Geo-Congress 2020: University of Minnesota 68th Annual Geotechnical Engineering Conference

Numerical modeling is used extensively in the design and evaluation of projects that involve the behavior of rock, soil, and groundwater. There are many potential pitfalls for the modeler, particularly if inexperienced, some of which are discussed. The importance of understanding mechanisms is stressed, and this point is illustrated with examples that show the advantages of building simplified models in addition to detailed models for site analysis.

Despite the advances in knowledge in geotechnical engineering over the last decades, failures during construction of structures still occur. This paper describes four such failures related to excavation support, and discusses their causes, from both technical and non-technical standpoints. The failures described include both limit and serviceability state failures. One can expand the notion of redundancy to include non-technical issues and it is in this context that the failures are discussed. The case histories show that if redundancy had been properly employed during design and construction, the failures probably would not have occurred.

A mixed low-level radioactive waste (LLRW) and Resource Conservation and Recovery Act (RCRA) waste on-site disposal facility (OSDF) was constructed as part of the remediation of the U.S. Department of Energy Feed Material Production Center in Fernald, Ohio. The 56-acre (lined area) OSDF is fully constructed, filled with waste, and closed. The facility was operated from November 1997 until October 2006, with operational data available from January 2000 to the present, a period of nearly 20 years. Post-closure monitoring is ongoing. This paper presents the design, construction, and performance of the OSDF, with a primary focus on its operational performance. Available performance data for the OSDF’s leachate collection system, leakage detection system, and horizontal-well vadose zone monitoring system are presented and evaluated. Observations from post-closure monitoring of the OSDF are described. The data and information indicate that the OSDF is performing well and is not impacting groundwater or surface water at the site.

We present the analysis carried out to investigate the cause of a gas explosion that occurred in Minneapolis. A gas pipeline leaked, resulting in the explosion, and the objective of the analysis is to determine whether the leak resulted from failure of a water main existing below the pipeline, or from differential settlements causing the failure of couplers that connected sections of the pipeline. An analysis based on the theory for a beam on an elastic foundation makes the differential settlements the most likely cause of the explosion.

Engineering Partners International LLC (Engineering Partners) has designed over 30 projects where steel sheet pile walls have been used as permanent perimeter foundations to support buildings with one to three levels of below grade parking and up to 16 above grade floor levels. Similar to conventional foundation walls, the sheet pile walls provide lateral support for soil and surcharge loads, and vertical support for below grade parking slabs and perimeter walls and columns of the superstructure. The sheet pile foundations have been designed to support uniform wall loads up to 12 kips per linear foot and point loads up to 380 kips. When Engineering Partners completed their first sheet pile foundation design in 2005, the use of sheet pile foundations together with the construction sequencing described in this paper was among the first of its kind in the United States. This paper summarizes projects completed in the 15 years since, including results of several high strain dynamic tests completed on sheet pile using a pile driving analyzer (PDA). The majority of these projects have been completed in and around Minneapolis, Minnesota.

Before we build a predictive groundwater flow model, we first identify candidate conceptual models. To cull our collection of candidate models, we ask the seemingly simple but key question: Where is the water coming from? We argue for the use of a Bayesian framework to address this question. We start with an uninformed prior distribution that says all directions are equally likely. We then incorporate the available information (usually error-prone, noisy information of varied quality) and appropriately update the characterization of the uncertain direction. When the added information is extensive and internally consistent, a clear flow direction emerges. On the other hand, if the added information is minimal or internally inconsistent, the uninformed prior is only slightly modified.

Minnesota Department of Transportation (MNDOT) and the Ontario Ministry of Transportation (MOT), started construction of a new border crossing to replace the existing historic bridge in Baudette, Minnesota. The new bridge was designed to bear on drilled shafts instead of driven pile like the adjacent railroad bridge and previously constructed bridge. The bridge design includes four river piers supported on 8’ diameter drilled shafts up to 100’ below river elevation. Piers one and two are located on the United States side of the river and piers three and four are location on the Canadian side. The piers were constructed in the existing river from barges. The technical challenges and extreme remote location required strong coordination between multiple agencies, engineers, and two federal governments. This case study details the technique shaft, bi-directional test, concrete design and production (mass, tremie, and self-consolidating concrete), thermal integrity profiler results, obstructions, drilling techniques, clean out procedures, and a dynamic working environment in an extreme northern environment.

This paper presents a series of computational tools for quick estimation of factors of safety and critical circular failure surface location for embankments. The embankments are assumed to be made of homogeneous frictional-cohesive dry soils, obeying a linear Mohr-Coulomb shear strength criterion. Cases of a firm horizon that the failure surface cannot cut through are considered to exist at the base of the embankment or at great depth. The latter case corresponds to the case of a slope in homogeneous soil. Dimensionless functions defining the factor of safety and position of the critical failure surface are constructed using the Bishop method of slices in the commercial limit equilibrium software SLIDE. Results are implemented in a computer spreadsheet that is made available for free downloading. Formulas for quick evaluation of factors of safety are also presented. The effect of the cohesion and friction angle of the soil and the position of the firm horizon on the resulting factors of safety and depth of the failure surface are discussed. An application example solved with the proposed tools and with the shear strength reduction technique implemented in the commercial software FLAC is presented. Although the tools provided in this paper have been developed for teaching basic slope stability analysis in an undergraduate soil mechanics course, the tools could be useful when performing preliminary estimates of factors of safety of embankments in actual geotechnical design practice.

The north abutment and approach of the Eisenhower Bridge has settled significantly since completion in 1960 and is currently several feet lower as compared to the 1960 construction plan profile. The north approach originally consisted of a 25-foot-high embankment placed on a soil profile of loose/soft alluvial sands, silts, and clays (alluvium) underlain by a very dense layer of sands and gravels; the north abutment was supported by 26 timber piles driven through the embankment fill and alluvium to practical refusal bearing on very dense sands and gravels. Given the timber piles were driven to practical refusal in competent materials, the movement in the abutment appears to be a result of downdrag loads. As the approach embankment adjacent to the abutment settled from consolidation of the soft alluvium, drag load developed on the timber piles and induced excessive internal stress and end bearing pressure on the piles. It is believed the yield strength of the timber piles was exceeded in compression resulting in damaged or broken piles. The purpose of this study is to demonstrate how the drag loads on the timber pile foundation system caused excessive stress and movement of the abutment and provide comparative analyses to model the effect if steel piles had been selected in place of the timber piles.

The Portland Water Bureau (PWB), until recently, had operated two open drinking water reservoirs at Washington Park, located in Portland, Oregon. The open reservoirs were part of a gravity-fed water system constructed during the 1890s. During original construction at the site, a large ancient landslide was re-activated. A project to improve the reliability and protection for the water supply is currently in progress, and includes a buried 12.4-million gallon (47 M liter) concrete reservoir. When completed, the project will provide a seismically resilient supply of drinking water to 360,000 people, including all of downtown Portland. The buried reservoir will be isolated from continued landslide movements using an innovative compressible inclusion. The compressible inclusion will deform in response to continued landslide movement, and in turn will impart relatively small, predictable loads on the buried reservoir walls. During a seismic event, the compressible inclusion will absorb landslide displacement as well as dampen reservoir displacements, reducing the demands on drilled shaft foundations and reservoir walls.

The Minnesota Department of Transportation (MnDOT), other transportation asset owners, and geotechnical consultants are embracing sensing and monitoring to add project value and reduce risk. Historically, measurements to assist with field observations of performance have occurred since the 1930s and 1940s, and in those early years, engineers in the field relied on hydraulic and mechanical systems to measure field behavior or performance of geotechnical designs during and after construction. In the decades since, there have been significant advancements made in communications, and computing technologies which have enhanced monitoring capabilities in terms of accuracy, precision, reliability, size, cost, networking, and functionality. With these advances and benefits have also come unexpected challenges and opportunities. A retrospective on geotechnical monitoring at MnDOT, a discussion on technologies for applications today, and thoughts on the future of the geotechnical monitoring practice in the geotechnical transportation discipline will be presented.

This paper presents preliminary results of an experimental campaign aimed at mapping the dependence of the cutting force on the depth of cut in scratch tests performed with a sharp cutter. Tests conducted in Berea sandstone and Indiana limestone confirm that the scaling of the force with the depth of cut depends on the cutting regime. They also show a dependence of the nature of the frequency distribution of the cutting force on the modes of failure.

Column-supported embankments with rigid inclusions have been popular for many transportation projects that include a combination of rapid timeline, environmental issues, and/or specific subsurface conditions. In traditional embankments built on soft ground, the most frequent uses of instrumentation are to monitor the progress of consolidation and to determine its stability. Conversely, in column-supported embankments, different mechanisms and modes of failure emerge despite minimal consolidation. Thus, the use of instrumentation must be altered to address the anticipated behavior of column-supported embankments to effectively monitor performance. Instrumentation programs for rigid inclusions are not standardized and highly varied. This paper discusses the overall practices in instrumentation for drilled full displacement type rigid inclusions for both national and international case studies followed by local experience in Minnesota. After 3.5 years of continuous monitoring, the data were reassessed to evaluate the actual soil-structural interaction and the maximum loads acting within several columns. These findings have been used as an instrumentation and monitoring framework for several other embankments supported on rigid inclusions in the Minneapolis/St. Paul metro area.

Several hundred high mast light towers (HMLTs) throughout the state of Minnesota have foundation systems, that are typically concrete-filled steel pipe piles or steel H-piles, with no construction documentation (e.g. pile lengths) and soil stratigraphy information. Reviews of designs within current standards suggest that many of these foundations may have insufficient uplift capacity in the event of peak wind loads. Without knowledge of the in situ pile length, an expensive retrofit or replacement program would need to be conducted. Thus, providing a screening tool to determine in situ pile length—as compared to a bulk retrofit of all towers with unknown foundations—would provide significant cost savings. The goal of the project is to establish a non-destructive field testing technique, including data analysis algorithm, for determining in-place pile lengths by way of seismic waves. A unique feature of the proposed work is the use of computational modeling to account for the effects of soil profile and ground conditions (e.g. moisture) on the sensitivity of the method. The length of each pile supporting an HMLT will be identified through a systematic sensing approach that includes (i) collection and classification of the pertinent foundation designs and soil conditions; (ii) three-dimensional (3D) simulation of dynamic soil-foundation interaction; (iii) parametric studies of the 3D pile vibration problem; (iv) field testing; and (v) analysis-driven data interpretation.

The Sky Harbor Airport Runway in Duluth, Minnesota, is in the process of being realigned to increase safety with the runway approach. In 2007, the Duluth Airport Authority identified several obstructions to the Runway 32 approach surface. As a result, the Authority completed a multi-year federal Environmental Assessment and a state Environmental Assessment Worksheet to evaluate alternative solutions with respect to the runway approach. The obstructions consisted of old growth trees, the majority of which were located within a scientific and natural area (SNA) protected by the Minnesota Department of Natural Resources. The selected solution consisted of shortening the runway from 3,050 feet to 2,600 feet and rotating it 5 degrees into Superior Bay. The geotechnical design of the runway considered slope stability and settlement of the embankment fill. Staged preloading of the site was completed using surcharge fill with a geotextile reinforcement base layer to improve stability of the fill. In addition to monitoring settlement with settlement gauges, drone surveys were completed of the surcharge fill embankment. Underwater surveys, using a multi-beam echo-sounder (MBES) to provide depth measurements, detailed surface model, and point cloud, were completed to verify the riprap embankment protection dimensions placed in Superior Bay. This case study presents the geotechnical investigation and design of the runway relocation along with results of construction monitoring of fill placement and settlement.

This paper is concerned with the failure mechanism predicted from the cylindrical cavity expansion model for a rock mass around the sidewalls of axially loaded sockets. The failure mode, which depends on the initial lateral stresses, load-induced radial stresses, and tensile strength of the rock, is critical to identify so that the axial capacity and stiffness of the foundation can be determined. A comprehensive database of in situ load tests, specifically drilled shaft, anchor, and plug load tests in weak rock provides validation of the model. The initial lateral stresses in the rock mass are estimated using the coefficient of earth pressure and knowledge of the average vertical stresses at each load test elevation. The back-calculated shear stress–shear displacement relationships for socket sidewalls in each load test are used to calculate load-induced radial stresses developed on the socket sidewalls. The tensile strength is obtained from tabulated ratios of uniaxial compressive strength to tensile strength and knowledge of the measured compressive strength. The failure mechanism around the socket sidewalls is analyzed by applying the Fairhurst (parabolic) failure criterion. Theory shows that the rock mass generally fails by circumferential tension, which is in agreement with field observations.

If you choose to read this whole story, without interruption, it will not take you 25 years to do so. You will also not need to read this story while you are sopping wet, covered in snow, unbearably hot, freezing cold, trapped underwater, perspiring profusely, or pelted by frozen spheres. You will still not need to read this story with mud oozing around you, wind piercing through you, machinery driving over you, lightning trying to strike you, stray critters nipping at you, insects crawling all over you, artists spraying paint on you, thieves trying to steal stuff from you, or pigeons dropping surprises on you. This story, however, is about that story. Specifically, it is about what it takes to read (or record) such a story; while enduring the most extreme of environments, without interruption, for 25 years. And why we would do so. But don’t worry, we will try to keep it brief; after all, this is a bridge version. So, fasten your fall protection, and enjoy the ride.

Load and resistance factor design (LRFD) is a reliability-based limit state design methodology. In this study we investigate the concept, application, and implementation status of LRFD in shallow and deep foundation designs for the South Dakota Department of Transportation. Challenging the use of LRFD in South Dakota and neighboring states are the frequent occurrence of difficult geologic and geotechnical conditions including expansive soil and rock, deep seated landslides, soft and/or highly weathered shales, intermediary geologic materials, highly corrosive soils, and lack of available high-quality engineered fill materials. For deep foundations, pile load testing data are available to evaluate effectiveness of LRFD methodologies in the geologic materials present within the state. However, whereas deep foundation design parameters are relatively well established for LRFD methods, local calibration of shallow foundation design parameters has not yet been performed in most states. This paper examines the current status of LRFD and discusses its implementation for shallow and deep foundation design in South Dakota and provides a set of recommendations that engineers can consider as they pursue implementation of LRFD on construction projects with the ultimate goal of economical designs incorporating quantitative estimations of failure.

Braun Intertec Corporation is one of many geotechnical consulting firms supporting oil and gas production by helping establish, maintain, and/or restore stability to production pads, pipelines, and associated infrastructure. This is accomplished through a variety of means that include site reconnaissance, exploration and material testing, geotechnical instrumentation and monitoring, analytical modeling, and geotechnical and structural solutions. This paper highlights work performed on sites both pending and in production. The featured sites lie within badlands type terrain host to geologic hazards related primarily to slope instability, which have been mitigated with traditional mass grading, and/or ground improvement, retention, buttressing, and drainage techniques. The intent of this paper is to: (1) clarify the role of geotechnical consultants in resource procurement efforts and operational efficiency; (2) describe the components of effective site characterization and hazard identification/qualification programs; and (3) initiate dialogue specifically around the inheritance or initiation of geologic hazards.