Development of geotechnical resistance fgactors for piles designed using a direct CPT method

Steven Coulter, David Tara

In the proceedings of: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Geotechnical Practice / Risk Assessment and Reliability

ABSTRACT: Pile design is generally carried out using static analysis based on traditional indirect methods or by directly using the results of CPT or CPTu profiling. With direct methods the cone resistance is used to infer the pile resistance, often with some additional knowledge of soil type interpreted from the CPT/CPTu. Two commonly used methods in Canada include the LCPC (or French) method and the Eslami and Fellenius CPTu method (EF). The LCPC method uses the cone tip resistance and soil type whereas the EF method includes pore pressure measurements taken at the shoulder of the cone. These methods calculate different estimates of resistance from each other. Our review shows that there is little information on the reliability of these methods and, in fact, most design methods. This paper proposes new resistance factors for use specifically with direct pile design using CPTu data. The resistance factors were evaluated using Monte Carlo simulation and the data included in the data set used to develop the EF method. 1 INTRODUCTION Load and Resistance Factor Design (LRFD) is becoming widespread in geotechnical engineering and is the basis for the Canadian Highway Bridge Design Code (CHBDC). As the American Association of State Highway and Transportation Officials (AASHTO) LRFD Bridge Design code has developed, it has applied LRFD to an increasing scope of design elements. The purpose of these methods is to provide a reliable infrastructure. From a geotechnical perspective, further development of appropriate resistance factors for pile design using static analysis methods is required. The value of the resistance factors depends on the variability of the resistance, which also depends on the variability of the site, the quality of the investigation and the method of analysis. Currently the CHBDC and AASHTO code do not include resistance factors based on the method of analysis. For example, the CHBDC provides a resistance factor of 0.4 for piles designed using static analysis methods. In British Columbia, possibly to reflect the higher quality data obtained from the cone penetration test (CPT) and piezocone penetration test (CPTu), the British Columbia Ministry of Transportation and Infrastructure (MoTI) allows a resistance factor of 0.45 to be used if the site investigation includes these tests. 2 PILE DESIGN USING CPT DATA The Eslami-Fellenious (EF) direct CPTu method uses the cone stress and pore pressure measurements taken at the shoulder of the cone to determine an effective cone stress. The effective cone stress is used to directly calculate the calculation of pile resistance The EF method was developed by calibrating 142 cone penetration testes to measured pile resistances (Eslami 1996). The calibration defined the data in three groups: Types I, II, and III. Types I and II were electric cone penetration test and type III were mechanical cone penetration tests. The E-F method defined the pile capacity as the plunging resistance of the pile. When plunging was not observed, the capacity was determined using the Brinch Hansen 80% criteria. Testing was carried out using quick and slow static load tests. The EF pile resistance can be calculated using Equations 1 and 2. [1] [2] Where is the unit toe resistance; is a toe correlation coefficient; is the average corrected cone tip resistance; is the unit toe resistance; is a shaft correlation coefficient; is the corrected cone tip resistance. Table 1. Shaft correlation coefficient. Soil Type Cs (%) Soft sensitive soils 8.0 Clay 5.0 Silty clay, stiff clay and silt 2.5 Sandy silt and silt 1.5 Fine sand or silty sand 1.0 Sand and gravel to sand gravel 0.4 The selection of the shaft correlation coefficient is made using the categories of soil shown in Table 1. The average corrected cone tip resistance is the average tip resistance over an 'influence' zone that depends on the sequence of soil strength. The toe correlation coefficient

RÉSUMÉ: opment of geotechnical resistance fgactors

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Cite this article:
Steven Coulter; David Tara (2014) Development of geotechnical resistance fgactors for piles designed using a direct CPT method in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper277,author = Steven Coulter; David Tara,title = Development of geotechnical resistance fgactors for piles designed using a direct CPT method,year = 2014}