GIS-based decision support system for evaluating renewal technologies for municipal sewer and water pipelines

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GIS-based decision support system for evaluating renewal technologies for municipal sewer and water pipelines

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http://irc.nrc-cnrc.gc.ca

GI S-ba se d de c ision suppor t

syst e m for eva luat ing re ne w a l

t e chnologie s for m unic ipa l

se w e r a nd w at e r pipe line s

N R C C - 5 0 8 0 8

H a l f a w y , M . R . ; B a k e r , S .

2 0 0 8 - 0 9 - 2 3

A version of this document is published in / Une version de ce document se trouve dans:

60th Annual Western Canada Water and Wastewater Association Conference

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Protecting Our Water – 60 Years of Service

60th Annual WCWWA Conference and Trade Show

September 23 – 26, 2008 Delta Regina Hotel Regina, Saskatchewan

GIS-BASED DECISION SUPPORT SYSTEM FOR EVALUATING RENEWAL TECHNOLOGIES FOR MUNICIPAL SEWER AND WATER PIPELINES Mahmoud R. Halfawy and Samar Baker

Centre for Sustainable Infrastructure Research, Institute for Research in Construction, National Research Council of Canada, 6 Research Drive, Regina, SK.

ABSTRACT

This paper describes an ongoing research project to develop a GIS-based decision support system (DSS) for evaluating and selecting feasible renewal methods for sewer and water pipelines. Renewal methods were grouped into four main categories: replacement (conventional open cut or trenchless methods), and structural, semi-structural, or non-structural lining methods. Data about various renewal methods are stored in an external database that can be maintained and updated separately, thus enabling easy modification of technology parameters to reflect local practices or adopted best practices. The software applies applicability criteria to identify feasible renewal categories and technologies. The applicability criteria are determined by the technology limitations (e.g., soil type, existing defects, or the diameter and material of the existing or desired pipeline), site characteristics (e.g., work area requirements, ground water level), and other social or environmental criteria. The expected costs and benefits of each applicable method are then calculated and used to optimize the selection of the methods with the best cost/benefit ratio. The paper also presents an example application of the prototype DSS using data sets from the City of Regina.

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INTRODUCTION

Renewal methods include a wide range of replacement and rehabilitation technologies. Replacement technologies are typically used when the pipe’s structural integrity is severely compromised, or to significantly increase its hydraulic capacity. Rehabilitation technologies aim to extend the pipe’s service life by restoring its structural integrity and improving its hydraulic performance, and are generally used when the pipe’s structural integrity and hydraulic capacity are not severely compromised.

Renewal technologies are advancing rapidly and becoming more efficient and cost-effective. The use of trenchless methods has been steadily increasing around the world. New materials and construction methods are constantly introduced into the market, causing the process of evaluating these technologies and selecting the most appropriate solution to become a daunting task for municipal practitioners. This process remains, by far, heuristic and subjective, and is still largely performed in a manual fashion, with limited software support.

Different renewal technologies exhibit different capabilities, limitations, costs, and benefits, which collectively serve as the main criteria for evaluating alternative options. The specific characteristics of the pipe (e.g., material, diameter, depth, age, etc.) and site conditions (e.g., soil, groundwater table, traffic condition, etc.), along with other operational, social, and environmental factors typically determine the applicability and feasibility of alternative technologies in a particular situation. In any given scenario, some technologies are more suitable and cost-effective than others, and therefore, a systematic decision-making procedure for selecting feasible technologies is needed (WEF and ASCE 1994, WRc 2001, and InfraGuide 2005).

The paper proposes a decision-support model and GIS-based software tool that could potentially support the evaluation of alternative renewal technologies in any given scenario, taking into consideration the applicability, limitations, costs, and benefits of each possible technology. The proposed model defines a procedure to quantitatively assess and evaluate the costs and benefits of alternative renewal methods, which would reduce the subjectivity typically employed in the decision-making process. The implementation and example application of the software system is also presented.

CLASSIFICATION OF RENEWAL TECHNOLOGIES

Renewal technologies include a wide range of replacement and rehabilitation methods. Figure 1 provides a classification of the technologies considered in this study. Replacement methods use either dig or trenchless technologies. Depending on the width and side support of the trench, dig technologies are usually classified as open cut or semi-open cut (Najafi 2004, Montero et al. 2002). Trenchless replacement methods

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include off-line and in-line replacement technologies. Off line replacement involve installing a new pipe in a new line underground without digging a trench for the entire length of the pipe (Montero et al. 2002). In-line replacement methods install a new pipe of the same diameter, or up to three times larger (Simicevic and Sterling 2001). In-line replacement technologies can be grouped into pipe displacement and pipe removal methods. The displacement methods involve destroying the old pipe or cutting and displacing it into the surrounding soil (Montero et al. 2002, ISTT 2006, Simicevic and Sterling 2001, Najafi 2004, Atkinson 2000), while removal methods involve removing the old pipe, in whole or in pieces, and installing a new pipe in place. The pipe removal methods are modifications of off-line replacement methods.

Pipe Bursting Pipe Removal

Replacement

Sliplining CIPP Close fit pipe Formed in place Thermoformed Spiral wound Panel lining UCL Fully Structural Semi-Structural Non-Structural Renewal Technologies

Open-Cut(Dig) In-Line Off-Line

Horizontal Directional Drilling (HDD) Pipe Jacking Micro-Tunelling Auger Boring Rehabilitation - Lining Pipe Bursting Pipe Removal Replacement Sliplining CIPP Close fit pipe Formed in place Thermoformed Spiral wound Panel lining UCL Fully Structural Semi-Structural Non-Structural Renewal Technologies

Open-Cut(Dig) In-Line Off-Line

Horizontal Directional Drilling (HDD) Pipe Jacking Micro-Tunelling Auger Boring Rehabilitation - Lining

Fig. 1: Classification of renewal technologies for gravity and pressure pipes

Rehabilitation technologies generally involve the installation or application of a new liner or coating inside the existing pipe. The most commonly used lining technologies include sliplining, modified sliplining, close fit pipe, thermoformed pipe, and cured-in-place pipe methods. Coating technologies (e.g., shotcrete and gunite) involve manual or mechanical application of a coating layer to the inner surface of the pipe mainly to prevent or delay deterioration and seal minor cracks.

Several rehabilitation methods can be designed to provide full structural, semi-structural, or non-structural capabilities. Design requirements for gravity liners are different from those for pressure liners. Liners for pressure pipelines (e.g., force mains or water mains) are designed to resist the internal pressure, while liners in gravity pipelines are designed to mainly resist soil and groundwater loads (Heavens 2007). Based on the structural

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capabilities of the liner and its interaction with the host pipe, Gumbel et al. (2004) defined four rehabilitation classes for pressure pipelines: A, B, C and D. This classification combined earlier classifications defined by American Water Works Association (AWWA) and European Standards.

Design standards of liners for gravity sewers are defined by American Society of Testing and Materials (ASTM) and by Water Research Council (WRc). The characteristics of these liners typically vary based on the bonding between the liner and the host sewer and the loads to be carried by the liner. ASTM defines two liner classes based on the condition of the pipeline: fully and partially deteriorated design (Najafi 2004). The WRc’s Sewer Rehabilitation Manual defines two main rehabilitation standards for gravity liners: Type I and Type II, which can be designed with additional check to provide additional structural capabilities for the liner (WRc 2001).

AN APPROACH FOR EVALUATING RENEWAL TECHNOLOGIES AND DEVELOPING OPTIMAL RENEWAL PLANS

The municipal sewer and water network renewal planning process can be defined as follows: what are the renewal actions (what assets to rehabilitate or replace, what methods to use, and when) for a specific planning horizon that would optimize the allocation of renewal budget by maximizing the network’s average condition and minimizing risk of failure, subject to condition, risk, and budget constraints. A typical renewal plan establishes, for a given year and for each pipe segment, the most appropriate and cost-effective renewal action, if any is required.

A new algorithm for optimal renewal planning was developed. This algorithm is described in (Halfawy et al. 2008), with a detailed application to the sewer renewal planning process. The algorithm starts by identifying and prioritizing candidate pipes for renewal. This involves classifying and subdividing the network into a set of homogeneous groups of pipes in terms of their current condition and deterioration pattern as well as their criticality (or expected consequence of failure). Then, for each group, a renewal plan is developed for each planning period (e.g., one or more years). At the beginning of each period, condition indices are evaluated for each pipe using established deterioration models. The condition indices and deterioration models are used to estimate the remaining service life and calculate the likelihood of failure index. The consequence of failure is then determined, and used with the calculated likelihood of failure index to estimate the risk level of each pipe segment. Based on the condition and risk levels, pipes are prioritized according to the urgency of needed intervention. For each pipe on the priority list, feasible and cost-effective renewal actions are selected based on their technical compatibility with the problem in hand and their economical merits. Then, a set of optimal renewal plans is generated using a multi-objective optimization approach, and further evaluated by the user to select the one that best meets the budget, condition, and

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risk constraints. This paper discusses the evaluation and selection of appropriate renewal technologies, a critical step in the renewal planning process. The proposed technique builds on several of the earlier efforts by other researchers to develop procedures, guidelines, and decision models, and attempts to harmonize and integrate several of these procedures into an integrated and flexible decision support model and software tool.

IDENTIFYING APPLICABLE RENEWAL CATEGORIES

Renewal methods can generally be grouped into four main categories: replacement (conventional open cut or trenchless methods, with same or larger diameter), and full-structural, semi-full-structural, and non-structural rehabilitation methods. Each renewal category includes a number of renewal methods. Criteria and guidelines for selecting renewal categories for pressure pipelines were provided in Gumbel et al. (2004) and Heavens (2007). However, there is a lack of similar guidelines for gravity pipelines. The structural capabilities of required renewal categories for gravity pipelines could be identified based on the pipe structural condition and the possibility of soil loss. The WRc condition grade can be used as an indicator of the sewer defect size. The possibility of surrounding soil loss can be assessed on a high, medium, and low scale. The “high” rating would indicate that the pipe is expected to lose soil support and may experience accelerated deterioration or collapse in the near future; while the “low” rating would indicate that the soil is expected to provide adequate support over the long term with minimum or no effect on the pipe’s deterioration rate. The rate of soil loss would generally be higher in cohesionless soils and high levels of the groundwater, and lower in cohesive soils especially where groundwater level fluctuations do not exist or occur below the pipe. The possibility of soil loss would also depend on the pipe defects size, where a larger defect size increases the rate of soil loss. Table 1 provides guidelines for selecting renewal categories based on the condition index and the possibility of soil loss.

P PoossssiibbiilliittyyooffSSooiillLLoossss Condition Index _L_L_o_o_w_w _M_M_e_e_d_d_i_i_u_u_m_m _H_H_i_i_g_g_h_h 2 2 _N_N_o_o_n_n_-_-_s_s_t_t_r_r_u_u_c_c_t_t_u_u_r_r_a_a_l_l _o_o_r_r _S_S_e_e_m_m_i_i_- -s sttrruuccttuurraall 3 3 N Noonn--ssttrruuccttuurraall oorr S Seemmii--ssttrruuccttuurraall S Seemmii--ssttrruuccttuurraall oorr s sttrruuccttuurraall S Seemmii--ssttrruuccttuurraall o orrssttrruuccttuurraall 4 4 aanndd 55 _S_S_t_t_r_r_u_u_c_c_t_t_u_u_r_r_a_a_l_l_o_o_r_r_R_R_e_e_p_p_l_l_a_a_c_c_e_e_m_m_e_e_n_n_t_t

Table 1: Selection of appropriate renewal categories for gravity pipelines.

Replacement is generally considered when collapse has occurred or is imminent, or when hydraulic capacity is inadequate. Pipes with current or predicted inadequate hydraulic

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Protecting Our Water – 60 Years of Service

capacity are typically replaced with pipes of larger diameter size. Otherwise, replacement is performed with a similar diameter pipe. Where collapse is not imminent, the residual structural capacity of the pipe should be considered in the design of the liner or coating (WRc 2001, Infraguide 2003). Full-structural and semi-structural rehabilitation is appropriate if the pipe’s structural integrity is severely compromised, but not yet collapsed (Infraguide 2003, Plenker 2002). Full-structural rehabilitation is appropriate if the pipe is currently unable to support hydrostatic pressure, and may not be able to support soil and traffic loads due to high deterioration rate or expected higher loading conditions. Semi-structural rehabilitation is appropriate if the pipe is unable, or will soon be unable, to support the hydrostatic pressure. Non-structural rehabilitation is appropriate if the pipe has adequate structural capacity, and is used mainly to resist corrosion, minimize further deterioration, improve hydraulic conditions, and seal small cracks in gravity sewers (Najafi 2004, Heavens 2007).

ELIMINATION OF INAPPLICABLE METHODS

Once renewal categories are determined, the renewal methods within each category are further evaluated in terms of their “applicability.” The applicability criteria are mainly determined by the technology limitations and compatibility with the pipe physical characteristics, site characteristics, and other social or environmental criteria and user requirements. These criteria are used to “eliminate” inapplicable methods by comparing the specific job conditions with the technology limitations (e.g., existing or desired pipe material, diameter, depth of cover and profile, defects and condition state, number of service connections, surrounding soil type, groundwater level, and work area requirements). The applicability criteria are examined for each pipe to determine whether a particular renewal method should be eliminated to reduce the number of possible alternatives that need to be thoroughly evaluated.

In addition, a number of compatibility rules can be formalized to further guide the screening process. An example of these rules states that the mechanical folding and diameter reduction close fit pipe methods are only applicable for pressure pipes, while modified sliplining methods are only applicable for gravity pipes. Another rule states that pipes with non-circular cross section can only be rehabilitated using the modified sliplining (i.e., spiral wound, panel lining, and insitu formed pipe), the underground coating and lining, and the dig methods. Similar rules can be formulated and used in the elimination process to reflect environmental criteria or preferences of a particular utility.

EVALUATION OF COSTS AND BENEFITS OF RENEWAL METHODS

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optimize the selection of the methods with the best cost/benefit ratio. Accurate assessment of the benefits should consider the post-rehabilitation impact of a renewal method or category on various defects, and modify the defect scoring and condition rating calculations accordingly. To date, there is no established way to accurately calculate expected condition improvements. Clearly, the development and validation of such a model would require significant pre- and post-rehabilitation data, which are not generally available. Abraham et al (1998) estimated the condition improvement in terms of extension of the sewer service life (e.g., shotcrete extends the service life by 20 years, while CIPP extends it by 50 years). Our proposed approach estimates benefits in terms of condition improvement (or recovery) by deducting certain values from the current condition index. The condition index is measured on a 1-5 scale. For gravity pipelines, the WRc condition rating protocol is used. However, there are no standard protocols for assessing the condition of pressure pipelines. Therefore, we employed a simplified rating scheme for pressure pipelines based on the breakage rate. This scheme will be extended in the future to include other condition and performance indicators (e.g., leakage levels). Condition improvement values are estimated for each renewal category or renewal methods. Default values of 0.5, 1.0, and 2.0 are assumed for non-structural, semi-structural, and structural lining, respectively, while replacement restores the condition index to “1.0” by default. These default values can be overridden for a particular method as it is applied to a specific pipe or group of pipes.

In assessing the costs of renewal methods, all relevant costs (direct, indirect, social, and environmental costs) need to be considered. In this study, unit costs (cost per unit diameter per unit length) for various renewal methods were estimated based on a literature review of available cost data. Six main sources for that data were studied: Ariaratnam et al (1999), USEPA (1999), Selvakumar et al (2002), Zhao and Rajani (2002), Garcia et al (2002), and Najafi (2004). These studies reported costs using different units (mm/m and in/ft), different currency (US and Canadian Dollar), and at different years (1999-2002). Therefore, the collected data had to be adjusted to establish approximate unit costs for each renewal method. Also, costs were not available for all renewal methods and in many cases costs of different methods were aggregated under different groupings. For example, USEPA (1999) considered close fit pipe methods and thermoformed pipe methods under modified cross section lining and provided a cost range for the group.

Also, reported costs were approximated to define method-specific cost ranges for different condition states and renewal categories. For example, if the reported cost of a renewal method ranges between $0.5 and $1.5 per mm/m, we assumed that the non-structural liner for condition “2” would cost $0.5, while a non-structural liner for condition “4” or “5” will cost $1.5. In the same manner, the cost of a structural liner is expected to be higher than that of a semi- or non-structural liner, due to extra design, testing, and material costs. These unit costs can be used as default values, especially where accurate

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and specific data are not available, for performing order-of-magnitude estimates for comparative assessment of alternative technologies. The default costs could also be modified by a decision maker based on more accurate estimates, actual project experience, or specific site conditions.

Since the break down of total costs into direct and indirect cost components was not given in the reported costs, these costs were assumed to include both cost components. For more accurate cost estimates, social and environmental costs could also be estimated and added to the cost assessment of renewal options. A lifecycle cost component can also be estimated as the expected yearly maintenance cost throughout the service life discounted to the analysis year. However, the scope and purpose of the renewal plan would determine the level of accuracy required for the cost estimates. While project-level or short-term planning would require more accurate assessment of direct, social, environmental, and lifecycle costs; network-level or long-term planning could be reasonably conducted using approximate total cost figures such as those compiled or estimated from the literature. Once the cost and benefit of feasible renewal methods are determined, the renewal trade-offs can be further evaluated and optimized using the multi-objective optimization model to identify a set of feasible renewal solutions as described in (Halfawy et al. 2008).

DSS IMPLEMENTATION

During the past two years, an integrated and modular integrated municipal asset management software environment has been under development in collaboration with the City of Regina, Saskatchewan, Canada. The software environment aims to support various processes conducted by different functional groups within typical municipal sewer and water departments. Several applications have already been developed and integrated into the environment. Examples include inventory data analysis, query, and reporting, condition assessment, deterioration modeling, risk assessment, asset prioritization, and renewal planning. The integrated environment was implemented as a set of loosely coupled applications, each addressing a specific process. Each application was implemented as an add-on to ESRI ArcGIS software using the ArcObjects class library (ESRI 2001). The modular architecture of the application would help accommodate future enhancement and extension of the application (Halfawy et al. 2002). This section describes the implementation of the renewal methods evaluation application based on the approach proposed above.

An object-oriented integrated data model for sewer and water networks were developed (Halfawy et al 2006, Halfawy 2007, Halfawy 2008). The data model defined classes that represented spatial, inventory, inspection, condition, risk, and renewal data. The data model was defined using the Unified Modeling Language (UML) notation and was based on ESRI’s water utilities spatial data models. The UML data model was used to generate

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the database tables, fields, and data types. The implementation of the centralized integrated data repository is described in Halfawy and Figueroa (2006).

The DSS was also designed to allow for easy customization to the specific practices and rules used in a particular municipality. All data and settings that can be considered as municipality-specific (e.g., costs of renewal methods) were not hard-coded into the software and were stored in an external database for possible editing. For example, information about various renewal methods is stored in a database that can be customized to the specific practices and data available at a particular municipality.

EXAMPLE

The prototype DSS was used to develop renewal plans for a sewer network in the City of Regina. The City has an inventory of approximately 860 km of sanitary sewers and 755 km of storm sewers. The network was subdivided into a set of homogeneous groups. This example demonstrates the application of the proposed approach to one of these groups. The group was defined to include vitrified clay sanitary sewers, with 200 mm diameter, and constructed between 1950 and 1955. This group included 249 sewer segments with a total length of 19.86 km. The condition assessment, deterioration modeling, risk assessment, and asset prioritization applications were used to identify and prioritize the sewer segments that require intervention. This process is explained in detail in Halfawy et al. (2008). For the 249 sewers in this group, 11 sewers were found to need immediate intervention, 8 sewers with medium priority, 230 sewers with low priority. Sewers identified for renewal are then considered for further evaluation and optimization of possible renewal actions.

The renewal methods selection procedure starts by identifying the applicable renewal category for each sewer, and retrieving the methods within these categories from the renewal technologies database. This database stores default information about renewal methods including their limitations (diameter range, soil type, pipe material, etc.), expected condition improvement, and cost. The default cost and improvement values are specified for each condition grade, since these values would depend on the type and severity of the defects. The user can eliminate some renewal methods or override their default values as they apply to a particular sewer or sewer group. For each sewer in the group, the system will then evaluate the applicability of various renewal methods and calculate costs and condition improvements. Figure 2 shows a screenshot from the renewal methods evaluation DSS. The multi-objective optimization application was then used to identify optimal renewal plans.

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Sewers selected

for the analysis,

with a list of

their attributes.

Methods stored in the database,

with the option to modify their

default parameters for the

selected sewers.

Methods selected for a particular

sewer (ID= 8306), with estimated

costs and benefits. Users could

modify the estimated values or

perform more detailed assessment

for any sewer.

Fig. 2: A screenshot of the renewal methods evaluation and selection application

CONCLUSION AND FUTURE DIRECTIONS

Integrated renewal planning of water and sewer networks can play a critical role to improve aspects of management and operation. This paper presented an approach to support the evaluation of alternative renewal technologies for water and sewer networks, taking into consideration the applicability, limitations, costs, and benefits of each possible technology. The proposed approach has been implemented into a prototype decision-support system that has been under development and evaluation in collaboration with the City of Regina during the past two years. The software has been tested using a number of examples for sewer renewal planning, and the results were found to be consistent with expectations and decisions possibly made by a professional asset manager. However, substantial work still needs to be done to refine and extend the

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approach and fully develop the software. Through industrial partnerships, the approach and software will be further extended, refined, and tested. The next step primarily focuses on the development and testing of the software to support water main renewal scenarios.

The long-term objective of this research project is to integrate the renewal planning and management processes for water, sewer, and road networks to optimize the overall allocation of funds across these spatially co-located linear infrastructure assets. The integrated software environment under development will need to be extended to integrate and coordinate the renewal planning processes across these asset classes.

ACKNOWLEDGEMENTS

The authors thank the City of Regina Engineering and Works staff Loretta Gette, Ken Wiens, and Andrea Weston, for providing data and guidance throughout this project. The authors also wish to thank Dr. David Hubble and Dr. Imran Syed for their valuable feedback and suggestions.

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