Groundwater disturbance and surface settlements around a de-watered sand and gravel quarry

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Froggatt, I.J., 1997. Groundwater disturbance and surface settlements around a de-watered sand and gravel quarry. PhD, Nottingham Trent University.

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Abstract

This study has examined the causes of apparently recent ground and structure movements around a de-watered sand and gravel quarry in Hoveringham, Nottinghamshire. The effects of de-watering on the sand and gravel aquifer itself and the response of groundwater levels around the quarry were also examined.

An initial literature study demonstrated the current uncertainty of the effects of de-watering on shallow sand and gravel aquifers. It was clear that spatial variation of permeability and "suffosion" (particle movement due to laterally flowing water) were given little or no consideration at the design stage. This study has focused on these two mechanisms as potential causes of the apparent ground movement in Hoveringham.

A geological desk study on the area of interest was followed by structural defect investigations on the structures of Hoveringham village close to the de-watered quarry in order to classify the type of defects existing. The results from the structural investigations suggested a correlation between proximity to the de-watered void and the occurrence of ground defects due to settlement.

Soil samples taken from very close to the quarry face during de-watering showed a significant lack of fine particles compared to samples taken from similar locations prior to de-watering.

A full scale pumping out test was used to examine the drawdown and settlement response of a section of the aquifer showing a large range of electromagnetic conductivities. The results showed that no significant variation in the distances of influence occurred within the zones showing different conductivities and that no significant ground settlement occurred during the test. This latter result suggested that even if suffosion was occurring remote from the pumped source, it had not led to significant ground settlement.

Numerical modelling of various de-watering scenarios were then carried out both in plan ("quasi three-dimensional") and axi-symmetric fashion. The objective of these studies was to examine the effects of spatial permeability variation on groundwater levels around the quarry and to examine groundwater velocities (termed "unit fluxes" in the thesis) under conditions of vertical permeability variation. Derived unit fluxes were compared to values published by previous researchers for which suffosion had first been observed. The results from the quasi three-dimensional analyses showed that even under extreme boundary and permeability conditions, the resulting drawdown was unlikely to cause structurally significant settlements. The axi-symmetric investigations showed that other than very close to the quarry wall, unit fluxes were unlikely to reach levels at which suffosion would occur.

The original contribution to knowledge contained in this study is in two parts. The first is the in-depth case study into the local effects of groundwater abstraction in the Hoveringham area, the conclusion from which is that groundwater withdrawal is highly unlikely to have caused structurally significant settlement within Hoveringham village. The second is the conclusion that in shallow alluvial aquifers subjected to de-watering operations on a similar scale to that studied, the previously little understood mechanism of suffosion is unlikely to cause any ground settlement. Indeed, the results suggest the mechanism is unlikely to occur at all other than very close to the de-watered zone.

Item Type:

Thesis

Creators:

Froggatt, I.J.

Date:

1997

ISBN:

9781369313017

Identifiers: