Soil mechanics

The most commonly used laboratory test in Geo-engineering research and in routine testing is the cylindrical compression test. This test is widely used to evaluate the stiffness and strength parameters of soils and other geologic materials. At the Minnesota Department of Transportation a MTS cyclic loading triaxial system is used for testing various types of subgrades and pavement materials under cyclic impulsive loading that corresponds to traffic conditions. Following some discussions between members of the Mn DOT Research Group and the University of Minnesota Soil Mechanics Group, it was decided that some improvements and modifications of the original triaxial set-up are feasible The second part of this report is devoted to the presentation of the modifications and improvements of the triaxial apparatus that have been done in due course of the research period. In the third part the improved testing capabilities of the Mn DOT triaxial apparatus are demonstrated on the basis of one conventional and two unconventional experiments done on fat clay and on dry sand specimens, respectively.

An earth pressure cell (EPC) is a device designed to provide an estimate of normal stress in soil. The practice of designing and manufacturing stress measurement devices revolves around the study of the interaction between the measuring device – the earth pressure cell – and the host material. However, distribution of normal stress is not necessarily uniform across a given surface. Consequently, output from an EPC may be different under soil loading conditions than under fluid pressure. In addition, depending upon the design, as the cell deflects, an arching-type phenomenon may develop. The objectives of this study were to devise a scheme for calibration of earth pressure cells and to recommend a procedure for field installation. A new testing device was designed to permit the application of uniaxial soil pressure to the earth pressure cell using various types of soil and load configurations. Sensitivities computed from soil calibrations varied from those determined from fluid calibrations by as much as 30%. A field installation procedure was developed from model tests. In the laboratory, a thin-walled steel cylinder with a geotextile bottom was filled with uniform silica sand in a medium dense state and the earth pressure cell was placed within the sand. The entire apparatus (earth pressure cell, cylinder, and sand) was carried into the field and installed in the desired locations. Once in place, the steel cylinder was pulled up out of the ground, leaving the cell, sand, and geotextile behind. Preliminary field data indicate that the soil calibration and placement procedure provide reasonably accurate measurements of the change in vertical stress. Content Note: This is the author’s version of a work that was accepted for publication in the Transportation Research Record: Journal of the Transportation Research Board, Issue Number: 1772, Publisher: Transportation Research Board ISSN: 0361-1981. The final version can be found at https://doi.org/10.3141/1772-02.

The Minnesota Road Research Project (Mn/ROAD) has completed time domain reflectometry measurements of soil moisture since 1993. But questions about interpretation about the data remain, because of soil characteristics at the site, the unusual design of waveguides, and long transmission lines that are known to degrade TOR system performance. This study's objectives included developing a procedure to recover moisture content data measured in situ with TOR since 1993 at the Mn/ROAD site. Researchers calibrated the relationship between TOR system response and soil water content and characterized the influences of unusual waveguide design, high soil bulk density, high soil clay content, cable length, and soil temperature. Cable lengths greater than 33 m caused errors in the TOR calibration relationship. In addition, soil temperature had a small effect on TDR measurements. Researchers developed correction equations to correct TOR measurements of water content for long cable lengths and soil temperature. Past TDR moisture measurements taken at the Mn/ROAD site must be interpreted using this report's calibration equation and cable length/soil temperature corrections. Additionally, present and future interpretations of soil moisture using the existing TDR system at the Mn/ROAD site must use these equations. This report is unpublished. 15 copies were produced and distributed.

This Implementation Summary pertains to Report 2018-32, “Cone Penetration Test Design Guide for State Geotechnical Engineers,” published November 2018.

This Technical Summary pertains to the LRRB-produced Report 2015-31, “Minnesota Steel Culvert Pipe Service-Life Map,” published June 2015.

This Implementation Summary pertains to Report 2016-26, “Development of a MnDOT Foundation Boring Mobile Application Gateway, GeoApp,” published August 2016.

For a road to perform well over the long term, its subgrade and base layers need to provide a stable, uniform foundation for the pavement. This is achieved by compacting underlying layers (typically with a roller) and performing inspections to ensure that compaction is sufficient.

For a road to perform well over the long term, its subgrade and base layers need to be compacted to provide a stable, uniform foundation for the upper pavement. Intelligent compaction is a promising technology for improving the uniformity and quality of soil compaction: IC allows for real-time measurements of the subgrade and the automatic adjustment of the force of compaction to soil conditions.

Phase 4 Slope Vulnerability Assessments is a continuation of Phases 1-3 previously conducted by WSB and MnDOT to determine the risk of slope failure along state highways. This phase includes 16 counties located in districts 1 and 3 and the Metro. The three main components of the model are 1) identify past slope failures, 2) model the causative factors of past failures and how they vary locally, and 3) model the risk of new slope failures. The vulnerability factors and expected failure types reflect the diverse geomorphology of Northeast Minnesota. Vulnerability factors for the new study area include slope angle, curvature, relief, slope orientation (aspect), and water table depth. The preliminary model developed prior to the field visit was underestimating slope vulnerability and was not effectively capturing all historical failure types. WSB determined the solution was to re-design the historical failures model. The model was refined and led to an improved vulnerability output. Model results were ranked into four proposed risk-management categories: action recommended, further evaluation, monitoring, and no action recommended. The risk estimation process was considered preliminary; further consideration of risk tolerance and consequence definitions should be conducted. Preliminary risk results indicated that 499 management areas and 1.4% of the total area was categorized as action recommended under the proposed risk matrix. The results of this study were intended to be the first step of actions required in minimizing slope failure effects including expensive mitigation and maintenance repairs and threats to public safety.

Phase 3 Slope Vulnerability Assessments is a continuation of Phases 1 and 2 previously conducted by WSB and MnDOT to determine the risk of slope failure along state highways. This phase includes 27 counties located in MnDOT districts 1; 2; 3; and 4. The three main components of the model are 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 angle; terrain curvature; and water table depth. Field verification validates the model's capability of identifying risk in regions with different geology; geomorphology; and hydrology including deep-seated slides. Model results were ranked into four proposed risk management categories: action recommended; further evaluation; monitoring; and no action recommended. 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 370 management areas; and 3% of the total area falls into the action recommended category under the proposed risk matrix. The results of this study are intended to be the first step of actions required in minimizing the effects of slope failure including expensive mitigation and maintenance repairs and threats to public safety.