CHAPTER 2

SUBSURFACE EXPLORATION

2.1 Introduction

As discussed in Chap. 1, the first step in the foundation investigation is to obtain preliminary information on the project and to plan the work. The next step is typically to perform the subsurface exploration. The goal of the subsurface investigation is to obtain a detailed understanding of the engineering and geologic properties of the soil and rock strata and groundwater conditions that could impact the foundation.

Specific items that will be discussed in the chapter are as follows:

1. Document review

2. Purpose of subsurface exploration

3. Borings, including a discussion of soil samplers, sample disturbance, field tests, boring layout, and depth of subsurface exploration

4. Test pits and trenches (Sec. 2.5)

5. Preparation of logs (Sec. 2.6)

6. Geophysical techniques (Sec. 2.7)

7. Subsurface exploration for geotechnical earthquake engineering (Sec. 2.8)

8. Subsoil profile (Sec. 2.9)

2.2 Document review

Prior to performing the subsurface exploration, it may be necessary to perform a document review.

Examples of the types of documents that may need to be reviewed are as follows:

Prior Development. If the site had prior development, it is important to obtain information on the history of the site.

Aerial Photographs and Geologic Maps. During the course of the work, it may be necessary for the engineering geologist to check reference materials, such as aerial photographs or geologic maps. Aerial photographs are taken from an aircraft flying at prescribed altitude along pre established lines. Interpretation of aerial photographs takes considerable judgment and because they have more training and experience, it is usually the engineering geologist who interprets the aerial photographs. By viewing a pair of aerial photographs, with the aid of a stereoscope, a three-dimensional view of the land surface is provided. This view may reveal important geologic information at the site, such as the presence of landslides, fault scarps, types of landforms, erosional features, general type and approximate thickness of vegetation, and drainage patterns. By comparing older versus newer aerial photographs, the engineering geologist can also observe any man-made or natural changes that have occurred at the site.

Geologic maps can be especially useful to the geotechnical engineer and engineering geologist because they often indicate potential geologic hazards (e.g., faults landslides and the like) as well as the type of near surface soil or rock at the site.

A major source for geologic maps in the United States is the United States Geological Survey (USGS). The USGS prepares many different geologic maps, books, and charts and these documents can be purchased at the online USGS bookstore. The USGS also provides an “Index to Geologic

Mapping in the United States,” which shows a map of each state and indicates the areas where a geologic map has been published.

Topographic Maps. Both old and recent topographic maps can provide valuable site information. As shown in Fig. 2.3, the topographic map is to scale and shows the locations of buildings, roads, freeways, train tracks, and other civil engineering works as well as natural features such as canyons, rivers, lagoons, sea cliffs, and beaches. The topographic map in Fig. 2.3 even shows the locations of sewage disposal ponds, water tanks, and by using different colors and shading, it indicates older versus newer development. But the main purpose of the topographic map is to indicate ground surface elevations or elevations of the sea floor, such as shown in Fig. 2.3. This information can be used to determine the major topographic features at the site and for the planning of subsurface exploration, such as available access to the site for drilling rigs.

Building Code and Other Specifications. A copy of the most recently adopted local building code should be reviewed. Usually only a few sections of the building code will be directly applicable tofoundation engineering. For example, the main applicable geotechnical section in the International Building Code (2009) is Chap. 18, “Soils and Foundations.” Depending on the type of project, there may be other specifications that are applicable for the project and will need to be reviewed. Documents that may be needed for public works projects include the Standard Specifications for Public Works Construction (2003) or the Standard Specifications for Highway Bridges (AASHTO, 1996).

Documents at the Local Building Department. Other useful technical documents include geotechnical and foundation engineering reports for adjacent properties, which can provide an idea of possible subsurface conditions. A copy of geotechnical engineering reports on adjacent properties can often be obtained at the archives of public agencies, such as the local building department. Other valuable reference materials are standard drawings or standard specifications, which can also be obtained from the local building department.

Forensic Engineering. Reports or other documents concerning the investigation of damaged or deteriorated structures may discuss problem conditions that could be present at the site (Day, 1999b, 2000b, 2004). Table 2.2 presents a summary of typical documents that may need to be reviewed prior to or during the construction of the project.