB) is calculated by the following equation: ( ) ( )g m m t r m x m F d f u u d d B + + Ω Ω + − ≅ cos 2 .. (1) where, md = mass of the drum (kg); xd = vertical displacement of the drum (m); TRB 2007 Annual Meeting CD-ROM Paper revised from original submittal. Rahman, Hossain, Hunt, and Romanoschi 4 .. d x = acceleration of the drum (m/s ); mf = mass of the frame (kg); mu = unbalanced mass (kg); ru = radial distance at which mu is attached (m); muru = static moment of the rotating shaft (kg.m); and Ω = f ⋅ ⋅π 2 ; f = frequency of the rotating shaft (Hz). The force acting on the positive direction (downward) has a plus sign and the inertia force d d x m shows negative value with respect to the corresponding coordinate. If the subsoil is described as a spring and dashpot system, the equation of soil-drum-interaction-force can also be written by: ' d B d B B x d x k F + ≅ (2) By setting equation (1) equal to equation (2), the soil stiffness kB can be obtained since all other parameters are known except the damping ratio which is considered 20 percent according to the manufacturer. Alternatively, the slope of the plotted force-settlement curve on the loading portion can be considered as the dynamic stiffness of the material being compacted. FIGURE 1 Soil reaction vs. roller amplitudes (after 4). Bomag Corp., one of the manufacturers of IC rollers, has developed a compaction quality measure, vibration modulus, EVIB (MN/m ). Soil stiffness, kB, is used as the basis for calculation of the vibration modulus. Since the modulus is a true independent soil parameter, the following relationship between kB and EVIB is proposed based on the Hertz and Lundberg theories: ( ) ( ) ( )                 ⋅ ⋅ + ⋅ ⋅ − ⋅ ⋅ ⋅ + ⋅ − ⋅ ⋅ ⋅ =