Large Calibration Chamber Research Articles

An update on soil gas harvesting technique as a non-depleting source of 222Rn for a large volume calibration chamber for long-term exposure experiments

ObjectiveTo demonstrate the versatility of the natural soil gas harvesting technique, coupled with a semi-dynamic injection algorithm, as an efficient and reliable source for continuous 222Rn delivery to a large-volume calibration chamber across different soil conditions and climatic conditions. MethodsLong-term experiments were performed during three seasons of a year in a tropical monsoonal climatic region with high rainfall on the Southwest Coast of India. Soil gas extraction was performed using soil-gas probes, inserted to ∼ 1 m deep into the ground. Soil gas was harvested at a flow rate of 60 lpm, passed through a moisture trap, a 222Rn progeny filter, and a delay volume and pumped into a 222Rn calibration chamber of volume 22.7 m3. Continuous monitoring of 222Rn concentration in the chamber was performed using ionization chamber-based reference monitors. To achieve and maintain the desired 222Rn concentration values in the calibration chamber, the semi-dynamic injection method was adopted, in which the 222Rn concentration in the soil gas was monitored periodically, and injection into the chamber was controlled using a well-defined pumping algorithm. Three ranges of 222Rn concentration values were chosen as the target values to be maintained in the calibration chamber for long-term experiments (15 d): (i) low concentration (500 - 1,000 Bq/m3), (ii) medium concentration (1,000 - 10,000 Bq/m3), and (iii) high concentration (10,000 – 25,000 Bq/m3). ResultsThe achieved 222Rn concentration values were in good agreement with the target values, with deviations of 12%, 8% and 5% for the low, medium and high concentration exposures, respectively, during the summer season. During the monsoon season, the deviations between the target and the achieved concentration values were 12%, 10% and 5% for low, medium and high concentration exposures, respectively, and the corresponding deviations were 12%, 4%, and 5% for the winter season. These deviations are well within the cumulative uncertainty associated with the measurements. ConclusionsThe results of this study conclusively demonstrate that the soil gas harvesting method, when coupled with the semi-dynamic injection approach, is a reliable method for generating and maintaining the desired 222Rn concentration in the large volume calibration chamber during different seasons and environmental conditions.

An experimental study of Horizontal Directional Drilling (HDD) in sand in a large flexible calibration chamber

Horizontal Directional Drilling (HDD) is a trenchless technique for underground infrastructure installation that uses pressurized drilling fluid. Excessive loss of fluid to the formation can cause problems not only for the execution of HDD but also to nearby structures and must, therefore, be avoided. Hydraulic fracture is one of the possible causes and theories to predict the ultimate pressure have been developed, mostly for clays. Luger and Hergarden (1988, apud Keulen, 2001) developed a hydraulic fracture model particularly for HDD. The present study presents an experimental program carried out on a large flexible calibration chamber in sand using drilling fluid and confining pressures compatible with HDD field conditions. Results show that while local fracturing may occur, causing injection pressure to decrease for a while, permeability is still the dominating factor.

Use of tactile sensors for horizontal stress measurements on a large calibration chamber

Large flexible calibration chambers (CCs) to evaluate and develop interpretation methods for cone penetration tests (CPTs) in cohesionless soils have been used since the late 1960s. Nevertheless, only average boundary stress values are known in almost all tests already performed. Tactile pressure sensors (TPS) represent a useful tool to assess the stress distribution in an area under loading and have been applied in some geotechnical studies. This study presents the results of CPTs carried out on a large flexible CC in a very loose and a medium-dense fine quartz sand. TPS were placed on the walls of the CC, allowing the measurement of horizontal stresses and their spatial distribution in all test phases: sample formation, lateral and cavity cell filling, piston filling, sample stressing and CPT. In particular, TPS were able to measure the horizontal stresses before the cavity and lateral cell filling – that is, when no information is available about the horizontal stresses in regular tests. The measurements contribute to the discussion on the influence of the CC boundaries on the test results. The results encourage the use of TPS on a more routine basis in large flexible CC testing.

A periodic pumping technique of soil gas for 222Rn stabilization in large calibration chambers: part 2\u2014theoretical formulation and experimental validation

In an adjoining publication, we demonstrated the novel technique to harvest soil gas of natural origin as a highly efficient source of 222Rn for calibration applications in a large volume 222Rn calibration chamber. Its advantages over the use of conventional high strength 226Ra sources, such as the capability to serve as a non-depleting reservoir of 222Rn and achieve the desired concentration inside the calibration chamber within a very short time, devoid of radiation safety issues in source handling and licensing requirements from the regulatory authority, were discussed in detail. It was also demonstrated that stability in the 222Rn concentration in large calibration chambers could be achieved within ± 20% deviation from the desired value through a semi-dynamic mode of injection in which 222Rn laden air was periodically pumped to compensate for its loss due to leak and decay. The necessity of developing a theory for determining the appropriate periodicity of pumping was realized to get good temporal stability with a universally acceptable deviation of ≤ ± 10% in the 222Rn concentration. In this paper, we present a mathematical formulation to determine the injection periods (injection pump ON and OFF durations) for the semi-dynamic operation to achieve long term temporal stability in the 222Rn concentration in the chamber. These computed pumping parameters were then used to efficiently direct the injection of soil gas into the chamber. We present the mathematical formulation, and its experimental validations in a large volume calibration chamber (22 m3). With this, the temporal stability of 222Rn concentration in the chamber was achieved with a deviation of ~ 3% from the desired value.

Study of plugging of steel H piles, jacked in sand, in a calibration chamber

When predicting the bearing capacity of an H pile, the design engineer is faced with the questions of which perimeter of the pile to consider for lateral resistance, and which base area for tip resistance. These questions derive from the doubt regarding the occurrence of ‘plugging’. The phenomenon, in which soil adheres strongly to the pile, causes failure to occur not at the steel–soil interface but at an enveloping area. There is little experimental data specifically on the plugging of H piles, in which the contribution of shaft and tip loads were properly separated. Different test interpretations by researchers and designers have led to different recommendations for the design of such piles, some considering and others dismissing soil plugging. The research herein described was conducted on instrumented model H piles jacked in fine sand in a large flexible calibration chamber. The results indicate that – even in loose sand – plugging does occur in H piles close to the pile tip and that the tip resistance is an important component of the bearing capacity of such piles.

LOAD-SETTLEMENT BEHAVIOUR OF MODEL PILE GROUPS IN SAND UNDER VERTICAL LOAD

Model pile load testing is effective to study the load-settlement behaviour of pile foundations given the con­trolled environment in which the testing is done. This paper reports a testing program in a large calibration chamber involving individual piles and pile groups installed in sand samples of three different densities. Tests on both nondis­placement and driven piles are evaluated to assess the influence of the pile installation process on pile load-settlement response. A method is proposed to predict the load-settlement response of a pile group based on the response of a single pile. The method is shown to produce estimates that are in good agreement with measurements. The influence of pile group configuration, pile spacing, soil density and method of pile installation is discussed.

CPT calibration and analysis for a carbonate sand

This paper describes a project-based cone penetration test (CPT) calibration on a carbonate sand fill that was hydraulically placed and subsequently densified by vibrocompaction. The project involved the development of an artificial island constructed offshore from the United Arab Emirates for oil and gas production. Carbonate sands have crushable grains and can be significantly more compressible than silica sands, so it was determined that the semi-empirical CPT-based correlations for silica sands were not applicable and a soil-specific calibration was needed for post-densification characterisation of density, shear strength and compressibility. The CPT calibration investigation was primarily undertaken in a centrifuge, and then checked with supplementary tests in a large calibration chamber. In this paper, analysis of the calibration data follows three different threads for comparison with more typical siliceous sands: (a) relationships between CPT tip resistance, void ratio and vertical effective stress; (b) a relationship between CPT tip resistance and state parameter; and (c) an approach based on cavity expansion theory, which provides predictive capability for the CPT once soil properties have been measured in the laboratory. The test results are compared with silica sands, in terms of void ratio or the state parameter ψ.

Development of a Miniature Cone Penetrometer for Calibration Chamber Testing

AbstractCone penetration testing (CPT) has become the industry standard for in situ testing of cohesionless soils, and in particular, field liquefaction evaluation. The empirical methods for the interpretation of CPT data are either based on field data or the observation of CPT measurements in laboratory samples. In this study, a miniature cone penetrometer (with a diameter of 6 mm) is developed for understanding the response of loose to medium-dense sands. A modified triaxial cell is used for sample preparation and containment of the sample during cone penetration. The miniature cone can measure cone tip resistance, sleeve friction, and excess pore water pressure developed at the cone tip. While cone tip resistance is measured by a separate load cell, sleeve friction is obtained by subtracting cone tip resistance from a combined measurement of tip resistance and sleeve frictional force. Due to the free-draining nature of the sand tested in this study, no excess pore water pressure is developed during cone penetration. The measured data from the miniature cone are verified by comparison with CPT resistances measured in several other calibration chamber experiments on similar sands. Compared to a large calibration chamber with a standard size cone, the miniature cone allows quicker and less expensive CPT experiments in a more uniform sample.

Small strain stiffness of carbonate Kenya Sand

In this paper results of a series of tests performed in a large calibration chamber (CC), consisting in the propagation of seismic shear and compression waves in dry carbonate oolitic sand from Kenya, are presented and analysed. The tests were aimed at assessing how the state parameters affect the small strain shear and constrained moduli.The tested sand was characterised by the determination of the index properties, its mineralogy and grain crushing, the latter investigated by oedometer tests.The calibration chamber houses a cylindrical specimen 1.2m diameter and 1.5m high. Within the specimens, reconstituted by dry pluviation at relative densities of 30% and 85%, were embedded arrays of three miniaturised geophones, used alternatively as source and receiver of seismic body waves. Shear and compression waves were propagated along the vertical, horizontal and inclined planes. By assuming a cross-anisotropic medium with a vertical axis of symmetry, the seismic tests allowed the assessment of the elastic parameters.The effects of the fabric, of the stress induced anisotropy as well as the stress history on the velocity of propagated seismic body waves are quantified and described in details.Semi-empirical correlations are calibrated; they can be used in engineering practice to analyse body wave velocity profiles measured in situ and to evaluate the state parameters and anisotropy properties of carbonate sand deposits.

Application of shear wave velocity for evaluation of equivalent radius of penetrometers

As a penetrometer is pushed into saturated soft soils, excess pore water pressure affected by the disturbed zone is generated. This excess pore water pressure, which dissipates radially, is related to the probe size and shape. The probe size and shape may be represented by an equivalent radius. The aim of this paper is to evaluate the equivalent radius of a penetrometer using the horizontal coefficient of consolidation. Three penetrometers including the cone penetrometer, dilatometer, and Field Velocity Probe (FVP), are used within a large laboratory calibration chamber and in the field. For the calibration chamber tests, a clay–sand mixture is prepared and consolidated under a vertical stress of 160 kPa. These dissipation tests are conducted at a depth that is equal to half the height of the chamber. For the field tests, three penetrometers are penetrated into soft soils through a gravel layer and a sand mat layer. These dissipation tests are implemented at a depth of 24 m. The cone penetrometer, dilatometer, and FVP measure the change of the pore water pressure, the closing pressure value, and the shear wave velocity, respectively. From these six dissipation tests, the equivalent radii of the dilatometer and the FVP are obtained. In addition, an empirical equation for the calculation of the equivalent radius of the penetrometer is suggested.

Statnamic testing of model piles in a clay calibration chamber

The rate of load application during a Statnamic pile test lies between that for static and dynamic pile tests. Current analysis methods offer excellent correlation between Statnamic and static test results for coarse-grained soils. However, over-prediction of pile load capacity can be obtained for fine-grained soils. It is very difficult to explore this anomaly using full scale field tests because of natural soil variability, so a study has been carried out using a model pile installed in similar clay beds prepared in a large triaxial calibration chamber. The instrumented model pile has been loaded at varying rates by a computer controlled servo-hydraulic actuator. The tests have simulated static loading, constant rate of penetration at various rates of penetration, and Statnamic loading. Results suggest that components of the rate effect associated with increased pile penetration rates may be described by a power law relationship.

Prediction of Ground Movements due to Pile Driving in Clay

This paper evaluates theoretical predictions of ground movements caused by the installation of driven (or jacked) piles in clay. The predictions are based on an approximate analysis framework referred to as the shallow strain path method (SSPM) that simulates undrained pile penetration from the stress-free ground surface. Large strain conditions close to the pile are solved numerically, and closed-form analytical expressions are obtained from small strain approximations at points further away. These results show that, for closed-ended cylindrical piles of radius R and embedment L, the normalized displacements are functions of their dimensionless position x/L. In contrast, for a planar sheet pile or unplugged open-ended pile, the far-field soil displacements at x/L depend only on the wall thickness. The proposed analyses show favorable agreement with data from a variety of available sources including field measurements of building movements caused by installation of large pile groups; uplift of a pile caused by driving of an adjacent pile within a group; and spatial distributions of ground movements caused by installation of a single pile (both cylindrical closed-ended and sheet pile wall), including a particularly detailed set of measurements in a large laboratory calibration chamber. The comparisons show that the proposed analysis is capable of reliably predicting the deformations within the soil mass but generally underestimates the vertical heave measured at the ground surface. Further investigation suggests that this discrepancy may be related to the occurrence of radial cracks observed at the ground surface during pile installation and is consistent with tensile horizontal strains computed in the SSPM analyses.

Bearing capacity of model piles driven into dense overconsolidated sands

Model piles were driven into dense siliceous sand samples and tested in a large calibration chamber. Axial tension and compression tests were performed on open-ended pipe piles. The objective of this research was to study the effect of overconsolidation on the bearing capacity of piles driven into dense sands representative of North Sea soil conditions. Emphasis was put on points of interest for the offshore petroleum industry in particular: dense to very dense normally consolidated (NC) and overconsolidated (OC) sands, unit end bearing and unit skin friction capacities, and comparison with tip resistances from cone penetration tests. Design parameters are proposed for computing the axial bearing capacity of piles driven into dense to very dense siliceous sands. They are compared with those given in the current American Petroleum Industry's Recommended Practice 2A document. A relationship between CPT cone resistance and ultimate unit end bearing and skin friction capacities of piles is also proposed.Key words: model test, dense sand, offshore pile driving, axial capacity, end bearing, skin friction, design parameters, cone penetrometer.

Anisotropy of small strain stiffness in Ticino sand

Comprehensive tests performed in a large calibration chamber and consisting of the propagation of seismic body waves in dry Ticino silica sand are presented. By virtue of the fact that both shear and constrained compression waves have been propagated in vertical, horizontal and oblique directions, five independent constants of the cross-anisotropic elasticity model of the test have been established. The tests have also made it possible to evaluate the influence of the structural (inherent) and stress-induced anisotropy on the velocity of the propagated seismic body waves. The influence of the principal effective stresses on the velocity of these waves is also described. De nombreux essais, réalisés dans une grande chambre d'étalonnage et consistant en la propagation d'ondes sismiques dans des sables siliceux et sec de Ticino, sont présentés. Les ondes de cisaillement et de compression ayant W émises dans des directions verticales horizontales et obliques, les 5 constantes indépendantes du modéle éastique d'anisotropie croiséle testéés. Ces essais ont ainsi permis une én de Pinfluence de Panisotropie induite de la structure (naturelle) et de la contrainte sur la vitesse de propagation des ondes sismiques. L'influence des contraintes principales effectives sur la vitesse de ces ondes aussi décrite

Driven model piles tested in calcareous sand in a large calibration chamber : Parkin, A K; Yee, Y W; Tan, C P; Willoughty, D R Proc 22nd Annual Offshore Technology Conference, Houston,7–10 May, 1990 V1, 389–397. Publ Richardson: OTC, 1990

Driven model piles tested in calcareous sand in a large calibration chamber : Parkin, A K; Yee, Y W; Tan, C P; Willoughty, D R Proc 22nd Annual Offshore Technology Conference, Houston,7–10 May, 1990 V1, 389–397. Publ Richardson: OTC, 1990

Design of a large calibration chamber : Sweeney, B P; Clough, G W Geotech Test JV13, N1, March 1990, P36–44

Design of a large calibration chamber : Sweeney, B P; Clough, G W Geotech Test JV13, N1, March 1990, P36–44

Design of a Large Calibration Chamber

AbstractThe design and operation of a conceptually simple, low-cost chamber for calibration of cone penetrometers and other in situ devices is described. The chamber weighs over 5500 kg and houses a 4400-kg soil sample that is 1.5 m in diameter and height. Operations in the chamber are automated using a self-equilibrating hydraulic insertion frame, a soil pluviation system for sand placement, a vacuum unit for removing sand, and an automatic data acquisition system. The results from a series of cone penetration tests in Monterey No. 0/30 sand, using standard and miniature cones, are also discussed. These tests indicate that the chamber design was successful and also that nearly homogeneous samples could be prepared at different relative densities using interchangeable rainer plates.

Interpretation of moduli from self-boring pressuremeter tests in sand

The pressuremeter is a unique method for assessing directly the in situ shear stiffness of soils. However, the correct interpretation and application of the measured modulus must account for the relevant stress and strain level acting around the pressuremeter during the test. A method to correct the measured unload-reload shear modulus from self-bored pressuremeter tests in sands is proposed. The method has been evaluated using extensive data obtained from 47 tests performed in a large calibration chamber using pluvially-deposited silica sand and from 25 tests performed in situ in a natural deposit of relatively clean silica sand at the River Po, Italy. A consistent relationship was obtained between the corrected unload-reload shear modulus and the small strain shear modulus determined from resonant column tests and field cross-hole tests. Suggestions are given to link the measured moduli with moduli values required for geotechnical design problems. The importance of strain level, stress-strain model, yield and number of load cycles is discussed. Le pressiomètre représente une méthode unique pour évaluer directement in situ la rigidité en cisaillement des sols. L'interpretation correcte et l'application du module mesuré doivent expliquer le niveau exact des contraintes et des déformations agissant autour du pressiomètre pendant l'essai. L'article propose une méthode pour corriger le module mesuré de cisaillement déchargement-rechargement à partir d'essais au pressiomètre auto foreur dans des sables. La méthode fut etablie sur la base de données detaillées obtenues par 47 essais effectués dans une grande chambre d'étalonnage en employant du sable silicieux depo-sée en pluie et de 25 essais effectués in situ dans un dépôt naturel de sable silicieux relativement propre du Pô (Italie). Une relation constante fut obtenue entre le modèle corrigé de cisaillement déchargement-rechargement et le module bas de cisaillement de déformation déterminé à partir d'essais de colonne résonante et d'essais à trous croisés sur chantier. L'article donne des suggestions pour relier les modules mesurés aux valeurs modulaires nécessaires pour des problèmes de construction géotechniques. On discute l'importance du niveau de déformation, le modele des contraintes et des déformations, le déplacement et le nombre de cycles de chargement.

A large spectrometer calibration chamber for SURF-II

A large spectrometer calibration chamber is under construction and scheduled for installation in early autumn this year. This system will be able to accomodate large spectrometer systems to be flown on the Space Shuttle Missions. The chamber is set up so that both translational and angular motions can be achieved to calibrate the optical system over its entire area. The control and calibration of the system will be under computer control.