J. David Frost

A MULTISLEEVE FRICTION ATTACHMENT FOR THE CONE PENETROMETER

Accurate knowledge of the strength of soil-geomaterial interfaces is becoming of increasing importance in geotechnical engineering. Systems whose performance is heavily dependent on soil-geomaterial interfaces include deep foundations, synthetic impervious liners, trenchless technologies, and an assortment of earth retaining structures. The strength of the interface is typically estimated by applying adjustment factors to values of soil or interface strength measured in laboratory tests. These adjustment factors are intended to correct for differences between the test and anticipated operating conditions such as variations in soil type and density, strain rate, surface roughness, or confining stress and are often empirically based with little theoretical underpinnings. Of these adjustment factors, the surface roughness is considered to be of utmost importance in that it has the potential to alter the interface strength by 100% or more. This paper describes the development of a new multisleeve Friction attachment for the cone penetrometer that allows for direct in situ measurement of the relationship between interface strength and surface ronghness. As discussed herein, the ability of the attachment to quantify this relationship, in conjunction with additional ongoing research, provides the opportunity to improve the design of friction dependent systems, as well as site characterization. A key characteristic of the penetrometer attachment is the ability to obtain four individual sleeve friction (fs) measurements at each elevation within a sounding, in addition to the conventional Cone Penetration Test (CPT) fs measurement. This allows for direct in situ analysis of the effects of sleeve roughness on the fs measurement. Considerations pertinent to the development of the device including assessments of the conventional CPT fs measurement and soil geomaterial interface mechanisms are first presented. A description of the new penetrometer attachment including key characteristics and capabilities follows. Finally, validation of the operation of the device through laboratory and field tests is described, and future applications of the attachment are discussed.

Interface Behavior of Granular Soils

Interface shear zones between particulate materials and continuum elements of man-made or natural materials have not been traditionally considered as shear bands. However, the results of recent micro-scale experimentation and numerical simulations have shown that they are indeed well developed partial shear bands. Further, these studies have indicated that significant similarities can be identified between these partial shear bands and what are more traditionally considered to be shear bands that evolve wholly within particulate materials. This paper presents results from physical and numerical experiments that suggest there is significant merit to this opinion. The physical experiments include quantitative analysis of the particle deformation following shear adjacent to continuum surfaces of different roughness as well as analyses of sand specimens sheared to different global strain levels. Complimentary 2-D Discrete Element Method modeling of particulate-continuum surface interfaces illustrates this parallel behavior.

Seismic Performance of Earth Structures during the February 2010 Maule, Chile, Earthquake: Dams, Levees, Tailings Dams, and Retaining Walls

The 27 February 2010 Maule, Chile, earthquake occurred during the driest time of the year, which implied that most of the soils in the slopes were not saturated and that the dams had extra freeboard. This may explain the small number of slope failures caused by the earthquake. However, two important earth dams suffered seismically induced permanent ground movements, but no catastrophic damage was reported because the reservoirs levels were low. Five medium-sized mine tailings dams failed due to liquefaction; one of them tragically caused four casualties. Retaining structures of all types performed well and no failures were observed.

Shear Banding and Microstructure Evolution in 2D Numerical Experiments

A limitation of using laboratory experiments to study the micromechanics of soils is that detailed information about the specimen microstructure is typically available at only one state in a test sequence due to the destructive nature of the forensic process. To study microstructure evolution, characterization of replicate specimens tested to various global axial strain levels has been undertaken, but this procedure presents some practical and theoretical problems. Accordingly, a numerical program was undertaken using the discrete element method to model the micromechanical deformation response of particulate assemblies in two dimensions. The simulated assemblies failed via regions of high localized strain. The microstructures of these assemblies were studied as a function of global axial strain to assess evolution of local and mesoscale void ratio distributions and mean free paths. Local void ratio distributions were modeled statistically and mesoscale measurements were used to assess microstructure inside and outside of the shear bands.

Experimental Study of Shear Zones Formed at Sand/Steel Interfaces in Axial and Torsional Axisymmetric Tests

The interface shear behavior of granular materials is central to many engineering applications, including the performance of structures like deep foundations, landfills, and retaining walls. Consequently, it is paramount to understand the behavior of construction material-soil interfaces involved in these applications. Furthermore, it has been shown that the study of interface behavior, in the laboratory and in-situ, can provide robust information about the soils properties and engineering performance. This paper presented laboratory evaluations of micro and meso-scale shear deformation of medium-sized sands aimed at developing an improved fundamental understanding of granular-continuum stress-strain behavior. A comparison of interface testing results from two different shear directions—axial and torsional—demonstrated that the evolution and progression of shear zone formation was affected differently by changes in the interface surface roughness and particle angularity. In particular, it was observed that torsional shear is a more dilative process that induces a larger degree of soil shearing and is greatly affected by particle angularity. Studies of shear-induced volume changes also revealed that the influence zone for torsional shearing is larger than that for axial shearing, with soil dilation occurring inside the shear zone in contact with the material counterface and soil contraction in a surrounding outer zone. Fundamental micromechanical processes that aim to explain the differences between the behavior of axial and torsional tests are proposed.

Size effects on the void ratio of loosely packed binary particle mixtures

Studies of binary particle mixtures provide useful insight into the effects of fine particles on the void ratio of natural multi-sized geomaterials. The relative amount of fine particles in a mixture significantly changes the void structure and influences the behavior of such materials. This paper presents complimentary experimental evaluations and numerical simulations that show how void ratios change non-linearly as additional fine particles are included in binary mixtures. Experimental results for a range of particle size ratios are presented. The paper also demonstrates that there are particular percentages of fine particles by weight at which the lowest values of void ratio are achieved. Loosely packed binary mixtures are simulated by a gravitational sphere packing method to further examine the effect of different weight percentages of fine particles on the void structure. The numerical studies are based on Monte-Carlo simulations wherein spherical particles are randomly packed. The complex pore structure obtained by random packing prevents any pre-defined or repetitive packing arrangements, which can lead to the computation of non- representative void ratio values. Results obtained from the numerical simulations are compared with experimental results and confirm the viability of the gravitational sphere packing method to efficiently reproduce realistic packed soil particle systems.

A Multi Piezo Friction Attachment for Penetration Testing

A new in situ testing device has been developed, the Multi Piezo Friction Attachment (MPFA), to allow for the direct characterization of geotechnical interface behavior in situ within the context of an effective stress framework. The MPFA simultaneously provides four independent measures of interface response (fa1, fa2, fa3, and fa4) and five independent measures of dynamic pore pressure along the shaft (ua0, ua1, ua2, ua3, and ua4) in addition to conventional cone penetrometer measurements (qt, fs, and u2). This paper summarizes the rational behind the development of an in situ test device to directly characterize interface strength in situ while accounting for the influence of penetration and shear induced pore pressures and varying the geomaterial counterface roughness. An overview of the device specifications is presented along with selected results from field tests demonstrating the capabilities of the MPFA device across a broad range of in situ testing applications.

Axisymmetric Shearing of Sand-Steel Interfaces Under Axial and Torsional Loading

Interfaces of soils with man-made materials play a major role in geotechnical systems, from deep and shallow foundations to landfills and retaining walls. Accordingly, it is of paramount importance to understand the behavior of interfaces involving different construction and natural materials. For soil-man made material surface interfaces, a linear increase in strength occurs with increasing surface roughness until the interface shear strength reaches that of the soil. Any subsequent increases in surface roughness result in no additional change of observed system interface strength. In this paper a series of laboratory axisymmetric interface shear tests involving axial and torsional loading are presented. This study involves interface shear tests between Cone Penetration Test (CPT) friction sleeves with different roughnesses and different sands. CPT sleeves with surface roughness ranging from that of the conventional smooth sleeve to those with texture elements of 1 mm in height are tested with sub-rounded and sub-angular medium sized sands. This laboratory study is part of an effort to improve soil characterization and pile design by means of the CPT that utilizes multi-sleeve modules. The next generation system expands the use of friction sleeves with different roughnesses to include response measurement under both axial and torsional loading.

Apparatus for Geosynthetic Interface Testing and Evaluation Under Elevated Temperature Conditions

Temperature is one of many important environmental variables that can impact the long-term performance, strength, and deformation characteristics of many man-made construction materials, including geosynthetics. The functional engineering properties of these materials must remain within acceptable limits during their service life to ensure that the overall design and performance are acceptable. In the case of geosynthetics used in landfills and other applications, laboratory interface shear tests are performed under standard test conditions, including temperature. Information emerging today shows that geosynthetic interfaces (i.e., in landfill liner applications) experience elevated temperatures resulting from exothermic reactions occurring in the waste body, amongst other factors. To this end, the field conditions at elevated temperatures should also be simulated in the laboratory during physical/mechanical laboratory tests in order for researchers to better understand in situ functional engineering properties and operational performance of manmade geo-construction materials. For this purpose, a temperature-controlled chamber was designed and developed to allow the shear behavior of geosynthetic–geosynthetic and soil–geosynthetic interfaces to be evaluated at different temperatures. This paper describes both the development and the validation of the test system. The results of experimental investigations are presented to illustrate how the shear behavior of interfaces between nonwoven polypropylene geotextile and smooth and/or textured high density polyethylene geomembrane, as well as those between rounded and/or angular sand and geomembranes, change with temperature. The results provide insight into the importance of being able to independently control this variable during mechanical testing in the laboratory.

Axial-Torsional Multi-Sleeve Friction Penetration System for Lunar Subsurface Studies

The multi-sleeve friction penetration system is an in-situ testing device that is derived from the well established cone penetration test. It incorporates a series of friction sleeves with varying surface texture in addition to the standard smooth friction sleeve located directly beneath the tip. The multiple measurements made with this device allow it to provide new insight into soil type and stratigraphic variations as well as insitu shear strengths as a function of sleeve texture height. This paper describes a next generation version of this device that incorporates torsional load sensing capabilities in addition to the standard axial load sensing capabilities. In this manner, the effects of different vertical and horizontal stress states on measured sleeve stresses can be explored. This device offers significant benefits as an alternative to the portable bevameter, which was used in the past to measure the mechanical response of lunar soils under normal and shear loading conditions that were likely to be imposed by various wheel and track systems.