1.                    Introduction

1.1                   Scope of Work

This note provides a summary of the proposed engineering strategy and water management issues associated with the development of Midland Quarry.  The following organisations have contributed directly to this note:

•          Entec UK

•          Blackwells

•          JMP Consultants

The preparation of this note has also included consultation with a number of organisations and companies with a direct interest in the proposed development, including Wardell Armstrong (preparing the Environmental Statement) and informal consultation with British Waterways and a national housebuilder who have a keen interest in these proposals.

Reference should be made to the figures in section B of this document when reading this summary note.

1.2                   Overall Proposals

The end use requirement dictates the Engineering considerations for the Project, the end use is to be mixed commercial and housing within Dev 11 area and employment within Dev 5. In addition, the potential use of the existing quarry as a marina introduces specific issues to be addressed.

 In summary the principal considerations include:

•          Preparation of the shallow void area to receive fill material

•          Excavation of overburden material to form development areas DEV5

•          Deposition of site sourced overburden material within the shallow void

•          Construction of INF3 and INF 3.1, foot/cycle way and four way duct corridor including junctions at Pool Road and Tuttle Hill Rd. of street lighting to INF 3

•          Installation of surface water drainage to INF3 & 3.1

•          Controlled import of fill to the shallow void area

•          Provision of highway access to service main void area @ 80m AOD

•          Foul water connections from DEV5 & DEV 11 including provisional sum allowance for pumping station if required

•          Design and Design Warranty

British Waterways are assessing the feasibility of the connection between the main quarry waterbody and the Coventry Canal and the engineering issues associated with this concept are not considered further in this note.  However, reference is made to potential water levels in the main quarry void which will impact on the feasibility and design of this proposed connection.

This note is structured as follows:

i)                         Comment on the engineering strategy for the Land Reclamation proposals

ii)                       Comment on water management issues

iii)                      Conclusions to date

 

2.                    Land Reclamation

The aim of the engineering strategy is to establish a sustainable development platform for the proposed end use on DEV5 and DEV11. In formulating the Engineering solutions for the scheme the long-term consolidation and compression of the fill is of prime concern, to monitor this and ensure the potential for compression is minimised the following measures will be adopted.

 

Material will be compacted to a bespoke specification established from laboratory earthworks testing and preliminary site compaction trials and based on the Highways Agency Specification for Highway Works. The control of the earthworks during the main operations will be validated by prescribed regular testing to ensure adequate compaction of the placed fill.

 

The existing face would be benched back in appropriate steps with all loose and overhanging rock removed as the fill is progressed. This loose rock would be processed for reuse within the works.

 

Material testing has already been undertaken on the available material and this indicates that it  is acceptable in physical and contamination terms. This testing will be supplemented by further advance testing prior to the compaction trials.

Long-term settlement of the fill will be measured from settlement plates incorporated into the upper layers of fill.

Unsuitable materials, for example oversize organic materials will be excluded from the fill.  

 

Prior to infilling the shallow void, the water level will be reduced to around 55m AOD or lower. This will expose the existing granite bench at around 60m AOD which will form the foundation for the construction of the reinforced earth embankment.

Currently there is a quantity of  unsuitable material present within the existing shallow void overlying the granite bench. This material will be removed.

The current information indicates that the scheme is approximately self-balancing in terms of earthworks materials. Should a shortfall of suitable material be experienced in the final proposal this

 

 shortfall in material will be made up from imported inert material surplus from other local contracts. This material will be tested to ensure compliance prior to being incorporated into the works.

In order to establish a development plateau within Dev 11 it is necessary to construct a reinforced earth structure to the shallow void fill area.

This earth structure will be founded on the existing bedrock and be comprised of specialist fill material with horizontal primary and secondary reinforcement placed at regular intervals, a typical section is shown overpage.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

	Secondary Reinforcement Layers

 

 

 

 

 

 

 

 

Construction of the reinforced embankment will be undertaken by a specialist subcontractor  concurrently with the filling to the shallow void.

The reinforced structure is formed by laying the primary reinforcing geotextile of approx. 16.5m lengths every metre lift. This primary reinforcement is interspersed at 250mm intervals with shorter secondary reinforcement.

The design relies on a granular material as the selected fill which minimises the length of geotextile required. This is necessary due to the limited working area at the extremities of the fill. In order to form the reinforced structure it will be necessary to establish a level bench at the 60m AOD contour along the base of the structure.

To prevent erosion and to provide an aesthetically pleasing face hydroseeding of the front face can be undertaken.

The above measures will ensure that a development platform can be achieved to support the proposed end use.

3.                    Water Management

3.1                   Key Water Management Issues

An assessment of the water related environmental impacts associated with the development is covered in the Environmental Statement (section C) provided by Wardell Armstrong and provides a summary of the baseline hydrology and hydrogeology of the location.  The Environmental Statement should be consulted to understand the location and interactions between the key surface water features (Bar Pool Brook, Holly Stitches Brook, Coventry Canal and Midland Quarry) referred to in this note. The objectives of this additional water management review were as follows:

•          Assessment of proposed surface water drainage from paved surfaces (development and highways) and the implementation of drainage measures which confirm with PPG25 Development and Flood Risk recommendations.

•          Assessment of a sustainable main quarry void water level which takes account of proposed end use and is acceptable for engineering purposes, and considers the wider hydrogeological issues in the vicinity.

•          Ensuring water management proposals are broadly acceptable to British Waterways and the Environment Agency and other drainage authorities, but recognising the likelihood to consult further on some of the more complex issues.

•          Identification of potential strategic benefit of Midland Quarry to with respect to Holly Stitches and Bar Pool Brook and the wider catchment, and as an opportunity to enhance the conservation and landscaping potential of this location.

•          Identify information gaps and propose further investigations to help address any uncertainties.

3.2                   Surface Water Drainage

The proposed surface water drainage from the development will be separated as follows:

i)                         The highway drainage from INF3 will discharge to a new balancing pond adjacent to Willow Road.  This area will also accept part of the highway drainage from DEV5 highway. The balancing pond, will also provide for an area of conservation in terms of reed beds that can be transplanted from their existing location in DEV5. 

ii)                       The remainder of the highway drainage from DEV5 will discharge to the main water body of the quarry via suitably-sized by-pass interceptors. 

iii)                      The highway and surface water from INF3 will also be collected to a point and discharged via a suitably sized by-pass interceptor to either the canal basin or the main water body within the quarry.

iv)                     A proportion of the foul and surface water from the DEV11 entrance can outfall to the public sewers in Tuttle Hill Road.  Further back into the site, the surface water will outfall, (via an interceptor) to the main body of quarry water.

 

Early indications are that as long as the water quality is maintained, there should be no problem discharging surface water run-off to the main quarry water body.

The aim of the drainage proposals is to ensure that the development is ‘stand-alone’ with no adverse impact on water levels in the wider catchment.

 

The industrial units served by DEV5 will need to take account of their own surface water attenuation to greenfield run-off in accordance with PPG 25 Flood Risk & Development.

 

The foul water drainage provision for the site will be an existing gravity sewer located at the South East corner of the site.  Sewer capacity has been discussed with Severn Trent Water and the indications are that a discharge to the sewer on the Southern end (Pool Road) of the site would be acceptable.

3.3                   Sustainable Water Levels in Midland Quarry

The current end use proposals for the main quarry void assume a sustained groundwater level between 70m and 80m AOD.

Hydrogeological Context

As described in a previous report by W.S Atkins (March 1999) Cambrian rocks underlie much of Midland Quarry and nearby Judkins Quarry.  These are likely to be of relatively low permeability but there may be some hydraulic connection between groundwater at Midland Quarry and within the Judkins site.

Groundwater levels within the Sherwood Sandstone in the area to the east of Midland Quarry are likely to be controlled in part by drainage to the River Anker which crosses the 80 m AOD contour around 1 km east of Midland Quarry.  Consistent with this understanding, groundwater level data from an observation borehole around 1.5 Km southeast of Midland Quarry show that groundwater levels in the Sherwood Sandstone fluctuate between approximately 80.5 to 81.5 m AOD at this location.  Since the Sherwood Sandstone crops out within the south eastern part of the Midland Quarry void it is possible that while water levels in the quarry remain below around 80 m AOD groundwater may discharge to the quarry from the Sherwood Sandstone.  However it should be noted that this assertion is based up on the 1:50 000 geological map of the area and should be treated at this stage with caution given the complexity of the site geology. For example, the hydraulic significance of the fault running just to the northeast of the quarry is not known.

Water Balance

The estimation of a sustainable water level for the main quarry void requires an understanding of the water balance.  The water balance of the site is likely to made up of the following components:

RF + GW_in + SW_in = AE + GW_out + <S

Where

RF = rainfall

GW_in and GW_out = groundwater input and output respectively

SW_in = surface water inflow from runoff

AE = actual evaporation

<S = change in water storage i.e. quarry water level

Further work is proposed to identify the potential to enhance this component, by diversion of surface water drainage or by diversion of flood flows from the surrounding catchment (e.g. Holly Stitches and Bar Pool Brooks) which will also potentially benefit the wider catchment flood risk.  The quarry water levels are likely to fluctuate seasonally due to the balance between rainfall and evaporation through the year.  Long term annual average rainfall for the site is likely to be around 658 mm/a (for the River Anker catchment in the period 1966-95 taken from the Hydrometric Register).  Water levels in the main quarry void currently recover to around 65 m AOD when pumping from the main void ceases.  It may be that water levels will recover above this level in the long term.  However ongoing site investigations and review of historical data from Judkins Quarry will help refine likely groundwater level recovery.  The information required to improve this understanding of groundwater levels is recommended below.

3.4                   Further Information Requirements

•          Obtain data to confirm site water balance

•          Collate and interpret geological logs and groundwater level data for observation boreholes at Judkins quarry.

•          Draw together information regarding the location of seepages and springs into the quarry voids from groundwater.  This will be anecdotal evidence from periods when the quarry water levels were kept low (by pumping to the canal) and also more recent observations made during pumping the shallow and main voids. 

•          Historical information regarding required pumping rates during extraction would also be useful.

•          Assess hydrogeological data from the site investigation currently being carried out.

Appropriate interpretation of these data may aid the understanding of the hydrogeological regime of the site and help confirm the sustainable water level for the development.

3.5                   Water Management Plan

Further analysis of available data and formal consultation with British Waterways, Environment Agency, Camp Hill Urban Village Project, Local Authorities and Waste Recycling Group (operators of Judkins Quarry) will be undertaken to agree a water management plan for Midland Quarry. The objective of this further work and management plan is to ensure that a long term sustainable water management regime for the main quarry waterbody:

•          Confirmation of site water balance and confirmation of sustainable main void water level taking account of the wider aquifer interactions

•          Ensure any flood risk to DEV5 from upstream flows is addressed and mitigated to the agreement of the Environment Agency

•          Ensure British Waterways requirements to maintain flows in the Bar Pool Brook for support to levels in the Coventry Canal are met, including any future proposals to develop a connection between the canal and main quarry void.

•          Identification of the potential benefit of Midland Quarry for catchment flood management and as a dry weather reservoir for supply to the Coventry Canal

•          The need for remedial engineering works to sustain water levels to the optimal level, taking into account final end use as a Marina, water resource, potential flood balancing scheme and as an opportunity to enhance conservation value.

•          Propose a monitoring programme of water levels and water quality

4.                    Conclusions to date

The engineering proposals for the end use of the quarry have taken account of the current understanding of the water management regime, and recognise the complexities on the recovery of groundwater levels.  Current engineering proposals assume a recovery of groundwater in the main void to 80m AOD, however, further analysis of groundwater data is required before this can be confirmed. The engineering design has catered for variability in groundwater levels.

The overall water management principle for the development is one of ‘self containment’ by keeping surface water drainage self-contained by directing it to the lake to assist sustaining a higher surface level. Any surplus water in the main quarry void will be directed to the Coventry canal through the existing discharge agreement with British Waterways.

A lower groundwater level in the main quarry void will not adversely affect overall primary end use proposals or engineering stability. Water inputs to the void may be enhanced by changes to the water management regime, possibly by flow augmentation from the surrounding catchment and surface water drainage.  A water balance study and water management plan is proposed to address this issue in more detail.  British Waterways views and interest in connecting the Coventry Canal to the main quarry void also need to be subject to further feasibility study depending on the confirmation of sustainable water levels.

 

Author:

 

 

Chris Hughes, Principal Consultant

 

Reviewer:

 

 

Tim Haines, Associate Director