James K. Mitchell

Energy-Based Evaluation and Remediation of Liquefiable Soils

The state-of-practice for performing remedial ground densification and evaluating earthquake liquefaction potential of loose saturated sands have evolved relatively independent of each other. This is in spite of the fact that the induction of liquefaction is typically requisite for remedial ground densification of sands. Simple calculations are presented herein for estimating the mechanical energy required to densify a unit volume of clean, loose sand using deep dynamic compaction, vibro-compaction, and explosive compaction. These computer energies are compared with that required to induce liquefaction during an earthquake using the Green-Mitchell energy based liquefaction evaluation procedure. The comparison highlights the importance of the efficiency of the method in which the energy is imparted to the soil and the importance of the mode of dissipation of the imparted energy (e.g., possible modes of energy dissipation/expenditure include: breaking down of initial soil structure, ramming soil particles into denser packing, and radiating away from the treatment zone). Additionally, the comparison lays the preliminary groundwork for incorporating the vast knowledge base gained from fundamental studies on earthquake induced liquefaction into the design procedures of remedial ground densification techniques.

Influence of colloidal silica grout on liquefaction potential and cyclic undrained behavior of loose sand

Cyclic triaxial tests were performed to investigate the influence of colloidal silica grout on the deformation properties of saturated loose sand. Distinctly different deformation properties were observed between grouted and ungrouted samples. Untreated samples developed very little axial strain prior to the onset of liquefaction. However, once liquefaction was triggered, large strains occurred rapidly and the samples collapsed within a few additional loading cycles. In contrast, grouted sand samples experienced very little strain during cyclic loading. Additionally, the strain accumulated uniformly throughout loading rather than rapidly prior to collapse and the samples never collapsed. Cyclic triaxial tests were done on samples stabilized with colloidal silica at concentrations of 5, 10, 15, and 20%. In general, samples stabilized with higher concentrations of colloidal silica experienced very little strain during cyclic loading. Sands stabilized with lower concentrations tolerated cyclic loading well, but experienced slightly more strain. Thus, treatment with colloidal silica grout significantly increased the deformation resistance of loose sand to cyclic loading.

Coseismic deformation of the 2002 Denali Fault earthquake: Insights from GPS measurements

[1] We estimate coseismic displacements from the 2002 M w 7.9 Denali Fault earthquake at 232 GPS sites in Alaska and Canada. Displacements along a N-S profile crossing the fault indicate right-lateral slip on a near-vertical fault with a significant component of vertical motion, north-side up. We invert both GPS displacements and geologic surface offsets for slip on a three-dimensional (3-D) fault model in an elastic half-space. We restrict the motion to right-lateral slip and north-side-up dip slip. Allowing for oblique slip along the Denali and Totschunda faults improves the model fit to the GPS data by about 30%. We see mostly right-lateral strike-slip motion on the Denali and Totschunda faults, but in a few areas we see a significant component of dip slip. The slip model shows increasing slip from west to east along the Denali Fault, with four localized higher-slip patches, three near the Trans-Alaska pipeline crossing and a large slip patch corresponding to a M w 7.5 subevent about 40 Ion west of the Denali-Totschunda junction. Slip of 1-3 m was estimated along the Totschunda Fault with the majority of slip being at shallower than 9 km depth. We have limited resolution on the Susitna Glacier Fault, but the estimated slip along the fault is consistent with a M w 7.2 thrust subevent. Total estimated moment in the Denali Fault earthquake is equivalent to M w 7.89. The estimated slip distribution along the surface is in very good agreement with geological surface offsets, but we find that surface offsets measured on glaciers are biased toward lower values.

Temperature Distributions Around Buried Cables

Methods for analysis of temperature distributions around buried power cables are reviewed. None accurately models the true geometric conditions or properties or enables prediction of the time- dependence of temperature change.

Apollo 11 soil mechanics investigation

The fine-grained surface material at the Apollo 11 landing site is a brownish, medium-gray, slightly cohesive granular soil, with bulky grains in the silt-to-fine-sand range, having a specific gravity of 3.1 and exhibiting adhesive characteristics. Within the upper few centimeters, the lunar soil has an average density of about 1.6 grams per cubic centimeter and is similar in appearance and behavior to the soils studied at the Surveyor equatorial landing sites. Althouglh considerably different in composition and in range of particle shapes, it is similar in its mechanical behavior to terrestrial soils of the same grain size distribution.

TESTING OF REINFORCED SLOPES IN A GEOTECHNICAL CENTRIFUGE

An evaluation of the use of centrifuge modeling as a tool for analyzing the behavior of reinforced soil slopes is presented in this paper. A review of the state-of-the-art indicates that previous centrifuge studies have focused mainly on the performance of reinforced soil vertical walls and that limit equilibrium approaches (used in the design of reinforced soil slopes) have not been fully validated against the failure of models in a centrifuge. As part of an evaluation of the conditions of similarity governing the behavior of reinforced soil structures at failure, scaling laws are specifically derived by assuming the validity of limit equilibrium. It is demonstrated that an Nthscale reinforced slope model should be built using planar reinforcements having 1/N the strength of the prototype reinforcements in order to satisfy similarity requirements. A description of the experimental testing procedures implemented as part of a recent centrifuge testing program is presented, and an example dataset from this investigation is used to illustrate typical results. These include the g-level at failure, visual observation of failure development, and post-failure analysis of reinforcement breakage. The pattern observed in the geotextile reinforcements retrieved after testing indicates that the boundary effects were negligible.

Acoustic penetrometer for subsoil investigation

A quasi-static cone penetrometer for subsoil investigation by simultaneously generating three sets of data, namely cone tip penetration resistance, sleeve friction resistance, and acoustical information, all as a function of depth. The penetrometer has a substantially smooth cylindrical outer surface terminating in a cone tip. The lower portion of the smooth cylindrical outer surface is provided by a friction sleeve immediately above the cone tip insulated from it by acoustic attenuation means. A microphone in the tip (or elsewhere in the penetrometer) is responsive to acoustical input generated, for example, by the tip moving through the soil, and sound barrier means holds said microphone firmly in place. The sound barrier means, the acoustic attenuation means, and the acoustical dampening means substantially isolate the microphone from the core ring and from the friction sleeve. A tip load cell is joined to the cone tip and insulated from it acoustically by acoustical-dampening means. A friction load cell has its upper end connected to the tip load cell and the remainder spaced away from it, while its lower end is secured to the friction sleeve.

Geotechnical Surprises—Or Are They? : The 2004 H. Bolton Seed Lecture

The nature and scope of geotechnical engineering are such that the problems and projects we deal with are a never ending source of both challenge and excitement. In most cases we do very well in defining the problems and developing suitable solutions resulting in successful completed projects. Unfortunately, however, there remain far too many cases in which something goes wrong. Often a failure or other bad outcome comes as a complete surprise. However, in a significant number of cases perhaps the unexpected or surprise outcome might have been anticipated. Three illustrative case histories are reviewed and then examined to see if what went wrong might reasonably have been anticipated. The first involves a well-known and much studied stability failure along the composite double liner system of a hazardous waste landfill. The second is concerned with unusual soil types that were the cause of major difficulties during earthwork construction of a large embankment dam. The third is about a very large, slow-moving landslide that caused major distress to a roadway and impacted the safety of a large bridge. Several reasons why unexpected adverse outcomes may occur are stated and some possible means for reducing their frequency and severity in the future are proposed.

Rapid Chemical Stabilization of Soft Clay Soils

Since World War II, the military has sought methods for rapid stabilization of weak soils for support of its missions worldwide. Over the past 60 years, cement and lime have been the most effective stabilizers for road and airfield applications, although many nontraditional stabilizers also have been developed and used. The most effective stabilizer to increase the strength of two soft clay soils within 72 h for contingency airfields to support C-17 and C-130 aircraft traffic needed to be determined. The treatment of one clay with cement resulted in relatively high unconfined compressive strengths (UCS), whereas treating the same clay with quicklime and calcium carbide resulted in lower UCS. The treatment of another clay with higher plasticity resulted in similar UCS for cement, quicklime, and calcium carbide. Secondary stabilizers, including sodium silicate, superabsorbent polymers, a superplasticizer, and an accelerator, were ineffective in increasing the UCS of a soil treated with cement, quicklime, or calcium carbide.

FIELD TESTING OF CABLE BACKFILL SYSTEMS

ABSTRACT Full scale field tests of ten simulated buried cables in different backfill materials are being carried out for the purpose of evaluating backfill treatments for the development and maintenance of low backfill thermal resistivity, comparison of measured and predicted temperature distributions, and the study of moisture movement under thermal gradients. Some results obtained so far are presented and interpreted, and the design of three new test sections for study of cable size effects is described.