All posts by Robert Thompson

NCHRP micropile study report published – NCHRP Report 989

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:

 

https://www.trb.org/Publications/Blurbs/182710.aspx

Welcome Aaron Leopold, P.E.!

Aaron Leopold, P.E. joined the team this May with 8 years of geotechnical engineering experience.  He received a BS and MS in Civil Engineering from the University of Illinois at Urbana-Champaign.  His previous work at Shannon & Wilson mainly focused on the design and construction of deep foundations and retention systems.  Aaron was often on the road, observing complex geotechnical projects throughout the Midwest and Western United States consisting of drilled shafts, ground anchors, micropiles, augercast piles, and other deep foundation and earth retention systems.  He has supported numerous landslide stabilization projects utilizing his knowledge of 2D and 3D numerical modeling and has worked on large design-build projects from the pursuit through construction in the Rockies.  Aaron is also heavily involved within ASCE and other professional organizations in Colorado and will be based in Denver.

Welcome Adam blazejowski and frank russell!

We are starting 2022 with two new faces at DBA – a big welcome to Adam Blazejowski, EI and Frank Russell, EI.  Both will be based in our office in Knoxville, Tennessee, but will soon be like the rest of us at DBA – traveling to interesting project sites all over the U.S.  They will be working on many of the deep foundation and earth retention projects that are our staples.

Adam is  from London, Canada where he completed his B.S. degree in civil engineering at Western University in 2020.  He came to the United States to complete an M.S. in geotechnical engineering at Virginia Tech, where he performed research on the cyclic strength of sands.  Adam is also interested in risk-based design and reliability in geotechnical engineering.

Frank  is from Hickory Flat, Georgia and graduated from Auburn University with his B.S. in 2019 and his M.S. in 2021 in civil engineering.  During graduate school, he was a recipient of the Long Family Endowed Civil Engineering Graduate Study Scholarship from the ADSC – The International Association of Foundation Drilling. His graduate school research evaluated the methods used in pile load testing across Alabama Department of Transportation projects. 

DBA Helps MnDOT Manage Risk for Dam Site

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:

Finally, some photos of the finished bridge:

 

TRB paper by andy boeckmann and erik loehr on Thermal requirements for drilled shafts

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.

Boeckmann, A.Z., El-tayash, Z., and Loehr, J.E. (2021). “Establishing and Satsifying Thermal Requirements for Drilled Shaft Concrete Based on Durability Considerations”, Transportation Research Record, March 2021.

Instrumentation at US 231 bridge and Slide

(Written by Andy Boeckmann – DBA)

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:

Thompson, W.R. and Dapp, S.D. (2021). “Innovative Landslide Solution”, Foundation Drilling, Vol XLII, No. 3April 2021, pp51-62.

US 231 Emergency Slide Repair – Laceys Spring, Alabama

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 drill rigs performing exploratory drilling (DBA)

 

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. 

 

ALDOT UAV flight June 16, 2020

ALDOT UAV flight July 07, 2020

ALDOT UAV flight July 23, 2020

ALDOT UAV flight July 28, 2020

ALDOT UAV Flight Aug 08, 2020

ALDOT UAV Flight Sep 2, 2020

ALDOT UAV Flight Sep 15, 2020

 

Excavating first shaft on the site (DBA)

 

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:

Thompson, W.R. and Dapp, S.D. (2021). “Innovative Landslide Solution”, Foundation Drilling, Vol XLII, No. 3April 2021, pp51-62.

Click HERE for some of the photos DBA team members have taken during construction.

To see aerial views from ALDOT’s UAV flight taken on July 10, 2020, click HERE.

 

 

DBA Announces Officer Transitions

DBA is excited to announce recent transitions within the company.

Dan Brown, Ph.D., P.E., D.GE, is turning over the reigns of leadership to the next generation in the firm and stepping down as President.   While stepping back from day-to-day management of the company, Dan will remain fully involved in technical aspects and client service in the role of Chief Engineer where he will continue to focus on developing practical solutions to complex and challenging foundation issues.

Three Senior Principal Engineers will assume the roles of the officers of DBA.  Tim Siegel, P.E., G.E., D.GE  has been promoted to Chief Executive Officer.   Paul Axtell, P.E., D.GE has been promoted to Chief Operating Officer.  Robert Thompson, P.E., D.GE has been promoted to Chief Financial Officer.   Tim, Paul, and Robert will provide the management and leadership for the continued growth of DBA and for DBA to provide value to its clients and continue to be a key contributor to the practice of geotechnical engineering.

Tim Siegel, CEO Paul Axtell, COO Robert Thompson, CFO

 

DBA Grows – 2020 Edition!

DBA is happy to announce our two latest additions to the team: Dan Ding, Ph.D., P.E. and Andy Boekmann, Ph.D., P.E.  Dan joined us as a Project Engineer in October of 2019 while Andy came on board as a Senior Engineer in February of this year.   Read a little bit about both of them below.  Go to our About us page to see their resume as well as everyone else on the DBA Team.

Dan Ding, Ph.D., P.E.

Dan received her Ph.D. degree in Civil Engineering from the University of Missouri. Dan specializes in site characterization, geotechnical laboratory testing, Load and Resistance Factor Design, and reliability analysis. Before joining DBA, Dan worked as an adjunct assistant professor at the University of Missouri to teach undergraduate and graduate engineering courses as well as conduct national and state research projects. She also worked as a laboratory geotechnical engineer at Geocomp in Acton, MA for a year after graduation.  Dan is based in Columbia, Missouri.

Andy Boeckmann, Ph.D., P.E.

Prior to joining DBA, Andy was a research engineer at the University of Missouri, where he performed research on topics including post-grouted drilled shafts, reliability of geotechnical designs, foundation reuse, and geotechnical asset management. Andy also earned a Ph.D. at the University of Missouri; his dissertation examined the reliability of foundation designs based on site-specific load tests. Andy is the lead author of two NCHRP Synthesis reports, including Current Practices and Guidelines for the Reuse of Bridge Foundations. Prior to working at MU, Andy was a consulting geotechnical engineer for URS Corp. in St. Louis, where he performed design and analysis for large projects, including post-Hurricane Katrina levee design in New Orleans. Andy is an active member of the Deep Foundations Institute, including serving as the vice-chair of the Subsurface Characterization Committee.  He is also based in Columbia, Missouri.

FHWA GEC 10 Update for 2018 Released!

UPDATE!  Posting of the PDF to the FHWA Resource page has been delayed while the formatting issues noted below are worked out. 

CLICK HERE for a Final DRAFT for use until FINAL document is posted by FHWA.

The long anticipated update to FHWA GEC 10 Drilled Shafts: Construction Procedures and Design Methods has finally been released by FHWA, The same team that authored the major update in 2010 that shifted design from ASD to LRFD also completed this update: Dr. Dan Brown, P.E., D.GE, and Dr. John Turner, P.E., D.GE of DBA, Dr. Erik Loehr, P.E. of the University of Missouri and DBA, and Mr. Ray Castelli, P.E. of WSP.

This version is an update of the 2010 publication.  A complete list of changes made since 2010 is in the opening chapter.  Some of the revisions include:

  • streamlining materials covered in other GEC publications (for example, site investigation and lateral loading) to focus on aspects particular or unique to drilled shafts;
  • updates to reflect the evolution of construction procedures, tooling, materials, drilling fluids, and concrete placement;
  • updated design equations for axial loading, particularly for earthquake loading;
  • updated group design to reflect recent changes to AASHTO design guidelines;
  • updates on integrity testing (including use of Thermal Integrity Profiling, or TIP); and,
  • an outline for a process for assessment and acceptance of drilled shafts based on inspection records and integrity tests.

You can download the new PDF here.  The PDF posted is “preliminary” with some minor formatting and other items to be cleaned up by the fall.