NCHRP 24-51 [Anticipated]
Effects of Construction Installation Methods on the Design and Performance of Drilled Shaft Foundations
| Project Data
||California Department of Transportation and Federal Highway Administration|
|This project has been tentatively selected and a project statement (request for proposals) is expected in June 2021. The project statement will be available on this site. The problem statement below will be the starting point for a panel of experts to develop the project statement. |
Some state DOTs have been conservative in their design approach and have discounted partially or completely either the estimated side or end resistance due to concerns with the effects of the various installation methods. For example, AASHTO (2014), Section 10.8, provides minimal guidance or commentary on using casing reduction factors to estimate the side friction of a cased drilled shaft. The predicted values of axial resistance for drilled shafts should be reduced accordingly when permanent steel casing is to be used along the partial or entire length of the element. AASHTO LRFD Bridge Design Speciﬁcations (2014), Section 10.8.3.5.2b (side resistance), provides the following: “Steel casing will generally reduce the side resistance of a shaft. No specific data is available regarding the reduction in skin friction resulting from the use of permanent casing relative concrete placed directly against the soil…Casing reduction factors of 0.6 to 0.75 are commonly used. Greater reduction in the side resistance may be needed if oversized cutting shoes or splicing rings are used.” For some state DOTs, the recommended factors vary from the range presented in the above-referenced section of AASHTO; for others, the factors are considerably lower or even zero, thereby eliminating any contribution from side resistance. During the past three to four decades, design demands have increased as the magnitude of axial, lateral, and flexural loadings have considerably increased due to larger superstructures, updated design codes, etc. As such, the diameter and depths to which drilled shafts can be constructed have increased substantially during this same time as a result of advancements in equipment and tooling technology and capabilities. As the methods, equipment, and tooling have evolved, there are remaining concerns regarding how the various installation methods affect the design (i.e., load transfer characteristics via side and end resistance) and the long-term performance of drilled shafts.
Experience has demonstrated that different types of installation methods, use of steel casing, and/or the drilling support fluid (i.e., slurry) may have significant effects on geotechnical load transfer characteristics (e.g., side and end resistance) and long-term performance. Given the widespread use and dependency on drilled shaft foundations, ongoing concerns regarding the effect of construction installation methods warrant a focused research investigation. Moreover, there are other construction effects that are just being recognized in the industry, such as the effect of a natural-synthetic (i.e., bentonite-polymer) blend of drilling support fluid (slurry) and the influence of rotator/oscillator-installed steel casing. FHWA GEC-10 provides emphasis on the importance of understanding the various construction methods used to install drilled shaft foundations, noting the effective use and design of drilled shafts requires knowledge of the construction methods used for these foundation elements. Drilled shaft construction is sensitive to the ground conditions encountered at the site, and the costs and magnitude of effort involved are closely tied to the ground conditions and the construction techniques that must be used for a particular circumstance. Performance is related to the effectiveness of the construction technique in preserving the integrity of the bearing materials and ensuring the structural integrity of the cast-in-place reinforced concrete drilled shaft foundation. AASHTO LRFD Bridge Design Speciﬁcations (2014), Section C10.5.3.4, further reinforces the position statement in FHWA’s GEC-10, noting the design of drilled shafts … should include the effects of the method of construction, including construction sequencing, whether the shaft will be excavated in the dry or if wet methods must be used, as well as the need for temporary or permanent casing to control caving ground conditions. The design assumptions regarding construction methods must carry through to the contract documents to provide assurance that the geotechnical and structural resistance used for design will be provided by the constructed product.
The AASHTO LRFD Bridge Design Speciﬁcations establish the national standard for design and construction for public infrastructure projects. However, various state departments of transportation (DOTs) have developed their own state-specific standards of practice based on experience, differences in local/regional geology, etc. Understandably, the design of and construction methods used for drilled shaft foundations vary widely across the U.S. and extend to various components of the construction methods, including dry method vs. wet method; natural vs. synthetic drilling support fluids; temporary vs. permanent steel casing (e.g., cast in drilled hole [CIDH] and cast in steel shell [CISS] including driven vs. drilled vs. spun into the ground; auger vs. bucket vs. grab methods of excavation; rotary drilling methods vs. vibratory / impact hammer, and rotator / oscillator methods).
Owens and Reese (Owens, M.J., and Reese, L.C. The Influence of a Steel Casing on the Axial Capacity of a Drilled Shaft. Report No. 255-1F. Texas State Department of Highways and Public Transportation Center for Transportation Research, Bureau of Engineering Research, University of Texas at Austin, Austin, TX) evaluated the effects on side resistance due to different construction methods of installing steel casing at two sites with different soil conditions. The authors concluded that (1) the effect of different casing installation methods on the unit side resistance was considerable (e.g., unit side resistance decreased by as much as 70 to 90% when casing was used compared to no casing); (2) vibratory means of installation caused densification of the sand deposit, resulting in an increase in unit side resistance; and (3) at one location, the loose-to-medium dense sand was densified to a degree such that the steel casing could not further penetrate. Camp et al. (Camp, W.M., Brown, D.A., and Mayne, P.W. Construction Methods Effects on Drilled Shaft Axial Performance. Deep Foundations 2002, Geotechnical Special Publication No. 116, M.W. O’Neil and F.C. Townsend, eds., ASCE, Reston, VA, 193-208) reported the results of a load testing program that was performed to evaluate the axial load transfer characteristics of three partially permanently cased drilled shafts. The authors reported significant differences in drilled shaft performance for dry, slurry, or casing methods in a cemented clay and that the unit side resistance decreased by an average of about 66% (range of 42% to 80%), as compared with uncased drilled shafts. Brown et al. (Brown, D.A., Turner, J.P., and Castelli, R.J. Drilled Shafts: Construction Procedures and LRFD Design Methods. Report No. FHWA-NHI-10-016. U.S. Department of Transportation, Federal Highway Administration, Washington, D.C., 2010) reported that polymer slurry can outperform bentonite slurry, specifically in terms of side resistance.
The LRFD Bridge Design Speciﬁcations, Section 10.8.3.5.2b (side resistance), provides minimal guidance or commentary on using casing reduction factors to estimate the side friction of a cased drilled shaft. The predicted values of axial resistance for drilled shafts should be reduced accordingly when permanent steel casing is to be used along the partial or entire length of the element. The code provides the following commentary: “Steel casing will generally reduce the side resistance of a shaft. No specific data is available regarding the reduction in skin friction resulting from the use of permanent casing relative concrete placed directly against the soil… Casing reduction factors of 0.6 to 0.75 are commonly used. Greater reduction in the side resistance may be needed if oversized cutting shoes or splicing rings are used.”
Lee et al. (Lee, J., Basnett, C., and Muhammad, S. Case History: Effects of Permanent Casing on the Axial Resistance of Drilled Shafts. Proceedings, Geotechnical and Structural Engineering Congress 2016, ASCE, Reston, VA, 151-162) reported on the findings from a load test program on eight drilled shafts, in which the casing was installed using vibratory means. The results from the dynamic load tests were compared with predicted values of unit side resistance (using AASHTO, 2010, and Brown et al., 2010). The authors reported that the unit side resistance of the installed casing was less than the predicted values as much as 35% to 47% in both the sand and clay strata. Li et al. (Li, Q., Stuedlein, A.W., and Marinucci, A. Axial Load Transfer of Drilled Shaft Foundations With and Without Steel Casing. DFI Journal - The Journal of the Deep Foundations Institute, 2019, 11:1, p. 13-29, DOI: 10.1080/19375247.2017.1403074) reported that the uncased shafts outperformed the cased shafts and the uncased shafts exhibited significantly greater unit side resistances than did the cased shafts. The axial resistances for the two cased test shafts were fully mobilized prior to the completion of the testing and at loads considerably lower than the maximum anticipated test load. However, the axial resistances for the two uncased shafts were not fully mobilized (i.e., did not achieve a peak or ultimate resistance). Compared with the cased shafts, the enhanced axial load transfer characteristics for the uncased shafts were attributed to an enhanced bond at the concrete-to-soil interface than it was for steel-to-soil interface; to the rougher soil-to-concrete interface; to larger as-built shaft diameters for the uncased test shafts; and to the presence of possible gaps between the soil and casing for the cased shafts. The authors reported that the steel cased drilled shafts indicated a reduction in unit side resistance ranging from about 75% to 95%, as compared with uncased drilled shafts. Recent research performed at the University of South Florida (Mobley, S., Costello, K., and Mullins, G. The Effect of Slurry Type on Drilled Shaft Cover Quality. DFI Journal - The Journal of the Deep Foundations Institute, 2019, 11:2-3, p. 91-100, DOI: 10.1080/19375247.2018.1468522) suggested that drilled shafts installed with bentonite slurry exhibited more significant flaws in the concrete cover than comparison shafts installed with polymer slurry.
Unpublished load test results from the signature Harbor Bridge currently under construction in Corpus Christi, Texas, indicated that the side resistance decreased as the time of exposure of the bentonite slurry within the borehole increased. Furthermore, locally established correlations based on the results of in-situ testing tended to overpredict the side resistance of the drilled shafts. The LRFD Bridge Design Speciﬁcations do not explicitly account for some of the numerous construction effects that have already been identified herein. This proposal addresses four of the eight strategic objectives identified in NCHRP Project 20-07/Task 335. Namely, this research will enhance the AASHTO Specifications for many aspects of drilled shaft design (No. 4), accelerate bridge delivery and construction (No. 5), optimize structural systems that are affected by drilled shaft performance (No. 6), and contribute to national policy (No. 8).
The main objectives of the research program are to quantify the installation effects on side and end resistances for different types of drilled shaft foundations (e.g., uncased, cast in drilled hole [CIDH], and cast in steel shell [CISS]) in different soil conditions due to various installation methods (e.g., dry method vs. wet method; natural vs. synthetic vs. blend drilling support fluids; temporary vs. permanent steel casing; driven vs. drilled vs. spun into the ground; rotary drilling methods vs. vibratory/impact hammer vs. grab methods; etc.). Ultimately, a report will be developed that will present how the various construction techniques affect design, which will form the basis for design guidance for state DOTs regarding technique-dependent installation effects.