Design of Flexible Pavements for Low Volume Rural Roads
Content
D.V .Bhava .Bhavanna nna Rao Rao M.Tech., M.Tech., F.I.E., Retired Retired R&B R &B Chief Engine E ngineer er E-mail: [email protected] P hone ho ne:: +919494 919494440 440202 202 E gis gis-india -india consulting cons ulting engine engineer ers s pvt ltd.
for all my presentat presentatio ions ns,, pleas pleas e visit visi t http://aproadbuildqa.blogspot.com
shoulder
Base Sub base eeeeEeeee Sub-Grade (300mm)
shoulder
Embankment
Typical Pavement Cross Section
Earlier Design Approach as per IRC:SP:20-2002 As per IRC: SP:20-2002, traffic is evaluated in Commercial Vehicles Per Day grouping together all types of vehicles weighing 3.0t or more. The percentage of laden, un-laden or over loaded vehicles was not considered. sub-Grade strength is evaluated in terms of 4-day soaked CBR values except in areas with annual rainfall less than 500mm and where the water table is too deep.
Pavement thickness chart as per IRC: SP 20 2002
Pavement thickness as per IRC: SP 20 2002 CBR
2%
3%
4%
5%
6%
Crust
0 to 15 CVPD
15 to 45
45 to 150
150 to 450
CVPD
CVPD
CVPD
Base
150
150
225
225
Sub base
275
365
370
455
Base
150
150
150
225
Sub base
200
265
330
320
Base
150
150
150
150
Sub base
125
200
260
315
Base
150
150
150
150
Sub base
100
165
210
260
Base
150
150
150
150
Sub base
60
115
150
175
Pavement thickness as per IRC: SP 20 2002 CBR
7
10%
15%
20%
150 to 450
0 to 15 CVPD
15 to 45
45 to 150
CVPD
CVPD
Base
150
150
150
150
Sub base
60
115
150
175
Base
150
150
150
150
Sub base
30
70
85
125
Base
150
150
150
150
Sub base
nil
30
50
75
Base
150
150
150
150
Sub base
nil
nil
30
50
Crust
CVPD
Recommended Design Approach as per IRC:SP:72-2007 Low volume roads are divided into three categories 1) Gravel/Aggregate –surfaced (Un-paved) roads 2) Flexible Pavements (Paved roads) 3) Rigid Pavements note: AASHTO guide for Design of Pavement structures was considered in this publication.
Recommended Design features in IRC:SP:72-2007 Pavement design for new roads as well well as for the up gradation/rehabilitation gradation/rehabilitation of existing roads have been included. Maximising the use of localised materials. Simple procedures for traffic counts, computing ADT(Average Daily Traffic) and ESAL (Equivalent Standard Axle Loads) in design life, selected as 10 years. 5 classes of sub-grade strength and 7 ranges of design traffic. Warrants Warrants for providing bituminous surface treatment. Long term performance monitoring by systematic condition surveys at regular intervals.
Design Traffic parameter parameter It has been expressed in terms of cumulative 80kN (8.16 tonnes) ESAL (Equivalent Standard Axle Loads) applications during the design life (10 years). Seasonal variations by way of enhanced traffic during the harvesting seasons have also been considered. Traffic growth rate shall be assessed by conducting traffic survey at least for 3 days each during peak and non peak seasons. In the absence of details, a traffic growth rate of 6% may be taken.
Composition of rural traffic traffic It is not only the traffic volume but also its composition that plays an important role in determining the pavement thickness and its composition. Heavy Commercial Vehicles (HCV) like full sized trucks and buses and the Medium-heavy commercial Vehicles (MCV) with a gross laden weight 3.0 tonnes and above shall be considered. Procedure has also been suggested to evaluate and consider the effect of Solid Wheeled Vehicles (SWC/iron-rimmed) in computing the design traffic.
Equivalency Factors Factors for different axle loads Axle l Load tonnes
kN
Equivalency factors
3.0
29.4
0.02
4.0
39.2
0.06
5.0
49.1
0.14
6.0
58.8
0.29
7.0
68.7
0.54
8.0
78.5
0.92
9.0
88.3
1.48
10.0
98.1
2.25
11.0
107.9
3.30
12.0
117.7
4.70
13.0
127.5
6.40
14.0
137.3
8.66
15.0
147.1
11.42
Axle equivalency factor = (W/Ws)4 W is single axle load in kN Ws is Standard Axle Load of 80kN(8.16t)
Vehicle Damage Factor 1) Fully Fully load loaded ed Heavy Heavy Vehi Vehicle cle (HCV (HCV): ): Rear Axle=10.2t Axle=10.2t and Front axle=5.0t. VDF= 2.44+0.14=2.58 2) Un-laden/partially loaded HCV: Rear Axle=6.0t Axle=6.0t and Front axle=3.0t. VDF=0.29+0.02= 0.31 3) Overloaded (20%) HCV: Rear Axle=12.3t Axle=12.3t and Front axle=6.0t. VDF= 5.06+0.29= 5.35 4) Unladen/partially loaded MCV Rear Axle=3.0t Axle=3.0t and Front axle=1.5t. VDF=0.018+0.001= 0.019
Equivalent Standard Axle Load applications +.
N=T0×365×
.
− ×
L
Where T0 = ESAL per day = number of commercial vehicles per day in the year of opening ×VDF rate( 6%) = annual traffic growth rate(6%) L = lane Distribution Factor: L = 1 for single lane or intermediate lane L = 0.75 for two-lane roads n = design life in years(10 years)
Seasonal Variations in Rural Traffic
Average Average Annual Daily Traffic (AADT) in CVPD
Correction Factor for Solid-Wheeled (iron rimmed)Cart rimmed)Cart Traffic
100
1.70
150
1.25
200
1.20
300
1.15
400 500
1.10 1.07
In the Equivalent Standard Axle Load calculations, the above correction factor Is to be applied for Sold-Wheeled Carts Ca rts if there Is significant volume of SWC(Iron Rimmed Vehicles). Vehicles). Damaging effect to the road surface by SWC is twice the damaging effect of a laden 6-8 t MCV.
Table 4 of SP:62 Guidelines for provi pr oviding ding a Bituminous Surface Treatment Type of Surfacing Traffic category
Annual Rain fall T1 ADT< 100
T2 T3 ADT= ADT= 100to150 150to200
T4 ADT> 200
Over 1500mm/ year
Gravel
BT
BT
BT
1000-1500mm/ 1000-1500mm/ year
Gravel
Gravel
BT
BT
Less than 1000mm/ year
Gravel
Gravel
Gravel
BT
Sub Grade Strength 4-day soaked CBR values for new roads can be assessed by one of the following. 1) Based on Soil Classification and table 1 of SP:72 2) Using a Nomo graph (appendix C of SP:62) 3) using 2 sets of equations, based on classification test data, one for plastic soils and other for non plastic soils (appendix D of SP:62) 4) By conducting actual CBR tests in the laboratory.
Table 1 of SP:62: Typical Presumptive Design CBR values
Description of sub grade soil
IS Soil Classification
Typical Soaked CBR Values %
Highly Plastic Clays and Silts
CH, MH
2 – 3*
Silty clays and sandy clays
ML, MI
4-5
Clayey sands and Silty sands
SC, SM
6 - 10
*Expansive soils like BC soil may have a soaked CBR of less than 2%. Free Swelling Index test as per IS:2720 – part 40 should be conducted.
Quick Estimation of CBR (appendix D) Plastic Soil CBR = 75/(1+0.728WPI) Where WPI = Weighted Plasticity Index = P 075× PI PI = Plasticity Index of the soil in % P075 = % passing 0.075mm sieve in decimal. Non-Plastic soil CBR = 28.091(D60)0.3581 Where D60 = diameter in mm of the grain size corresponding to 60% finer.
Expansive Soil Sub Grade Expansive soil should be compacted to 95% Standard Compaction at a moisture content 1 to 2% higher than optimum. When the CBR of subgrade is less than 2%, the economic feasibility of replacing 300mm sub grade with suitable soil and CBR of improved sub grade shall be considered for design. Alternatively, a capping layer of thickness less than 100mm of modified soil with CBR not less than10% should be provided. CBR of 2% may be considered for design
IRC: SP 72 – 2007, Pavement Design Catalogue. CBR = 2%
Cumulative ESAL applications
Bituminous Surface treated WBM/CRMB
Base of gravel, WBM, CRMB of CBR not less than 100%
10,000 to 30,000
Nil
nil
200
nil
100
30,000 to 60,000
75
nil
150
nil
100
60,000 to 1,00,000
75
100
nil
100
100
1,00,000 to 2,00,000
75
100
nil
100
150
2,00,000 to 3,00,000
75
100
nil
150
150
3,00,000 to 6,00,000
75
100
nil
225
150
6,00,000 to10,00,000
75
150
nil
200
225
Gravel Base of CBR not less than 80%
Granular Sub Base of CBR not less than 20%
Modified soil or sub grade of CBR not less than 10%
IRC:SP IRC:S P :72 :72 – 2007, Pavement Design Catalogue.CBR = 3 to 4%
Cumulative ESAL applications
Bituminous Surface treated WBM/CRMB
Base of gravel, WBM, CRMB of CBR not less than 100%
10,000 to 30,000
Nil
nil
200
nil
nil
30,000 to 60,000
nil
nil
275
nil
nil
60,000 to 1,00,000
75
100
nil
150
nil
1,00,000 to 2,00,000
75
100
nil
100
100
2,00,000 to 3,00,000
75
100
nil
100
150
3,00,000 to 6,00,000
75
150
nil
100
150
6,00,000 to10,00,000
75
150
nil
150
150
Gravel Base of CBR not less than 80%
Granular Sub Base of CBR not less than 20%
Modified soil or sub grade of CBR not less than 10%
IRC: SP 72 – 2007, Pavement Design Catalogue. CBR = 5 to 6% Cumulative ESAL applications
Bituminous Surface treated WBM/CRMB
Base of gravel, WBM, CRMB of CBR not less than 100%
10,000 to 30,000
Nil
nil
175
nil
nil
30,000 to 60,000
nil
nil
250
nil
nil
60,000 to 1,00,000
nil
nil
275
nil
nil
1,00,000 to 2,00,000
75
100
nil
125
nil
2,00,000 to 3,00,000
75
100
nil
150
nil
3,00,000 to 6,00,000
75
100
nil
100
100
6,00,000 to10,00,000
75
150
nil
100
100
Gravel Base of CBR not less than 80%
Granular Sub Base of CBR not less than 20%
Modified soil or sub grade of CBR not less than 10%
IRC: SP S P 72 72 – 200 007 7, Pavem P avement ent Des Des ign Cata C atallog ogue. ue. CB C BR =7 to 9%
Cumulative ESAL applications
Bituminous Surface treated WBM/CRMB
Base of gravel, WBM, CRMB of CBR not less than 100%
10,000 to 30,000
Nil
nil
150
nil
nil
30,000 to 60,000
nil
nil
175
nil
nil
60,000 to 1,00,000
nil
nil
225
nil
nil
1,00,000 to 2,00,000
75
100
nil
100
nil
2,00,000 to 3,00,000
75
100
nil
125
nil
3,00,000 to 6,00,000
75
100
nil
150
nil
6,00,000 to10,00,000
75
150
nil
150
nil
Gravel Base of CBR not less than 80%
Granular Sub Base of CBR not less than 20%
Modified soil or sub grade of CBR not less than 10%
IRC: SP S P 72 72 – 20 2007 07,, Pavem P avement ent Des Des ign Cata atallog ogue. ue. CB C B R =10 to 15% 15%
Cumulative ESAL applications
Bituminous Surface treated WBM/CRMB
Base of gravel, WBM, CRMB of CBR not less than 100%
10,000 to 30,000
Nil
nil
125
nil
nil
30,000 to 60,000
nil
nil
150
nil
nil
60,000 to 1,00,000
nil
nil
175
nil
nil
1,00,000 to 2,00,000
75
150
nil
nil
nil
2,00,000 to 3,00,000
75
100
nil
100
nil
3,00,000 to 6,00,000
75
100
nil
125
nil
6,00,000 to10,00,000
75
150
nil
125
nil
Gravel Base of CBR not less than 80%
Granular Sub Base of CBR not less than 20%
Modified soil or sub grade of CBR not less than 10%
for all my presentat presentatio ions ns,, pleas pleas e visit visi t http://aproadbuildqa.blogspot.com
shoulder
Base Sub base eeeeEeeee Sub-Grade (300mm)
shoulder
Embankment
Typical Pavement Cross Section
Earlier Design Approach as per IRC:SP:20-2002 As per IRC: SP:20-2002, traffic is evaluated in Commercial Vehicles Per Day grouping together all types of vehicles weighing 3.0t or more. The percentage of laden, un-laden or over loaded vehicles was not considered. sub-Grade strength is evaluated in terms of 4-day soaked CBR values except in areas with annual rainfall less than 500mm and where the water table is too deep.
Pavement thickness chart as per IRC: SP 20 2002
Pavement thickness as per IRC: SP 20 2002 CBR
2%
3%
4%
5%
6%
Crust
0 to 15 CVPD
15 to 45
45 to 150
150 to 450
CVPD
CVPD
CVPD
Base
150
150
225
225
Sub base
275
365
370
455
Base
150
150
150
225
Sub base
200
265
330
320
Base
150
150
150
150
Sub base
125
200
260
315
Base
150
150
150
150
Sub base
100
165
210
260
Base
150
150
150
150
Sub base
60
115
150
175
Pavement thickness as per IRC: SP 20 2002 CBR
7
10%
15%
20%
150 to 450
0 to 15 CVPD
15 to 45
45 to 150
CVPD
CVPD
Base
150
150
150
150
Sub base
60
115
150
175
Base
150
150
150
150
Sub base
30
70
85
125
Base
150
150
150
150
Sub base
nil
30
50
75
Base
150
150
150
150
Sub base
nil
nil
30
50
Crust
CVPD
Recommended Design Approach as per IRC:SP:72-2007 Low volume roads are divided into three categories 1) Gravel/Aggregate –surfaced (Un-paved) roads 2) Flexible Pavements (Paved roads) 3) Rigid Pavements note: AASHTO guide for Design of Pavement structures was considered in this publication.
Recommended Design features in IRC:SP:72-2007 Pavement design for new roads as well well as for the up gradation/rehabilitation gradation/rehabilitation of existing roads have been included. Maximising the use of localised materials. Simple procedures for traffic counts, computing ADT(Average Daily Traffic) and ESAL (Equivalent Standard Axle Loads) in design life, selected as 10 years. 5 classes of sub-grade strength and 7 ranges of design traffic. Warrants Warrants for providing bituminous surface treatment. Long term performance monitoring by systematic condition surveys at regular intervals.
Design Traffic parameter parameter It has been expressed in terms of cumulative 80kN (8.16 tonnes) ESAL (Equivalent Standard Axle Loads) applications during the design life (10 years). Seasonal variations by way of enhanced traffic during the harvesting seasons have also been considered. Traffic growth rate shall be assessed by conducting traffic survey at least for 3 days each during peak and non peak seasons. In the absence of details, a traffic growth rate of 6% may be taken.
Composition of rural traffic traffic It is not only the traffic volume but also its composition that plays an important role in determining the pavement thickness and its composition. Heavy Commercial Vehicles (HCV) like full sized trucks and buses and the Medium-heavy commercial Vehicles (MCV) with a gross laden weight 3.0 tonnes and above shall be considered. Procedure has also been suggested to evaluate and consider the effect of Solid Wheeled Vehicles (SWC/iron-rimmed) in computing the design traffic.
Equivalency Factors Factors for different axle loads Axle l Load tonnes
kN
Equivalency factors
3.0
29.4
0.02
4.0
39.2
0.06
5.0
49.1
0.14
6.0
58.8
0.29
7.0
68.7
0.54
8.0
78.5
0.92
9.0
88.3
1.48
10.0
98.1
2.25
11.0
107.9
3.30
12.0
117.7
4.70
13.0
127.5
6.40
14.0
137.3
8.66
15.0
147.1
11.42
Axle equivalency factor = (W/Ws)4 W is single axle load in kN Ws is Standard Axle Load of 80kN(8.16t)
Vehicle Damage Factor 1) Fully Fully load loaded ed Heavy Heavy Vehi Vehicle cle (HCV (HCV): ): Rear Axle=10.2t Axle=10.2t and Front axle=5.0t. VDF= 2.44+0.14=2.58 2) Un-laden/partially loaded HCV: Rear Axle=6.0t Axle=6.0t and Front axle=3.0t. VDF=0.29+0.02= 0.31 3) Overloaded (20%) HCV: Rear Axle=12.3t Axle=12.3t and Front axle=6.0t. VDF= 5.06+0.29= 5.35 4) Unladen/partially loaded MCV Rear Axle=3.0t Axle=3.0t and Front axle=1.5t. VDF=0.018+0.001= 0.019
Equivalent Standard Axle Load applications +.
N=T0×365×
.
− ×
L
Where T0 = ESAL per day = number of commercial vehicles per day in the year of opening ×VDF rate( 6%) = annual traffic growth rate(6%) L = lane Distribution Factor: L = 1 for single lane or intermediate lane L = 0.75 for two-lane roads n = design life in years(10 years)
Seasonal Variations in Rural Traffic
Average Average Annual Daily Traffic (AADT) in CVPD
Correction Factor for Solid-Wheeled (iron rimmed)Cart rimmed)Cart Traffic
100
1.70
150
1.25
200
1.20
300
1.15
400 500
1.10 1.07
In the Equivalent Standard Axle Load calculations, the above correction factor Is to be applied for Sold-Wheeled Carts Ca rts if there Is significant volume of SWC(Iron Rimmed Vehicles). Vehicles). Damaging effect to the road surface by SWC is twice the damaging effect of a laden 6-8 t MCV.
Table 4 of SP:62 Guidelines for provi pr oviding ding a Bituminous Surface Treatment Type of Surfacing Traffic category
Annual Rain fall T1 ADT< 100
T2 T3 ADT= ADT= 100to150 150to200
T4 ADT> 200
Over 1500mm/ year
Gravel
BT
BT
BT
1000-1500mm/ 1000-1500mm/ year
Gravel
Gravel
BT
BT
Less than 1000mm/ year
Gravel
Gravel
Gravel
BT
Sub Grade Strength 4-day soaked CBR values for new roads can be assessed by one of the following. 1) Based on Soil Classification and table 1 of SP:72 2) Using a Nomo graph (appendix C of SP:62) 3) using 2 sets of equations, based on classification test data, one for plastic soils and other for non plastic soils (appendix D of SP:62) 4) By conducting actual CBR tests in the laboratory.
Table 1 of SP:62: Typical Presumptive Design CBR values
Description of sub grade soil
IS Soil Classification
Typical Soaked CBR Values %
Highly Plastic Clays and Silts
CH, MH
2 – 3*
Silty clays and sandy clays
ML, MI
4-5
Clayey sands and Silty sands
SC, SM
6 - 10
*Expansive soils like BC soil may have a soaked CBR of less than 2%. Free Swelling Index test as per IS:2720 – part 40 should be conducted.
Quick Estimation of CBR (appendix D) Plastic Soil CBR = 75/(1+0.728WPI) Where WPI = Weighted Plasticity Index = P 075× PI PI = Plasticity Index of the soil in % P075 = % passing 0.075mm sieve in decimal. Non-Plastic soil CBR = 28.091(D60)0.3581 Where D60 = diameter in mm of the grain size corresponding to 60% finer.
Expansive Soil Sub Grade Expansive soil should be compacted to 95% Standard Compaction at a moisture content 1 to 2% higher than optimum. When the CBR of subgrade is less than 2%, the economic feasibility of replacing 300mm sub grade with suitable soil and CBR of improved sub grade shall be considered for design. Alternatively, a capping layer of thickness less than 100mm of modified soil with CBR not less than10% should be provided. CBR of 2% may be considered for design
IRC: SP 72 – 2007, Pavement Design Catalogue. CBR = 2%
Cumulative ESAL applications
Bituminous Surface treated WBM/CRMB
Base of gravel, WBM, CRMB of CBR not less than 100%
10,000 to 30,000
Nil
nil
200
nil
100
30,000 to 60,000
75
nil
150
nil
100
60,000 to 1,00,000
75
100
nil
100
100
1,00,000 to 2,00,000
75
100
nil
100
150
2,00,000 to 3,00,000
75
100
nil
150
150
3,00,000 to 6,00,000
75
100
nil
225
150
6,00,000 to10,00,000
75
150
nil
200
225
Gravel Base of CBR not less than 80%
Granular Sub Base of CBR not less than 20%
Modified soil or sub grade of CBR not less than 10%
IRC:SP IRC:S P :72 :72 – 2007, Pavement Design Catalogue.CBR = 3 to 4%
Cumulative ESAL applications
Bituminous Surface treated WBM/CRMB
Base of gravel, WBM, CRMB of CBR not less than 100%
10,000 to 30,000
Nil
nil
200
nil
nil
30,000 to 60,000
nil
nil
275
nil
nil
60,000 to 1,00,000
75
100
nil
150
nil
1,00,000 to 2,00,000
75
100
nil
100
100
2,00,000 to 3,00,000
75
100
nil
100
150
3,00,000 to 6,00,000
75
150
nil
100
150
6,00,000 to10,00,000
75
150
nil
150
150
Gravel Base of CBR not less than 80%
Granular Sub Base of CBR not less than 20%
Modified soil or sub grade of CBR not less than 10%
IRC: SP 72 – 2007, Pavement Design Catalogue. CBR = 5 to 6% Cumulative ESAL applications
Bituminous Surface treated WBM/CRMB
Base of gravel, WBM, CRMB of CBR not less than 100%
10,000 to 30,000
Nil
nil
175
nil
nil
30,000 to 60,000
nil
nil
250
nil
nil
60,000 to 1,00,000
nil
nil
275
nil
nil
1,00,000 to 2,00,000
75
100
nil
125
nil
2,00,000 to 3,00,000
75
100
nil
150
nil
3,00,000 to 6,00,000
75
100
nil
100
100
6,00,000 to10,00,000
75
150
nil
100
100
Gravel Base of CBR not less than 80%
Granular Sub Base of CBR not less than 20%
Modified soil or sub grade of CBR not less than 10%
IRC: SP S P 72 72 – 200 007 7, Pavem P avement ent Des Des ign Cata C atallog ogue. ue. CB C BR =7 to 9%
Cumulative ESAL applications
Bituminous Surface treated WBM/CRMB
Base of gravel, WBM, CRMB of CBR not less than 100%
10,000 to 30,000
Nil
nil
150
nil
nil
30,000 to 60,000
nil
nil
175
nil
nil
60,000 to 1,00,000
nil
nil
225
nil
nil
1,00,000 to 2,00,000
75
100
nil
100
nil
2,00,000 to 3,00,000
75
100
nil
125
nil
3,00,000 to 6,00,000
75
100
nil
150
nil
6,00,000 to10,00,000
75
150
nil
150
nil
Gravel Base of CBR not less than 80%
Granular Sub Base of CBR not less than 20%
Modified soil or sub grade of CBR not less than 10%
IRC: SP S P 72 72 – 20 2007 07,, Pavem P avement ent Des Des ign Cata atallog ogue. ue. CB C B R =10 to 15% 15%
Cumulative ESAL applications
Bituminous Surface treated WBM/CRMB
Base of gravel, WBM, CRMB of CBR not less than 100%
10,000 to 30,000
Nil
nil
125
nil
nil
30,000 to 60,000
nil
nil
150
nil
nil
60,000 to 1,00,000
nil
nil
175
nil
nil
1,00,000 to 2,00,000
75
150
nil
nil
nil
2,00,000 to 3,00,000
75
100
nil
100
nil
3,00,000 to 6,00,000
75
100
nil
125
nil
6,00,000 to10,00,000
75
150
nil
125
nil
Gravel Base of CBR not less than 80%
Granular Sub Base of CBR not less than 20%
Modified soil or sub grade of CBR not less than 10%
