1DAB Geotechnics Ltd.,
3, Tweed Avenue,
Ellington,
MORPETH,
Northumberland,NE61 5ES.
Tel. No. 07711 168524
01670 861709
(0161 330 8617)
DAB/DAB/09059/07
10th March 2016
H. J. Banks & Company Ltd.,
Inkerman House,
St. John’s Road,
Meadowfield,
DURHAM,
DH7 8XL.
FAO Gemma Dickenson
Dear Gemma,
General
You asked me to review and comment, as appropriate, on the points raised by the Environment Agency (EA) in its response to the consultation by Northumberland County Council on the proposed Highthorn site (EA letter ref NA/2015/112875/01-L01 dated 30 November 2015). In particular, you asked me to provide a response to the Environment Agency’s comments with regard to the approach taken in the submitted Environmental Statement to options for groundwater management for the proposed site.
As explained in the Environmental Statement (Chapter 16) and discussed below, at this time there are several options available to the developer for groundwater management. I therefore welcome the position of the EA that final details of the option chosen should be made the subject of a subsequent submission.
HIGHTHORN SITE - GROUNDWATER MANAGEMENT OPTIONS
1.Drawdown facilitated by pumping at Lynemouth
There has been substantial groundwater recovery on and around the proposed Highthorn site following the closure of Ellington Colliery in 2005. The effects have been described in Chapter 16 and technical appendix 5 of the Environmental Statement. The Coal Authority has now established pumping operations at Lynemouth with the intention of controlling the level at 34m below Ordnance Datum to protect inland aquifers and prevent uncontrolled discharges of mine water. An optimum abstraction rate is being determined so that the necessary level of pumping can be implemented on a permanent basis, when required, even if this follows a further period of recovery.
DAB Geotechnics VAT. No. 126 1766 18
As noted in the ES, negotiations have been held with the Coal Authority with the intention of increasing the current rate of abstraction to achieve the desired level of drawdown at Highthorn if planning permission is granted. This will minimizethe pumping task at the site, improve stability and eliminate the risk of inrushes. If the groundwater levels are drawn down, it will be possible to recover the coal reserves around the margins of the pillared and longwall workings that extend from Ellington Colliery to the south east of the site. The water that enters the surface excavations will then comprise surface runoff, together with minor groundwater ingress along the Grange Moor Fault.
2.Partial or no further drawdown by pumping at Lynemouth
The Coal Authority is currently stabilizing the groundwater levels at Lynemouth. Regular monitoring of the piezometers at Highthorn has shown that apart from the localized effects of recharge due to record levels of rainfall and restricted flow through the abandoned mine workings, the groundwater levels are now showing little change. If only some or no further drawdown can be achieved by pumping at Lynemouth, the operator will be left with two options:
(a)leave a barrier of solid coal around the flooded workings; or
(b)dewater the flooded workings that lie within the proposed excavation area by pumping at the site.
Option (a)
The Quarries Regulations (1999) do not provide any guidance with regard to the provision of coal barriers around flooded workings. However, the Mines Regulations (2014), Part 3, Regulations 33 to 35 stipulate the need to assess the risk of an inrush where underground mining operations are likely to extend within 37m of any disused workings measured in any plane. These regulations replace The Mines (Prevention of Inrushes) Regulations of 1979, but inherit much older guidelines intended to account for inaccurate mine records. Clearly, the risks and consequences of any inrush into an underground mine will be far greater than those of a surface mining operation. Exploratory drilling has proved that the longwall panels and associated headings shownon the most recent abandonment plans for Ellington Colliery have a relatively high level of accuracy. Further investigations during the operation of the site may well allow a narrower standoff or pillar to be formed without compromising safety. Members of the public and adjacent properties would in any event be immune from any unintended inflows of water into the excavations.
Table 1 has been taken from an Excel spreadsheet and provides details of the flow rates that would occur through the coal pillars from the flooded workings. These have been calculated using Dupuit’s equation; the groundwater levels currently observed in each of the affected seams; and an average head measured along the coal pillars. The hydraulic conductivity of the coal pillars was assumed to be 10-6 m/s which is considered to be reasonable estimate given the likely effects of mining subsidence and surface excavation. The results show that even if the conductivity was one order of magnitude higher, the inflow rates would still be relatively low and, based on experience gained at other sites, certainly manageable. The same conclusion can be drawn if the width of the coal pillars is reduced, especially where the head of water is lower (i.e. in the areas around the current rest levels). Where there is no head of water it will be possible to expose the old workings.
Table 1 Estimated Inflow Rates from Protected Areas of Mine Workings
Given that measures have already been taken to exclude those parts of the site that have already been worked, the provision of these protective coal pillars will not have a significant effect on the reserves at the site. There is very little impact on those identified in the Ashington (4D20) and High Main (E000) seams for example.If only partial drawdown can be achieved by continued pumping at Lynemouth, any reduction in the levels at Highthorn will clearly reduce the length and possible width of the protective coal pillars and hence the loss of reserve.
Option (b)
If the developer chooses to substantially dewater the flooded mine workings around Highthorn, a relatively high level of abstraction will be required to remove the water already held in storage and counter the natural recharge. The workings will also have to be dewatered before the surface excavations are extended into the affected areas.
An estimate has been made of the volumes of water that lie between the present levelsof flooding and thoseto which the water will have to be drawn down. The likely effects of mining subsidence have been taken into consideration. Clearly, where avoid created by coal extraction has effectively migrated above the level of flooding as a consequence of roof collapse, there will be some reduction in the volume of stored water. The storage is otherwise equivalent to the product of the extraction thickness and the area of extraction. The degree and extent of mining subsidence above the areas of longwall extraction will obviously be far greater than those overlying pillared workings where there is a greater support of the roof strata, but it is not possible to determine precise figures that express these effects.The volumes presented in Table 2 can therefore only be considered to be very approximate.
.
Seam Name / Seam Code / Type of Working / Area of Flooded Workings (m2) / Extraction Thickness (m) / Extraction Volume (m3) / Volume of Stored Water (m3)Ashington / 4D20 / Pillared / 0* / 1.25 / 0* / 0*
High Main / E000 / Longwall / 11,351
11,947
16,955
(to -39m AOD) / 1.07 / 43,071 / 10,768
Top of Ashington / F200 / Longwall / 189,014
(to -50m AOD) / 0.97 / 183,344 / 45,836
Yard / G210/G120 / Longwall / 129,929
63,768
2,140
(to -58m AOD) / 1.22 / 238,921 / 59,730
G120/G110 / Pillared / 4,064
(to ca -50m AOD) / 1.20 / 2,438
(at 50%) / 853
Total / 117,187
(117 million litres or 25.7 million gallons)
* Assumes the present level of flooding is -26m AOD (as for the High Main (E000).
Remaining void space assumed to be 25% in longwall workings and 35% in pillared workings.
Table 2 Estimated Minimum Volume of Stored Water Requiring Abstraction
The calculations assume that there is sufficient separation between the workings that extend within Highthorn site and those that lie beyond it, but which nevertheless hold water. The old workings plans indicate that there are pillars of coal and unworked areas across which groundwater flow will occur, but at rates that can be adequately matched by pumping. On the otherhand, there could still be a significant level of interconnection along former roadways to allow the more immediate mobilisation of larger volumes of stored water coupled with a higher rate of recharge. This would have to be matched by more pumping at the site.
The quantity of water that can be stored in the Yard (G120/G110) old workings that extend from the former Ferneybeds Colliery is more restricted as this lies up-dip of the solid coal that surrounds and underlies the longwall panels in the Yard (G210/G120)seam worked from Ellington.
Exploratory boreholes can provide important flow paths for groundwater, but by far the majority of those drilled on the site were cement grouted to rockhead. Most of the boreholes were extended below the proposed excavation pavement, but the quantities of cement used were closely checked for contractual purposes and there is some certainty that the work was properly executed. Consequently, the flow of water through the excavation pavement should be minimal.
It is likely the developer would utilise submersible pumps located in deep wells to carry out the dewatering. Similar deep wells were used to dewater mine workings at the former East Chevington and Stobswood sites. The estimated volumes of stored water were 237 and 74 million gallons respectively. In both cases, it proved possible to locate the wells where the submersible pumps could be adequately supplied: specifically along main roadways. At this stage the work has not been undertaken to establish the optimum location for any deep well pumps. Such work would only take place if this option were to be pursued by the developer.
I trust that these comments regarding the various options available to the developer are helpful. If I can be of any further assistance, please do not hesitate to contact me at the above address.
Yours sincerely,
David Blythe