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Home My WebLink Aboutmem_thunderhills_risk analysis_Memo 6_20150224 CITY OF RENTON Technical Memorandum No. 6 Risk Analysis Thunder Hills Sanitary Sewer Interceptor Alternative Analysis February 24, 2015 Prepared By: Stantec Consulting Services Inc. 11130 NE 33rd Place Suite 200 Bellevue, WA 98004 425.869.9448 TECHNICAL MEMORANDUM NO. 6 February 24, 2015 ew v:\2002\active\2002003607\analysis\risk analysis\mem_thunderhills_risk analysis_20150224.docx i Contents 1.0 INTRODUCTION ............................................................................................................. 1 2.0 REMAINING USEFUL LIFE (RUL) ASSESSMENT ................................................................ 1 3.0 CRITICALITY ASSESSMENT ............................................................................................. 2 4.0 RISK ANALYSIS ............................................................................................................... 3 TECHNICAL MEMORANDUM NO. 6 February 24, 2015 ew v:\2002\active\2002003607\analysis\risk analysis\mem_thunderhills_risk analysis_20150224.docx ii TECHNICAL MEMORANDUM No. 6 February 24, 2015 ew v:\2002\active\2002003607\analysis\risk analysis\mem_thunderhills_risk analysis_20150224.docx 1 1.0 INTRODUCTION This memorandum includes the risk analysis of the Thunder Hills Alternative Analysis project. The Alternative Analysis aims to find the best alternative that will improve the functionality of the Thunder Hills Interceptor, particularly the section between Grant Avenue South and Benson Road South. The Interceptor was originally constructed in 1965, and serves a portion of the City’s Rolling Hills neighborhood southeast of Renton City Hall. Several portions of the existing interceptor have been replaced over time as additional development (interties) has occurred, or as required due to erosion from Thunder Hills Creek. The goal of the risk analysis section of the Alternative Analysis is to quantify the risk associated with various reaches along the exiting alignment. Risk is defined as the existing pipes Remaining Useful Life (RUL) multiplied by the pipes Criticality. Each of these factors were determined and quantified as part of separate technical memorandums. Technical Memorandum No. 3 assessed the existing pipes RUL over identified reaches, while Technical Memorandum No. 5 assessed the criticality, or the potential for erosion to expose or damage the pipe. For the purposes of comparing the existing interceptors RUL and Criticality, the interceptor was separated into eight reaches based on either similar geologic conditions or pipe material changes which would affect either the RUL or criticality for each reach. The eight reaches were separated as follows: Reach Location Approximate Gradient (%) 0+00 to 1+00 Not Applicable (Culvert) 1+00 to 5+50 15.1 5+50 to 8+30 4.3 8+30 to 11+00 8.2 11+00 to 12+30 10.4 12+30 to 18+90 5.2 18+90 to 26+75 7.1 26+75 to 28+00 14.4 2.0 REMAINING USEFUL LIFE (RUL) ASSESSMENT As part of the analysis conducted for Technical Memorandum No. 3, the remaining useful life for each reach of existing interceptor has been designated as either “Good”, “Moderate”, or “Poor”. For the purposes of completing the RUL analysis a pipe section with a “Good” RUL was given a numerical value of 1 and it represents a pipe that is constructed of ductile iron (DI) pipe that still has the majority of its CML lining and limited corrosion spots. A section of pipe with a “Moderate” RUL was given a numerical value of 2 and it represents a portion of asbestos cement (AC) pipe that appears to be in good shape, or a DI pipe that is showing more significant CML scaling and corrosion. A section of pipe with a “Poor” RUL was given a numerical value of 3 and it represents a portion of pipe that has severe corrosion or a physical TECHNICAL MEMORANDUM No. 6 February 24, 2015 ew v:\2002\active\2002003607\analysis\risk analysis\mem_thunderhills_risk analysis_20150224.docx 2 defect in the pipe that will cause flow constraints which would potentially reduce the remaining lifespan of the pipe. These are locations identified as the most likely to fail and cause damage to the environment which would require emergency repairs. Table 2-1 summarizes the amount of pipe in each reach with Good, Moderate, and Poor RUL and provides a designated numerical value for the RUL for each reach. Table 2-1 Remaining Useful Life (RUL) Assessment Reach Location Length of Good RUL (feet) Length of Moderate RUL (feet) Length of Poor RUL (feet) RUL Designation 0+00 to 1+00 100 0 0 1 1+00 to 5+50 437 13 0 1 5+50 to 8+30 277 3 0 1 8+30 to 11+00 253 17 0 1 11+00 to 12+30 124 0 6 1 12+30 to 18+90 619 32 9 1 18+90 to 26+75 65 684 36 2 26+75 to 28+00 0 125 0 2 It should be noted that these reaches do not include the existing pipe from Manhole 035 to 033 because this section of pipe was not analyzed as part of the erosion evaluation due to it being under I-405. Likewise, a small portion of pipe from Station 28+00 to approximately 28+75 between Manhole 051 and 051A was not susceptible to erosion from Thunder Hills Creek and was not included in the reaches described above. Both of these portions of pipe would be given an RUL designation of Moderate, or 2, due to corrosion in the DI pipe from Manhole 035 to 033 and the condition of the concrete pipe from Manhole 051 to 051A. These sections of pipe will be included in the Risk analysis. Due to the limited amount of Poor RUL pipe, the majority of the Thunder Hills Interceptor falls under the Good RUL designation. The only reaches that are considered Moderate RUL is due to the presence of existing asbestos cement (AC) pipe. 3.0 CRITICALITY ASSESSMENT The analysis conducted as part of Technical Memorandum No. 5 identified the potential for erosion to occur across the length of the existing interceptors. The analysis estimated the erosion potential at critical cross section locations and extrapolated that information into generalized erosion potentials for each of the reaches identified in Section 1.0 of this memorandum. The potential for erosion at each reach was designated as “Stable”, “Marginal” or “Unstable”. Table 3-1 summarizes the erosion analysis designations for each reach with respect to 20, 40 and 80 years of stream erosion. TECHNICAL MEMORANDUM No. 6 February 24, 2015 ew v:\2002\active\2002003607\analysis\risk analysis\mem_thunderhills_risk analysis_20150224.docx 3 Table 3-1 Generalized Erosion Potential at Reach Locations Reach Location 20 Years 40 Years 80 Years 0+00 to 1+00 Stable Stable Stable 1+00 to 5+50 Marginal Unstable Unstable 5+50 to 8+30 Stable Stable Stable 8+30 to 11+00 Stable Stable Marginal 11+00 to 12+30 Stable Marginal Unstable 12+30 to 18+90 Stable Stable Stable 18+90 to 26+75 Stable Stable Stable 26+75 to 28+00 Marginal Unstable Unstable One of the overriding goals of the Thunder Hills analysis has been to finish with a new interceptor that has at least an 80 year design life. As such, the erosion potential for the reaches at 80 years was used to determine the criticality value for each reach. Using the simple designation of Stable equaling 1, Marginal equaling 2, and Unstable equaling 3, Table 3-2 summarizes the criticality for each of the designated reaches: Table 3-2 Criticality at Reach Locations Reach Location 80 Year Erosion Potential Criticality Designation 0+00 to 1+00 Stable 1 1+00 to 5+50 Unstable 3 5+50 to 8+30 Stable 1 8+30 to 11+00 Marginal 2 11+00 to 12+30 Unstable 3 12+30 to 18+90 Stable 1 18+90 to 26+75 Stable 1 26+75 to 28+00 Unstable 3 4.0 RISK ANALYSIS The risk of a reach of interceptor is defined as the reach’s RUL multiplied by the reach’s criticality. Based on the analyses conducted for Technical Memorandum No. 3 and Technical Memorandum No. 5, the RUL and Criticality has been given designations from 1 to 3 as described above. Table 4-1 provides the Risk Analysis for each designated reach based on the multiplication of the two numbers: TECHNICAL MEMORANDUM No. 6 February 24, 2015 ew v:\2002\active\2002003607\analysis\risk analysis\mem_thunderhills_risk analysis_20150224.docx 4 Table 4-1 Risk Analysis Reach Location RUL Designation Criticality Designation Risk Designation 0+00 to 1+00 1 1 1 1+00 to 5+50 1 3 3 5+50 to 8+30 1 1 1 8+30 to 11+00 1 2 2 11+00 to 12+30 1 3 3 12+30 to 18+90 1 1 1 18+90 to 26+75 2 1 2 26+75 to 28+00 2 3 6 The risk analysis designation shown above will be used, in part, by the City to determine the level of comfort with the existing interceptor remaining in its current location whether it is rehabilitated or not, based on the potential risk to the pipeline at that location.