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HomeMy WebLinkAbout11/02/1999, 2 - MODIFICATION OF STREET DESIGN STANDARDS Council `°i( _Z _4H j agenda Rpm CITY OF SAN LUIS OBISPO FROM: Michael D.McCluskey,Public Works Director Prepared by: Daniel Van Beveren, Civil Engineer�,,/ SUBJECT:MODIFICATION OF STREET DESIGN STANDARDS CAO RECOMMENDATION • Amend Policy 2.1 of the Pavement Management Plan to reflect a 50-year design life for all new Collector&Arterial streets. • Amend City design standards to add an option, with City approval on a case by case situation, for new Collector&Arterial streets to be constructed out of Portland cement concrete. • Affirm the existing 50-year design standard for new Local streets, and the existing 20-year design standard for all reconstructed and resurfaced Local,Collector&Arterial streets. DISCUSSION Background The City Council has asked staff to investigate the possibility of modifying the adopted Pavement Management Plan to designate that all city streets be designed for a 50-year design life. Response to the request was delayed until paving contracts that were either in construction or preparation were completed This report will summarize the City's current policy of street design, discuss the logic behind current policy, and discuss the proposed modifications and other alternative changes to the standards. The Council, through the adoption of the Pavement Management Plan in April 1998, approved the current city design standards for pavement design. These standards call for new Local asphalt streets to be designed for a 50-year life, and for. a) all new Collector&Arterial (C/A)asphalt streets, and b) all reconstructed asphalt streets to be designed for a 20-year life. Staff has conducted an in-depth study of these existing standards in response to the Council request Should new Collector&Arterial streets have a 50-year design life? A 50-year designed C/A street would have a higher initial cost than a 20-year designed C/A street. A street designed for a 50-year life would, in theory, not need an overlay until the end of its 50-year design life. Regardless of its design life, an asphalt street requires a seal coat (slurry seal) every few years. An analysis of the lifetime costs of a pavement's initial construction and required periodic maintenance and rehabilitation shows that a 50-year designed street would have lower costs than a 20-year designed street. A thorough discussion of the costs associated with street design is included in the Fiscal Impact section of this report. A new CIA asphalt street designed for a 50-year life would be approximately 20% thicker than a 20-year designed CIA asphalt street. This would affect any future trenching in the street. The thicker asphalt section would require more work in the sawcutting, and replacement O� Council Agenda Report—Modification of Street Design Standards Page 2 asphalt. This impact, in the opinion of Public Works staff, is worth it when one considers the benefits of longer-designed streets that would require less money to be spent on future rehabilitation. Should.reconstructed or resurfaced.streets have a 50-year design.life? The Pavement Management Plan addresses this issue and recommends a design life of 20years for all reconstructed or resurfaced streets. This is primarily due to the existing underground utilities, which will most likely require maintenance in the future.. The issues .regarding the recommended design life of 20 years have not changed. Staff, therefore, is recommending that all reconstructed or resurfaced streets continue to be designed for a 20-year life. 50-year design fife:-Concrete vs..Asnhalt In performing the analysis of the lifetime pavement costs, staff also included a .study of the economics of using concrete streets as an alternative to asphalt streets. The analysis showed that the higher initial cost of concrete was not economical for Local streets. For Collector&Arterial streets, however, the study of the long-term costs of 50-year streets showed that the cost of an asphalt street with a slurry seal applied every 8 years was about the same as the cost of a concrete street with little or no required maintenance. Although asphalt is used far more often than concrete,both are viable options that present different advantages(and disadvantages). A thorough discussion of the advantages & disadvantages of both asphalt and concrete streets is included in the appendix of this report. In the opinion of Public Works staff, these advantages and disadvantages, although different for each, are of equal weight. The staff recommendation.is to build all new streets out of asphalt. If a developer would like to use concrete staff would require Council approval during the development review process. For an in-depth explanation of the City's history of.street design, an explanation of current street design policies,discussion on the theory of pavement design,and further discussion of the alternatives to the current policy,.see the appendix.of this.report. CONCURRENCES The City Utilities Department is in concurrence with the Public Work's Staff Recommendation to modify the current street design standards as stated in this report. FISCAL R"ACT The City Council adopted the Pavement Management Plan in April 1998. The plan allocates a set dollar amount to be spent annually on pavement rehabilitation, reconstruction and preventative maintenance. The plan's budget would not be affected by the proposed change, however many years from now, pavementrehabilitation costs could drop due to the proposed change in design:standard. The following table, which presents expected annual costs for the first 50 years of a street's life, shows how money is spent on a street. The costs shown in the table represent present day fps Council Agenda Report—Modification of Street Design Standards Page 3 (no inflation factor), and assume that the relative future costs of labor and materials will not change significantly. Although this assumption may not hold true, it is a"best guess" for future paving costs. Likewise, this assumes no street cuts by utilities and therefore no accelerated degradation of street condition due to that activity. NEW STREETS (COST/M2) Actual total lifetime cost is shown within the chart LOCAL STREETS COLLECTOR &ARTERIAL STREETS 20-year 50-year 50-year 20-year 50-year 50-year Asphalt Asphalt Concrete Asphalt Asphalt Concrete Initial Cost $19 $23 $35 $30 $35 $40 20-year - - - resurfacin $16 $20 8-year surface - treatment $5 $6 $5 $6 Total Cost over life of $40 $29 $35 $55 $41 $40 pavement Total Annual $0.86 $0.58 $0.70 $1.10 $0.82 $0.80 Cost Notes: These average costs are for a 50-year period&do not include fugue reconstruction costs. This table does not reflect costs associated with trenching,pothole repair or striping replacement. The cost of the concrete does not reflect possible resurfacing with asphalt. Minimize City Maintenance cost versus Minimal Overall cost? It is important to note that the costs of a new street shown in the above table are not always paid for by the City. Typically, a new street is added into the City's inventory by a developer who is usually required to pay for the street construction. Should the City set its standard solely on expected future maintenance costs? They are real costs and it is important to verify that money is being spent on a street in a cost-effective manner—whether the developer or the City pays for it. It is important to recognize that the above chart shows that arterial and collector streets would cost less over their life time and that all streets would cost the City much less when the initial street is built of concrete and is built by the development community. The basis of the staffs recommendation is that asphalt streets have been shown to be more acceptable to citizens as they are easier to stripe (and therefore more legible), provide a quieter ride, and they "look better". Once in place, a 50 year asphalt street, requires minimal expenditure ($0.12 per square meter of pavement per year)in order to maintain the quality of riding surface citizens expect 2-3 Council Agenda Report—Modification of Street Design Standards Page 4 ALTERNATIVES Additional alternatives to the staff's recommendation to increase the design life of all new streets to 50 years include the adoption of one or more of the following alternatives: 1. Keep the existing standards unchanged. (50-year design on new local streets and 20-year design on all other new streets and all reconstructed or resurfaced streets) Keeping the existing standards unchanged for new C/A streets would result in lower initial costs of pavement and thinner pavement section that are easier to work with. This option is not recommended due to the following reason: The analysis of long-term costs shows that while the initial investment of a 50-year designed street may be somewhat higher, the average annual cost of a street averaged out over 50 years is higher for a street designed for a 20-year life as compared to a 50-year life. 2. Adopt a 50-year standard for all Reconstructed Streets. Currently all new Local streets are designed for 50 years. Switching from a 20-year standard to a 50-year standard on all existing Local streets that are reconstructed or resurfaced would theoretically result in a pavement section that would last for at least 50 years,thus reducing future maintenance and rehabilitation costs for the next 50 years. The main disadvantage to a 50-year design standard on a Reconstructed Residential Street is the fact that an existing street will usually already contain aged underground utility lines which will require replacement and/or maintenance—most certainly within less than 50 years. The disadvantages of longer-designed reconstructed streets are further discussed in the main body and the appendix of this report. 3. Adopt a 50-year design standard that ENuires the use of concrete as a paving material for any street classification. The adoption of a 50-year design standard for any of the classifications of City streets is achievable through the use of concrete. Concrete has the advantage that it does not require a seal coat every few years like asphalt does. Concrete has also been proven to last for over 50 years. The cost analysis has shown that a new C/A street made of concrete will have a total life cost similar to that of asphalt. Staff does not recommend requiring streets to be constructed or reconstructed out of concrete due to its disadvantages. These disadvantages, which are thoroughly discussed in the appendix of this report, have resulted in a drastic decrease in usage as a paving material in this part of the country. If the Council would like to further explore this option it should direct staff to meet with representatives of the development community that would be asked to carry the additional up front cost. Attachment: 1. Appendix—Pavement Design Theory 2-4 AT rACB31ENT#1 Page 1 APPENDIX—PAVEMENT DESIGN THEORY Currently, through the cooperation between the Public Works department and the City Utilities Department,pavement rehabilitation in-a given neighborhood takes place approximately one year after routine underground utility maintenance/replacement is performed. If a street is found to require pavement rehabilitation, and that street has recently had all of its underground utilities replaced, then that street could be considered to be a new street in this instance and would be designed as.such. Utilities are typically designed for a 50-year life. A street designed to match the 50-year life expectancy of its underground utilities makes good sense. The current City standard for new arterial & collector streets is based on a 20-year design life..A Collector/Arterial street that has a 50-year design life, as shown in the body of this report, would be. beneficial to the City's economics of street rehabilitation. This economic benefit would not be recognized.for many years—when rehabilitation of a 20-year street would have been needed. The City's design standard fora reconstructed or resurfaced street is based on a 20-year design life. The lower design life is due to the fact than with older existing underground utilities.present,the street will most likely need to be trenched within a.shorter periodof time compared to a.stmt with new underground utilities. As stated earlier in this report, whenever a trench is dug through an asphalt street,whatever.remami mi g life the street had is greatly reduced. Street design is based on many factors, one of those being the expected traffic load Each street category of street is given a Traffic Index (TI), a number that is a function of the total number 9-ton axle loads that the street will be subjected to over its design life. The life of the pavement is governed by the time it takes for that number to accumulate. The T1 is similar to a logarithmic number in that. twice the number of axle loads does not result in twice the TL The figure on the.next page illustrates the relationship between TI and 9-ton axle loads. With the rate of 9-ton.axle loads (based on truck traffic)held constant, a TI based on a 50-year design life would be approximately 10% to 15% higher than a TI based on a 20-year design We. The actual rate and weight of future truck traffic can only be estimated. Because of these unknowns, the City uses conservative-estimations on future heck traffic and the_respective weight of future trucks. These conservative estimations give us values for the TTs an which we base pavement design. Due to these conservative TI values,the pavement may actually last longer than its design life. .TI os.. 9- Toa •z3a Loads 9 8 6 r ' 5 �— 3 - = 2 — 00 00 00 00 00 00 00 00 o0 00 00 00 00 b tib ,fib ,,9 bb bb bS ,fib bb 9b yob' yyb y1b saa bar at 9-Tea asL• per Saar 2_5 TI (20 year)'� TI (5'0 year)' Page 2 The following table compares 20-year design to 50-year design of both asphalt and concrete streets. Shown in this table are typical pavement sections that may be used based on various design criteria and soil conditions. Comparison of 20-Year and 50-Year Asphalt Pavement Approx.Cost Street 20-Year Desi 50-Year Design Increase Classification TI Pavement Section 11 Pavement Section 50 yr.vs.20 yr. New Asphalt75 mm(3 in.)AC 90 mm(35 in.)AC& Residential Street 55 &300 nun(12 in.) 6.5 375 corn(15 in.) 25% R=5 Class H Base Class II Base New Concrete 100 mm(4 in)PCC 110(4.5 in)PCC Residential Street 55 & 150 mm(6 in.) 6.5 &150 mm(6 in.) 10 R=5 Class 11 Base Class 11 Base Resurfaced Residential 55 40 mm AC Overlay 6.5 60 mm AC Overlay 25% (typical overlay)** (15 inches) (2.5 inches Reconstructed Residential,R=5 55 200 mm AC 65 350 mm AC 20% full thickness A 8 inches 10 inches Reconstructed Residential,R=30 55 165 mm AC 6.5 200 mm AC 20% full thickness A 65 inches 8 inches New 125 mm(5 in)AC 150 mm(6 in)AC Collector/Arterial, 85 &500 mm(20 in) 9.5 &550 rnm(22 in)Class 15% R—�5 Class H Base 11 Base New Concrete 110& (45 in) PCC 150 mm(6 in)PCC Collector/Arterial, 85 &150 nim(6 in) 95 & 150 mm(6 in) 15% R=5 Class H Base Class R Base New 125 mm(5 in)AC 150 mm(6 in)AC Collector/Arterial, 85 &300 mm(12 in) 95 &350 mm(14 in)Class 15% R=30 Class 11 Base II Base New Concrete175 mm(7 in)PCC 190 mm(8 in)PCC Collector/Artmial, 85 (No Base) 95 (No Base) 10% R=30 Resurfaced 60 mm AC Overlay 75 mm(3 inches)AC Collector/Arterial 85 (25 inches) 95 Overlay& 30% ( cal overlay)** I l la erglasgrid Reconstructed 300 mm AC 300 mm AC CollectodArterial, 85 (12 inches)&1 layer 95 (12 inches)&2 layers 10% R=5 of glasgdd*** of glasgrid*** full thickness A Reconstructed Collector/Arterial, 85 265 inin AC 9.5 300 mrn AC 10% R=30 (105 inches) (12 inches) full thickness A * Total cost of project,including asphalt,striping,manhole adjusting,traffic control,etc. 2-6 Page 3 ** Actual overlay thickness may be somewhat different—based on deflection testing. *** Due to difficulties in underground utility maintenance created by thicker pavement sections, 300mm is the maximum thickness currently used. Additional strength, when needed, is achieved through the use of a pavement-reinforcing grid(i.e.glasgrid). City History of Pavement Design The current condition of many of the City streets, and consequently the large amount of funds currently being used to rehabilitate these streets,is primarily due an older standard of constructing thin asphalt streets. When many of the residential streets were originally built in the years following World War U, the standard was to build a street with 1 r/i or 2 inches of asphalt over 4 or 6 inches of base. The base material used on many streets in those years was not a crushed aggregate as used today but a somewhat weaker material called red rock, which deteriorates over time. This design is significantly substandard compared to the City's current design. Many of these streets have lasted 40 years or more but now are in need of repair. Today, with better traffic projections, better understandings of what is good pavement design, and with the required TI (50 year design) of 6.5, the typical residential street section above the City's common clay soil subgrade is 90 mm (3.5 inches) of asphalt over 375 mm (15 inches) of base. A typical reconstructed local street with a TI(20-year design) of 5.5 would contain 75 mm (3 inches) of asphalt over 300 mm (12 inches) of base or as an option, a full-thickness asphalt section of 225 mm (9 inches). Today's 20-year design standard for reconstructed asphalt residential streets results in a street far more durable than a typical street that was built years ago, and has lasted much longer than 20 years. Full-Thickness Asphalt The option of using a full-thickness asphalt section for street reconstruction does not require as deep of an excavation compared to a reconstructed street comprised of asphalt concrete and a structural base material. The deeper excavation required in using a gravel base under the asphalt results in a greater risk of damaging underground utilities as compared to the full-thickness asphalt method. Because of this,Public Works staff prefers the full-thickness option. There are, however, some difficulties to the full thickness design. When an underground utility breaks or needs maintenance or replacement, trenching through a full-thickness asphalt section can be very difficult; the thicker the section of asphalt is, the more difficult and expensive it is to sawcut, remove, and replace. Additionally, when a waterline break occurs under a section of full-thickness asphalt, the escaped water can actually build up pressure beneath the surface of the street. If this wager cannot surface through the pavement, underground water pressure can build to the point where the pavement can actually lift up from the subgrade, causing excessive damage. The thicker the pavement section is, the greater chance there is for this kind of pavement damage to occur. A section based on a 50-year design would be thicker than a section based on a 20-year design life, and consequently, would increase these drawbacks to using the full-thickness design. Because of the difficulties stated above, the maximum pavement thickness currently used by the City is 300mm (12 inches). Additional pavement strength, when needed, can be gained through 2-7 Page 4 the use of one or more layers of a pavement-reinforcing grid. The grid is effective in increasing the strength of the pavement, however, if the pavement ever needs to be trenched for underground work, the structural benefit of the grid is lost in the vicinity of the underground work- The orkThe design life of an asphalt street is based on the assumption that the street will not be cut open at any time during its design life. As stated above, the remaining life an asphalt street may have is greatly compromised when the street is cut open. Because we cannot assume that a reconstructed street will not need to be cut open for 50 years, the use of asphalt for a 50-year designed street may not even be realistically achievable. Portland Cement Concrete Pavement There are many experts in the pavement industry that believe that Portland Cement concrete is a preferable material to use for street paving — particularly when designing for a longer life. An asphalt street, even with a seal coat applied every few years (such as the City's 8-year seal coat program), may not last as long as concrete. At some point, a breakdown of the physical characteristics of the asphalt will occur, resulting in a pavement failure within a period of time that may be less than 50 years. The use of Portland Cement concrete as a structural element for City streets is an option that could result in streets that can realistically be designed to last for 50 years or more. Concrete does not age the same way that asphalt does; a concrete street can be trenched and then repaired in such a way that the loss of the street's structural strength and remaining service life is minimal. Although there are advantages to using concrete as a structural element in city streets, there are also many disadvantages. Concrete streets have historically resulted in generating more noise by vehicle tires (particularly on high-speed roads). The white appearance of concrete does not provide a good contrast for painted pavement markings. Pavement markers wear out faster. Many people simply prefer the dark look of asphalt as compared to concrete. An older concrete street, although structurally sound, can develop a rough ride quality (evident in some of the City's older concrete streets). And when it comes to resurfacing or reconstructing a street out of concrete, the long cure time requires that a street be kept closed for an extended period of time (at least one day),which can cause a great inconvenience to the public. Because of these disadvantages, staff does not recommend changing the City standard paving material from asphalt to concrete. Composite Concrete with Asphalt Pavement In the first half on this century,many of the city's downtown streets were initially constructed out of concrete. Most of these streets have been since overlaid with asphalt, Maintenance of this combination of concrete and asphalt has worked out well. When one of these streets is in need of repair, the asphalt is simply milled off and replaced with new asphalt. The design life of a newly resurfaced composite concrete/asphalt street is approximately 20 years, however, the street only needs resurfacing(not full reconstruction)at the end of its design life. 2-8 Page 5 A program could be implemented that when a street is in need of reconstruction, the.new street could be constructed with 50mm (2 inches) of asphalt over approximately 150mm (6 inches) of concrete. This would result.in a.street that would remain structurally intact for well over 50 years, only needing resurfacing approximately every 20 years. This approach, however, also has many disadvantages. The same drawbacks relating to the long cure time of concrete, and the difficulties in patching trenches exist with this approach. The initial construction would be more extensive as it would require an initial pour of the concrete,. followed by the laying down of the asphalt.layer. This approach would contain the highest-initial cost of all the options discussed (approximately 40% more than asphalt alone), and would result in the greatest amount of public inconvenience during construction. Comparison:.Concrete vs.Asphalt-New Collector/Arterial Streets As stated in the body of this report, there are unique advantages and disadvantages of both asphalt and concrete related to their use as a paving material on newly constructed Collector & Arterial Streets. For longer-designed streets (50-years) concrete has been proven to be an extremely durable and long-lasting paving material. This is evident in the fact that some of the City's older streets, constructed out of concrete over 80 years ago, are still structurally intact (although a bit rough). Concrete does not require a routine surface treatment (such as a slurry seal required on asphalt streets). Concrete also has the advantage that when the condition of the surface does become a bit rough after many years of use, it can be overlaid with a thin layer of asphalt. Although somewhat costly, this approach provides a smooth riding surface while benefiting from the structurally sound concrete layer below. The disadvantages of using concrete as a paving material are discussed on the previous page. Most of these disadvantages apply to a newly constructed Collector/Arterial street. However, the disadvantage of the long cure time of concrete would not be a disadvantage when the street is brand new and not yet open to traffic. Today,most city streets are constructed outof asphalt due to its advantages over concrete. These advantages include the fact that initially construction-is less expensive. After construction,traffic. can be let on the street is a short period of time (approximately one hour). Although it does require more routine maintenance than concrete, this maintenance results in an.asphalt street that contains a fresh, black surface throughout the life of the pavement. Asphalt has proven to be a cost effective paving material that is greatly accepted by the public and by most of the pavement industry. The disadvantages associated with asphalt being used as a paving material on Collector and Arterial streets relates to the trenching that will inevitably occur on these streets in the future. These busier streets often serve as corridors for public and private utilities where installation and/or replacement of utilities is likely. A good example of this is installation of fiber optic communication lines that is currently being done in the City—primarily on Collector and Arterial t streets. The sawcutting and trenching of a street causes a street to lose some of its remain service life. Exactly how much life is lost.is difficult to determine, and is based on factors such as 2-9 i Page 6 existing soil type, pavement thickness and age of pavement. We do how,however,that-some of the life of the pavement is lost when the street is trenched, andthat some of the extra money spent on the longer life of the:pavement is lost when this occurs. Due to the structural.properties of concrete streets, it appears that this loss of pavement life due to trenching-is.far less severe.on a concrete street as compared to an asphalt street.. These advantages and disadvantages of asphalt and concrete are quite different for each. In the opinion of Public Works staff, asphalt pavement offers some clear advantages in the eyesof the public that uses the streets. Therefore, staff has recommended that the City adopt a policy that m&es:asphalt the pavement of choice but allows the-City's review process some latitude to allow concrete pavement on a case by case basis. This would give a developer a certain amount of flexibility in neighborhood design without compromising any aspect of current or future pavement condition and related expenses. MCOUNMACOMA RFPOM MYEAR DEMNL E 2-10 MEE, AGENDA DATE 'I=V ITEM # C®utilco4ion memoRAnOum COUNCIL OCOD DIR IYACAO ❑FIRE CHIEF November 1, 1999C3'AT70^^IEv ❑PW DIR B'�:.0 .:'1=3 ❑POLICE CHF ❑I.;h'„`T_'-A'I ❑RECDIR TO: Council Colleagues E30 UTIL DIR ❑- ❑PERS DIR 19 FROM: John Ewan 9491.5, SUBJECT: San Luis Obispo County Day in Washington. D.C. This trip was very similar to our legislative trips to Sacramento, only in D.C. The people we met were at the highest levels of government, and showed true interest in our County. This was spurred on by our being there, and by our great representation in D.C. by Lois Capps. A Few Notes: Sam Farr: "We live in a County in California on land that can grow and do what no other land in the world can do.” "Sustain our tourism industry by sustaining our `special' place." • Congressman Farr referred to our ability to grow such a variety of vegetables, and our economic strengths in high tech commerce. He also mentioned the availability of highway and transportation funds for economic growth(Prado Overpass?). Kelly Carnes, Economic Senior Administrator, learned our name and location in California, although not how to pronounce! Carnes spoke about all the areas in the U.S. and the world, around which clusters of high tech industry are moving. Put SLO in that same sentence or paragraph, and get us our investment of capitol in research and development. Small specialized R&D clusters are strengthening. This is an area in which we can achieve distinction. Cal Poly is already known for bio and computer technology. A shortage of trained tech persons is presenting a dilemma for higb tech industries. CEO's are concerned about available tech skilled workers. Literacy above eighth grade is needed. There is no simple answer. We have the foundation for technical growth—and now the ear of Washington. Money is available for infrastructure for economic development. Planning pro rural areas. $50,000.00 for determining an economic plan. $100,000.00 for Cu sta? ECEIVED N U V 1 - 1999 SLO C:;7 Y CLERK .. .:� 1. Dan Glick--man, Education: There are 428,000'neW students in California.- Dick alifornia.Dick Gephardt: Education begins in the home with the family. Not.all kids are afforded.this basis of education—from low to high income. Our children receive 1/3 less time than our parents' generation did: How do we help families to do what they need to do? Revolutionize: Parent training, preschool, after school, and•teacher training. Technology in the school Pencil Labs? Move through the process and do it. Time is.wasting. We need to celebrate people(teachers). Agriculture: We need to promote sustainable vineyard practices, findingalternativesto methyl-bromide and other toxic:chemicals. I