Maureen A Kestler

Determining When to Place and Remove Spring Load Restrictions on Low-Volume Roads: Three Low-Cost Techniques

Damage to low-volume roads in seasonal frost areas can be kept to a minimum by implementing seasonal load restrictions (SLRs). However, not all agencies responsible for road management implement such restrictions, and among those that do, there is no standard technique. Some use qualitative methods, such as observation or dates. Others use quantitative methods, such as measuring or estimating stiffness or subsurface temperature. Loss and recovery of summertime pavement strengths can be directly measured with a falling-weight deflectometer (FWD). Alternatively, mathematical models of varying levels of complexity, using meteorological data, are in existence or are undergoing development for SLR placement. However, initial investment for an FWD can be prohibitive for agencies such as the U.S. Department of Agriculture Forest Service (FS), for which road management is not a primary mission. Similarly, such agencies have limited personnel to conduct detailed calculations required of complex mathematical models. Consequently, easy-to-use, low-cost alternatives are needed. In cooperation with other partnering agencies, FS has recently been evaluating several techniques for determining when to place and remove SLRs. Three methods that appear promising include (a) subsurface instrumentation for temperature and moisture, (b) portable or lightweight FWDs, and (c) the thaw index. Technology-and-development efforts in each of these areas are outlined here; field test programs, observations, and analysis procedures are discussed; and recommendations for implementing each method are provided.

Spring Thaw Predictor and Development of Real Time Spring Load Restrictions

In the fall of 2006, the State of New Hampshire Department of Transportation (NHDOT) began a two - year study to develop a Real Time Spring Load Restriction Methodology. The methodology is intended to guide the Maintenance Districts in their management of spring load restrictions by identifying the beginning and duration of spring thaw. Two methods will be used to determine how long load restrictions will be needed after the date of actual thaw: frost tube readings and computer model forecasting. A total of nine sites in the central part of the state were instrumented with frost tubes, water wells, subsurface temperature and moisture sensors, and weather stations. Falling Weight Deflectometer (FWD) tests were conducted at those sites from January 2008 through June 2008 with an additional test in August 2008 and another in October 2008. The U.S. Forest Service is continuing its work with the current developer of the Enhanced Integrated Climatic Model (EICM), which is embedded in the Mechanistic Pavement Design Procedure (MEPDG). It is proposed to use the EICM (isolated from the MEPDG) with the 7–10 day forecast temperatures to predict thaw. This paper describes the overall project, data collected to date, and preliminary results.

Calibration of a Freeze-Thaw Prediction Model for Spring Load Restriction Timing in Northern New England

A major problem with low-volume roads located in seasonal frost areas is their susceptibility to damage from trafficking during spring thaw. Therefore, seasonal load restriction (SLR) policies that limit the axle loads of heavy trucks during the spring thaw period have been implemented in many countries in an effort to minimize costly roadway damage. Several agencies have been addressing the question of when to place and remove SLRs and have expressed the need for a prediction model to aid them in the process of posting roads. Models are available which predict the depth of frost and thaw penetration based upon air freezing and thawing indices, requiring only air temperature data for input. Various forms of these models have been used by transportation agencies in the United States and Canada. When using any prediction model, a key element is model validation and calibration for local conditions. The purpose of the research described herein was to calibrate a freeze-thaw index model for use in SLR timing in northern New England. Atmospheric weather data and measured subsurface temperature data obtained from nine field test sites in New Hampshire over a period of three years were used in this analysis. Frost and thaw coefficients for the model were calibrated on a site-specific basis. Results suggest that frost-thaw patterns were reasonably estimated at most of the nine test sites using this model, although the model tended to be too conservative in estimating end-of-thaw dates, with estimated end-of-thaw dates falling after measured dates in many instances.

Techniques for Extending the Life of Low-Volume Roads in Seasonal Frost Areas

Major highways are designed to withstand heavy vehicles and high traffic volumes year round. However, low-volume roads (LVRs) in seasonal frost areas are extremely susceptible to damage from trafficking by heavy vehicles during spring thaw. As a result, the maintenance-free life of an LVR in a seasonal frost area averages less than half that of a similar road in a nonfrost area. This study serves as a practical primer on addressing thaw weakening of LVRs: it offers guidance for identifying frost-susceptible soils, summarizes methods used and currently undergoing research to determine when conditions are critical, and provides several solutions for avoiding the costly impact of spring thaw on LVRs. Diagnostic tools and recommended road-usage techniques are provided for existing roads; alternative design techniques are presented for new and reconstructed roads. Tools and techniques are applicable across much of Europe, North America, and Asia; some also apply to wet areas outside seasonal frost areas.

Keeping Springtime Low-Volume Road Damage to a Minimum: Toolkit of Practical Low-Cost Methods for Road Managers

There are approximately 3 million miles of low-volume roads (LVRs) in the United States, and approximately half of them are located in seasonal frost areas. Limiting or prohibiting loads during spring thaw can keep damage to a minimum. However, methods of determining when to place and remove spring load restrictions, particularly on LVRs, are often highly subjective—if restrictions are imposed at all. In partnership with several other agencies, the U.S. Department of Agriculture Forest Service has been compiling a toolkit of practical low-cost diagnostic techniques for determining conditions under which spring load restrictions should be placed and removed. This paper expands on techniques reported in a previous paper from a TRB low-volume roads conference and reports on further developments of additional methods. Techniques discussed include (a) subsurface instrumentation, (b) lightweight deflectometer, (c) thaw index, (d) climatic thaw predictor model, and (e) length of time. Requirements and equipment needed to use each of the techniques are described, strengths and weaknesses of each are outlined, and recommendations on various combinations of methods are provided to enable road managers to optimize placement of spring load restrictions.

Estimating When to Apply and Remove Spring Load Restrictions

A significant percentage of roads in seasonal frost areas are not designed to carry normal loads during the spring thawing process. Determining dates to apply and remove seasonal load restrictions (SLR) is of great interest to many federal, state and local agencies. In a recent study for the USDA Forest Service, Berg (2004) developed a method which estimated SLR application dates at a site in Minnesota and another in Ohio. In this report, we apply the same method to estimate SLR application dates on a road in New Hampshire and an airport parking apron in Vermont. The calculated results are compared to dates the SLR was actually applied and computed results were nearly the same as the application dates which were established by state DOT personnel. The method is also extended to estimate the time that the SLR can be removed. The Modified Berggren Equation was used to develop a procedure, but a regression equation developed by Rutherford, Mahoney, Hicks and Rwebingira (1985) was more accurate when compared to measured data.

EVALUATING MOISTURE SENSORS AND MONITORING SEASONAL MOISTURE VARIATION IN LOW-VOLUME ROADS

For the past several years, the Forest Service of the U.S. Department of Agriculture has been evaluating a quantitative technique for the application and removal of load restrictions by observing relationships among pavement stiffness, pavement damage, soil moisture, and seasonal freezing and thawing. Laboratory tests of time-domain reflectometry (TDR) and radio frequency (RF) sensors showed them to be reasonably accurate and repeatable when compared with known moisture values in several soil types. Laboratory tests of the probes under repeated adverse freezethaw cycling showed the probes to be durable. Although the field survival rate of TDRs surpassed that of RFs, analysis of field data collected at seven locations in four national forests showed that permanently installed sensors strategically located on a forest road network can provide an affordable method for quantitatively determining the beginning and end of critical periods of pavement weakening associated with springthaw. This information would be useful in administering periods of spring-thaw load restrictions. The laboratory and field test programs conducted are outlined. The field technique is applicable to any secondary road subjected to seasonal freezing.

Removing Spring Thaw Load Restrictions from Low-Volume Roads: Development of a Reliable, Cost-Effective Method

Low-volume roads in areas of seasonal freezing are highly susceptible to damage from trafficking during spring thaw. To minimize pavement damage, many agencies and states impose load restrictions during periods in which damage is most likely to occur. However, the magnitude and duration of reduced or prohibited hauling vary widely among agencies, and an optimal balance between maximizing local economy and minimizing road damage is rarely achieved. The U.S. Department of Agriculture Forest Service and the U.S. Army Cold Regions Research and Engineering Laboratory are evaluating a quantitative technique for removing load restrictions by developing correlations between pavement stiffness and soil moisture. Laboratory tests of the moisture sensors showed them to be accurate and repeatable under adverse freeze-thaw cycling. Preliminary analysis of field data showed that permanently installed time domain reflectometry and radio frequency soil moisture sensors strategically located throughout the forest road network will provide an affordable method for quantitatively determining when to remove load restrictions. Load restriction practices are reviewed, economic ramifications on the forest industry are briefly discussed, and laboratory and field test programs conducted to monitor soil moisture and pavement stiffness are outlined. In addition, instrumentation used for the study is described, observations from one of four national forest pavement test sites are presented, and the ongoing research to develop a method to remove load restrictions is discussed.

Initial Analysis of the New Hampshire Spring Load Restriction Procedure

In late 2006, the New Hampshire Department of Transportation (NH DOT) initiated a study to develop a Real Time Spring Load Restriction (SLR) Methodology to guide the Maintenance Districts in their management of spring load restrictions by identifying the beginning and duration of the spring thaw period. The study includes selection of test areas, purchase and installation of instrumentation, collection and analysis of data from the instruments and development and implementation of the methodology, Eaton, et al (2009), discuss details of the project. In this paper the authors discuss the initial procedure, the installed instrumentation, observations from the instruments, and refinements indicated by the observations and initial analysis of the data.

Reducing damage to low-volume roads by using trucks with reduced tire pressures

Heavy-volume highways in seasonal frost areas are designed to resist the effects of spring thaw. However, timber access roads, county roads, and other low-volume roads with thin bituminous surfaces can be quite susceptible to pavement damage during midwinter- and spring-thaw periods. To reduce damage to low-volume roads, towns, cities, and states typically either post reductions in allowable load or completely prohibit hauling during damage-susceptible periods. Associated economic impact can be significant. To evaluate the effects of tire pressure on cumulative road damage, a mechanistic pavement design procedure developed by the U.S. Army Corps of Engineers for use in seasonal frost areas was used on a matrix of tire pressures, low-volume pavement cross sections, and environmental conditions. A series of computer simulations showed (a) trucks operating with conventional tire pressures can cause excessive damage, particularly in the form of cracking, to low-volume roads with thin bituminous surfaces during relatively short thaw periods; (b) pavement damage could be reduced substantially by restricting hauling to trucks operating with reduced tire pressures; and (c) there are “threshold” tire pressures under which only minimal damage occurs, even during critical spring thaw. These results could influence guidelines for hauling restrictions and, in turn, associated economics.