Partial Factors of Safety

Transcription

1 APPENDIX A Partial Factors of Safety A.I Introduction The capacity of single piles in axial compression should provide an adequate safety factor against failure due to insufficient soil-pile interface strength or structural (material) capacity. The minimum overall factor of safety applied to ultimate soil-pile interface strength (F s ) may be determined as the product of partial safety factors that consider: (1) the uncertainties inherent in determination of pile capacity, Factor F,, from Table A.I, and (2) the uncertainties inherent in the ability to install the pile without structural defects, Factor F 2, from Table A.2 where F s = F, X F 2. This method of partial factors of safety allows the use of a global factor of safety more proportionally reflecting the uncertainties in the pile foundation design and construction than a single uniform factor of safety. This method allows for revisions to the usual arbitrary factors of safety when specific criteria are met. Some of these criteria are: (1) a reliable knowledge of the subsurface conditions; Capacity Determination Method From Pile Load Test coupled with wave equation and o (2) a, construction load testing, dynamic analysis and testing; and (3) inspection by qualified personnel under the direction of the Engineer (see Sec. 1.4). The provisions for these minimum partial safety factors do not consider high-risk construction environments, uncertainty of maximum transient loads (tornado, hurricane, wave, seismic, etc.), or a consequence of failure that is unusually great, or any other abnormal condition. The foundation design should consider aspects relative to (1) the variability of subsurface conditions across the site; (2) the reliability of soil strength data, confidence in the magnitude of structural loads; (3) the longevity of the structure (temporary, normal service, monument, etc.); (4) environmental effects; (5) confidence in the magnitude of design loads; Pile Type TABLE A.2 Partial safety factor F 2 (minimum Concrete-filled, closed-end steel Steel H and open-ended steel pipe Timber, precast concrete, and concrete-filled shell piles Uncased cast-in-place concrete piles with temporary casing Augered, pressure grouted piles Uncased cast-in-place concrete piles without temporary casing TABLE A.I Partial safety factor F, (minimum values). values). with Inspection* * Full time, on-site monitoring by a qualified engineer. (See Sec. 1.4.) Design Axial Loads <15 T 16 T -40 T 41 T -100 T >100 T (<133kN) ( kn) ( kn) (>889kN) From Dynamic Measurements coupled with wave equation and * From wave equation and static analysis'" From driving formulas and static analysis H or other methods o per Sec to be established by the Engineer 21

2 STANDARD GUIDELINES FOR THE DESIGN AND INSTALLATION OF PILE FOUNDATIONS TABLE A.3-a less than 15 tons (< 133 kn). Concrete-filled, closed-end steel Steel H and open-ended pipe Timber, precast concrete, and Uncased cast-in-place concrete piles Augered pressure grouted piles * * Uncased cast-in-place concrete piles From pile load test coupled with wave equation and 3. From wave equation and TABLE A.3-C 41 to 100 tons (364 to 889 kn). Concrete-filled, closed-end steel Steel H and open-ended pipe O Precast concrete and O Uncased cast-in-place concrete piles O Augered pressure grouted piles * * Uncased cast-in-place concrete piles O 1. From pile load test coupled with wave equation and 3. From wave equation and 0 Not recommended; however may be established by the Engineer for pile types and situations in which he or she has confidence. TABLE A.3-b 16 to 40 tons (142 to 356 kn). Concrete-filled, closed-end steel Steel H and open-ended pipe Timber, precast concrete, and Uncased cast-in-place concrete piles Augered pressure grouted piles * * Uncased cast-in-place concrete piles From pile load test coupled with wave equation and 3. From wave equation and TABLE A.3-d over 100 tons (> 889 kn). Concrete-filled, closed-end steel O O Steel H and open-ended pipe O O Precast concrete and O O Uncased cast-in-place concrete piles O O Augered pressure grouted piles * * Uncased cast-in-place concrete piles O 1. From pile load test coupled with wave equation and 3. From wave equation and O Not recommended; however may be established by the Engineer for pile types and situations in which he or she has confidence. 22

3 STANDARD GUIDELINES FOR THE DESIGN AND INSTALLATION OF PILE FOUNDATIONS (6) number of piles in a group; (7) pile type; (8) installation means; and (9) testing, construction surveillance, and integrity verification. A.2 Application The partial safety factor F, is taken from Table A. 1 as a function of the method used to predict the ultimate geotechnical capacity of a pile, P u]1, and the design load category. These factors assume (1) information has been obtained and documented to reflect confidence in predicting the variability of the subsurface conditions and the strength/deformation properties of the subsurface materials; (2) reasonable confidence that the pile material will exceed the design service life of the structure; and (3) reasonable confidence in service and transient loads. The factor F 2 is selected from Table A.2 as a function of the proposed pile type, and method of installation. Factor F 2 assumes full-time on-site monitoring of the installation by a qualified representative of the Engineer (see Sec. 1.4). The global factor of safety F s is then calculated as the product of F, and F 2. Tables A.3-a through A.3-d show the various global factors of safety resulting from partial factors of Tables A. 1 and A.2. The resulting P u u/f s must be consistent with the original design considerations. Subsequent to the determination of the design capacity for soil/rock resistance, the structural design capacity of the pile should be evaluated according to provisions of Sec. 3 and 6 of this Standard. The lowest pile capacity determined from either the pile shaft structural strength using the allowable design stresses from this Standard, or the pile capacity based on soil-rock interface strength analyses including all the factors of safety from this Appendix A, should control the single pile design capacity. These factors of safety relate to structural and soil-pile interface strength. Pile settlement behavior must also be considered. Acceptance of lower global factors of safety may increase settlement magnitude of pile foundations. 23

4 COMMENTARY Appendix A Partial Factors of Safety A.3 Commentary A method of partial factors of safety allows the use of a global factor of safety more appropriately reflecting the uncertainties in pile foundation design and construction than a single uniform factor of safety. This method allows for revisions to the usual arbitrary factors of safety when specific criteria are met. These criteria are: (1) pile load tests taken to failure; (2) dynamic measurements coupled with wave equation analysis; (3) from reliable subsurface information; (4) full time inspection under the direction of the Engineer; (5) integrity testing performed in conjunction with full-time inspection under the direction of the Engineer; and (6) an understanding that, under some conditions, use of lower global factors of safety may increase settlement magnitude of pile foundations. A.3.1 Factor F, Considerations integrated into partial safety factor F, are: (1) variability of subsurface conditions across the site; (2) the quality and reliability of the soil strength data used to estimate the pile capacity, as well as knowledge of soil setup or relaxation with time; (3) the longevity of the structure (temporary, normal service, monument, etc.); (4) environmental effects; and (5) confidence in the magnitude of the loads. The five factors presented have all been combined and presented as one numerical value in Table A. 1 with the specific capacity determination method noted. The following discussion is presented as guidance for gauging when a factor F, must be adjusted from the values shown in Table A.I. The variability of the subsurface conditions across the site is a complex issue, embodying the concepts of stratigraphic continuity, and strength continuity within each particular stratum, the occurrence of obstructions that will prevent installation of a pile to a desired level, and the level at which rock or other unyielding materials are encountered that constitute a very high degree of end-bearing capacity. In considering whether F, may require adjustment, emphasis should be given to the qualitative assessment of strata continuity and soil strength within each stratum, particularly if the pile under consideration is a friction pile. Sites where subsurface and/or published geologic information indicate channel deposits, rapid facies changes, faults, formational contacts, and the like, represent a greater risk of at least some percentage of the piles encountering dramatically different subsurface conditions than have been likely assumed in the pile design. Furthermore, the occurrence of potential obstructions, both from geologic conditions and manmade fill, must be considered. Values in Table A. 1 assume that only modest or predictable variability is expected in the subsurface conditions based upon geologic literature review, a subsurface exploration consistent with good geotechnical engineering practice, and knowledge of subsurface conditions and/or pile installation on adjacent sites. If the exploration indicates highly variable subsurface conditions, only a marginal exploration was performed, geologic literature review indicates formational contacts beneath the site, and so on, then F, should be increased accordingly. Note that the assessment of variability must include all facets of the pile design, namely, lateral, uplift, negative skin friction, and compressive capacity. For example, if the primary design consideration for a pile or pile group is uplift capacity, obstructions or the occurrence of shallow competent rock could have a severe impact on the uplift capacity of the piles. Also, changes in near-surface soil conditions could significantly affect the lateral capacity of piles and pile groups even though deeper subsurface conditions are uniform and provide reasonably consistent uplift and/or compressive capacity. The quality and reliability of the soil strength data embody issues such as the type of test used to obtain or estimate soil strength and the method or manner in which the test was performed. The standard penetration test has been considered the "standard method" assumed in arriving at estimates of 24

5 COMMENTARY strength. Although the standard penetration test can rightly be considered crude and only an index to the strength determined by laboratory or in situ means, it has nonetheless provided a considerable backlog of empirical data and has been the basis for numerous static pile capacity analysis techniques. Some of the primary concerns in evaluating the reliability and quality of standard penetration test results are the type of hammer used (i.e., safety, automatic, donut, pin) and an assessment of the boring techniques used in sandy materials sampled and tested below the water table. The proliferation of hollowstem augers and their frequent use without regard to the subsurface conditions encountered are a significant concern. Another consideration is whether the standard penetration resistances have been obtained using a rope and cathead or have been obtained with one of the currently available automatic hammers. Site-specific adjustment factors should be employed to relate automatic hammer standard penetration resistances to databases obtained with a rope and cathead system. Other techniques, such as use of an electronic cone penetrometer, unconfined compressive strength testing of clays, vane shear testing, borehole shear testing, and use of a dilatometer are all considered superior methods of assessing in-place strength when compared to the standard penetration test. A prior history of instrumented piles in the area often provides information on changes of the pile's capacity due to soil setup or relaxation. If, in the opinion of the Engineer, this information is considered consistent and reliable, it can be used in the static analysis. The factor F, can be reduced accordingly, provided there is local experience with these techniques for the specific pile type and subsurface conditions under consideration. However, F! should be no lower than 1.5. The longevity of the structure affects the magnitude of F,. "Normal" service has been assumed in the formulation of F,. Certainly a temporary structure can reasonably have an F, adjusted lower, whereas a monumental structure, such as the Golden Gate Bridge, the Washington Monument, and the like, reasonably should have an F, adjusted upward, however, again no lower than 1.5. Environmental effects include degradation of the pile material with time, potential effects of freeze-thaw, and exposure of the pile after construction by physical forces such as scour. The factor F, assumes that the soil chemistry is compatible with the pile type selected and there is reasonable confidence in the stability and permanency of the materials that support the pile shaft. If these normal confidence levels are problematic, the factor F, should be increased accordingly. Confidence in the magnitude of the applied loads primarily deals with the confidence in predicting the live load component that must be supported by the piles. Compilation of factor F, assumes reasonable prediction of such live load components as wind, snow, and seismic forces. Code determination of these loads is considered "normal" confidence. A.3.2 Factor F 2 Considerations integrated into the partial safety factor F 2 include: (1) the pile type; (2) the installation means; (3) construction surveillance; and (4) shaft integrity verification. Review of Table A.2 indicates that two of the four previously discussed factors (pile type and construction surveillance) can be identified. Integrity verification is not included in this version of the table. Integrity verification is a rapidly evolving field, with current techniques constantly undergoing refinement and new techniques being developed. For example, in the case of, a broad definition of integrity testing would involve lowering an inclinometer probe into the pile to determine that there is acceptable curvature prior to concrete placement. The fact that any kind of integrity testing provides additional scrutiny under the direction of the Engineer implies a higher degree of confidence in the pile. The partial safety factor shown in Table A.2 can be reduced if integrity verification tests are used, but in no case should factor F 2 be less than