Guidelines for design of New Flexible pavement and overlay
Introduction: Pavements are designed as per guidelines published by Indian Roads Congress, New Delhi. In order to attain economical design and better performance, Indian Roads Congress revises these Guidelines from time to time. Indian Roads Congress recently revised the existing flexible pavement design IRC: 37-2001 and published the new guidelines IRC: 37 – 2012 “Tentative Guidelines for the design of flexible pavements” during January 2013.
In IRC: 37-2012, several changes were made in the flexible pavement design parameters. Hence, henceforth, flexible pavement shall only be designed as per IRC: 37 – 2012. Under these circumstances, the need for simplified design procedure and design plates for CBR values ranging from 3 to 15 was felt. Accordingly, design guideline for flexible pavement design, design plates for CBR 3 to 15 and guidelines for overlay design has been prepared.
The guidelines are prepared to make the pavement design easy and simple. The factors involved in the design have been discussed briefly. For additional details / specifications relevant codes / MORTH specifications shall be referred. Design plates are given for the unbound sub-base and unbound base layers. IRC: 37-2012 recommends the use of bound sub-base, bound base and Recycled asphalt pavement layers for economical pavement design. For these methods, design plates and recommendations given in the IRC: 37-2012 shall be adopted.
Pavement thickness plates are prepared assuming the plates given in IRC 37-2012 as the basis. Thickness of GSB and WMM layers are maintained in such a way that in each and every case the thickness adopted are in harmony with the adjacent plates.
1. Traffic growth rate (r). The present day traffic has to be projected for the end of design life at growth rates (‘r’) estimated by studying and analyzing the following data: a. Past trends of traffic growth; and b. Demand elasticity of traffic with respect to macro-economic parameters (like GDP or SDP) and expected demand due to specific developments and land use changes likely to take place during design life. c. If the data for the annual growth rate of commercial vehicles is not available or if it is less that 5 percent, a growth rate of 5 percent should be adopted. 2. Vehicle damage factor (VDF). a. VDF value shall be arrived by carrying out specific axle load surveys on the existing roads. Minimum sample size for survey shall be as given in table 4.1-Sample size for Axle Load Survey and the same is given below; Total number of commercial vehicles per day
Minimum percentage of commercial traffic to be surveyed
< 3000
20 per cent
3000 – 6000
15 percent
>6000
10 percent
b. If axle loads data not available then the default indicative values given in table 4.2, IRC-37-2012 shall be used. Initial traffic volume in terms of Commercial Vehicles per day
Terrain Rolling / Plain
Hilly
0 – 150
1.5
0.5
150 – 1500
3.5
1.5
More than 1500
4.5
2.5
3. Distribution of commercial vehicle over the carriageway.
Road category
Number of commercial vehicles
a. Single-lane roads.
Total number of Commercial vehicles in both directions.
b. Two-lane single
50 percent of the total Commercial vehicles directions. 40 percent of the total commercial vehicles directions.
carriageway roads. c. Four-lane single carriageway roads.
number of in both number of in both
d. Dual two-lane carriageway roads.
75 percent of the total number of commercial vehicles in each direction.
e. Dual three-lane carriageway roads.
60 percent of the total number of commercial vehicles in each direction.
f. Dual four-lane carriageway roads.
45 percent of the total number of commercial vehicles in each direction.
Note: 1.
There is significant difference between traffic in each direction; pavement thickness in each direction shall be made separately. 2. For Two lane single carriageway higher VDF value shall be considered. 3. For divided carriageway, each direction shall be designed adopting respective VDF value.
4. Subgrade: a. Top 500mm of soil layer immediately below the bottom of pavement. b. Subgrade may be in-situ material, select soil or stabilised soil. c. The difference between bottom of subgrade and the level of water table / high flood level should, generally, not less than 1.0m for new roads and 0.6m for existing roads which have no history of being overtopped.
5. Minimum CBR value. Minimum CBR value for in-situ subgrade soil and select soil forming the subgrade is a. In-situ soil b.
– 3 CBR, Select material – 8 CBR for roads having traffic of 450 commercial vehicles per day or higher.
6. Effective CBR. a. In the case of select soil forming the subgrade, effective CBR value shall be determined from fig.5.1.of IRC: 37-2012. b. For other compacted subgrade thickness, reference 4 of IRC: 37-2012 (Page No:90 ) consulted for guidance. 7. Design CBR a. Where different types of soils are used in subgrade, a minimum of six to eight average CBR values (average of three tests) for each soil type along the alignment is necessary for determination of design CBR. b. 90th percentile of these value should be adopted as Design CBR for Expressway, National and State highways. c. 80th percentile of these values should be adopted as Design CBR for other categories of roads. d. Method of computation of 90th percentile CBR is given in Annexure IV, IRC: 37-2012. e. The soil classification chart given in IRC: 36-1970 may be used for the classification soil.
8. Sub-base layer – Clause 7.2.1, IRC:37-2012. I) Unbound layer. a. It is a normal sub-base layer. b. It may consist of crushed aggregate etc. c. Sub-base layer composed of two layers. d. Lower separation / filter layer. e. Upper drainage layer. f. Drainage layer shall be designed to satisfy Grading 4, Table-V-1, Page 71, IRC: 37-2012. g. When coarse graded sub-base is used as a drainage layer it shall have Los Angeles abrasion value less than 40 and Fines passing 0.075mm should be less than 2 percent. h. The GSB should be extended to entire formation width where the subgrade soil has low permeability and it is to be ensured that its exposed ends do not get covered by the embankment soil.
II) Bound sub-base layer (Clause 7.2.2.) a. It is a stabilised cementitious sub-base layer. b. Stage construction not recommended in the case of stabilised sub-base layer. c. The GSB should be extended to entire formation width where the subgrade soil has low permeability. 9. Base layer- Clause (Clause 7.3) I) Unbound base layer – clause 7.3.1. a. It is a normal base layer. b. Wet mix macadam or crusher run macadam or reclaimed concrete etc. c. It must satisfy relevant IRC/MORTH specifications. II) Bound base layer – Clause 7.3.2. a. It is a stabilised base layer. b. Wet mix macadam or crusher run macadam or reclaimed concrete etc. c. It must satisfy relevant IRC/MORTH specifications. d. The gradation for bound cementitious base layer shall be as per Table 400-4. GRADING LIMITS OF MATERIAL FOR STABILISATION MORTH specification. 10. Bituminous surfacing. a. b. c. d. e.
The bituminous surfacing will consists only of DBM and wearing course. BM layer not recommended. VG 30 bitumen for design traffic up to 30msa. VG 40 bitumen for design traffic above 30 msa. Minimum layer thickness of DBM for design traffic of 5 msa or above is 50 mm.
11. Wearing course. a. SDBC for design traffic up to 5msa. b. BC for design traffic more than 5 msa. 12. Shoulder. a. Paved shoulder for major highways. b. The base should be constructed 300-450 mm wider than the required bituminous surfacing. c. Shoulders should be well-shaped to aid proper drainage. 13. Special category (Soil having CBR value less than 3): If the CBR value of the subgrade soil is less than 3 then
1. Existing subgrade soil may be stabilised or fill material with minimum CBR value of 8 may be used as select sub-grade soil or Geo-fabric inter layer may be used immediately over the in-situ subgrade soil. 2. Effective CBR value shall be the design CBR for stabilised and select subgrade soil. 3. Pavement thickness required for 3 CBR to be provided over the Geo-fabric layer. 4. In the case of Geo-fabric interlayer, there shall not be any road cutting at this particular stretch. Hence, permanent warning sign shall be erected to avoid road cutting. 5. Lime, Fly ash, Cement and any other cementitious chemical stabiliser could be used to stabilise the subgrade soil. Relevant IRC guidelines shall be followed while selecting the stabiliser. 6. In the case of cementitious chemical stabilisers, laboratory or field performance evaluation done either by CRRI or HRS is essential. 7. The minimum CBR value of the stabilised soil when tested at 28th day from the date of stabilization shall be 8. 17. Pavement design: Pavement thickness for the specific design CBR and design traffic could be read from the design plates provided which was prepared using “IITPAVE” software. For economical design, IRC: 37-2012 suggested five different layer combinations and recommended the designer to choose his own combinations, instead of using the design catalogues. The most cost effective pavement design shall be adopted. Even in the case of adopting the design plates, horizontal tensile strain in bituminous layer and vertical compressive strain at bottom of granular layer shall be arrived for each and every case using “IITPAVE” software. The strain value obtained shall be less than the
allowable tensile and vertical strain values. If the strain value obtained is higher than the allowable value then the layer thickness shall be modified as recommended under chapter 13 of IRC: 37-2012. Similarly, for design traffic other than given in the design plates, pavement composition shall only be arrived using “IITPAVE” software. Clause 9.2 and 9.3 of IRC: 37-2012 shall be followed while designing the pavement thickness. 19. Internal Drainage in pavement: IRC: 37-2012 recommend constructing the pavement as far above the water table as economically practicable. The difference between bottom of subgrade and the level of water table / high flood level should, generally, not less than 1.0m or 0.6 m in case of existing roads which have no history of being overtopped. In water logged areas, where the subgrade is within the zone of capillary saturation, consideration should be given to the installation of suitable capillary cut-off as per IRC: 34 at appropriate level underneath the pavement. The filet/separation layer of sub-base should satisfy the criteria given in 11.2, 11.2 and 11.3 in IRC: 37-2012. For additional information reference shall be made to chapter 11 of IRC: 37-2012.
20. Overlay design from BBD test. IRC: 37-2012 guidelines shall only be applicable for the design of new flexible pavements for Expressway, National Highways, State Highways, Major district roads and other categories of roads predominantly carrying motorized vehicles.
For overlay design, IRC: 81-1997 or a more suitable procedure based on evaluation of in situ properties of pavement layers by Falling weight deflectometer should be used.
But, the factors involved in calculating the design traffic shall only be as recommended in the IRC 37-2012. Hence, the same is mentioned below,
a. Traffic growth rate shall be as per IRC: 37-2012. b. Lane distribution factor shall be as per IRC: 37-2012. c. Overlay thickness in terms of BM shall be arrived as per IRC: 81-1997. d. Arrived BM thickness shall be converted to equivalent overlay thickness using the equivalency factor of 1cm BM = 0.7 cm of DBM/AC/SDC.
21. IITPAVE software:
IITPAVE software shall be obtained either at circle office concerned or at QAR Chennai.
The operating procedure for IITPAVE is given in chapter 13 of IRC: 37-2012.
Allowable strain shall be calculated using appropriate equation given in IRC: 37-2012.
The resilient modulus of subgrade soil, granular layer shall be calculated using equation 5.2, 7.1 / 7.3, IRC: 37-2012.
In the case cementitious layers, allowable strain shall be arrived using equation 6.6 and 6.7 in IRC: 37- 2012.
22. Maintaining records:
IRC: 37- 2012 suggested to keep detailed record of the
1. Year of construction, 2. Subgrade CBR, 3. Soils characteristics including resilient modulus, 4. Pavement composition and specifications, 5. Traffic and 6. Pavement performance, 7. Overlay history, 8. Climatic conditions etc
