Utah Admin. Code R655-11-5C - Method of Analysis

A. Procedures are available for selecting design earthquakes and associated site-specific motions and for assessing the resistance of dams to these earthquake motions. Procedures and techniques for evaluating the effects on dams from estimated earthquake ground motions range from simplified concepts to comprehensive dynamic analyses. When the degree of sophistication of analytical procedures is far advanced, however, uncertainty is produced in the results by imperfect knowledge of input parameters obtained through field exploration and laboratory testing programs.

B. The extent or scope of studies, investigations, tests and analyses which may be required to adequately determine the seismic safety of a dam will vary from site to site. In general, the following physical factors will indicate a high priority and a greater degree of investigations and analysis:

1. Proximity to known active faults.

2. Indications of low-density materials in the dam or foundation.

3. Zones of high pore pressures or potential liquefaction.

4. Indications of marginal static stability.

5. Lack of adequate construction records for existing dams.

C. Regardless of these factors, however, one of the major considerations will be the "consequences of a failure". High and moderate hazard structures with permanent pools which could result in loss of life or extensive property damage from a failure will, in general, require a greater scope of investigation and analyses.

D. Following are the general analysis requirements, unless otherwise stipulated by the State Engineer, for MCE design earthquakes:

1. Embankments, foundations, and abutments not subject to liquefaction or significant strength loss:

a. For a maximum acceleration of 0.2g or less, or a maximum acceleration of .35g or less if the embankment consists of clay on a clay or bedrock foundation, a pseudo-static coefficient which is at least 50 percent of the maximum peak bedrock acceleration at the site should be used in the stability analysis. The minimum factor of safety in an analysis should be 1.0.

b. For a maximum peak acceleration greater than indicated above, a deformation and settlement analysis should be performed to estimate anticipated total crest movement. The evaluation should consider the potential for excess pore pressure generation and be performed for both the upstream and downstream slopes of the dam. Total crest movement should consider settlement and potential accumulation of movement from both sides. The minimum factor of safety against overtopping should be 2.0.

2. Embankment, foundation, or abutment soils subject to liquefaction or significant strength loss:

a. A liquefaction/strength loss analysis should be completed with enough detail to establish the boundaries of the liquefiable/strength loss soils and the physical characteristics of the soil during and immediately following the design earthquake.

b. A post earthquake stability analysis should be performed to show that the embankment is stable after liquefaction/strength loss occurs with a minimum factor of safety of 1.2. The potential for excess pore pressure generation will be considered.

c. Calculated deformation and settlement of the embankment total crest movement should result in a minimum factor of safety, against overtopping, of 3.0. Analyses will consider liquefaction/strength loss and the potential for excess pore pressure generation.

3. Other more sophisticated analytical procedures may be required at the discretion of the State Engineer, where conditions warrant greater detailed studies.

E. In addition to analysis of deformation and liquefaction, it will be necessary to assess the potential for internal erosion and cracking. Judgment must be used to decide whether or not erosion would tend to be self- healing as a result of filtering.

F. Construction of dams on active faults will not be allowed unless evidence is presented to, and approved by, the State Engineer that the dam can safely withstand the anticipated offset.

G. Evaluation of a dam under OBE conditions should be completed by similar methods to those described for the MCE. Under the OBE loading conditions the dam should experience no significant damage.

Notes

Utah Admin. Code R655-11-5C