Module 12: Geotechnical and Pavement Design on Mohammad Movahedi

Soil Classification for Pavements and Subgrades

Mon, 04 May 2026 00:00:00 +0000

Soil Classification for Pavements and Subgrades

Soil classification is a fundamental topic on the PE Civil Transportation exam. In transportation engineering, soils serve as the subgrade that supports pavement structures. Understanding the engineering behavior of these soils based on index properties—such as grain size distribution and plasticity—is critical for design, compaction control, and drainage.

1. Soil Index Properties

Before classifying a soil, we must determine its index properties through laboratory testing. The two primary laboratory tests are:

Soil Phase Relationships (Weight-Volume Relations)

Mon, 04 May 2026 00:00:00 +0000

Soil Phase Relationships (Weight-Volume Relations)

A soil mass is a three-phase system consisting of solid particles (soil solids), water, and air. The relative proportions of these phases govern the physical behavior, compaction characteristics, and strength of the soil. On the PE Civil Transportation exam, soil phase calculations are highly common, requiring a systematic understanding of weight-volume relationships.

1. The Soil Phase Diagram

To solve phase relationship problems, it is helpful to visualize the three phases using a block diagram (the phase diagram). Weights are typically written on the left side of the diagram, and volumes are written on the right.

Soil Compaction

Mon, 04 May 2026 00:00:00 +0000

Soil Compaction

Compaction is the densification of soil by the application of mechanical energy, which expels air from the void spaces. Densifying the soil increases its shear strength, decreases its compressibility, and reduces its permeability, making it a critical step in constructing highway subgrades, embankments, and structural fills.

1. Laboratory Compaction Tests

To determine the maximum dry density and optimum moisture content of a soil, standard laboratory tests are performed.

Shear Strength and Lateral Earth Pressures

Mon, 04 May 2026 00:00:00 +0000

Shear Strength and Lateral Earth Pressures

Soil strength and stability are core geotechnical topics on the PE Civil Transportation exam. These concepts govern the design of retaining walls, abutments, excavation support systems, and highway embankments.

1. Shear Strength of Soil

Soil is a particulate material that derives its shear strength from friction between particles and cohesion (cementation or electrostatic bonding) between clay minerals.

Mohr-Coulomb Failure Criterion

The shear strength ($\tau$) of a soil at any point along a failure plane is modeled by the Mohr-Coulomb failure criterion:

Permeability and Drainage in Soils

Mon, 04 May 2026 00:00:00 +0000

Permeability and Drainage in Soils

Water flow through soil is a key parameter in civil engineering design. It affects seepage under dams, stability of slopes, settlement of clay layers, and the structural integrity of pavements. Water trapped in pavement layers can lead to base softening, pumping of fines, and premature cracking. On the PE Civil Transportation exam, understanding permeability (hydraulic conductivity) and drainage design is essential.

1. Darcy’s Law

Water flows through soil pores due to differences in total energy (hydraulic head). The rate of water flow is governed by Darcy’s Law, which states that flow velocity is directly proportional to the hydraulic gradient.

Earthwork and Mass Balance

Mon, 04 May 2026 00:00:00 +0000

Earthwork and Mass Balance

Earthwork operations are a major component of highway construction projects. Designers must balance the volume of soil excavated from cut sections with the volume of soil required for fill (embankment) sections. Because soil volume changes when it is excavated, hauled, and compacted, engineers must apply volumetric conversion factors to calculate hauling costs and borrow-pit requirements. These concepts are frequently tested on the PE Civil Transportation exam.

Flexible Pavement Design Concepts

Mon, 04 May 2026 00:00:00 +0000

Flexible Pavement Design Concepts

Flexible pavements consist of a bituminous (asphalt) surface layer underlaid by granular base and subbase courses, resting on the prepared subgrade. They distribute traffic loads through a layered system, dispersing stresses with depth so that the stress reaching the subgrade does not exceed its bearing capacity. On the PE Civil Transportation exam, flexible pavement design is governed by the AASHTO 1993 Guide for Design of Pavement Structures.

Rigid Pavement Design Concepts

Mon, 04 May 2026 00:00:00 +0000

Rigid Pavement Design Concepts

Rigid pavements are constructed using Portland cement concrete (PCC) slabs resting on a subbase or directly on the subgrade. Unlike flexible pavements, which distribute loads through a layered system, rigid pavements distribute traffic wheel loads over a wide area through the bending stiffness (flexural rigidity) of the concrete slab itself. On the PE Civil Transportation exam, rigid pavement design is based on the AASHTO 1993 rigid pavement design methodology.

Pavement Distress and Rehabilitation

Mon, 04 May 2026 00:00:00 +0000

Pavement Distress and Rehabilitation

Over time, pavements deteriorate due to the combined effects of traffic loading and environmental conditions. Pavement rehabilitation involves restoring an existing pavement’s structural capacity or functional performance. On the PE Civil Transportation exam, you must be able to identify types of pavement distress, select appropriate rehabilitation strategies, and calculate structural overlay thickness requirements.

1. Pavement Distress Types

Pavement distresses are categorized into structural distresses (related to load-carrying capacity) and functional distresses (related to ride quality and safety).

Traffic Characterization for Pavements

Mon, 04 May 2026 00:00:00 +0000

Traffic Characterization for Pavements

Pavements must be designed to withstand the repeated applications of traffic loads over their design life. Because traffic streams contain a diverse mix of vehicles—ranging from passenger cars to heavy multi-axle semi-trucks—engineers must characterize this traffic into a single design parameter. On the PE Civil Transportation exam, the primary traffic input is the 18-kip Equivalent Single Axle Load (ESAL).

1. The Equivalent Single Axle Load (ESAL) Concept

The damage caused to a pavement structure is not linearly proportional to the wheel load. A single passage of a heavy truck causes thousands of times more damage than a passenger car.