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Home My WebLink About14 - Design Report - Valley Meadows - Pavement PAVEMENT LESION REPORT VALLEY MEADOWS SUBDIVISION Prepared for: Fortin Construction, LLC. P.O. Box 11811, Bozeman, MT 59719 Prepared by: C&H Engineering and Surveying, Inc. 1091 Stoneridge Drive, Bozeman, MT 59718 (406) 587-1115 i T it p fjw ll= �§�� LS� C- C7 UZ f� 0f �a� ` Project Number: 14328 November 2014 PAVEMENT DESIGN REPORT—VALLEY MEADOWS SUBDIVISION LOCAL STREETS WITHIN SUBDIVISION PUBLIC RIGHT-OF-WAY SOIL CONDITIONS A total of I test pits were excavated with a backhoe across the proposed subdivision on September 19, 2014. The I test pits revealed very similar soil profiles. The subsurface conditions were generally observed to consist of 6 to 12 inches of an Organic Soil of Low Plasticity (OL) followed by a layer of Lean Clay with Sand (CL) to depths ranging from 2.0 feet below grounds surface (bgs) to 4.0 feet bgs, followed by a layer of Silty Gravel with Sand and Cobbles (GM) to depths ranging from 3.5 feet bgs to 5.0 feet bgs, after which Poorly Graded Gravel with Sand and Cobbles was encountered to the end of each excavation at approximate depths of 9.1 feet bgs to 11.5 feet bgs. Groundwater was encountered at the end of each excavation, with the seasonal high groundwater elevation estimated to be 4.5 feet bgs. Penetration tests were performed on the Lean Clay with Sand using a static cone penetrometer. The estimated CBR value is obtained by the equation Qc = 3.3(CBR), or CBR= Q"13.3, with Qc being the cone index reading. The average cone index reading obtained was 10.0, resulting in an estimated CBR value of 3.33. A conservative value for the CBR of 3 was used for this report for Meriwether Avenue and Villard Street to provide for possible inconsistencies that may be found in the soils during construction, and the approximate testing methods used. The Standard Test Method for CBR (California Bearing Ratio) of Soils in Place, based on ASTM Designation D 4429-4, requires complex and specialized equipment, the expense of which is not warranted for the local streets with low projected use. STREET DESIGN Criteria for design: Bozeman Municipal Code, Section 38.24.060 and City of Bozeman Design Standards and Specifications Policy, Addendum No. 4, Section IV.G: pavement thickness design will be based on the current AASHTO Guide for Design of Pavement Structures, or the current Asphalt Institute Manual Series No.I (MS-1). The design shall be based on a minimum 20 year performance period traffic volume, with the minimum design lane based on a minimum of 50,000 ESAL. According to a recent traffic study, the estimated traffic after subdivision build-out is expected to be approximately 360 vehicle trips per day within the subdivision during the average weekday. All of the roads in the proposed subdivision contain two driving lanes (one in each direction) so the number of trips per day is divided in half to calculate the ESAL value for each lane. Average daily traffic per lane equates to 360/2 = 180 vehicles per lane per day (vplpd), which equates to 180 vplpd x 365 days/year=65700 vehicles per lane per year. The following assumptions were made while calculating the Total ESAL: 2% of the AYT will consist of heavy trucks or buses Growth rate =4% over 20 years 2000 lb axle load for cars, and 10,000 lb axle load for trucks. 2 axles per vehicle Based on 2% of the traffic being trucks/buses, this yields 64,386 cars per lane per year, and 1314 trucks/buses per lane per year at full build out. Traffic Estimate for Local Streets within Subdivision Vehicle Type Vehicles Growth Design Vehicles ESAL Factor Design per year Factor (20 years) ESAL (4%,20yrs) Passenger Car 64,386 29.78 1,917,415 0.0003*2=0.0006 1,150 2 axle/6 tire 1314 29.78 39,130.92 0.118*2=0.236 9,235 truck/bus Total ESAL 10,385 The calculated estimate of the equivalent 18,000 lb Single Axle Load (ESAL)= 10,385 The calculated ESAL is less than the minimum 50,000 ESAL design requirement. Therefore, ESAL=50,000 shall be used for all calculations. According to the California Bearing Ratio (CBR) Test (ASTM-D 1883/AASHTO T193) performed by C&H Engineering Inc.,the CBR used for the subgrade soil is 3.0. CBR can be related to the subgrade Resilient Modulus MR by the following: (Sec. 3.5.4, Highway Engineering Handbook, McGraw Hill, 1996) Subgrade Resilient Modulus MR (psi): MR= 1,500 CBR(Shell Oil Co.) This value used by Asphalt Institute. MR= 5,409 CBR0-"1 (United States Army Waterway Experiment Station) MR=2,550 CBRO.64 (Transport&Research Laboratory, England) With CBR= 3.0 MR= 1,500 CBR= 1,500 (3.0)=4,500 psi MR= 5,409 CBR0-"' = 5,409 (3.0)"" = 11,812.73 psi MR=2,550 CBRo.64=2,550 (3.0)0.64 = 5,151.07 psi Use most conservative value=4,500 psi USING THE AASHTO METHOD OF FLEXIBLE PAVEMENT DESIGN The AASHTO method utilizes a value known as the Structural Number (SN) which relates the below variables to the wear surface,base, and sub-base depths. Structural Number Equation (EQ 1): la APS1 to 147 Z S + 9.36 o Rwr+ 1 g 2.7 � i$ _ � Q [1 g( 1094 +2.32 log MR—8.07 0.40+ (SiNr+1�s19 Variables: 1. ESAL(W18)= 50,000 2. Level of Reliability (ZR) _ -1.282 for Local Streets used for Local Streets based on 90% reliability from Part I, Table 4.1, and Part II, Table 2.2, AASHTO Guide. Level of reliability is based on the cumulative percent of probability of reliability with a standard normal distribution. 3. Standard Deviation (So)= 0.49 for flexible pavements. See Part I, Sec. 4.3, AASHTO Guide. The standard deviation is the statistical error in the estimates for firture values within the formula. Typical values range from 0.40-0.50 for flexible pavements, with a value of 0.49 used to ensure a conservative solution. 4. Serviceability Loss (APSI) =2.2 for Local Streets. The designed allowable deterioration of the roadway is represented by the serviceability loss. A new road is usually assigned a serviceability index of 4.2 and the final index is based on the type of roadway. Local streets are normally allowed to deteriorate to 2.0. The resulting difference in the initial to final indexes is the total serviceability loss. 5. Soil Resistance Modulus (MR)=4,500 psi Solution: using (EQI), the SN for Local Streets =2.57 Pavement Design Equation(EQ2): Sept = a,D, +a;D;M,2 +a;;D; 1;, 1. Layer Coefficients: a, =0.44 (Hot-mix asphalt concrete) a2= 0.14 (Base Course- 1 1/2" minus crushed gravel) a3 =0.11 (Sub-base Course - 6" minus crushed stone) 2. Drainage Coefficients: m2 = 1.00 (good drainage 5-25%) m3 = 1.00 (good drainage> 25%) %of time base &sub-base will approach saturation 3. Layer Depth Assumptions: Di =3" for Local Streets D2=6"for Local Streets Solution: using the values given for Dl and D2, and solving (EQ2),D =3.69" for Local Streets Use a standard street sub-base section of 6" on the Local Streets. This results in an asphalt section of 3", a base course of 6", and sub-base course of 6" for the Local Streets. SPECIAL CONSIDERATIONS Field data we have obtained shows groundwater depths across the site vary between 9.1 to 11.5 feet bgs, with the seasonal high groundwater elevation estimated at 4.5 feet bgs. Past experience with road construction in the Gallatin Valley has shown us that the lean clay encountered is highly moisture sensitive. All subgrade material should be proof rolled prior to placement of pit- run to check for structural competence. Where saturated materials or soft spots are encountered one of three alternatives should be pursued. The first alternative is to allow the subgrade to dry out. For fine-grained soils, this is best accomplished by frequently scarifying them during warm climactic conditions. This is the least expensive method but can cause delays to the project with even minimal precipitation. Prior to stopping work for the day it is recommended that all scarified soils be re-rolled with a smooth drum roller to seal them against moisture infiltration. Option 2 is the over-excavation of the subgrade soils and replacement with compacted engineered fill. When this option is exercised we recommend that the over-excavation occur down to native sandy gravels. This option is most effective when native sandy gravels are found within a reasonable depth from bottom of sub-base. Option 3 includes over-excavation of subgrade soils and installation of a geotextile fabric and approved geogrid. This option should be utilized whenever there isn't time for the subgrade to dry out and when native sandy gravels are located at excessive depths. Project 4 14328 Title: Valley Meadows Subdivision To solve for minimum required Sub-Base depth,we first need to calculate the Structural Number(SN). Calculating SN can be accomplished by formula or graphically(AASHTO Guide for Design of Pavement Structures) Required Values For SN Calc WSB(ESAL) Equivalent Single Axle Load R(%) Probability serviceability will be maintained over the design life(R is used for graphical solution) Za Probability serviceability used in numerical solutions(Equated to R by table below) 5, Standard Deviation in estimates for ESAL,typically 0.30.0.50 APSI Serviceability loss over design life MR Soil Resistence Modulus of subgrade soil EQ 1: logAPSi log Wig= ZRSo+ 9.36[log(SN+ 1))— 0.20+ 2.7 1094 + 2.32109 MR— 8.07 0.40+ SN+ 1 s.19 Equivalent Single Axle Load ADT 360 Peak A.M. Peak P.M. Total �— 360 AYT 65700 (per lane) Assumptions: 2 %of AYT Consisting of Heavy Trucks 4 %over 20 Years Growth Rate Lb/Axle 2000 for cars Lb/Axle 10000 for trucks Initial SN 3 AASHTO tables for ESAL Factor are based on SN and above listed axle loads Vehicle Type Vehicles Per Year Growth Factor Design Vehicles ESAL Factor Design ESAL (01.,20 years) (20 years) Passenger Car 64386.00 29.78 1917415 0.0006 1150 2 Axle/6 Tire Truck 1314.00 29.78 39130.92 0.236 9235 Total ESAL 10385 or Use Minimum Value of �0,000 Level of Reliability(R and ZR) R to ZR Conversion Chart R ZR 90 1.2820 95 -1.6450 97.5 -1.9675 99 -3.0800 R 90 (Conservative estimate) ZR= -1.282 Standard Deviation(So) So= 0.49 Serviceability Loss(APSI) Road Type vs.TSI Present Serviceability Index(PSI)= 4.2 Highways 3.0 Terminal Serviceability Index(TSI)= 2.0 Arterials Local Roads 2.0 APSI Resistance Modulus(MR) CBR —31 Determined on basis of soil analysis MR= 4500 Shell Oil Co. (Should not be used for CBR>10) MR= 11812.73 U.S.Army Waterway Experimentation Station MR= 5151.07 Transport&Research Laboratory,England Use most conservative value of the three methods to calculate MR 4500.00 Structural Number(SN) SN= 2.566311756 Calculated by EQ 1 Once SN is determined,the thickness of the wearing surface,base,and subbase layers can be determined by EQ 2. EQ 2: SN= a,D1+ a2D2M2+ a3D3M3� al,a2,a3 structural layer coefficients of wearing surface,base,and subbase M21 M3 drainage coefficients of base and subbase Di,D2,D3 thickness of wear surface,base,and subbase in inches Structural Layer Coefficients(a) Pavement Component Coeffie_cient Wearing Surface Sand-mix asphaltic concrete 0.35 Hot-mix asphaltic concrete 0.44 Base Crushed Stone 0.14 Dense-graded crushed stone 0.18 Soil cement 0.2 Emulsion/aggregate-bituminous 0.3 Portland cement/aggregate 0.4 Lime-pozzolan/aggregate 0.4 Hot-mix asphaltic concrete 0.4 Subbase Crushed Stone 0.11 a2= 0.44 (Hot-mix asphaltic concrete) a2= 0.14 (11/2"Minus crushed gravel) a3= 0.11 (6"Minus crushed stone) Drainage coefficients(M) M2= 1.00 Good Drainage M3= 1.00 Good Drainage Layer Thickness Di= 3 Assumed(in inches) D2= 6 Assumed(in inches) Solve for D3= 1 3.69