Case history of a dewatering and recharge system in chalk

Abstract

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.

Wet winter pore pressures in railway embankments

“…Summary of soil properties used in finite-element model. Labels A-C indicate models to which relationships apply (see Figure 5) (Powrie and Roberts, 1995), and van Genuchten (1980) parameters for a sandy gravel, to represent full underdrainage by chalk or river terrace deposits.…”

Test and numerical research on wall deflections induced by pre-excavation dewatering

“…However, k s actually reflects the discharge flow rate via a well (q), but there is no parameter with respect to q in Eq. (2). Therefore, it is inappropriate to use Eq.…”

“…In a congested urban environment, dewatering is typically performed inside the excavation to reduce the impact of dewatering on the adjacent structures and facilities [2,3]. As to excavations that require a drawdown exceeding 5-10 m (e.g., typical metro excavations), the deep well is often employed [4].…”

Contributions to Géotechnique 1948–2008: Groundwater

“…Roberts & Preene (1994) refer to the groundwater control systems at Kilsby Tunnel, the Tees Barrage and the Pontsticill water treatment works in relation to the use of the observational method in design. Powrie & Roberts (1995) describe an unusual dewatering and groundwater recharge system in Chalk at Winchester, in which geochemical effects caused major difficulties towards the end of the project.…”