INTRODUCTION OF SOILS INVESTIGATION
Geological forces and processes often result in inhomogeneous and discontinuous formations that signifi cantly infl uence the stability and costs of civil engineering works. The amount of investigation needed to characterize a site economically, the type and methods of construction, and natural geological hazards such as earthquakes, volcanic activity, and groundwater conditions are important geological factors that must be considered in the practice of geotechnical engineering. Many failures of structures, causing loss of lives and property, have resulted from unrealized geological conditions.
Consider the geology at a potential construction site in a county, as shown in Figure To map these geological features requires applications of geophysical methods and a series of closely spaced boreholes. The precise size of each geological feature is diffi cult to ascertain. In building a skyscraper, for example, you must have knowledge of the geological features under and within the vicinity of the building to design a safe and economical foundation.
Most of the theories we will be using in later chapters to predict and understand the response of a soil are based on the assumption of a homogeneous soil mass. But soils are rarely homogeneous, as illustrated in Figure 3.1. Thus, we will be treating soils as ideal or hypothetical materials and use statistical average properties. However, in many cases, statistical average values could mislead because a weak or discontinuous soil layer at a particular location may control the stability of a geotechnical system (e.g., a foundation).
A soils investigation is an essential part of the design and construction of a proposed structural system (buildings, dams, roads and highways, etc.). Soils are identifi ed, observed, and recovered during a soils investigation of a proposed site. Usually soils investigations are conducted only on a fraction of a proposed site because it would be prohibitively expensive to conduct an extensive investigation of a site. We then have to make estimates and judgments based on information from a limited set of observations, and from field and laboratory test data that will have profound effects on the performance and costs of structures constructed at a site. A practical situation is as follows.
A subdivision consisting of 3000 homes, a shopping center, water and sewer plants, utilities, and an offi ce complex is planned for a 200-hectare (approximately 300-acre) site in your neighborhood. As part of the permitting, preliminary design, and construction processes, a soils report is required. You are assigned to plan and execute the soils investigation and write the report.
Soil profi le at a construction site. (Source: McCammon and Golder, 1970.)
DEFINITIONS OF KEY TERMS:
SPT is the standard penetration test.
N is the number of blows for the last 305-mm penetration of an STP sampler.
Soil sensitivity (St) is the ratio of the intact strength to the disturbed strength.
PURPOSES OF A SOILS INVESTIGATION:
A soils investigation program is necessary to provide information for design and construction, environmental assessment, and project due diligence (due diligence is the process of evaluating a prospective project to facilitate business decisions by the owner). The purposes of a soils investigation are:
1. To evaluate the general suitability of the site for the proposed project.
2. To enable an adequate and economical design to be made.
3. To disclose and make provision for diffi culties that may arise during construction due to ground and other local conditions.