Integration of Structural Information into an Urban Asset Management System
The Strategic Services Branch of the City of Saskatoon (COS) has observed that neighborhoods such as Erindale and Briarwood, exhibited more fatigue cracking than neighborhoods that were much older. Many of these roads appeared to be structurally deficient even though they were less than twenty years old. Asset management systems utilize data collection and analysis to derive cost-effective solutions for preservation of assets. Since 1993, the City of Saskatoon (COS) has employed an asset management system that focuses on surface deterioration and ride quality to prioritize treatments. However, this research has found that reliable prediction of the structural condition of roads based on surface distress information can be difficult to obtain and inaccurate. The current COS surface condition ratings, as collected, can’t be analyzed to accurately determine if a roadway is structurally deficient since surface cracking can be from multiple causes. Additionally, even if the surface rating accurately identified fatigue cracking, the damage to the roadway sub-structure has already typically occurred and the repair options are more limited. Given structural road issues are the most costly deficiency to correct, this component of the current COS asset management system needs to be addressed if optimized life cycle road preservation is to be attained. Therefore, the incorporation of a more reliable structural measure would improve the life cycle funding optimization within the COS’s asset management system. This is particularly the case for the local and collector road systems as they comprise of over seventy percent of the COS road network. The objective of this research was to demonstrate the value of road structural condition survey information obtained from heavy weight deflectometer (HWD) measures into the existing COS asset management system across local and collector road classes on a pilot project basis. The structural integrity of a roadway is a critical characteristic to its long-term performance. Roadways that are structurally deficient will not be able to remain safe and are more costly to maintain over their life cycle compared to structurally sound roadways. Many Saskatoon roads, particularly locals and collectors, from 1985 to present were constructed with uniform layer thickness regardless of sub grade condition (COS, 2010). This has resulted in structural failures or reduced life cycles in areas with poor sub grade conditions. As part of this research starting in 2006, the COS used HWD measurements to assess the structural condition of their road network to provide a fundamental engineering mechanics approach to characterizing the insitu condition of COS streets. This research utilizes network level HWD measurements from 2006 to 2010 on the local and collector road classes. Using COS network level surface distress data and condition ratings from their pavement management system, the effect of using structural data as measured by HWD was examined. This research established structural primary response condition states based on peak deflection thresholds utilized to divide the roads into good, fair, and poor structural condition at primary weight loading. The structural response of the local pavements was found to be highly dependent on the sub grade characteristics. Most local and collector streets in Saskatoon have been built using the Equivalent Single Axel Loading (ESAL) design method and a standard thickness structure corresponding to a soaked California Bering Ratio (CBR) of 5. Thin pavement structures, such as those utilized on Saskatoon’s local and collector streets have been unsuccessful, in certain instances, at protecting weaker sub grades from critical state loading, particularly during spring thaw. These designs are based on the ESAL method that has been used by the Ministry of Highways for decades. Heavy truck traffic such as transit, solid waste trucks, and construction vehicles such as cement and tandem trucks commonly use residential and collector roadways. On thick structures these vehicles simply add more ESAL loads but a thin roadway structure may not adequately protect the sub grade and these vehicles may cause damage with a single pass under specific conditions. This could result in structural failures in neighborhoods where the sub grade condition is poor, therefore inflicting significantly higher strains than those originally accounted for in the ESAL design. This research found that local and collector streets in neighborhoods such as Briarwood and Erindale, which were constructed in the 1990s, have several streets that are rated as structurally poor and reconstruction has occurred on some of the streets. Most of the local and collector streets in these neighborhoods, based on the HWD data within this study, will reach an average deflection that will result in poor structural condition in less than 30 years. The percentage poor local streets, based on this structural research, in Briarwood and Erindale are 82.7 and 56.3 respectively. By contrast, neighborhoods such as Meadowgreen and Parkridge were primarily constructed before 1983 and no roads were found to be structurally poor based on this research. The local and collector streets in these neighborhoods are expected to last more than 60 years prior to requiring reconstruction largely due to their improved sub grade conditions. As of 2012, no local or collector streets in Meadowgreen and Parkridge have been reconstructed. The new neighborhood of Willowgrove, through HWD testing, was found to have 59.8% of local roadways that were structurally poor and 5.1% of roads tested were found to be structurally good based on the thresholds established within this study. In 2008 there was no observed fatigue cracking in Willowgove which is used as a structural indicator within the current COS asset management system. In the spring of 2011 severe fatigue cracking was found in multiple locations on four separate road segments in Willowgrove. In 2010, Saskatoon experienced one of the wettest years in a century. This resulted in the water table in Willowgrove to increase by several meters higher than experienced in the past 60 years were data was available. There is a good chance that some of these roads will have to be partially reconstructed in the foreseeable future. Currently the City of Saskatoon pavement asset management system is based solely on surface condition data and it is utilized to select roadway preservation, restoration, and rehabilitation treatments based respectively on ravelling, depression score, and cracking measurements. However, unexpected road failures have recently occurred on segments that had not qualified for treatment based on these surface condition measures. The lifecycle performance of roads constructed in areas with poor sub grades is worse than roads constructed in good sub grade areas. This has resulted in increased annual expenditures in these poor sub grade areas. From 2000 to 2007, the City of Saskatoon spent $4.88 million on the reconstruction of roadways and over $3 million, 61%, has been spent on reconstructing roads that were less than 25 years old (COS Public Works, 2007). Based on results heirin, it is projected that savings will result from incorporation of structural information into the COS asset management system. Initial and immediate savings would result from avoiding the application of surface treatments on structurally deficient roads. Secondly, the most significant savings were found to result from changes to the structural design of low volume residential and collector roads. The current costs associated with the construction of historically designed locals and collectors in poor sub grade areas is estimated at $24.8 million and $2.6 million respectively. The future costs, over a period of twenty years, assuming construction of the historic structures was estimated at $49.2 million for local and $4.5 million collector roadways. Therefore significant savings will be realized with improvements to the local and collector structural standards in terms of future mitigated structural failures and costly structural repairs. This research has shown that HWD structural condition assessments complement and enhance surface condition assessments within the COS asset management system. The use of structural asset management will reduce the risk of unexpected road failures and will allow for treatment intervention or improved management. The HWD information will improve treatment selection particularly with respect to avoiding preservation and restoration treatments on roads that have structural issues.
Asset Mangement, Urban Asset Management, Pavement Management, HWD, Structural Asset Management
Master of Science (M.Sc.)
Department of Civil and Geological Engineering