Nicholas J. Yafrate

Evaluation of Undrained Shear Strength Using Full-Flow Penetrometers

Full-flow penetrometers (the T-bar and ball) are increasingly used on sites with thick deposits of soft clays, particularly prevalent offshore. Full-flow penetration tests were performed at five international well-characterized soft clay test sites to assess the use of full-flow penetrometers to estimate undrained shear strength. Field vane shear data were used as the reference undrained strength. Statistical analyses of strength factors indicates that full-flow penetrometers provide an estimate of undrained shear strength at a similar level of reliability compared to the piezocone. Relationships for estimating the strength factor and soil sensitivity using only full-flow penetrometer data obtained during initial penetration and extraction are developed. A strong dependence of the strength factor on sensitivity was identified and can be used for the estimation of undrained strength. The effectiveness and use of the developed correlations are demonstrated through their application at an additional test site.

Evaluation of Remolded Shear Strength and Sensitivity of Soft Clay Using Full-Flow Penetrometers

The undrained remolded shear strength of soft clays is of importance in geosystem design, particularly for offshore structures. Common methods to estimate remolded shear strength, such as correlations with cone penetration data, direct measurement with an in situ field vane shear device, and laboratory measurements, produce varied results and can be particularly costly and time consuming. Full-flow penetrometers (T-bar and Ball) provide an alternative rapid method to estimate remolded shear strength and soil sensitivity through remolding soil by repeated cycling of the penetrometer up and down over a given depth interval. The cyclic penetration resistance degradation curve inherently contains information regarding remolded strength and sensitivity. The objective of this paper is to assess the ability of full-flow penetrometers to predict remolded strength and soil sensitivity, and to develop a suite of predictive correlations in which these properties can be estimated in the absence of complementary laboratory or in situ test data. To accomplish this, full-flow penetration profiles and cyclic tests were performed at five well characterized soft clay sites, which together represent the broad range of soils in which the penetrometers will be often used. A previously developed model for the reduction in penetration resistance with cycling is modified to predict the entire degradation curve, including the remolded penetration resistance using only measurements obtained during initial penetrometer penetration and extraction. Using field vane shear strength as the reference measurement, correlations are developed to predict soil sensitivity and remolded shear strength based solely on full-flow penetrometer data, which is particularly useful in site investigation programs where site specific data are not yet available or are sparse. Finally, the usefulness of these relationships is demonstrated by implementing them for two additional soft clay sites.

Recommended Practice for Full-Flow Penetrometer Testing and Analysis

The increasing use of full-flow penetrometers for estimating the undrained and remolded shear strength as well as soil sensitivity of soft sediments by both industry and researchers has resulted in a rather rapid maturation of this new in situ test method over the past decade. Experimental, analytical, and numerical analysis results for full-flow penetrometers are now sufficient to provide recommended practices regarding equipment design, testing procedures, and data analysis. Equipment design must consider both physical attributes of the penetrometers and electronic design and data acquisition. The testing procedures presented are modified from standard test methods for the piezocone with additions for evaluation of remolded strength by cycling and rate effects through variable penetration rate testing. Data reduction includes methods for normalizing the penetration resistance data for comparison between test sites and depths and methods for estimating undrained and remolded shear strength as well as soil sensitivity. All recommendations are summarized in a guidance table.

Detection of Stratigraphic Interfaces and Thin Layering Using a Miniature Piezoprobe

Assessment of the depth location of stratigraphic interfaces and the depth and thickness of thin layers can be critical to the proper performance of geosystems. For example, stratigraphic interfaces can promote anisotropic soil strength response and potentially provide preferential slip planes that create slope instability. Similarly, the presence of thin, high permeability layers can alter groundwater flow regimes and rates of consolidation, which can hinder or accelerate methods of ground improvement. A miniature piezoprobe (MPP) with an apex filter location and a measurement frequency of 64 Hz was recently developed in an effort to detect and characterize these stratigraphic features. The piezoprobe’s functionality is through the measured excess pore pressure profile, which reflect s changes in the drainage conditions, and therefore soil co nditions. This paper evaluates the piezoprobe’s ability relative to the conventional piezocone penetrometer (CPTu) to characterize distinct interfaces and thin layers through the analysis of soundings performed at two highly characterized test sites. One test site consists of a near normally consolidated clay layer with distinct interfaces with the overlying backfill and interbedded layers of sand and silt below. The second site consists of varved clay, which is comprised of a repeating structure of thin pairs of silt and clay layers. Results demonstrate that the piezocone detects distinct interfaces at a higher resolution than the CPT u and that thin layers may go undetected by the CPTu.

Design of a miniature piezoprobe for high resolution stratigraphic profiling

The stratigraphic features present in all natural deposits, which reflect their deposition environment, can significantly influence the response of these deposits to engineered construction. This paper presents the development of a miniature piezoprobe optimized for the continuous detection of stratigraphic features within a varved soil deposit where individual layers are less than one centimetre in thickness. The study uses previous research to identify parameters in the design of a field-deployable miniature piezoprobe and to present a framework that enables stratigraphic detection from pore pressure measurements. Laboratory experiments on the ability of fluids to saturate the filter and the influence of the fluid-filter system on piezoprobe sensitivity reveal that low to moderate viscosity immiscible fluids (e.g., 100 cS silicone oil) provide optimal performance considering practical implementation aspects. Laboratory and field studies that examine the soil deformation characteristics within the vicinity of such probes indicate that an apex filter location is preferable for stratigraphic detection and that soil deformations around the probe are primarily limited to less than one probe radii. Proof of concept results from an in situ profile obtained with the miniature piezoprobe compares well with measurements from a continuous core sample from a Connecticut Valley varved clay deposit.