My Section visits begin next week with a visit to the San Francisco Section on Tuesday, Oct. 14th (details available at this link), where I'll be talking about my involvement with the geo-investigation for the Hoover Dam Bypass Bridge (in addition to general AEG business). On Thursday evening, Oct. 16th, I'll be joining the Sacramento Section for drinks at a brewery in downtown Sacramento (details in image).
Generally, as President I will be giving a technical talk plus some general discussion of AEG business. I have 4 technical talks plus 1 general applied geology talk that it appropriate for students and the general public (titles are below).
- Someone is Paying Me to do This! The Geologic, Geohazard, and Geotechnical Field Investigation for the Hoover Dam Bypass Bridge
- Seismic Refraction as a Tool for Geotechnical, Geologic, and Geohazard Investigations
- Characterization, Rehabilitation, and Monitoring of a Subsidence-Impacted Dam: A Case Study at Powerline FRS, Arizona
- Application of Satellite-Based Interferometry (InSAR) to Geologic and Geotechnical Investigations
- Between a Rock and Geologic Disaster. Working as an Applied Geologist
Below the cut I have added the text for the full abstracts.
Someone is Paying Me to do This!
The Geologic, Geohazard, and Geotechnical Field Investigation for the Hoover Dam Bypass Bridge
This presentation is a retrospective discussion of the field investigation portion of the geohazard investigation and geotechnical design of the Hoover Dam Bypass Bridge that spans the border of Arizona and Nevada, crossing the Colorado River immediately downstream of the Hoover Dam. Or to put it another way, a collection of ‘war stories’ from a young field geologist who, on his first year on the job, had the opportunity to work on a career-defining project. This project was awarded the 2014 Outstanding Environmental and Engineering Project Award from AEG at the 57th Annual Meeting in Scottsdale, Arizona.
The river bridge structure — the Mike O’Callaghan-Pat Tillman Memorial Bridge — is as grand as its safety, security and economic impact. The 1,900-foot-long Colorado River crossing is the centerpiece of the project, which included 3.5 miles of new approach roadway on both sides of the river and seven other bridges. It is the highest and longest arched concrete bridge in the Western Hemisphere and features the world’s tallest precast concrete columns. The innovative hybrid structure is designed to complement the dam with the high-performance concrete arch while limiting the load demands with a modern steel superstructure. It is the first steel-concrete hybrid arch bridge in the United States.
The spectacular setting provides a backdrop for one of America’s most significant modern civil engineering projects but also proved to be the greatest challenge. The Black Canyon below the dam is an 800-foot-gorge with dramatic rock cliffs, steep to vertical canyon walls and a vast geological palette. Working in such a setting required rock cuts and fills exceeding 100 feet in height, accounting for winds up to 70 miles per hour and setting concrete at night to avoid desert heat reaching more than 120 degrees.
Major challenges faced by the geotechnical investigation included
- · Extreme heat (on site temperatures in excess of 130°F in the shade were measured – there was very little shade),
- · Extreme access – mountaineering rope work techniques were required, drill rigs were mobilized via helicopter, crane, track, and ‘spyder’,
- · High voltage overhead electrical transmission lines constraining helicopter access were present in many locations,
- · Security – initial field work began the week after September 11, 2001, and
- · Accelerated project schedule due to security concerns.
The project consisted of several investigative phases – 1) Geologic mapping and seismic hazard investigation, 2) Canyon wall mapping, 3) Preliminary drilling investigation at river bridge foundation locations and potential tunnel location, 4) Arizona approach drilling, 5) Colorado River bridge foundation drilling, and 6) Nevada approach drilling. Total time in the field was a little over 6 months over a 1-year period with up to 4 drill rigs on site at a time. Several thousand feet of corehole sampling with optical borehole surveys were drilled. Additional characterization included early adoption of LiDAR scanning to map the canyon walls, pushing the technology at the time, and other techniques such as Goodman Jack testing, surface refraction and downhole seismic surveys, helicopter reconnaissance, and fixed-wing aerial search for any previously unknown area faults.
Seismic Refraction as a Tool for Geotechnical, Geologic, and Geohazard Investigations
When characterizing sites, especially in areas of shallow rock, geoprofessionals typically need to know depths to and profile of harder materials, and to quantify the strength or resistance to excavation (rippability) of those materials, the assessment of foundation conditions, the stability of the geomaterial, and/or scour potential. As practical, cost effective tools to address these parameters, signal enhancement refraction seismographs have been available to geotechnical engineers since the 1970s. Using a typical single channel instrument in the early years, data could be obtained using the one geophone set near the instrument and moving the sledgehammer energy source at set spacing distances to obtain the seismic refraction first arrival time-distance data one point at a time. Interpretations were performed using slide rule, chart or hand calculator (by mid to late 1970s) methods to calculate seismic compression wave (p-wave) velocities and depth to harder horizons (often bedrock). Although field equipment, seismic methods and computational interpretation technologies have improved exponentially since that time, some of the geoprofessional’s basic data needs from seismic geophysics have remained the same: depth to and profile of harder horizons (often rock or bedrock), excavatability of those harder horizons, foundation conditions for load bearing, stability of the geomaterials, and/or scour potential.
Recent advances in surface wave measurement capabilities now permit collection of both seismic refraction compressional wave (p-wave) and Rayleigh surface wave (for s-wave) data using the same physical field equipment and (thankfully!) multi-geophone arrays. Typical energy sources include sledge hammer for p-wave and spectral-analysis or multi-analysis of surface wave data, and field vehicle, jogging or background ambient noise for refraction microtremor (ReMi) data. Seismograph field sampling rate settings are changed from p-wave to surface wave data collection; the rest of the field setup may remain the same. Interpretation of p-wave and s-wave data sets uses standard procedures. However, results of each data set are used to constrain or verify the results of the other data set. P-wave interpretations proceed first to interpret interface depths and p-wave velocities in two dimensions across the seismic line profile. The p-wave results provide an initial model for interpreting the s-wave profile from the dispersion curve. Upper or shallow interface depths based on p-wave results are honored, and s-wave velocities for the shallow layers are initially estimated as about one-half the p-wave velocities. Interpretation of the deeper portion of the s-wave profile based on the dispersion curve then proceeds. The p-wave results provide an effective constraint on the upper portion of the s-wave interpretation to reduce the non-uniqueness of dispersion curve interpretation. At the same time, the s-wave interpretation can be used to identify the absence or presence of a significant velocity reversal condition or water table, and typically provides a considerably greater depth of investigation for the final interpretation. Information concerning subsurface geotechnical profile geometries, site seismicity classification, geomaterial strengths and excavation conditions, and estimates of in-situ unit weights to assist in determining shrink / swell earthwork factors are obtained.
This presentation is given from the perspective of the engineering geologist utilizing small-scale surface seismic methods as another tool in the toolbox for efficient and effective characterization. Examples of geotechnical characterization for highway projects in Arizona, Utah and New Mexico using these complementary surface seismic concepts are presented, including earthwork factors, excavability, scour potential, slope stability and landslides.
Characterization, Rehabilitation, and Monitoring of a Subsidence-Impacted Dam: A Case Study at Powerline FRS, Arizona
Powerline Flood Retarding Structure (FRS) is a 2.5-mile long earthen dam located in Apache Junction, about 20 miles east of Phoenix, Arizona in Pinal County. Constructed by the U.S. Department of Agriculture Soil Conservation Service from 1967 to 1968, it is operated and maintained by the Flood Control District of Maricopa County (District). A study in 2002 identified earth fissures about 900 feet downstream of and trending towards the dam. In 2007 additional investigations determined that an earth fissure was present immediately downstream of the dam and likely passed underneath. Other studies have included the development of earth fissure risk zones, failure modes and effects analysis, and performance of an alternatives analysis. Providing a new dam segment as an Interim Dam Safety Measure (IDSM) to bypass zones of higher risk emerged as the preferred alternative; elevated monitoring was not considered sufficient to reduce the risk associated with the known earth fissure. The District’s goals were to have an interim solution in place in 2 to 3 years, and for the interim solution to have a life of up to 15 years. The design consists of an engineered earthen fill with an over-excavated cut-off and upstream filter aggregate and geotextile. IDSM construction was completed in early 2014.A final design for the larger region, including channels and 2 additional structures is currently underway. Powerline FRS will be abandoned and flood protection will be provided by a new channel that will flow to an adjacent rehabilitated and raised FRS. Additionally, a comprehensive monitoring plan has been developed. It includes conventional survey leveling, GPS data collection, regular acquisition and analysis of high-resolution digital imagery and satellite-based interferometric synthetic-aperture radar interferometry (InSAR), ground inspection, and regional groundwater monitoring. The presentation will show the project’s evolution through time and conclude with data acquired in the monitoring process and some lessons learned.
Application of Satellite-Based Interferometry (InSAR) to Geologic and Geotechnical Investigations
Interferometric synthetic aperture radar (InSAR) has the potential for measuring deformation of the earth’s surface with very high accuracy. In addition, when archived (post-1992) raw data is available, recent historic movement may be quantifiable. Future InSAR data can be utilized as a monitoring tool. InSAR utilizes satellite-based data acquired at two different times along orbits of a similar trajectory to detect changes in the ground surface elevation. This technique can be used to measure ground deformation for rectangular areas as large as 100 kilometers on a side, often with higher-resolution data available for smaller areas. Production of an interferogram from raw satellite data and quantification of differential ground movement (‘unwrapping’) is a complex computational process. Successful interpretation of the interferometric results requires applying available knowledge or inference of topography, geology, atmospheric conditions, ground movement trends and mechanics of existing ground conditions to address issues at hand. InSAR can also be utilized to create high quality and repeatable digital elevation models (DEMs).
The most important information that InSAR can provide is a direct observation of ground deformation as measured from the satellite’s line of site. This includes ground deformation from
· Ground subsidence (groundwater withdrawal, petroleum withdrawal, mine collapse),
· Landslides and slope instability,
· Erosion events,
· Earthquakes, and
· Other geologic events.
This presentation will show examples of InSAR use and application to geologic and geotechnical investigations and monitoring for
· Flood control structures impacted by land subsidence due to groundwater withdrawal (Arizona),
· Pilot study for evaluating InSAR and landslides as part of a geotechnical asset management plan in Colorado,
· Highway infrastructure impacted by sinkholes, potential sinkholes, dissolution mining, and underground mine works (New Mexico),
· Mine infrastructure impacted by ground subsidence related to pit dewatering (Nevada),
· Pit slop stability in large, open-pit mines (British Columbia, Arizona, and Nevada), and a
· Landslide hazard study for a tailings dam location (Western US).
Between a Rock and Geologic Disaster
Working as an Applied Geologist
This presentation is intended for students and/or a general audience. If you are interested in this talk, I can generate an abstract/description based on the audience of the presentation.