Sample Disturbance, Numerical Model and Experiments

Sample disturbance, numerical model and experiments: S. J. M. Van Eekelen &0026; P. Van den Berg, in: Geotechnical engineering; emerging trends in design and practice, ed K.R. Saxena, (Balkema), 1994, pp 1–32

It is typically challenging to model numerically soil-structure interaction problems in sandy soils with high accuracy. Difficulties include employing competent but practical soil constitutive models and dealing with issues associated with sample disturbance. In this paper, the Nor-Sand model is shown to be a promising candidate capable of overcoming these problems. The PISA pile tests performed at Dunkirk were selected as a case study. The ground conditions were reproduced by the Nor-sand model, and selected PISA lateral pile tests were back-analysed. A parametric study was performed by using both the parameters derived from laboratory tests on reconstituted samples and from in situ testing that captured the sand deposits long-term fabric and ageing effects. Recommendations are provided for achieving representative sand characterisation. It is emphasised that for numerical modelling, the most important steps are careful characterisation and parameterisation of the soil model, rather than the numerical modelling per se.

Current deep excavation applications, which pose risks for constructing high-rise buildings and infrastructures, are increasing. Therefore, the increasing urbanization, underground infrastructure requirements, and time and cost constraints in construction projects have led to a growing demand for rapid, economical, and safe deep excavation designs. Although numerical modeling tools enable rapid analyses, the reliability of soil engineering parameters remains a challenge due to natural variability, sample disturbances, and differences between laboratory and field test conditions. In this study, PLAXIS 2D (Version 24) was used to model a deep excavation, allowing for the assessment of soil–structure interaction and excavation-induced deformations. The objectives are to compare field data with the numerical model and identify which soil parameters are critical for excavation. Through the sensitivity analysis, the study highlighted that the variations in shear strength parameters, such as cohesion and internal friction angle, are crucial and shall be precisely determined. The performed analyses revealed that even minor changes in the internal friction angle can dramatically impact displacements by doubling them and highlight the significant disparity between the minimum and maximum margins. The numerical analysis underscores the need for precise parameter measurement and careful analysis to achieve reliable results and ensure safer, more effective designs. The comparison of numerical results with field measurements confirmed the model’s accuracy.

This paper critically compares the use of laboratory tests against in situ tests combined with numerical seepage modeling to determine the hydraulic conductivity of natural soil deposits. Laboratory determination of hydraulic conductivity used the constant head permeability and oedometer tests on undisturbed Shelby tube and block soil samples. The auger hole method and Guelph permeameter tests were performed in the field. Groundwater table elevations in natural soil deposits with different hydraulic conductivity values were predicted using finite element seepage modeling and compared with field measurements to assess the various test results. Hydraulic conductivity values obtained by the auger hole method provide predictions that best match the groundwater table’s observed location at the field site. This observation indicates that hydraulic conductivity determined by the in situ test represents the actual conditions in the field better than that determined in a laboratory setting. The differences between the laboratory and in situ hydraulic conductivity values can be attributed to factors such as sample disturbance, soil anisotropy, fissures and cracks, and soil structure in addition to the conceptual and procedural differences in testing methods and effects of sample size.

Investigation on the disturbance of the bedding information of lunar soil samples in flexible tube coring missions

The California bearing ratio (CBR) test evaluates the structure of the layers of pavements. Such a test is laborious, time-consuming, and its results are generally affected by sample disturbance and tests conditions. The main objective of this research was to build a numerical model for the prediction of CBR tests that might substitute laboratory tests. The model was based on structural and physical parameters of granular bases. Four different materials from the central region (Querétaro) and north (Mexicali) of Mexico were used for the experimental work. Using the above-mentioned materials, 36 samples were fabricated, and six of them were used for the evaluation of the model presented in this research. Numerical and experimental comparisons demonstrated the adequacy of the model to predict the result of CBR tests from soil parameters.

Predicting behavior of soil in the field in response to construction activities and natural events is the reason why so much effort in the past 50 years has been put into developing constitutive models. It is the field responses of soils that are of interest to geotechnical practitioners, and this aspect of the problem has not received nearly enough attention. There are many logical reasons for this, e.g., the need for establishing behavior under well-defined boundary conditions, the difficulty of obtaining high quality samples, the effects of sample disturbance, the effort required to obtain field performance data of sufficient quality and quantity such that it can be correlated with a causative construction activity, to name a few. The author and his colleagues have collected detailed field performance data from a number of supported excavations in the Chicago area and have correlated the responses with construction activities. As part of the effort, block samples have been cut as the excavations have proceeded. Detailed laboratory evaluations of the stress-strain strength responses have been conducted on specimens from these blocks and from thin-wall tube samples. Inverse analyses have been conducted on results of finite element simulations of the excavations using the field observations as a basis of comparison so that best fit parameters for several constitutive models were determined based on the field data. This paper summarizes the techniques used and describes the lessons learned from this effort. Comments are made regarding applicability of laboratory data, the relation between the capabilities of the constitutive model and selection of observations for inverse analyses and numerical modeling.

With the advancement of computational geomechanics over the past decades, most of the activities typical of site characterization, such as in situ testing or sampling, can be realistically simulated using appropriate constitutive models and balance equations for geomaterials. These numerical simulations are not only informative of the processes that take place during testing but can be used to assess the reliability of current practice empirical interpretation techniques or even propose new interpretation techniques. In this paper, we present two numerical analyses of long-standing geotechnical problems in which new insights are gained by means of advanced numerical modelling. The first analysis corresponds to cone penetration testing in undrained, brittle geomaterials; we describe the effect of the constitutive parameters on cone metrics, and we propose a novel procedure to estimate the initial state parameter from CPTu based on a wide-ranging parametric analysis. The second analysis involves tube sampling in undrained geomaterials. A total stress analysis allows us to describe the kinematics of the soil during tube insertion and evaluate the effect of the tube geometry on the strain path of the problem. The analysis is then extended by considering a fully coupled hydromechanical formulation and a critical state constitutive model for cemented soils; by simulating conventional laboratory tests on the soil that has entered the tube we can quantify sampling disturbance in terms of geotechnical design parameters (e.g. undrained shear strength and yield stress). The possibilities offered by numerical modelling for site characterization are far from being fully exploited. It is envisaged that in the future site-specific numerical analyses will become available, enabling a more comprehensive understanding of the subsurface conditions.

Characterization of the near-surface gas-phase chemical environment in atmospheric-pressure plasma chemical vapor deposition of diamond

This paper focuses on ground investigation by deepwater cone penetration tests (CPT). All current offshore CPT systems deploy subtraction-type cone penetrometers. Their use now includes record-breaking deepwater applications. This paper presents results of geotechnical field and laboratory measurements that explore the operational limits of deepwater cone penetration tests in very soft soils. The review of field repeatability studies shows the feasibility of excellent performance of the subtraction penetrometer. The results of specific laboratory experiments indicate a relatively small influence of deepwater ambient pressures on its performance. A further consideration for deepwater applications is the zeroc-orrection to hydrostatic conditions prior to the start of a test. Seabed-based CPT systems allow zero-correction with an uncertainty approaching the resolution of the CPT system. Downhole CPT systems latch into the lower end of a drill pipe. The pressure conditions in the drill pipe may not be in full equilibrium with the surrounding ground water pressure and zero-correction will be subject to increased uncertainty. The main points are:currently available evidence indicates that a high-quality subtraction-type cone penetrometer is adequate for very soft soil characterisation to a water depth of 3000 metres and probably beyond,Class 1 accuracy results (according to ISSMGE) in very soft soil are likely to be feasible for deepwater seabed systems anddeepwater downhole systems are less accurate and will probably give Class 2 results. Introduction The drive for resource development of deepwater regions provides great challenges to the engineering profession. This also applies to deepwater geotechnical investigation practice. Operational limits, very soft soils and integration of the disciplines of oceanography, geology, geophysics and ground investigation are some of the specific challenges (Kolk and Campbell, 1997). The cone penetration test (CPT) has become the most widely used in-situ testing technique for offshore geotechnical investigations. Detection capabilities for distinct and gradual changes in stratigraphy are generally excellent. There now exists a comprehensive theoretical and empirical database world-wide (Lunne et al., 1997) that provides a high level of confidence in correlations between CPT cone resistance and laboratory measurements of undrained shear strength and other geotechnical parameters. Recent results of large-strain numerical modeling of cone penetration testing in undrained soil implicitly support the common empirical correlations (Fugro, 1998). This observation implies that the CPT has the ability to achieve a high level of accuracy, where accuracy is defined as "the closeness of the agreement between the result of a measurement and the true value of the measurand" (ISO, 1992). The use of the CPT in a deepwater environment is significant, as the role of CPTs in providing parameter values for analytical design models becomes increasingly important. Conventionally, laboratory testing provides the reference strengths. Laboratory testing of very soft soils requires highquality samples. This is an important consideration for deepwater sites, as the effects of sample disturbance increase with an increase in pressure release upon sampling. This is leading to a greater reliance on the results of in-situ tests.

Based on a review of numerical models it would appear that debate is still open as to which combination of volumetric and shear strains is responsible for destructuration of intact soils. Direct experimental evidence is rather limited and mostly related to sedimentary clays. A saprolitic soil from Hong Kong that had been block sampled was tested to investigate the effects of sample disturbance and more in general of destructuration. The amount of destructuration was quantified by measuring the elastic shear stiffness by means of bender elements mounted axially in triaxial cells. The tests were designed to investigate separately the effects of volumetric and shear strains, by way of loading cycles reaching increasingly larger strains. The loading cycles were carried out under isotropic conditions and at constant mean effective stress for the volumetric and shear strains, respectively. In addition, the results obtained were compared with those from a test on a reconstituted specimen that was carried out in a similar fashion. In general, a reduction in shear stiffness was observed for the intact specimens, while a slight increase was measured for the reconstituted specimen at the same strain levels. The same held true also after normalising for void ratio, which was necessary as this changed as a result of straining hence having an effect on the shear stiffness. The reduction in stiffness observed for the intact specimens, which indicates loss of structure, was observed for both test types. However, at a given strain threshold, the reduction was larger for the specimen that underwent cycles of volumetric strains.