HomeMy WebLinkAbout15 - Design Report - Westbrook - Pavement PAVEMENT DESIGN REPORT
WESTBROOK SUBDIVISION
Prepared for:
Rosa-Johnson Development, LLC.
125 Central Avenue, Bozeman, MT 59718
Prepared by:
C&H Engineering and Surveying, Inc.
1091 Stoneridge Drive, Bozeman, MT 59718
(406) 587-1115
�'e;;•!y'v;'r;;rCr �n'S
Project Number: 14693
JANUARY 2015
PAVEMENT DESIGN FOR LOCAL STREETS WITHIN SUBDIVISION
PUBLIC RIGHT-OF-WAY SOIL CONDITIONS
On November 27 and 29, 2001 seven (7) test holes were excavated with a backhoe across the
proposed subdivision site by Kolnik Excavation. The subsurface conditions in the vicinity of the
interior roadway generally consist of a 12 to 24 inch layer of a soft to medium stiff, dark brown to
black, organic clayey silt topsoil followed by a 1.5 foot to 2.5 foot thick layer of hard to soft light
brown, clayey silt containing minimal sands,pebbles or small gravels. Underlying the clayey silt
layer, sandy gravel materials of alluvial fan origin were encountered having an undetermined
depth. Groundwater was also encountered at depths ranging from 4 to 5 feet below the ground
surface and it is worth noting that high groundwater levels are known to exist in the area during dry
periods.
Laboratory tests were performed on the silty-clay material below the topsoil to estimate the
California Bearing Ratio (CBR) and are provided in the geotechnical report from 2002 submitted
by Allied Engineering for Laurel Glen Subdivision. The CBR determined by Allied in this report
is 2.95. The CBR value used for this report, however, is 1.0, which is a conservative estimate
based on field experience gained by C & H Engineering while working on adjacent subdivisions.
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.l (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 conducted by Abelin Traffic services,the estimated traffic after
Design Report-Page 2 of 11
subdivision build-out is expected to be approximately 2230 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 2230/2 = 1115 vehicles
per lane per day (vplpd), which equates to 507 vplpd x 365 days/year = 406975 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 406,975 cars per lane per year, and 3,701
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 406,975 29.78 11,877,321 0.0003*2=0.0006 7,126
2 axle/6 tire 3,701 29.78 242,394 0.118*2=0.236 57,205
truck/bus
Total ESAL 64,331
The calculated estimate of the equivalent 18,000 lb Single Axle Load (ESAL) =64,331
The calculated ESAL is more than the minimum 50,000 ESAL design requirement. Therefore,
ESAL=64,331 shall be used for all calculations.
Design Report-Page 3 of I
According to the Allied Engineering Geotechnical Report for Laurel Glen Subdivision the CBR
was determined to be 2.95 for the clayey silt layer below the topsoil. However, for this report, a
CBR = 1 was used to give a more conservative sub-base thickness based on field experience
gained by C and H Engineering during construction of adjacent subdivisions.
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 CBR'-"�t (United States Army Waterway Experiment Station)
MR= 2,550 CBR'�64 (Transport &Research Laboratory, England)
With CBR= 1.0
MR = 1,500 CBR= 1,500 (1) = 1,500 psi
MR= 5,409 CBR0-"' = 5,409 (If"' = 5,409 psi
MR=2,550 CBRo.64=2,550 (1)1.64 =2,550 psi
Use most conservative value= 1,500 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(EQI):
tog APSI
log W18 = ZRSo + 9.36[log(SN + 1)] - 0.20+ 2.7 1094 + 2.32 log MR - 8.07
0.40+ (SN + 1)5.19
Variables:
1. ESAL (Wig) =64,331
Design Report-Page 4 of I i
2. Level of Reliability VR) _ -1.282 for Local Streets used for Local Streets based on 90%
reliability from Part I, Table 4.1, and Part Il, 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 future 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 (OPSI) =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) = 1,500 psi
Solution: using (EQI), the SN for Local Streets = 3.91
Pavement Design Equation (EQ2):
SN = a1D1 + a2D2M2 + a3D3M3
1. Layer Coefficients: al = 0.44 (Hot-mix asphalt concrete)
a2 =0.14 (Base Course - 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 D, and D2, and solving (EQ2), D3 = 15.91" for Local Streets
Design Report-Page 5 of 11
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 18" for the Local Streets.
Design Report-Page 6 of 11
PAVEMENT DESIGN - COLLECTOR ROADS BORDERING SUBDIVISION
PUBLIC RIGHT-OF-WAY SOIL CONDITIONS
On November 27 and 29, 2001 seven (7) test holes were excavated with a backhoe across the
proposed subdivision site by Kolnik Excavation. The subsurface conditions in the vicinity of the
interior roadway generally consist of a 12 to 24 inch layer of a soft to medium stiff, dark brown to
black, organic clayey silt topsoil followed by a 1.5 foot to 2.5 foot thick layer of hard to soft light
brown, clayey silt containing minimal sands,pebbles or small gravels. Underlying the clayey silt
layer, sandy gravel materials of alluvial fan origin were encountered having an undetermined
depth. Groundwater was also encountered at depths ranging from 4 to 5 feet below the ground
surface and it is worth noting that high groundwater levels are known to exist in the area during dry
periods.
Laboratory tests were performed on the silty-clay material below the topsoil to estimate the
California Bearing Ratio (CBR) and are provided in the geotechnical report from 2002 submitted
by Allied Engineering for Laurel Glen Subdivision. The CBR determined by Allied in this report
is 2.95. The CBR value used for this report, however, is 1.0, which is a conservative estimate
based on field experience gained by C & H Engineering while working on adjacent subdivisions.
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.l (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 conducted by Abelin Traffic services,the estimated traffic after
Design Report-Page 7 of I I
subdivision build-out is expected to be approximately 2230 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 2230/2 = 1115 vehicles
per lane per day (vplpd), which equates to 507 vplpd x 365 days/year=406975 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 406,975 cars per lane per year, and 3,701
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 406,975 29.78 11,877,321 0.0003*2=0.0006 7,126
2 axle/6 tire 3,701 29.78 242,394 0.118*2=0.236 57,205
truck/bus
Total ESAL 64,331
The calculated estimate of the equivalent 18,000 lb Single Axle Load (ESAL) =64,331
The calculated ESAL is more than the minimum 50,000 ESAL design requirement. Therefore,
ESAL=64,331 shall be used for all calculations.
Design Report-Page 8 of 11
According to the Allied Engineering Geotechnical Report for Laurel Glen Subdivision the CBR
was determined to be 2.95 for the clayey silt layer below the topsoil. However, for this report, a
CBR = 1 was used to give a more conservative sub-base thickness based on field experience
gained by C and H Engineering during construction of adjacent subdivisions.
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 CBR 1.64 (Transport& Research Laboratory, England)
With CBR= 1.0
MR= 1,500 CBR = 1,500 (1) = 1,500 psi
MR = 5,409 CBR'-"' = 5,409 (If"' = 5,409 psi
MR =2,550 CBR°-64=2,550 (1)0.64 =2,550 psi
Use most conservative value= 1,500 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 (EQ1):
to APS1
log W18 = ZRSo + 9.36[log(SN + 1)] - 0.20+ g .7 + 2.32 log MR - 8.07
0.40+ 1094
(SN + 1)5.19
Variables:
6. ESAL (W18) = 64,331
Design Report-Page 9 of 11
7. Level of Reliability VR) _ -1.282 for Local Streets used for Collector Streets based on
95%reliability from Part I, Table 4.1, and Part Il, Table 2.2, AASHTO Guide.
Level of reliability is based on the cumulative percent of probability of reliability with a
standard normal distribution.
8. 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 future 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.
9. Serviceability Loss (OPSI) =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.
10. Soil Resistance Modulus (MR) = 1,500 psi
Solution: using (EQ1), the SN for Collector Roads= 4.15
Pavement Design Equation (EQ2):
SN = a1D1 + a2D2M2 + a3D3M3
4. Layer Coefficients: a, = 0.44 (Hot-mix asphalt concrete)
a2 = 0.14 (Base Course - 1 '/z" minus crushed gravel)
a3 = 0.11 (Sub-base Course - 6" minus crushed stone)
5. Drainage Coefficients: m2 = 1.00 (good drainage 5-25%)
m3 = 1.00 (good drainage> 25%)
%of time base & sub-base will approach saturation
6. Layer Depth Assumptions: D, =4" for Collector Roads
D2=6" for Collector Roads
Solution: using the values given for Di and D2, and solving (EQ2), D = 14.05" for Collector
Design Report-Page 10 of 11
Roads
This results in final values used for this design of 4 inches of asphalt, 6 inches of 1" minus road
mix, and 18 inches of 6"minus gravel for all collector roads.
Design Report-Page 11 of 11