bluuebunny/crossref_metadata_2025 huggingface dataset:
The stability of shallow tunnels and underground openings in cohesive material
Authors: Davis E. H. , Gunn M. J. , Mair R. J. , Seneviratine H. N.
Published: October 2009

A basic engineering decision to be made in designing a tunnel in soft clay is whether or not the tunnel can be excavated without internal support. The safety of constructing a shallow heading in soft clay can be assessed in terms of the fluid support pressure which may be required to maintain stability. This can be estimated by means of the lower and upper bound theorems of plasticity. The Paper considers three different shapes of shallow underground opening relevant to tunnelling and upper and lower bound stability solutions are derived for collapse under undrained conditions. Solutions are also derived for assessment of the risk of blow-out failure caused by excessively high fluid pressures. Conditions are considered under which local collapse can occur, independent of the cover above the tunnel.

Une décision fondamentale doit être prise lorsque l'on veut réaliser un tunnel dans de l'argile molle; il s'agit, en effet, de décider si un support interne sera ou non nécessaire pour l'excavation du tunnel. La sécurité de la construction d'une galerie peu profonde dans de l'argile molle peut être évaluée en fonction de la pression du fluide support qui pourrait être nécessaire au maintien de la stabilité. Cette estimation peut se faire à l'aide des théorèmes à limites inférieure et supérieure. L'article envisage trois formes différentes d'ouverture souterraine peu profonde intéressant le creusement de tunnels, et des solutions de stabilité à limites inférieure et supérieure sont trouvées pour l'affaissement dans des conditions non drainées. Des solutions sont également trouvées en ce qui concerne l'évaluation du risque de rupture par éruption due aux pressions excessives du fluide. Les conditions dans lesquelles un affaissement local peut se produire sont envisagées, indépendamment de la couverture située audessus du tunnel.
Behaviour of a propped embedded retaining wall in stiff clay at Bell Common Tunnel
Authors: Tedd P. , Chard B. M. , Charles J. A. , Symons I. F.
Published: October 2009

Extensive instrumentation has been installed at Bell Common Tunnel to monitor the performance of a propped embedded retaining wall and the behaviour of the stiff clay in which it has been constructed. Earth and porewater pressures, ground and wall movements, and roof loading have been monitored during the construction period. The deformations and stresses measured at each stage of construction are presented and the performance of the wall is discussed.

Un système important d'instrumentations a été installé au tunnel de Bell Common pour contrôler le comportement d'un mur de soutènement encastré et celui de l'argile raide dans laquelle il a été construit. Pendant la période de construction on a contrôlé les pressions du sol et de l'eau interstitielle, les mouvements du sol et du mur et le chargement du mur. L'article presente les déformations et les contraintes mesurées à chaque étape de la construction et analyse le comportement du mur.
An experimental and theoretical comparison between static and dynamic torsional soil tests
Authors: Bolton M. D. , Wilson J. M. R.
Published: October 2009

The dynamic properties of dry Leighton Buzzard sand have been investigated using a resonant column test apparatus. These data are compared with very low frequency cyclic tests on identical specimens of sand. The comparison indicates that the properties of dry sand are independent of frequency. A simple one-dimensional model of kinematic hardening plasticity is used to predict the dynamic behaviour of the sand. The input parameters for this model are based on the results of static tests. These may be conducted on standard laboratory equipment with only minor modifications. The predictions are in good agreement with the measured data. In particular, the frequency response close to resonance was correctly shown to be asymmetric and the reduction in the resonant frequency with increasing strain amplitude was also correctly represented. The practical consequences of these findings are assessed with regard to soil testing procedures and dynamic analyses of foundations. Particular emphasis is placed on the dynamic analyses of the foundations for vibrating machinery and offshore gravity structures.

Lea propriétés du sable set de Leighton Buzzard ont été étudiées a l'aide d'un appareil de laboratoire employant une colonne de résonance. Ces données sont comparées avec des essais cycliques de fréquence trés basse effectués sur des échantillons identiques du sable. La comparaison indique que les propriétés dn sable set ne dépendent pas de la fréquence. Un modéle trés simple unidimensionnel de la plasticité de durcissement cinématique a été employé pour prédire le comportement dynamique du sable, les parametres d'entrée pour ce modèle étant basés sur les réultats d'essais statiques, qui peuvent s'effectuer sur des appareils normaux de laboratoire avec des modifications de faible importance. Les prédictions s'accordent s bien aux donées mesurées. En particulier la réponse de fréquence asymétrique prés de la résonance et la réduction de la fréquence de résonance avec des amplitudes croissantes de déformation ont été rep résentées de façon exacte. Les conséquences pratiques de cette étude sont examinées en fonction des procedures pour tester le sol aussi hien que des analyses dynamiques des fondations. On examine plus particuliérement les fondations pour les machines oscillantes et pour les structures gravitationelles au large.
Design aspects and permeability testing of natural clay and sand-ben tonite liners
Authors: Van ree C.C.D.F. , Weststrate F.A. , Meskers C.G. , Bremmer C.N.
Published: October 2009

A methodology has been developed to evaluate the permeability performance of natural clay and sand-bentonite liners using undisturbed field samples of the actual liner. The tools necessary to apply it to the design and inspection of new liners are described. Test results are presented and the influence of the testing procedures is described. The overall performance of liner systems are only partially characterized by their permeability. Other factors, including physico-chemical processes, are equally or even more important, and have to be considered in the design phase. To demonstrate this simulation results on contaminant transport taking geochemical reactions into account as well as some results of leachate compatibility testing of a sandbentonite liner are presented.

Une méthodologie a été développée pour évaluer les performances en perméabilité des revêtements d'argile naturelle et de sable/bentonite en utilisant des échantillons de chantier du revêtement même. L'article décrit les instruments nécessaires pour son application à la construction et au contrôle des nouveaux revêtements. Des résultats expérimentaux sont présentés et l'influence exercée par les essais est précisée. Les performances générales des systèmes de revêtement ne sont caractérisées qu'en partie par leur perméabilité. D'autres facteurs, compris des phénomènes physico-chimiques, ont une importance égale ou même supérieure et doivent être pris en considération dans l'établissement des projets. Pour illustrer cette simulation l'article présente des résultats concernant la migration des polluants tenant compte des réactions géochimiques et aussi des résultats obtenus au cours d'essais effectués pour étudier la compatibilité des lessivages dans le cas d'un revêtement de sable-bentonite.
Interaction behaviour of steel grid reinforcements in a clayey sand
Authors: Bergado D. T. , Shivashankar R. , Alfaro M. C. , Chai Jin-Chun , Balasubramaniam A. S.
Published: October 2009

The interaction behaviour between steel grid reinforcements and a clayey sand has been studied in laboratory and field pullout tests. The clayey sand is potentially useful as cheap, low quality, locally available and cohesive-frictional backfill in the construction of mechanically stabilized earth walls and embankments, especially in coastal areas. The laboratory tests were conducted under undrained conditions at three compaction moisture conditions. The field tests were conducted on dummy reinforcements embedded at different elevations in a full-scale test embankment resting on soft clay foundation. The laboratory tests revealed that the moisture content of the compacted soil, compaction stress, applied normal stress level, diameter and spacing to diameter ratios of the transverse members of the steel grid, all affect the soil-reinforcement interaction, and thereby, also the magnitudes of the pullout resistances. Interferences between the bearing transverse members of the grid were found to be less significant for spacing to diameter ratios above 75. The laboratory tests, in general, gave conservative values of the pullout resistances as compared with the field tests. The proposed prediction equations agreed well with the experimental data.

L'interaction entre les renforcements par treillis métalliques et les sables argileux a été étudiée à l'aide d'essais de traction en laboratoire et sur chantier. Le sable argileux est utilisé pour construire des murs en terre mécaniquement stabilisés et des remblais, dans les zones littorales notamment. Ce type de matériau est en effet un matériau de remplissage cohérent-frottant bon marché, de qualité médiocre et disponible localement. Les essais de laboratoire ont été menés, dans des conditions non-drainées, avec trois teneursen eau de compactage. Les essais de chantier ont été réalisés sur des renforcements ancrés à différentes profondeurs dans un remblai grandeur nature reposant sur des argiles molles. Les essais de laboratoire ont montré que la teneur en eau du sol compacté, la contrainte de compaction, le niveau de contrainte normale, le diamètre et les rapports espacement-diamètre des composants transversaux du treillis métallique jouent sur les interactions renforcement-sol, et par conséquent, sur la grandeur de la résistance à la traction. Les interférences entre les éléments porteurs transversaux du treillis semblent moins importantes pour des rapports espacement-diamètre supérieurs à 75. Les essais de laboratoire donnent, en général, des valeurs de résistance en traction plus reproductibles que celles obtenues dans les essais de chantier. Les équations proposées pour la prédiction du comportement sont en bon accord avec les données expérimentales.
Experimental study of damage in granite
Authors: Su K. , Ghoreychi M. , Chanchole S.
Published: October 2009

The results presented here on damage of a granite from the centre of France are obtained using new experimental devices and a specific methodology in which the relation between strain and stress is obtained at different stages of damage using local strain measurements under high pressure. Mechanical tests have been undertaken under the load paths such as deviatoric compression and isotropic compression. Thus, at different stages of damage, the evolutions of elastic and plastic characteristics are obtained. Variations of Young's modulus, damage criterion and strength are presented as functions of irreversible volumetric strain considered as an internal damage variable. Induced anisotropy of mechanical properties clearly appears under isotropic compression loading. For example, the ratio of lateral strain over axial strain can reach 2·5 when the irreversible volumetric strain is equal to 1%.
The effect of the development of undrained pore pressure on the efficiency of compaction grouting
Authors: Kovacevic N. , Potts D. M. , Vaughan P. R.
Published: October 2009

Compaction grouting involves the injection of stiff grout that does not penetrate the ground. Bulbs of grout are formed. It is applied to free-draining granular soils. The aim is to increase the density of the soil being treated by applying high local pressure. The design of compaction grouting is based largely on experience and empiricism. If the soil is not sufficiently permeable for consolidation to occur as it is treated, excess pore pressures may be generated, which will dissipate after treatment. The potential effect of such excess pore pressures on the compaction achieved is considered here. It is found that the efficiency of treatment may be reduced substantially.
Drainage and reinforcement of soft clay tank foundation by sand columns
Authors: Al-Khafaji Z. A. , Craig W. H.
Published: October 2009

A series of centrifuge model tests, at acceleration levels of 105 g, is reported, in which loosely placed vertical sand columns and densified sand columns have been used to improve the performance of oil storage tank foundations. A simulated tank of 34 m diameter, with a flexible base, has been subjected to incremental fluid loadings typically to 160 kPa, with foundation drainage permitted. Model preparation included the installation of as many as 572 sand columns in a single foundation. Comparison is made between the experimental results and the analysis of Priebe, which indicates that that analysis will overestimate the improvements if three-dimensional considerations are not taken into account. Comments are also included as to the extent to which model column formation may be deficient.
Analysis of wave-induced liquefaction of sand beds
Authors: Sassa S. , Sekiguchi H.
Published: October 2009

This paper describes finite element analyses of wave-induced liquefaction of sand beds. The authors were motivated by previous experimental results in which a series of centrifuge wave tank tests were performed on loosely packed fresh deposits of sand with viscous scaling introduced. The principal finding was that the resistance to liquefaction under progressive waves was considerably smaller than the resistance exhibited under standing waves. An important factor for this marked difference was suggested to be the rotation of the principal stress axes that occurs under progressive wave loading. The present study extends an existing cyclic-plasticity constitutive model to account for the effect of stress axis rotation of sands. This model was incorporated into a finite element analysis procedure, which was applied to soil responses to progressive wave and standing wave loading. The predicted results compare well with observed soil behaviour. The predicted effective stress paths and associated stress–strain curves for the representative soil element are discussed, illustrating how the effects of rotations of the principal stress axes manifest themselves in the course of liquefaction under progressive wave loading. The prediction of the propagation of the liquefied zone is also discussed as a subject for future study.
Reduction of equipment-generated damping in resonant column measurements
Authors: Meng J. , Rix G. J.
Published: October 2009

Equipment-generated damping in the resonant column (RC) test due to interaction between the magnets and solenoids in the drive system causes a bias error in measured values of material damping ratio. Calibration and post-measurement corrections have been used to address this problem. In this study, an electromechanical model of the RC test is developed to aid in understanding the cause of equipment-generated damping and to identify possible real-time solutions. The model explicitly includes the signal source, which plays a key role in determining the amount of equipment-generated damping in the system. Furthermore, the model indicates that a current-mode source with large internal resistance rather than a more commonly used voltage-mode source is capable of reducing equipment-generated damping significantly in real time. A current-mode source is implemented using a voltage-to-current converter. The effectiveness of this approach is demonstrated using RC tests on aluminium, polymethyl methacrylate (PMMA), polyvinyl chloride (PVC), and soil specimens.
Parameters that influence the pressure filtration characteristics of bentonite grouts
Authors: BEZUIJEN A. , SANDERS M.P.M. , DEN HAMER D.
Published: October 2009

The pressure filtration of grout, pressurised against fine sand, has been investigated in a laboratory set-up. The grout mixtures that were tested consisted of water and bentonite, together with cement, fly ash, silica flour and/or retarder. Samples were subjected to a pressure of 100–400 kPa. The permeability of the grout cake formed during pressure filtration was calculated from the results. It appears that the permeability of the grout can differ by more than three orders of magnitude, depending on the grout sample tested. Adding cement increases the permeability. The grain size of the material in the mixture had minimal influence. Mechanisms are discussed.
Basic creep of unsealed hardened cement paste at temperatures between 20°C and 725°C
Authors: Dias W. P. S. , Khoury G. A. , Sullivan P. J. E.
Published: October 2009

Synopsis

A definition for basic creep at elevated temperatures is arrived at, based on the time at which load is applied. Experimental results are presented for the basic creep of hardened cement paste and comparisons are made with the behaviour of various concretes. The time function of basic creep is shown to follow a power law relationship, whereas the temperature function indicates that a rheological property, seated in the cement paste itself, would limit the structural usefulness of Portland cement-based concretes to below 600°C. Preheating to temperatures higher than the creep test temperature reduces the creep potential in general.
Carbonation of concrete containing pfa
Authors: Hobbs D. W.
Published: October 2009

Synopsis

A limited investigation of the influence of 35% replacement of one OPC by two pfa's upon rate and depth of carbonation, strength development and water retention of concrete cubes is described. It is shown that both water retention and strength development of OPC/pfa concretes may be more adversely affected by short-term curing than those of OPC concretes and that the depth of carbonation in concretes of a similar water/binder ratio or similar binder content can be substantially higher in OPC/pfa concretes than in OPC concretes. The depth of carbonation of the OPC/pfa concretes tested after 3 years is similar or slightly higher than in the OPC concretes of similar 28 day compressive strength.
A galvanic sensor for monitoring corrosion of steel in carbonated concrete
Authors: Short N. R. , Page C. L. , Glass G. K.
Published: October 2009

The construction and operation are described of a galvanic sensor which responds to changes in the corrosion intensity of mild steel in carbonated mortars or concretes. The sensor is simple to construct and operate; it is robust and has potential for development as an inexpensive method of long–term continuous nondestructive corrosion monitoring of steel in concrete. Its possible applications include laboratory and field studies of effects of environmental exposure conditions and cement matrix variables on corrosion rates of steel in reinforced concrete structures. Information derived from such investigations is needed to provide an improved basis for service–life prediction.
Compressive strength of concrete at high temperatures: a reassessment
Authors: Khoury G. A.
Published: October 2009

Based on experience with siliceous aggregate/OPC paste concrete it is generally believed that the compressive strength of unsealed ‘concrete’ declines sharply above 300°C. This is too pessimistic a view. A reassessment of the subject is given in this Paper, which considers material and environmental factors/mechanisms influencing the strength of concrete during the heat cycle and after cooling, not all of which necessarily result in strength loss. Design of concrete for better performance at high temperatures should aim at minimizing contributions to strength loss, while exploiting the processes responsible for gain in strength. It appears that, in its hydraulic state of binding, a rheological criterion limits the structural usefulness of Portland cement concrete to temperatures of 600°C. Today, many commonly used concretes lose considerable strength at temperatures above about 300°C. There is, therefore, scope for improvement in design within the temperature range 300— 600°C. Raising the ‘working’ temperature of the material means that a significantly larger proportion of a structure exposed to high temperatures will remain serviceable and reparable, thus bringing about significant economic benefits.
Development of patch test to study behaviour of shallow concrete patch repairs
Authors: Austin S. A. , Robins P. J.
Published: October 2009

The work reported is concerned with developing an understanding of the behaviour of shallow concrete patch repairs and methods of assessing the bond between cementitious repair materials and a concrete substrate. A new patch repair test specimen is described which may be loaded in a variety of configurations that model typical structural elements containing a patch repair. In particular, the specimen loads the repair material indirectly, as would occur in practice; this is in contrast with other bond tests, such as the slant shear and core pull-off, with which the patch test is compared in a subsequent paper. Two sandkement and two proprietary polymer-modified repair mortars were investigated. Following initial development work on the specimen geometry and failure modes in compression, the effects of surface roughness, moisture condition, repair stifiess and partial debonding were examined. The results conjrmed the sensitivity of the test specimen to changes in bond quality and highlighted the influence of relative material stifiess on patch repair behaviour. There was also good agreement with more complex tests in which the repair was applied to a pre-compressed specimen which was subsequently released to put the repair in tension.
Push-off shear strength with inadequately anchored interface reinforcement
Authors: Baldwin M. I. , Clark L. A.
Published: October 2009

An essential requirement for composite construction, consisting of precast and in situ concrete elements, is that the two component parts act together as a single unit under all load conditions. It is unclear how, or even this is achieved when the reinforcement crossing an interfacial shear plane is inadequately anchored. In order to provide experimental data, 63 single-bar push-off shear tests have been performed with anchorage lengths less than those currently required. The anchorage lengths ranged from 2·5 to 20 bar diameters. Two bar types and two covers were tested. The results indicate thathe interface shear strength increases as the bond length increases. This has been attributed to an increase in the concrete-to-concrete friction, brought about by an increase in the pressure normal to the interface. In order to predict the shear strength a modified shearfriction equation is proposed, where the coefficient of friction is a function of the anchorage length of the reinforcement crossing the interface. This approach is also used to predict the residual strength.found to remain after failure. Comparisons are also made with predictions from the UK concrete bridge assessment code (BD44195) and itis shown that this code is conservative.
Trials of grade 100 high–strength concrete
Authors: Wang S.-D. , Read A. S.
Published: October 2009

This paper describes the site trials for a grade 100 high–strength concrete project conducted in Hong Kong. The data presented include cube strength, core strength, the effect of different methods for core end preparation and in situ temperature monitoring. The results have shown that a concrete having a water–to–binder ratio of 0·22 and a binder content of 700 kg/m3 can achieve an average in situ cube strength of 116 MPa and a core strength of 105 MPa at 28 days. The cube strengths have a standard deviation of 8 MPa, giving a characteristic strength of 102 MPa. The peak in situ temperature can be controlled to below 85°C with appropriate cooling measures. Significantly, this work has confirmed the notion that high–strength concrete is more robust against elevated–temperature curing and found that high–strength concrete may have a higher core/cube ratio than conventional concrete.
Quantification of the influence of cracks in concrete structures on carbonation and chloride penetration
Authors: De Schutter G.
Published: October 2009

In order to study the influence of existing cracks in concrete structures on the carbonation and chloride penetration, small–scale experiments were carried out on mortar prisms. Six different mortar compositions were investigated, stored in five different environmental conditions. At regular times, the carbonation and chloride penetration of cracked and uncracked specimens were examined experimentally. On the basis of the experimental results, a tentative formula is established, quantifying the influence of the cracks as a function of crack width and crack length. The statistical variation of the influence factor can be modelled by means of a log–normal distribution.
Early age strength and workability of slag pastes activated by sodium silicates
Authors: Collins F. , Sanjayan J. G.
Published: October 2009

This article reports the results of an investigation on the activation of blast furnace slag with emphasis on the achievement of equivalent one-day strength to Portland cement at normal curing temperatures and reasonable workability. The effects of varying dosages of sodium silicate activators are discussed in terms of strength of mini cylinders and also workability by the mini slump method. The results are mainly based on pastes but comparisons are also made with mortar and concrete results. The effects of preblended gypsum dosage within the slag, as well as the effect of ultra fine slag on workability are reported. The results of trials with various water reducing admixtures and superplastisers and their effects on strength and workability are reported.
Analysis of shrinkage-induced stresses in concrete pavements
Authors: Zhang J. , Leng B.
Published: October 2009

In this paper, the effect of pavement material properties as well as slab–base interfacial characteristics on the shrinkage-induced stresses in concrete pavements is investigated by application of an analytical shrinkage stress model. The model is based on the interfacial friction stress transmitted between slab and base when concrete shrinks. A bilinear model describing the frictional stress-slippage behaviour of a slab–base interface is adopted in the modelling. By model calculations, the influences of the time-dependent material parameters, such as shrinkage strain, elastic modulus and strength of concrete, as well as the time-independent parameters such as slab–base interfacial characteristics and slab length, on the development of shrinkage-induced stresses in concrete pavements are analysed. The closed-form expression of the minimum slab length or transverse crack spacing based on the assumption that the maximum shrinkage stress in the slab falls below the tensile strength of concrete under a given concrete tensile strength and shrinkage strain is obtained. The minimum shrinkage strain and the minimum slab length required to crack the concrete pavements are provided.
Influence of drying on concrete sorptivity
Authors: Dias W. P. S.
Published: October 2009

The experimental programme described in this paper indicated that concrete grade and specimen preconditioning were the major factors influencing sorptivity, whereas specimen size and coating had relatively little effect. The paper recommends that sorptivity be carried out on specimens coated on their side surfaces—that is, the curved surface in the case of cylinders. A 25 mm thick slice incorporating the original cast surface of a cylinder or core could be used as a sorptivity specimen, leaving the rest of the specimen for strength testing. A moderate amount of oven drying (e.g. 3 days at 50°C) is also recommended in order to increase the sensitivity of the sorptivity index. This research also demonstrated that sorptivity results from a variety of conditions could be generalised as a function of the desorption ratio; this is an index of the amount of drying that has taken place at the time of sorptivity testing.
Seismic performance of mechanically coupled reinforcing bars
Authors: Ingham J.M. , Bai A.S.H.
Published: October 2009

Seismic performance verification of a spiral thread mechanical reinforcement connection system for use in reinforced concrete structures was investigated by conducting both in-air component tests and structural subassembly tests. Current performance verification criteria listed in NZS 3101:2006 were found to be outdated and, following a review of various international protocols for testing mechanical connection systems, it was concluded that DIS 15835 protocols should instead be considered for adoption. Two external beam–column joint subassemblies having spiral thread mechanical reinforcement couplers installed at the column–joint interface were subjected to simulated seismic cyclic loading. It was found that bar curvature and the development of a non-uniform stress distribution around the perimeter of the reinforcement–coupler connection led to coupler slippage. However, the measured pinched hysteretic response owing to coupler slippage closely matched the response of companion tests where strength degradation was instead a result of excessive beam shear deformations. It was concluded that uniaxial tensile tests are unable accurately to demonstrate probable coupler performance in plastic hinge zones and that such tests should therefore not be used to predict likely ductile response in potential plastic hinge zones of reinforced concrete members.
Modelling shrinkage of concrete from moisture lost using moisture diffusion theory
Authors: Mu R. , Forth J.P.
Published: October 2009

This paper proposes a model to predict the shrinkage of concrete based on the moisture lost from the concrete in the process of drying. The drying of concrete causes the decrease in relative humidity within concrete, which is usually considered responsible for the shrinkage of concrete. The fall in relative humidity was correlated to the moisture lost according to diffusion theory. Incorporating existing relative humidity–shrinkage relationships, the shrinkage of concrete was then modelled as a function of the moisture lost. The predicted shrinkage was compared with the experimentally measured results and good agreement was achieved. The paper also illustrates that the model can represent non-uniform shrinkage and the effect of specimen size on shrinkage. However, further development of the model is required to accommodate the influence of aggregate and changes in environmental conditions.
The overlapping crack model for uniaxial and eccentric concrete compression tests
Authors: Carpinteri A. , Corrado M. , Mancini G. , Paggi M.
Published: October 2009

An analytical/numerical model, referred to as the overlapping crack model, is proposed in the present paper for the analysis of the mechanical behaviour of concrete in compression. Starting from the experimental evidence of strain localisation in uniaxial compression tests, the present model is based on a couple of constitutive laws for the description of the compression behaviour of concrete: a stress–strain law until the achievement of the compression strength and a stress–displacement relationship describing the post-peak softening behaviour. The displacement would correspond to a fictitious interpenetration and therefore the concept of overlapping crack in compression is analogous to the cohesive crack in tension. According to this approach, the slenderness and size-scale effects of concrete specimens tested under uniaxial compression are interpreted from an analytical point of view. Then, implementing the overlapping crack model into the finite element method, eccentric compression tests are numerically simulated and compared with experimental results. The influence of the size-scale, the specimen slenderness, as well as the degree of load eccentricity, is discussed in detail, quantifying the effect of each parameter on the ductility of concrete specimens.
Predicting Sorption-Induced Strain and Permeability Increase With Depletion for Coalbed-Methane Reservoirs
Authors: Clarkson C.R.. R. , Pan Z.. , Palmer I.. , Harpalani S..
Published: October 2009

It is well known that absolute permeability changes occur in coalbed methane (CBM) reservoirs during primary depletion or enhanced recovery/CO2 sequestration operations. Sorption-induced strain in CBM reservoirs, also known as matrix shrinkage or swelling, may dominate permeability changes at low pressures, as is the case for CBM wells undergoing primary depletion in the Fruitland coal fairway of the San Juan basin.

Several analytical models have been developed to predict changes in coal permeability as a function of stress and sorption. Most models, however, utilize an empirical method for estimating sorption-induced strain. Recently, a theoretical model for sorption-induced strain was developed and applied to single-component adsorption/strain experimental data. The new model was developed from basic thermodynamic principles and is more predictive than the empirically based approaches. In this paper, the theoretical model is expanded to incorporate multicomponent adsorption models that are more rigorous, and sometimes more accurate, than the commonly applied extended Langmuir (EL) equation. This improves predictions of multicomponent gas sorption-induced strain, as demonstrated by comparison to experimental data. The new sorption-induced strain model is then used to calculate the sorptionstrain component of the popular Palmer and Mansoori (P&M) equation, which, in turn, can be used to model permeability changes during both primary (single- or multicomponent gas) and enhanced recovery operations. Finally, the coupled sorption-strain/permeability model, incorporated into an analytical simulator, is used to predict and match permeability growth in a producing CBM well in the Fruitland coal fairway, which has a binary (CH4 + CO2) sorbed/produced gas composition.

Matches to field-derived permeability growth using the new model are accurate but nonunique because of the lack of available data, particularly rock mechanical properties. Given the availability of rock mechanics and adsorption isotherm data, the rigorous thermodynamic basis of the new model should allow for more accurate predictions of coalbed permeability changes, but further testing is required.
Specially Formulated Drilling Fluid Prevents Differential Sticking, Mud Losses, and Wellbore Instability in Oil and Gas Wells in Saudi Arabia
Authors: Al-Ansari Adel A. , Al-Mutair Faisal M. , Al-Kilani M. H. , Al-Haidary Saleh A. , Al-Baloosh Mishal S. , Al-Nimer A. A. , Al-Zahrani Walid G. , Veloo Vickneswaran , Villalobos Mario
Published: October 2009

Differential sticking can be a significant cause of lost time and non-productive cost where formations with varying permeability are drilled. In some instances, operators have limited choice and drilling overbalanced through lower pressured formations is necessary to meet well design targets. Higher permeability sections result in high rates of fluid loss into the formation, thick filter cakes and a significant risk of the drillstring becoming differentially stuck.

A novel approach to preventing such incidents was introduced in Saudi Arabia in 2006. The treatment/system relies on a combination of two new products added to the mud system; a synthetic graphite and an organic polymer which, in combination, provide an effective seal against high permeability formations and substantially reduce occurrences of differential sticking.

The synthetic graphite and the polymer, (added in the form of an aqueous emulsion) sizes are important factors in the success of this approach. Strategies to maintain the correct level of products in the mud system during drilling are discussed in conjunction with examples of successful field applications.

Laboratory work, designed to improve on previously existing guidelines for product application in different formation types and a proposed method of determining the graphite concentration in the mud by analytical analysis are also presented. Data from all field applications to date are summarized, showing that the system described is highly effective at eliminating differential sticking, mud losses and wellbore stability issues.
Costly Shoe-Track Drilling Practices Using Conventional and Rotary-Steerable Systems Evaluated: A Summary of Lessons Learned, Proposed Best Practices, and Software-Driven Recommendations
Authors: Rogers Hank , Webb Earl , Tilley Jim , Fears Brett
Published: October 2009

Drilling operations performed through the shoe-track are generally considered duplicated effort by many operators. Nonetheless, shoe-track drillout is an operation that must be performed on all surface or intermediate casing strings or liners. Slow penetration rates are often experienced, even when drilling through so-called drillable casing equipment. Today's costly drilling operations force operators to attempt to reduce nonproductive operations whenever possible. Therefore, improved shoe-track drillout performance can improve operators' overall drilling cost and schedule. Thus, a better understanding of downhole dynamics is necessary to develop improved drilling procedures.

A review of jobs from the North Sea database of cement jobs and shoe-track drillouts revealed that, of the more than 1,200 data points available to the authors, 83% of the drillout times were less than three hours; 70% of the drillout times were less than two hours; and 50% of the drillout times were less than one and one-half hours, with the overall average being 93 minutes. When a single type of cementing casing equipment was drilled in some wells in 30 minutes and other wells in three hours, questions were raised as to what major contributing factors determine actual drillout time.

Two case histories are presented with close attention paid to drilling parameters that adversely affect actual weight applied by the bit to the target being drilled. A better understanding of weight on bit (WOB) and weight on target (WOT) is needed to best determine drilling procedures to be used for any given drillout.

This paper documents lessons learned from successful drillouts performed with conventional and rotary-steerable drilling assemblies. Software-driven recommendations are provided for improved interpretation of downhole forces applied to the target being drilled.
Preventing Shallow Water Flow in North Sea Exploration Wells Possibly Exposed to Shallow Gas
Authors: Opseth T. Landbø , Ribesen B. T. , Tran T. N. , Hinton A. , Laget M. , Lende G. , Saasen A.
Published: October 2009

Originally, shallow water flow was severely hindering further drilling in a North Sea exploration well. The water depth was 111.5m. The 20" casing was set at 590m; above a shallow gas zone. After mounting the Blow Out Preventer (BOP), drilling the 17 ½" section started and continued for a few 100 meters. The wellhead was routinely inspected using an ROV (Remotely Operated Vessel). After some time, a tiny flow was observed around the wellhead. The flow increased in strength and later a large wash out area was observed, and further drilling had to be terminated. Most likely, the water flow originated from a zone at a depth of 172m MSL.

To cure the shallow water flow it was decided to grout cement on the outside of the casings. Regular well cements was not desirable to use since these cements are somewhat retarded in itself by mineralogical composition. It was decided to use standard construction industry cement with a very short curing time. Before the grouting could start the BOP had to be removed. Because of the shallow gas zone underneath the 20" casing, two barriers had to be included in the well for well control. Two packers were used, one drillable and one retrievable. The BOP was removed and the grouting operation was performed successfully. No water flow or gas flow were observed while the cement was setting. After the flow was cured drilling resumed, first by drilling and retrieving the packers, then continuing to drill the 12 ¼" section.

The lessons learned from this particular well are to focus on using rapid hardening industry cement when cementing the conductor and a foamed cement system around the surface casing. Furthermore, a riserless mud return system has been used on later wells to be able to drill with weighted drilling fluid prior to setting the riser. These solutions to prevent shallow water flow are further discussed and experiences from later wells are presented in the following paper.
Dealing with the Challenges of UBD Implementation in Southern Iranian Oilfields
Authors: Shadravan Arash , Nabaei Mohammad , Amani Mahmood
Published: October 2009

Approximately 90% of the discovered fields in Iran are in carbonate reservoirs making this country as one of the largest carbonate producers in the world. Iranian depleted zones are increasing, as most of the fields become mature. High pressure drop in most of these fields such as Parsi, Karanj, Aghajari, Gachsaran, Bibi Hakimeh, Lab Sefid, Dehloran, Danan and Mal Kouh caused several drilling and production problems. The current condition for new operation challenges in Iranian oilfields emphasizes the significance of utilizing new tools and methods.

Implementing underbalanced drilling (UBD) technology in Parsi oilfield, located in south western part of Iran, may offer a solution to the challenges of operations of the field. Parsi has undergone substantial pressure loss over its history of nearly 50 years, and it is characterized by the highly fractured, vuggy Asmari formation. That combination of a challenging reservoir and low pressure has led to increasing problems for traditional overbalanced approaches. Many papers have been published discussing potential UBD advantages for southern Iranian oilfields; however, few papers are available about the challenges of this technology. Despite the increasing experience in this field, the value of UBD has not been fully acknowledged. This paper investigates the main UBD challenges in Parsi oilfield and summarizes the solutions to deal with them.
Pressurized Mud Cap Drilling Drastically Reduces Non-Productive Time in Soka Field, South Sumatera
Authors: Runtuwene Michael , Suprihono Sanny , Rizka Dany , Prasetia Andi Eka , Toralde Julmar Shaun , Nas Steve
Published: October 2009

The wells in Soka field in onshore South Sumatra, Indonesia are drilled through a fractured carbonate reservoir (Baturaja formation) where severe circulation losses and kicks while drilling are commonplace. Drilling in one of the wells in the field, Soka 2006-1 was suspended for two years due to total losses combined with gas kicks. Almost two months were spent after total loss of circulation was experienced trying to control and drill Soka 2006-1 to no avail. The well was plugged and abandoned on July 2006 with the intention of returning to it and finishing it once an appropriate approach can be developed to address the drilling issues that were encountered.

The approach chosen for re-entry operations in Soka 2006-1 was a managed pressure drilling (MPD) technique called Pressurized Mud Cap Drilling (PMCD). A rotating control device (RCD) is the main component used in PMCD operations, the objective of which is to eliminate the non-productive time (NPT) associated with drilling when loss – kick scenarios occur.

A new well, Soka 2006-6, was also planned to be drilled in the area using the same technique, but with the addition of a downhole isolation valve that was to be installed and cemented together with the last casing string above the section where losses are expected. The casing valve allows for safer and faster tripping operations and more importantly, can serve as a downhole lubricator that will help facilitate the running of the completion assembly in PMCD mode. A casing valve could not be installed in Soka 2006-1 as the casing string above the loss zone was already in place.

This paper describes the planning and implementation of the PMCD technique in both the Soka 2006-1 and 2006-6 wells and discusses the results of the drilling operations. Furthermore, it explains how drilling in PMCD mode allowed re-entry operations in Soka 2006-1 to reach the target depth in less than a day after total loss of circulation was again experienced, and how the completion assembly was run and cemented in PMCD mode.
A Unique Cement Slurry Overcoming Low Temperature, Shallow Water/Gas Flows in Deepwater Cementing
Authors: Wang Chengwen , Wang Ruihe , Bu Yuhuan , Zhou Weidong
Published: October 2009

The hydrocarbon resources in deepwater areas are very captivating. The trend of developing and utilizing the hydrocarbon resources is necessary. Unfortunately, severe challenges have been proposed on deepwater cementing, i.e., the low temperature, weak formations, shallow water/gas flow, environmental protection and highly day rate for deepwater rigs. Conventional cements and low density cement slurries usually take a long time to set and can't allow significant compressive strength development at these low temperatures. In addition, conventional cement additives are often environmentally unacceptable subject to more stringent environmental regulations. Therefore, it is stringent to design environmental friendly low density cement slurry to overcoming low temperature and shallow water/gas flow during deepwater cementing.

As the low temperature and shallow water/gas flow is concerned in deepwater cementing, a unique deepwater cement system DWC, combining the sulfoaluminate cement and class-G oil well cement, was developed for the first time. And, various environmental friendly additives, including cement dispersant, fluid loss control agent and accelerator, were developed. Finally, a unique low density deepwater cement slurry was designed. Laboratory tests show that the unique deepwater cement slurry has exhibited low density, excellent fluid loss control, no free fluid, right-angle-set, short time of wait-on-time at low temperature, and a shortened transition time of static gel strength. Results presented in this paper should help resolving problems in deepwater cementing.
Integrating Learning Curves in Probabilistic Well Construction Estimates
Authors: Jablonowski C. , Ettehadtavakkol A. , Ogunyomi B. , Srour I.
Published: October 2009

For multiple well drilling and completion campaigns, cost and schedule performance tend to improve over time. This trend in improvement is commonly referred to as a "learning curve." When a learning curve is assumed, the campaign cost and schedule estimates may be reduced dramatically (relative to an assumption of constant performance). Many operators consider the use of learning curves a best practice, and provide procedures for estimation and implementation in their cost estimating guidelines.

This paper investigates methods for systematic integration of learning curves in probabilistic estimates. Brief reviews of probabilistic estimating methods and learning curves are provided. A general method and specific procedures for integrating learning curves in probabilistic estimates are then provided. For each method, the key assumptions are itemized and discussed and a demonstration is provided. While no single procedure will fit every situation, it is concluded that the general method is straightforward, transparent, and can be implemented using off-the-shelf spreadsheet software. The proposed procedures generate results that provide engineers and decision-makers with a refined representation of uncertainty, and can improve capital investment valuation and decision-making.