Don J. DeGroot

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.

Structural Numbers for Reclaimed Asphalt Pavement Base and Subbase Course Mixes

The 1993 AASHTO pavement design guide equation for flexible pavement structures requires the determination of a structural number, which is a function of the layer coefficient, thickness, and drainage coefficient of each layer of pavement. Empirical correlations for layer coefficients have been developed based on resilient moduli. The principal factor affecting the drainage coefficient is the hydraulic conductivity. The objective was to determine how the layer and drainage coefficients of Massachusetts Highway Department (MHD) base and subbase aggregates would be affected by the introduction of reclaimed asphalt pavement (RAP). MHD gray, dense, graded, crushed-stone base and gravel-borrow subbase were combined with 0 to 50 percent RAP (by weight); 9 mixtures were tested, and 27 resilient modulus and 12 hydraulic conductivity tests were conducted. Results from the resilient modulus tests indicate that the resilient moduli of the RAP/base and RAP/subbase mixtures generally increased with an increase in the percentage of RAP. Therefore, the layer coefficient and the structural number increased. The hydraulic conductivity of the RAP/crushed-stone base was not affected by the percentage of RAP in the mixture, but the hydraulic conductivity of the RAP/gravel-borrow subbase increased with an increase in the percentage of RAP. Nonlinear elastic analyses of typical MHD pavement structures indicated a small increase in the structural number with an increase in the percentage of RAP in the base and subbase. Therefore, the addition of RAP to the base and subbase materials tested is considered to be beneficial.

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.

Measurement of Suction in a Marine Clay as an Indicator of Sample Disturbance

This paper describes a test program that investigated the potential of residual porewater pressure, or suction, to be a quick and nondestructive measure of sample disturbance. A portable 35 mm diameter suction probe, consisting of a high air -entry porous ceramic stone and pressure transducer capable of direct suction measurements on soil samples, was developed for this study. The suction probe was used at a test site located in Newbury, Massachusetts where a layer of low to medium plasticity Boston Blue Clay (BBC) exists. Six Sherbrooke block samples and several 76 mm split spoon samples were collected. Conso lidation tests were conducted on all samples to assess sample quality based on volumetric strain methods. The high quality Sherbrooke block samples had much higher suctions than the poor quality split spoon samples. The results suggest that the suction p robe could be a practical tool for making assessments of sample quality in the field immediately after sample collection and during selection for laboratory consolidation and strength testing.

The Multidirectional Direct Simple Shear Apparatus

The paper describes a new simple shear testing device, the multidirectional direct simple shear (MDSS) apparatus, for testing soil specimens under conditions that simulate, at the element level, the state of stress acting within the foundation soil of an offshore Arctic gravity structure. The MDSS uses a circular specimen that is consolidated under both a vertical effective stress (σ′vc) and a horizontal shear stress (τ1). The specimen is subsequently sheared undrained by applying a second independent horizontal shear stress (τ2) at an angle ϑ relative to the horizontal consolidation shear stress τ1. Evaluation of the MDSS first compares conventional K0-consolidated undrained direct simple shear (CK0UDSS) test data (τ1 = 0) on normally consolidated Boston blue clay (BBC) with results obtained in the Geonor DSS device. The MDSS gives lower secant Youngs modulus values and on average 8% lower strengths, but produces remarkably less scatter in the test results than the Geonor DSS. Kinematic proof tests with an elastic material (rubber) confirm that the setup procedure, application of forces, and strain measurement systems in the MDSS work properly and produce repeatable results. Results from a MDSS test program on BBC wherein specimens were first normally consolidated with σ′vc and τ1 = 0.2σ′vc and then sheared undrained at ϑ varing in 30° increments from zero (shear in same direction) to 150° show dramatic differences in the response of the soil as a function of θ. The peak undrained strength varies almost twofold from θ = 0 to 120°, while the deformation behavior varies from very brittle at low θ angles to becoming ductile at higher angles. The experimental results indicate that dramatic changes in foundation response at different θ angles will be an important design issue.

Slug Tests in the Presence of Background Head Trends

We extend Bouwer and Rice (1976) slug test theory to incorporate background head trends that may be important in incompressible material of low permeability k. The extension, which features a convolution integral of the background head, is closed form for linear trends. A sensitivity study suggests that a rising background head can diminish the head changes associated with a slug-out test and underestimate k if it is ignored, as does falling background trend with a slug-in test. A falling background head can reinforce slug-in test head change and, if ignored, can overestimate k, as does a rising background head with a slug-out test. The simple extension is verified by field tests in glacial till and stratified drift deposits in eastern Massachusetts.

Sensitivity of the Dynamic Response of Monopile-Supported Offshore Wind Turbines to Structural and Foundation Damping

The prediction of ultimate and fatigue demands for the design of offshore wind turbines (OWTs) requires accurate simulation of the dynamic response of OWTs subject to time-varying wind and wave loads. The magnitude of damping in an OWT system significantly influences the dynamic response, however, some sources of damping, such as foundation damping, are not explicitly considered in design guidelines and may increase damping significantly compared to commonly assumed values in design. Experimental and analytical studies have estimated the magnitude of foundation damping to be between 0.17% and 1.5% of critical, and this paper investigates how increased damping within this range affects load maxima and fatigue damage for a hypothetical 5MW OWT subjected to a variety of wind, wave, and operational conditions. The paper shows that increased damping effects the greatest percentage reduction of ultimate moment demands and fatigue damage when the OWT rotor is parked and feathered. In such cases, the aerodynamic damping is relatively low, allowing for additional damping from the foundation to account for a relatively larger proportion of the total system damping. Incorporating foundation damping in design guidelines may lead to more efficient structures, which is a crucial factor in overcoming the high cost barrier associated with offshore wind development.

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.

Measurement of Small Strain Shear Modulus Anisotropy on Unconfined Clay Samples Using Bender Elements

This paper presents results of an investigation of the potential to evaluate small stain shear modulus (Gmax) anisotropy for unconfined clay samples using a simple test procedure. Vertical and horizontal shear wave velocities (Vvh, Vhv and Vhh) and residual isotropic effective stress were measured on saturated samples of Boston Blue Clay, Australian Burswood Clay, and Norwegian Onsoy Clay using portable piezoceramic bender element equipment and a suction probe. The measured data, which show significant Gmax anisotropy for the Boston Blue Clay and Onsoy Clay sites, are evaluated and compared with Gmax anisotropy of clays from the literature.