Have you ever wondered how Load and Resistance Factor Design (LRFD) is implemented in relation to deep foundations? Or why there is not a standard LRFD method for deep foundation design in all the states that you work in? At Braun Intertec, we are familiar with how to implement resistance factors and analysis methods in numerous states, including, but not limited to: Iowa, Kansas, Louisiana, Minnesota, Missouri, North Dakota, South Dakota, Texas, and Wisconsin . In this article, we will briefly discuss the difference between LRFD and ASD design methods and examine how three states employ/calibrate resistance factors and design methods in drilled shaft design.
LRFD vs. ASD
The geotechnical community has adopted the use of LRFD for foundations and slope stability in federal and state-funded projects; however, most privately funded projects still use the allowable stress design (ASD) method. Instead of using factor of safety per the ASD method, the LRFD uses load and resistance factors to limit the probability that the foundation loading is greater than the foundation resistance, as seen in Figure 1. For a foundation, American Association of State Highway and Transportation Officials (AASHTO) limits the probability of failure to 0.0003 (i.e., the failure region encompasses 1 failure in 3,500 structures).
Once AASHTO adopted LRFD for foundations and slopes in 2004, each state was required to either adopt the federal resistance factors or calibrate their own resistance factors to receive federal funding on certain public projects. Below, we give a broad overview of the drilled shaft axial resistance factors for Texas, Missouri, and Minnesota, three of the states that Braun Intertec operates in.
Minnesota
Minnesota Department of Transportation (MnDOT) specifies the use of resistance factors provided by AASHTO publications for deep foundation design in Minnesota, except when using MPF12 dynamic driving formula for driven piles. We summarize the resistance factors for geotechnical axial compressive resistance of drilled shafts based on the analysis method and soil type from AASHTO in Table 1.
Table 1. Recommended geotechnical resistance factors for drilled shafts (modified from AASHTO LRFD Bridge Design Specifications, 9th Edition, Table 10.5.5.2.4-1)
Missouri
In the Missouri Department of Transportation (MoDOT) Engineering Policy Guide (EPG), MoDOT recommends determining the resistance factors based on the mean soil property (i.e., undrained shear strength for clay soils, blow count value for sand soils, or uniaxial compressive strength in rock), the coefficient of variation (COV) of the soil property, and the type of bridge. In general, as the variability of the site and/or soil properties increase, the resistance factors decrease, see Figure 2. In addition to the state-calibrated resistance factors, MoDOT EPG recommends using the alpha method, per Reese, et al. (2006), to evaluate the nominal unit side resistance with alpha (a) calculated as in Eq. 1, where is the mean undrained shear strength of the soil layer.
As a numerical example, for a clay layer with a mean undrained shear strength of 1,000 pounds per square foot (psf) and a site standard deviation of 250 psf, the COV would be 0.25. For a bridge on a major road, the resistance factor for the alpha method is 0.57 using Figure 2. Furthermore, using the alpha method and the resistance factors in MoDOT EPG 751.37, the nominal and factored unit side resistance of the layer is 750 psf and 427.5 psf, respectively.
Texas
Texas Department of Transportation (TxDOT) bases the design of drilled shaft and pile parameters on Texas Cone Penetration (TCP) data and/or laboratory data. TCP test is an in-situ test method using a hammer to drive a 3-inch diameter cone into the soil or rock formation. The total blow count (NTCP) is the number of blows required to drive the cone to 12-inches of penetration after an initial seating increment. In hard materials (i.e. stiff/dense soil or rock), the NTCP value is the penetration of the cone per 100 blows. TxDOT Geotechnical Manual provides correlations for the allowable skin friction and point bearing based on the NTCP (see Figure 3 for the allowable skin friction). However, the allowable skin friction values have a factor of safety (FS) of at least 2 applied to the correlations, still referring to the ASD method of assessing failure risk.
For example, a lean clay (CL) soil layer with a NTCP value of 30 blows per 12 inches has an allowable skin friction of 0.5 tons per square foot [tsf] (1.0 kips per square foot [ksf]) with a FS of 2. For a general reference, a CL soil with NTCP of 30 blows has an undrained shear strength of approximately 1 tsf based on a TxDOT relationship of where su is the undrained shear strength in tsf.
Figure 3. Allowable skin friction in a) soils and b) stiff/dense soil or rock (Figures 5-1 and 5-3 in TxDOT Geotechnical Manual, dated July 2020)
These three states demonstrate the range of practice for designing deep foundations based on ASD and LRFD methodologies, and just for one type of deep foundation. Braun Intertec can help you navigate deep foundation design using either methodology as it best suits your project. Contact us if you have any questions.