Thermal conductivity of the ground is an important parameter in the design of ground energy systems, which have an increasing role to play in providing renewable heat to the built environment. For larger schemes, the bulk thermal conductivity of the ground surrounding the system is often determined in situ using a thermal response test. Although this test method is commonly used, its limitations are often not fully understood, leading to an over-simplistic interpretation that may fail to identify key facets of the ground thermal behaviour. These limitations are highlighted using data from an instrumented thermal response test carried out in a 150 m deep borehole in east London. It is shown that a single, unique value of bulk thermal conductivity may not be appropriate, as stratification of the ground can lead to differences in thermal performance, depending on the direction of heat flow. Groundwater flow within the Chalk aquifer is also shown to have an important effect on the long-term heat transfer characteristics.
The paper describes the performance of a dewatering and groundwater recharge system for a large excavation in chalk. The assessment of the effective bulk permeability of the chalk, and the probable hydraulic boundary conditions are discussed. The measured extraction and recharge flow rates, and the porewater pressures in certain locations, are in reasonable agreement with those calculated using a simple flownet analysis. The influence of groundwater chemistry on the effectiveness of the dewatering system is discussed with reference to an incident towards the end of the pumping period which led to the clogging of the pumps due to calcium carbonate precipitation following a rise in the pH of the groundwater. KEYWORDS: basements; case history; chalk; chemical properties; groundwater; permeability. L'article décrit les performances du système de drainage et de recharge des nappes mis en oeuvre dans une excavation de grande taille réalisée dans de la craie. I.'évalulation de la perméabilité effective de la craie et les conditions hydrauliques probables aux limites sont discutées. Les débits mesurés lors de l'extraction et lors de la recharge, ainsi que les pressions interstitielles d'eau en certains endroits, sont en accord avec ceux calculés à l'aide d'une analyse simple de réseaux d'écoulement. L'influence du propriétes chimiques de l'eau sur l'efficacité du système de drainage est étudiée au travers d'un incident ayant eu lieu en fin de période de pompage et qui a conduit à un colmatage de la pompe par un précipité de carbonate de calcium dû à une augmentation du pH de la nappe.
An 8·2m diameter, 40m deep shaft for Crossrail is being constructed below the 10m deep basement of the Moorhouse development near Moorgate in the City of London. The depth of the shaft is such that it will penetrate through stiff London Clay and will be founded at the bottom of the Lambeth Group. The shaft is being constructed after the Moorhouse structure has been completed and the design of the Moorhouse foundations places tight constraints on acceptable ground movements due to construction of the shaft. Furthermore, the shaft needs to be designed to accommodate future ground movements associated with construction of Crossrail. The paper describes the complex relationship between the foundations of Moorhouse, the draught relief shaft and the future Crossrail assets. The optimised design includes extensive slip coating and base grouting of the Moorhouse piles, a complex temporary works dewatering system around the shaft and the option to carry out additional dewatering from within the shaft during construction. Control of ground movements through the Lambeth Group was perceived to be a particular problem in relationship to destressing the ground around the Moorhouse piles. To prevent longterm settlement of these piles, provision was made for radial grouting to “restress” the ground should the need arise.
This is the accepted version of the paper.This version of the publication may differ from the final published version. that support Moorhouse and the presence of these foundations placed tight constraints on acceptable ground movements associated with construction of the shaft. The depth of the shaft is such that it penetrates through stiff London Clay and is founded at the bottom of the Lambeth Group. The paper describes the contingency measures to deal with potentially difficult ground conditions including the water bearing layers of the Lambeth
Permanent repository linkGroup. The construction processes included a complex temporary works dewatering system around the shaft with the option to carry out additional dewatering from within the shaft during excavation. Provision was also made for radial grouting to "restress" the ground, to prevent long-term settlement of the Moorhouse piles, should the need arise. The success of the project was due, in no small part, to the detailed planning and consideration of contingency measures to deal with perceived risk.
To facilitate construction of the Channel Tunnel Rail Link through Ashford in cut-and-cover tunnels and retained cut, it was necessary to control pore water pressures in the relatively low-permeability, laminated Weald Clay. This was achieved by means of an ejector well dewatering system. This paper describes and discusses the investigations carried out to characterise the in situ permeability of the Weald Clay, the design and performance of the ejector well system installed, and the associated soil surface settlements. The correlation between the changes in pore pressure and settlements at the site is compared with that given by Preene et al. This comparison is used to evaluate the method, and to provide some insights into the selection of appropriate parameter values of soil permeability and stiffness.
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