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OCR for page 71
71 CHAPTER 4 CONCLUSIONS AND SUGGESTED RESEARCH 4.1 CONCLUSIONS 4.2 SUGGESTED RESEARCH-- KNOWLEDGE-BASED DESIGNS The data and analyses presented in this report lead to the following major conclusions: 4.2.1 Statement of Problem Variability in the parameters used in the design, site con- 1. The compilation of large databases allows for the quan- ditions, construction quality, and previous experience are all titative assessment of pile capacity evaluation methods important factors. The present study bypassed some of the during both design and construction. In addition, data- difficulties by calibrating specific design methods and corre- bases (combined with the application of mechanics lations as a unit. A more complete design process based on principles) allow the determination of the controlling LRFD can recognize the contribution of different factors-- parameters of capacity evaluation methods that require such as subsurface variability, site-specific technology, and calibration. Databases are essential therefore for the previous experience--as well as amount and type of testing examination of any design methodology and hence during construction. A framework for such an approach is enable the testing of the factors of safety used by WSD presented here; further development, however, will require (i.e., their validity and effect on costs) and the devel- additional databases, e.g., for the correlation between soil opment of other methodologies, such as LRFD. parameters and field tests, as well as subjective judgments. 2. LRFD facilitates a design methodology that is more suitable for geotechnical applications than WSD. The ability to determine design factors while quantifying 4.2.2 Framework for LRFD Design the significance of their outcome is a powerful tool in for Deep Foundations engineering. The development of load and resistance factors utilizing reliability-based calibration and data- In order to fully exploit the potential of the LRFD method- bases is a major necessary step toward objective quan- ology for geotechnical purposes the aforementioned issues tification of the LRFD parameters. More so, it allows a must be addressed. Many of the affecting factors are in fact meaningful utilization of the LRFD principles (in con- being considered in the design (e.g., previous experience) but trast to parameter fitting to WSD) and sets a base for need a framework to allow future progress. A proposed solu- future developments that will further rationalize design tion is to establish knowledge-based factors for both the and lead to more economical construction. design and the construction (independent) capacity evaluation 3. The findings presented are the first of their kind in methods. These factors can be accounted for by a modifying the development of resistance factors for LRFD design constant to be multiplied by the resistance factor. methodologies in geotechnical engineering. A review of the recommended resistance factors must be compatible R > L (40) with the fundamental principle of the methodology, i.e. engineering design with a consistent level of reliability. Where: Existing LRFD codes worldwide were developed, by = 1 2 3 4 limit and large, to be compatible with previous WSD parame- 1 = factor adjusting for the variability of site conditions ters based on different concepts. While radical changes 2 = factor adjusting for the quality of soil parameter cannot be expected in construction practices, a shift in estimates both directions (more and less conservative depending 3 = factor adjusting for construction quality control on the specifics) should be expected and accepted when 4 = factor adjusting for previous site or construction adopting a true LRFD design. experience
OCR for page 71
72 limit = an upper limit on the factor that will be determined medium clay from SPT values), and thus the confidence in the from computing the components of (and judg- accuracy of the soil parameter values is low. Average values ment). The limit should have some real value larger reflect normally variable soil conditions adequately inves- than 1.0, such as 1.10. tigated. High values mean that the correlation is good (e.g., when parameters are estimated based on laboratory test of A short description of the knowledge-based factors follows. undisturbed samples or direct in situ testing like a field vane 1 applies to the spatial variation of soil properties, strati- test), and thus the confidence in parameter values is high. fication across a site, and the extent to which that variation 3 applies to the extent of measures taken to control con- has been categorized by the subsurface investigation pro- struction quality and testing the integrity and capacity of the gram. The factor is relevant for deep foundations capacity constructed deep foundations. Low values mean that few evaluation during both design and construction. Low values measures are to control construction quality; and no static, mean that the site is more erratic than normal or that little dynamic, or integrity testing results are available. High val- exploration and testing has been done. Average values reflect ues mean that extensive measures are taken to control con- normally variable soil conditions adequately investigated. struction quality, and multiple pile testing results are avail- High values mean that the site is more uniform than normal able (e.g., integrity and capacity of drilled shafts, static and and that an extensive program of boring and testing has been dynamic tests of driven piles). The high quality control also conducted. The extent of exploration can be evaluated via relates to the number of tested deep foundations as a ratio of number of borings per substructure unit. the number of piles installed per substructure. 2 applies only to deep foundations capacity methods 4 is to be used during the design to account for previous employing calculations based on soil parameters. This factor experience accumulated either on a specific construction site accounts for the manner in which soil parameters are estimated or from a specific construction technology. Low values are from field and laboratory test data and the exactness of those used if no previous experience is known at the site and a new estimates. Low values mean that the correlation between soil unfamiliar construction technology is used. High values mean parameters and the measurements they are based upon is poor that previous deep foundation testing results similar to the (e.g., when estimating undrained shear strength of soft to one designed (type and installation) are available.