New drilled-shaft design to improve safety, save money

There are thousands of bridges in the State of Illinois that connect us to where we need to go. Much of what makes these structures able to safely carry the load of your vehicle, and thousands of other cars and trucks every day, lives underground. 

A bridge needs a good foundation to not subside or fall down. There are several types of foundations that engineers can use to stabilize bridges, but the use of drilled shafts as a foundation element is increasing in the state of Illinois.

Project R27-099, “Drilled Shaft Design in Weak Illinois Rocks (Phase 1 Study) and Modified Standard Penetration Test-Based Drilled Shaft Design Method for Weak Rocks (Phase 2 Study)”— which was conducted in cooperation with the Illinois Center for Transportation, Illinois Department of Transportation and the U.S. Department of Transportation’s Federal Highway Administration — sought to improve the design methods for axial capacity of drilled shafts embedded in weak sedimentary rock, develop appropriate reliability-based resistance factors, and modify the standard penetration test (SPT) method to allow in-situ measurements to be used for final design instead of laboratory test results. A drilled shaft is a deep foundation that is constructed by pouring fluid concrete into a drilled or augered hole that contains an assembly of reinforcing steel bars. Although these shafts are typically designed based on the results of prior field load tests in similar rocks or soils, this method has a great deal of uncertainty because of its empirical nature and the different subsurface conditions between the load test sites and IDOT bridge sites.

“The SPT did not work for rocks because the sampler could not be driven the required 18 inches in 100 drops of the 140 pound hammer,” University of Illinois at Urbana-Champaign (UIUC) Professor and Principal Investigator for the project Timothy D. Stark said. “As a result, the SPT blowcount could not be determined to estimate the unconfined compressive strength for drilled shaft design in weak rocks. This required modifying the SPT for use in Illinois rocks.”

The modified standard penetration test (MSPT) has already helped reduce the need for rock coring and laboratory triaxial compression testing to measure the unconfined compressive strength required for final drilled shaft design.

“Traditional rock coring often damages weak rock and shale samples, which can either prevent testing or produce reduced strength and rock quality designation (RQD) values, increasing foundation costs because the design is too conservative,” retired IDOT Bureau of Bridges and Structures Foundations and Geotechnical Engineer William Kramer said. Kramer was the Technical Review Panel Chair for the project. “The new modified standard penetration test (MSPT) developed by this research avoids these problems by providing an in-situ test that yields the necessary design parameters for shale deposits.”

Researchers drilled two borings at 21 IDOT bridge sites where weak shales are present.  One boring was used to obtain shale core samples for triaxial compression testing, and the other boring was used to perform MSPTs at the location of the shale core samples.  This allowed for the development of an empirical correlation between laboratory unconfined compression strength and the MSPT penetration rate that is being used for drilled shaft design on future IDOT projects.  As a result, designers can perform MSPTs and reduce, or eliminate, shale coring because the required unconfined compressive strength can be obtained directly and quickly from the in-situ MSPTs. The MSPT strengths are a lot less conservative than laboratory-measured values.

In addition, Illinois-specific design procedures were developed for drilled shafts founded in weak shale or rock. The MSPT was verified using drilled shaft load tests for drilled shaft design in weak shales and rocks in Illinois. The new design procedure will improve safety and reduce IDOT’s deep-foundation costs when constructing future bridges or remediating existing bridges.

“When fully implemented, I anticipate the time and expense of rock coring and testing will be reduced, but the majority of savings will result from less conservative deep foundations for IDOT structures,” Kramer said.

Stark says some of that time and cost savings is already taking place. “The use of MSPT penetration rates for drilled-shaft design is already reducing the design time and costs by decreasing or eliminating shale coring and laboratory triaxial compression testing previously performed by IDOT and other public and private owners,” Stark said.

James H. Long and Ahmed H. Baghdady of UIUC and Abdolreza Osouli of Southern Illinois University at Edwardsville also contributed to this project.