Engineering and Construction

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INTRODUCTION
As discussed previously, the actual rupture of the ground due to fault movement could damage a structure. Secondary effects, such as the liquefaction of loose granular soil and slope movement or failure could also cause structural damage. This chapter will discuss some of the other earthquake-induced effects or structural conditions that can result in damage to foundations and retaining walls.

Topics will include earthquake-induced settlement and foundation alternatives to mitigate earth-quake effects.

Earthquakes throughout the world cause a considerable amount of death and destruction.

Earthquake damage can be classified as being either structural or nonstructural. For example, the Federal Emergency Management Agency (1994) states:

Damage to buildings is commonly classified as either structural or non-structural. Structural damage means the building’s structural support has been impaired. Structural support includes any vertical and lateral force resisting systems, such as the building frames, walls, and columns. Non-structural damage does not affect the integrity of the structural support system. Examples of non-structural damage include broken windows, collapsed or rotated chimneys, and fallen ceilings. During an earthquake, buildings get thrown from side to side, and up and down. Heavier buildings are subjected to higher forces than light-weight buildings, given the same acceleration. Damage occurs when structural members are overloaded, or differential movements between different parts of the structure strain the structural components. Larger earthquakes and longer shaking durations tend to damage structures more. The level of damage resulting from a major earthquake can be predicted only in general terms, since no two buildings undergo the exact same motions during a seismic event. Past earthquakes have shown us, however, that some buildings are likely to perform more poorly than others.

There are four main factors that cause structural damage during an earthquake, as follows:

1. Strength of shaking. For small earthquakes (magnitude less than 6), the strength of shaking decreases rapidly with distance from the epicenter of the earthquake. According to the United States Geological Survey (2000b), the strong shaking along the fault segment that slips during a earthquake becomes about half as strong at a distance of 8 mi, a quarter as strong at a distance of 17 mi, an eighth as strong at a distance of 30 mi, and a sixteenth as strong at a distance of 50 mi.

In the case of a small earthquake, the center of energy release and the point where slip begins is not far apart. But in the case of large earthquakes, which have a significant length of fault rup- ture, these two points may be hundreds of miles apart. Thus for big earthquakes, the strength of shaking decreases in a direction away from the fault rupture.

2. Length of shaking. The length of shaking depends on how the fault breaks during the earthquake. For example, the maximum shaking during the Loma Prieta earthquake lasted only 10 to 15 sec But during other magnitude earthquakes in the San Francisco bay area, the shaking may last 30 to 40 sec. The longer the ground shakes, the greater the potential for structural damage. In general, the higher the magnitude of an earthquake,  the longer the duration of the shaking ground (see Table 13.2).



3. Type of subsurface conditions. Ground shaking can be increased if the site has a thick deposit of soil that is soft and submerged. Many other subsurface conditions can cause or contribute to structural damage. For example, as discussed in Sec. 13.4, there could be structural damage due to liquefaction of loose submerged sands.

4. Type of building. Certain types of buildings and other structures are especially susceptible to the side-to-side shaking common during earthquakes. For example, sites located within approxi- mately 10 mi (16 km) of the epicenter or location of fault rupture are generally subjected to rough, jerky, and high frequency seismic waves that are often more capable of causing short buildings to vibrate vigorously. For sites located at greater distance, the seismic waves often develop into longer period waves that are more capable of causing high-rise buildings and buildings with large floor areas to vibrate vigorously (Federal Emergency Management Agency, 1994).

Much like diseases will attack the weak and infirm, earthquakes damage those structures that have inherent weaknesses or age-related deterioration. Those buildings that are nonreinforced, poorly constructed, weakened from age or rot, or underlain by soft or unstable soil are most susceptible to damage. The next section will discuss some of these susceptible structures.

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INTRODUCTION
Geotechnical earthquake engineering can be defined as that subspecialty within the field of geotechnical engineering that deals with the design and construction of projects in order to resist the effects of earthquakes. Geotechnical earthquake engineering requires an understanding of basic geotechnical principles as well as an understanding of geology, seismology, and earthquake engineering. In a broad sense,  seismology can be defined as the study of earthquakes. This would include the internal behavior of the earth and the nature of seismic waves generated by the earthquake.

For geotechnical earthquake engineering of soils and foundations, the types of activities that may need to be performed by the geotechnical engineer include the following:

• Subsurface exploration for geotechnical earthquake engineering, i.e., the screening investigation and the quantitative evaluation.
• Total stress and effective stress analyses for geotechnical earthquake engineering
• Bearing capacity analyses for geotechnical earthquake engineering
• For the design earthquake, determining the peak ground acceleration and magnitude of the earthquake
• Investigating the possibility of liquefaction at the site
• Investigating the stability of slopes for the additional forces imposed during the design earthquake
• Calculating the settlement of the foundation caused by the design earthquake
• Performing calculations to assess the effect of the earthquake on retaining walls
• Considering foundation alternatives to mitigate the effects of earthquakes

In terms of the accuracy of the calculations used to determine the earthquake induced soil move-
ment, Tokimatsu and Seed (1984) conclude:

It should be recognized that, even under static loading conditions, the error associated with the estimation of settlement is on the order of ±25 to 50%. It is therefore reasonable to expect less accuracy in predicting settlements for the more complicated conditions associated with earthquake loading.... In the application of the methods, it is essential to check that the final results are reasonable in the light of available experience.

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The documented cases of bearing capacity failures during earthquakes indicate that usually the following three factors (separately or in combination) are the cause of the failure:

1. Soil shear strength. Common problems include an overestimation of the shear strength of the underlying soil. Another common situation leading to a bearing capacity failure is the loss of shear strength during the earthquake, because of the liquefaction of the soil or the loss of shear strength for sensitive clays.

2. Structural load. Another common problem is that the structural load at the time of the bearing capacity failure was greater than that assumed during the design phase. This can often be the case when the earthquake causes rocking of the structure, and the resulting structural overturning moments produce significant cyclic vertical thrusts on the foundation elements and underlying soil.

3. Change in site conditions. An altered site can produce a bearing capacity failure. For example, if the groundwater table rises, then the potential for liquefaction is increased. Another example would be the construction of an adjacent excavation, which could result in a reduction in support and a bearing capacity failure.

The most common cause of a seismic bearing capacity failure or excessive settlement is due to liquefaction of the underlying soil. Later will present the analyses used to determine if a soil will liquefy during the design earthquake.

When presenting the recommendations for the allowable bearing pressures at a site, it is common practice for the geotechnical engineer to recommend that the allowable bearing pressure be increased by a factor of one-third when performing seismic analyses. For example, the International Building Code (2006) states:

In soil reports, it is commonly recommended that for the analysis of earthquake loading, the allowable bearing pressure may be increased by a factor of 1/3. The rational behind this recommendation is that the allowable bearing pressure has an ample factor of safety and thus for seismic analyses, a lower factor of safety would be acceptable. Usually the above recommendation is appropriate for the following materials:

1. Massive crystalline bedrock and sedimentary rock that remains intact during the earthquake
2. Dense to very dense granular soil
3. Heavily overconsolidated cohesive soil, such as very stiff to hard clays

These materials do not lose shear strength during the seismic shaking and therefore an increase in bearing pressure is appropriate.

A one-third increase in allowable bearing pressure should not be recommended for the following materials:

1. Foliated or friable rock that fractures apart during the earthquake
2. Loose soil subjected to liquefaction or a substantial increase in excess pore water pressure
3. Sensitive clays that lose shear strength during the earthquake
4. Soft clays and organic soils that are overloaded and subjected to plastic flow

These materials have a reduction in shear strength during the earthquake. Since the seismic shaking weakens them, the static values of allowable bearing pressures should not be increased for the earthquake analyses. In fact, the allowable bearing pressure may actually have to be reduced to account for the weakening of the soil during the earthquake. The remainder of this section will deal with the determination of the bearing capacity of soils that are weakened by the seismic shaking.

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An engineering geologist is defined as an individual who applies geologic data, principles, and interpretation so that geologic factors affecting planning, design, construction, and maintenance of civil engineering works are properly recognized and utilized (Geologist and Geophysicist Act, 1986). In some areas of the United States, there may be minimal involvement of engineering geologists except for projects involving such items as rock slopes or earthquake fault studies. In other areas of the country, such as California, the geotechnical engineer and engineering geologist usually performs the geotechnical investigations jointly. The majority of geotechnical reports include both engineering and geologic aspects of the project and both the geotechnical engineer and engineering geologist both sign the report. For example, a geotechnical engineering report will usually include an opinion by the geotechnical engineer and engineering geologist on the engineering and geologic adequacy of the site for the proposed development.

Table 1.3 (adapted from Fields of Expertise, undated) presents a summary of the fields of expertise for the engineering geologist and geotechnical engineer, with the last column indicating the areas of overlapping expertise. Note in Table 1.3 that the engineering geologist should have considerable involvement with foundations on rock, field explorations (such as subsurface exploration and surface mapping), groundwater studies, earthquake analysis, and engineering geophysics. Since geologic processes form natural soil deposits, the input of an engineering geologist can be invaluable for nearly all types of foundation engineering projects.

Because the geotechnical engineer and engineering geologist work as a team on most projects, it is important to have an understanding of each individual’s area of responsibility. The area of responsibility is based on education and training. According to the Fields of Expertise (undated), the individual responsibilities are as follows:

Responsibilities of the Engineering Geologist

1. Description of the geologic environment pertaining to the engineering project
2. Description of earth materials, such as their distribution and general physical and chemical characteristics
3. Deduction of the history of pertinent events affecting the earth materials
4. Forecast of future events and conditions that may develop
5. Recommendation of materials for representative sampling and testing
6. Recommendation of ways of handling and treating various earth materials and processes
7. Recommendation or providing criteria for excavation (particularly angle of cut slopes) in materials where engineering testing is inappropriate or where geologic elements control stability
8. Inspection during construction to confirm conditions

Responsibilities of the Geotechnical Engineer

1. Directing and coordinating the team efforts where engineering is a predominant factor
2. Controlling the project in terms of time and money requirements and degree of safety desired
3. Engineering testing and analysis
4. Reviewing and evaluating data, conclusions, and recommendations of the team members
5. Deciding on optimum procedures
6. Developing designs consistent with data and recommendations of team members
7. Inspection during construction to assure compliance
8. Making final judgments on economy and safety matters