Design Guide for Improved Quality of Roadway Subgrades and Subbases (Civil Project)

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The performance of a pavement depends on the quality of its subgrade and subbase layers; these foundational layers play a key role in mitigating the effects of climate and the stresses generated by traffic. Therefore, building a stable subgrade and a properly drained subbase is vital for constructing an effective and long lasting pavement system.

This manual has been developed to help Iowa highway engineers improve the design, construction, and testing of a pavement system’s subgrade and subbase layers, thereby extending pavement life.

The manual synthesizes current and previous research conducted in Iowa and other states into a practical geotechnical design guide (proposed as Chapter 6 of the SUDAS Design Manual) and construction specifications (proposed as Section 2010 of the SUDAS Standard Specifications) for subgrades and subbases.

Topics covered include the important characteristics of Iowa soils, the key parameters and field properties of optimum foundations, embankment construction, geotechnical treatments, drainage systems, and field testing tools, among others.


A. General information:

This section summarizes the basic soil properties and definitions required for designing pavement foundations and embankment construction.  Basic soil classification and moisture-density relationships for compacted cohesive and cohesion less soil materials are included. The standard for soil density is determined as follows:

Coarse-grained soil:   The required minimum relative density and moisture range should be specified if it is a bulking soil.

Fine-grained soil:   The required minimum dry density should be specified; then the acceptable range of moisture content should be determined through which this density can be achieved.

Inter-grade soils:   The required minimum dry density or relative density should be specified, depending on the controlling test.  Moisture range is determined by the controlling test.

B. Soil types:
Soil:  Soils are sediments or other unconsolidated accumulation of solid particles produced by the physical and chemical disintegration of rocks, and which may or may not contain organic matter. Soil has distinct advantages as a construction material, including its relative availability, low cost, simple construction techniques, and material properties which can be modified by mixing, blending, and compaction.

Rock: Rocks are natural solid matter occurring in large masses or fragments.

Iowa soils: The three major soils distributed across Iowa are loess, glacial till, and alluvium, which constitute more than 85% of the surface soil.


Soils are classified to provide a common language and a general guide to their engineering behavior, using either the Unified Soil Classification System (USCS) (ASTM D 3282) or the AASHTO Classification System (AASHTO M 145).

Moisture-density relationships for soils:

Compaction is the densification of soils by mechanical manipulation. Soil densification entails expelling air out of the soil, which improves the strength characteristics of soils, reduces compressibility, and reduces permeability. Using a given energy, the density of soil varies as a function of moisture content. This relationship is known as the moisture-density curve, or the compaction curve.


A. General information:

There are three major types of soils in Iowa:

Loess: A fine-grained, unstratified accumulation of clay and silt deposited by wind (37.5%).Glacial till: Unstratified soil deposited by a glacier; consists of clay, silt, sand, gravel, and boulders (28.5%).
Alluvium: Clay, silt, sand, or gravel carried by running streams and deposited where streams slow down (20.1%).

Iowa geology:

The Iowa landscape consists mainly of seven topographic regions:

  • Des Moines Lobe
  • Loess Hills
  • Southern Iowa Drift Plain
  • Iowan Surface
  • Northwest Iowa Plains
  • Paleozoic Plateau
  • Alluvial Plains


General information:

A subsurface exploration program is conducted to make designers aware of the site characteristics and properties needed for design and construction. The horizontal and vertical variations in subsurface soil types, moisture contents, densities, and water table depths must be considered during the pavement design process.

 Program phases:

The objective of subsurface investigations or field exploration is to obtain sufficient subsurface data to permit selection of the types, locations, and principal dimensions of foundations for all roadways comprising the proposed project.  These explorations should identify the site in sufficient detail for the development of feasible and cost-effective pavement designs.

Site characterization:

  • Frequency and depth of borings
  • Depth requirements for borings
  • Types of borings


Disturbed sampling

Disturbed samples are those obtained using equipment that destroys the macrostructure of the soil without altering its mineralogical composition.  Specimens from these samples can be used to determine the general lithology of soil deposits, identify soil components and general classification purposes, and determine grain size, Atterberg limits, and compaction characteristics of soils.

Undisturbed sampling

Clay and granular samples can be obtained with specialized equipment designed to minimize the disturbance to the in-situ structure and moisture content of the soils. Specimens obtained by undisturbed sampling methods are used to determine the strength, stratification permeability, density, consolidation,  dynamic properties, and other engineering characteristics of soils.


General information:

Several testing methods can be used to measure soil engineering properties.  The advantages, disadvantages, and measured soil properties for each test.

Field testing:

  • Types of in-situ equipment
  • Correlations with soil properties.

Laboratory testing:

Index testing and soil classification:  AASHTO and ASTM standards for frequently used
laboratory index testing of soils.

Shear strength testing: AASHTO and ASTM standards for frequently used laboratory strength properties testing of soils.

Settlement testing: AASHTO and ASTM standards for frequently used laboratory compression properties of soils.


Geotechnical report:

The results of the explorations and laboratory testing are usually presented in the form of a geology and soils report. This report should contain sufficient descriptions of the field and laboratory investigations performed, the conditions encountered, typical test data, basic assumptions, and the analytical procedures utilized; to allow a detailed review of the conclusions, recommendations, and final pavement design.


General information:

This section addresses the importance of pavement foundations and the potential for pavement problems due to deficient foundation support.

Pavement support:

The prepared subgrade is the upper portion (typically 12 inches) of a roadbed upon which the pavement and subbase are constructed. Pavement performance is expressed in terms of pavement materials and thickness. Although pavements fail from the top, pavement systems generally start to deteriorate from the bottom (subgrade), which often determines the service life of a road.

Pavement problems:

There are a number of ways that a pavement section can fail as well as many mechanisms which lead to distress and failure.


General information:

Quality embankment construction is required to maintain smooth-riding pavements and to provide slope stability.  Proper selection of soil, adequate moisture control, and uniform compaction are required for a quality embankment.  Problems resulting from poor embankment construction have occasionally resulted in slope stability problems that encroach on private property and damage drainage structures.

Site preparation:

  • Clearing and grubbing
  •  Stripping, salvaging, and spreading topsoil

Design considerations:

  • Slope stability evaluation.
  • Causes of slope instability
  • Slope stability problems in Iowa.
  • Stabilization methods.


It provides suggested compaction equipment and compacted lift thicknesses for coarse- and fine-grained soils, according to the USCS and AASHTO soil classification systems.


General information:

The subgrade is that portion of the pavement system that is the layer of natural soil upon which the pavement or subbase is built. Subgrade soil provides support to the remainder of the pavement system. The quality of the subgrade will greatly influence the pavement design and the actual useful life of the pavement that is constructed.


General information:

Pavement systems generally consist of three layers: prepared subgrade, subbase, and pavement. This section will deal with the proper design and construction of subbases. The subbase is the layer of aggregate material that lies immediately below the pavement and usually consists of crushed aggregate or gravel or recycled materials (see Section 6C-1, Pavement Systems, for more information).

Granular subbases:

  • Purpose
  • Materials
  • Gradation
  • Construction
  • Thickness requirement

Recycled materials:

Recycled materials with the required particle distribution, high stiffness, low susceptibility to frost action, high permeability, and high resistance to permanent deformation can be successful subbases. Recycled aggregate can solve disposal problems,conserve energy, and lower the cost of road construction.


General information:

Subsurface drainage is a key element in the design of pavement systems. Indiscriminate exclusion of this element will assuredly lead to the premature failure of pavement systems, thereby resulting in high life-cycle costs. Faulting and associated pumping in rigid pavements systems, extensive cracking from loss of subgrade support in flexible pavements, and distress from frost heave are clear signs of inadequate drainage.


 General information:

Soft subgrade and moisture-sensitive soils such as expansive soils, frost-prone soils, and collapsing soils present a construction challenge as well as a pavement performance challenge. Proper treatment of problem soils and the preparation of the foundation are important to ensure a long-lasting pavement structure that does not require excessive maintenance.

Source: Iowa State University
Authors: Vernon R. Schaefer | David J. White | Halil Ceylan | Larry J. Stevens

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