--- title: "Factors affecting pavement design" subject: "Transportation Engineering II" chapter: 2 key: "semester_7/transportation_engineering_ii:pavement-design-factors" source: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/pavement-design-factors site: BCT Engineering --- # Factors affecting pavement design Transportation Engineering II · Chapter 2: Highway Pavement - Asked in: 2081 Bhadra, 8 marks; 2076 Chaitra, 8 marks; 2075 Asoj, 8 marks (asked 3 times) - Video: [Factors affecting pavements design](https://www.youtube.com/watch?v=QBAL_ABsJJI) — Caesar Laban - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/pavement-design-factors - Whole chapter: https://www.bctengineer.com/md/semester_7/transportation_engineering_ii/chapter-2 ## Question Explain the various factors affecting the pavement design. ## Short answer Design depends on wheel load and repetitions, traffic (CVPD, growth, VDF, lane distribution), subgrade CBR/M_R or k, material properties, climate and drainage, reliability, and construction/maintenance. Cumulative standard axles are computed and used with CBR-based catalogues. ## Answer A pavement must carry traffic loads for its design life without excessive cracking, rutting or roughness. The factors affecting the design (IRC:37-2018 for flexible and IRC:58-2015 for rigid pavements; Nepal: DoR Pavement Design Guidelines / Nepal Road Standard 2070, based on CBR charts) are: #### 1. Design wheel/axle load - **Magnitude of load and axle configuration** (single, tandem, tridem). The standard axle is an 80 kN (8.16 t) single axle with dual wheels. - **Tyre pressure and contact area** govern the surface stress; a contact pressure of about 0.56 MPa is used in IRC:37. - **Repetition of loads:** damage increases with the number of passes. #### 2. Traffic - Present commercial traffic (vehicles of more than 3 t laden weight) in each direction, expressed as CVPD. - **Growth rate $r$** and **design life $n$** (15 years for NH/SH in IRC:37-2012; at least 20 years in IRC:37-2018, and longer for expressways). - **Vehicle damage factor (VDF)**, the equivalent standard axles per commercial vehicle, and the lane distribution factor (LDF). - Cumulative standard axles: $$ N_s = \frac{365\left[(1+r)^n - 1\right]}{r} \times A \times D \times F $$ where $A$ = initial traffic at opening, $D$ = lane distribution factor and $F$ = VDF. The pavement is designed for $N_s$ in million standard axles (msa). #### 3. Subgrade strength - Soaked **CBR** of the subgrade (at the expected field density and moisture), or the resilient modulus $M_R$ (IRC:37: $M_R = 10\,CBR$ MPa for CBR $\le$ 5 % and $17.6\,CBR^{0.64}$ MPa for CBR > 5 %); for rigid pavements, the modulus of subgrade reaction $k$. - Soil type, plasticity, compaction, depth of water table and swelling potential. #### 4. Pavement materials - Strength and stiffness of the granular sub-base, base (WBM/WMM), bituminous layers (modulus, fatigue, rutting) or cement concrete (flexural strength, modulus of elasticity). - Availability and cost of local materials. #### 5. Climate and environment - **Temperature** (bitumen stiffness, thermal stresses in concrete), rainfall and moisture variation of the subgrade; in the hills, frost action and landslides. - **Drainage** and ground-water level: poor drainage greatly reduces the subgrade strength. #### 6. Other factors - Design period, required reliability (80-90 percent) and failure criteria (fatigue cracking, rutting). - Construction quality, method and maintenance level. - Economy: initial cost and life-cycle cost; stage construction. - In rigid pavements: joints and load transfer, slab thickness and the temperature differential. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the standard axle used in IRC pavement design? A. 80 kN single axle with dual wheels B. 40 kN single axle C. 120 kN tandem axle D. Any legal axle load of the vehicle Answer: A. The standard axle is 80 kN (8.16 t) single axle with dual wheels. (Key point: Standard axle) 2. Which vehicles are counted as commercial traffic (CVPD) for design? A. All vehicles including cars B. Vehicles of more than 3 t laden weight C. Only two-wheelers D. Only vehicles above 20 t Answer: B. Commercial vehicles are those of more than 3 t laden weight. (Key point: Traffic for design) 3. In Ns = 365[(1+r)^n - 1]/r x A x D x F, what does F represent? A. Friction factor B. Vehicle damage factor C. Frost factor D. Fatigue factor Answer: B. F is the vehicle damage factor, the equivalent standard axles per commercial vehicle. (Key point: Cumulative standard axles) 4. Subgrade strength for flexible pavement design is mainly represented by: A. Soaked CBR (or resilient modulus M_R) B. Dry density only C. Liquid limit only D. Aggregate impact value Answer: A. Soaked CBR at expected field density and moisture, or M_R derived from it, is used; k is used for rigid pavements. (Key point: Subgrade strength measure) 5. Why is drainage an important design factor? A. It increases tyre pressure B. Poor drainage greatly reduces subgrade strength C. It raises the VDF D. It changes the standard axle Answer: B. Moisture and a high water table weaken the subgrade, so drainage controls the strength assumed in design. (Key point: Effect of drainage) ## Related in Transportation Engineering II - [Pavement design factors; IRC rigid design steps](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Apavement-factors-irc-rigid-steps) — Transportation Engineering II, chapter 2, asked 2× - [Pavement design factors in AASHTO method](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Apavement-design-factors-aashto) — Transportation Engineering II, chapter 2 - [Traffic and loading factors controlling pavement design](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Atraffic-loading-factors-controlling-pavement-design) — Transportation Engineering II, chapter 2 - [Traffic capacity; factors affecting capacity and LOS](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Atraffic-capacity-factors-los-timing) — Transportation Engineering II, chapter 1 - [Flexible pavement design: four lane, ADT 8000 trucks](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aflexible-design-adt-8000-four-lane) — Transportation Engineering II, chapter 2 - [Flexible pavement design with 8 CBR values](https://www.bctengineer.com/question.md?key=semester_7%2Ftransportation_engineering_ii%3Aflexible-design-eight-cbr-340-cvd) — Transportation Engineering II, chapter 2 ## Related in other subjects - [Factors affecting foundation design, improving bearing capacity](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Afactors-design-improve-bc) — Building Technology, chapter 2 - [Factors affecting choice of BIL](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Abil-choice-factors) — Transmission and Distribution System Design, chapter 3 - [Factors affecting air clearance](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Aair-clearance-factors) — Transmission and Distribution System Design, chapter 3 - [Factors affecting system expansion planning](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Asystem-expansion-factors) — Power Plant Design, chapter 2