--- title: "Pavement design factors in AASHTO method" subject: "Transportation Engineering II" chapter: 2 key: "semester_7/transportation_engineering_ii:pavement-design-factors-aashto" source: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/pavement-design-factors-aashto site: BCT Engineering --- # Pavement design factors in AASHTO method Transportation Engineering II · Chapter 2: Highway Pavement - Asked in: 2081 Baisakh, 1+7 marks - Video: [AASHTO Method of Flexible Pavement Design, Complete procedure in just 15 minutes, #AASHTO guide 1993](https://www.youtube.com/watch?v=dXWFEFvw7fE) — Transportation Engg Talks | Satish IITR - 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-aashto - Whole chapter: https://www.bctengineer.com/md/semester_7/transportation_engineering_ii/chapter-2 ## Question What are the factors that affect in pavement design? How are they considered in AASHTO method? ## Short answer Key factors are traffic, subgrade, materials, climate/drainage, and performance criteria. AASHTO method accounts for them through design ESALs, subgrade resilent modulus Mr, structural number SN from layer and drainage coefficients, serviceability loss ΔPSI, and reliability level ZR S0. ## Answer **Factors affecting pavement design** 1. **Traffic and loading** - axle loads, axle configuration, repetitions, tyre pressure, growth and design life. 2. **Subgrade** - strength and volume change of the roadbed soil. 3. **Pavement materials** - strength, stiffness and durability of each layer. 4. **Climate and environment** - rainfall, drainage, temperature and frost. 5. **Serviceability, reliability and failure criteria** - the performance level expected during the life. **How AASHTO (1993) considers them** - the design is based on the structural number (SN) equation $$ \log_{10}W_{18}=Z_RS_0+9.36\log_{10}(SN+1)-0.20+\frac{\log_{10}\left[\frac{\Delta PSI}{4.2-1.5}\right]}{0.40+\frac{1094}{(SN+1)^{5.19}}}+2.32\log_{10}M_R-8.07 $$ | Factor | Considered through | |---|---| | Traffic | Design ESALs $W_{18}$ (18-kip axle) from load equivalency factors, lane and direction factors, growth and design period | | Subgrade | Effective roadbed resilient modulus $M_R$ (psi), seasonally weighted; $M_R\approx1500\,CBR$ | | Materials | Layer coefficients $a_i$ from layer moduli, so $SN=a_1D_1+a_2D_2m_2+a_3D_3m_3$ | | Drainage and climate | Drainage coefficients $m_i$ (0.4 to 1.4) from drainage quality and time near saturation | | Performance | $\Delta PSI=p_0-p_t$ (initial 4.2, terminal 2.0 to 2.5) | | Reliability | Reliability R (50 to 99.9 percent) through $Z_R$ and overall standard deviation $S_0$ (0.40-0.50 for flexible) | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. How does AASHTO represent traffic? A. As design ESALs W18 (18-kip axles) using load equivalency, lane, growth and design period B. As CVPD only C. As daily vehicle count of all vehicles D. As speed Answer: A. W18 is the design ESAL from equivalency factors, factors for lane and direction, growth and period. (Key point: Traffic in AASHTO) 2. Subgrade strength enters the equation as: A. Effective roadbed resilient modulus M_R (about 1500 CBR in psi) B. Liquid limit C. Layer coefficient a1 D. Drainage coefficient Answer: A. A seasonally weighted effective M_R is used, with M_R approximately 1500 CBR psi. (Key point: Subgrade in AASHTO) 3. Drainage and climate are considered through: A. Drainage coefficients m_i applied to base and sub-base layer terms B. Z_R C. Delta PSI D. The standard axle Answer: A. m_i (0.4 to 1.4) depend on drainage quality and time near saturation: SN = a1D1 + a2D2m2 + a3D3m3. (Key point: Drainage coefficient) 4. Reliability is introduced into the AASHTO equation by: A. Z_R and overall standard deviation S0 B. SN only C. M_R only D. Layer thickness D Answer: A. R (50 to 99.9 percent) gives Z_R, combined with S0 (0.40-0.50 for flexible pavements). (Key point: Reliability) 5. Delta PSI is: A. Initial serviceability minus terminal serviceability (p0 - pt) B. Traffic growth rate C. Subgrade CBR loss D. Rutting depth Answer: A. Delta PSI = p0 - pt, with p0 = 4.2 and pt = 2.0 to 2.5. 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