Self-weight Stresses Research Articles

Assessment of in-service stresses in steel bridges for high-frequency mechanical impact applications

The application of high-frequency mechanical impact (HFMI) treatment to improve the fatigue performance of composite steel and concrete road bridges was studied through a state-of-the-art review in conjunction with simulations of variable amplitude in-service stresses in four case-study bridges in Sweden. Empirical stress range spectra with associated mean stresses were characterised for HFMI-treated bridges. It was shown that the fatigue-critical locations in HFMI-treated bridges remain unchanged compared with conventional bridges and that compressive overloads pose no detrimental effect that requires additional attention in the fatigue assessment. Calculations also showed a considerably better fatigue performance if HFMI treatment is performed on site, after the application of self-weight stresses.

Centrifuge study on the runout distance of submarine debris flows

ABSTRACTIn this paper, a series of mini-drum centrifuge experiments on motion of submarine debris flow, which are able to correctly reproduce the self-weight stresses and gravity-dependent processes, are presented. These tests were performed using artificial submarine clay with high water content, from 93 to 149%. The extremely low shear strength made the debris material behave as idealistic lubricating material when it was deposited, resulting in a linear relationship between water content and runout distance of strongly coherent debris flow. On the other hand, the dilation of the flow body and hydroplaning was observed for weakly coherent debris flow, which further increased the mobility of flow body. A densimetric Froude number Frd was used to indicate the threshold of hydroplaning, which occurs if the Frd is greater than 0.2. Finally, two simple analytical models based on prototype debris flow under 1 g condition was used to validate the experiment results, which further prove the effects of soft marine clay on the high mobility of submarine debris flow. On the other hand, when the water content exceeded 120%, the experiment results deviated from the analytical solution due to the effects of hydroplaning.

Laboratory, in situ and full-scale load tests to assess flood embankment stability on peat

The low submerged unit weights of peats usually lead to low effective self-weight stresses, stiffnesses and undrained shear strengths. These features, in combination with high compressibility, a propensity to creep and the uncertain effects of fibrous inclusions, make foundation stability hard to assess reliably. It is usual to apply high safety, or strong material reduction, factors in foundation design. However, over-conservatism can lead to undesirable environmental and financial costs. This paper describes full-scale field tests conducted on peat, with and without pre-loading, at Uitdam on the borders of Lake Markermeer, north of Amsterdam. The experiments investigated the peat layers' consolidation behaviour and their response under loading, including full shear failure. Noting the complex final test geometries and the large displacements developed, simple numerical analyses were undertaken to help interpret the failures within a Tresca and ‘consolidated undrained shear strength' framework. The trials that included modest pre-loading developed large vertical consolidation strains (up to 35%) and significant bearing capacity improvements. The field experiments provide a rich resource for testing advanced numerical techniques. They also allowed a range of practical characterisation techniques to be assessed and calibrated for flood dyke applications.

Practical Model of Frost Heave in Clay

Freezing behavior of clay differs from that of silt. This difference stems primarily from the low permeability or hydraulic conductivity of clay, and the higher water content of saturated clay. Freezing effects include simultaneous heave and consolidation. Six small physical model columns of clay were frozen: one at 1g; and five on a centrifuge at various scales and with corresponding accelerations, to bring self-weight stresses into similarity with a full scale column of clay 4m in height. The experimental results demonstrated the importance of replicating the prototype stress conditions in a model. They demonstrated the importance of local water content on development of heave in clay, and the relative insensitivity of heave to location of the phreatic surface. Low permeability caused the clay to behave essentially as a closed system with regard to water flow. A simple analytical model was developed to explain observed soil response. Further research is recommended to provide more guidance in selecting input parameters.

A continuity method for bridges constructed with precast prestressed concrete girders

A method of making simply supported girders continuous is described for bridges with spans of 30-45 m. The splicing method takes advantage of an induced secondary moment to transform the self-weight stresses in the precast simply supported girders into values representative of a continuous girder. The secondary moment results from prestressing of continuity tendons and detensioning of temporary tendons in the girders. Preliminary sections are selected for spliced U-girder bridges with a range of span lengths. Use of the proposed technique results in girder depth reductions of 500-800 mm when compared to standard simply supported I-girder bridges. The flexural behavior of an example bridge with 40-m spans is examined to illustrate the necessary considerations for determining the optimum sequence of splicing operations.

Consolidation of naturally gassy soft soil

Biogenic gas is produced and contained in fine-grained soils in environments such as river estuaries and landfill sites. Planning dredging operations or predicting the volume capacity of a landfill requires information about the amount of gas that can be held in the soil, and about the effect that the gas has on the soil behaviour. The purpose of this paper is to present some experimental results and interpretation relevant to these and related topics. A series of experiments has been carried out at Oxford University with soil of estuarine origin from the Slufter disposal site in the Netherlands. The soils were introduced in settling columns, and the different stages of consolidation were documented by measuring height, densities, excess pore pressures and amount of gas. At the end of primary consolidation the gassy soil was consolidated to higher stress levels by applying a hydraulic gradient. The growth rates of bacteria producing gas within the soil were accelerated or reduced by controlling the temperature in the range 10–30°C. The experimental results show a sequence of events during consolidation. In the first phase, gas was produced and accumulated within the soil. The overall density of the gassy soil decreased and pore pressures fluctuated unpredictably. At the end of this phase the amount of gas within the soil reached a critical threshold value and thereafter began to escape through cracks and fissures. This marked the start of a second phase of events. Although the gas production continued to be high, the total amount of gas within the soil slowly decreased. The soil began a new phase of consolidation, with the settlement accelerating, as the cracks and fissures provided a quick route for pore water dissipation. The self-weight stresses in these experiments do not exceed 1 kPa, so that higher stress levels were achieved by the application of a hydraulic gradient. During this stage, the gas within the soil became less influential as the soil gained in strength.

Consolidation of naturally gassy soft soil

Biogenic gas is produced and contained in fine-grained soils in environments such as river estuaries and landfill sites. Planning dredging operations or predicting the volume capacity of a landfill requires information about the amount of gas that can be held in the soil, and about the effect that the gas has on the soil behaviour. The purpose of this paper is to present some experimental results and interpretation relevant to these and related topics. A series of experiments has been carried out at Oxford University with soil of estuarine origin from the Slufter disposal site in the Netherlands. The soils were introduced in settling columns, and the different stages of consolidation were documented by measuring height, densities, excess pore pressures and amount of gas. At the end of primary consolidation the gassy soil was consolidated to higher stress levels by applying a hydraulic gradient. The growth rates of bacteria producing gas within the soil were accelerated or reduced by controlling the temperature in the range 10–30°C. The experimental results show a sequence of events during consolidation. In the first phase, gas was produced and accumulated within the soil. The overall density of the gassy soil decreased and pore pressures fluctuated unpredictably. At the end of this phase the amount of gas within the soil reached a critical threshold value and thereafter began to escape through cracks and fissures. This marked the start of a second phase of events. Although the gas production continued to be high, the total amount of gas within the soil slowly decreased. The soil began a new phase of consolidation, with the settlement accelerating, as the cracks and fissures provided a quick route for pore water dissipation. The self-weight stresses in these experiments do not exceed 1 kPa, so that higher stress levels were achieved by the application of a hydraulic gradient. During this stage, the gas within the soil became less influential as the soil gained in strength.

Geotechnical centrifuge modelling of gelifluction processes: validation of a new approach to periglacial slope studies

AbstractHere we compare scaled centrifuge modelling of gelifluction processes with earlier full-scale physical modelling experiments. The objective is to assess the validity of the centrifuge technique for cryogenic slope-process investigations. Centrifuge modelling allows correct self-weight stresses to be generated within a small-scale physical model by placing it in an elevated gravitational field. This paper describes an experiment in which a scaled frozen-slope model was thawed in a gravitational field equivalent to ten gravities. After four cycles of thawing, during which soil temperatures, pore pressures, thaw settlement and downslope soil displacements were continuously monitored, a series of marker columns were excavated to reveal profiles of soil movement. Comparison of these data with those from an earlier full-scale laboratory simulation experiment indicates that thaw-related gelifluction was successfully reproduced during centrifuge modelling. It is concluded that rates of soil shear strain during gelifluction were not time-dependent? since soil displacements in the centrifuge tests were of a similar magnitude to or greater than those observed in the much longer-duration full-scale simulation. This suggests that no transition occurred in soil behaviour from a frictional plastic to a true viscous fluid during the period of high moisture contents immediately following thaw.

Self-Weight Stresses in Hyperbolic Cooling Towers of General Shape

Explicit analytical results are developed for self-weight membrane stresses in hyperbolic cooling towers of general shape, for a number of different patterns of shell-thickness variation along a meridian of the shell of revolution. The very general nature of the results makes them particularly useful in obtaining quick estimates of dead-weight stresses for structures of this type, and any desired parametric studies may readily be undertaken, permitting the designer to choose the most optimum shell-thickness variation for a given combination of geometric parameters of the hyperboloid of revolution. The results are intended for use during the preliminary phase of the design process. Other equally important effects (such as wind loading and thermal gradients) are best taken into account numerically using the finite element method, and will therefore not be treated in this paper. Some numerical results for self-weight stresses are then obtained on the basis of special but very practical cases, and an analysis of these results reveals interesting design implications.

Measurement of Stress and Strain in an Unsurfaced Haul Road at a Soft Clay Site in Scotland

Strain coil sensors and pressure cells were installed in relatively large numbers in a trial haul road, constructed in Scotland, that was subjected to controlled trafficking. The instruments and their installation are described and their operation is discussed. The strain coils are shown to give a reasonably good measurement of both transient and plastic strains when placed in both unbound aggregate and cohesive subgrade soil, and data collection is shown to be facilitated by a new electronic apparatus. Sensors also have good survivability and are cheap to make and install. Pressure cells performed somewhat less well, because of apparent environmental survivability defects and inherent difficulties of installation and operation resulting from inhomogeneity generated in the surrounding ground. The cells were unable to monitor any (small) self-weight stresses.

Conventional and centrifuge model studies of the moment carrying capacity of short pier foundations in clay

An experimental investigation into the moment carrying capacity of short rigid pier foundations in saturated clay is described. Scale models of square piers with different breadths and depths were used in both conventional and centrifugal studies. The results show that the relationships between moment and rotation are nonlinear but do not exhibit any peak values, and that moment limits, defined by limiting angular rotations, increase with increases in pier depth and breadth. Empirical equations are derived between moment carrying capacity and pier geometry, for a range of limiting rotations, and a very close fit is demonstrated between the moment–rotation relationships obtained using these equations and the actual data obtained from the model tests. It is shown that, at the same pier rotations, the moment carrying capacities observed in the centrifugal model tests are significantly larger than those in the conventional model tests. Numerical analyses of the prototype geometries were also carried out using a three-dimensional nonlinear finite-element computer program. The results are shown to provide satisfactory agreement with the moment–rotation behaviour and working limits observed in the centrifuge model tests. Thus, even though conventional modelling is usually legitimate for determining the immediate bearing capacity of rigid foundations in saturated clay, their rotational stability is shown to be significantly affected by self-weight stresses. Some of the existing methods for designing short piers subjected to moments are examined and compared with the results from the centrifuge model tests. Key words : pier foundation, clay, moment capacity.

The Theory of One-Dimensional Consolidation of Saturated Clays

Synopsis The equation governing the one-dimensional consolidation of a fully saturated clay layer are derived here on the basis of assumptions more general than those usually adopted. The limitation of small strains has not been imposed and the variation of soil compressibility and permeability during consolidation has been taken into account. Furthermore, although Darcy's law is assumed to be valid, it is recast in a form in which it is the relative velocity of the soil skeleton and the pore fluid that is related to the excess pore fluid pressure gradient. The consolidation of a thin clay layer, the self-weight stresses of which are negligible compared with those applied, is examined in detail. Non-homogeneity, time-effects intrinsic to the soil skeleton and compressibility of the pore fluid are incorporated in the theory, but a detailed consideration of their importance is deferred to later papers in which the case of the thick clay layer will also be presented. Les équation régissant la consolidation à une dimension d'unne couche d'argile pleinement saturée sont déduites ici en se basant sur des suppositions plus générales que celles adoptées habituellement. La restriction des petites déformations n'as pas été de la perméabilité du sol pendant la consolidation a été prise en considération. En outre, quoique la loi de Darcy soit supposée être valable elle est énoncée sous une forme squelette du sol et l'eau interstitielle sont exprimées en fonction de la pente excédentaire de la pression de l'eau interstitielle. La consolidation d'unne couche d'urgile mince, dont les constraintes de son poids propre sont négligeables en comparaintes de celles qui lui sont appliquées, est examinée détail. La non homogénéité, les effets du temps intrinsèques au squelette du sol et à la compressibilité de l'eau interstitielle sont incorporés à la théorie, mais une considération détailée de leur importance est remise à des exposés ultérieurs dans lesquels sera aussi présenté le cas de la couche épaisse d'argile.