Chapter 6 – Dynamic Earth Pressures

For a complete version of this document click here.

6.1. Overview of Earthquake Loads

6.1.1. Limit States

A broad look at the problem of seismic safety of waterfront structures involves the three general limit states shown which should be considered in design.

1. Gross site instability: This limit state involves lateral earth movements exceeding several feet. Such instability would be the result of liquefaction of a site, together with fail- ure of an edge retaining structure to hold the liquefied soil mass in place. Liquefaction of backfill is a problem associated with the site, mostly independent of the type of retaining structure. Failure of the retaining structure might result from overturning, sliding, or a failure surface passing beneath the structure. Any of these modes might be triggered by liquefac- tion of soil beneath or behind the retaining structure. There might also be a structural failure, such as failure of an anchor- age that is a common problem if there is liquefaction of the backfill.

2. Unacceptable movement of retaining structure: Even if a retaining structure along the waterfront edge of a site remains essentially in place, too much permanent movement of the structure may be the cause of damage to facilities immediately adjacent to the quay. Facilities of potential con- cern include cranes and crane rails, piping systems, ware- houses, or other buildings. Tilting and/or sliding of massive walls or excessive deformations of anchored bulkheads may cause permanent outward movement of retaining structures. Partial liquefaction of backfill will make such movements more likely, but this limit state is of concern even if there are no problems with liquefaction.

3. Local instabilities and settlements: If a site experiences liquefaction and yet is contained against major lateral flow, buildings and other structures founded at the site may still experience unacceptable damage. Possible modes of failure include bearing capacity failure, excessive settlements, and tearing apart via local lateral spreading. Just the occurrence of sand boils in buildings can seriously interrupt operations and lead to costly clean-up operations.

6.1.2. Key Role of Liquefaction Hazard Assessment

The foregoing discussion of general limit states has empha- sized problems due to soil liquefaction. Backfills behind waterfront retaining structures often are cohesionless soils, and by their location have relatively high water tables. Cohesionless soils may also exist beneath the base or on the

waterside of such structures. Waterfront sites are often devel- oped by hydraulic filling using cohesionless soils, resulting in low-density fills that are susceptible to liquefaction. Thus, liq- uefaction may be a problem for buildings or other structures located well away from the actual waterfront. Hence, evalua- tion of potential liquefaction should be the first step in analy- sis of any existing or new site, and the first step in establish- ing criteria for control of newly placed fill. The word “lique- faction” has been applied to different but related phenomena. To some, it implies a flow failure of an earthen mass in the form of slope failure or lateral spreading, bearing capacity failure, etc. Others use the word to connote a number of phe- nomena related to the build-up of pore pressures within soil, including the appearance of sand boils and excessive move- ments of buildings, structures, or slopes. Situations in which there is a loss of shearing resistance, resulting in flow slides or bearing capacity failures clearly are unacceptable. However, some shaking-induced increase in pore pressure may be acceptable, provided it does not lead to excessive movements or settlements.

Application of the procedures set forth in this book may require evaluation of: (a) residual strength for use in analyz- ing for flow or bearing capacity failure; or (b) build-up of excess pore pressure during shaking. As a general design principle, the predicted build-up of excess pore pressure should not exceed 30 to 40 percent of the initial vertical effec- tive stress, except in cases where massive walls have been designed to resist larger pore pressures and where there are no nearby buildings or other structures that would be dam- aged by excessive settlements or bearing capacity failures. With very loose and contractive cohesionless soils, flow fail- ures occur when the residual excess pore pressure ratio reach- es about 40 percent46 . Even with soils less susceptible to flow failures, the actual level of pore pressure build-up becomes uncertain and difficult to predict with confidence when the excess pore pressure ratio reaches this level.

6.1.3. Choice of Design Ground Motions

A key requirement for any analysis for purposes of seismic design is a quantitative specification of the design ground motion. In this connection, it is important to distinguish between the level of ground shaking that a structure or facil- ity is to resist safely and a parameter, generally called a seis- mic coefficient that is used as input to a simplified, pseudo- static analysis. Design Seismic Event

Most often a design seismic event is specified by peak acceleration. However, more information concerning the ground motion often is necessary. Duration of shaking is an important parameter for analysis of liquefaction. Magnitude is used as an indirect measure of duration. For estimating per- manent displacements, specification of either peak ground velocity or predominant period of the ground motion is essential. Both duration and predominant periods are influ- enced strongly by the magnitude of the causative earthquake, and hence magnitude sometimes is used as a parameter in analyses.

Unless the design event is prescribed for the site in ques- tion, peak accelerations and peak velocities may be selected using one of the following approaches:

1. By using available maps for the contiguous 48 states. Such maps are available for several different levels of risk, expressed as probability of non-exceedance in a stated time interval or mean recurrence interval. A probability of non- exceedance of 90 percent in 50 years (mean recurrence inter- val of 475 years) is considered normal for ordinary buildings.

2. By using attenuation relations giving ground motion as a function of magnitude and distance. This approach requires a specific choice of a magnitude of the causative earthquake, requiring expertise in engineering seismology. Once this choice is made, the procedure is essentially deterministic. Generally it is necessary to consider various combinations of magnitude and distance.

3. By a site-specific probabilistic seismic hazard assessment. Seismic source zones must be identified and characterized, and attenuation relations must be chosen. Satisfactory accom- plishment of such an analysis requires considerable expertise and experience with input from both experienced engineers and seismologists. This approach requires selection of a level of risk.

It is of greatest importance to recognize that, for a given site, the ground motion description suitable for design of a building may not be appropriate for analysis of liquefaction. Local soil conditions

The soil conditions at a site should be considered when selecting the design ground motion. Attenuation relations are available for several different types of ground conditions, and hence the analyses in items (2) and (3) might be made for any of these particular site conditions. However, attenuation rela- tions applicable to the soft ground conditions often found at waterfront sites are the least reliable. The maps referred to under item (1) apply for a specific type of ground condition: soft rock. More recent maps will apply for deep, firm alluvi- um, after revision of the document referenced in item (1). Hence, it generally is necessary to make a special analysis to establish the effects of local soil conditions.

A site-specific site response study is made using one- dimensional analyses that model the vertical propagation of shear waves through a column of soil. For any site-specific response study, it first will be necessary to define the ground

motion at the base of the soil column. This will require an establishment of peak acceleration for firm ground using one of the three methods enumerated above, and the selection of several representative time histories of motion scaled to the selected peak acceleration. These time histories must be selected with considerable care, taking into account the mag- nitude of the causative earthquake and the distance from the epicentre.

If a site response analysis is made, the peak ground motions will in general vary vertically along the soil column. Depending upon the type of analysis being made, it may be desirable to average the motions over depth to provide a sin- gle input value. At each depth, the largest motion computed in any of the several analyses using different time histories should be used.

If finite element analyses are made, it will again be neces- sary to select several time histories to use as input at the base of the grid, or a time history corresponding to a target spec- trum. Seismic Coefficients

A seismic coefficient (typical symbols are kh and kv) is a dimensionless number that, when multiplied times the weight of some body, gives a pseudo-static inertia force for use in analysis and design. The coefficients kh and kv are, in effect, decimal fractions of the acceleration of gravity (g). For some analyses, it is appropriate to use values of khg or kvg smaller than the peak accelerations anticipated during the design earthquake event.

For analysis of liquefaction, it is conventional to use 0.65 times the peak acceleration. The reason is that liquefaction is controlled by the amplitude of a succession of cycles of motion, rather than just by the single largest peak.

In design of buildings, it is common practice to base design upon a seismic coefficient corresponding to a ground motion smaller than the design ground motion. It is recognized that a building designed on this basis may likely yield and even experience some non-life- threatening damage if the design ground motion actually occurs. The permitted reduction depends upon the ductility of the structural system; that is, the ability of the structure to undergo yielding and yet remain intact so as to continue to support safely the normal dead and live loads. This approach represents a compromise between desirable performance and cost of earthquake resistance.

The same principle applies to earth structures, once it has been established that site instability caused by liquefaction is not a problem. If a retaining wall system yields, some perma- nent outward displacement will occur, which often is an acceptable alternative to significantly increased cost of con- struction. However, there is no generally accepted set of rules for selecting an appropriate seismic coefficient. The displace- ment-controlled approach to design is in effect a systematic and rational method for evaluating a seismic coefficient based upon allowable permanent displacement. The AASHTO seis- mic design for highway bridges47 is an example of design…