7th International Conference on Managing Pavement Assets (2008)

Transcription

1 Asset Management of Gravel Airstrips in The Yukon Canada Donaldson R. MacLeod, Ph.D., P.Eng Manager, Highways and Bridges Public Works and Government Services Canada PdP lll 8A1 11 Rue Laurier Gatineau, Qc, Canada Tel Fax Wally Hidinger, P.Eng Manager, Transportation Planning and Programming Yukon Territorial Government 461 Range Road Whitehorse Yukon, Canada Y1A 3A4 Tel Fax ABSTRACT The Yukon Department of Highways and Public Works operates a series of gravel airstrips in isolated parts of the Yukon. In addition to the primary function of providing services to general aviation, these isolated airstrips support medical emergency evacuations and act as temporary bases for forest fire fighting aircraft and equipment. The Department has well-established bridge inspection/ management, pavement and bituminous surface treatment management systems for its highway network and a pavement management system for its major airports. Lacking however was an inspection and rating system for its gravel airstrips. This paper describes the establishment of a rating system for these low volume gravel airstrips. It identifies the distresses that should be monitored during an annual inspection. A number of composite indices were developed similar to the PCI used for pavement management, for use with the gravel airstrips. Due to the variation of gravel surfaces depending on weather and the last gravel blading, a general condition index based on the overall visual observation of the airstrip was judged to be more effective than composite indices based on individual distresses. The major feature of the management system for these airstrips is the identification of maintenance items required for the operation and preservation of the airstrip and the identification of the time frame required for major capital investments (gravel resurfacing). INTRODUCTION The aviation system administered by the Yukon Government is unique. It includes one national/international airport at Whitehorse, two regional airports at Watson Lake and Dawson

2 MacLeod and Hidinger Page 1 City, 11 community airports at other centres of population and an additional 13 local airstrips. Figure 1 shows the locations of all Yukon airport facilities. This paper deals with 10 of the community airports. The eleventh is located at Old Crow which does not have highway access and as a special case because of its isolation is excluded from this study. The remaining 10 airports can be characterized as having good land access, gravel surfaces and low traffic volumes. Most runways are 30 metres wide and are between 1000 and 1500 metres in length. Some have Community Aerodrome Radio Service (CARS) operators on a varying shift schedule while others are unmanned. The fact that all have good land access distinguishes these facilities from other remote airports in Northern Canada. As such they do not have scheduled passenger service nor do they serve major cargo re-supply functions. Although the usage varies from airport to airport, these community airstrips provide for; 1. Emergency medical evacuations. 2. Temporary bases for forest fire fighting aircraft. 3. Supply bases for local outfitters and mining exploration camps. 4. Emergency landing facilities during poor weather. 5. General Aviation services for the local community. The requirements for medical evacuations and forest fire fighting are controlling factors in the level of service at these airports and is what distinguishes them from the 13 other local airports. In 1997, the Government of Yukon, with extensive technical assistance from Public Works and Government Services Canada, developed surface management systems for paved and gravel airports within the Territory. These asset management systems complemented the Bituminous Surface Treatment (BST) management system (1) and pavement management system (2) used for the Yukon highway network. The highway surface management systems have proven to be successful tools for both the short and long term planning of rehabilitation and maintenance of highway facilities. The surface management system for gravel airports was designed to complement the studies of other Yukon infrastructure assets. GRAVEL MANAGEMENT SYSTEM Compared to pavement management systems, there are very few existing systems for the management of gravel roads or airports. This gravel surface management system was based on developmental work by the Ontario Department of Highways (3) and the US Army Corp of Engineers (4).

3 MacLeod and Hidinger Page 2 FIGURE 1 Airports in the Yukon The objectives of the management system were to: 1. Inspect the gravel airstrips on an annual basis, in conjunction with the pavement and BST rating of adjacent highways. 2. Provide guidance for local maintenance personnel as these are the only inspections carried out by technical personnel. 3. Identify short-term rehabilitation projects that are beyond the capability of local maintenance forces. 4. Develop prediction models similar to pavement management systems to forecast long

4 MacLeod and Hidinger Page 3 term rehabilitation needs. 5. Establish priorities for major rehabilitation needs. FIGURE 2: Typical Community Airstrip INSPECTION A review of highway literature and a consensus of pavement and maintenance personnel identified 14 distresses (Table 1) that should be evaluated in terms of severity and extent on the gravel airstrips. Most of the distresses are self-explanatory, surface drainage refers to the drainage of the gravel/traveled portion, whereas airfield drainage refers to the drainage of the facility as a whole. Maintainability refers to the ease with which the airfield can be maintained with normal on-site equipment.

5 MacLeod and Hidinger Page 4 TABLE 1 Weighting Values for Gravel Distresses Distress Weighting Value (w i ) Factors Independent of Rating Timing Gravel Supply 3 Cross Section 3 Subgrade Failures 3 Settlements/Distortions 3 Airfield Drainage 2 Maintainability 3 Factors Dependent on Timing of Rating Rutting 2 Potholes 1 Washboard 1 Surface Drainage 2 Loose Gravel 1 Slipperiness 1 Dust 1 Patching 1 A rating guide was prepared to aid in consistent evaluation of the extent and severity of the individual distresses. With the implementation of the Airstrip Gravel Surface Management (AGSM) system it was noted that a common problem in the evaluations of the gravel facilities was that many of the distresses were dependent on weather and existing surface moisture conditions at the time of inspection and on the timing of the last maintenance intervention. For example, dust conditions depend on whether the airport was rated during a dry spell or following a rain. Unfortunately this is a limitation of any gravel surface rating system. In order to address the problem the rating factors in Table 1 were established to distinguish between distresses that were dependent or independent of the time of rating. Those distresses that were more independent and related to the inherent physical properties of the materials on site were given higher rating values w i for the calculation of the overall condition index GCI described below, than those factors which were more dependent on conditions at the time of rating. Table 2 is a summary of the output of the AGSM. It includes comments on maintenance and maintenance strategies that are of interest to the local airport operators and rehabilitation comments and strategies that are of interest to more senior planners and managers. GUIDANCE FOR LOCAL MAINTENANCE Local maintenance forces at the airstrips are limited in the equipment available normally a grader, a compactor and a water truck. The inspections revealed a rather short list of miscellaneous deficiencies as for the most part the airstrips were well maintained from this point of view. The list varies from year to year and from airport to airport, however, a common issue was the lack of drainage at the junction of the taxiway and runway.

6 MacLeod and Hidinger Page 5 SHORT TERM REHABILITATION PROJECTS Although it is sometimes difficult to separate rehabilitation from maintenance, comments for rehabilitation were considered to apply when equipment additional to that used in normal maintenance was required, and comments were considered as maintenance when normal on-site forces could handle the problem. The most common short-term rehabilitation project, not surprisingly, was gravel resurfacing either due to a perceived lack of quality and/or lack of gravel. A common concern at all airports was the quality of the surfacing gravel. At those airports where there were sufficient fines to bind the gravel so that there was no loose gravel, most operators complained that the runway was soft and rutted when wet thereby increasing the drag on departing aircraft. On other runways where fines were insufficient to knit the surface together, operators complained of propeller damage and/or flying stones. Within limits, there is only so much that can be done to control the amount of fines in the surfacing material, as the fines are a function of local gravel pits. Fines can be added to clean gravel to make the surface "knit" together limiting the problems of flying stones but at the expense of soft surfaces and rutting during inclement weather. As a general rule, the fewer the fines, the better the structural support. As a result of the AGSM system, a revised gradation was developed for airport applications that tried to mitigate the issues, realizing that in some cases there was no local aggregate that could meet all the requirements. LONG TERM ANALYSIS The data collected in the study identify short-term work activities requiring action such as gravel supply, subgrade failures, surface drainage, airfield drainage, and lack of cross section. In addition to providing information for maintenance and immediate rehabilitation projects as above, a gravel surface management system should also provide other information for long term planning for major rehabilitation where funding must be established well in advance of the project. For these longer-term needs, an index can be used to predict future behaviour. Such an index is useful for determining priorities between different airstrips, establishing long term budgets and for comparing the average condition of an airstrip from year to year, allowing an evaluation of whether the system is improving or deteriorating. Two indices are used for reporting the overall condition of Yukon gravel airports. The GCR (General Condition Rating), an overall rating for gravel airstrip, and the GCI (Gravel Condition Index) that is calculated based on the rating of individual distresses. The GCR system is simplest to use and gives a quick evaluation of the airfield condition. It is an index based on the raters' evaluation of the general condition of the airfield. The GCR is rated using the guidelines in Table 3. It is very subjective and the rating levels are so coarse that comparing a rating from one airfield to another or between years at the same airport may not be meaningful.

7 MacLeod and Hidinger Page 6 TABLE 2 Airside Gravel Surfaces Distress Summary Airport Facility Runway Condition GCR Gravel Supply Beaver Creek Runway VERY GOOD 85 VERY GOOD Main Rehabilitation Comments Dust suppressant evident over center portion of runway for entire length. Vegetation control on airfield in progress. Main Comments Water ponding on airfield at east end. Vegetation control required on stub taxiway. Rehabilitation Strategy Strategy Burwash Landing Runway GOOD 70 POOR Oversize showing on runway surface. Drainage problem near taxiway/runway intersection. Vegetation control on apron. Extensive lack of gravel of appropriate size. Thin Resurface next year - Vegetation Control Carcross Runway FAIR 60 FAIR Gopher holes in runway to be filled. Sand area at west end. Vegetation control. Carmacks Runway GOOD 70 VERY GOOD Some oversize gravel on surface. Cousins Runway GOOD 60 Fair Drainage on apron needs to be improved. Close of alternative access to prevent unauthorized traffic Recommend extra compaction when grading to help with loose gravel. Drainage problem exists at junction of taxiway and runway. - Drainage Improvements - Re-establish Crown More Blading

8 MacLeod and Hidinger Page 7 TABLE 2 Airside Gravel Surfaces Distress Summary (cont d) Airport Facility Runway Condition GCR Gravel Supply Main Rehabilitation Comments Ross River Runway FAIR 70 FAIR Depression across strip near east end wind sock (located at fifth light from end). Coarse area near centerline between lights 3 and 4 needs repair. Main Comments Rehabilitation Strategy Excessive loose gravel needs Spot compacting and blading. East Reconstruction end wind sock needs replacement. Gopher holes outside light line. Ruts in taxiway (east). Access road needs gravel and grading. Strategy More Blading

9 MacLeod and Hidinger Page 8 TABLE 3 GCR Rating Guidelines Level Description The airfield is well shaped with well defined shoulders. There are no surface distress manifestations, and there is no more than a slight classification for dust and loose gravel. There are no frost heaves or soft spots and there is good drainage for surface run off The airfield surface is well shaped. There are some distress manifestations in the slight to moderate category, such as loose gravel, dust and potholes. There may be a few soft spots in the spring. There is good drainage for surface run off A mixture of properly shaped runway surface and improperly shaped areas. Distress manifestations such as surface drainage are in the slight to moderate class, as are various surface distress manifestations, including washboard and potholes. There may be localized soft spots. Increased routine maintenance is required and spot gravel application may be necessary The majority of the airfield is improperly shaped. Surface drainage is impeded. There are some localized break up areas and various distress manifestations such as washboards, potholes or distortions are in the severe or very severe classification. with the addition of gravel is necessary. Some portions may need rehabilitation A flat or reverse crown characterizes the surface or there are severe airfield distresses such as washboards, loose gravel and potholes. Water is trapped along the edge of the airfield. There is little or no gravel. Rehabilitation is necessary. The GCI is a composite index based on the individual distresses. The index is more refined than the GCR and it was hoped that it would allow the comparison of different airstrips. It is an adaptation of the PCI used for pavements. (2) It is calculated from the Gravel Distress Manifestation Index (GDMI), which is based on utility theory, thereby enabling all distresses to be put on the same scale and their contribution is combined to give the GDMI. n GDMI = Σ w i (s i + e i ) (1) i =1 Where: GDMI = Gravel Distress Manifestation Index. GDMI is an overall characteristic describing surface condition in terms of distress manifestations. w i = weighting value representing the relative weight of each distress manifestation. Those distresses that are considered more serious are given larger weighting values (Table 1). The rationale behind these weighting values was that distresses requiring major work were assigned 3, distresses requiring additional work for improvement are assigned 2, and distresses that can be resolved with a grader or are not sufficient in themselves to justify rehabilitation were assigned 1. s i = severity of a distress manifestation expressed on a scale from 0-4. e i = extent of a distress manifestation expressed on a scale from 0-4.

10 MacLeod and Hidinger Page 9 n = the total number of distress types. The GCI is an overall rating of the gravel section on a scale of 0 to 100. The higher the GCI, the better the condition of the pavement. The GCI is calculated using the following formula: GCI = (100 x ((0.1 x RideScore) 05 ) x ((192-GDMI)/192) x 0.924) (2) Figure 3 shows a comparison of the two methods. The correlation between the GCR and GCI is good and is consistent over the entire range of gravel surface conditions y = x R 2 = GCR FIGURE 3 Comparison of GCR and GCI Given the reasonable correlation between the two indices, the question of which one to use arises. The GCR by its general definition is a course index and does not indicate annual changes unless there has been a very significant traumatic event in the past year. The hope was that the GCI would fill this role. However, variations due to weather and the last maintenance on the airstrip mask any changes in the overall condition of the airport. The main advantage of continuing to gather the data for the GCI is that it forces the rater to look at all of the individual distresses that affect the airstrip performance. Over the long term, Table 4 indicates that the overall condition of the gravel airport system had slowly decreased between 1999 and However, there was a significant increase in overall system performance in 2005, due largely to runway resurfacing and dust control application at Beaver Creek, resurfacing at Pelly airstrip and reconstruction at Dawson airport. GRAVEL REQUIREMENTS One of the major improvements in the managing of the gravel airstrips with the

11 MacLeod and Hidinger Page 10 implementation of the AGSM system was the recognition of a need to identify current and future aggregate requirements. Previously, gravel requirements for airports were identified on an ad-hoc, relatively reactionary and short term basis. If gravel is required for airports under this surface management system, the quantities, availability, locations and costs of gravel for each airport are determined. Table 5 outlines gravel requirements for the airports included in this report and the approximate dimensions, location of the nearest pit and stockpile amount. SUMMARY AND CONCLUSIONS The development of a surface management system for Yukon gravel airstrips dealt with challenges encountered in dealing with runway surface conditions that changed frequently due to maintenance activities and/or the weather. The system indicates that for the most part, the airsides of the airports are in good condition and indicates the priorities for rehabilitation. Although the system puts an emphasis on the description of current distresses and required immediate corrective work rather than on numerical predictions of future conditions, it allows for the identification of future priorities. The system provides the framework for a longer-term network type analysis to develop a five-year plan to ensure that each airport meets the current and anticipated demand and operational requirements. TABLE 4 Performance of Yukon Community Airstrips Beaver Creek Runway Burwash Landing Runway Carcross Runway Carmacks Runway Cousins Runway Dawson City Runway Faro Runway Haines Junction Runway Mayo Runway Pelly Crossing Runway Ross River Runway Teslin Runway Average Runways

12 MacLeod and Hidinger Page 11 TABLE 5 Gravel Requirements Gravel Thickness Length Width Volume Nearest Stockpile Airport Required (mm) (m) (m) req d(m 3 ) Pit # Available(m 3 ) Beaver Creek No Burwash Landing , No Carcross No Carmacks No Cousins No Dawson City No Faro No Haines Junction Mayo No Pelly Crossing No Ross River , Teslin REFERENCES 1. MacLeod,D.R., Walsh R. and Hidinger W. Development of a Multi-Year Operational Plan of a Low Volume Pavement Network. Proc. 5th International Conference on Managing Pavements, Seattle, Washington, Transportation Research Board, Washington D.C., MacLeod, D.R. Integration of Pavement Management into the Strategic Plan for the Reconstruction of the Alaska (Alcan) Highway, Proc. 6th International Conference on Managing Pavements, Brisbane, Australia, Transportation Research Board, Washington D.C., Manual for Condition Rating of Gravel Surface Roads. Ontario Ministry of Transportation, Research and Development Branch Report SP-025, Toronto, Ontario, Eaton R., Gerard S, and Cate D,Rating Unsurfaced Roads A Field Manual for Measuring Problems. US Army Corps of Engineers, Cold Regions Research and Engineering Laboratory. Special Report 87-15, August 1987.