2.1 Introduction

Soil is arguably the most critical component of any transportation system, since all transportation systems are built either on, in, or with soil and products from the ground. The characterization and evaluation of soil is critical to the performance of pavement structures. The guidelines provided herein will only address geotechnical considerations necessary for the design and evaluation of pavement structures.
These guidelines are prepared to provide department personnel, consultants, and contractors with guidance in:
  • Determining soil properties and characteristics to be used in pavement design. These properties include, but are not limited to, soil strength, applicable modulus (or stiffness), and volumetric stability of a pavement structure; and
  • Determining the influencing site characteristics that might require modifications to the pavement structure or adjacent works to accommodate those characteristics.
From this information, a report should be prepared that documents the findings from the geotechnical investigation.
2.1.1 Applicability
The guidance provided is intended for use by department personnel, consultants, and contractors involved in the planning, designing, evaluating, or construction of soil subgrade to be used or considered in pavement structures.
Although intended for all levels of involvement, the decisions that are made from an investigation are critical to the performance of the roadway. Contact the district pavement engineer (DPE) and/or materials engineer for further assistance and recommendations.
2.1.2 Background
It is important to determine the levels of investigation and when to perform them. From project conception to construction and throughout the operation and maintenance phases, geotechnical information is essential. Geotechnical investigations can be very general and cover broad geographic areas, such as an initial site investigation. They can also be very detailed and specific, such as identification of properties and characteristics of a single soil, as is often done in forensic studies.
Some of the frequently asked questions with regard to conducting soil investigations for pavement design are:
  • How do I get started?
  • What information is needed?
  • What test data do I need?
  • At what interval do I need to retrieve material for testing?
  • What test values are acceptable?
There are a number of variables that must be defined in an attempt to answer these questions. The direction of the investigation often depends on the nature of the project and the engineering properties desired (e.g., projects on new locations, reconstruction, reclamation of roadway materials, and resurfacing or overlay, required cuts, or fills).
2.1.3 Scope of Guidelines
These guidelines are intended solely for pavement applications.
There are numerous geotechnical and soils investigation guides that are relevant to other construction activities; most pertain to evaluations and analyses applicable to structures, slope and global stability, and retaining walls. For information on these subjects, refer to the .