, Former Professor, Civil, Environmental and Geo-Engineering
Joseph Labuz, Professor, Civil, Environmental and Geo-Engineering
Deflection testing and analysis is routinely used to evaluate the spring load capacity of pavements, and to design structural overlays. The current process used by Mn/DOT and implemented in the computer program TONN is not very reliable. The program underestimates the allowable axle load for soft clay subgrades and may overestimate it for stiff sand or granular subgrades. This was recently demonstrated by analysis of the test cells on the MnROAD Low Volume Loop.
In this study, researchers developed a procedure to determine the structural adequacy and need of seasonal axle load restrictions for Minnesota low-volume roads. This procedure has been implemented into a new program, TONN2010. Since it is anticipated that the results of this study will be widely used by Mn/DOT, city, and county engineers, as well as consulting engineers involved in analysis of the falling weight deflectometer (FWD) data collected by the transportation agencies, an emphasis was made on development of a simple, easy to implement procedure.
To simplify the procedure's implementation, the number of inputs was minimized. TONN2010 utilizes pavement layer thicknesses, FWD deflection basins, air temperature of the previous day, pavement surface temperature at the time of testing, pavement location, and anticipated traffic. All the inputs required by TONN2010 can be easily obtained by the user. Using these inputs, TONN2010 proceeds to 1) back-calculate layer moduli using the back-calculation procedure developed in this study, 2) adjust the back-calculated moduli using MnPAVE temperature and seasonal adjustment factors, and 3) estimate pavement axle load capacity by mechanistic-empirical analysis. In addition to detailing TONN2010, the researchers further describe the selection of the damage models, development of the back-calculation design procedure, determination of the critical structural responses, development of new structural rating indexes, and finally the calibration and validation of the proposed procedure.