HomeMy WebLinkAbout15 - Design Report - Boulder Creek - Pavement PAVEMENT DESIGN REPORT
BOULDER CREEK 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
t f..
,
Zt .
Project Number: 14624
JUKE 2015
PAVEMENT DESIGN FOR LOCAL STREETS WITHIN SUBDIVISION
PUBLIC RIGHT-OF-WAY SOIL CONDITIONS
On November 27 & 29, 2001, 11 test holes were excavated with a backhoe across the proposed
subdivision 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 topsoil, which overlays a layer of brown, clayey silt having a non-uniform
thickness and containing minimal sands, pebbles or small gravels. Underlying the clayey silt
layer is sandy gravel (locally called pit run) of alluvial fan origin. Groundwater was also
encountered at depths ranging from 4 and 5 feet below the ground surface, and it is worth noting
that high groundwater levels are known to exist in the area even during dry periods.
Penetration 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. 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.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 conducted by Abelin Traffic services,the estimated traffic after
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
Design Report-Page 2 of 10
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.
According to the Allied Engineering Geotechnical Report for Laurel Glen Subdivision the CBR
was determined to be 2.9. However,for this report CBR= 1 was used to give a more conservative
sub-base thickness based on field experience gained by C &H Engineering during construction of
adjacent subdivisions.
Design Report-Page 3 of 10
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 CBRo-"1 (United States Army Waterway Experiment Station)
MR= 2,550 CBR0.64 (Transport& Research Laboratory, England)
With CBR= 1.0
MR= 1,500 CBR= 1,500 (1) = 1,500 psi
MR= 5,409 CBRo-"' = 5,409 (1)"" = 5,409 psi
MR= 2,550 CBRo.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 (EQ 1):
to OPSI
log W18 = ZRSo + 9.36[log(SN + 1)] — 0.20+ g .7 + 2.32log MR — 8.07
0.40+ 1094
(SN + 1)5.19
Variables:
1. ESAL (Wls) = 64,331
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.
Design Report-Page 4 of 10
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 (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)= 1,500 psi
Solution: using (EQ 1), 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 1/2" minus crushed gravel)
a3 = 0.11 (Sub-base Course - 6" minus gravel)
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 Di and D2, and solving(EQ2), D3 = 15.91" for Local Streets
Use a standard street sub-base section of 18" 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 5 of 10
PAVEMENT DESIGN FOR OAK STREET (FUTURE PRINCIPAL ARTERIAL)
PUBLIC RIGHT-OF-WAY SOIL CONDITIONS
On November 27 & 29, 2001, 11 test holes were excavated with a backhoe across the proposed
subdivision 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 topsoil, which overlays a layer of brown, clayey silt having a non-uniform
thickness and containing minimal sands, pebbles or small gravels. Underlying the clayey silt
layer is sandy gravel (locally called pit run) of alluvial fan origin. Groundwater was also
encountered at depths ranging from 4 and 5 feet below the ground surface, and it is worth noting
that high groundwater levels are known to exist in the area even during dry periods.
Penetration 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. However, the CBR value used for this report was 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.1 (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 in 2013, the estimated
traffic after subdivision build-out is expected to be approximately 2230 vehicle trips per day
(VTPD) within the subdivision during the average weekday. For the purpose of this report in
Design Report-Page 6 of 10
determining thickness of sub-base of gravel at Oak Street, a future 5-lane principal arterial, we
looked at Figure 3-17 of the 2007 Greater Bozeman Area Transportation Plan (2030 traffic
projections). Projected traffic volume in VTPD as shown on Figure 3-17 is 9,200. Since Oak
Street will eventually be a four lane arterial with the eastbound lane bordering Boulder Creek
Subdivision to the north we divided 9,200 VTPD by 4. Therefore, the average daily traffic per
lane equates to 9,200/4 = 2,300 vehicles per lane per day (vplpd), which equates to 2,300 vplpd x
365 days/year= 839,500 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
2,000 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 822,710 cars per lane per year, and
16,790 trucks/buses per lane per year at full build out.
Traffic Estimate for Collector Streets within Subdivision
Vehicle Type Vehicles Growth Design Vehicles ESAL Factor Design
per year Factor (20 years) ESAL
(4%,20yrs)
Passenger Car 822,710 29.78 24,500,304 0.0003*2=0.0006 14,700
2 axle/6 tire 16,790 29.78 500,006 0.118*2=0.236 118,001
truck/bus
Total ESAL 132,702
The calculated estimate of the equivalent 18,000 lb Single Axle Load (ESAL) =132,702
The calculated ESAL is more than the minimum 50,000 ESAL design requirement. Therefore,
ESAL=132,702 shall be used for all calculations.
Design Report-Page 7 of 10
According to the Allied Engineering Geotechnical Report for Laurel Glen Subdivision the CBR
was determined to be 2.9. However,for this report 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 CBRo.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 (1)0-"1 = 5,409 psi
MR= 2,550 CBRo.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 (EQ 1):
to APSI
log W18 = ZRSo + 9.36[log(SN + 1)] — 0.20+ g .7 + 2.32log MR — 8.07
1094
0.40+ (SN + 1)s'`'
Variables:
6. ESAL (W18) = 132,702
Design Report-Page 8 of 10
7. Level of Reliability (ZR) _ -1.645 used for Arterials and Collector Streets based on 95%
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.
8. Standard Deviation(So) = 0.49 for flexible pavements.
See Part 1, 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 (APSI) = 1.7 used for Arterial and Collectors See Part 1I, Sec. 2.2,
AASHTO Manual. 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. Arterial and collector roads such as Oak Street are normally allowed to
deteriorate to 2.5. The resulting difference in the initial to final indexes is the total
serviceability loss.
10. Soil Resistance Modulus (MR) = 1,500 psi
Solution: using (EQI), the SN for Collector Roads =4.62
Pavement Design Equation (EQ2):
SN = a1D1 + a2D2M2 + a3D3M3
4. Layer Coefficients: al = 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)
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: Di =4" for Collector Roads
D2 = 6" for Collector Roads
Design Report-Page 9 of 10
Solution: using the values given for Di and D2, and solving (EQ2), D3 = 21.05" for Collector
Roads
This results in final values used for this design of 4 inches of asphalt, 6 inches of 1" minus road
mix, and 21 inches of 6"minus gravel for all collector roads.
Design Report-Page 10 of 10