Jerry D. Higgins

Comparison of Chilled-mirror Measurements and Filter Paper Estimates of Total Soil Suction

A comparison of chilled-mirror total suction measurements with those estimated from filter paper total suction data show general agreement. However, there are significant discrepancies that are similar to those previously reported by others. The nature and magnitude of the discrepancies are consistent with: (a) possible errors in chilled-mirror total suction measurements due to incomplete equilibration in the sealed test chamber of the chilled-mirror device, and (b) possible errors in estimated filter paper total suction values due to natural variations of the zero-water-content intercept in the log total suction versus water content relationship. The possible errors in chilled-mirror measurements are easily avoided. The possible errors in estimated filter paper measurements can only be minimized by avoiding the need for an assumed zero-water-content intercept. This can be accomplished far more easily with the chilled-mirror device than with the filter paper method.

Methods for Predicting Shale Durability in the Field

Durability of shales and other weak rock is an important parameter describing the materials susceptibility to breakdown upon exposure to water or during construction work. While laboratory methods have been developed to measure durability, no correlations between laboratory results and simple field tests have been determined. The results of this study show that certain field parameters can predict slake durability with acceptable accuracy. The recommended field tests, in order of desirability, are slake index, jar slake, hammer rebound, and NGI “Q” value. Single variable and multiple variable linear regression equations are given, including equations conservatively modified so that 95% of the slake durability values are underpredicted.

Relationships between size and velocity for particles within debris flows

Estimation of the impact forces from boulders within a debris flow is important for the design of structural mitigation elements. Boulder impact force equations are most sensitive to the inputs of particle size and particle velocity. Current guidelines recommend that a design boulder should have a size equal to the depth of flow and a velocity equal to that of the flow. This study used video analysis software to investigate the velocities of different sized particles within debris flows. Particle velocity generally decreased with increasing particle size, but the rate of decrease was found to be dependent on the abilities of particles to rearrange within debris flows.

Pore-water extraction from unsaturated tuff by triaxial and one-dimensional compression methods, Nevada Test Site, Nevada

The hydrologic system in the unsaturated tuff at Yucca Mountain, Nevada, is being evaluated for the US Department of Energy by the Yucca Mountain Project Branch of the US Geological Survey as a potential site for a high-level radioactive-waste repository. Part of this investigation includes a hydrochemical study that is being made to assess characteristics of the hydrologic system such as: traveltime, direction of flow, recharge and source relations, and types and magnitudes of chemical reactions in the unsaturated tuff. In addition, this hydrochemical information will be used in the study of the dispersive and corrosive effects of unsaturated-zone water on the radioactive-waste storage canisters. This report describes the design and validation of laboratory experimental procedures for extracting representative samples of uncontaminated pore water from welded and nonwelded, unsaturated tuffs from the Nevada Test Site.

Preparing Geologists for Careers in Engineering Geology and Hydrogeology

Many classically- educated geologists have found employment as engineering geologists or hydrogeologists, gaining the technical knowledge and skills they need through experience and self-education. Ideally, these individuals would learn the necessary subjects in their undergraduate classes and have a shorter learning curve on the job. There are many degree programs that prepare students specifically for these fields, but students may also be prepared within a standard geology program, with some modifications. The purpose of this paper is to suggest ways of preparing students to enter work or graduate studies in engineering geology or hydrogeology through the standard geology curriculum. Critical technical skills, such as use of the Unified Soil Classification System or air photo interpretation, must be taught as additional topics in existing classes. Problem-solving and analytical thinking skills can be taught through a variety of exercises that enhance the geology curriculum without adding new topics, including in-class discussion questions, homework and laboratory problems, and add-ons to mapping and semester projects. Other educational experiences to prepare undergraduates for careers in engineering geology and hydrogeology include interaction with professional associations, internships and co-ops, and professional registration.

Prediction of piezometric surfaces and drain spacing for horizontal drain design

Horizontal drains, used independently or as part of a more complex remediation scheme, are frequently installed to mitigate the effects of increased groundwater in slope stabilization projects. Due to a general trial and error approach to their design, the need for improved design practices has been recognized. The procedures established by Crenshaw and Santi in 2004 made some advances in this direction, but did not account for slopes with drains that were not horizontal or for sloping low-permeability layers underneath the slide mass. Furthermore, the method outlined by Crenshaw and Santi is time-consuming and requires some trial and error calculations to achieve convergence. Therefore, the method has been modified to account for nonhorizontal elements, and a horizontal drain spreadsheet has been developed to streamline the design for projects where horizontal drains will be installed. The horizontal drain spreadsheet may be used to: (1) predict a conservative piezometric profile in a drained slope for use in slope stability analyses, (2) predict piezometric heads in any single piezometer in a drainage field, and (3) predict drain spacing for design purposes. This document explains the revisions to Crenshaw and Santi’s procedures and provides instructions for applying the method. The instructions may be used for hand calculations, but are specifically intended for use with the horizontal drain spreadsheet. The spreadsheet may be used for slopes composed of silty or clayey sands, silts, and silty or sandy clays.

GEOTECHNICAL CHARACTERIZATION OF THE STEEPLY DIPPING PIERRE SHALE

This study compares alternative approaches for differentiating and characterizing the swelling potential of steeply dipping strata in the Pierre Shale along the Colorado Front Range Urban Corridor. The hazard associated with this geologic setting is commonly referred to as heaving bedrock because adjacent strata expand to different degrees and may cause damaging differential deformation to residential and light commercial structures. This study is part of a larger investigation aimed at determining the feasibility of using reflectance spectroscopy, a remote sending technology, to identify and characterize the swelling potential of expansive soils and bedrock. A trench in the Upper Pierre Shale was logged and 252 samples were taken horizontally with thin-walled brass tubes. Grain size, Atterberg Limits, filter paper suction, and clod tests provided, for each of the samples, the swell potential indices and rankings according to schemes proposed by Seed, Chen, and McKeen. Swell-consolidation tests provided percent swell pressure indices under in-situ surcharge pressures. All the indices are consistent in that they suggest that trench wall can be differentiated into two zones of steeply dipping strata overlain by a horizontal and shallow overburden soil. However, the schemes differ in that McKeens indices vary over a significantly broader range of categories compared with the Seed and Chen indices. McKeens indices and also the swell-consolidation indices have two advantages compared with Seeds and Chens indices. They are obtained on relatively undisturbed materials and they are properties involved in quantitative heave prediction models. These advantages suggest that McKeens scheme is providing the most sensitive and useful basis for differentiating semi-quantitative categories of swelling potential at this site.