Geotechnical Subsurface Explorations – Purpose, Factors and Stages

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Purpose of geotechnical subsurface exploration, factors influencing the scope of such explorations and stages of subsurface earthquake explorations for geotechnical earthquake analysis are discussed.

Purpose of Geotechnical Subsurface Exploration

The purpose of subsurface exploration is to determine and specify earthquake hazards and dangers at the construction site. If it is found that there is possible seismic hazard, the investigation should clearly define the seismic danger to be able to establish a proper mitigation approach for the described seismic hazard.

Factors influencing Scope of Geotechnical Earthquake Exploration

• Type of a structure to be constructed on the site
• The nature and complexity of geologic hazard that may influence the site, during earthquake
• Economic considerations
• Degree and level of the possible risk
• Specific requirements for instance applicable codes and specifications.

Stages of Geotechnical Subsurface Earthquake Exploration

Subsurface earthquake investigation is composed of two major stages including screening investigation and quantitative investigation. These will be explored in the following sections:

Screening Investigation

What is the aim of screening investigation?

It is the first step of the geotechnical earthquake investigation. Screening exploration will specify whether the construction site would face seismic hazard or not. If it is found that seismic danger would not occur, then the second stage of the earthquake investigation will not be carried out. However, if it is proofed that seismic hazard is present, then screening investigation should describe and determine the seriousness of the danger that may occur.

How screening investigation is conducted?

In this type of investigation, the influence of new construction activities on the probable seismic hazard should be clarified. For instance, bedding planes as shown in Figure 1, which are in bad conditions, may be exposed and land slide hazards would probably occur.

Fig.1: Bedding Planes

So, not only it is necessary to assess the influence of present construction condition but also the effect of final construction condition on the seismic hazards should be evaluated. Apart from onsite condition effects, offsite condition influences on the seismic hazards need to be accounted for. The first step need to be taken in this investigation is the review of available data. For example, seismic history of the area and special study maps. There are various documents that provide valuable information about history of earthquakes of the area such as seismology maps and reports which provide explanation regarding differences in ground shaking intensity based on the 50; 100; 250 acceleration data, type of the structure, and landmarks.

Fig.2: Typical Seismic Map

These data may involve vibration period, magnitude, ground acceleration, intensity of earthquakes occurred in the past. Another significant source of information is the geographical maps and reports. These provide information such as pattern, type, and movement of active faults and its closeness to the area. Regarding special study maps, it may provide local seismic hazards, potentially liquefiable soil, abandoned mines, and Landslides. Finally, site reconnaissance should be conducted after the research has been finished. In this stage, the conditions and recent changes, which are not available in the reviewed documents, should be observed and documented. After that, geotechnical engineer and geologists would analysis the data and proof whether the area is seismic hazard free or not.

Quantitative Evaluation

What is the Purpose of Quantitative Evaluation?

The purpose of quantitative evaluation is to achieve sufficient information about the nature and seriousness of the seismic hazard to establishes proper mitigation recommendation.

What are the Components of Quantitative Evaluation?

• Geologic mapping
• Subsurface exploration which should specify the nature and extent of seismic hazard.

Subsurface exploration should extent to a depth adequate to show subsurface conditions and ground water that influence possible liquefaction and slope stability. Finally, liquefaction exploration, which is conducted by geotechnical earthquake engineers, should extend to a depth below which liquefaction would not anticipate to occur.