Soil mechanics

This project; Phase 2 Slope Vulnerability Assessments; is a continuation of the Phase 1 project previously conducted by WSB and MnDOT to determine the risk of slope failure along state highways. The Phase 2 study area includes 32 counties located in districts 4; 6; 7; 8; and Metro. This reports outlines the methods and results of this project including the new landforms; geomorphic processes; and causative factors influencing slope failures in this part of the state. The three main components of the model are the same 1) identify past slope failures; 2) model the causative factors of past slope failures and how they vary locally; and 3) model the risk of new slope failures. Vulnerability factors; failure types; and model results reflect the geomorphology of this region. Vulnerability factors for the new study area include slope; terrain curvature; incision potential; and local relief. Field verification results validate the model's capability of identifying risk in regions with different geology; geomorphology; and hydrology including ravines and bluffs located along the Minnesota River Valley. Model results are ranked into four proposed risk management categories: action recommended; further evaluation; monitoring; and no action recommended. Risk incorporates the model outputs with consequence to infrastructure including distance to roads and populated areas. The risk estimation process for this phase is considered preliminary; further consideration of risk tolerance and consequence definitions should be conducted. Preliminary risk results indicate that 858 management areas; or 0.7% of the total area; would be recommended for mitigation under the proposed risk matrix. Next steps include field visits and a site-specific mitigation program. The results of this study are intended as the first step of actions required in minimizing the effects of slope failure including expensive mitigation and maintenance repairs and threats to public safety.

The objectives of this project are focused on a new cone penetration testing (CPT) geotechnical design manual for highway and transportation applications based on recent research and innovation covering the period from 2000 to 2018. A step-by-step procedure is outlined on how to use CPT data in the analysis and design of common geotechnical tasks. Previous manuals are either very outdated with information from 1970-1996; or not appropriately targeted to transportation works. This design document introduces modern and recent advancements in CPT research not otherwise captured in legacy manuals from the 1990's and earlier. Examples and case studies are provided for each topic interpreted using CPT measures. In the manual; a step-by-step procedure is outlined on how to use CPT data in analysis and design for typical geotechnical practices. These topics; which are applicable both to state highways and local roads; include bridge foundations (including shallow footings and deep foundations) and soil characterization (including determination of standard soil engineering properties).

A custom application "app" has been created for use on "smart devices" (phones and tablets on iOS and Android platforms) that will allow users to easily access MnDOT geotechnical asset information in the field. Through the app, users can access MnDOT foundation boring metadata and download PDF files of boring logs of interest. In addition to the development of a mobile-friendly tool [supplementing an existing website], this research effort is partly to determine a proof-of-concept related to development time, effort, documentation, and interdisciplinary coordination for development of similar systems. Through this tool, an interactive map, using boring GPS locations, or search queries, will allow improved access to subsurface information in the field in real time. This will provide a broader benefit to consultants, contractors, local units of government, researchers, and other groups especially when making decisions on-site and in the field (i.e., project scoping, site review, construction inspection, forensic analysis, etc.) This effort builds on efforts to create a data warehouse of geotechnical information and make it more easily sharable and useful to the engineering community.

The goal of this project was to develop a series of steel pipe service-life maps for the state of Minnesota. The California Method 643 is utilized to estimate steel pipe service life at locations throughout the state. Over 560 soil resistivity and pH samples were collected statewide during summer 2014 along embankments of state-trunk and county highways. Concurrent observations of soil texture, surrounding landscape, roadway type, and water presence were also made; water pH and conductivity measures were made where applicable. Data verification efforts to build confidence in field-measured soil pH and soil resistivity included comparing data to other available datasets including geology, soil pH, electrical conductivity, and soil texture, as well as observations available from district and county engineers. Field-measured soil pH data, with some exceptions in Districts 2 and 6, generally aligned with the available STATSGO soil pH data, indicating that this layer could reasonably be used in service-life calculations as it has greater resolution than provided by field data. In the absence of a statewide soil resistivity or electrical conductivity map, field-classified soil textures and the statewide STATSGO soil texture map were used to estimate soil resistivity values. Calculations of service life using the above data were completed for 18-, 16-, 14-, 12-, 10- and 8-gage galvanized and aluminized steel pipe across Minnesota. These maps were then compiled into a zone map and table that presents the 90th percentile service-life estimate for various gages and types of steel pipe. Caveats and limitations to this analysis are also discussed.

The objective of this project was to provide a qualitative assessment of the Minnesota Department of Transportation's Intelligent Compaction (IC) Specifications. IC is an attractive approach to evaluate the compaction quality because it involves continuous and instantaneous evaluation of the soil through machine-drive power or drum vibration monitoring. Four construction sites utilizing IC were visited: (1) TH 36 in North St. Paul, involving both granular and nongranular soils; (2) US 10 in Staples, with granular soil; (3) TH 60 in Bigelow, with nongranular soil; (4) US 10 in Detroit Lakes, involving both granular and nongranular soils. The report integrates comments from the four site visits and provides an interpretation on the use of IC at each site. As the technology now exists on the equipment used at these locations, IC provides only an index, which is specific to the conditions associated with a particular site. An interpretation of comments provided the basis for the following recommendations: Use light weight deflectometers (LWD) for quality assurance of stiffness; Establish a procedure to determine the target LWD value; Eliminate calibration areas (control strips); Simplify IC data evaluation and presentation; Calibrate the IC roller and related transducers; Support development of alternative IC methodologies; Simplify or eliminate moisture corrections.

Pavements are constructed on compacted soils that are typically unsaturated. The negative pore-water pressure (soil suction) due to the ingress of water in between soil particles has a significant effect on pavement foundation stiffness and strength. The study characterized the effects of soil suction on shear strength and resilient modulus of four soils representing different regions of Minnesota. The deviator stress in shear strength measurements followed a power function relationship with soil suction. Resilient modulus also followed the power function relationship with suction but these relationships fell within a narrow range. We present models for incorporating suction effects in shear strength and resilient modulus measurements of highly compacted subgrade soils. We also briefly outline a framework for incorporating these models in the resistance factors of MnPAVE. Since soil water content and the resulting soil suction under the pavement varies with season, adjustments are needed to account for increased strength and stiffness of the material as a result of unsaturated soil conditions. These adjustments will not only reflect the more realistic field conditions but will result in more reliable performance predictions than the current pavement design method.

The successful implementation of intelligent compaction technology into earthwork construction practice requires knowledge of the roller-integrated compaction measurements and their relationships with the engineering and index properties of soil that may be used for pavement design (e.g. California bearing ratio, elastic modulus, resilient modulus). These relationships were studied at three earthwork construction projects in Minnesota. In these field studies, intelligent compaction and in-situ test data were collected to demonstrate use of the various technologies, characterize the variation associated with each measurement system, and ultimately aid performance of regression analyses. For the pilot study at TH 64, a GIS database was created with roller data and parallel quality assurance data to demonstrate one method for managing large quantities of data. Spatial statistics were also determined using variogram modeling and discussed with regards to their potential for characterizing uniformity. A laboratory compaction study using different compaction methods (e.g. static, impact, gyratory, and vibratory) was conducted to show different moisture-density-compaction energy relationships for granular and cohesive soils. Resilient modulus test results showed that vibratory and impact compaction methods produce higher-modulus samples than static compaction. The findings from field studies of intelligent compaction systems provide the basis for developing QC/QA guidelines regarding effective and appropriate use of the technology. These recommendations, along with a brief summary of European specifications for continuous compaction control, are provided in the report.

This report describes an intelligent compaction demonstration project on Mn/DOT TH 53 in Duluth, MN, and the associated field and laboratory testing. The project was conducted during September 2005, using a Caterpillar Model CS-563E vibratory soil compactor, equipped with Intelligent Compaction (both Compaction Meter Value (CMV) and energy or power) and global positioning system (GPS) technology. A Prima light-weight deflectometer (LWD), dynamic cone penetrometer (DCP) and Humboldt GeoGauge were used to collect in situ companion test data at 42 locations. Mn/DOT conducted gradation, moisture content and Procter tests. Location and Compaction Meter Value (CMV) were downloaded for comparison with the in situ testing. CMV data was compared to the in situ data on a point-by-point basis and on the basis of the overall distribution. In general, poor correlations were obtained on a point-by-point basis, likely due to the depth and stress dependency of soil modulus, and the heterogeneity of the soils. Good correlations were obtained between CMV values and DCP measurements for depths between 8-inches and 16-inches deep. The Caterpillar Compaction Viewer software, although still in development at the time of testing, is functional and is well integrated with GPS. It is easy to extract data and do more sophisticated analyses. Surface-covering documentation adds value by identifying potential problem areas where compaction is limited by material, moisture or subgrade deficiencies. LWD testing protocol must be followed to obtain useful results, since measurements vary significantly between successive tests. Relatively good correlations were obtained between LWD and GeoGauge. The GPS technology used for the demonstration is not adequate to distinguish between lifts.

In September 2004, engineers conducted a Continuous Compaction Control (CCC) demonstration at MnROAD, an outdoor pavement test facility. Continuous Compaction Control (CCC), also called Intelligent Compaction (IC), is a new technique in the United States construction market that uses an instrumented compactor to measure soil or asphalt compaction in real time and adjusts compactive effort accordingly to control the level of compaction. This demonstration used the BOMAG Compactor and focused on Young's soil modulus as the soil parameter of interest. CCC may potentially provide substantial benefits, including improved quality due to more uniform compaction, reduced compaction costs because effort is applied only where necessary, reduced life-cycle cost due to longer pavement life, and a stronger relationship between design and construction. State departments of transportation have expressed interest in exploring this method as a way of meeting quality-assurance requirements within a tight budget environment. In general, this study found CCC to be an effective quality-control mechanism for soil compaction. However, further questions arose as a result of the study and certain variables affected the results and measurements, including moisture content and the use of different measurement tools. Further research is needed to determine the level of uniformity in using CCC and the extent of reliability in achieving target values when using this method.

Soil water retention refers to the relationship between the amount of soil water and the energy with which it is held. This relationship is important for characterizing water movement through granular materials. In this project, we generated soil moisture retention data of 18 non-recycled and 7 recycled materials used in pavement construction. The results showed that water retention of non-recycled materials was nearly similar. The major differences among the curves were in the inflection points (air entry values) and in the water contents either near saturation or at 15,300 cm of suction. Using this database, we also developed Pedo-transfer functions that can predict (1) water retention or (2) the parameters of functions that describe water retention from easily measurable properties of the pavement materials. Water retention of concrete with and without shingles was only slightly different. This is partially because shingle chips imbedded in the concrete were large. Traditionally, the influence of matric suction has not been directly considered in pavement design. The water retention data in this report will be helpful in developing resistance factors for Minnesota Flexible Pavement Design Program either through physical modeling or through statistical relationships between design criteria and the water contents.