This report provides guidance for factoring deep foundation passive structural resistance for use in two-dimensional limit-equilibrium SSA, and is intended to serve as a consensus document on this subject. The report is divided into two main sections. The first section provides an overview of the basic framework for incorporating deep foundation elements into global stability analyses, followed by a discussion of the different possible methods for factoring (or not) structural resistance at different stages of the analysis. From this discussion, various plausible combinations of methods for including or not including load and resistance factors are identified, including a simple example. In the second section of the report, the various factoring methods are applied to three case studies in order to analyze the influence of factoring method on reliability. The report concludes with a summary of the recommended approach for incorporating deep foundation resistance in SSA, informed by the conclusions presented in the earlier sections.
The report can be downloaded for free from DFI at the Committee Project Fund page (https://www.dfi.org/cpf) . Scroll down and look for the Landslides and Slope Stabilization Committee. The DFI committees fund a lot of projects that result in reports such as this that benefit our industry and the state of practice.
While the report is free, you can access so much more, including the DFI Journal, by becoming a member.
FHWA has published (posted the PDF!) of Geotechnical Engineering Circular 015 – Acceptance Procedures for Structural Foundations of Transportation Structures. Work began on this in 2019 and was delayed due to COVID. Andy Boeckmann, Dan Brown, Erik Loehr, and John Turner of DBA are the authors. They worked hard with Silas Nichols and the team at FHWA to produce a great guidance document for accepting deep foundations supporting transportation structures. Here is a bit from the Introduction in Chapter 1 that gives the “big picture” of this GEC:
Foundation acceptance is a crucial component of the design and construction process used to develop transportation infrastructure in the United States today. As considered in this circular, foundation acceptance refers to a process resulting in approval of payment to the constructor for installation of a deep foundation element. The process should involve the following actions by an owner agency, or entity acting on its behalf:
1. Establishment of measurable and achievable acceptance criteria that serve as assurance that a foundation element will fulfill all appropriate performance requirements, and
2. Documented evaluation of the constructed foundation element to demonstrate that the established acceptance criteria have been satisfied.
Foundation acceptance is the culmination of quality assurance (QA) efforts that, when appropriately implemented, provides the owner agency with confidence that a foundation element will fulfill all appropriate performance requirements. In some instances, the foundation acceptance process may include provisions for cost adjustments for foundation elements that do not strictly satisfy established acceptance criteria, but that are nevertheless judged to satisfy all appropriate performance requirements and which the owner agency agrees to accept.
Topics covered in this GEC include the framework for accepting deep foundations (project delivery, participants, role of QA/QC,etc.) , roles of inspection and testing, and specific items of concern for drilled shafts, driven piles, micropiles, and continuous flight auger piles. You can download the PDF at the link below or on the FHWA Geotechnical Publications page HERE.
At long last, the report for the NCHRP micropile study performed by Erik, Dan D., and Andy is published. The report, Reliability-Based Geotechnical Resistance Factors for Axially Loaded Micropiles, is the result of a considerable research effort that aims to rework AASHTO’s micropile design methods. Highlights of the research tasks are listed below.
Compile a database of micropile load tests and organize the database by micropile type and ground conditions.
Develop new presumptive and predictive models for micropile design. The presumptive models are based only on micropile type and ground condition; the predictive models further consider soil or rock strength.
Calibrate probabilistic resistance factors for micropile design based on presumptive and predictive models, and for designs based on site-specific load tests. If adopted, the resistance factors for designs based on load tests would be the first for AASHTO to be based on probabilistic calibration rather than fitting to historical practices.
The report can be downloaded for free from TRB’s website:
DBA recently completed construction observation of pile driving and earthwork for TH-84 across Norway Brook in Pine River, MN. The structure may look like a run of the mill bridge, but the project was replete with geotechnical challenges associated with constructing a new bridge at the toe of an active dam. (We use “run of the mill” with all due affection; there’s no such thing as a boring bridge to DBA.)
During the design phase, DBA designed an instrumentation system and used the resulting piezometric information to calibrate seepage models for the site. The calibrated models were used to analyze conditions during and after construction of the new bridge. DBA also developed an emergency action plan (EAP) that established items to observe during construction, defined levels of distress leading up to all potential failure mechanisms, and designated response actions associated with the distress levels. During construction, DBA was on-site to implement the EAP, coordinating with the contractor, Schroeder Construction, Inc., and MnDOT to quickly respond to any evidence of distress.
Throughout analysis, EAP development, and EAP implementation, DBA collaborated with the bridge designer, Parsons, and MnDOT to identify, explain, and manage the risks associated with this unique and challenging project. We are pleased to see live traffic crossing the dam in the picturesque Minnesota North Country!
Here is a link to a video shot by Dan Ding of DBA during construction:
DBA is pleased to be part of the Kiewit Massman Construction (KMC) design-build team delivering the first transportation design-build project in the State of Arkansas, 30 Crossing. The project will improve traffic patterns and capacity along a 3-mile urban corridor of Interstate 30 from the Interstate 630 interchange south of downtown Little Rock to the Interstate 40 interchange in North Little Rock. The project includes replacing the existing bridge over the Arkansas River with two bridges supporting 12 lanes of traffic. 30 Crossing is the capstone project of the Connecting Arkansas Program (CAP), one of the largest highway improvement programs ever undertaken by the Arkansas Department of Transportation (ArDOT).
Teamed with designers Burns & McDonnell and HDR, DBA is the lead geotechnical and foundation designer for the navigation unit and north approach unit of the new river bridges. The bridge foundations are 10-ft diameter drilled shafts socketed into shale bedrock. Due to the project’s proximity to the New Madrid Seismic zone, design of the bridges included seismic considerations for critical operational class structures. Geotechnical seismic analysis included liquefaction triggering and lateral spreading evaluation. DBA worked closely with the structural designers to consider kinematic loads induced by lateral spreading and the interaction of substructure and superstructure components during lateral spreading events. The north abutment includes a column supported MSE wall embankment (CSE). The CSE ground improvement is included to mitigate performance and stability issues associated with shallow, soft alluvial silts and liquefaction hazards associated with deeper, alluvial sands.
KMC is self-performing the drilled shaft construction and CSE installation. To date, all of the eastbound bridge drilled shafts have been installed in the river and for the north approach. The ground improvement elements as part of the CSE beneath the eastbound embankment has also been completed. Once the eastbound bridge is ready for traffic in 2022, both directions of Interstate 30 will be temporarily shifted to the eastbound bridge, and the existing bridge will be demolished to allow for construction of the westbound bridge in its place.
Senior Engineer, David Graham, P.E., is the geotechnical engineer of record and has been involved in all aspects of the project form pursuit phase to current construction activities with significant contribution and guidance of Senior Principal Engineer and COO, Paul Axtell, P.E., D.GE. Senior Engineer, Ben Turner, Ph.D., P.E., G.E., lead the geotechnical seismic design and analysis effort. Project Engineers, Dan Ding, Ph.D., P.E. and Nathan Glinski, P.E., continue to be heavily involved in construction support and observation.
Eastbound Pier 13
Westbound Pier 13 drilled shafts
Looking north from Pier 13 at Eastbound Piers 14 and 15
Andy Boeckmann, Ph.D., P.E. (DBA Senior Engineer) and Erik Loehr, Ph.D., P.E. (DBA Senior Principal Engineer) have published a paper on the topic of thermal testing of drilled shafts in the Transportation Research Board (TRB) journal Transportation Research Record. Their co-author was Zakaria El-tayash of Burns & McDonnell.
As the drilled shaft diameters have increased in size over the years, designers and owners have had questions or concerns about the issues of temperature impacts to concrete durability similar to the issues with mass concrete placement for large structural elements. Some transportation agencies have recently applied mass concrete provisions to drilled shafts, such as limits on maximum temperatures and maximum temperature differentials. The temperatures commonly observed in large diameter drilled shafts have been observed to cause delayed ettringite formation (DEF) and thermal cracking in above-ground concrete elements. This has led to the practice of applying to drilled shafts the control provisions that are based on dated practices for above-ground concrete. However, the reinforcement and confinement (embedded in soil and/or rock below grade) unique to drilled shafts should provide resistance to thermal cracking and possibly other effects of mass concrete temperatures.
Conceptual illustration of crack development in early age concrete with time from internal restraint. Adapted from Bamforth (2018) with permission from CIRIA
The paper reviews current requirements of several state DOTs for addressing DEF and thermal cracking, then establishes a rational procedure for design of drilled shafts for durability requirements in response to hydration temperatures. DEF is addressed through maximum temperature differential limitations while thermal cracking is addressed through calculations that explicitly consider the thermo-mechanical response of concrete for predicted temperatures. The recommended procedure includes a detailed five step evaluation process. Additional alternate steps for mitigation techniques and/or monitoring temperature are detailed as well. The procedures allow for explicit account of project-specific characteristics, including ground conditions, concrete mix design characteristics, drilled shaft geometry, and the quantity of steel reinforcement.
Temperature differential between center and edge of shaft versus time from thermal model and from temperature measurements
The methodology was developed from guidance established by ACI and CIRIA and provides a rational means for designing drilled shafts for durability without imposing unnecessary constraints that may exacerbate challenges with effective construction of drilled shafts. Results from application of the procedure indicate consideration of DEF and thermal cracking potential for drilled shafts is prudent, but provisions that have been applied to date are overly restrictive in many circumstances, particularly the commonly adopted 35 ?F maximum temperature differential provision.
You can get the paper from The Transportation Research Record at the link below.
After successful design and construction of the US 231 emergency slide repair in Lacey’s Spring, Alabama, DBA shifted gears to install a state-of-the-art monitoring system for the project. The monitoring system allows DBA and ALDOT to remotely detect any movement of the drilled shafts, changes in groundwater levels, and movement of the slope, itself.
The monitoring system includes ShapeAccelArray (SAAV) devices to measure displacement profiles with depth. SAAVs, which are manufactured by Measurand, consist of a chain of rigid segments, each 1.5-ft long and about 1-inch diameter. DBA installed 27 SAAV devices at US 231. Each of the 24 drilled shafts has one SAAV, which DBA installed in a 1-inch conduit welded to the drilled shaft reinforcement and emerging from the top of the grade beams connecting the shafts. The other three SAAVs are “free-field” SAAVs, installed in the soil between bridge bents. DBA worked with ALDOT’s drill crews to install the free-field SAAVs.
ALODT drill crew installing a free-field SAAV under the Northbound bridge.
Completed free-field and foundation instruments at Bent NB4.
DBA also worked with the ALDOT drill crews to install vibrating wire piezometer devices at six locations across the site. Each location includes two piezometers, one in the soil and one just below the top of rock. The piezometers were installed using the fully-grouted method. The piezometers measure pore pressure, which DBA uses to interpret groundwater conditions at the site.
Datalogger atop a vibrating wire piezometer.
All of the instruments are connected wirelessly to two central hubs that collect the data. The hubs are solar powered. One of the hubs is equipped with a cellular modem that facilitates remote collection of the data. RST Instruments manufactures the monitoring equipment as well as the vibrating wire piezometers.
Housing for SAAV devices installed in drilled shafts.
R-star hub and solar panel mounted to SB Bent 6.
Inside of data collection hub.
Results of the monitoring program indicate the foundation system is performing as designed. The US 231 structure has passed its first wet season with flying colors. Despite several periods of heavy rain that resulted in localized slope movement, the drilled shafts have shown only very small movement, typically less than 0.05 inch. The movement shown in the shafts indicates they are resisting loading from the slope movement, but with plenty of reserve capacity. The monitoring system has successfully captured realistic results from all instruments, including the drilled shaft and free-field SAAVs and piezometers.
Piezometer data shows strong correlation between rainfall and increases in groundwater levels.Example of SAAV drilled shaft displacement. Shaft displacements are very small, typically less than the stated accuracy of the SAAV devices.
The monitoring system is more than just bells and whistles: it is an integral part of DBA’s design philosophy for the US 231 project. DBA engineers were able to implement the innovative strategy of drilled shafts through an active landslide because we knew performance of the foundation system would be actively monitored. This strategy represents a modern take on the observational method, which has represented best geotechnical engineering practice since the profession originated. DBA will also use results of the monitoring program to inform future designs, consistent with our commitment to using state of the art to improve the state of practice.
To read more in detail about the design and construction of the bridge foundations, we published an article i nthe April 2021 issue of Foundation Drilling Magazine:
DBA had the great fortune to be working with the Alabama Department of Transportation (ALDOT) on a very interesting bridge project in Lacey’s Spring, Alabama just south of Huntsville, Alabama. On February 12 and 13, 2020 a large landslide occurred on SR-53 (US-231) at milepost 301.7 in Morgan County approximately 1.7 miles south of the Laceys Spring Community. The slide completely severed the 4-lane divided highway which is a major commuting route between Huntsville and several communities south of the city. Many of the workers at the U.S. Army Redstone Arsenal, NASA Marshall Space Flight Center, and the contractors and vendors that support these two major installations live in the communities impacted by the closure of the highway. Detours were established on existing state and county roads, but these added 30 to 60 minutes to commute times, depending on time of day. ALDOT was under significant pressure from the impacted communities to quickly solve the problem and reopen the road.
ALDOT drilling crews were immediately mobilized to the site to begin drilling exploratory borings and install slope inclinometer casings for monitoring slide movements. The Department of Civil Engineering at Auburn University was engaged to perform geophysical testing in conjunction with an existing research project for ALDOT. Geotechnical engineering firm TTL also assisted with field investigation efforts.
DBA and ALDOT immediately began evaluating several alternate concepts for stabilizing the slide and reopening the road during the soil and rock exploratory drilling. The design team looked at several retaining wall options, a complete rebuild of the roadway, and bridges. ALDOT selected a solution that removed most of the existing roadway embankments (built in 1947 and 1970) to reduce loading on the slope and then spanning the slide area with bridges built on the existing road alignments, with the bridges designed to withstand future movements of the slope. Excavation was begun by Reed Contracting before bridge design was complete in order for the rough grading to be done before the bridge contractor mobilized.
The bridges are two-lane structures, one Northbound and one Southbound, each about 947 ft in length. The superstructure is AASHTO BT-72 concrete girders and a concrete deck. There are seven spans in each bridge each about 135ft long. The grading work was begun while the bridge was still being designed to accelerate the schedule and shorten the time the road would be closed.
The foundations for each pier are a pair of 9.5ft diameter, permanently cased drilled shafts with 9ft diameter rock sockets. The sockets are 14ft long into the limestone and shale bedrock. The limestone uniaxial compressive strengths range from 10,820 psi to 28,100 psi.
After much design and analysis in a highly compressed schedule, a bridge contract was let for bid in early May 2020, less than 3 months after the slide occurred. Brasfield & Gorrie was the successful bidder and awarded a $15 million contract that has incentives for finishing early, and disincentives for finishing late.
A.H. Beck (Beck) was the drilled shaft contractor, drilling each shaft, placing reinforcement, and placing concrete. The 9.5ft diameter permanent casing is 5/8 inch wall thickness spiral weld 60ksi steel fabricated by Nucor in Birmingham, Alabama. The shafts are reinforced with a 1.5inch wall thickness, 8ft diameter, 60ksi steel pipe. These pipes were rolled and welded by Favor Steel in Birmingham, Alabama before being trucked to the site. The steel plate was manufactured by SSAB in Axis, Alabama near Mobile. So, the structural steel pipes were completely Alabama-made and the steel travel almost the length of the state!
8ft diameter x 1.5in wall steel pipe for shaft reinforcement (DBA)
Inner structural pipe (1.5in) and outer casing (5/8in) (DBA)
The pair of shafts for each pier is connected by a reinforced concrete grade beam 10ft wide by 7ft high by 46ft long. To connect the shafts to the grade beam, a 14ft long reinforcement cage is placed in each shaft, embedded 8ft into the shaft with 6ft embedded in the grade beam. The cage consists of 28 No.18 Grade 75 bars.
Grade beam at NB Bent 7 with column steel (DBA)
Completed shaft with reinforcing cage to embed in grade beam (DBA)
The project includes a robust instrumentation plan with ShapeArray inclinometers installed in each shaft and in the slope, supplemented by traditional inclinometers in the slope and vibrating piezometers to monitor groundwater levels. DBA and ALDOT will monitor the bridge and slope, intending to be able to measure displacement and calculate strain and loads in the shafts should the slope move again in the future.
Foundations were completed a few days ahead of schedule at the end of July 2020. The deadline to have the bridge open to traffic was early December, 2020, but Brasfield and Gorrie had an aggressive plan to complete the project early and earn the bonus for early completion. The bridge was open to traffic September 28, 2021 to much rejoicing among the commuters and others that use this route. Volkert was the CE&I Consultant on the project for ALDOT, providing construction management and inspection services for the project, ensuring all requirements were met to build the bridges.
To read more in detail about the design and construction of the bridge foundations, we published an article i nthe April 2021 issue of Foundation Drilling Magazine:
I-44 Construction Aerial View; video courtesy of Emery Sapp & Sons
DBA has partnered with bridge designer Parsons and prime contractor Emery Sapp & Sons on a design/build project in Southwest Missouri being administered by MoDOT. Design is complete and the project is in construction phase. The project involves replacing 13 bridges and rehabilitating another six bridges along a 30-mile stretch of I-44 between Sarcoxie and Halltown. The $36 million project is progressing nicely with construction beginning in 2019 and on schedule to be completed by December 15, 2021. To get a birds-eye view of some of the work, check out the video at the top of the post (from Emery Sapp & Sons)
Although smaller bridges than DBA typically works on, challenging subsurface conditions and unique structure types have made things interesting with respect to foundation design and construction. Foundation types for various structures include driven H-piles installed with high-strain dynamic testing, drilled shafts with rock sockets in various rock formations, and spread footings bearing on near surface bedrock where applicable. Pinnacle bedrock surface and karstic foundation conditions are prevalent in the area and this project is no exception. Foundation design had to anticipate the complex subsurface conditions and consider constructability throughout the entire design process.
DBA has been fortunate to be involved as a consult to Alabama Department of Transportation (ALDOT) for the Mobile River Bridge and Bayway Project. This project represents Alabama’s largest ever investment for a single infrastructure project. The project includes a cable stayed bridge over the Mobile River and seven miles of bridge over Mobile Bay. Bridge foundations therefore represent a major component of the estimated $2 billion project cost. DBA serves as a foundation consultant under subcontract to Thompson Engineering, Inc.. Thompson is one of the ALDOT Advisory Team partners, the other partners being HDR and Mott MacDonald.
With the tremendous volume of foundations required for the project, the DBA/Thompson team worked with ALDOT’s Geotechnical Division to develop a pre-bid load test program to help reduce some of the risks that would face both ALDOT and prospective concessionaires. Performing a deep foundation load test program during the procurement phase of a Public Private Partnership (P3) project can help the prospective concessionaires better define foundation design parameters and reduce uncertainties and risks related to constructability of the foundations. The reduced risk leads to reduced costs by allowing the concessionaire to develop a more efficient design while minimizing contingency costs and potential delays related to foundation constructability or performance.
The load test program was designed to include the most likely foundation types that the prospective teams might use. Several types of driven piles were installed and tested, including typical square and cylinder concrete piles used on the Alabama coast plus steel H-piles and an open-ended steel pipe pile.
All driven piles were subject to dynamic testing with a Pile Driving Analyzer during driving. Restrikes with dynamic testing were conducted on all driven piles to evaluate potential strength gain with time. Jetting techniques were specified for some piles to evaluate this installation technique which could potentially be used during construction.
Traditional axial static load tests were performed on steel HP14x89 and 18in Precast Prestressed Concrete (PPC) square piles. Rapid (Statnamic) axial load tests were performed on 36 in PCC square piles, 54in PCC cylinder, and 60in steel open-end pipe piles.
A 72in diameter drilled shaft foundation was also installed and tested. Axial load testing was done using a bi-directional load cell (AFT A-Cell). Lateral load testing was done using the Statnamic device.
Here are some videos of the Statnamic testing, with slow motion action!
Foundation contractors that are part of a concessionaire team pursuing the project were allowed to bid the load test program. Jordan Pile Driving was the successful bidder for the $3.7 million test project. AFT provided the testing services for the project – dynamic, static, Statnamic, and A-Cell.
A 72in diameter drilled shaft foundation was also installed and tested. Axial load testing was done using a bi-directional load cell (AFT A-Cell). Lateral load testing was done using the Statnamic device.