Transportation Engineering-II Principles of Pavement Design

Pavement Design What is a Pavement? Pavement is the upper part of roadway, airport or parking area structure It includes all layers resting on the original ground It consists of all structural elements or layers, including shoulders

Pavement Design What is Design? What is a pavement design? Conceived/developed plan for something to serve a specific function. What is a pavement design? It is the process by which the structural components of a road segment are determined, taking into account the nature of the subgrade, density and traffic composition.

Pavement Design ‘OR’ It is the process of developing the most economical combination of pavement layers (in relation to both thickness & type of materials) to suit the soil foundation and the traffic to be carried, during the design life.

Pavement Design ‘OR’ It is a process of selection of appropriate pavement and surfacing materials to ensure that, the pavement performs adequately and requires minimal maintenance under the anticipated traffic loading for the design period adopted. This selection process involves adoption of material types, thicknesses and configurations of the pavement layers to meet the design objectives.

What is the goal of pavement design? Provide the most cost-effective structure while optimizing the level of service provided to road users.

Pavement Design

Pavement Design Phases Highway design consists of three stages Geometric design (route selection or alignment design) Capacity design (number of lanes to meet traffic demand) Structural design (to withstand loads and environment) Structural design consists of 3 steps Selection of materials (types of pavement) Proportioning of materials Layer thickness design

Pavement Design Principles Design objectives are to: Provide safe and comfortable riding conditions to all road users, being motor vehicles, cyclists and pedestrians, optimized for the road’s intended functions and the level of use. Provide low cost of ownership (i.e. minimum whole of life cost) to the Government. Comply with the Standards and relevant State Road Authorities’ Guidelines and/or Standards.

Fundamental Parameters Following are the fundamental parameters to be considered: Subgrade Loads Environment

Subgrade Characterized by strength and/or stiffness California Bearing Ratio (CBR) Measures shearing resistance Units: percent Typical values: 0 to 20 Resilient Modulus (MR) Measures stress-strain relationship Units: psi or MPa Typical values:3,000to40,000 psi

Subgrade Some Typical Values Classification CBR MR (psi) Typical Description Good ≥ 10 20,000 Gravels, crushed stone and sandy soils. GW, GP, GM, SW, SP, SM soils falls in this category. Fair 5 – 9 10,000 Clayey gravel and clayey sand, fine silt soils.  GM, GC, SM, SC soils falls in this category. Poor 3 – 5 5,000 Fine silty sands, clays, silts, organic soils.  CL, CH, ML, MH, CM, OL, OH soils falls in this category.

Loads Load characterization Tire loads Axle and tire configurations Load repetition Traffic distribution Vehicle speed

Load Quantification Equivalent Single Axle Load (ESAL) Converts wheel loads of various magnitudes and repetitions ("mixed traffic") to an equivalent number of "standard" or "equivalent" loads Based on the amount of damage they do to the pavement Commonly used standard load is the 18,000 lb. equivalent single axle load Load Equivalency Generalized fourth power approximation

Typical LEFs Notice that cars are insignificant and thus usually ignored in pavement design.

LEF Example The standard axle weights for a standing-room-only loaded Metro articulated bus (60 ft. Flyer) are: Axle Empty Full Steering 13,000 lb. 17,000 lb. Middle 15,000 lb. 20,000 lb. Rear 9,000 lb. 14,000 lb. Using the 4th power approximation, determine the total equivalent damage caused by this bus in terms of ESALs when it is empty. How about when it is full? Empty (13,000/18,000)4 = 0.272 (15,000/18,000)4 = 0.482 (9,000/18,000)4 = 0.063 Total = 0.817 ESALs Full (17,000/18,000)4 = 0.795 (20,000/18,000)4 = 1.524 (14,000/18,000)4 = 0.366 Total = 2.685 ESALs Increase in total weight = 14,000 lb. (about 80 people) or 39% Increase in ESALs is 1.868 (229%)

Environment Temperature extremes Frost action Frost heave Thaw weakening

Secondary Parameters Following are the secondary parameters for pavement design; Soil Classification Soil horizons Parent materials Moisture-solid relationship Material Characterization Soil and Base Stabilization Sub-grades Strength Studies Compaction Strength-Density-Moisture Considerations

Secondary Parameters Base and Subbase Courses Surfaces Feasibility Grading Construction Surfaces Functions Types of materials Skid qualities Cracking Materials Considerations

Design factors

General Framework of Pavement Design