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31 CASE 5. Load Transfer Efficiency The procedure is documented in ODOT's Manual for Aban- doned Mine Inventory and Risk Assessment (1998). An example of a forensic study using LTE was conducted in Michigan (Peng et al. 2005). A time history analysis of the deflection data showed that the dowels were likely loose. CASE 7. Spring Load Restrictions Deflection testing showed that permanent loss of slab con- tact with the base (void) existed near the doweled joint. The North Dakota DOT imposes limits on the per-axle weight trucks may carry during the spring thaw. These spring load restrictions are imposed to save the pavement CASE 6. VOID DETECTION layers from otherwise-avoidable and significant damage. A North Dakota DOT website ("Implementation of Spring FWD data are used to detect voids where pavement layers Load Restrictions . . ." 2007) details the three main factors have no support. used to determine when the restrictions should be posted. Direct strength measurements, interpreted from FWD data, Undersealing are combined with long-range weather forecasts and tem- perature probes. To fill voids under a PCC pavement, injection holes are drilled into the pavement and a grout of cement, fly ash, and water is SDDOT adopted using FWD data for spring load restric- pumped through the holes. This procedure is referred to as tions in 1996. SDDOT had recorded centerline miles sub- "undersealing" by the South Dakota DOT (SDDOT). Before jected to spring load restrictions since 1969. In the years and after drilling holes, voids in the pavement are detected since spring load limits were instituted in South Dakota, the using FWD data. The FWD loading plate is placed as close percentage of road network mileage requiring load restric- as possible to the slab corner, and the LTE to the adjacent tions during the spring thaw has generally decreased. When joint is measured. If the measured deflections fall out of a FWD data became a criterion for spring load restrictions range determined by the state engineer as acceptable, under- in 1996, the number of lane-miles subject to restriction sealing procedures begin (Standard Specifications for Roads increased temporarily (12.7% of the roadway network in & Bridges 2004). 1996 versus 11.1% in 1994 and 1995), but continued their downward trend thereafter. By 2007, 3.5% of SDDOT's In a research report, MoDOT disseminates their FWD roadway network was subject to spring load restrictions. void detection efficiency findings. Because voids under PCC- SDDOT attributes the additional limits to FWD data ("2007 bridge approach slabs contribute to premature cracking, early Spring Load Restriction Summary" 2007). detection of these voids is crucial to avoid costly replacement and rehabilitation measures. Based on the study's findings, NYSDOT utilized FWDs to study the seasonal variabil- MoDOT recommends that FWD should be used to determine ity of pavement layer moduli. Regions experiencing winter- voids under PCC slabs. This recommendation assumes that freeze, spring-thaw conditions in the soil undergo severely the FWD and operator are available, undersealing is being weakened subgrade layers during the thaw season. Because considered as a preventive maintenance treatment, and one such seasonal differences in pavement layer moduli severely or more of the following conditions are met ("Void Detection affect pavement surfaces, pavement designers must com- with the Falling Weight Deflectometer" 2004): pensate for them. Six possible seasons were identified (Orr 2006): Long lane closures for proof-roll testing are not desir- able (e.g., at bridge approaches with reduced shoulder Freezing, when frost is present in less than 100 mm widths and high-volume routes). (4 in.) of the subgrade layer. Fewer personnel than required with proof rolling are Winter, when at least 100 mm (4 in.) of the subgrade available for testing. layer contains frost and no thaw is present. The pavement shoulder is unstable for accurate proof- Spring thaw, when any thaw in the unbound layers is rolling measurements. present and some portion remains frozen. More clear and quantifiable indications of undersealing Spring recovery, when resilient modulus increases improvements than proof rolling can provide are desired quickly because of drainage. (i.e., AASHTO rapid void detection procedure). Spring, when all frost has thawed, but precipitation outpaces evaporation. Summer, when evaporation outpaces precipitation. Abandoned Mine Detection To identify when such seasonal parameters are necessary, The Ohio DOT is experimenting with the use of FWD data Cornell University and NYSDOT developed a geographical to supplement investigations of abandoned mine detection. model that shows which portions of New York are subject