CHAPTER V GEOTECHNICAL INVESTIGATIONS AND STUDIES APRIL 1991 Chapter V GEOTECHNICAL INVESTIGATIONS AND STUDIES (Dams, Dam Sites or Appurtenant Structures) 5-0 Contents Title Page 5-1 General ............................................................ 5-1 5-2 Purpose and Scope ................................................... 5-1 5-3 References .......................................................... 5-2 5-4 Sources of Pre-existing Data and Information ..................................................... 5-2 5-5 Need for Supplemental Information ................................... 5-4 5-6 Geological Investigation and Review .................................. 5-5 5-7 Intensity of Investigations ............................................ 5-5 5-7.1 Preliminary Investigations ........................................... 5-6 5-7.2 Initial Design Investigations .......................................... 5-6 5-7.3 Final Design Investigations .......................................... 5-7 5-8 Methods of Investigations ............................................ 5-7 5-8.1 Constructed Projects .............................................. 5-7 5-8.2 Unconstructed Projects ............................................ 5-8 5-8.2.1 Types of Exploration .......................................... 5-8 5-8.2.1.1 Geologic Reconnaissance and Mapping ....................... 5-8 5-8.2.1.2 Borings ............................................... 5-8 5-8.2.1.3 Special Excavations ...................................... 5-8 5-8.2.1.4 Geophysical ............................................ 5-8 5-8.2.2 Location of Explorations ....................................... 5-9 5-8.2.3 Laboratory Tests............................................. 5-9 5-8.2.4 Field Tests ................................................. 5-9 5-i 5-0 Contents (Cont.) Title Page 5-9 Instrumentation and Monitoring ..................................... 5-10 5-9.1 General .................................................... 5-10 5-9.2 Geotechnical Instrumentation ....................................... 5-12 5-10 Special Geologic Hazard Studies ..................................... 5-13 5-10.1 Volcanic Hazards ................................................ 5-13 5-10.2 Seismic Hazards................................................. 5-14 5-11 Submittal of Geotechnical Report .................................... 5-15 5-12 Summary .......................................................... 5-17 5-13 References ......................................................... 5-17 5-14 Appendices......................................................... 5-19 Appendix V-A Types of Exploration Appendix V-B Field Permeability Tests Appendix V-C FERC Parameters for Developing Stability Analyses List of Tables 5-1 Causes of Deficient Behavior, Means of Detection ....................... 5-11 5-2 Inventory of Geotechnical Instruments................................. 5-13 5-ii Chapter V GEOTECHNICAL INVESTIGATIONS AND STUDIES (Dams, Dam Sites, or Appurtenant Structures) 5-1 GENERAL An adequate assessment of site geologic and geotechnical conditions is one of the most important aspects of a dam safety evaluation. Evaluation of the safety of either a new or an existing dam requires, among other things, that its foundation has been adequately examined, explored, and investigated so that it is as fully understood as possible. Foundation explorations should be directed towards obtaining only such information as may be important to an evaluation of the dam. The exploration program should identify the factors that critically affect the safe performance of the dam, and not develop extraneous information. The following sections of this chapter briefly identify the principal methods commonly employed in foundation investigations, and are intended to suggest approaches and scopes of investigations which, when properly implemented, should comply with FERC requirements and expectations. It is emphasized that, because of the almost infinite variety of geologic conditions from site to site, it would be unreasonable and impractical to attempt to set forth in these Guidelines specific investigation programs. While this chapter is principally directed toward dams and dam sites, the types of investigations and studies discussed are also applicable to other water retention structures and appurtenant structures of hydropower projects. 5-2 PURPOSE AND SCOPE The purpose of this chapter is to present guidelines for use by FERC staff for determining the appropriateness and level of geotechnical investigations and studies for dams, water retention structures and other appurtenances. Reports on such investigations and studies must be included in the technical exhibits supporting; (a) applications for license, (b) final design reports, and (c) Part 12 reports (FERC Regulations, 18 C.F.R. Part 12) by independent consultants, when such reports have not been previously submitted or referenced. The scope of this chapter is intended to outline the desirable quantity and quality of those investigations required to support design or evaluation conclusions. It is fully recognized that the specific investigation programs and studies for individual projects cannot realistically be standardized, and will vary widely according to site conditions, type of dam, hazard classification, and design phase. 5-3 REFERENCES Selection of field investigation procedures, and use of data evaluation procedures supporting geologic or geotechnical reports, are acknowledged to be best guided by criteria and procedures available in qualified literature, as well as by proven local practice. Reliable sources include publications and manuals authored by the U.S. Corps of Engineers, U.S. Bureau of Reclamation, American Society of Civil Engineers, U.S. Committee on Large Dams, and other widely recognized engineering organizations. In each particular case, variations based on good judgment and experience are encouraged and differing approaches of scope and detail between government and private practice are realistically inevitable. Selected references are listed in Section 5-13 5-4 SOURCES OF PRE-EXISTING DATA AND INFORMATION The geotechnical information and data presented in a licensee's or applicant's design report and in Independent Consultant's (Part 12) report, should incorporate references, where applicable to available FERC reports. The following reports may be useful: • Operation Inspection Reports • Construction Inspection Reports • Independent Consultants' Safety Inspection Reports • Other Inspection and/or Special Reports on existing dams or sites that are available One or more of the above listed reports can be expected to be available for licensed projects. If a license has not previously been issued, the FERC staff engineer performing the review may have available the Prelicense Inspection Report prepared by the responsible FERC regional office. For existing dams, geologic and geotechnical data may be available from the facility owner, previous owners, state or local agencies if the facility is a publicly owned project, and from the state agency responsible for dam safety. Also, geological information may be available from Corps of Engineers Phase I Inspection Reports of public or private entities having impounding structures upstream or downstream of the facility. For proposed dams, the source of geotechnical information will generally be the licensee and/or the applicant's consultants and engineers. For all proposed dams, the applicant will be required to provide those data necessary to evaluate whether the foundation of the proposed structure is adequate to safely construct the structure proposed for that site. Data to be made available should include, where applicable: 5-2 • Logs of borings, test pits, and exploratory trenches • Site geologic maps and reports • Site seismicity reports • Site geophysical reports • Materials exploration and testing reports • Reservoir rim conditions • Reports and papers published by Geological Societies and Departments in their Bulletins • Correspondence that may highlight geological changes or problems in the foundation • Design drawings and specifications for foundation excavation and support • Local landslide history • Inspection records • Maintenance records • Aerial photography • Seepage history • Licensee's reports • Construction photographs • Concrete materials and mix design • As built drawings • Instrumentation and monitoring data 5-5 NEED FOR SUPPLEMENTAL INFORMATION 5-3 The preferred approach for assessing the adequacy of dam foundations should be to minimize the use of general assumptions as to foundation conditions and strength parameters. The objective of reviewing existing data is self-evident. Site-specific information and data-based analyses should be the prevalent basis for judgments on dam safety. If potentially hazardous foundation conditions are believed or determined to exist, and the existing data are insufficient to resolve the problem, it will be necessary to conduct supplemental investigations and analyses, or develop additional information to complete the evaluation. Appendix V-C provides step-by-step procedures for developing the parameters needed in a stability analysis. The supplemental information will usually involve additional explorations and testing, materials testing and seismic information. Typical conditions that would require additional foundation explorations for existing dams are suggested below: • Significant cracking, settlement or sloughing of dams or related nearby structures. • Increase in settlement rates or indications of downstream movement. • Uncontrolled seepage conditions under, through, and around foundations or abutments, and at the toe area of any water retention structures. • Sudden or steady increase in observed seepage. • Credible foundation data is insufficient to support stability analysis. • Unexplained high or rapid rise in piezometric pressures either in the foundation or abutment material or within an embankment structure. • Highly fractured, jointed rock. • Rock formations that are known to be susceptible to seepage problems, solution activity or erodible material. • Rock formations that are conducive to weak seams or planes with low strength characteristics and adverse orientation (i.e. downstream dip). • Use of assumed high shear-strength parameters in a stability analysis that are not justified or supported. • Request by an owner to allow use of a reduced fact or of safety from the criteria normally required in Table 2 of Chapter 3. 5-4 5-6 GEOLOGICAL INVESTIGATION AND REVIEW Geological Investigations should be conducted for new projects and reviewed for existing structures to determine the following: • The general geologic setting of the area at and near the project. • The geologic conditions related to selection of the site. • The characteristics of the foundation soils and rocks. • Any other geologic conditions that may influence design, construction, and long term operation. • Seismicity of the area. • The sources of construction material. The extent of the investigations will depend on whether the project is proposed or existing and/or the design and the complexity of the local geology. The methods used for the investigations are dependent on the data that needs to be obtained to fully understand the foundation for both constructed and proposed projects. These investigative methods also depend on the types and size of the structures involved, and on the extent and quality of the information needed. 5-7 INTENSITY OF INVESTIGATIONS The extent of required investigations should be dictated by hazard classification, nature of structures, and quantity of data already available. Existing dams without adequate data should be evaluated as carefully as proposed structures; not to do so is to be dangerously presumptive. Geotechnical investigations for proposed sites should be generally divided into three separate phases to minimize costs and for developing the necessary data at each stage of the approval, design, and construction of a project: • Preliminary Investigations (Adequate information to justify site selection and preliminary cost estimates). • Initial Design Investigations (Information necessary to obtain regulatory approvals, refine cost estimates, and develop engineering and environmental data). • Final Design Investigations (Information necessary for developing plans and specifications, obtaining bids, and constructing the project). 5-5 For existing dams, the extent of data needed may be relatively limited, depending upon the adequacy of existing data and construction documentation. Evaluation of an existing structure generally requires detailed foundation data that may only be obtained by drilling, sampling, and testing that is concentrated on specific site areas or problems. Such investigations, when needed, should be planned to provide the engineer with information and data to answer questions on specific dam safety problems and to perform dam safety analyses. 5-7.1 Preliminary Investigations (Adequate information to justify site selection and preliminary cost estimates). This investigation should provide a first general impression of the engineering and geological aspects of the proposed site, and should determine if further study of the site is warranted. The field work generally would include preliminary field geologic mapping, some preliminary hand auger holes for soil and overburden sampling, a limited number of core holes into rock and possibly some preliminary seismic refraction lines. This information would be used to answer questions raised by an office study. The data would also be used to plan the type, location, and amount of explorations and laboratory testing required for future, more detailed investigations. 5-7.2 Initial Design Investigations (Information necessary to obtain regulatory approvals, refine cost estimates, and develop engineering and environmental data). These investigations would be undertaken to provide more detailed information on foundation characteristics on a particular site or several sites, and to provide data for preliminary considerations of the design requirements and construction methods. This type of information is usually developed for inclusion in the license application or in reports providing conceptual analyses of existing project structures. This phase of field investigation should include surface and subsurface exploration and sampling through borings, test pits, test trenches, material testing, geologic mapping, and additional geophysical surveys to supplement drilling. Data developed from these activities should be used to compare alternative sites, to analyze different types of structures that might serve the same purpose, and to develop economic evaluations of the sites. An end product of this investigation usually is an application for license, which includes a specifically identified site and appurtenant structures. 5-7.3 Final Design Investigations (Information necessary for developing plans and specifications, obtaining bids and constructing projects). These investigations would be primarily composed of detailed drilling, sampling, and testing concentrated on specific features at the selected project site; and should be specifically planned to provide the engineer with information that is necessary to design structures, estimate quantities, determine rates of construction progress, develop cost estimates, prepare plans and specifications, and obtain bids. 5-6 5-8 METHODS OF INVESTIGATIONS The adequacy of the analysis of an engineered structure will normally be primarily dependent on the extent of the information known about foundation conditions of the site and the physical properties of the foundation materials. To evaluate these properties, the type and application of sampling methods is important. There is no single sampling method or sampling device that will guarantee the recovery of satisfactory samples in all materials, but the less disturbance to the sample, the more accurate the results will be from testing that sample. Different devices and techniques have been developed for drilling and sampling a wide variety of material types. Proper sampling is a combination of science and art. Although many procedures have been standardized, the alteration and adaptation of techniques are often dictated by specific investigation requirements. 5-8.1 Constructed Projects For constructed projects, including evaluation of structures under Part 12 of the Commission's Regulations, the methods of investigation generally consist of researching available information as described in paragraph 5-4. However, if questions of project safety cannot be properly addressed by the use of existing data, then additional site specific field investigations should be required. This work will generally include explorations of the foundation, abutments, and structures themselves, using equipment and methods discussed in paragraph 5-8.2 below. 5-8.2 Unconstructed Projects 5-8.2.1 Types of Exploration The general types of explorations used to investigate potential project sites fall into four categories (1) geologic reconnaissance and mapping (2) borings, (3) special excavations, and (4) geophysical measurements. These types of exploration methods are discussed in U.S. Corps of Engineers EM 1110-1-1804, and EM 1110-2-1907 (See References, Paragraph 5-13), and briefly presented in Appendix V-A. 5-8.2.1.1 Geologic Reconnaissance and Mapping Geologic reconnaissance and mapping is crucial for understanding critical items influencing siting, design, and construction. It gives the overall picture. The geologic map is frequently the only "as built" drawing of the foundation conditions and is very useful in evaluating any stability, settlement or seepage problems that may occur during the operation of the project. 5-8.2.1.2 Borings 5-7