Sleeper end resistance of ballasted railway tracks

This paper describes model tests used to investigate how ballast shoulder width and height contribute to a railway sleeper’s resistance to lateral movement for a range of shoulder widths and heights. Deflection and resistance were measured and photographs taken during the tests.The photographs were analyzed using a digital image correlation technique to identify the zones of ballast surface disturbance, which demonstrated that a bulbed failure volume was mobilized at the ultimate limit state. An idealized three-dimensional failure mechanism is proposed, and resistances are calculated using the limit equilibrium approach. The calculation provides a reliable estimate of the measured resistance. The work identifies the optimum shoulder width and height. The calculations are extended to demonstrate that when a number of sleepers are moved simultaneously, the sleeper end resistance may be one-third less per sleeper than that indicated in tests on an isolated sleeper. Image analysis and limit equilibrium calculations show that this is caused by overlapping of mobilized failure volumes from adjacent sleepers

The Average Temperature of Energy Piles

The geotechnical design of energy piles requires confirmation that the foundations can continue to carry safely the required load from the overlying structure and that no detrimental effects from the additional imposed temperature changes will occur. These additional design checks require assumptions to be made about the temperature changes within the pile. However, there is no universal approach for determining these, and routine application of over-conservative pile temperatures can lead to unrealistically adverse geotechnical design scenarios. This paper considers how the average temperature of a pile can be determined based on the analysis steps already carried out for the thermal design. The aim is to be able use the calculated fluid temperatures, along with readily available pile and ground parameters, to provide better assessments of the actual pile temperature so that the outputs of the geotechnical design can be improved. Two dimensional numerical simulations are used to determine the average pile temperature for different pipe, pile and concrete properties. The results of the simulations are compared with analytical approaches, allowing these to be validated for use on a routine basis. It is shown that the temperature of the center of the pile, which can be determined easily by analytical methods, can be used as a proxy for the average pile temperature

2D thermal resistance of pile heat exchangers

Structural foundation piles are being used increasingly as heat exchangers to provide renewable heat for new buildings. To design such energy systems a steady state is assumed within the pile, which is conventionally characterised by constant thermal resistance. However, there has been little research regarding pile resistance and there are few published case studies. Numerical modelling results are presented here to provide typical values of pile resistance, depending on the details of the heat exchange pipes. Analysis suggests large diameter piles may take several days to reach steady state; in these cases a transient design approach may be more appropriate

Seasonal changes in pore water pressure in a grass covered cut slope in London clay

In temperate European climates, the season of peak water demand by vegetation (summer) is out of phase with the season of greatest rainfall (winter). This results in seasonal fluctuations in soil water content and, in clay soils, associated problems of shrinking and swelling that can in turn contribute to strain-softening and progressive slope failure. This paper presents field measurements of seasonal moisture content and pore water pressure changes within the surface drying zone of a cut slope in the London Clay at Newbury, Berkshire, UK. A climate station was installed at the site to measure the parameters needed to determine specific plant evapotranspiration. This information was used to carry out a water balance calculation to estimate the year-round soil moisture deficit caused by the vegetation. The calculated soil moisture deficit matches reasonably closely the field measurements of soil drying. The field measurements of seasonal changes in pore water pressure and suction are linked quantitatively to the measured changes in water content using the soil water characteristic curve for the London Clay. The suctions generated by the light vegetation cover at Newbury were found not to persist into the winter and early spring

Error analysis of the thermal cell for soil thermal conductivity measurement

Soil thermal conductivity is an important factor in the design of energy foundations and other ground heat exchanger systems. Laboratory tests in a thermal cell are often used to determine the thermal conductivity of soil specimens. Two interpretation methods have been suggested. Analysis can be based on the assumption of one-directional heat flow and the thermal conductivity calculated using Fourier's law. Alternatively the lumped capacitance method can be employed, using results generated as a specimen cools. In this study, six samples of London Clay were tested using a thermal cell. A finite-element model of the tests was then used to determine the validity of the assumptions made in analysis. The model showed substantial heat loss through the sides of the specimens, which would have a significant impact on the calculated thermal conductivity. The conditions required for the lumped capacitance method to be valid were also found not to be met. Consequently neither analysis method is recommended. A better approach would be to pursue apparatus with fewer heat losses or transient testing techniques

Seepage and pore pressures around contiguous pile retaining walls

Diaphragm and bored pile retaining walls are often used for the construction of basements, metro station boxes and cut-and-cover tunnels in urban areas. While diaphragm and secant pile walls are generally intended (and assumed in analysis) to be effectively impermeable, contiguous piles may allow through-the-wall seepage even when preventative measures have been undertaken. Provided the flowrates can be tolerated or dealt with, through-the-wall seepage should result in a reduction in pore water pressures behind the retaining wall compared with an impermeable construction, giving the potential for reductions in the depth of embedment and wall thickness, hence cost. However, this potential is rarely realised owing to the difficulty in quantifying with sufficient confidence the hydraulic regime associated with a leaky retaining wall. This paper reports the results of laboratory investigations and numerical analyses carried out to assess the effect of the inter-pile gaps on the pore pressure distribution around a contiguous pile retaining wall. The results show that the pore pressures behind the piles reduce significantly as the pile spacing is increased. Long-term field monitoring confirms that the pore water pressures are much lower than would be expected for an impermeable retaining wall in similar soil. The applicability of a simple expression linking the pile diameter, pile spacing and the effective permeability of an equivalent uniform wall is demonstrated

A new modelling approach for piled and other ground heat exchanger applications

Pile heat exchangers have an increasing role to play in the delivery of renewable heating and cooling energy. Traditionally the thermal design of ground heat exchangers has relied upon analytical approaches which take a relatively simple approach to the inside of the heat exchanger. This approach is justified while the heat exchanger diameter remains small. However, as larger diameter piled foundations are used as heat exchangers, the transient heat transfer processes operating within the pile become more important. To increase our understanding of these processes and ultimately lead to improved thermal design approaches for pile heat exchangers it is important to examine the heat transfer within the pile in detail. To accomplish this, a new numerical approach has been implemented within the finite element software ABAQUS. Coupling of the convective heat transfer due to fluid flow within the heat transfer pipes and the heat transfer by conduction within the pile concrete is the most important facet of the model. The resulting modelling approach, which is ready to generalise to other geothermal applications and to assess thermo-mechanical couplings, has been validated against a multi-stage thermal response test carried out on a test pile in London Clay

Thermal Conductivity of Simulated Soils by the Needle Probe Method for Energy Foundation Applications

Soil thermal conductivity is an important parameter in the design of ground source heat pump and energy foundation systems. A laboratory method for measuring the soil thermal conductivity is the needle probe method. Earlier, analysis of the needle probe test data has been simplistic, relying heavily on human judgment and rules of thumb. This article presents an alternative method of analyzing the needle probe data with the aid of MATLAB, which is a technical programming language and computing environment. Four agar–kaolin specimens of varying densities were prepared to resemble simple soils. These were tested using the needle probe for a range of heating times and heating powers, to see what effect these parameters would have on the results. The repeatability when keeping the heating time and heating power constant was within ±2%. When the heating time and heating power were varied, the variation in results from the average for a given specimen ranged from ±4% to +10%/–8%. This range is significantly higher than the repeatability. Possible reasons for this are discussed in this article